Hydroformylation of alkenes by use of rhodium complex catalysts

Abstract

The use of complexes of rhodium of the type trans-RhX(CO)(PR₃)₂, (X = halogen, R = aryl) as hydroformylation catalysts for alkenes is studied. It is shown that an inhibition period is removed by addition of hydrogen halide acceptors and that the halide complex undergoes hydrogenolysis to form a hydrido-species. The hydrido-species is only one of several complexes in equilibrium but it appears that the principal active catalytic species is RhH(CO)₂(PPh₃)₂. This species is formed by addition of carbon monoxide to the monocarbonyl complex RhH(CO)(PPh₃)₂. The latter is also formed by dissociation when the stable crystalline complex RhH(CO)(PPh₃)₃ is dissolved in benzene or other solvents. With RhH(CO)(PPh₃)₃ as catalyst, alkenes undergo rapid hydroformylation even at 25°/1 atm.; with alk-1-enes the ratio of straight- to branched-chain aldehyde formed is considerably higher than the ratios normally found in hydroformylation reactions, being ca. 20 at 25°. Hydrogen atom exchange and isomerisation reactions of alkenes with RhH(CO)(PPh₃)₃ are described.