Mechanism of the hydrogenation of 2,5-norbornadiene catalyzed by [Rh(NBD)(PPh3)2]BF4 in dichloromethane: a kinetic and spectroscopic investigation

A kinetic investigation of the selective hydrogenation of 2,5-norbornadiene to norbornene catalyzed by [Rh(NBD)(PPh3)2]BF4 (1) has been carried out in dichloromethane at room temperature. The reaction is independent of the substrate concentration, while it is first order in catalyst and hydrogen pressure. Furthermore, the addition of triphenylphosphine inhibits the reduction. Moreover, it has been observed that under hydrogen atmosphere and at low temperature, complex 1 is in equilibrium with the dihydrido cis–trans-[RhH2(NBD)(PPh3)2]BF4, and that the addition of one equivalent of [Rh(NBD){P(p-Tol)3}2]BF4 to a chloroform-d1 solution of 1 affords [Rh(NBD)(PPh3){P(p-Tol)3}]BF4 in 54% yield. On the basis of these observations and other spectroscopic results, we propose that the hydrogenation of 2,5-norbornadiene to norbornene catalyzed by 1 proceeds by the five-coordinate dihydrido [RhH2(NBD)(PPh3)]+, which is formed by oxidative addition of molecular hydrogen to both 1 and a tricoordinate species [Rh(NBD)(PPh3)]+, depending on the concentration of free phosphine in the catalytic solution.