MOLECULAR RHENIUM(V) OXOTRANSFERASES - OXIDATION OF THIOLS TO DISULFIDES WITH SULFOXIDES - THE CASE OF SUBSTRATE-INHIBITED CATALYSIS

Re(O)Cl-3(PPh3)(2), 1, and Re(O)Cl-3(OPPh3)(Me2S), 2, catalyze the oxi dation of thiols to disulfides with sulfoxides under mild conditions. Catalyst 1 exhibits an induction period which features PPh3 oxidation to OPPh3 prior to disulfide formation. This lag is absent when 2 is th e catalyst precursor. Otherwise, 1 and 2 display comparable kinetics a nd concentration dependencies. The catalytic reactions are first-order in catalyst, inhibited by thiol, and first-order in sulfoxide at low sulfoxide concentrations. Thiol inhibits the oxygen-transfer reaction because it competes with sulfoxide for coordination on rhenium. Sulfox ides must bind to rhenium in order to be activated for oxo transfer. L igand substitution reactions of 1 and 2 display kinetics that are cons istent with a dissociative (D) mechanism: the substitution rate is zer o-order in entering ligand and inhibited by departing ligand. The firs t-order rate constant for the formation of a 5-coordinate intermediate is 0.06 s(-1). As the sulfoxide concentration is increased, the react ion rate increases to reach a maximum and then begins to decline. The catalytic turnover rate at optimal conditions (maximum k(cat) for PhS( O)Me is 180 h(-1)) approaches the rate of ligand substitution in these rhenium(V) complexes. Rate retardation at high sulfoxide concentratio ns is due to catalyst deactivation; sulfoxides oxidize the rhenium(V) catalyst to ReO4-, which is inactive. Dimethyl sulfoxide (DMSO) is mor e efficient than aryl sulfoxides at oxidizing the catalyst, a fact tha t could be rationalized by the thermodynamics of S-O bond strength. Th us, aryl sulfoxides, such as PhS(O)Me, appear to be more reactive than alkyl ones. The oxygen-transfer reaction, therefore, is not involved in the rate-controlling step and the rate is limited by ligand substit ution. The rhenium(V) catalyst in these reactions acts as a Lewis acid and activates the sulfoxide via coordination: the sulfoxide ligand an d not the metal is the bearer of the transferred oxygen. A single-crys tal X-ray structure of Re(O)Cl-3(OPPh3)(Me2S), 2, has been solved: spa ce group Pcmn, a = 8.863(6) Angstrom, b = 14.269(9) Angstrom, c = 18.4 5(1) Angstrom, Z = 4; the structure was refined to final residuals R = 0.028 and R-w = 0.035.