Kinetics and mechanisms of homogeneous catalytic reactions: Part 7. Hydroformylation of 1-hexene catalyzed by cationic complexes of rhodium and iridium containing PPh 3

Abstract

Cationic rhodium and iridium complexes of the type [M(COD)(PPh 3) 2]PF 6 (M = Rh, 1a; Ir, 1b) are efficient precatalysts for the hydroformylation of 1-hexene to its corresponding aldehydes (heptanal and 2-methylhexanal), under mild pressures (2–5 bar) and temperatures (60 °C for Rh and 100 °C for Ir) in toluene solution; the linear to branched ratio ( l/ b) of the aldehydes in the hydroformylation reaction varies slightly (between 3.0 and 3.7 for Rh and close to 2 for Ir). Kinetic and mechanistic studies have been carried out using these cationic complexes as catalyst precursors. For both complexes, the reaction proceeds according to the rate law r i = K 1 K 2 K 3 k 4[M][olef][H 2][CO]/([CO] 2 + K 1[H 2][CO] + K 1 K 2 K 3[olef][H 2]). Both complexes react rapidly with CO to produce the corresponding tricarbonyl species [M(CO) 3(PPh 3) 2]PF 6, M = Rh, 2a; Ir, 2b, and with syn-gas to yield [MH 2(CO) 2(PPh 3) 2]PF 6, M = Rh, 3a; Ir, 3b, which originate by CO dissociation the species [MH 2(CO)(PPh 3) 2]PF 6 entering the corresponding catalytic cycle. All the experimental data are consistent with a general mechanism in which the transfer of the hydride to a coordinated olefin promoted by an entering CO molecule is the rate-determining step of the catalytic cycle.