Oxygen transfer reactions catalyzed by rhenium (VII) and rhenium (V) complexes
A new binuclear oxothiolatorhenium(V) compound, Re2O 2(MtP)3 (D1, mtp = 2-mercaptomethylthiophenol), was synthesized by reacting dirhenium(VII) heptoxide (Re2O 7) with H2mtp, and characterized spectroscopically and crystallographically. One Re-S bridge in D1 was opened, and sometimes D1 was monomerized, through ligand coordination. D1 was found to be an efficient catalyst for the oxidation of phosphines, triphenylarsine, triphenylantimony, sulfides and dimes by pyridine N-oxides, and unprecedently, by molecular oxygen. D1 also catalyzes the oxidation of phosphines by dimethylsulfoxide. The kinetics and mechanism for the oxidation of triarylphosphines by pyridine N-oxides and O2, as well as the relative reactivities of all substrates, were studied. The reaction was proposed to go through oxorhenium(VII) intermediates;Methyltrioxorhenium (MTO) catalyzes the two-step oxidation of thioketones by hydrogen peroxide to sulfines (thioketone Soxides) and to ketones releasing sulfur monoxide, which was trapped by a 1,3-diene. The kinetics and mechanism of both steps were studied. The substituted thiobenzophenones were found to attack the peroxo rhenium oxygen nucleophilically. The Hammett correlation of the second-order rate constants against the 2sigma values of the substituents is a straight line with a negative slope. Remarkably, a V-shaped Hammett plot was obtained for the oxidation of sulfines to ketones, with rate being accelerated with both electron-donating and withdrawing substituents. This suggests a mechanism for the sulfines in which the direction of the electron flow in the transition state changes with the electron demand of the substituents on the sulfines. The oxidation rate for the sulfine is less than 10 --3 times as fast as that of its parent thioketone. This makes the controlled oxidation of thioketones by the MTO/1 eq H2O 2 system a superior method for the synthesis of sulfines. A series of sulfines were synthesized in high yields with this method;The sulfur bridge of a binuclear platinum(I) complex, Pt2(mu-dppm) 2(mu-S)Cl2 (dppm = diphenylphosphinomethane), was oxidized step-wise to a sulfoxide, then sulfone by hydrogen peroxide catalyzed by MTO. The SO-bridged complex, Pt2(mu-dppm)2(mu-SO)Cl 2, was crystallographically characterized. The same compound was obtained by trapping the sulfur monoxide released from the oxidation of a sulfine by MTO/H2O2 with Pt2(mu-dppm)2Cl 2.