Vinyl Radicals. Stereoselectivity in Hydrogen Atom Transfer to Equilibrated Isomeric Vinyl Radicals¹

Abstract

Facile equilibration of the isomeric vinyl radicals derived from the cis-and trans-t-butyl a-methyl-and a-phenylpercinnamates (1-4) is observed at 110'. The olefin product mixture is much richer in the cis isomer than that expected from the relative stabilities of the cis and trans olefins, indicating stereoselectivity in the hydrogen atom transfer step. Stereoselectivity increases in going from the a-methyl to the a-phenyl system and also when the effective bulk of the transferring agent is increased in going from toluene or cyclohexene to cumene. The preparation of trans-a-methylcinnamoyl peroxide (9) is described. Thermal (80-90 ") decompositions and low-temperature photodecompositions (1 and -75 ") of 9 were carried out in toluene, cyclohexene, and cumene and the cisjtrans propenylbenzene product ratio compared with the thermal data (1 10") from the corresponding peresters. Plots of log (cisjtrans) us. 1/T for the propenylbenzene products from the perester and diacyl peroxide sources give straight lines for cyclohexene and cumene in the temperature range 110 to -75' and for toluene from 110 to 1 ". From the slopes of these lines are calculated the relative istereoselectivities of the three solvents in hydrogen atom transfer to the isomeric 1-methyl-2-phenylvinyl radicals. Some kinetic data and further product studies of the peresters are also included.ecently, considerable attention has been focused R on the question of the configuratio~ial stability of vinyl radicals. Studies on free-radical additions to terminal acetylenes indicate that the kinetiically determined product is much richer in the cis isomer than the equilibrium mixture. 2-4 Skell and Allen2 observed essentially 100% cis product from the addition of hydrogen bromide to propyne at -78" and ascribed this result to a stereospecific trans addition of bromine atom to propyne followed by capture of the configurationally stable vinyl radical. Oswald and co-workers3 found predominantly cis product from the addition of mercaptans and thioacetic acid to phenylacetylene. Kampmeier and Chen4 observed that the kinetic product from the addition of thioacetic acid to 1-hexyne contained 82 % cis isomer where the position of equilibrium for the cis and trans products is about 50 : 50.These latter results3r4 could arise from (i) a nonstereospecific addition of thiyl radical followed by capture of configurationally stable vinyl radicals, or (ii) capture of rapidly equilibrating vinyl radicals where the product ratio is independent of the stereochemistry of the addition step. These studies are further complicated by the facile isomerization of the olefinic products under the reaction conditions.A less tenuous approach to the problem of the configurational stability of vinyl radicals is i o generate isomeric radicals directly from perester sources and determine the cisltruns olefin product resulting from hydrogen atom transfer to the vinyl radicals. We previously reported our preliminary results utilizing this (1) For a brief account of this work see L. man, zbid....

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