synthesis and reactions of some

I

SYNTHESIS AND REACTIONS OF SOME POLYHALONITROSOETHANES

II SOME S TEREOCHEMICAL ASPECTS OF NUCLEOPHILIC SUBSTITUTION IN U NS AT UR AT ED FLUOROCARBONS

by

Edward Werner Cook A . B . , Cl a r k U n i v e r s i t y , 1959

A thesis submitted School

to the Faculty of the Graduate

of the U n i ver si ty of Colorado in partial

fulfillment of the requirements

for the Degree

Doctor of Philosophy De partment of Chemistry 1966

R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.

This Thesis for the Ph.D. degree by Edward Werner Cook

has been approved for the Department of Chemistry by

R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission.

Cook, Edward Werner (Ph.D., Chemistry) I.

Synthesis and Reactions of some Polyhalonitrosoethanes. II.

Some Stereochemical Aspects of Nucleophilic

Substitution in Unsaturated Fluorocarbons. Thesis directed by Professor Joseph D. Park. In Part I, the addition of nitrosyl chloride to fluorinated olefins was investigated. found to promote the additions.

Lithium iododichloride was A possible mechanistic

pathway for additions of this type is proposed.

Two new

reactions were shown to have potential synthetic utility for preparing fluorinated nitrosoalkanes:

photolysis of hexa-

fluoroacetone with nitric oxide, and reaction of perfluoroalkenyl Grignard reagents with nitrosyl chloride. Polyhalonitrosoalkanes were shown to be versatile and reactive intermediates because of their electrophilic electrons.

The addition of these nitrosoalkanes to olefins,

Grignard reagents, and triphenyl phosphine is discussed. Decomposition of fluorinated nitrosoalkanes to imines was investigated.

Evidence to show the intermediacy

of an N-nitrite was obtained.

A possible mechanistic path

way is discussed and clarified. In Part II, cis- and trans-2,3-dichlorohexafluorobutene-2 were isolated for the first time.

The stereochemis

try of their reaction with alkoxide ion was studied, as was their relative reactivities.

iii

R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited without perm ission.

The trans butene has a relative reactivity about three times that of the cis isomer.

Both isomers yielded vinyl

ethers with a net retention of configuration over the tem perature range studied, although retention was higher with the cis (901) than the trans (40t). The stereochemistry of alkoxide attack on 1,1,2trichlorotrifluoropropene-1 was also studied. The implications of this study on the stereochemistry and conformational requirements of the intermediate carbanion is discussed. This abstract of about 220 words is approved as to form and content.

I recommend its publication.

Signed Instructor m

cnarge of dissertation

iv


CONTENTS

ABSTRACT

iii

CHAPTER I.

SYNTHESIS AND REACTIONS OF SOME POLYHALONITROSOALKANES ..................................

1

Historical Review A.

Synthesis.of Polyhalonitrosoalkanes

B.

Physical Properties of Polyhalonitroso alkanes ....................................

16

Reactions of Polyhalonitrosoalkanes

17

C.

. . . .

2

. . . .

Discussion A.

New Syntheses of Polyhalonitrosoalkanes

B.

Reactions of Polyhalonitrosoalkanes

. .

33

. . . .

40

Preparation of 1-Nitroso-l,2-Dichlorotrifluoroethane ...............................

43

Preparation of l-Nitroso-2-Chloro-l,2,2Trifluoroethane ...........................

44

Attempted Reaction of Nitrosyl Sulfuric Anhydride with Trifluoroacetic Acid . . . .

44

Preparation of Nitrosotrifluoromethane by Photolysis of Hexafluoroacetone with Nitric Oxide ................................

45

Preparation of Nitrosotrifluoroethylene from Trifluorovinyl Magnjsiura Iodide and Nitrosyl Chloride .......................

46

Experimental A. B. C. D.

E.

F. Reaction of Triphenylphosphosphine with 1 ,2-Dichlorotrifluoronitrosoethane ........

v

R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout perm ission.

.

46

G.

Attempted Preparation of N - (1,2-dichlorotrifluoroethyl)-3-chlorotrifluoro oxazetidene, BatchProcess .................

47

Preparation of N-(1,2-dichlorotrifluoroethyl)-2,2,3,3-tetrafluorooxazetidene, Batch P r o c e s s ................

47

Attempted Preparation of N- (1,2-dichlorofluoroethyl)-3-chlorotrifluorooxazetidene, Flow S y s t e m ...............................

48

Attempted Preparation of N-(1,2-dichlorotrifluoroethyl)-3-broraotrifluoro oxazetidene, FlowS y s t e m ...................

48

Preparation of N-Trifluorovinyl-2,2,3,3tetrafluorooxazetidine ....... . . . . .

49

Attempted Preparation of N - (1,2-Dichlorotrif luoroethyl) -3,3-dichlorodifluoro oxazetidene ...............................

49

Reaction of 1,2-Dichlorotrifluoronitrosoethane with Cyclopentadiene ........

49

Disproportionation of 1 ,2-Dichlorotrifluoronitrosoethane. . . . . ............

50

R E F E R E N C E S ............................................

5.1

H.

I.

J.

K. L.

M. N.

CHAPTER II.

SOME STEREOCHEMICAL ASPECTS OF NUCLEOPHILIC SUBSTITUTION IN UNSATURATED FLUOROCARBONS ...

59

INTRODUCTION ...................................

59

D I S C U S S I O N .....................................

61

Experimental A. B. C.

Cis- and Trans-2,3-Dichlorohexafluorob u t e n e - 2 ...................................

74

Cis- and Trans-2-Methoxy-3-chlorohexafluorob u t e n e - 2 ...................................

75

Cis- and Trans-2-Ethoxy-3-chlorohexafluorob u t e n e - 2 ...................................

76

vi


D. E.

Cis- and Trans-2-Isopropoxyhexafluorobutene-2 .......................

78

Cis- and Trans-1-Ethoxy-1,2-dichlorotrifluoropropene-1 . .......................

79

TABLE I. II. III.

IR Double Bond Vibration Adsorption Frequencies in c m " l ....................

80

Proton NMR Data

81

...........................

Reaction of Alkoxide Ion with Cis- and Trans-2,3..................... Dichlorohexafluorobutene-2

82

IR Spectrum of Cis-2,3-Dichlorohexafluorobutene-2

83

IR Spectrum of Trans-2,3-Dichlorohexafluorobutene-2 . . . . . . . . . .....................

84

IR Spectrum of Cis-2-Ethoxy-3-chlorohexafluorobutene-2 ...............................

85

IR Spectrum of Trans-2-Ethoxy-3-chlorohexafluorob u t e n e - 2 ..........

86

V.

IR Spectrum of Cis-2-isoPropoxy-3-chlorobutene-2

87

VI.

IR Spectrum of Trans-2-isoPropoxy-3chlorobutene-2 .................................

88

IR Spectrum of Trans-l-Ethoxy-l,2-dichlorotrifluoropropene-1 . . . . . ...................

89

FIGURE I. II. III. IV.

VII. VII.

IR Spectrum of Cis-l-Ethoxy-l,2-dichlorotrifluoropropene-1 ...............................

REFERENCES

. . . *

....................................

J

vii


90 91

I

SYNTHESIS AND REACTIONS OF SOME POLYHALONITROSOETHANES

In 1936, Ruff

1 7 * announced the synthesis of tri-

fl uo ro ni tr o s o m e t h a n e , the initial member of an unusual series, by elemental fl uorination of silver nitratecontaminated silver cyanide.

Fluorinated nitrosoalkanes

were to remain a curiosity, however, until reported^

a more practical

1953 when Banus

and general route by p h o t o l

ysis of trifluoroiodomethane wit h nitric oxide. practical route established, vestigated and shortly ported

With a

their chemistry was soon in

thereafter

(1955) Haszeldine r e

the accidental systhesis of a liquid copolymer from

trifluoronitrosomethane and tetrafluoroethylene.^ Yakubovich,

Shpansky,

and Lemke^

ln 1954 hgj

Earlier,

obtained a

polymer from 1 , 1 -d if lu o r o - 1 ,2-dichloro-2-nitrosoethane (via 1, 1-difluorochloroethylene and nitrosyl chloride) and 1,1-difluorochloroethylene although the constitution of the polymer was unknown.

Several investigators

in this

country working concurrently on this material believed this copolymer mi ght possess

some unusual elastomeric

properties and initiated intensive research programs.

7 8 ’

One result is the existence of a variety of synthetic routes to these nitrosoalkanes

and a baffling and sometimes c o n

tradictory body of literature describing their reactions.


2

In reviewing prior work in this field, a subdivision into three major headings has been found most convenient: synthesis, physical properties, and reactions of polyhalo nitrosoalkanes. Historical Review A.

Synthesis of Polyhalonitrosoalkanes A discussion of the various methods employed is

best served by considering two broad classifications:

(1)

syntheses utilizing substitution reactions and (2) systheses utilizing addition reactions.

Despite this, there

remain one or two preparations of doubtful classification; these have been treated arbitrarily.

These synthetic meth

ods are treated chronologically where clarity is not sacri ficed. Following the initial synthesis of trifluorenitroso1 2 methane by Ruff in 1936 * and except for a brief report (without details) by Huckel

in 1946 on the oxidation of

trifluoronitrosomethane, no new synthesis of this compound appeared until 1953 when Dacey*^ reported its formation by trapping trifluoromethyl radicals (generated from trifluoroiodomethane) with nitric oxide.

At about the same time,

3 4 Banus ’ independently reported this as a practical method for the preparation of trifluoronitrosomethane.

Thus, the

concept that perfluoronitrosoalkanes could be prepared by trapping perfluoroalkyl free radicals with nitric oxide was born.

Haszeldine*1 demonstrated the generality of the


3

reaction by utilizing the perfluoroethyl and perfluoropropyl iodides while workers at the Minnesota Mining Mfg. g Co. have claimed that bromotrifluoromethane may be sub stituted for iodotrifluoromethane, although the yields are undoubtedly much lower.

These gas phase photolytic reac

tions are carried out in a quartz apparatus usually in the presence of mercury.

Unfortunately, the necessity for using

mercury, presumably to scavenge iodine and nitrogen dioxide, ia in doubt for Mason

12

Haszeldine^*claims

considers mercury detrimental while it essential.

As perfluoroalkyl iodides appear to be the most satisfactory source of perfluoroalkyl radicals, the most severe limitation is the availability of these iodides. There are only two generally satisfactory routes (Cf. Ref. 17).

The first is the well-known Hiinsdiecker reac

tion of silver perfluoroalkyl carboxylates with iodine. Although this reaction generally proceeds well there are only a restricted number of perf luoroalkyl carboxylic acids available.

Two more recent routes are 1) addition of

either a mixture of iodine pentafluoride and iodine

1 7a

2) potassium fluoride and iodine in dirnethylformamide to a fluoroolefin.

or

1 7K

The second however only goes well with

the more "activated" olefins.

For example, tetrafluoro-

ethylene gives disappointing yields while good yields may be obtained only with olefins like perfluoroisobutylene. The addition of the elements of iodine and fluorine always


4

yields the most substituted iodides while the HUnsdiecker reaction is mainly limited to primary iodides since only primary perfluorinated acids are easily available. Early workers believed the necessity of using iodides could be alleviated by reacting silver carboxylates directly with nitrosyl chloride.

However, Dale,

18

the initial

3 19 investigator, and subsequent workers ’ all reported poor yields of nitroso compounds by this method. It was only when Park, Rosser, and Lacher

20

were

able to isolate and purify the previously only suspected intermediates--perfluoroacyl nitrites--that substantial progress was made (Cf. Ref. 21). Wear

22

Later, Taylor, Brice, and

found other heavy metal salts, namely mercury and

lead, could also be used.

The necessity for using salts

was eliminated altogether however, when Park, Rosser, and Lacher

23

showed that the anhydride reacted with nitrosyl

chloride to produce equal molar quantities of acyl nitrite and acyl chloride.

Unfortunately, this concurrent acyl

chloride formation limited overall yields of nitrosoalkane to less than 50%. searchers at 3M Co.

This too was soon avoided when re24

and the University of

Colorado

25

demonstrated that substitution of dinitrogen trioxide for nitrosyl chloride gave excellent yields of acyl nitrite. Formation of perfluoroacyl nitrites by this last method appears to be a general reaction and has been ex tended to include a number of acid anhydrides, including


5

cyclic anhydrides such as perfluorosuccinic anhydride.

24

Although these acyl nitrites are quite reactive particularly with protonic substances, and may decompose violently; they can be isolated, purified, and stored in definitely if suitable precautions are taken. Decomposition of perfluoroacyl nitrites to perfluoronitrosoalkanes and carbon dioxide (Eq. 1) proceeds 25 smoothly by either pyrolysis or photolysis. (1)

RfC02N0 - -fry-- RfNO ♦ C02

An important exception is perfluorosuccinyl dinitrite, which reverts to starting anhydride and dinitrogen trioxide^**

(Eq, 2) unless it is irradiated in the solid

phase, in wh,ich case 2-nitroperfluoropropionic acid is obtained:^ (2)

CF2C02N0 c f 2c o 2n o

Although formation of perfluoroacyl nitrites, occurring only in the liquid phase, is considered to proceed by an ionic mechanism; decomposition, occurring in both liquid and gas phases, appears to be a radical process. Thus, Dick

27

was able to isolate trifluoroiodomethane by

pyrolyzing trifluoroacetyl nitrite with iodine.

This also

indicates that a true SNi process is not involved.

Gen

erally, however, the intermediate radical is elusive and


6

cannot be trapped. The major limitations of this reaction are: limited availability of perhaloalkyl carboxylic acids; possibility of reversibility with some diacyl nitrites on attempted decomposition, as with perfluorosuccinyl di nitrite; and possibility of explosion. Even so, they are today the best route in the open literature to many nitrosoalkanes, particularly trifluoronitrosomethane itself. A novel and unusual synthesis of chlorodifluoronitrosomethane has been recently reported by Knunyants, Fokin, and coworkers.

28

They first observed that this

nitrosomethane was produced when ethyl nitrodifluoroacetate was heated with hydrochloric acid.

Apparently, decarboxyl

ation to aci-nitrodifluoromethane occurs with subsequent protonation, loss of the elements of water, and finally attack by chloride ion. •0 (3)

it.

0,NCF,C07Et ----- * F - O h T l & i i \ 0H „H o — --- ► [F2O N - 0 ] * —

yr0 F-ON + L \ 0H 2

► CFZC1N0

This reaction has not yet been confirmed and no other examples are known. Knunyants and Sokol’skii

29

have also reported the

synthesis of trifluoronitrosomethane by pyrolysis of trifluoroacethydroxamic acid.


7

Several recent methods of preparing perhalonitrosoalkanes via di- and trihalomethylradicals are interest ing, even though they do not have any preparative utility. The first,

30

utilizing formation of trifluoromethyl radicals

by photolysis of hexafluoroacetone, gives trifluoronitro somethane from hexafluoroacetone and nitric oxide. yields are low.

The

This method has been studied in the present

work and will be discussed in greater detail later. The second study of interest here is the report by Henglein**1 on the synthesis of trichloronitrosomethane and dichloronitrosom'ethane by ionizing radiation.

Here, an

intense electron beam from a Van de Graaff generator is allowed to impinge on carbon tetrachloride or chloroform while bubbling nitric oxide through it.

The trichloro-

methyl (or dichloromethyl) radicals generated are then scavenged by nitric oxide and the nitrosomethane produced is separated*by rectification.

Although this technique

may later have some other interesting application, the non-chain nature of the reaction, with correspondingly low G-values, requires a large energy expenditure to produce a small amount of product.

This, coupled with the neces

sity of the use of expensive equipment not generally avail able to the synthetic organic chemist, makes this procedure presently unattractive. The importance of these two procedures then, is to reinforce the view point that the scavenging of polyhaloalkyl


8

radicals produced in many different ways with nitric oxide to synthesize halonitrosoalkanes has great practical utility. Dichlorofluoromethanesulfenyl chloride and nitric acid have been shown by Soviet workers of dichlorofluoronitrosomethane.

32

to give low yields

Chlorodifluoromethane-

sulfenyl chloride gave even lower yields, while trifluoromethanesulfenyl chloride was unsuccessful.

The reaction

has not been studied further. •f nr

Tarrant

*

*

J

has shown that bis(polyfluoroalkyl)

mercury compounds, obtained by adding mercuric fluoride to a fluoroolefin, react with nitrosyl chloride to form nitrosoalkanes.

The procedure offers little promise of

preparative value.

Haszeldine

35

has reported that tris(tri-

fluoromethyl) arsine and nitrosyl chloride gave low yields of the nitrosomethane. If the central theme of the substitutive methods involves the generation of the alkyl radicals with their subsequent trapping, then it must be said that the same theme also applies to the additive methods. Synthesis of polyhalonitrosoalkanes by addition methods is generally by addition of fluoroolefins to either nitrosyl halides or nitrogen oxides.

Because some aspects

of chloronitrosation must be appreciated before a complete picture of the nitrogen oxide addition can be developed, the addition of nitrosyl halides is considered first.


9

The fluoronitrosoation of fluoroolefins with nitrosyl fluoride is the only additive technique which will produce perfluoronitrosoalkanes. studied. dine,

36

et al.

57

It is also the least

Aside from the premature publication of Haszeland the preliminary communication of Knunyants, the only serious study of this compound was made

by Andreades.

38

In this study not only was a practical

route to nitrosyl fluoride from nitrosonium fluoroborate developed, but also reliable working procedures were de tailed, including the necessity for using "perfluorinated" Hastelloy apparatus, rather than the glass or stainless r X7 steel apparatus used by the earlier workers. * The reaction may be carried out in the vapor phase without catalysts. The chloronitrosation of terpenes and alkenes is a well established method for characterizing these com pounds.

Early w o r k e r s , i n

attempting to extend this

reaction to polyfluoroolefins, met with failure.

No

nitrosyl chloride adduct was found and only compounds assumed to arise from decomposition of the presumed inter mediate polyhalonitrosoalkane were isolated.

It was con

cluded that no nitroso compounds would be isolable by this route, except for those from vinyl fluoride and polychloroethylenes as only these olefins demonstrated suf ficient reactivity. It was soon recognized by Stefani,

39

however, that


10

with the rate of formation of nitrosoalkanes being slower than their decomposition, a catalyst system was needed to increase the rate of formation of the nitrosoalkanes.

A

flow system utilizing ferric chloride was found satis* factory for producing nitrosyl chloride adducts of fluoroolefins in acceptable yields and the feasability of this method was thus demonstrated. Subsequently, Tarrant

33

found that aluminum chloride

in dimethylformamide was useful, although the particular species existent in the dimethylformamide solution is not known, as aluminum chloride reacts vigorously with this solvent.

Ih’a less ambiguous manner, Titov**® has been able

to prepare polychloronitrosoalkanes from the polychloroethylenes, aluminum chloride, and nitrosyl chloride at -20°, Here, however, the addition is anomalous to the uncatalyzed addition.® **** It was also demonstrated by Park, McClure, and A 0

Lacher

that nitrosyl chloride adds to fluoroolefins photo-

chemically.

It was further demonstrated by McClure,

42

and

A 7

independently by Ginsburg,

that a chlorine-nitric oxide

mixture was as effective as nitrosyl chloride. Although the photolytic addition of nitrosyl chlor ide is undoubtedly of a free radical nature (Eq. 4), (4)

nocI

and/or 2N0C1

NO + Cl2N0 + Cl2


11

ci2 M-fay— 2ci* Cl* ♦ C ¥ 2 m C * 2 ..— - C1CF2-CX2cicf2-cx2 * ♦ n o ------ ► cicf2cx2no the ferric chloride or aluminum chloride-induced addition mechanism is much less certain. Briefly, the uncertainty arises because an ionic mechanism would require that the fluoroolefin undergo initial nucleophilic attack by chloride ion followed by neutralization by nitrosonium ion (Eq. 5). (5)

N0C1 ----- * NO* ♦ Cl” ci" ♦ cf2 - cx2 ------v cicf2cx2" cicf2cx2” ♦ no*

-> cicf2cx2no

Unfortunately, the chloride ion is not generally considered to be sufficiently nucleophilic to make this possible mechanism attractive.

Thus, Park, Stefani, and Lacher

conclude in their convincing argument that

44

. . a con

sistent over-all ionic process can be logically eliminated." The difficulty is compounded, however, for although ferric chloride and aluminum chloride do promote the reac tion, they are also consumed and cannot be considered only as catalysts.

Just how is not known with certainty, al

though additional insight shall be gained in this study, when this promoter system is examined in a related system. It must be observed however, that it is not possible to


12

simply analyze the spent systems as they contain a con siderable number of inseparable organic--including polymeric--and inorganic substances, some of which are highly reactive. One example shall suffice:

The most convenient

method for synthesizing 1,2-dichlorotrifluoronitrosoethane is a modification of the original ferric chloridepowdered glass system employing a binary solution of acetic anhydride and c h l o r o b e n z e n e . T h i s ferric chloride solu tion is heated to about 50* while equimolar amounts of nitrosyl chloride and chlorotrifluoroethylene are bubbled through and the nitro-soethane collected in a dry-ice cooled trap.

During the course of the addition a straw colored

precipitate forms, which can be filtered off at the con clusion of the addition.

(This precipitate is not amenable

to analysis for the reasons mentioned above, but does, at least, show the presence of ferrous ion).

This dark fil

trate may then be reused as a reaction medium with fresh ferric chloride to give improved yields to the nitrosoethane!

In fact, continual reuse of the solution will

consistently improve the yields. This problem is further compounded when it is re alized, as will be demonstrated in this study, that these nitrosoalkanes are a highly reactive species.

Indeed, some

of the nitrosoalkanes generated by this method are among the most highly reactive of the series.


13

Bromonitrosation of fluoroolefins has been studied superficially by Soviet workers

A\

who photolyzed a mixture

of bromine, nitric oxide, and fluoroolefins.

This reaction

is analogous to the addition of chlorine and nitric oxide mentioned above. The addition of nitrogen oxides to fluoroolefins has received considerable attention as a possible route to nitrosoalkanes.*6,47,48,49,50

An unusuai aspect of this

approach is the inability of nitric oxide to initiate addition to fluoroolefins.

This necessitates the utiliza

tion of some agent, such as the halogens mentioned above or nitrogen dioxide, that will add initially to the double bond.

Therefore, the nitrosoalkanes produced in this way

are always bifunctional in that they always have a halo or nitro B-substituent. (6)

(Eq. 6)

CP2»CX2 ♦ Y- ----- ► YCF2-CX2« *J YCF2-CX2* «• NO ----- YCF2CX2NO Y ■ halogen, N02 Several features of the addition of nitrogen

dioxide, then, are pertinent here.

It shall also be in

structive to interpret them in a new manner. Briefly, nitrogen dioxide attacks a fluoroolefin^* 52 53 with the formation of a nitroalkyl radical, * rather than an alkyl nitrite radical (Cf. Ref. 54).

From an un-

symmetrical olefin the intermediate radical is predominantly

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14

or exclusively the thermodynamically more stable isomer depending on the energy differences involved (Eq. 7). (7)

c f 2» c f c i

► [o 2n c f 2-c f c i ]

CF2-CFCF j ----- ► [CF2NO-CFCF3] mainly + [*CF2-CFN02CF3] little Termination of the intermediate radical with nitrogen dioxide may be either on oxygen (to give nitroalkyl nitrites) or on nitrogen (to give dinitroalkanes); the former predominating for the more stabilized radicals (Eq. 8 ) . ^ * ^

Andreades^® considered steric rather than

electronic factors to be most important--nitrite formation being favored at hindered positions--but his theory fails, for example, with Eq. 8.

This author, then, proposes that

electronic factors are of deciding influence. (8)

CF2-CF2 ♦ N02 ----- ► [CF2N02CF2] ----- ► CF2N02CF2N02 4-

CF2N02CF20N0 (1:4) CF2«CFC1 + N02 ----- *• [CF2N02CFC1] ----- ► CF2N02CFC1N02 ♦

CF2N02CFC10N0

(1 :1 )


IS

(9)

CF2-CFCF3 ♦ N02 --- ► [CF2N02CFCF31 ♦ {CF2CFN02CF31 1

1

c f 2n o 2c f c f 3

c f 2c f 2n o 2c f 3

major NO2

ONO

c f 2n o 2c f c f 3

| ONO

c f 2c f c f 3

| | N02N02

minor

([c f 2n o 2c f c f 3]> > [c f 2c f n o 2c f 3]) Where the nature of the olefin allows a bidirection al addition, the major amount of nitrite observed is on that carbon that would be expected to be a less stable radical (Eq. 9).52 If nitrogen cfioxide adds to fluoroolefins to yield dinitroalkanes and nitroalkyl nitrites, then the addition of dinitrogen trioxide, which, in the gas phase is

equi-

an

molar mixture of nitrogen dioxide and nitric oxide, should produce, among other products, nitronitrosoalkanes. Ginsburg, Yakubovich, and coworkers

A7

Indeed,

have been able to

obtain up to 60% yields of nitronitrosoalkanes by this route. Haszeldine•

claim to have added nitric oxide to

fluoroolefins to produce nitronitrosoalkanes is, in reality, the addition of nitrogen dioxide followed by nitric oxide (i.e., the addition of dinitrogen trioxide).

That

nitrogen dioxide can arise by careless handling of nitric

R ep ro d u ced with p erm ission of th e copyright ow ner. Further reproduction prohibited w ithout p erm ission .

16

oxide so as to allow admixture with air is conceded by Haszeldine in a later publication.

48

However, Crawford,

49

in an independent study reported at the same time, demon strated that only a low initial concentration of nitrogen dioxide is needed; the product, for example:

l-nitro-2-

nitrosotetrafluoroethane, reacts with excess nitric oxide to generate more nitrogen dioxide.

This reaction of nitroso

compounds with nitric oxide is considered in detail in this study. While nitric oxide, by itself, will not add to 46 47 fluoroolefins, Park, Stefani, Crawford, and Lacher * have been able to add nitric oxide with the help of ferric chloride.

The products are identical to those obtained by

chloronitrosation, the chlorine moiety being obtained from ferric chloride.

Further comment on this reaction

shall be deferred to the discussion of results of the present study. Physical Properties of Polyhalonitrosoalkanes The most striking and well-known feature of the nitrosoalkanes is their deep blue color.

Originally, this

color was thought to be due to a nitroso triplet ground state but recent work, particularly spectroscopic studies by Mason (nee Banus), (see Ref. 58 and references cited therein) hay$ demonstrated that the ground state is sing let, and that the red absorption (blue color) results in a nvl-»* transition.

This worker also demonstrated that


17

the H q -w * transition occurs at about 2700X and the *-w* transition begins at

220oX,

both in the ultra violet.

This blue color is persistent, indicating that these nitrosoalkanes are totally devoid of any tendency 59 to dimerize unlike their hydrocarbon analogs. The Trouton constant of 21,9 for trifluoronitrosomethane indi cating an unassociated molecule supports this observation 12 as does the low boiling point (-84*). An extensive analysis of all the major infrared absorption bands has been presented by Mason.^ * 6 0 C.

Reactions of Polyhalonitroso Alkanes The nitrosoalkanes may be considered to undergo

six major types of reactions:

rearrangement, oxidation,

reduction, addition (to the NO double bond or to a non bonding nitrogen electron), substitution (of the nitroso group), and elimination.

Several reactions are a combin

ation of several of the above arbitrary classes, for example, the decomposition of the nitrosoalkanes may be considered to be a combination of substitution, addition, and elimin ation.

Also, oxidation and reduction are redundant classi

fications, but it is more useful to treat them separately. The rearrangement of the polyhalonitrosoalkanes to hydroxamyl halides or oximes can occur only with aH-nitrosoalkanes ;6»-31»43 (10)

RfCHXN0 ---- ► RfCX-NOH; RfCH2N0 ---- ► RfCH-N0H


18

The initial study of this rearrangement and re actions of the resultant hydroxamyl halides was done in 1954 by Yakubovich, Shapansky, and Lemke.6

In their study

they found that nitrosoalkanes that would be expected to give the oxime gave instead the nitroalkane, while only primary nitrosoalkanes possessing an a-halogen rearranged. However, at this stage of the art the experimental condi tions required for the isolation of nitrosoalkanes were not known; such compounds were considered intermediates and generally oxidized in situ.

One may conclude, however,

that the rearrangement of o-H-nitrosoalkanes to hydroxamyl halides is more rapid than the rearrangement of o-H, a-Hnitrosoalkan’es to oximes. The a-halogen on the hydroxamyl halides is readily replaced under the action of nucleophilic agents, as with the formation of phenylaminofluoroacetohydroxamic acid anilide.6

(11)

NOH NOH II . II CHC1FCC1 ♦ «.NH2 ---- ► ♦NHCHFCNH* The preparation of nitrosohydropolyhaloalkanes from

nitrosyl chloride and hydropolyfluoroalkenes is part of the present study. A considerable number of examples of oxidations and attempted oxidations of the fluorinated nitroso com pounds has been reported in the literature.

Unfortunately,

a simple study of the various oxidizing agents is not


19

possible as the oxidation product, the nitroalkane, is also a major product of simple decomposition of these nitroso alkanes.

The most detailed study of the oxidation of poly-

fluoronitrosoalkanes was reported in 1962 in a paper read before the Academy of Sciences of the USSR by Knunyants and Kobachnkov.61

They reported that acidic oxidizing agents

such as nitric, manganic, and chromic acids; and halogens ft7 (used earlier by Haszeldine) were much less effective than basic oxidizing agents.' Thus, they were able to ob tain better than 901 yields of trifluoronitromethane by using 101 aqueous sodium hypochlorite. Banus^ has been able to prepare perfluoronitropropane in 681 yield from perfluoronitrosopropane with 301 hydrogen peroxide at 100*. The azoxy compounds are also formed in strongly alkaline sodium hypochlorite solution; undoubtedly by reduction such as that outlined above (cf. Ref. 61).

This

contrasts to oxidation observed in more nearly neutral hypochlorite solution.61

Base alone64 is effective for the

synthesis of these azoxy compounds; however, hypochlorite greatly improves the yield.61 Numerous examples of additions across the NO double bond of nitrosoalkenes are reported in the literature. Several features are common to all these additions:

they

are generally radical rather than ionic, and they involve nucleophilic species preferentially (i.e., the nitroso


20

group in these systems is strongly electrophilic).

The

importance of radical addition is best appreciated if one considers that a significant amount of triplet state nitroso is present, even at low temperatures.

The electrophilic

character of the nitToso group is the result of strong electron withdrawal by the perfluoroalkyl moiety. Addition of the nitroso group to fluorinated olefins is the best studied and understood addition.

At low

temperatures (below 0*) polymer formation is favored while 7 oxazetidine formation observed at higher temperatures. (14)

CF.NO ♦ CF.-CF, ---- ► (-N-0-CFCF,-} + CF--N--- 0 3 I L v j Z'n 3 | |

CFj

CF2-CF2

The polymerization mechanism has been thoroughly 7

studied by Crawford, Rice, and Landrum

who conclude that

chain initiation is by triplet trifluoronitrosomethane (Eq. 15). (15)

CFjN-0 ---- ► CF3N«0 CF j N-0 + CF2-CF2 ---- * .N-0-CF2CF2

Propagation is assumed to occur on one site prefer entially (the other being much less reactive) (16)

*n -o -c f 2 c f 2 ♦ CFj

c f 2-n -o

(Eq. 16).

► -n -o -c f 2c f 2-n -o * CFj

CFj


21

•N-0-CF2CF2-N-0* ♦ CF2-CF2 --- ► *N-0-CF2CF2-N-0-CF2CF2

cf3

cf3

cf3

ip3

Termination may be by either the radical chain end R^NO* or R£CF2.

They found that the rate of polymerization

showed a dependence on [CFjNO] somewhat greater than 1 and on [C2F4] less than 1. Haszeldine originally published an opinion that the polymerization was ionic.^*13,65

However, he later recanted

but then proposed an equally fallacious radical mechanism that initiation was by self-decomposition of trifluoronitrosomethane*'* (a possibility he discounted six years earlier when he attacked^ a statement of Banus). Crawford, 7 Rice and Landrum were able to show that trifluoronitrosoraethane does not, in fact, decompose under the reaction conditions (highly purified, below 0°, in the dark). A large number of nitroso compounds and fluoroolefins have been polymerized, but the simplest still re mains the most interesting: CF3NO/CF2CF2, "nitroso rubO ber." This rubber possesses several unusual and inter•i esting features: it is the first nonhydrogen-containing elastomer; it has high solvent resistance, the lowest refractive index, and the lowest glass transition temper ature (Tg ■ -51*) studied thus far.**6 The ease of copolymerization is reported

to de

crease with decreasing halogen substitution on the olefin,


22

e.g.:

CF2-CF2>CF2-CFH>CF2-CH2>CFH-CH2«CH2-CH2 , while alter

ation of the perfluoroalkyl moiety on the nitroso reactant has but little effect on the rate of polymerization.

Un-

symmetrical olefins such as CF2«CXY add to give polymers of the type: (N-0-CF2-CXY)n

I

X and Y » hal.

that is, the olefinic carbon capable of forming the more stable free radical is bonded to N.

This supports the di

radical mechanism mentioned above. At higher temperatures (ca. 50-100*) oxazetidine 5

formation is observed with a variety of olefins (Bq. 14). * 15,14,36,37,38,46,48.65,67,68,69,70,71

Some of the ol(jfin8

studied include the following: c f 2»c f 2

c f 2»c f c i

c f 2-c c i 2

c f 2»c f c f 3

III

IV

V

II c f 2- c h f

VI

c 6h 5c h -c h 270

VII

(c 6h s )2o

o o

70

VIII

Additional olefins are discussed in the present study.

In

contrast to polymer formation, the oxazetidine formed with unsymmetrical olefins has the difluoromethylene group bonded to nitrogen (IXa). R,-N - 0 f i i c f 2-c x y IXa

R*-N — 0 f i i c x y -c f 2 IXb


23

j This is interpreted

to mean that a greater amount of the

intermediate biradical X (rather than XI) is formed at higher temperatures; the Rr-N-CF~-CXY f I 2 O’

’N-0-CF9-CXY ( 2 r£

X

XI

biradical X leading to oxazetidines and the biradical XI to polymers, presumably for steric reasons.

As interesting

as this explanation is, this author does not consider it to be compelling.

For example, it would not appear likely

that one biradical is formed predominantly at one tempera ture, while the other is formed predominantly at some high er temperature.

It would be more reasonable to assume that,

at best, a limiting condition is achieved where both iso meric biradicals are formed in equal amounts.

As this

would then give considerable amounts of polymer along with the oxazetidine, or alternately, both possible isomers of the oxazetidine, IXa and IXb (none of which is observed); this explanation must be discounted. It would be better to believe that both the poly mer and the oxazetidine arise from the same biradical X. It would then appear reasonable to assume that the oxazeti dine IXa would not be energetically as favorable as the polymer I, thus leading to polymer formation at low temper atures (-80 to 0*).

As the temperature is increased and


24

enargy differences become less significant, oxazetidine formation becomes more important.

At higher temperatures

(50 to 125°) the most reasonable explanation that oxazeti dine formation is observed exclusively is that one or both reactants are often above their critical temperatures, re quiring addition to occur primarily in the gas phase; a condition known to hinder polymerization. It must be mentioned however, that the actual polymerization step is not in question.

Here, the biradical

initiator adds to the-nitrogen moiety (Eq. 16). The addition of fluoroolefins to fluorinated nitroso compounds to form either polymers or oxazetidines appears to be a general reaction.

The formation of the oxazetidines,

however, suffers when the less thermally stable nitroso compounds are employed, as their rate of decomposition at higher temperatures exceeds their rate of reaction with fluoroolefins.

This observation, along with a study

utilizing nonhalogenated olefins, forms part of the present study and will be discussed in greater detail elsewhere. Subsequently, and apparently independently, to the initial studies on the addition of the fluorinated nitroso compounds, Kresze and his coworkers

72 73

*

found that nitroso-

benzene could be employed as a dienophile in the DielsAlder reaction (Eq. 17).


25

Whether or not this

reaction bears any relation to oxazeti

dine formation is not known. It would seem however, that the fluorinated nitroso compounds do not present quite as simple a case as the nitrosobenzene example. In this connection, Soviet work70 ers have reported the cycloaddition of trifluoronitrosomethane to a number of nonfluorinated unsaturated compounds (VII, VIII, XII-XIV). CH, i CH2»CC02CH3

C2H s 0 2CN=NC02C2H 5

XII

XIII

CH2»CHOAc XIV

Styrene, in addition to an oxazetidine, produced 1,3,2,4-dioxadiazine on reaction with trifluoronitrosomethane.

XV

Presumedly, the addition proceeds as follows:


a

26

(18)

*-CH«CH2 +

*c h -c h 2 « CF3-N-0-

c f 3-n «o

c f 3-n *o

XV

In all cases, addition was vigorous.

This is

reasonable in view of what must be a highly electrophilic nitroso nitrogen. It seems likely, then, that any attempt to use fluorinated nitroso compounds as dieneophiles in a DielsAlder reaction must also be concerned with the possibility of 1,2-addition. The structural determination of these adducts is aided considerably by the thermal lability of the N-0 bond 74 (ca. 53 kcal/mole) . Thus, the polymer decomposes in the following man ner:

- (CF2-CX2-N* -0)n

n CF20 + n Rf-N-CX2 XVI


27

While the oxazetidines decompose by the following scheme: (20)

Rf-N'

Rf-N - 0

Rf-N-CF2 ♦ CX20 CF.-CX.

CF2- CX2 XVII

The pyrolysis of the C£H,.CH«CH2/2CF3NO adduct is somewhat more involved:

(21)

70

C6H 5

C C6H 5 ,N N, CF. \ O ' CF.

C6H 5CH-N-CF3

-CFj NO XVIII N -

0

i

+ CH20

CF.

In addition to being a simple but effective means of structural analysis, these pyrolyses have been employed for the preparation of various azomethines.*3,38,43,63,70,75 74 In many cases the yield is nearly quantitive. Several Lewis-type bases give adducts with trifluoronitrosomethane.61

Thus, triethyl phosphite and

trimethylamine readily form complexes with fluorinated nitroso compounds, which in the presence of excess nitrosoalkane precipitate the azoxyalkane.

On the other hand,

phosphinimides and nitrosoalkanes form azoalkanes:^ (22)

C6H 5P - NC6H5 ♦ CF3NO

—

43PO ♦ CF3-N-N-C6H 5 XIX

These additions have not been completely studied. Hexafluoroazoxymethane is formed by condensation of trifluoromethylhydroxyl amine with trifluoronitrosomethane


28

(Eq. 13).

Nonfluorinated N-alkyl hydroxyl amines were also

successfully reacted (Eq. 23). (23)

0 t RfNO ♦ RNHOH ---- ► Rf-N-N-R XX However, when higher alkyl groups were substituted

for methyl on the nitroso compound, disproportionation was observed to take place (Eq. 24, cf. Eq. 12). (24)

CF3NHOH ♦ RfCF2NO ---- ► CFjNO ♦ [R£CF2NHOH]

* RfCF-NOH XXI The higher perfluoroalkylhydroxyl amines, as men tioned earlier, are unstable and spontaneously lose the elements of H and F to form the perfluoroalkyl hydroxamyl halides. Fluorinated nitroso compounds, like their nonfluorinated aromatic analogs, oxide.?»48,49,63 works of Bamberger,

7 fi 77

*

react readily with nitric

by extrapolating the previous

76

Brown,

77

and of Hasxeldine

48

(the

latter made no acknowledgement of previous studies and had insufficient evidence) reasoned the addition to occur in the following manner: '^25)

RfNO ♦ NO ---- ► [Rf-N-0*] — NO

— » [Rf-N-ONO] NO XXII


29

NO(orNO-) [Rf *] ------- ^ RfNO (or N02) XXII ---- ►

♦

NOj —

N02

♦ N2 It remained, however, for the Soviet workers^ to isolate the proposed intermediate XXII by adding two equivalents, pf nitric oxide to a nitrosoalkane at -100*. On warming in solution to -50* the intermediate XXII de composed to the N-nitrosohydroxylamine XXIII (Eq. 26), which could (26)

be isolated as the salt.

CH-OH CF-N-ONO — — ►CF-N-OH I 5i NO NO XXII, Rf=CF3 —

XXIII CF7N-0‘ + HB+ 3| NO

- -

Further warming to room temperature resulted in the loss of the second nitroso group to give the hydroxyl amine XXIV (Eq. 2 7). (27)

CH-OH CF„-N-OH * , NO

CF--N-OH 3 , H XXIV

Notice that the reaction of nitric oxide with


30

nitrosoalkanes has two effects.

First, the nitroso com

pound catalyzes the disproportionation of nitric oxide to nitrogen dioxide and nitrogen.

This is important in the

addition of nitric oxide to fluoroolefins discussed earlieT. Second, the nitrogen dioxide formed in the disproportionetion ultimately achieves a concentration high enough to produce the nitro alkane in significant amounts.

Thus,

the admixture of a nitroso compound with nitric oxide gen erates the nitro compound, a side reaction which must be borne in raind whenever nitroso compounds are synthesized and if nitric oxide is one of the reactants, i.e., prepar ations of this type suffer when long reaction times are *

i

employed. The original workers, Ruff and Giese, trifluoronitrosomethane readily decomposed.

2

noticed that They

posed that the product was perfluoroformamide. 4 proposal was discounted by Banus.

30

pro

However, this

Shortly thereafter, Haszeldine and his coworker

78

claimed that the decomposition product was a dimer formed in the following manner: (28)

CFjNO ---- ► [CFj*] + NO NO » [CFj*] + CFjNO ---- ► [ C F j - N - O - C F j ]

This claim was attacked by Ta'rte,

79

Nn

I

CFj - N - O - C F j

who considered their

evidence as incapable of proving anything.

R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited w ithout p erm ission

31

Although Haszeldine

80

defended his position, he

reversed himself within the year.

81 82 *

Subsequent investi

gations by Knunyants and his w o r k e r s , M a s o n , a n d Andreades have demonstrated that the following reaction occurs: (20)

RfN0 ----- ►[R£-] ♦ NO [Rf «] ♦ Rf-N0 ---- ► Rf -N-0* ---- ► Rf-N-ONO I | Rf Rf XXV XXVI

Blackley and Reinhard

85

have successfully isolated the

intermediate nitroxide radical (XXV, R^-CFj) as a stable purple gas which reacted with nitric oxide to form the N-nitrite XXVI. The rate determining step appears to be formation of an alkyl radical; Tarrant

33 85 ' has shown that the order

of thermodynamic stability of several nitroso compounds is: c f 2c i c f 2n o >c f 3c f c i n o >c f 2c i c f c i n o >c f 3c c i 2n o

Tjy2

49 hr.

4.0 hr.

1.95 hr.

1.1 hr.

This corresponds to the stability of the corres ponding radicals: c f 3c c i 2 ‘>c f 2c i c f c i ->c f 3c f c i ->c f 2c i c f 2 -

Therefore, in the opinion of this author, the over all ease of formation, of the "dimer" may be directly re lated to the thermodynamic stability of the polyfluoroalkyl


32

radical generated by hoinolysis of the C-N bond. The N-nitrite can be readily isolated only from the first member of the series (R^-CFj, Eq. 29).

With the

higher homologs decomposition readily occurs: (30)

ONO RfCX2-N-CX2Rf ---- ► [RfCX2-ti-CX2Rf] -«• RfCX2-N-CXRf XXVII This decomposition is not quite this simple for

many other products are formed.

Thus, Mason,

84

in the

most extensive study reported, states that carbon dioxide, silicon tetrafluoride, nitric oxide, nitrous oxide, and nitrogen dioxide were often found in considerable amounts. In addition, the nitro compound (XXVIII) the acyl halide, (XXIX) and an N-nitro compound (XXX) could also be identi fied.

They may be considered to arise in the following

manner: 131')

^2 RfCX2NO ---- ► [R£CX2*]---- =-*» RfCX2N02 «■ R£CX2ONO XXVIII 0 -N-°-X~ RfCX XXIX

(32) r £c x 2n o

NO 2 -N07X NO-X =-*■ R£CX2-N»CXR£ — =-*- R£CX2-N-CX2Rf XXVII

XXX


33

With proper precautions however, the decomposition of the nitroso compounds may be utilized as a preparative method for the synthesis of fluorinated aza-alkenes.

38 87 *

Discussion This research

program

has had thedual objectives

of (1)

development of alternate synthetic routes to fluor

inated

nitrosoalkanes and (2)

as starting materials A.

for new

utilizationof nitrosoalkanes synthetic preparations.

New Syntheses of Polyhalonitrosoalkanes Any study in organic halogen chemistry must contend

with the restricted availability of starting materials. Because of the unique industrial foundation of fluorine chemistry, only fluoroolefins are generally readily avail able and this must be considered.

Hence, the synthesis of

nitrosoalkanes from fluoroolefins would be a suitable area ‘> of study. The first problem was to determine what different catalyst systems might be capable of effecting the addition of nitrosyl chloride to fluoroolefins.

Several potential

catalysts tried, but without much success, were cobaltic chloride, manganic chloride, and cupric chloride. However, a solution of lithium iododichloride in N,N-diraethylformamide was found to be about as effective as ferric chloride in acetic anhydride-chlorobenzene.

Neither

lithium chloride nor iodine monochloride alone promoted the


34

reaction. It is likely that the reaction proceeds through a charged intermediate or a transition state of high electron density rather than by a free radical process.

It is un

likely, however, that chloride ion is an attacking species for it is generally not sufficiently nucleophilic to initi ate attack on fluoroolefins under the mild conditions em ployed.

Of course, a case may be made for other more

electrophilic olefins under different reaction conditions but it is not relevant here. Instead, the most reasonable explanation appears to be formation of a t complex with either ferric chloride or iododichloride ion.

Nitrosyl chloride can then add to the

complex, either concertedly or step-wise. The concept of a ir-complex facilitating addition to a fluoroolefin is not without precedent.

Miller

88

has

postulated such a complex with the mercuric fluoride-catalyzed addition of hydrogen fluoride to fluorinated ethylenes. This w-complex with mercuric fluoride, he proposed, enabled electrophilic addition of hydrogen fluoride to the fluor inated ethylenes to occur. It is not unreasonable to assume that here also a ir-complex facilitates the addition of nitrosyl chloride, known to be electrophilic, to chlorotrifluoroethylene. the electrophilic nitrosium ion (N0+) , generated in situ

R ep ro d u ced with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout p erm ission

Thus,

35

from nitrosyl chloride could attack the complex; the inter mediate nitrosoalkyl cation would then terminate with chloride ion: (5*)

r CF2-CFC1 + :M ---- ►

-A 0.-, +CF2CFC1N0

/

CF2

«' CFC1

C1CF2CFC1N0

The v complex concept plausibly explains the addi tion of electrophiles to fluoroolefins, normally unreactive to this type of addition. Should the addition of nitrosyl chloride be a ir-complex-assisted, electrophilic addition, the structure of the product can be predicted by simple carbonium ion theory, even though only charge localization rather than a free carbonium ion is involved.

This theory, summarized

go

by Miller,

postulates that the carbonium ion of greatest

stability is bonded to the largest number of fluorines, that is, for perfluorinated carbonium ions the order of preferred formation is 1*>2*>3*.

This order is the result

of the possibility of overlap of a non-bonding fluorine orbital with the vacant carbon orbital.

The inductive

withdrawal of electrons away from carbon, although not decisive in determining relative stabilities, nevertheless does detract from the resonance stabilization considerably; hence the iraperviousness of fluoroolefins to electrophiles


36

under normal conditions.

Carbonium order (l*>s*>3°) is

the reverse, of carbanion stability order (3#>2#>1#) and the product of electrophilic addition is the same as an hypo thetical nucleophilic addition. In addition to chlorotrifluoroethylene, several hydrogen-containing fluoroolefins were also studied, namely vinylidene fluoride and trifluoroethylene.

Vinylidene

fluoride was found to be inert under the conditions employed and was recovered unchanged.

The reluctance of vinylidene

fluoride to enter into additions is undoubtedly reflected here.

In a similar fashion, trifluoroethylene was found to

react sluggishly and in low conversion. The product from trifluoroethylene, identified by infrared spectral analysis, vas__shown to be the nitroso alkane XXXIa (Eq. 34). (34)

That the correct structure is the

CF2-CFH ♦ N0C1 ---- ► CF2C1-CFHN0 XXXIa

a-H nitrosoalkane XXXIa and not the isomeric B-H nitrosoalkane, CFC1H-CF2N0, cannot be decided by examination of the N-0 stretching frequency as this absorption is rather insensitive towards minor alterations on the carbon.

34

problem can be solved by observing the recently reported results of Tarrant, however.

He and his students, in

attempting to synthesize the nitrosoalkane XXXIa in the same manner were unsuccessful, and instead obtained only


The

37

the known dichlorotrifluoronitrosoethane (XXXII): (35)

CFZ-CFH ♦ N0C1

CF2C1CFC1N0 XXXII

Obviously, the failure of these workers to obtain the desired product was the result of an unfortunate choice of reaction conditions whereby the a-H nitroso alkane was chlorinated: (36)

CF2C1-CFHN0 + »C1 (from N0C1) --- ► CF2C1CFC1N0

Furthermore, Soviet workers

42

were able to synthesize this

a-H nitrosoethane under carefully controlled free-radical conditions (using chlorine and nitric oxide), and were able to prove the structure by chemical methods.

Since free

radical and ionic additions give the same product the nitrosoethane obtained by the Soviet workers is identical to that synthesized in the present study (XXXIa). It was somewhat surprising that this a-H nitroso ethane XXXIa did not rearrange to the hydroxamyl fluoride (Eq. 37), assuggested by infrared spectral analysis, but difficulty in obtaining this compound precluded more de tailed study. (37)

c f 2c i c f h n o

c f 2c i c f » n o h

(if at all) Several other methods of synthesizing fluorinated nitroso compounds were attempted.

The first utilized

nitrosyl sulfuric anhydride and trifluoroacetic acid.


Nitrosyl sulfuric anhydride is often considered to react as a mixture of the anhydrides of nitrous and sulfuric acids, that is, as a mixture of dinitrogen trioxide and sulfur trioxide.

Thus, it was believed that trifluoroacetic acid

could be dehydrated and nitrosated to trifluoroacyl nitrite in one step.

Neither trifluoroacetic acid nor trifluoro-

acetic anhydride could be altered by refluxing over nitrosyl sulfuric anhydride for extended periods, however. Another method, which met with limited success, was the photolysis of hexafluoroacetone with nitric oxide (Eq.

38).

The photolyses were conducted by charging

hexafluoroacetone and nitric oxide into an evacuated flask equipped with a quartz insert and a high pressure

(38)

450

watt

CFjCOCFj «• 2N0 ---- »• 2CF3NO + CO

mercury laBip.

Unfortunately, the photolysis of hexafluoro

acetone to trifluoromethyl radicals is efficient only at wave lengths below about 280oX.

An experiment utilizing a

7740 Pyrex filter, which stops all transmission below 2800^ and 501 at 310oX, resulted in a greatly decreased yield.

At these low wave lengths, nitric oxide dispro-

portionates to nitrous oxide and nitrogen dioxide at a significant rate, particularly at higher pressures.

Further

more, the rate of photolysis of hexafluoroacetone is not rapid enough to generate adequate quantified of trifluoronitrosomethane, and the extended times required lead to

with p erm ission o f th e copyright ow ner. Further reproduction prohibited w ithout perm ission.

39

secondary reactions.

The method, undeT the present condi

tions, cannot be considered to have preparative utility. Nevertheless, it is an important link in understanding nitroso chemistry. In 1909, Oddo

90

reported the preparation of nitroso-

benzene in 56% yield from phenyl magnesium bromide and nitrosyl chloride.

An attempt was made here to extend this

reaction to fluorinated Grignard reagents and in particular *J 71 trifluorovinyl magnesium iodide. Although Haszeldine claimed to have prepared nitrosotrifluoroethylene from nitric oxide and trifluorovinyl iodide, one has never been able to duplicate this work.

Preparation of this com

pound b.y Oddo’s method: (39)

CF2»CFI

CF2»CFMgI -

CF2-CFNO + MgClI

would be expected to provide a more reliable route to this interesting monomer. Although there is some evidence that the desired compound was formed and trapped in a dry-ice cooled trap (by its blue color) , the material had completely decom posed at -80* within 30 minutes." Nitrosotrifluoroethylene could not be expected to be stable in view of the known propensity of nitroso compounds to react with double bonds. A further complicating factor in this synthesis is the high reactivity of nitroso compounds with Grignard reagents.

Although this reaction will be discussed in


40

greater detail later, it is sufficient to mention here that the experimental conditions were modified to reduce the possibility of this side reaction by removing the nitroso compound as rapidly as it was formed.

Further modifications

may be desirable in the future, such as reversing the order of addition. B*

Reactions of Polyhalonitrosoalkanes In the historical discussion it was seen that the

highly reactive polyhalonitrosoalkanes may be utilized in novel synthetic applications if one realizes that these compounds behave as electrophilic free radicals by virtue of their low-energy triplet state.

Unfortunately, the rel

ative ease of cleavage of the N-0 bond with the resultant instability of the products must also be considered. Several reactions were undertaken which utilized the nucleophilic triplet nitrogen electron, but products could not be completely identified because of thermal instability. Thus, 1,2-dichlorotrifluoronitrosoethane was found to react vigorously with methyl magnesium bromide.

The

addition could follow the course indicated below: (40)

.

OH | =* CF j CICFCI-N-CH j

h \ h ,o

CF2C1CFC1-N0 ♦ CHjMgBr ---- ► as summarized by Kharasch,

92

but only a tar was obtained.

Similarly, triphenyl phosphine was found to rapidly decolorize the same nitrosoethane.

Again, only an


41

intractable tar was found. Major synthetic effort was also expended on the possibility of the cycloaddition of olefins to the nitroso group as the first step in the following scheme: (41)

,!

C-F.Cl CF,C1CFC1N0 -■ =CF-CICFCI-N

2

2

0

CF2~CFC1 ► CF~aCF-N 0 ^ | |

1-CF,«CF-N»CF~

c*

L

CF2-CFC1 Unfortunately, the thermal stability of the required nitrosoethane was such that disproportionation was inevit ably more rapid than cycloaddition.

Because of this, the

desired oxazetidene could never be obtained in greater than 21 yield, if at all. Because of these difficulties, it was hoped that utilization of nonfluorinated olefins might allow for oxazetidene formation under much milder conditions.

In

order to verify this, the nitrosoethane was slowly added to ice-cold, freshly prepared cyclopentadiene.

A very

vigorous and highly exothermic reaction occurred.

The

product rapidly decomposed and within an hour only an in tractable black tar remained.

Furthermore, an unpublished

report had reached this laboratory that several attempts to add fluorinated nitrosoalkanes to olefins resulted in explosions; this procedure was discarded.


42

Because decomposition of 1 ,2 -dichlorotrifluoronitrosoethane occurred with such facility, it was more extensively investigated.

Reports of several other workers

in this field, appearing after the conclusion of this aspect of the project, are corroborative. 1,2-Dichlorotrifluoronitrosoethane decolorized within several hours at reflux.

1,1,2-Trichlorotrifluoroethane

and 1 ,2 -dichlorotrifluoronitroethane were major components of the fractions boiling below 80°.

The higher boiling

fractions, however, were more complicated.

By carefully

distilling in vacuo this higher boiling fraction, the major component was found to be an N-nitrite by IR analysis and chemical methods.

Distillation of this N-nitrite at

atmospheric pressure occurred with extensive decomposition and a nitrosoalkane was obtained in low yield (ca. 1 0 1 ). This nitrosoalkane was shown to be identical to the original material by VPC analysis.

On the basis of these results

it is likely that the following reactions occurred: (42)

ONO CF 2 C1CI}C1N0 --- ► CF 2 ClCFCl-il-CFClCF2Cl + other products

(43)

ONO I CF 2 C1CFC1-N-CFC1CF2 C1 ---- ► CF 2 C1CFC1N0 + other products

One of the ’’other products" (the major one) indicated in Eq. 43 was shown by IR analysis to be identical to the major high boiling product.formed by decompositions at high


43

temperature.

IR analysis further indicated that this

compound was an imine, and on the basis of previous work, the following reactions have occurred: (44)

CF 2 C1CFC1N0 CF.C1CFC1N-CFCF-C1 ONO cf 2 cicfcincfcicf2ci It follows that the N-nitrite is an isolable inter

mediate in the formation of the imine from the nitrosoethane; this result obtained in the present study provides further evidence that the decomposition of fluorinated nitrosoalkanes proceeds through the mechanism presented earlier. Both the N-nitrite and the imine are highly reactive compounds:

the N-nitrite attacks glass with alarming

rapidity while the vile smelling imine fumes in moist air, and rapidly hydrolyzes to a number of compounds. Experimental A*

Preparation of 1-Nitroso-l,2-Dlchlorotrifluoroethane A solution of 15 g. lithium chloride and 2 ml.

iodine monochloride in 230 ml. N,N-dimethylformamide was placed in a cylindrical flask equipped with a gas inlet tube and a reflux condenser connected to a dry-ice cooled trap. The solution was maintained at 50° and an equimolar mixture of chlorotrifluoroethylene and nitrosyl chloride was bubbled through the solution.

The color of the solution changed


44

rapidly from pale yellow through blue-green to deep blue while a deep blue overgas developed.

The blue liquid col

lected in the trap was then fractionated to remove unreacted olefin.

The liquid remaining was washed with water and

distilled, CF 2 C1CFC1N0, b.p. 31-3°, 625 mm. (Lit: 31.7*, 630 mm.4^ -8 °, 125 mm.*'*

36.2*,68

The yields were usually

erratic and the product decomposed rapidly if not worked •*

up immediately, but yields of the order of 201 could be attained. B,

Preparation of l-Nitroso-2-Chloro-lt2,2-Trifluoroethane In the apparatus mentioned above, a solution of

15 g. lithium chloride and 2 ml. iodine monochloride in 200 ml, N,N-dimethylformamide was placed.

Equiraolar

quantities of trifluoroethylene and nitrosyl chloride were bubbled through the solution as the temperature was gradually increased.

At 70° reaction commenced, as evidenced by a

blue coloration.

After a total of 85 g. nitrosyl chloride

and 93 g. trifluoroethylene were passed through, the material

collected in the trap

was distilled.There was

obtained

5 g. CF 2 C1CFHN0, b.p.

14-18*,

31 yield.

This compound was stable for at least several

hours in

the gas phase at room

C.

Attempted Reaction of

(Lit .43 10°, 432 mm.),

temperature.

Nitrosyl Sulfuric Anhydride with

Trifluoroacetic Acid To a 500 ml., 3-neck round bottom flask equipped


45

with an addition funnel, stirrer and Friedrichs condenser and containing 25,0 g. (0 . 1 0 mole) nitrosyl sulfuric anhy dride was added 13 ml. acid.

(21.4 g., 0.10 mole) trifluoroacetic

As there was no reaction during addition, the mixture

was stirred under reflux for 1 week.

No reaction was ob

served. Another reaction was conducted in the same manner, but also utilizing 14.2 g. (0.10 mole) phosphorus pentoxide. Again, no evidence of reaction was noted. D*

Preparation of Nitrosotrifluoromethane by Photolysis of Hexafluoroacetone with Nitric Oxide An evacuated 5-1, three-neck flask, fitted with a

manometer and quartz well containing a 450 watt high-pressure mercury lamp, was charged with 26 mm. hexafluoroacetone and 60 mm. nitric oxide.

The contents of the flask were

irradiated for 60 min., during which time the pressure in creased an additional 60 mm.

At the conclusion of the

irradiation the gas was pale-blue colored.

Comparison of

the IR spectrum of this material with the IR spectra of the authentic material indicated that the gas was a mixture of nitrosotrifluoromethane and nitrotrifluoromethane along with some nitric oxide, nitrogen dioxide and carbonyl difluoride.

Only traces of carbon monoxide were detected.

A 159 min, irradiation of a mixture of 20 mm. hexafluoroacetone and 40 mm. nitric oxide was also successful


46

in producing nitrosotrifluoromethane; although a 120 min. irradiation of 170 mm. hexafluoroacetone and 340 mm. nitric oxide gave no indication of reaction at any time.

The best

conditions utilized 2 mm. (CF^^CO and 4 mm. NO with 30 min. irradiation time.

IR spectral analysis suggested

that this reaction mixture was 40% COF 2 .

501 CF^NO,

5-101 NO, and

A 7740 Pyrex filter around the lamp was used

in one instance; only a trace of product could be detected. Pyrex 7740 stops all transmission below 2800X and 50% at 3100X

E.

Preparation of Nitrosotrifluoroethylene from Trifluorovinyl Magnesium Iodide and Nitrosyl Chloride Trifluorovinyl magnesium iodide was prepared by the

method of Park, Seffl, and Lacher.

91

Nitrosyl chloride was

slowly bubbled into the cold, stirred, ethereal Grignard solution until there was no further up-take.

During the

addition, a considerable amount of tar formed in the pot and a small amount ( lected in the trap. faded.

2-3 ml.) of a pale blue liquid col Within 1 hour the color had completely

Limitatidn of starting material prevented further

study. F.

Reaction of Triphenylphosphine with 1 ,2-Dichlorotrifluoronitrosoethane A 100 ml. round-bottom flask containing 5 g.

triphenylphosphine and 10 g. 1 ,2 -dichlorotrifluoronitrosoethane


47

in 50 ml. ethyl ether and equipped with a reflux condenser was heated to reflux temperature for 1 hour.

Within 0.5 hr.

the blue coloration of the nitroso compound completely disappeared.- » After removal of the ether, only a tar re mained* G.

Attempted Preparation of N - (1,2-dichlorotrifluoroethyl)3-chlorotrifluorooxatctldene, Batch Process Fifty grams (0.34 mole) of 1,2-dichlorotrifluoro-

nitrosoethane was sealed in a 0.5-1. Parr autoclave; the autoclave was chilled, evacuated, and charged with 50 g. (0,42 mole) of chlorotrifluoroethylene.

The bomb was then

heated to 100* for 24 hours, cooled, the liquid product washed once with water, once with 1 0 % sodium carbonate solution, dried over sodium sulfate and distilled.

Only

decomposition products of the starting nitroso compound were found. H.

Preparation of N - (1 ,2-dichlorotrifluoroethyl)-2,2,3,3tetrafluorooxazetidcne, Batch Process To an evacuated 0.5-1. Parr autoclave containing

50g. (0.34 mole) of 1,2-dichlorotrifluoronitrosoethane was charged 40 g. (0.40 mole) of tetrafluoroethylene. was then heated to 110° for 10 hours.

The bomb

Rectification

(Podbielniak Concentric Tube column) of the liquid product afforded 1.5 g. of oxaretidene, p.b. 81.6-2.6°, 625 mm. (Lit.

84.6°), 1.71 yield.

The remainder of the products


48

consisted of compounds arising fTom decomposition of the starting nitroso compound. I.

Attempted Preparation of N-(lt2-dichlorotrifluoroethyl)3-chlorotrifluorooxazetidenet Flow System A vertical 25 mm. by 60 cm. glass tube packed with

4 mm. diameter glass beads and heated to 125° with a winding of glass cloth heating tube was used as the reactor. A total of 50 g. of 1 ,2 -dichlorotrifluoronitrosoethane was slowly bled into a stream of chlorotrifluoroethylene and the whole passed through the tube.

Identification of the liquid

products showed only decomposition products of the nitroso compound. J,

Attempted Preparation of N-(1,2-dichlorotrifluoroethyl)3-bromotrifluoTooxazetidene, Flow System A vertical 25 mm. by 60 cm. glass tube packed with

4 mm. diameter glass beads was used as the reactor.

The

tube was heated to 150* and SO g. 1 ,2-dichlorotrifluoronitrosoethane slowly bled into the reactor under a stream of bromotrifluoroethylene.

The liquid product was collected

in a flask at the bottom of the reactor while the excess starting materials were trapped in a dry-ice cooled trap for recycling.

At the conclusion of the run, the liquid

was washed with water and 5% sodium bicarbonate solution and then dried over dodium sulfate.

Distillation afforded

only 1 ,2 -dichlorotrifluoronitroethane (identified by


49

comparison of its IR spectrum with authentic material) and a black tar which could not be further worked up. K.

Preparation of N-trifluorovinyl-2,2, 3 ,3 -tetrafluorooxazetidine To 4 g. (0.063 mole) of zinc in 50 ml. dioxane

contained in a 100 ml. three-neck flask was slowly added with stirring 1.5 g. (0,053 mole) of N - (1,2-dichlorotrifluoroethyl) tetrafluorooxazetidene (b.p. 81.6-2.6°, 625 mm.) dissolved in 3 ml. dioxane.

The stirred mixture was heated

on the steam bath for 5 hours.

Analytical VFC analysis of

the supernatent liquid indicated that dechlorination had occurred with a yield of about 501.

Unfortunately, the

material could not be isolated with the techniques available. L.

Attempted Preparation of N - (1,2-Dichlorotrifluoroethyl)3,3,-dichlorodifluorooxazetidene A 0.5-1. Parr autoclave was charged with 100 g.

(0.55 mole) freshly distilled 1,2-dichlorotrifluoronitrosoethane and 100 g. (0.76 mole) 1,1-dichlorodifluoroethylene and then heated to 75° for 18 hours.

After cooling and

venting there remained 181 g. of liquid material.

Distil

lation VPC, and IR analysis indicated a complex mixture of decomposition products but no exazetidene. ‘

M.

2

Reaction of 1,2-dichlorotrifluoronitrosoethane with cyclopentadiene To 11 g. (0.14 mole) of freshly distilled

R ep ro d u ced with p erm ission of th e copyright ow ner. Further reproduction prohibited w ithout p erm ission .

50

cyclopentadiene in 25 ml. benzene cooled in an ice bath was slowly added 22 g. (0.14 mole) 1,2-dichlorotrifluoronitrosoethane. A vigorous reaction ensued with the addi') tion of each drop of nitroso compound, as evidenced by instantaneous loss of blue coloration. of heat was generated.

A considerable amount

At the conclusion of the addition

the solution was red; but after allowing to warm to room temperature overnight only a tacky-gummy tar was obtained. N.

Disproportionation of 1,2-Dlctilorotrifluoronitrosoethane In a sealed Pyrex tube equipped with a pressure

gauge was heated 62 ml. (94 g., 0.52 mole) 1,2-dichloro trif luoronitrosoethane at 70* overnight.

The green liquid

was distilled to give 27 g. (0.15 mole, 314 yield) 1,1,2trichlorotrifluoroethane and 38 g. (0.19 mole, 374 yield) 1 ,2 -dichlorotrifluoronitroethane

as the only major products.

A small amount of higher boiling material gave, on attempted distillation, the starting nitroso compound and other de composition products (by VPC analysis).


51

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59

II SOME STEREOCHEMICAL ASPECTS OF NUCLEOPHILIC SUBSTITUTION IN UNSATURATED FLUOROCARBONS Introduction Following the classical studies of Coffman, Raasch, Rigby, Barrick, and Hanford* on addition reactions of tetrafluoroethylene, the susceptibility of fluorinated olefins to nucleophilic (Nuc") attack has been demonstrated by a number of workers and is now considered a general reaction: (1)

Nuc" ♦ -«CF-CX

► -C(Nuc)«CXor -CF(Nuc)-CXH-

The most extensive study, particularly of the halogenated cyclobutenes, has been undertaken by Park and 2

his students

who were generally interested in the effects

of structure on the mode of nucleophilic attack. Although the reaction is well studied, there was, until recently, considerable doubt as to the exact nature of the reaction path.

However, it now appears certain that

attack of the nucleophile is followed by formation of a carbanion (Eq. 2), the fate of which is governed by several factors. (2)

F I ♦ -CF-CX------ ► -C - C-X -

Nuc

I

Nuc

-

I


Should the carbanion be sufficiently basic and a proton be available, proton abstraction occurs (Eq. 2a). On the other hand, carbanions that are sufficiently stabilized will decay be elimination of a 6-halogen (Eq. 2b). Quite recently, additional support was given to this view by Andreades^ who studied the effect of basicity on monohydrofluorocarbons.

He found that 1*, 2*, and 3*

monohydrofluorocarbons had relative rates of hydrogen exchange in sodium methoxide/methanol of about 1, 6 x 10,** Q and 1 x 10 respectively. Scrambling, a phenomenon to be expected in elimination-addition type mechanisms, was not observed. In spite of a considerable body of work establishing the carbanion as an intermediate in these reactions and of 2

the studies by Park and his students

on the influtnce of

various halogen substituents, a study of the stereochem istry of these reactions has had to await the availability of modern analytical tools for isolation and identification

R ep ro d u ced with p erm ission o f the copyright ow ner. Further reproduction prohibited without perm ission.

t

61

of the reactants and products. Furthermore, the more fundamental and more diffi cult problem of the stereochemistry of carbanions has been little studied.

The’ difficulty is that there is no known

experimental method capable of unambiguous interpretation. Thus, a carbanion may be described as either trifonal (and hence symmetric), or tetrahedral (and hence asymmetric) or tetrahedral with rapid inversion (and hence symmetric).

For

those carbanions not stabilized by overlapping of orbitals-where a planar configuration is 'assumed--the rapidly in4 verting tetrahedral is the presumed configuration. Irre futable experimental evidence supporting this hypothesis is not available, however. While the stereochemistry of carbanions has been but little studied, the stereochemistry of nucleophilic displacements in haloalkenes has received almost no atten tion.

In fact, while the problem has been examined by

several workers, most notably Jones, et all,^ who examined the reaction of ethyl B-chlorocrotonates with mercaptides, earlier workers studying fluoroalkenes have not mentioned or apparently were not cognizant of the problem. It was to be expected, therefore, that solution of this problem should provide some interesting answers. Discussion

-

Two halogenated olefins were chosen for the present study:

2,3-dichlorohexafluorobutene-2, and 1,1,2-trichloro-

R ep ro d u ced with p erm ission of the copyright ow ner. Further reproduction prohibited w ithout p erm ission .

62

trifluoropropene-1.

The first exhibits cis/trans isomer

ization while the second has none.

Both olefins are de

scribed in the literature (although the butene had never been resolved into its isomers) and both react readily with ethoxide ion to form the vinyl ethers (although the isomers were not identified or isolated).

These two ole

fins may be described as typical polyhalogenated olefins. Both are liquids at room temperatures and thus easy to handle; they are also readily obtainable. Although 2,3-dichlorohexafluorobutene-2 is available from specialty supply houses, previous undisclosed work** in this laboratory has shown that dechlorination of 2,2,3,3-tetrachlorohexafluorobutane, which is commercially available, should produce approximately equal quantities of the cis and trans isomers.

On the other hand, the

distribution of the commercially available butene was not known. This expectation was borne out; there was obtained after distillation of the dechlorinated produce a mixture consisting of 461 trans and 541 cis isomers of 2,3dichlorohexafluorobutene-2: CF

(3) c f 3c c i 2c c i 2c f 3

Zn

Cl

/ Cl I

+

c-c

\ CF3

II-trans

CF, •\

CF /

/

\

c-c

Cl

Cl

II -cis


63

The identification of the isomeric butanes II-trans and II-cis was expedited by infra-red spectroscopic analy sis.

The trans butene possesses a center of symmetry which

also coincides with its center of mass. on the other hand, has no such center of

The cis isomer, symmetry

(although it does have a plane of symmetry).

Since the

trans isomer has no dipole change associated with the vi bration of its

double bond, this vibration should be

infra

red inactive.

By the same reasoning, the cis isomer would

* J

be expected to show strong absorption in the carbon-carbon double bond vibration region. As expected, one compound had no absorption in the 1500-1700 cm * region and was thus considered to be the trans isomer.

The other compound possessed a strong

ab

sorption band at 1605 cnT* and was, therefore, considered to be the cis isomer. The purified olefins were then reacted with several potassium alkoxides:

potassium ethoxide and potassium

isopropoxide, first on a large preparative run in order to isolate and identify the products, and then later on a small scale under more carefully controlled conditions in order to determine the isomer distributions. Isolation of the desired isomers by means of preparative vapor phase chromatography was facile, and identification proceeded along the same general principles


64

outlined above for the starting butenes, but with several important modifications.

First, the replacement of an

alkoxyl-moiety for a chloro-moiety in the butenes alters the center of symmetry of the trans molecule to an approx imate center of symmetry.

This means that both the cis and

trans isomers will display C-C double bond vibration absorption bands, but with predictably different intensities. Thus, trans-2-ethoxy-3-chlorohexafluorobutene-2 exhibits a weak band at 1642 cm’* while the cis isomer exhibits a much stronger absorption band at 1626 cm *.

The shift to higher

frequencies for the trans isomers is also significant. Secondly, proton NMR analysis is of great assistance in helping to determine the configuration of the isomeric ethers.

As the data in Table II indicate, protons a to

the ether oxygen are deshielded by about

0 .0 7 t

when the

trifluoromethyl moiety is trans rather than cis.

Although

this shift is small, it is well outside any experimental error.

Quite recently, Brace

7

and Machleidt and Wessendorf

have arrived at the-same conclusions.

8

Furthermore, in those

cases where there are 6-protons, it will be observed that they, too, are deshielded more by the trans-trifluoromethyl group than the cis although in these cases the difference is 0.03 t . In order to minimize any possible errors in deter mination of isomer distribution of the vinyl ethers several


65

precautions were undertaken.

First, reactions were con

ducted under nearly isothermal conditions.

Second, there

was always an excess of starting olefin present so that any isomerization under reaction conditions could be detected by observing if, for example, the trans butene would con tain any cis butene at the conclusion of the reaction. As the relative reactivities of these two olefins are not greatly different (determined by utilizing a mixture of cis and trans olefin as starting material) this was judged to be a valid criterion. scrambling noted.

In no case was any evidence of

Third, the reaction product was analyzed

directly after dilution with water in order to avoid any possibility of later isomerization.

Several products were

also saved for periods of up to two weeks for further analy sis, but no change of composition was observed. To determine the effect of temperature on product distribution, several runs were done at different tempera tures.

However, as indicated in Table III, the differences

between the isomer distribution at the various temperatures cannot be considered significant. The most reasonable interpretation of this insen sitivity to temperature is that there is no duality of mechanism over the temperature range studied.

If several

simultaneous pathways to products were being utilized, then a temperature-dependent product distribution would have been


66

expected.

It follows that a single pathway is involved in

each reaction.

Hopefully, an analysis of the product dis

tribution would then provide an insight to the nature of this pathway. The product distribution of cis-trans vinyl ethers obtained from reacting 2 ,3-dichlorohexafluorobutene-2 with several alkoxides is summarized in Table III. The most striking observation is that both the cis- and trans-butenes react with net retention of config uration.

Furthermore, if one assumes that any inverted

product which is formed arises from an intermediate that has lost its asymmetry, then the cis-butene is calculated to have reacted with 90% retention of configuration, and the trans-butene 40%, Several hypothetical mechanisms may be immediately discounted on> the basis of this data.

An S^2 substitution

must, by definition, produce an inverted product: (4)

X

\/

R

C II

I

X

x » •*C»••Y + Y" ---- ► II

c /\ X

R

c X V

R

X

►

R

R

\/

Y

C !l

c /\ X -R

But, if configuration is to be retained, the nucleophile in a hypothetical concerted mechanism must attack on the same side as the leaving group:


This formulation cannot adequately explain the facts, how ever, as both isomers are observed.

Since the reaction

appears to proceed through a common intermediate (vide supra), a concerted mechanism does not appear likely. Furthermore, an addition-elimination type mechanism would involve an intermediate compound (threo and erythro) that would be identical whether generated from cis or trans olefin.

Of necessity then, there could be no difference

in isomer distribution: (6 )

R

H

X

R

X

\/

C

c

||

X

/\

R

and/or

R

|| C R

/\

Y

The only plausible explanation is that a carbanion intermediate has been irreversibly generated.

Thus, the

problem is the stereochemical and confirmational require ments of a carbanion. This must be so, as the proton which attached it self simultaneously with the alkoxide to the olefin must come from a separate solvent molecule. Hence, the addition is not exclusively cis; nor is it likely to be trans (which would be the result of asymmetric solvation) as the elimin ation would also have to be concerted (or else the addition would not have been) and would then most likely be trans itself, the net result being inversion.


68

If one regards a carbanion's conformational require ments as similar to other nonbonding-electron species (such as nitrogen) then one is thrust into the midst of 9 what Miller calls the "lone-pair ambiguity.” One supposes that a carbanion stabilized only by induction would be in a tetrahedral configuration, rather than a planar configuration required for a carbanion stabilized by resonance.

Although resonance stabilization

with a d-orbital of chlorine bonded to the carbanion is a remote possibility in this particular study, it does not appear important. Earlier, it was mentioned that the reactivities of the cis and trans butenes were not greatly different.

It

is instructive to determine to a first approximation what the reactivity ratio is. Measurement of the relative rate of reaction was conducted with a cis-trans mixture of 2,3-dichlorohexafluoro2-butene.

This technique eliminated, as much as is possible,

gross experimental errors arising from inaccuracies in re action time, fluctuations in temperature, discrepancies in concentrations, etc.

The technique is valid in this system

as there is no interconversion of isomers. In order to simplify the calculations as much as possible, two objectives were met. cis-trans mixture was £a. 1:1.

First, the intitial

Second, for a given reactivity


69

ratio, there exists an optimum stage during the reaction where determination of r has the least error.

As r was

unknown, a compromise value of 201 conversion was selected; this was later found to be reasonably close to the actual optimum conversion (ca. 301). The following data were obtained with ethoxide ion and a cis-trans mixture of 2,3-dichlorohexafluoro-2-butene at 25*:

Amount of both isomers rn,.n n e i H n T , ( ci5: Coraposition(trans;

______ Olefin______

Ether

initial

remaining

found

80.1%

19.9%

5 ^.9 45?

The relative rate of reaction

ot

ratio of specific

rate constants for a reaction of the present type is given by the equation: ^

10

_ kt _ log ( t / trt) r cC Tbi r c / o O' & s Substituting from the above data:

(8}

- - kt _ log C .362 / .517 ) t ~ — r a w ) .483 j V*

Whence: (9)

r - 3.82 '

>

In order to check the above calculation the follow ing approximate relationship may be made; given the isomer distribution for the isomers:


70

(10)

0.199 (0.945 kc + 0.299 kt) *> 0.478

and: (11)

0.199 (0.561 k + 0.700 k ) - 0.522 v L

solving: (12 )

k r -

■ 2.62 c

This result agrees well with the more accurate cal culation (9) . Of course more precise work could improve the ac curacy of r, but the main conclusion is nevertheless unmistakenly clear:

the trans butene reacts about three times

more rapidly with ethoxide ion than the cis butene. Interestingly, Dougherty,

11

in the only study of

reaction rate and stereochemistry in this field, found the solvolysis of trans-1 ,3 ,5 ,7,7,7-hexafluoro-l-heptene 20 times faster than the cis isomer (Eq. 13).

This study

involved simultaneous allylic rearrangement (internal return) and hydrolysis. (13)

c f 3c h 2c h f c h 2c h f c h =c h f

—

>- c f 3c h 2c h f c h 2c h =c h c h f 2

•I +• CF3CH2CHFCH2CH»CHCHO Observations'based on hydrogen-containing olefins are not directly relatable to the present situation, however.

In

the above case, it may well be that hydrogen bonding with the vinylic fluorine, which could only be present in the cis olefin, may play an important role in the activation


71

energies for the two reactions.

Viehe,

12

who recently

synthesized several isomeric partially-fluorinated buta dienes, has proposed’that the greater stability of the cis isomers is from just this sort of hydrogen bonding. Nevertheless, it would be interesting to speculate on the greater reactivity of the trans isomer in the pres ent case, although no sufficient data are available to entertain any definite conclusions.

It would appear, how

ever, that any explanation involving steric factors as a predominant force would be invalid, for one would naturally expect results opposite to those actually observed.

One

is left, then, to consider electronic factors to be of deciding influence. A reasonable explanation is that the electron den sity of the v bond is lower in the trans isomer than the cis and hence the trans isomer is more electrophilic. This hypothesis is borne out by IR spectral data (Table III), which clearly indicates that the juxtaposition of electron withdrawing moieties about a double bond in a trans arrangement inevitably produces a higher double-bond stretching frequency than the cis isomer. As the frequency of absorption is, in this case, directly proportional to the bond force constant, then the cis olefins are probably thermodynamically more stable than a the corresponding trans olefins. *

It will be remembered that the cis butene was

R ep ro d u ced with p erm ission of the copyright ow ner. Further reproduction prohibited w ithout p erm ission.

72

One would expect then, that the vinyl ether formed would be predominantly the cis isomer.

Indeed, this is so

with the cis olefin as 901 retention is observed; however, the trans olefin does react with 401 retention. Although it is obvious that the products are deter mined by kinetic rather than thermodynamic control, it is important to attempt to understand why net retention of configuration is observed. If an asymmetric carbanion is involved, one may visualize either cis or trans carbanion formation and either cis or trans carbanion collapse to products.

While one may

not necessarily expect either path to be followed, it has been widely assumed

13

that both trans addition and elimin

ation is favored in reactions involving asymmetric carbanions. We shall consider all alternatives, and call these modes of addition as cis- and trans-coplanar respectivtly. Now, it can be shown that if elimination of the chloride occurs from a cis-coplanar configuration of the cis-coplanar generated carbanion and trans-coplanar from the trans anion, then inversion must be faster than rotation and elimination must occur only from the inverted anion to formed in greater amounts than the trans (541 vs. 461).

This

does not appear to be a spurious result, but rather one example of a very general and unrecognized trend in many types of fluorinated compounds, including those of Viehe mentioned earlier.


12

73

retain configuration.

Alternatively, a trans-coplanar

elimination from the cis-coplanar generated carbanion (and vice versa) means that the rotational frequency must be at least two to six times the inversion frequency.

But,

there is no a priori argument that can entertain such a reality. Therefore, one must consider a planar p-orbital carbanion:

,f w RO CF

CF Cl

Here, if the elimination is coplanar then it is clear that the preferred product will have retained its configuration and the degree of retention will, to some extent, be a *

*

reflection of the rotational barrier of the intermediate. Presumedly, inversion would result if the inter mediate carbanion is rotated 240*, causing the intermediate to pass through a squew conformation. It is readily seen then that, in the squew con formation, the carbanion from the cis olefin has a CFj,0R interaction while the trans gives a CFj,Cl interaction. While the difference between the two cannot be considered large, the CFj,0R interaction appears to be less favorable. In order to explore this possibility more com pletely, nucleophilic substitution on 1 ,1 ,2 -trichlorotri-


74

fluoropropene-1 was examined: CF

OEt

CF

Cl

+

CF 3 CC1-CC1 2 ♦ OEt Cl

Cl 26.71

Cl

OEt

73.3%

Like the butene examples, the most reasonable intermediate would be: Cl

Cl

Cl

Again, it would appear that rotational barriers are respon sible for much of the difference in isomer distribution observed in the product, and that the CFj,OR repulsion is greater than the CFj,Cl repulsion. Experimental A.

Cis- and Trans-2,3-Dichlorohexafluorobutene-2 To a heated, and stirred suspension of 65 g. (1.0

mole) zinc dust in 200 ml. n-butyl alcohol in a 1-1. Morton flask equipped with an additional funnel and distillation adapter was added 1 ml. of a solution of 300 g. (1.0 mole) CFjCC^CC^CFj

(Halocarbon Products Corp.) in 200 ml.

n-butyl alcohol.

After 0.5 hr. addition was resumed and

heating discontinued; the addition was then continued at a rate sufficient to maintain reflux.

The product was distilled


75

from the flask during the course of the addition.

This

crude product was washed once with water and dried over sodium sulfate.

Rectification through a short, vacuum

jacketed, Heli-Pak column yielded 180 g. (0.78 mole) of CFjCCI-CCICFj, 781 of theory; b. 59-62°, 625 nun., lit .14 b. 67-78° (mixture of cis and trans isomers). Resolution of this distillate on both a ’'TCP'1 and a MFS-1265M column showed 45.91 of the trans isomer and 54.11 of the cis to be present. The isomers were cleanly separated and collected on a Wilkins Instrument Autoprep (automatic preparative gas chromatograph) employing a 20' x 5/8" Dow-Corning FS-1265 treated column at 50°; trans:

b.p. (micro, inverted capil

lary method) 61.1° (625 rara.), n * 5 1.3439, dj 5 1.6100; cis: b.p.

(micro) 64.5° (625 mm.), n * 5 1.3449, dj 5 1.6141. Infrared absorption spectra of the neat isomers

shows a double bond stretching absorption at 1605 cm "1 with the cis isomer and no absorption in this region with the trans. ®*

Cis- and Trans-2-Methoxy-3-Chlorohexafluorobutene-2 To a solution of 40 g. (0.172 mole) CF j CCI-CCICFj

(911 trans, 9.2% cis) in 75 ml. methanol in a 250 ml. three-neck round bottom flask equipped with an additional funnel, stirrer, and a thermometer and reflux condenser on a MU" adapter was slowly added dropwise with stirring,


76

a solution of 9.7 g. (0.17 mole) potassium hydroxide in 125 ml. methanol. After about 151 of the addition was 'J completed, a white precipitate formed. At about the same time, an ice bath was employed to maintain the reaction temperature at 25*.

At the conclusion of the addition,

the reaction slurry was poured into 100 ml. of ice cold water.

The lower organic layer was separated, and the

aqueous phase extracted twice with 50 ml. portions of methylene chloride.

The organic layer and extracts were

combined, washed twice with 50 ml. portions of water, and then dried over calcium chloride (to also remove any methanol present, later found to be unnecessary). Distillation yielded a "heart cut" b. 78-83*, 625 mm., lit. 15 b.p. 84-5* (745 jnm.) 6.4 g., 0.028 mole (161 theory).

A considerable amount of material was dis

carded to insure that a pure fraction was obtained.

NMR an

alysis indicated a singlet at t6.13 (cis isomer) and a singlet at t6.19 (trans isomer).

VPC analysis indicated,

and NMR confirmed, that the mixture was approximately 601 trans and 40% cis. C.

Cis- and Trans-2-Ethoxy-3-Chlorohexafluorobutene-2 A solution of 28 g. (0.5 mole) potassium hydroxide

in 200 ml. ethanol was slowly added dropwise to a stirred solution of 116.5 g. (91 cis, 911 trans)

(0.5 mole)

CFjCC1-CC1CF 3 in 125 ml. ethanol in a 500 ml. three-neck Morton flask equipped with an additional funnel, stirrer,


77

and a "U" adapter fitted with a reflux condenser and a ther mometer immersed in the flask’s contents.

The temperature

was maintained between 25® and 35° during the addition with an ice bath. The resultant white slurry was stirred for 3 hours and then poured into 500 ml. ice water.

The lower organic

layer was separated and dried over 8 -mesh calcium chloride. VPC analysis indicated that 26.21 of the starting butene remained unreacted, although the isomer distribution was altered to 254 cis and 754 trans.

The butenyl ether was

an isometric mixture of 66.64 trans and 33.34 cis. The isomers were cleanly separated and isolated on a Wilkens Autoprep; trans:

b.p.

n£E

1.3442, dj 5 1.3863; cis: b.p.

lip5

1.3441, d ^ 5 1. 3959. Analysis:

(micro) 92.3® (625 ram.) (micro)

Calcd. for C 6 H 5 C1F 6 0:

14.624 Cl, 47.004 F.

Found:

98.1® (625 mm.),

29.714 C, 2.084 H,

29.634 C,2.184 H,

14.924 Cl,

46.974 F. NMR data are collected in Table II.

IR data are

collected in Table I. Further studies of cis/trans isomer distribution were conducted on a semi micro scale.

In a typical experi

ment, 1.16 g. (50 mole) cis CF^CCl-CClCFj in 2.5 ml. ethanol was stirred magnetically in a 18 x 250 mm. test tube set in a constant temperature bath.

A 1.02 ml. (30 mmole)


78

aliquot of 1.68 g. of potassium hydroxide in 10 ml. ethanol was slowly added from a 2.5 ml. Hamilton "Gas-Tight" syringe.

About 15 minutes after the addition was completed,

ca. 20 ml. water was added and stirring was stopped.

The

lower organic layer was transferred to a smaller test tube containing 10 mg. of sodium sulfate with the "Gas-Tight" syringe and a 6 inch needle. liquid was analyzed by VPC. was employed.

After 30 minutes drying the In all cases an excess of butene

The results are tabulated in Table III.

Following the general procedure used above, a 1.00 ml. aliquot of 0.84 g. (15 mmole) potassium hydroxide in 10 ml. ethanol was added to 1.00 ml. (1.39 g., 6 mmole) of a 48.31 cis/51.7% trans mixture of CFjCCI^CCICFj in 5.0 ml. ethanol at 25®. CF j CCI-CCICF j : C(OEt)CFj: D.

The product contained:

80.11

54.91 cis, 45.2% trans; 19.91 CFjCCl-

47.8% cis, 52.2% trans.

Cis- and Trans-2-Isopropoxy-Hexafluorobutene-2 An amount of 13 g. of potassium hydroxide in 100

ml. isopropyl alcohol was added slowly to a solution of 50 g. (91%) trans- CF j CC1«CC1CFj in 100 ml. isopropyl alco hol at 25®.

When the addition was completed, the slurry

was poured into 1 1. ice water.

The organic layer was

separated, dried over sodium sulfate, and separated directly in a Wilkins Autoprep. Analysis:

Calcd. for CyH^ClPgO:

32.771 C, 2.75%


79

H, 13.821 Cl, 44.431 F.

Found:

32.97% C, 3.01% H, 13.48%

Cl, 44.18% F. The cis isomer exhibited a strong double bond ab sorption at 1629 era’* while the trans isomer exhibited a weak absorption at 1637 cm"*. Trans:

b.p.

25 (micro) 117* (765 mm.), n^ 1.3572,

d^

1.3434; cis:

b.p. (micro) 110.5° (765 mm.).

E.

Cis- and Trans-1-Ethoxy-1,2-Dichlorotrifluoropropene-l To 50 g. (0.2S mole) C C ^ ^ C l C F j in 100 ml. ethanol

in a 500 ml. Morton flask was added dropwise 13 g. of potassium hydroxide in 100 ml. ethanol over a period of 2 hours with stirring at 25°.

The reaction mixture was then

poured into 1 1 . ice water and the product separated and dried.

The mixture, 26.7% cis and 73.3% trans isomer, was

separated with a Wilkins Autoprep.

Trans:

132* (625 mm.), nj 5 1.4044, d * 5 1.3939; cis:

b.p.

(micro)

b.p. (micro)

127° (625 mm.), n {*5 1.4002 , [lit.16 b.p. for cis/trans mixture 129° (630 ram.)].


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TABLE II PROTON NMR DATA CF3 \

/ CF3 OC^ Cl^ OCHj 0

singlet, 6 . 1 3 t

*i

Cl ^

CF, 'C«C^ 0 CFj^ ^OCH3 CF ^)C-C^ ^ CF ^ O C H 2 CH 3

singlet, 6.19r

CF

C1\

X CF3 C-Cv. CF^ ^ O C H 2 CH3

CF3 \

Cl

C-C

/

C1

^ O C H 2 CH 3

^

/Cl C-C. CF3^ ^ O C H 2 CH 3

quadruplet, 5.92x; J-7.0 cps (CH3: triplet, 8.62t; J-7.1

quadruplet, 6 . 0 1 t ; J-7.0 (CH3: Triplet, 8 . 6 5 t ; J-7.0)

quadruplet, 5 . 7 9 t ; J-7.2 (CH3 : triplet, 8 .6 6 t ; J-7.2)

Cl \

/CF C-C. Cl^ OCH(CH 3 ) 2

quadruplet, 5,86r; J-7.2 (CH3:" triplet, 8.69t; J-7.2)

CF3^

C3K CF^

C-C

/ CF3 ^OCH(CH3) 2

septuplet, 5.40t ; J-6.0 (CHj: doublet, 8 . 6 3 t ; J-6.3)

septuplet, 5 . 4 8 t ; J-6.0 (CH3: doublet, 8 . 6 6 t ; J-6.3)


TABLE III REACTION OF ALKOXIDE ION WITH CIS AND TRANS 2.5,-DT CHLOROHEXAFLUOROBUTENE-2 OLEFIN CP3 \

/ CF3

Cl

^C1 PRODUCTS CF,^ 5

Cl^ R-CH25° 5 R-CH-CH0° L b 25° 50° R«CH(CH _ ) 7 25* L

CP.

OR

CF.OR ^^c-ccC CK CF

92.81

7.34%

97.2% 94.5% 94.9%

2.89% 5.61% 5.18%

96.5%

3.49%

30. 7%

69.3%

29.9% 32.9%

70.0% 67.3%

28.1%

71.9%

Cl C-C

Cl

CF.

R-CH, 25° 3 R-CH-CH25° 2 3 50*

R"CH(CHt)25° 5 L


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