Substituent Effects on Vinyl Radical Cyclizations onto Aryl Rings

Regioselective toluenesulfanyl radical addition to the C-1 carbon of propynyl benzyl ethers 2a-i leads to vinyl radicals 3a-j which mainly give methyl sulfides 9a-j and methyl ethers 8a-j through stereoselective 5-(pi-endo)exo and 5-(pi-exo)exo cyclization, respectively. Additionally, minor amounts of pi-endo 6-membered cyclization products 5a-j and hydrogen abstraction products 4a-j are also formed. The role of steric and stereoelectronic factors in the 5-membered vs 6-membered and pi-endo vs pi-exo cyclization as well as the role of stabilization and polar factors has been studied. The substituent effect on the 5-(pi-exo)exo cyclization has been estimated by the relative rate constants, k(R), calculated for several substituents in the 4- and 3-position. Results show that stabilization and polar factors slightly affect the rate of the vinyl radical cyclization onto arene rings, which appears to be rather unselective with respect to the nature of the substituent. The nature of polar effects indicates that vinyl radicals are slightly electrophilic in character.