Reaction of thiols and disulfides with phosphite radicals. A chain mechanism and RS.cntdot./PO32-.cntdot. equilibrium

Abstract

Thiyl and phosphite radicals exist in equilibrium, PO/sub 3//sup 2/. + RSH reversible RS. + HPO/sub 3//sup 2 -/, with equilibrium constants of 800 where RSH is ethyl mercaptan and 1500 where RSH is penicillamine. Rate constants for the respective forward reactions are 3.0 x 10/sup 8/ M/sup -1/s/sup -1/ for both compounds, and for the back reactions, 2.0 x 10/sup 5/ and 3.8 x 10/sup 5/ M/sup -1/s/sup -1/. In solutions containing both phosphite and disulfide an S/sub H/2 reaction, PO/sub 3//sup 2 -/. + RSSR ..-->.. RSPO/sub 3//sup 2 -/ + RS., yields phosphate thioester and thiyl radicals. At higher phosphite concentrations, a chain reaction mechanism is established, based on re-formation of PO/sub 3//sup 2 -/. radicals in the reverse reaction of the above equilibrium. G values of up to about 30 are observed for thiol formation and disulfide destruction in these systems. One of the factors controlling the extent of the chain mechanism seems to be the thiyl/phosphite radical equilibrium.