Abstract
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.
Highlights
Thiol–disulfide exchange reactions are essential for a number of biochemical transformations
We report here a series of experiments conducted to investigate reaction of disulfide with phosphine gold(I) thiolates in order to gain mechanistic insight into the competing pathways of gold(I) thiolate–disulfide exchange and phosphine oxide formation
We investigated the reaction in DMSO in more detail to determine the influence of the phosphine, thiolate and disulfide on the amount of phosphine oxide formed
Summary
Thiol–disulfide exchange reactions are essential for a number of biochemical transformations. The mechanism of this reaction in solution (in the absence of metals) is described as SN2 between thiolate and disulfide [1,2]. We have been studying how metals, such as a phosphine gold(I) thiolate complexes, alter the mechanistic reaction pathways for thiol–disulfide exchange. Our work to date has shown that the phosphine gold(I) thiolate–disulfide exchange reaction is overall second order; first order in gold–thiolate and first order in disulfide (see Equation (1)) [3,4,5]. The initial products of exchange are the unsymmetrical disulfide (i.e., R*SSR) and a new gold–thiolate complex (i.e., R3PAuSR*). Neither free thiolate nor gold(III) (from possible oxidative addition of disulfide) [6] appear to be involved in the reaction
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