Radical intermediates in the photoreaction between disulfides and acylsilanes

Photolysis of cyclopropane solutions of a number of disulfides RSSR (R = Me, n-Bu, CFJ in the presence of acylsilanes R'C(0)SiMe3 led to radicals of general structure R'C(OSiMe3)SR which have been detected by EPR spectroscopy. The structure of the observed paramagnetic species has been ascertained by producing PhC(OSiMe3)SMev ia an alternative route, i.e., by reacting Me3& radicals with the thio ester PhC(0)SMe. The lifetime of these radicals varies considerably with the bulkiness of the substituents in the neighborhood of the radical center. The decay rates as a function of temperature, together with the corresponding activation parameters, have been determined for the hindered t-BuC(OSiMe3)SCFr3a dical; it is suggested that the decay process involves an equilibrium with a dimeric species. A number of possible reaction sequences leading to these adicals have been examined, although the actual mechanism is as yet unclear.