Homogeneous Hydrogenation of Benzo[b]thiophene by Use of Rhodium and Iridium Complexes as the Catalyst Precursors: Kinetic and Mechanistic Aspects

The complexes [M(COD)(PPh3)2]PF6 (M ) Rh, 1a; Ir, 1b) are highly efficient precatalysts for the homogeneous hydrogenation of benzo[b]thiophene (BT) to 2,3-dihydrobenzo[b]- thiophene (DHBT). Both complexes react rapidly with BT and H2 to produce the corresponding [M(H)2(è1(S)-BT)2(PPh3)2]PF6 (M ) Rh, 2a; Ir, 2b), which enter the catalytic cycle. In the case of Ir, the catalysis is more conveniently carried out by use of 2b, which is stable enough to be isolated pure. Kinetic and mechanistic studies of BT hydrogenation were carried out by using 1a and 2b as the catalyst precursors in 1,2-dichloroethane solution at 40 °C. For both complexes, the reaction proceeds according to the rate law r ) kcat [M] [H2], where kcat ) k1K3/(1 + K3[H2]). All of the experimental data are consistent with a general mechanism in which the transfer of the hydrides to coordinated BT in [M(H)2(è2(CdC)-BT)- (PPh3)2]PF6 is the rate-determining step of the catalytic cycle. The complex [Ir(H)2(è1(S)- DHBT)2(PPh3)2]PF6 was obtained as a stable end product of the Ir catalysis and was characterized by spectroscopic analysis and by X-ray diffraction. The coordination geometry around the iridium atom consists of a distorted octahedron with mutually cis S-bonded DHBT, cis hydrides, and trans triphenylphosphines.