Kinetics and mechanism of oxygen atom abstraction from a dioxo-rhenium(VII) complex.

Abstract

The kinetics of reaction between triarylphosphanes and two newly prepared dioxorhenium(VII) compounds has been evaluated. The compounds are MeRe(VII)(O)(2)("O,S") in which "O,S" represents an alkoxo, thiolato chelating ligand. With MeReO(3), ligands derived from 1-mercaptoethanol and 1-mercapto-2-propanol form MeRe(O)(2)(met), 2, and MeRe(O)(2)(m2p), 3. These compounds persist in chloroform solution for several hours at room temperature and for 2-3 weeks at -22 degrees C, particularly when water is carefully excluded. They were obtained as red oils with clean (1)H NMR spectra, but attempts to obtain pure, crystalline products were not successful because one decomposition pathway shows a kinetic order >1. The fastest reaction occurs between P(p-MeOC(6)H(4))(3) and 2; k(298) = 215(7) L mol(-1) s(-1) in chloroform at 25(1) degrees C. The other rate constants follow a Hammett correlation against 3sigma, with rho = -0.69(7). This study relates to oxygen atom transfer reactions catalyzed by MeReO(mtp)PPh(3), 1, in which MeRe(O)(2)(mtp), 4, is a postulated intermediate that does not build up to a measurable concentration during the catalytic cycle. Compound 2 does not react with MeSTol, but MeS(O)Tol was formed when tert-butyl hydroperoxide was added. This suggests that equilibrium lies to the left in this reaction, 2 + MeSTol + L = MeReO(met)L + MeS(O)Tol, and is drawn to the right by a reaction between MeReO(met)L and the hydroperoxide. Triphenyl arsane does not react with 2, but thermodynamic versus kinetic barriers were not resolved.