1986
MICH.4EL
M. MARTIN
and a basic group in the non-enzymatic catalysis indicates that a similar, but more efficient, as-
[CONTRIBUTION FROM THE
1701. s1
sistance may be responsible for the enzyrnatic catalysis.
DEPARTMENT OF CHEMISTRY, THEUNIVERSITY
OF
MICHIGAN, A N N ARBOR, MICH.]
The Mechanism of the Decompositions of &Butyl 3-Phenylperpropionate and &Butyl 4-Phenylperbutyrate BY MICHAEL M. MARTII’; RECEIVED DECEMBER 5, 1961
A kinetic study of the free radical decompositions of t-butyl 3-phenylperpropionate and 4-phenylperbutyrate has established that these peresters decompose by a two-step, non-concerted mechanism. This is evidence against the existence of bridged, “non-classical” free radicals.
Introduction who found that the degree of rearrangement of the I n the course of studies currently in progress 2-phenyl-2,2-dimethylethylradical, generated by relating to polar effects in free radical-forming re- the decarbonylation of 3-phenyl-3,3-dimethylproactions, i t became essential that the question of the pionaldehyde, depends upon the concentration of existence or lack of existence of bridged, “non- the hydrogen donor, thus implicating the simulclassical’’ radicals, analogous to the phenonium taneous existence of a t least two interconvertible ions which intervene during some of the solvolytic radical species. However, i t is still possible that reactions of P-phenyl-substituted benzenesulfonates the first-formed radical is bridged, and that this and substituted benzenesulfonates,lS2 be settled. radical may then abstract a hydrogen atom from The transition state leading to such a bridged the hydrogen donor or rearrange to a non-bridged radical formed by the addition of a free radical radical. Thus, although this work precludes a to allylbenzene is depicted in structure I, or, em- bridged radical as the product-determining species, ploying the polar model for the transition state of it does not rigorously exclude bridging a t the ratea free radical-forming reaction, suggested by determining stage of radical formation. Overberger and Gainer’ studied the rates of Walling and Mayo,3 and utilized by many others,4 decomposition of three p-substituted 2-azo-bis-3by structure 11. methyl-3-phenylbutanes a t 255O, and found the rate to be insensitive to the electronic nature of the 9-substituent. This result is consistent with the lack of participation by a P-phenyl group, but does not rigorously exclude it, since, as‘ Leffler8 has pointed out, the rates of a reaction for a series of To date, no evidence has been presented which related compounds can be relatively insensitive requires phenyl participation in radical-forming to changes in structure or solvent when the particreactions a t a carbon atom beta to a benzene ring, ular reaction is observed near the isokinetic tembut neither has any been presented which rigorously perature because of compensating changes in AH* excludes it. I n their studies on the decarbonyla- and AS”. Nowhere has the behavior of AH* and A S * been tion of P-phenylated propionaldehydes, Curtin and Hurwitz5 observed that for a 1,2-phenyl utilized so effectively in gaining an understanding of shift to occur, the migration origin must either be the nature of a transition state than in perester tertiary, or if secondary, both groups must be aryl. decompositions (reaction 1). In the elegant work The authors recognized that no conclusions re- of Bartlett’s group,g the factors which cause a garding the concerted or non-concerted nature of perester to decompose in a stepwise (path A) or concerted fashion (path B) are established. n’hen the decarbonylation could be drawn, however. The most compelling evidence against the exist0 ence of bridged radicals was presented by Seubold,6 0 path A I1 (1) D. Cram, J. A m . Chem. Soc., 71, 3863 (1949); 74, 2129, 2137 2159 (1952). (2) (a) S. Winstein, B. K. Morse, E. Grunwald. K . C. Schreiber and J. Corse, i b i d . , 74, 1113 (1952); (b) S. Winstein and B . K. Morse, ibid., 74, 1133 (1952); (c) S. Winstein, M . Brown, K . C. Schreiber and A. H. Schlesinger, ibid., 74, 1140 (1952); (d) S. Winstein and K . Scbreiber, ibid., 74, 2165, 2171 (1962). (3) (a) F. R . Mayo and C. Walling, Chem. Reus., 46, 191 (1950); (b) C. Wal1ing;’Free Radicals in Solution,” John Wiley and Sons, Inc., 1957; ( c ) C. Walling and B. Miller, J . A m . Chem. iYew York, N. Y., SOC.,79, 4181 (1957); (d) C. Walling and B. Jacknow, ibid.. 82, 1756 (1960). (4) (a) G. A. Russell, J . Org. Chem., 23, 1407 (1958); (b) E. S. Huyaer, J . A m . Chem. Soc., 82, 394 (1960); (e) P . D. Bartlett and C. RBchardt, i b i d . , 82, 1756 (1960). (6) D. Y . Curtin and M. J. Hurwitr, i b i d . , 74, 5381 (1052). (6) F. H. Scubold, Sbid., 76, 1682 (lQ6’d).
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