Sept. 20, 1959
DIACYLPEROXIDE DECOMPOSITION : EFFECT OF RINGSIZE
[CONTRIBUTION FROM THE
4801
KEDZIECHEMICAL LABORATORY, MICHIGAN STATE UNIVERSITY]
The Effect of Ring Size on Diacyl Peroxide Decompositions's2 BY
HAROLD
HARTAKD DONALD P.WYMAN
RECEIVED FEBRUARY 28, 1959 Cycloalkaneformyl (13- 7 ) and cycloalkaneacetyl (II&6 ) peroxides were prepared and their products and rates of decomposition in carbon tetrachloride studied. A large variation in rate (102) was observed for type I peroxides, IS being remarkably slow. This is attributed to the difficulty with which a free-radical on a three-membered ring is produced. B u t the cyclepropyl radical, once formed, reacted with the solvent (giving cyclopropyl chloride) rather than isomerize to the more stable allyl radical. I n anhydrous carbon tetrachloride containing iodine, II gave mainly cyclopropyl iodide, but with water present, a 41y0yield of cyclopropanecarboxylic acid was obtained (capture of acyloxy radicals by iodine). The ester from 1 3 showed no alkyl rearrangement (cyclopropyl cyclopropanecarboxylate). Peroxides II((- 6) decomposed at about equal rates, and appreciably slower than corresponding peroxides I+ ,) ; the difference is attributed to the formation of primary r3ther than secondary alkyl radicals. All the rate data may be rationalized in terms of considerable C-C as well a s 0-0bond stretching in the transition state for radical peroxide decomposition. Peroxide 1 1 3 was erratic in rate behavior, in most instances being remarkably fast. N o product corresponding to abstraction of chlorine atoms from the solvent was detected. the major (85%) product being cyclopropylcarbinyl cyclopropanecarboxylate. Possible explanations are discussed.
Carbonium ions with a positive charge either directly on a cyclopropane ring, or on a carbon atom adjacent to one, exhibit distinctive chemical beha~ior.~ For example, ions with the positive charge directly on the three-membered ring are reluctant to form when compared with equivalently substituted carbonium ions (slow acetolysis of cyclopropyl us. isopropyl p-toluene~ulfonate~) and give rearranged products (allyl acetate4). In contrast, reactions which involve cyclopropylcarbinyl ions proceed unusually r a p i d l ~ ~ - ~ they ; often, but not always,6,8.0 give rearranged products. It was the initial purpose of this work to study similarly constituted (k.,cyclopropyl and cyclopropylcarbinyl) free radicals, to determine whether they are in any way remarkable. Several reactions already described in the literature may involve these radicals as intermediates, and give some indication of the behavior to be anticipated. Photochemical chlorination'0 and vapor phase nitration1' of cyclopropane gave good yields of chloro- and nitrocyclopropane, respectively. In these reactions, the intermediate cyclopropyl radical maintained its structural identity, and did not rearrange to the (presumably more stable) allyl radical. Brominative decarboxylation of silver cyclopropanecarboxylate to bromocyclopropane'2may be another example. But there are also several cases in which cyclopropyl radicals apparently rearrange to allyl radicals. Kolbe electr~lysis'~ of potassium cyclopropanecar(1) Taken in part from the Ph.D. thesis of Donald Paul Wyman. Michigan State University, 1957. (2) Presented a t the 133rd national Meeting of the American Chemical Society, San Francisco, Calif., April, 1958. (3) For a general review, see E. Vogel, Fcrfschr. chtm. Forsch., 3 , 430 (1955). (4) J. D. Roberts and V. C. Chambers, THIS JOURNAL, 73, 5034 (1951). (5) J. D. Roberts and R. H . Mazur, ibid., 7 3 , 2509 (1951). ( 6 ) C. G. Bergstrom and S. Siegel, i b i d . , 7 4 , 145 (1952). (7) See A. Streitwieser, J r . , Chcm. Reus., 66, 571 (1956),for a review and discussion. (8) H . Hart and J. M. Sandri, THISJOURNAL, 81, 320 (1959). (9) R . G. Pearson and S. H. Langer, ibid., 76, 1065 (1953). (10) J. D. Roberts and P. H. Dirstine, i b i d . , 6 7 , 1281 (1945). (11) H. B. Hass and H. Shechter, i b i d . , 76, 1382 (1953). (12) J. D. Roberts and V . C. Chambers, ibid., 73, 3176 (1951). (13) See C . I,. Vilson and W. T . Lippincott, i b i d . , 7 8 , 4290 (1956), and C. G. Overberger and P. Kabasakalian, J. Org. Chem., 21, 1124 (1956), for recent discussions of the radical mechanism for Kolbe electrolysis.
boxylateX4is claimed to give allyl cyclopropanecarboxylate. Photolysis of methyl cyclopropyl ketone'j gave, among products ascribable to initial alkyl-carbonyl cleavage, only structures related to allyl rather than cyclopropyl radicals.16,17 Trotman-Dickenson and Steacie showed that methyl radicals, generated from acetone photolysis, found it about equally difficult to remove a hydrogen atom from cyclopropane or benzene.lS These results were confirmed by McNesby and Gordon19 who found approximately 4 kcal./mole difference in activation energy for removing hydrogen atoms from cyclopropane and cyclopentane. Products from the former were structurally related to allyl, not cyclopropyl radicals. There are fewer data on the cyclopropylcarbinyl radical. It presumably was an intermediate in the photochemical chlorination of methylcyclopropane where, a t - 20' in the liquid phase, cyclopropylcarbinyl chloride was obtained.*O The vapor phase reaction (
