Photodisulfidation of alkenes with linear disulfides: Reaction scope and kinetics

Thiol-ene and thiol-yne photomediated conjugations have received substantial attention in research and in practice. Herein is presented the photodisulfidation of alkenes based on the radical-mediated 1:1 reaction of a disulfide and a vinyl ether, which provides an additional route for the formation of the types of sulfides seen in thiol-ene and thiol-yne polymers. Although similar linkages are formed, this approach starting with disulfides is expected to have benefits over the thiol-ene and thiol-yne reactions including extended shelf life of disulfides compared to thiols, reduced shrinkage stress, and increased refractive index of the resulting materials. It was determined that vinyl ethers were the only alkenes capable of undergoing photodisulfidation under ambient conditions and in reasonable timescales. The reaction between vinyl ethers and disulfides performed well in a variety of solvents providing modest to excellent yields (100% for bis(1-methylacetate) disulfide (DSMA)/triethyleneglycol divinylether (TEGDVE)) for numerous disulfide substrates evaluated. It was determined that the mechanism of the photodisulfidation reaction involves an auto-propagating cycle of thiyl radicals which add into either end of a vinyl ether to form thio-ether and thio-acetal linkages in the final product. Finally, although the reaction rate is slower than that of the thiol-ene reaction, the photodisulfidation reaction proceeds relatively rapidly under the explored conditions.