Synthesis and Molecular Structure of Silylated ...

Synthesis and Molecular Structure of Silylated Ethenes and Acetylenes Christoph Riidinger, Holger Beruda, H ubert Schmidbaur* Anorganisch-chemisches Institut der Technischen Universität München, Lichtenbergstraße 4, D-85747 Garching Z. Naturforsch. 49b, 1348-1360 (1994); received May 20, 1994 Silanes, Trichlorosilanes, Polysilylethenes, Carbon H alogenated Silylolefines Disilylacetylene (1) has been obtained from LiA lH 4 reduction of bis(trichlorosilyl)acetylene (2) and bis[(trifluoromethylsulfonyloxy)silyl]acetylene (4). The catalytic hydrosilylation of 2 with HSiCl3 affords tris(trichlorosilyl)ethene (5) and l.l,2-tris(trichlorosilyl)ethane (6). The synthesis of 6, fra/«-bis(trichlorosilyl)ethene (8) and l,l-bis(trichlorosilyl)ethene (9) has been accomplished by hydrosilylation of trichlorosilylacetylene (7) which was synthesized by the reaction of trichloro(trifluoromethylsulfonyloxy)silane with sodium acetylide. R e­ ductive elimination of halogen from l,l,l,2-tetrachloro-bis(trichlorosilyl)ethane ( 10) and 1,2dibrom o-l,l-bis(trichlorosilyl)ethane (13) gave the corresponding ethenes 1,1-dichloro-bis(trichlorosilyl)ethene (11), trichloro-trichlorosilylethene (12), l,l-bis(trichlorosilyl)ethene (9) and l-chloro-2,2-bis(trichlorosilyl)ethene (14). Tetrakis(trichlorosilyl)ethene (15) has been obtained in a three step synthesis starting from chloromethyl-trichlorosilane or dichloromethyl-trichlorosilane. By LiA lH 4 reduction of trichlorosilylethenes under various reaction conditions, the silylethenes rra/is,-dichloro-di(silyl)ethene (16), l,l-dichloro-di(silyl)ethene (17), trichloro-silylethene (18), 1-brom o-l-silylethene (19), rra«5-di(silyl)ethene (20), l-chloro-2,2-di(silyl)ethene (21), tri(silyl)ethene (22) and l,l,2-tri(silyl)ethane (23) could be generated. Silylethyne and silyl-chloroethyne were identified as side products. The crystal and molecular structures of 2, 5 and 15 have been determined by single crystal X-ray diffraction. 2 and 5 crystallize from the melt in the m onoclinic space groups Cc and P2j/n, respectively. 15 has been crystallized by sublimation (orthorhombic, space group Pbca). 5 and 15 feature strongly distorted ethene skeletons with a double bond twist of 28.1° in 15.

Polysilylmethanes [1-6] and polysilylbenzenes [7-9] have received considerable interest as p re­ cursors for chemical vapour deposition (CVD) of silicon/carbon alloys [1-3] and for the synthesis of various silicon/carbon polymers [9], By varying their stoichiometry these compounds can be used as single source feedstock gases for layers with tailor-made composition. In a new approach to the epitaxial deposition of silicon carbide, halogencontaining feedstock gases are expected to show improved properties owing to a greater reversi­ bility of the deposition process. W hereas polysilylalkanes halogenated at carbon are well rep­ resented in the literature [1 0 , 1 1 ], only limited information is available on the corresponding alkenes. The low thermal stability of polysilylated haloalkanes makes these compounds unsuitable for practical processes. The ethene analogues are expected to exhibit greater thermal stability and to be more amenable regarding storage and con­

* Reprint requests to Prof. Dr. H. Schmidbaur. 0932-0776/94/1000-1348 $06.00

tinuous handling. Although several sterically crowded polysilyl ethenes have been the subject of investigations of their molecular structure, dy­ namics and energy characteristics [12-16], the num ber of structurally characterized compounds is still small. Following work on silylalkanes, our more recent studies have therefore focused on silyl olefines. In the course of these investigations we have undertaken the X-ray structure determ i­ nations of bis(trichlorosilyl)acetylene and of the two most crowded trichlorosilylethenes, tris(trichlorosilyl)ethene and tetrakis(trichlorosilyl)ethene. Results

Preparation and properties o f the compounds For the synthesis of disilylacetylene (1) two new routes have been investigated. The first route starts from bis(trichlorosilyl)acetylene (2 ), which is read­ ily accessible on a large as well as on a laboratory scale [17-19]. The compound can be reduced to disilylacetylene using phase transfer techniques

© 1994 Verlag der Zeitschrift für Naturforschung. All rights reserved.

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1349

Ch. Rüdinger et al. • Silylated Ethenes and A cetylenes LiAIH* / BzEt3NCl Cl3 Si— C«=C— SiCl3

H 3 Si— C = C — SW3

2

1

PhH 2 Si— C = C — S H 2Ph

c f 3 s o 3h ---------►

(CF 3 S 0 3 )H2 Si— C —C - S H 2 ( 0 S 0 2 CF3)

(1)

(CF 3 S 0 3 )H2 Si— C = C — S H 2 ( 0 S 0 2 CF3)

LiAH, -------- ►

H 3 Si— C = C — StH3

H \

HSiCl, Cl3 Si— C —C— SiCl3

/

SiCl 3 3

C=C \s i c l 3 c ^ s i'./

Speier catalyst

(3)

H

H

H- C - C ... . C ^sf

5

[5, 20] in m oderate yields. The second one is based on a selective Si-aryl cleavage by triflic acid. Bis[(trifluoromethylsulfonyloxy)silyl]acetylene (4) can be obtained starting from bis(phenylsilyl)acetylene (3) [7] upon reaction with two equivalents of triflic acid. The silyltriflate functions are further converted into silyl groups via LiAlH 4 reduction. The catalytic hydrosilylation of disilylacetylenes with silicochloroform and Speier catalyst turned out to be very slow [21,22]. Significant amounts of m onoaddition products could only be obtained at high tem perature and pressure. In an effort to accomplish double addition even m ore forcing re­ action conditions had to be used in order to add a second molecule of the hydrosilane to the trisilylethene interm ediate. The reaction of 2 with an ex­ cess of trichlorosilane at tem peratures between 80 and 110 °C and reaction times up to 24 d gave no double addition products. The yields of tris(trichlorosilyl)ethene (5) are lower as compared to those obtained previously [2 1 , 2 2 ], and 1 ,1 ,2 -tris(trichlorosilyl)ethane (6 ) is also produced. The amounts of this by-product increase as the reac­ tion conditions become more forcing. The reaction pathway which leads to 6 is not clear, but the detection of hexachlorodisilane as a by-product makes a catalytic addition-elimination

(2)

(4)

SiCl; 6

cycle the most likely origin of this by-product. For the preparation of bis(trichlorosilyl)ethenes and tris(trichlorosilyl)ethanes trichlorosilylacetylene (7) has been synthesized using the S i-C (aryl) cleavage of phenyltrichlorosilane with triflic acid [23, 24] and further reacting the resulting trichloro(trifluoromethylsulfonyloxy)silane with sodium acetylide. This procedure is more convenient for labora­ tory scale preparations than the high tem perature gas-phase reaction of various chloroethenes with trichlorosilane [19]. The hydrosilylation of 7 with trichlorosilane and Speier catalyst at room tem ­ perature leads to quantitative conversion of the acetylene. The main product is trans-bis(trichlorosilyl)ethene (8 ). The hydrosilylation is not fully regioselective, and the product of the 1 ,1 -ad­ dition, l,l-bis(trichlorosilyl)ethene (9), is pro­ duced in quantities up to 25%. The use of excess trichlorosilane, more catalyst, and forcing reaction conditions leads to l ,l , 2 -tris(trichlorosilyl)ethane

(6). Preparation o f C-halogenated trichlorosilylethenes The reaction sequence of radical chlorination of trichlorosilylethanes followed by reductive elimi-

SiCI3

+

CF 3 S 0 3H

---------►

Cl3 Si—0 S 0 2 CF 3

+

Cl3 Si—0 S 0 2 CF 3

+

N a — C = C —H

---------►

CI3 Si— C = C - H

+

((

))

CF 3 S 0 3N a

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(5)

(6 )

Ch. Rüdinger et al. ■ Silylated Ethenes and A cetylenes

1350 SiCI,

HSiCi,

SiCI,

Cl,Si— C —C - H

(7 )

Speier catalyst

HSiCI3 (excess)

-c—C

Cl3 Si— C = C —H Speier catalyst

"SiCI, SiCI,

(8 )

nation of halogen with copper proved to be very useful in the synthesis of various trichlorosilylethenes. The complete chlorination of l,l-bis(trichlorosilyl)ethane [25] under UV-irradiation gives l , l , l , 2 -tetrachloro-bis(trichlorosilyl)ethane ( 10) in almost quantitative yields. 10 is a waxy, white solid, m oderately soluble in acetonitrile and pentane, which easily sublimes and melts at 187 °C with partial decomposition. Thermal elimination of tetrachlorosilane from 10 afforded trichloro-trichlorosilylethene (12). The reaction of 10 with copper powder at 170 °C gives l,l-dichloro-bis(trichlorosilyl)ethene ( 11) in good yields. This process also leads to thermal elimin­ ation of m inor amounts of trichloro-trichlorosilylethene ( 12), which are growing with increasing re­ action tem peratures. The partial brom ination of l,l-bis(trichlorosilyl)ethane with elemental bromine under UV-ir­ radiation proceeds only very slowly. To accelerate the reaction a slow passage of a stream of chlorine gas through the reaction apparatus proved to be of great advantage. The large num ber of by-products (different partially halogenated and mixed halogenated compounds in the form of various iso­ mers) makes the work-up of the reaction mixture

SiCI, 1 ~~C—C -

-SiCl,

C ^ /h v CCl,

Cl c i\\ Cl

P

r

SiCI3

Cl

SiCI, SiCl3 SiCI,

b/

10

Cl,Si.

SiCl3

/C

\-~SiCI3

Cl

Cl

10

+ Cu

Cl

- CuCl

Cl

C=C - CuBr

(1 3 )

SiCI,

H

+ HSiCl3 /NR 3

Cl

- HNR 3 Cl/SiCl,

CI3Si

+ H SCI 3 /NR 3

Cl,Si.

- HNR 3 Cl/SiCl4

C l3S i/

H

Cl CU \

SiCI,

+ Cu

SiCI,

(9 )

( 12)

- CuBr

H CIOC - C ...C

SiCb

\

quite difficult, and even the main product, 1 ,2 -dibrom o-l,l-bis(trichlorosilyl)ethane (13) could not be isolated in a pure state. 1 ,2 -dibrom o-l-chloro2 ,2 -bis(trichlorosilyl)ethane in particular remains as the major impurity. In the reaction with copper powder compunds 13 and 1 ,2 -dibrom o-l -chloro- 2 ,2 -bis(trichlorosilyl)ethane are converted into l,l-bis(trichlorosilyl)ethene (9) and l-chloro-2,2-bis(trichlorosilyl)ethene (14), respectively (equs. 12, 13). The first synthesis reported for tetrakis(trichlorosilyl)ethene (15) [26] started from dichlorobis(trichlorosilyl)methane, obtained from bis(trichlorosilyl)methane by chlorination. The prep­ aration of bis(trichlorosilyl)methane by the reac­ tion of trichlorom ethane with trichlorosilane and a tertiary amine base [27] can be notably improved by replacing the trichlorom ethane with chloromethyltrichlorosilane or dichlormethyltrichlorosilane. The photo-chlorination of bis(trichlorosilyl)methane in CC14 solution is slow but proceeds with­ out side reactions, and the copper reduction of dichloro-bis(trichlorosilyl)methane at 160-190 °C affords 15 in 64% yield. For the conversion of the simple halosilyl com­ pounds into the hydrides the classical reduction using lithiumaluminiumhydride in an ether solvent could be applied with some success, but the scope

A

(14) H

/C b

/C = C \

( 10)

SiCb

11

Cl3 Siv

C

c / SC1

H (1 5 )

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Ch. Rüdinger et al. • Silylated Ethenes and A cetylenes

1351

^iCl, H 2C ( S iC l3)2

HT SiC U

+

S 1C I3

(1 7 )

^iHjBr B r H j S i — C — S H 2B r

+

H"

B rH 2S i— 3

XC=C\ S iC lj

23

and 70% trisilyl­

H

C Q

11

H. ^ VS1CI3 H C =C

Cl^

VSiCl3

-----► H—C = C —S1CI3 + HSiClj +

14

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C f

(23)

1352

Ch. Rüdinger et al. • Silylated E thenes and A cetylenes

ci.

,S«3

S1H 3

c= c

c= c

C=C

H

\

/S H s,

/C=CN Br

SH 3

H3Si

H

SH ,

C=C SH ,

SH 3

~C—C... v"SH 3 SH , H,Si

C I23

CI21

CI32

Fig. 3. Molecular structure o f tetrakis(trichlorosilyl)ethene (15). (ORTER 50% probability ellipsoids). CI43

Fig. 1. Molecular structure of bis(trichlorosilyl)acetylene (2). (ORTER 50% probability ellipsoids).

CI33

Fig. 2. Molecular structure of tris(trichlorosilyl)ethene (5). (ORTER 50% probability ellipsoids for non-hydro­ gen atoms; arbitrary radii for hydrogen atom).

CI21

Fig. 4. Projection of the molecular structure of tetrakis(trichlorosilyl)ethene (15) along the axis of the double bond showing the twist of the silicon atoms. (O R T E P. 50% probability ellipsoids).

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Ch. Rüdinger et al. • Silylated Ethenes and A cetylenes

products. The generation of silylacetylene and chlorosilylacetylene can be easily rationalized by a hydride attack at silicon and subsequent trans-ßelimination of chloride. This reaction is neglegible if there are no ß-fra/is-located chloro substituents as in rra«s-dichloro-bis(trichlorosilyl)ethene. The C-halogenated polysilylethenes are all col­ orless, very volatile liquids with a characteristic sweetish odor. Their therm al stability exeeds that observed for polysilylated halomethanes. In the reduction of halogen substituted alkenes no hydrogenation of the double bond could be ob­ served. By contrast, the reduction of 5 leads to a conversion into the corresponding ethane in low yields, and the reduction with lithiumaluminiumhydride in di-/?-butyl ether at low tem pera­ tures also gave rise to double bond hydrogen­ ation (Table II).

1353

Attem pts to overcome this problem through substitution of lithiumaluminiumhydride by lithiumhydride in di-/?-butyl ether, lithiumaluminiumtri-f-butoxy-hydride in di-«-butyl ether, or diisobutylaluminiumhydride in THN did not lead to any improvement. Crystal and Molecular Structures Bis(trichlorosilyl)acetylene crystallizes from the melt in the absence of a solvent in large, clear col­ umns, from which small monoclinic single crystals, space group C2/c, could be cut. The individual molecules have a crystallographic twofold axis perpendicular to the C = C triple bond. With the S i- C = C - S i group not exactly linear, the overall symmetry of the molecule is close to D 3h. Bond distances and angles are within the expected

Table III. Crystal data and details of structure determination.

Formula Mol. mass Cryst. dimens, (mm) Temp (K) Cryst. system Space group

a (4)

b (A)

c (A ) a (deg) ß (deg) 7 (deg) V ( A ?) S e a le d ( g C m

3)

Z F(000) // (MoK„) (m m -1) Diffractometer Scan Scan range (0) (deg) h k l range No. of measd reflcns No. of unique reflcns

2

5

15

C2Cl6Si2 292.90 0 .3 x 0 .3 x 0 .3 223 m onoclinic C 2 Ic 17.076(1) 6.362(1) 9.740(1) 90 91.35(1) 90 1057.8(5) 1.839 4 568 1.781 Syntex P2j

C2HCl9Si3 429.37 0.1x0.2x0.45 199 monoclinic P2 x/n 6.840(1) 22.970(1) 9.499(1) 90 92.38(1) 90 1491(2) 1.910 4 832 1.905 C AD 4

C2C l12Si4 561.80 0.1 x 0 .2 x 0 .3 203 orthorhombic Pbca 12.144(2) 18.308(4) 16.474(3) 90 90 90 3663(2) 2.038 8 2176 2.053 CAD 4

0-0

0-20

oj

2 -2 7 ± 15/23/21 1946 975

2 -2 7 ± 8/29/12 3733 3629 0.016 2778 > 4 a (F 0) 127 a, = l/(a 2(F0)+k(F0)2) 1/0.000113

2 -2 7 15/23/21 3548 3447 0.0026 2980 —4 a (F Q) 163 co = l/(a 2(F0)+k(F0)2) 1/0.001328

R'\nt

0.021

No. of obsd reflcns

893 > 4 ct(F0) 46 co = l/( a 2(F0)+k(F0)2) 1/0.002513

F0 Refined params Weighting scheme Weighting param (1/k) H atoms (found/calcd) R (shift/error)max o 6 fin (max/min) (e A ~ 3)

0/1 0.0292 0.0498 0 .0 0 0

0 .3 0 /-0 .4 0

0.097 0.113 0.007 2.58/-5.67

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0.0396 0.0510 0.028 0.8 3 /-0 .4 2

1354

Ch. Rüdinger et al. • Silylated Ethenes and A cetylenes

Table IV. Selected bond distances (A ) and angles (deg) of compound 2. (Standard deviations in parantheses, see Fig. 1 for atomic numbering.) Atoms

Bond distance

Atoms

Angle

C ( l) - C ( la )

1.186(4) 1.818(2) 2.007(1) 2.006(1) 2.9003(1)

C l(l)-S i(l)-C l(2 ) Cl(2) —Si( 1) —Cl(3) C l(l)-S i(l)-C l(3 ) C (1) —Si( 1)—C l(l) C (l) -S i(l)-C l(2 ) C (l) -S i(l)-C l(3 ) C ( l ) - C ( la ) - S i( l)

109.9(1) 109.6(1) 109.7(1) 108.9(1) 109.1(1) 109.5(1) 179.2(2)

c ( i) - s i( i) S i(l)-C l( l) Si( 1) —Cl(2) S i(l)-C l(3 )

range. The C = C triple bond length is significantly shorther as compared to that of related com­ pounds [7,31]. The SiCl3 groups exhibit almost ideal tetrahedral geometry. The lattices of tris(trichlorosilyl)ethene 5, space groupe P 2 xln , and tetrakis(trichlorosilyl)ethene 15, space group Pbca, contain individual molecules with no crystallographic symmetry. For 15 the deviation from point groupe C 2 or even D 2ci is only small. A com­ parison of the m olecular structures of 5, 15, and

Table V. Selected bond distances (A ) and angles (deg) of compound 5. (Standard deviations in parantheses, see Fig. 2 for atomic numbering.) A tom s

Bond distance

Atom s

Angle

C ( l) - C ( 2 ) C (l) -S i( l) C (l) -S i( 2 ) C (2 )-S i(3 ) C (l)-H (l) S i(l) —C l( ll) S i(l) —Cl(12) S i(l)-C l(1 3 ) S i(2 )-C l(2 1 ) S i(2 )-C l(2 2 ) S i(2 )-C l(2 3 ) Si(3) —C l(31) S i(3 )-C l(3 2 ) S i(3 )-C l(3 3 )

1.28(1) 1.86(2) 1.85(1) 1.90(1) 1.20(1) 2.013(3) 2.011(3) 2.041(4) 2.017(3) 2.005(4) 2.043(4) 2.014(4) 1.999(4) 2.002(4)

C ( 2 ) - C ( l ) - S i ( l) C (l) -C ( 2 )-S i(2 ) S i(3 )-C (2 )-S i(2 ) C (l) -C ( 2 )-S i(3 ) C ( 2 ) - C ( l ) - H ( l) S i( l)- C ( l)- H (l) angles at S i(l) C ( l) - S i( l) - C l ( 1 3 ) min angles at Si(2) Cl(22) - Si(2) - C l(23)min angles at Si(3) C l(3 1 )-S i(3 )-C l(3 3 )min S i( l) - C (l) - C(2) - Si(2) H - C ( l) - C ( 2 ) - S i( 3 ) sum of angles at C (l), C(2)

139.4(9) 120.0(8) 122.3(5) 117.7(8) 105.7(8) 114.9(8) 105.1(4)-113.2(3) C ( l) - S i( l) - C l ( 1 2 ) max 106.0(1)-117.7(1) C ( l) - S i( 2 ) - C l( 2 2 ) max 104.9(1)-118.4(1) C(2) - Si(3) - C l(32)max 0.7 0.9 360.0

Table VI. Selected bond distances (A ) and angles (deg) of compound 15. (Standard deviations in parantheses, see Fig. 3 for atomic numbering.) A tom s

Bond distance

Atom s

Angle

C ( l) - C ( 2 ) C (l)-S i(l) C ( l) -S i( 2 ) C (2 )-S i(3 ) C (2 )-S i(4 ) S i( l)- C l(ll) S i(l) —Cl(12) S i(l)-C l(1 3 ) S i(2 )-C l(2 1 ) S i(2 )-C l(2 2 ) S i(2 )-C l(2 3 ) S i(3 )-C l(3 1 ) S i(3 )-C l(3 2 ) S i(3 )-C l(3 3 ) S i(4 )-C l(4 1 ) S i(4 )-C l(4 2 ) S i(4 )-C l(4 3 )

1.361(5) 1.913(4) 1.914(4) 1.907(4) 1.908(4) 2.016(1) 2.012(1) 2.032(1) 2.023(1) 2.017(2) 2.023(2) 2.016(2) 2.016(2) 2.027(2) 2.015(1) 2.013(2) 2.024(2)

C ( 2 ) - C ( l ) - S i ( l) C ( 2 )-C (l)-S i(2 ) S i( l) - C ( l) - S i( 2 ) C (l) -C ( 2 )-S i(3 ) C (l) -C ( 2 )-S i(4 ) S i(3 )-C (2 )-S i(4 ) angles at S i(l) C ( l) - S i( l) - C l ( 1 3 ) min angles at Si(2) C ( l) -S i( 2 )-C l(2 3 )min angles at Si(3) Cl(31) - Si(3) - Cl(33)min angles at Si(4) C (2 )-S i(4 )-C l(4 3 )min S i( l) - C ( l) - C ( 2 ) - S i( 3 ) Si(2) - C(1) - C(2) - Si(4) sum of angles at C (l), C(2)

124.1(3) 124.3(3) 111.6(2) 124.2(3) 123.8(3) 112.0(2) 104.3(1) —118.3(1) C ( l) - S i( l) - C l ( 1 2 ) max 104.8(1) -1 1 7 .7 (1 ) C( 1) - Si(2) - C l(22)max 104.9(1) —118.4(1) C(2) - Si(3) - C l(32)max 103.9(1) —117.1(1) C (2 )-S i(4 )-C l(4 1 )max 28.0 28.2 360.0

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Ch. Rüdinger et al. • Silylated Ethenes and A cetylenes

tetrakis(trim ethylsilyl)ethene [12,14] clearly shows gradual changes in the distortion of the ethene frame with increasing steric bulk of the substi­ tuents. In 5 the bond distances remain in the usual range, and the most significant distortion is the de­ formation of the angles at C (l) from 120° to values varying from 105.7(8)° to 139.4(9)°. Additional de­ formation occurs in the tetrahedral angles at sili­ con, but the steric requirem ents of three trichlorosilyl groups and one hydrogen substituent do not re­ quire any further structural changes. If the rem ain­ ing hydrogen substituent is replaced by another trichlorosilyl group as in 15, however, additional se­ vere deformations occur. The C=C and C -S i bond distances are now sig­ nificantly longer (e.g. C=C (5) = 1.28(1) vs. C=C (15) = 1.361(5)) and the C=C double bond shows a twist with dihedral angles S i ( l ) - C ( l ) - C ( 2 ) Si(3) - 28.0° and S i( 2 )-C (l)-C (2 )-S i(4 ) = 28.2°. The four threefold trichlorosilyl rotors are inter­ locked, with opposite groups in eclipsed positions (S i(l) vs. Si(4) and Si(2) vs. Si(3)). Summarizing the deformations in 15 it becomes apparent that this compount reaches quite closely the geometry of tetrakis(trimethylsilyl)ethene, indicating the great similarity of trimethylsilyl and trichlorosilyl groups regarding steric effects. Experimental All experiments were carried out under pure, dry nitrogen. Solvents were purified, dried, and stored over molecular sieves in a nitrogen atmosphere. NMR: CDC13, C 6 D 6 as solvents, tetramethylsilane as internal standard, JE O L JNM-PMX 60, Bruker WP 100 SY, JE O L GX 270, JE O L GX 400 spec­ trometers. IR: Nicolet FT 5 DX spectrometer. Mass spectra: Electron impact source (70 eV), Varian MAT 90 spectrometer. Bis I'(trifluoromethylsulfonyloxy jsilyl]acetylene (4) Bis(phenylsilyl)acetylene [7, 8 ] (1.0 g, 4.2 mmol) was dissolved in toluene (50 ml) and cooled to -7 8 °C. Triflic acid (1.26 g, 8.5 mmol) was added to this solution in one portion. The reaction mixture was allowed to warm to room tem perature. Accord­ ing to the NM R spectra the reaction was com­ plete after 1 h. Toluene and benzene were removed in a vacuum. Bis[(trifluoromethylsulfonyloxy)silyl]acetylene could not be distilled without de­ composition. Yield: 1.29 g (81%), yellow liquid.

!H NMR (C 6 D6):