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/?-Bromo Acids. 111. Heterolytic Fragmentation' Wyman R. Vaughan," William F. Cartwright,2 and Beat Henzi3 Contribution f r o m the Departments of Chemistry, The University of Michigan, Ann Arbor, Michigan 48104, and The University of Connecticut, Storrs, Connecticut 06268. Received September 3, 1971 Abstract: The kinetics of solvolysis of four cyclic 0-bromo acids, trans-2-bromocyclopentanecarboxylic acid, cis-2-bromocyclopentanecarboxylic acid, trans-2-bromocyclohexanecarboxylic acid, and cis-2-bromocyclohexanecarboxylic acid, have been examined in order to provide insight into the effects of conformation on the heterolytic fragmentation reaction (debrominative decarboxylation). Comparison of the behavior of the two trans acids with that of trans-10-bromodecalin-9-carboxylic acid strongly suggests that the latter compound provides a clear case of synchronous heterolytic fragmentation in the saturated 0-halo acid series. The comparatively facile isolation of a stable fused-ring 0-lactone, cis-cyclopentanecarbo-2-lactone, from the solvolysis of trans-2-bromocyclopentanecarboxylic acid raises the question as to whether @-lactonesare indeed intermediates in the usual solvolytic process. The available evidence suggests that unless there is comparable concomitant fragmentation no actual 0-lactone is formed.
lthough the dehalogenative decarboxylation of 6halo acids is one of the oldest recognized reactions of such compounds4 and has been investigated extensively in the senior author's 1aboratory5-l1 and elsewhere,12-22 the precise nature of the reaction has not emerged in its entirety for any save P-halo-a$-unsaturated acids, 21 which constitute a special case because of the rigid geometry involving the halogen, the carboxyl carbon atom, and carbons 2 and 3. This isolated case has been shown by GrobZ1to fit nicely into the general pattern of heterolytic fragmentations, which he has studied very extensively. The purpose of the present study is t o reexamine the fragmentation of saturated @halo acids (dehalogenative decarboxylation) to see if in any instance the reaction conforms completely to Grob's requirements for synchronous heterolytic fragmentation. Early work in the senior author's laboratory9 appeared to confirm implicit and explicit arguments that 6-halo acid fragmentation was essentially a trans elimination, but at the same cis fragmentations
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(1) Supported in part by a Faculty Research Grant from the Horace H. Rackham School of Graduate Studies of The University of Michigan and Public Health Service Research Grant No. CA 05406 from the National Cancer Institute to The University of Michigan. (2) Work done at The University of Connecticut. (3) Work done at The University of Michigan. (4) A. Schmitt, Ann., 127,319 (1863). ( 5 ) W. R. Vaughan and K. M. Milton,J. Amer. Chem. Soc., 74, 5623 (1952). (6) W. R. Vaughan, M. V. Andersen, Jr., and R. Q. Little, Jr., ibid., 76,4 130 (1954). (7) W. R.Vaughanand R. Q. Little, Jr., ibid., 76,2952(1954). (8) W. R. Vaughan andR. Q. Little, Jr., ibid., 76,4130(1954). (9) W. R. Vaughan and R. L. Craven, ibid., 77,4629 (1955). (lo) W. R. Vaughan and A. C. Schoenthaler, ibid., 79,5777 (1957). ( 1 1 ) W. R. Vaughan and A. C. Schoenthaler, ibid., 80, 1956 (1958). (12) E. Erlenmeyer, Ber., 13,303 (1880). (13) P. Pfeiffer, 2 . P h y s . Chem., 48,40(1904). (14) H. Johannson and S. M. Hagman, Ber., 55,647 (1932). (15) A. R. Olson and R. T. Miller, J. Amer. Chem. Soc., 60, 2687
A. R. Olson and J. L. Hyde, ibid., 63,2459 (1941). E. Grovenstein and D. E. Lee, ibid., 75,2639 (1953). S.J. Cristol and W. P. Norris, ibid., 75, 632 (1953). S. J. Cristol and W. P. Norris, ibid., 75,2645 (1953). E. R. Trumbull, R. T. Finn, K. M. Ibne-Rasa, and C. I
