Some free-radical addition reactions of norbornene and related compounds

Abstract

In the additions of methanethiol, 2-methylpropane-2-thiol, thioacetic S-acid, benzenethiol, and S-deuteriobenzenethiol to norbornene, products of exo–cis-addition are formed. exo-cis-Addition similarly occurs when benzenethiol adds to 2-methylnorborn-2-ene; the phenylthio-radical attacks the more sterically available 3-position, which affords the more highly substituted intermediate radical. In the addition of benzenethiol to both 2-methylene-norbornane and camphene, chain transfer of the intermediate radicals occurs from the exo-direction. When methylene bromide, methyl bromoacetate, and probably ethyl bromoacetate are added to norbornene, products of both trans- and exo–cis-addition are formed, with the latter predominant. These results are best rationalised on the basis of exclusive exo radical attack, since this minimises both steric and torsional strain effects. In the chain-transfer step stereoelectronic interactions between the attacking molecule and the group introduced in the initial radical attack control the exo : endo ratio for direction of chain transfer leading to product formation. In additions of methylene bromide and methyl bromoacetate these interactions are so great that some chain transfer from the endo-side of the radical centre occurs in spite of the increase of torsional strain experienced in the transition state.