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SECTION C Organic Chemistry Friedel-Crafts Acylations of Aromatic Hydrocarbons. Part VIII. t Monoand Di-acylation of Mesitylene By

P. H. G o r e * and J. A. Hoskins, Department of Chemistry, Brunel University, Woodlands Avenue, London

Published on 01 January 1970. Downloaded by 260809 on 30/04/2015 18:47:11.

w.3 The Friedel-Crafts acylation of mesitylene has been studied in detail. In the acetylation, propionylation or benzoylation reactions mono- or di-ketones may be formed. Mesitoylation leads only to the monoketone. A two-stage acylation procedure is described for the synthesis of acetylbenzoylmesitylene and acetylpropionylmesitylene. Reversibility has been shown to be a factor in these acylations, especially where an aliphatic acyl group is involved. The relationship between the stereochemistryof mesityl ketones and their u.v., i.r., and l H n.m.r. spectra is discussed.

WHENthe Friedel-Crafts acylation of a benzene homologue leads to substitution between two alkyl groups, as in mesitylene,ly2 durene,3,4 i ~ o d u r e n e , ~ or, ~(to a small extent) 1,2,4-trimethylben~ene,~ the consequent lack of coplanarity between acyl groups and the aromatic ring4*5s6"allows further acylation. We describe now our study of the diacylation of mesitylene.

(IIa),lY2but high yields of the former can be obtained by '-lo various acylation procedures and in several sol~ents.~s To obtain diacetylmesitylene (IIa) in good yield a sixfold excess of catalyst and the Elbs a addition sequence, have been r e ~ o m m e n d e d . ~ , ~ Details of our acetylations of mesitylene are given in Table 1.

TABLE1 Friedel-Crafts acetylations of mesitylene Expt. 1 2c 3"

Addition sequence Perrier Perrier Perrier

Acyt component ( M ) AcCl (1) AcCl (3) AcCl (2)

Catalyst ( M ) AICl, (1) AlCl, (6) AlC1, (3)

Conditions [temp. ; b duration (hr.)] 40'; 0.5

B.P.; 0-25 0-5

c

I

d

2 3 4

d

I 5f

6

Elbs Bouveault (i) Perrier (ii) Reverse Perrier Perrier Perrier Perrier

AcCl (3-8) Ac,O (0-5) AcCl (1) AcCl (2)

AlC1, AlCl, A1C1, AlCI,

AlC1, AcCl (1) AlCl, AcCl (1) AlCl, AcCl (1) ZnC1, Ac,O (2.2) Perrier AlCl, AcCl (4) Perrier AlCl, AcCl (4) AlCl, Perrier AcCl (4) a Reactions in carbon disulphide are only partially homogeneous. (ref. 10). d Mole-fraction, not absolute yields. e After 6 hr.: yield h Ketone (IIb) not detected. j Substrate: (Ia). 7

8 9 10 11J 12 13j

Acetylation of MesityZene.-Acetylmesitylene (Ia) was first obtained by the action of acetyl chloride and aluminium chloride on mesitylene.3 Acetylations usually affordmixtures Of the monoketone (Ia>and the Part vII, p. H. Gore, c. K. Thadani, and s. Thorburn, J . Chem. sot. ( c ) ,1968, 2502. Preliminary communication of the present work, P. H. Gore and S. Thorburn, Chern. Comm., 1969, 1487. 1

*

(5.4) (1.1) (2) (4)

B.p.; B.p.; B.p.; B.P.;

(1) (1) (1)

40"; 0.5 40"; 0.5 40'; 0.5 B.?; 6 50 ; 2 50'; 1.5 50'; 2

(1) (4) (6) (4) b

92

B.p.; 1

b

40

Products (%) (14 (114

1 0.5 0-5 0.5

50 34 33 32 31 29 9 1-95 85

1.4 96 50 d 66 67 68 69 71

82 92 92 70 g 6h

0.6 1.5 0.1 9 5 12

B.p. signifies: a t the boiling point. Method of Meyer f Method of Shirley (ref. 8). Tarry by-products.

9976.

With excess of reagents, vix. 2 molar proportions of acetyl chloride and 3 molar proportions of aluminium chloride, mixtures of monoketone (Ia) and diketone 5 6

R.C. Fuson and H, 0.House, J . Org, Chem., 1953, 18, 496.

P. H. Gore, in ' Friedel-Crafts and Related Reactions,' ed. 1g64J vO1' part ()'

G' A' Olah* Interscience, New

p'

' 1

t2i!b&;g; ~ ~ ~ ' ~ 4gi4, &f)&, D. A. Shirley, ' Preparations of Organic Intermediates,' 8

H. Weil, Ber., 1897, 30, 1285. L. I. Smith and C. Guss, J . Amer. Chem. Soc., 1937, 59, 804. C. Friedel and J. M. Crafts, Ann. Chirn. Phys., 1884, [6], 1,

Wiley, New York, 1951. R. T. Morrison and M. Wishman, J . Amev. Chem. Soc.,

V. Meyer and G. Pavia, Ber., 1896, 29, 1413, 2564.

1889.

449.

1954, 76, 1059. lo

C. R. Noller and R. Adams, J . Amer. Chewz. SOC.,1924, 46,

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518

excess of acetyl chloride and of aluminium chloride, in boiling carbon disulphide for 42 hr. The only compound isolated was the diketone (IIa). It is likely that overcrowding, due to ' buttressing ',13 prevents the site available for further substitution being accessible. A 6% yield of ketone (Ia) is formed (Expt. 10) with zinc chlonde as catalyst, which exerts its usual weak effect on acylation.14 Other Acylations of ~~esitylene.-Pro~ionylatiolz. Methods have been reported for the Friedel-Crafts preparation, separately, of propionylmesitylene (Ib) 7~15and of dipropionylmesitylene (IIb).1 Mesitylene afforded both the monoketone (Ib) (yield 63%) and the diketone (IIb) (15y0),on treatment with propionyl chloride and a 50% excess of aluminium chloride. Benxoylatiorc. Benzoylmesitylene (Ic) l6 has been obtained by Friedel-Crafts reactions, but no experimental details were furnished. We describe a preparation of this ketone in 83% yield, by the use of the Perrier reaction 6d in ethylene dichloride solution. Dibenzoylmesitylene (IIc) is best obtained (in 8936 yield) by using an excess of reagents in carbon disulphide suspension.17 An attempt to obtain tribenzoylmesitylene, by use of an excess of reagents at 120°, gave only dibenzoylmesitylene (IIc). Because of the known smaller steric requirements of the benzoylating species than of the acetylating species,l8 trisacylation might have been favoured here. 2,4,6-Trirnethylbenzoylation.Mesitylene reacted with trimethylbenzoyl chloride in carbon disulphide, essentially by the method of Weiler,lg to give dimesityl ketone (Ig), m.p. 141', in 89% yield.m Weiler reported his product to melt at 84-85', but dirnesityl ketone prepared by other than Friedel-Crafts procedures,21 has m.p. near 140'. An attempt to prepare the ketone (Id) by acylation in ethylene dichloride solution gave an intractable polymer. Attempts to prepare dimesitoylmesitylene (IId), under conditions analogous to the preparation of dibenzoylmesitylene (IIa),gave the ketone (Ig) as the only ketonic product. MeNO, ArC(Me):O,AlCl, Prefiaration of acetyl~ropio~zylnzesitylene.' Mixed ' MeNO,,AlCI, ArCOMe (1) diacylmesitylenes have proved elusive. In an attempt Attempts to achieve trisacylation of mesitylene failed to prepare acetylpropionylmesitylene (IIIa), Weil treated acetylmesitylene (Ia) with a large excess of (see also below). Mesitylene, or the diketone (IIa), "as subjected to forcing acylation conditions, vix. a large propionyl chloride and aluminium chloride but isolated only dipropionylmesitylene (IIb) ; analogously, propi11 B.-P. Susz and I. Cooke, Helv.Chim. Acta, 1954, 37, 1273; onylmesitylene (Ib) gave only diacetylmesitylene (IIa). B. P. Susz, I. Cooke, and C. Herschmann, ibid., 1280. Since diacylmesitylenes can be obtained in good yields 1% Cf.L.Schmerling, in ' Friedel-Crafts and Related Reactions,' ed. G. A. Olah, Interscience, New York, 1964, vol. 11, p. 1097; 1 7 W. H. Mills and T. H. Easterfield, J . Chem. Soc., 1902, 81,

(IIa) are formed. Under these conditions (Expt. 3) diacylation remains incomplete, and the constant proportion of 30% of monoketone (Ia) is formed after a reaction period of several hours. With a larger excess of acylating reagents (Expt. 21, in confirmation of earlier work,* diacetylmesitylene (IIa) is formed in quantitative yield. When this reaction is conducted in two acylation stages (Expt. 6), without isolation of ketone (Ia) after the first stage, the same product is formed, as anticipated. However, when acetylmesitylene (Ia) is similarly treated with excess of reagents, either in carbon disulphide suspension (Expts. 11 and 12) or in ethylene dichloride solution (Expt. 13), the extent of formation of diacetylmesitylene (IIa) is only slight. Diacetylmesitylene has previously been obtained from monoacetylmesitylene by the method of Expt. 12,4but no yield was reported. Our findings appear to show that acetylmesitylene (Ia), when prepared in. sit%, is much more reactive than when introduced as free ketone into a reaction mixture. An attempt was made to detect any gross structural differences between the two species by examining the perturbed carbonyl band in the i.r. spectrum in both nitromethane and ethylene dichloride solutions. The strong carbonyl band, occurring at 1703 cm.-I in the free ketone (Ia), is absent in all ketonealuminium chloride-solvent mixtures with molar equivalents of catalyst, due to formation of an oxonium complex of the type ArC(Me) =O, AlC13.11 The band a t 1703 cm.-l was shown to reappear on brief exposure to moist air, as the result of hydrolysis. The absorption frequency of the perturbed carbonyl function could not be found due to interfering solvent bands, but the i.r. spectra for the in sitzc prepared ketone complex, and the mixture of ketone and catalyst, appeared identical in the other parts of the spectrum. Removal of solvent was not considered feasible. The absence of the free carbonyl band in solutions in nitromethane shows that the expected equilibrium (1) must lie in favour of the ketone complex ;12 the significance of this is discussed below.

+

+

P. Gagnaux, D. Janjic, and €3.-P.Susz, Helv.Chim. ~ c t a 1958, , 41, 1322; N. N. Greenwood and K. Wade, in ' Friedel-Crafts

and Related Reactions,' ed. G. A. Olah, Interscience, New York, 1963, vol. I, p. 585; G. Hoornaert and P. J. Slootmaekers, Bull. SOC.chim. belges, 1969, 78, 257. l3 M. Rieger and F.H. Westheimer, J . Amer. Che.m. SOC., 1950, 72, 19; H. A. Kames, €3. D. Kybett, M. H. Wilson, J. L. Margrave, and M. S. Newman, ibid., 1965, 87, 5554. l4 P. H. Gore and J. A. Hoskins, J . Chem. SOC.,1964, 5666. l6 A. Klages, B e y . , 1902, 35, 2245. 16 E. Louise, Ann. Chim. Phys., 1885, 8, 202, 234; H. C. Brown, B. A. and F. R. Jensen, J . Org. Chem., 1958, 23, 417.

1311. 1s H. C. Brown, G. Marino, and L. M. Stock, J . Amer. Cheiit. SOC., 1959, 81, 3310; P. H. Gore, C. K. Thadani, and S . Thorburn, J . Chem. SOC.( C ) , 1968, 2502. 19 M. Weiler, Ber., 1899, 32, 1908. 20 P. H. Gore, J. A. Hoskins, R. J. W. Le Fbvre, L. Radoni, and G. L. D. Ritchie, J . Chewz. SOC.( B ) ,1967, 741. 21 ( a ) E. P. Kohler and R. Baltzly, J . Amer. Chem. SOC.,1932, 54, 4015; H. E. Zimmerman and D. H. Paskovich, zbid., 1964, 86, 2149; (b) R. I;. Rekker and W. Th. Nauta, Xec. Trav. chim., 1961, 80, 764; (c) R. C. Fuson, L. J. Armstrong, D. H. Chadwick, J . W. Kneisley, S. P. Rowland, W. J. Shenk, jun., and Q. F. Soper, J . Amer. Chem. SOC.,1945, 67, 386.

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519

by a separation of the two acylation stages (Expt. 6), the preparation of diacylmesitylenes with two different acyl residues became feasible. Acetylmesitylene was consequently prepared in situ (yield >go%) ; a solution of propionyl chloride and aluminium chloride in carbon disulphide was then added, and the reaction was allowed to proceed a t 50". The ketone mixture was isolated, and analysed by g.1.c. (Table 2, Expt. 14). This shows that the ketone (IIIa) is formed in substantial yield. In another experiment propionylmesitylene, prepared in sitzc, was similarly treated with acetyl chloridealuminium chloride. The results (Table 2) from the two experiments are essentially the same ; they show clearly that partial deacylation must here occur, under Friedel-Crafts acylation conditions. Even when the

instability. The monoketone retains the complexing aluminium chloride (see above), and is probably solvated by nitromethane molecules,22and thus very bulky. The space available a t the meta-positions is reduced by buttressing,13 and attack by a solvated acylating species, e.g. MeCOC1,A1C1,,MeN0,,6b~23 very difficult. Acylation may possibly occur by ' free ' acyl cations, of which a small proportion may be in equilibrium with the solvated complex. In other solvents, in contrast, less effective solvation will occur. In the ketone complex the buttressing effect will be smaller, and the smaller acylating species, vix. the addition complex MeCOC1,A1Cl,,6b*23 can effectively acylate the monoketone species. Preparation of A cetylbenzoylmesitylene. Attempts to effect benzoylation of ketones (Ia) or (IIa) with excess of

TABLE2 Forination of acetylpropionylmesitylene Products, molar proportions (%) Conditions I h > Reactants Expt. Solvent [temp. ; b duration (hr.)] (IIa) (Ia) (Ib) (IIIa) (IIb) B.P.; 0.7 0.6 1.2 35 A ,D 42 22 14 CS, B,C B.p.; 0.7 1.7 1.5 32 26 39 15 CS, A ,D 13.p.; 0.1 cs, 16 16 4.5 33 32 15 17 MeNO, 40"; 0.6 50 47 B,C 0-7 0.9 1.7 40"; 0.75 51 18 E,C,D, 11 16 5 18 C,H ,C1 19 MeNO, E,C,D 40"; 0-75 2.0 66 30 0.1 1.6 20 MeNO, 40"; 24 E,C,D 55 20 5.1 13 6.4 E : mesitylene. a B.p. signifies: a t a A : (Ia) prepared in situ; B : (Ic) preparcd in situ; C : AcCl-AlCI,; D : EtCOC1-AICI,; the boiling point.

propionylation of ketone (Ia) (prepared in sit%)is allowed to proceed for a short period only (Expt. 16), the proportions of diketones are high; significantly, the proportions of the ' foreign' ketones (Ib) and (IIb) are already relatively high. The relevance of these results to the concept of reversibility of acylation is discussed below. When a solution of acetyl chloride-aluminium chloride in nitromethane was added to ketone (Ib), preformed in sit%, monoacylation products predominate. Again, the formation of much acetylmesitylene is strong evidence of a deacylation step occurring under these conditions. The results of competitive acylations, in which mesitylene was added to a mixture of acetyl chloride, propionyl chloride, and aluminium chloride, are also given in Table 2. A yield of 15% of the diketone (IIIa) is formed in ethylene dichloride solution (Expt. 181, but only 1.6% in nitromethane solution (Expt. 19). By allowing the competition t o proceed in nitromethane solution for 24 hr. (Expt. 20), the proportion of diketones rises to ca. 25%, but much resinification takes place and at least 6 unidentified products (totalling ca. 20y0)were formed, possibly by migration of methyl groups. The low proportions generally formed of diacyl products in nitromethane solution are probably the result of low rates of formation, rather than of thermodynamic

reagents resulted in recovery of unchanged starting materials. However, when acetylmesitylene (Ia), prepared in sita, was employed, a 42% yield of acetylbenzoylmesitylene (IIIb), m.p. 81.5", could be isolated (Expt. 21). Analogous acetylation of in situ prepared benzoylmesitylene (Ic) gave the ' mixed ' diketone (IIIb) in S50,', yield. The results indicate that in this system an acetyl (or propionyl) group is labile, whilst a benzoyl group is comparatively stable. In this respect mesitylene is perfectly analogous to the (meso-) anthracene s ystem.14,z4 Reversibility .-The necessary conditions for reversible substitution a t an aromatic position are a high reactivity, coupled with a high degree of steric hindrance. The acetyl group in the ketone (Ia) was found by polarisability measurements to exhibit an effective conformational angle, in benzene solution, of 73", with the plane of the aromatic ring; 26 other physical methods gave lower values (51" or 63").27 With co-ordination of the carbonyl function to an aluminium chloride molecule this angle will probably increase. The conjugative stabilisation of mesityl ketones would thereby be much reduced, and in the presence of hydrogen halide the ketone-complex would be liable to deacylation. The results summarised in Table 2 can be understood in t e r n s either of a rapid deacylation of monoacylmesitylenes, or 6cy2s

S . M. Rivkin, Zhuv. obshchei Khim., 1935, 5, 277. P. H. Gore, Bull. Chem. SOC.Japan, 1962, 35, 1627. 24 P. H. Gore and C. K. Thadani, J . Chem. SOC.(C), 1966, es P. H. Gore, J. A. Hoskins, R. J. W. Le FBvre, L. Radom, 1729; 1967, 1498. and G. L. D. Ritchie, J . Chem. SOC.( B ) ,1969, 485. 2 6 (a) P. H. Gore, J . Oyy. Chenz., 1957, 22, 135; J . Chem. SOC., 27 (a) E. A. Braude and F. Sondheimer, J . Chem. SOC.,1955, 1957, 1437; (b) R. B. Girdler, P. H. Gore, and C. K. Thadani, 3754; ( b ) K. S. Dhami and J. R. Stothers, Carzad. J . Chem., J . Chem. SOC.( C ) , 1967, 26629. 1965, 43, 479. 23

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520 a rapid monodeacylation of diacylmesitylenes ; both may, in fact, occur. The alternatives of synchronous acylreplacement either at the same carbon atom, or a t a free meta-position, appear less likely. Compounds with labile acyl groups might be able to act as acylating agents under conditions of reversibility. Acetylmesitylene, preformed in situ in nitromethane solution was, therefore, allowed to react with naphthalene at 0" for 24 hr. Under these conditions traces only of 1- and 2-acetylnaphthalenes were found, in amounts readily accounted for had there been a direct reaction with traces of unchanged acetyl chloride. Similarly, diacetylmesitylene (11) was treated in nitromethane solution with mesitylene and aluminium chloride (molar equivalents) under various conditions, including the passage of a stream of hydrogen chloride through the reaction mixture,2*which possibly could promote deacylation ; only small amounts of acetylmesitylene (Ia) were, however, obtained. Diacetylmesitylene , prepared in situ in carbon disulphide, when treated with mesitylene and nitromethane, gave a 44% yield of ketone (Ia). This suggests that nitromethane may function by causing rapid deacylation of the diacylmesitylenes. Confirmation of this is given by a reaction in which diketones (IIa) and (IIb), separately prepared in situ, were mixed and nitromethane was added; no mixed diltetone (IIIa), but the monoketones [(Ia) yield 277'1 and [(Ib) yield 3%] were formed. COR

COR

(1) a; R = M e Et b; c; Ph d; 2,4,6-Me3C,H,

Ac

(111)Et a; R = b; Ph

The results given in Table 2, and discussed above, show for the first time the importance of reversibility in the acylation of a benzene derivative. TABLE 3 Formation of acetylbenzoylmesitylene Products, molar proportions (%) Expt. Reactants a (Ia) (IIa) (IIIb) (IIc) (Ic) A ,G 1 4 39 42 15 21 2 F,c Trace 85 1 22 11 a A , C , as in Table 2, footnote ( a ) ; F : (Ib) prepared in situ; G : BzCI/AlCl, ; solvent was carbon disulphide.

Spectra and Stereochemistry of Acylmesity1enes.-U.v. and i.r. absorption data are collected in Table 4. The short wavelength maxima (near 200 nm.) of these ketones are recorded for the first time. The nonplanarity of the D. P. N. Satchell, PYOG. Chem. Soc., 1960, 355. 29 K. Yates and B. F. Scott, Canad. J . Chem., 1963, 41, 2320; M. L. Josien, N. Fuson, J. M. Lebos, and T. M. Gregory, J . Chem. Phys., 1953, 21, 331; R. F. Rekker and W. Th. Nauta, Spectrochim. Acta, 1957, 8, 348. as

carbonyl function relative to the mesitylene ring (see above) causes a reduction in molar extinction (E) of the x-n* transition. Reduction in t, without an accompanying hypsochromic effect, is believed to signify a transition between a nonplanar ground state and a near-planar TABLE4 U.V. and i.r. light absorption of some acylmesitylenes v1llaa. (C=O) Ketone , , , ,A (nm.)/smmax. (in methanol) (cm.-l) E.c.a." (Ia) 205-5/13,300; 212 */12,200; 1703 73" 244 */2600 6 (Ib) 205*5/15,100; 211.5 */13,000; 1702C 242 */2400 (Ic) 201*5/41,100; 249/13,100; 1672 ~e go", 40" 332.5 */99 (Id) 203*5/49,700; 271*5/9,80Of 1656g.h 61" (IIa) 208/13,200; 244 */4100; 1705 299 */350 (IIb) 202/17,200; 206/17,100; 1705' 246 */3800 (TIC) 197/82,000; 202/84,100; 1675; 252/3300; 288 */450; 337/220 (IIIa) 201 */14,300; 206/15,700; 1701 C 246 */3700 (IIIb) 194/47,000; 200/49,000; 1706 (Ac) 251.5/18,200; 278 */3000 1677j (Bz) 1688 k Acetophenone Benzophenone 1664 gvl 42" * Inflection. a Effective conformational angle (see text). Where two angles are given, the first refers to the mesitylene ring. 6 I n cyclohexane: 202/21,700; 214 */14,500; 268 * / 3400; 276 */540. I n ethanol (P. Grammaticakis, Compt. vend., 1950, 231,278) : maxima at 247 and 295 * nm. c Film. R. F. Rekker and W. Th. Nauta (Rec. Trav. chim., 1961, 80, 747) quote, in iso-octane : 244/16,500; 270-5/2700; 350/90; and in ethanol: 250/16,000; 284 */3600; 335/140. de Roos (ref. 30) quotes, in chloroform; 1669-3 cm.-l, E 337. f Rekker and Nauta (footnoted) report, in iso-octane: 265/ 12,700; 341/205; and in ethanol: 271/12,100; 335 */330. CS, solution. A de Roos (ref. 30) quotes, in chloroform; 1651.2 cm.-l, E 353; or, in carbon tetrachloride: 1658.6 cm.-l, E 491. CCl, solution. k CHCl, solution. de Roos (ref. 30) quotes, in chloroform: 1659.9 crn.-l, E 394; or, in carbon tetrachloride; 1666.1 cin.-l, E 590.

excited state.27~ The relation between the effective conformational angle (e.c.a.) between the aromatic plane(s) and the C-CO-C plane, and U.V. light absorption of these ketones has been investigated in detail.27as2g The stretching frequency of the C=O function appears similarly to be related t o the e.~.a.~OThus, in ketone (Ia), known to exhibit an e.c.a. of 73" (see also above)F6 conjugation of the carbonyl function with the aryl residue is much reduced, and the frequency of the C=O band is increased relative to acetophenone (Table 4), becoming close to a typical aliphatic ketone. The position of this band is not significantly altered for higher alkyl ketones of m e ~ i t y l e n e . ~I n~ benzoylmesitylene (Ib) the likely conformation has the C-CO-C plane at 90" to the mesitylene plane, and at ca. 40" to the phenyl ~ l a n e , ~and * . ~this ~ involves loss of conjugation 30 A. M. De Roos, Rec. Trav. chim., 1968, 87, 1359; cf. A. T. Balaban, Omagiu Acad. Prof. Raluca R i p a n , 1966, 103. 31 P. H. Gore, J . A. Hoskins, and S. Thorburn, J . Chem. Soc. ( B ) ,1970, to be published. 32 P. H. Gore, J. A. Hoskins, R. J. W. Le Fhvre, L. Radom, and G. L. D. Ritchie, J . Chem. SOC.( B ) ,1969, 227.

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Org. relative to benzophenone (e.c.a. = 42"). With dimesityl ketone (Id), however, the low frequency suggests an overall increased conjugation, despite the high e.c.a. (61c).20 If conjugation can be measured by cc cos26 (where M. = conjugating power of the aryl position), this means that the increase in cc here more than compensates for the lowered cos20 term. Nonplanar conformations for ketones (Ia), (Ib), and (Ic) implies that diketones (IIa), (IIb), (IIc), (IIIa), and (IIIb) can exist as cis- or trans-geometric isomers. The available evidence, adduced from polarisability studies,26 suggests equilibrium mixtures (favouring the transisomer) a t ambient temperatures. In diketones with only one type of acyl group, (IIa), (IIb), (IIc), or where the acyl groups are not very different, (IIIa), only one carbonyl band was observed (cf. ref. 30). With acetylbenzoylmesitylene (IIIb), however, two bands were observed, one equivalent to that in acetylmesitylene (1706 cm.-l), and the other equivalent to that in benzoylmesitylene (1677 cm.-l). The lH n.m.r. data of mesityl ketones are summarised

U.V. light absorption was measured in methanol solution on an Optika CF,NI recording spectrophotometer. The 1H n.m.r. spectra were measured at 60 MHz. Gas Chromatography.-Analyses were carried out using column (A},2 m. x 2.2 mm, (i.diam.), packed with Apiezon L(20yo)on Chromosorb P (60-80 mesh); or a preparative column (B), 7 f t . long packed with S.E. 30 (20%) on silanised Chromosorb P (80-100 mesh), a t ca. 200°, with nitrogen as carrier gas. Column (B) was used with a 100 : 1 splitter. A flame-ionisation detector was used and mass corrections were applied where appropriate. Kovhts retention indices s6 of the ketones are given in a later ~ a p e r . 3 ~ T.l.c.-Analyses were carried out on plates of silica gel G (Mercli-),which were developed by ether-benzene (1: 19). The spots were made visible when sprayed with Rhodamine 6 G (0.005~0), and examined under U.V. light. Friedel-Cra fts A cylations.-Genera& procedure. The reactants, viz. substrate, the acyl component, and anhydrous aluminium chloride were brought together in the chosen solvent in one of the following ways : (1)Perrier procedure,a in which mesitylene was added as last reagent to a stirred mixture of acyl component and catalyst; (2) Elbs procedure,sdin which the catalyst is the component last added;

TABLE5 lH N.m.r. spectra Chemical shifts, Ketone (la) (Ib) (Ic) (Id)" (IIa) (IIb) (IIc) (IIIa)

Solvent 2-CH3 CC1, 7-87 CCl, 7-93 CCI, 7.99 CDC1, 7-89 7.97 CC1, 8-05 CC1, CCl, 8.17 7.97 CDC1, (IIIb) CCl, 8.07 0 H. Kwart and S. Alekman ment is uncertain.

3-H (or 5-) 3-30 3-32 3.21 3.17 3.24 3.22 3.07 3-22

T

(p.p.m.)

Aromatic inultiplet

Aliphatic Aliphatic 4-CH3 6-CH3 COCH, CH, (triplet) CH, (quartet) 7.79 7.87 7.70 7.81 7.93 8.90 7.45 7-74 7.99 2.1 5-2-93 7.72 7.89 7.88 7.88 7-63 7.89 8.87 7.40 7.89 7.90 7.90 2.05-2.84 7.83b 7.86a 7-63 8-80 7.40 7.98 7-79 3.16 7.64 2'20-2.76 ( J . Awev. Chern. Soc., 1968, 90, 4482) report signals (in CCl,) a t T 7.81, 7.64 and 3-10.

in Table 5. The signals for acetyl protons in mesityl ketones occur in the region T 7.63-7.70, which is much nearer the position of an RCOCH, signal ( T 7.80) than the PhCOCH, signal (7 7.37).33 Also, ortlzo-methyl groups are subjected to a diamagnetic shielding effect, relative t o para-methyl groups ( ~ f .PhCH, , ~ ~ : T 7-65). Both effects must be due to a steep e.c.a. of the carbonyl functions relative to the aryl moiety.% The chemicalshift differences (AT) of ortho- and para-methyl groups vary considerably; this effect has been interpreted in a related series of arylrnesitylenes 35 in terms of the .n-electron character of the rotating group (here the C=O function), and the angle of rotation. The much greater A T value for a benzoyl than an acetyl substituent, in the present system, enables one to assign the methyl signals in the diketone (IIIb) unequivocally. EXPERIMENTAL

Infrared spectra were obtained either in solution or as liquid films, on a Grubb-Parsons GS3 spectrophotometer. 33 R. M. Silverstein and G. C. Bawler, ' Spectrometric Identification of Organic Compounds,' Wiley, New York, 1963, p. 83. 34 N. E. Alexandrou, J . Chem. SOC.( C ) , 1969, 636.

AT

6

0.08 0.12 0.26 0.17 0.09 0.16 0.27 0.16, 0.11 0.09, 0.28 The assign-

(3) Bouveault procedure,sdwhere the acyl component is the last added reactant; (4) reversed Perrier procedure, where a solution of the catalyst-acyl chloride complex is added to a stirred solution of substrate. The general procedure followed was that given earlier; 25b the scale for diagnostic experiments was usually 0.025~. A cetylvnesitylene (2,4,6-Trimethylacetophenone)and Diacetylmesitylene (1,3-Diacetyl-2,4,6-trimethyZbenzene) .-Results of acetylation experiments are summarised in Table 1. Propionylmesitylene (Ethyl 2,4,6-Trimethylphenyl Ketone) and DipropionylmesiQlene (1,3-DipropionyZ~2,4,6-trimethylbenzene).-Mesitylene (0.1 mole) was added gradually to propionyl chloride (0-1 mole) and aluminium chloride (0.15 mole) in carbon disulphide (30 ml.), and the mixture was boiled for 10 min. Distillation of the crude product gave propionylmesitylene (11 g., 63%), b.p. 90"/2.2 mm. (1it.,lG125"/13 m.m.), nDle61-5125; the distillation residue, after chromatography on alumina (Spence, type H), afforded white crystals of dipropionylmesitylene (methanol), m.p. 100-100-5" (lit.,l 101-102"). M. Kuhr and H . Musso, Angew. Chem. Internat. Edn., 1969,

8, 147.

36 E. Kovits, Helv. Chim. A d a , 1958, 41, 1916; L. S. Ettre, Analyt. Chem., 1964, 36, 31A.

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522

J. Chem. SOC. (C), 1970

AcetylP~oPionylmesityZene (3-Acetyl-2,4,6-trimethylphenyl wzethylbemophenone).-(i) Diacctylmesitylene was preEthyl Ketone) (Expt. 13).-To a stirred solution of acetyl pared in situ in carbon disulphide; acetyl chloride (10mol.) chloride (0.1 mole) and aluminium chloride (0.1 mole) in and aluminium chloride (10 mol.) were added, and the carbon disulphide (30 ml.), mesitylene (0.1 mole) was mixture was boiled under reflux for 42 hr. At intervals added, and the stirring was continued for 30 min. (to give carbon disulphide was added to replace that which had in situ acetylmesitylene). To this mixture a slurry of evaporated. Diacetylmesitylene was the only ketonic aluminium chloride (20g., 0-15mole) was added, followed by material in the product. addition during 10 min. of propionyl chloride (8.7 nil., (ii) Dibenzoylmesityleiie was prepared in situ in carbon 0.1 mole) and carbon disulphide (5 ml.). The stirred mix- disulphide, the solvent was distilled off, and the residue was ture was then boiled under reflux for 30 min., and worked heated with benzoyl chloride (10 mol.) and aluminium up, to give a mixture of ketones (yield 19 g.), which was chloride (5 mol.) a t 120" for 22 hr. The only product analysed by g.1.c. [column (A)]. A pure sample of acetyl- isolated was unchanged dibenzoylmesitylene (61yo); there propionylmesitylene was obtained by preparative g.1.c. was no evidence (t.1.c.) for the formation of tribenzoyl[column (B)], and crystallisation (methanol), as white mesitylene. crystals, map.40-41" (Found: C, 76.6; H, 8.5. C,4H& (iii) Mesitylene was added with stirring to mesitoyl requires C, 77-0;H, 8.3%). chloride (3 mol.) and aluniiniuiii chloride (4mol.) in carbon The ketone could also be obtained from propionyl- disulphide solution, and the mixture was boiled under mesitylene prepared in situ. reflux for 3 hr. The product was dimesityl ketone, m.p. 140-5--141"; there was no evidence (t.1.c.)for the formation Benzoylmesitylene (2,4,6-Trimethylbenzophenone).-This ketone was prepared by a Perrier reaction in ethylene di- of diinesitoylmesitylene. chloride solution, with mesitylene (0.025molc), aluminium A cetylnaesitylene as a n A cylating Agent.-Acetylmesitylene chloride 10.028 mole), and benzoyl chloride (0.039 mole). was prepared in situ in nitromethane a t 0", the reaction The mixture was stirred at 25" for 1 hr. and then boiled beiiig allowed to proceed virtually to completion (3-5hr.). under reflux for 3 hr. The ketone (yield 83%) was ob- Naphthalene (1 mol.) was then added, and the mixture 189"/17 stirred a t 0" for 24 hr. Analysis (g.1.c.) of the product tained as an oil, b.p. 180-182"/8.5 mm. mm.), tzD20'5 1.5788. showed traces only of acetylnaphthalenes. Dibenzoylmesitylene had m.p. 116-5-1 17" (lit.,17 117"). Diacetylmesityleneas an A cylating Agent.-(a) To diacetylA cetylbenzoylmesitylene (3-Acetyl-2,4,6-trimethylbenzophen- inesitylene, prepared in situ in carbon disulphide (or a mixone).-Benzoylmesitylene was prepared in situ by Perrier ture of diacetylmesitylene and aluminium chloride) mesityladdition during 10 min. of mesitylene (0.1 mole) to a stirred ene was added, and the mixture was boiled under reflux for mixture of aluminium chloride (0.2 mole) and benzoyl 1 hr. Traces only of acetylmesitylene were detected. chloride (0.1mole) in carbon disulphide (20 ml.), followed Passage of dry hydrogen chloride through the reaction by boiling under reflux for 30 min. (Expt. 22). To the cooled mixture had no apparent effect. mixture were added with stirring aluminium chloride (0.15 (b) To diacetylmesitylene (0.05 mole), prepared as mole) and acetyl chloride (0.1 mole) during 5 min. The under ( a ) , mesitylene (0.05 mole) and nitromethane (0.15. mixture was then boiled under reflux for 30 min. and the mole) were added, and the mixture was stirred a t 50" for crude product was isolated by distillation as an oil (A),b.p. 4 hr. The product comprised acetylmesitylene (43%) and 150-168"/0-5 mm. The residue left after the distillation diacetylmesitylene (57yo). (c) An equimolar mixture of diacetylmesitylene and was chromatographed on alumina to give mixture ( B ) . The distillate (A), on crystallisation (methanol) gave dipropionylmesitylene, each prepared iut situ in carbon'diacetylbenzoylmesitylne (total yield 85%), m.p. 81-8 1.5" sulphide, was boiled under reflux for 4 hr. Acetylpro(Found: C, 80-8; H, 6.7. C,,H& requires C, 81.2; H, pionylmesitylene was formed only in traces (ca. 0.5% yield). ( d ) This was carried out as for (c), with addition of 6.8y0). Fraction (A) contained (g.1.c.) acetylbenzoylmesitylene (93%); (B) was a mixture of acetylbenzoyl- nitromethane ( 3 mole). The product contained acetylinesitylene (27%) and propionylmesitylene (3%). mesitylene (69%) and dibenzoylmesitylene (31%) . From a parallel experiment (Expt. 21), involving the The authors thank Mr. S. Thorburn and Dr. D. N. Waters benzeylation of acetylmesitylene prepared in situ (see above), the ketone (yield 42%) could be isolated by distillation, for helpful advice, and Mr. C. Simpson (University of Sussex) for the preparative g.1.c. followed by fractional crystallisation. Attempted Preparations of (i) Triacetylmesitylene(1,3,5-Tri[9/890 Received, May 28fh, 19691 acetyl-2,4,6-trimethylbenzene), (ii) Tribenzoylmesitvlene(1,3,5Tribenzoyl-2,4,6-trimethylbenzene),and (iii) DimesitoytR. C. Fuson and M. D. Armstrong, J. Amer. Chcm. SOC., mesitylene (3-(2",4",6'f-Trimet~ylbenzoyl)-2,4,6,2',4',6'-hexa- 1941, 63,2650.