Disulfide Competition for Phosphine Gold(I) Thiolates: Phosphine Oxide Formation vs. Thiolate Disulfide Exchange

Phosphine gold(I) thiolate complexes react with aromatic disulfides via two pathways: either thiolate–disulfide exchange or a pathway that leads to formation of phosphine oxide. We have been investigating the mechanism of gold(I) thiolate–disulfide exchange. Since the formation of phosphine oxide is a competing reaction, it is important for our kinetic analysis to understand the conditions under which phosphine oxide forms. 1H and 31P{1H} NMR, and GC-MS techniques were employed to study the mechanism of formation of phosphine oxide in reactions of R3PAu(SRʹ) (R = Ph, Et; SRʹ = SC6H4CH3, SC6H4Cl, SC6H4NO2, or tetraacetylthioglucose (TATG)) and R*SSR* (SR* = SC6H4CH3, SC6H4Cl, SC6H4NO2, or SC6H3(COOH)(NO2)). The phosphine oxide pathway is most significant for disulfides with strongly electron withdrawing groups and in high dielectric solvents, such as DMSO. Data suggest that phosphine does not dissociate from gold(I) prior to reaction with disulfide. 2D (1H-1H) NMR ROESY experiments are consistent with an intermediate in which the disulfide and phosphine gold(I) thiolate are in close proximity. Water is necessary but not sufficient for formation of phosphine oxide since no phosphine oxide forms in acetonitrile, a solvent, which frequently contains water.