Regioselectivity of Radical Addition of Thiols to 1-Alkenes: Regioselectivity of Radical Addition of Thiols to 1-Alkenes

The addition of dodecanethiol (1) to 1-alkenes H2C=CHR that have substituents R = Hex, (CH2)(8)COOMe, Ph, Bn, cHex, CH(2)cHex, tBu (2a-2g) with different steric effects was studied. Some few percent of branched Markovnikov addition product 4 was formed in all cases with the exemption of styrene (2c) in addition to linear anti-Markovnikov product 3 as the main product. The regioselectivities correlate well with the Taft steric parameter E-S of the R-substituent at the alkene. This gives evidence that the regioselective outcome of the radical addition of thiols to 1-alkenes is mainly steered by the steric effect of the R-substituent as it is well known for the addition of alkyl radicals to alkenes. In the case of thiol addition, however, the regioselectivity is strongly enhanced by the much faster fragmentation of the intermediate Markovnikov than of the anti-Markovnikov adduct radical.