OXIDATIVELY INDUCED REDUCTIVE ELIMINATIONS - KINETICS AND MECHANISM OF THE ELIMINATION OF ETHANE FROM THE 17-ELECTRON CATION RADICAL OF CP-ASTERISK-RH(PPH3)(CH3)2

The dimethylrhodium compound Cp*Rh(PPh3)(CH3)2 (1; CP* = eta5-C5Me5) undergoes an overall 2-electron oxidation at 0.04 Vvs the ferrocene/ferrocenium (Fc) couple in 9:1 acetonitrile/dichloromethane, 0.1 M Bu4N+PF6-. The oxidation results in the intramolecular reductive elimination of ethane and yields Cp*Rh(PPh3)(NCMe)22+ (2) as the Rh-containing product. A mechanistic and kinetic investigation of the reaction by derivative cyclic voltammetry (DCV) showed the reductive elimination to be a first-order reaction with respect to 1.+. Very small solvent effects (acetonitrile, acetone, THF, dichloromethane) on the reaction rate indicated the nonintervention of 19-electron species in the reductive elimination. The DCV analysis provided kinetic parameters k(20-degrees-C) = 96 +/- 3 s-1, DELTAH(double dagger) = 53.1 + 0.8 kJ/mol, and DELTAS(double dagger) = -25.9 +/- 3.3 J/(K.mol) in acetonitrile. The data are interpreted in terms of a direct, concerted elimination of ethane from the 17-electron cation radical 1.+. It is estimated that the rate of reductive elimination has been increased by a factor of at least 3 x 109 due to the oxidation of 1 to 1.+. In dichloromethane, the intramolecular elimination of ethane was initiated by an overall I-electron process. The major product in this case was Cp*Rh(PPh3)(CH3)X+ (X most likely is coordinated dichloromethane or the counterion PF6-) as a result of a secondary intermolecular methyltransfer process. The methyl transfer was mimicked by a quantitative (H-1 NMR) reaction between 1 and 2 which yielded Cp*Rh(PPh3)(NCMe)CH3+ (3).