STEREOCHEMISTRY OF FREE-RADICAL ADDITIONS OF THIOLS TO SUBSTITUTED CYCLOHEXENES

Radical chain addition of hydrogen bromide to 2-chloro-4-t-butylcyclohexene (1) gave exclusively 2°-chloro- 4°-t-butylcyclohexyl bromide (8), the product of irons-diaxial addition. AIBN- and uv-initiated free-radical additions of benzyl mercaptan (A), hydrogen sulfide (B), methanethiol (C), and thiolacetic acid (D) to 1- and 2-chloro- and 1- and 2-methyl-4-t-butylcyclohexene (3, 1, 4, and 5, respectively) were investigated. The relative proportions of the diastereomeric adducts produced under varying reaction conditions were determined for the following reactions: A and 1; B and 3 and 1; C and 3 and 1; D and 1and 3-5. In all cases, transdiaxial addition predominated, and the relative proportions of products were shown to be dependent upon the addendum to olefin ratio. Conformational energy factors associated with the transition state appear to be responsible for preferential axial attack by a thiyl radical. The lower stereoselectivity of thiol additions, as contrasted to those of hydrogen bromide, are best explained in terms of a reversal of the thiyl-radical addition step. Differences in the extent of this reversibility appear to be dependent upon the relative stabilitiesof the thiyl radicals and the 2-thiyl-1-substituted cyclohexyl radicals, as well as on the rate of chain transfer. Bridged thiyl radical intermediates are not considered important in these additions. The concentration and temperature effects observed for free-radical additions of thiols to 1- and 2-chloro-4-t-butylcyclohexene compared with those observed for additions to 1- and 2-methyl-4-t-butylcyclohexene are of the same order of magnitude, but in opposite directions. The energy difference between the cyclohexyl-radical intermediates arising from “equatorial” thiyl-radical attack is probably responsible for the opposing trends. The extent of reversibility of the initial radical addition step is greatest for hydrogen sulfide and thiolacetic acid, less for methanethiol, and nonexistent for hydrogen bromide. © 1969, American Chemical Society. All rights reserved.