Mechanism of the homogeneous hydrogenation of olefins catalysed by chlorocarbonylbis(triphenylphosphine)iridium(I) with and without basic cocatalysts

Abstract

The complex chlorocarbonylbis(triphenylphosphine)iridium(I), IrCl(CO)(PPh₃)₂, has been found to catalyse the hydrogenation of olefins, (RHCCHR), in dimethylformamide at 50°(R = H) and at 80°(R = CO₂Me or CO₂H), at a rate which increases to a maximum value during an initial induction period. The induction period does not occur in hydrogenations carried out in toluene at 80°(R = CO₂Me), and may be removed in dimethylformamide by addition of p-methoxyphenol. In the presence of basic cocatalysts in dimethylformamide, the catalyst is converted into trihydridocarbonylbis(triphenylphosphine) iridium (III), IrH₃(CO)(PPh₃)₂, during the induction period. The induction period in the absence of cocatalyst is attributed to the involvement of an iridium–olefin intermediate in a radical chain process depending on the solvent or its impurities. The mechanism of the reactions is discussed in terms of an iridium–olefin intermediate, the concentration of which is determined partly by the equilibria existing between the stable iridium complexes present and partly by the kinetic competition of the reactions forming and removing the intermediate.