6573
Molecular Rearrangements. V.’ Studies of the Rearrangements of 1-Chloro-cis- and -trans-4-methylcyclohexene Oxide. Stereospecific Chlorine Migration Richard N. McDonald and Theodore E. Tabor2
Department of Chemistry, Kansas State University, Manhattan, Kansas 66502. Received June 29, 1967 Abstract: A mixture of 1-chloro-cis- (2a) and -trans-4-methylcyclohexene oxide (2b) is obtained in the peracid epoxidation of 1-chloro-4-methylcyclohexene.Neat thermal rearrangements of this mixture give exclusively trans2-chloro-4-methylcyclohexanone(3a) through about the first 35 % of rearrangement. Essentially no rearrangement rate differenceis observed for 2a and 2b. The results are interpreted in terms of a common a-ketocarbonium ionchloride ion-pair intermediate. Zinc chloride catalyzed rearrangements give rise to a mixture of 3a and the cis isomer (3b).
0
ur previous reports of a-chloro epoxide rearrangements with preferential chlorine migration in the peracid oxidation of a-chlor~stilbenes~ and the Darzens condensation of benzaldehyde and methyl dichloroacetatelb were based on product studies and assumed the intermediacy of a-chloro epoxides. In the hope of gaining information about the mechanism of this novel chlorine migration, the synthesis of stable a-chloro epoxides became an obvious necessity. Of particular interest in this regard were the reports by Mousseron and his co-workers4 on alicyclic a-chloro epoxides and their relative stability allowing for purification by distillation under reduced pressure. Under the influence of heat or acid catalysts, these a-chloro epoxides readily rearranged to ketones. 1-Chloro-5-methylcyclohexene oxide was reported to rearrange to 2-chloro5-methylcyclohexanone on exposure to air or in contact with palladium black in acetic No yield data were given and no discussions advanced of chlorine as the migrating group in the rearrangement. The vinyl chloride chosen for our studies was l-chloro4-methylcyclohexene (1). 4-Methylcyclohexanone was treated with phosphorus pentachloride to give good yields of l . 5 Epoxidation of 1 using either perbenzoic acid or m-chloroperbenzoic acid gave l-chloro-4methylcyclohexene oxide (2) in ca. 65 % yield. The a0
e1
c1
1
2
(1) (a) A portion of this work was communicated in Chem. Commun., 655 (1966). (b) For paper I11 in this series, see R. N. McDonald and P. A. Schwab, J . Org. Chem., 29,2459 (1964). (2) NDEA Fellow, 1962-1965. This represents a portion of a dissertation presented by T. E. Tabor to the Graduate School of Kansas State University in partial fulfillment of the requirements for the Ph.D. degree. (3) R. N. McDonald and P. A. Schwab, J . A m . Chem. Soc., 85,4004 (1963). (4) (a) M. Mousseron and R. Jacquier, Bull. Soc. Chim. France, 698 (1950); (b) M. Mousseron, F. Winternitz, and R. Jacquier, ibid., 81, (1947); (c) M. Mousseron, F. Winternitz, and R. Jacquier, ibid., 260 (1948); (d) M. Mousseron, F. Winternitz, and R. Jacquier, Compf. Rend., 223, 1014 (1946). (5) The general procedure is that described by M. S. Newman, G. Fraenkel, and W. N. Kim, J . Org. Chem., 28, 1851 (1963).
chloro epoxide 2 could readily be distilled at reduced pressure and could be stored under Dry Ice for relatively long periods of time with little or no rearrangement. However, when allowed to remain at room temperature and atmospheric pressure, the epoxide soon began to undergo rearrangement as was evidenced by development of a strong carbonyl absorption in its infrared spectrum. Decomposition and/or polymerization were also indicated by development of a deep red color and evolution of hydrogen chloride fumes. The products considered possible from the rearrangement of a-chloro epoxide 2 were 2-chloro-4-methylcyclohexanone (3), formed by chlorine migration, and 2-chloro-5-methylcyclohexanone(4), formed by hydrogen migration. These a-chloro ketones have been prepared by chlorination of 4-methylcyclohexanone and 3-methylcyclohexanone, respectively. 4 . 0
*
c1_ +/c1
€ I +
CHJ 4
CH, 3
Two isomers of 2-chloro-4-methylcyclohexanone(3) have been reported previously in the literature,‘j-g but have been described6 only as a “low-boiling isomer” and a “high-boiling isomer.” A mixture of these two isomers was prepared by chlorination of 4-methylcyclohexanone using either sulfuryl chloride or aqueous chlorine.6 Quite pure samples of each isomer could be obtained by distillation of the mixture through a semimicro spinning band column. The “low-boiling isomer” exhibited bp 74.0-74.7” (10 mm) and the “high-boiling isomer” had bp 107-108” (10 mm). In a study of the carbonyl absorption in the infrared spectra of solutions of various a-chloiocyclohexanones, Corey and Burkeg observed that carbonyl absorption is displaced from that of the parent ketone, +26 to +31 cm-1 for equatorial chlorine and 10 to +18 cm-l for axial chlorine substitution. In the present study the carbonyl absorption frequencies of
+
( 6 ) E. Campaigne and R. D. Lake, ibid., 24, 478 (1959). (7) M. Godchot and P. Bedos, Compt. Rend., 180, 295 (1925). (8) M. S. Newman and W. T. Booth, J . Org. Chem., 12, 737 (1947). (9) E. J. Corey and H. J. Burke, J . Am. Chem. Soc., 77, 5418 (1955).
McDonald, Tabor
Molecular Rearrangements
6574
the two isomers of 3 appeared at 1727 cm-l in the “low-boiling isomer” and 1736 cm-l in the “highboiling isomer,” compared to 1718 cm-’ for the parent 4-methylcyclohexanone. This suggested that the chlorine substituent was axial in the “low-boiling isomer” ( A Y ~ +9 = ~cm-’) and equatorial in the “highboiling isomer” ( A V ~ = 18 ~ cm-l).l0 The neat nuclear magnetic resonance (nmr) spectra of the two isomers of 3 confirmed the conclusions obtained from the infrared spectra. In the nmr spectrum of the “low-boiling isomer,” the proton on the carbon bearing the chlorine substituent appeared as a triplet centered at T 5.44, with an over-all spacing of 7.6 cps and with each peak further split into a doublet, showing this proton to be equatorially oriented (axial chlorine). In the nmr spectrum of the “high-boiling isomer,” this proton appeared as a four-line multiplet centered at 7 4.88, with an over-all spacing of 18.4 cps, showing this proton to be axially oriented (equatorial chlorine). These results are in excellent agreement with the report of Garbischll for the conformationally homogeneous cis and trans isomers of 2-bromo-4-phenylcyclohexanone and 2-bromo-4-t-butylcyclohexanone. Also, Wellman and Bordwell12 have discussed the shielding of a protons in cyclohexanones and have shown that, in contrast to cyclohexanes, the absorption for an axial proton a to the carbonyl group in cyclohexanones appears downfield with respect to its equatorial counterpart. Assuming the methyl group to be equatorial in the two isomers of 3 allows a configurational assignment of trans-2-chloro-4-methylcyclohexanone(3a) to the “low-boiling isomer” and cis-2-chloro-4-methylcyclohexanone (3b) to the “high-boiling isomer.” l 3
+
6
CI.
CH,
C H Hj f i 0 C1 3a
+ CH
plet centered at r 5.54, with an over-all spacing of 16.8 cps. Again assuming the methyl group to be equatorial, the results show that this solid isomer is trans-2-chloro-5-methylcyclohexanone (4a). No further attempts were made to isolate the liquid isomer, which is now indicated to be cis-2-chloro-5-methylcyclohexanone (4b).
A 4b
4a
In the epoxidation of 1-chloro-4-methylcyclohexene (I), the peracid can approach the double bond from either side and thus should give rise to two isomeric a-chloro epoxides in which the methyl group is either cis (2a) or trans (2b) t o the epoxide ring.I5 The nmr spectrum of the epoxidation product confirms this by showing a pair of methyl doublets at 7 9.11 and 9.15. The chemical shift difference for the methyl groups of the isomers is believed t o be due to a long-range shielding effect of the a-chloro epoxide ring on the methyl group. However, it is not known whether the methyl group cis or trans to the epoxide ring is the most shielded. The multiplicity of the epoxide ring proton was also approximately that expected for a mixture of 2a and 2b.
Q
fi0 H
H
3b
Chlorination of 3-methylcyclohexanone with chlorine gave rise to two isomers of 2-chloro-5-methycyclohexanone (4), one being a liquid, bp 90” (6 mm), and the other a solid, mp 55-560.14 In the present study the chlorination procedure was executed using sulfuryl chloride, and gc analysis of the reaction mixture showed two major products. The solid isomer 4 readily separated from this mixture upon cooling and after recrystallization showed mp 5 6.5-57.0 ”. The infrared spectrum showed the carbonyl absorption displaced +18 cm-l from that of 3-methylcyclohexanone. In the nmr spectrum of this solid isomer of 4 in carbon tetrachloride solution, the proton on the carbon bearing the chlorine substituent appeared as a four-line multi(10) The difference in the magnitudes of the shifts compared with those in ref 9 is that the present spectra were determined on neat liquid samples. (11) E. W. Garbisch, J . Am. Chem. Soc., 86, 1780 (1964). (12) I
