Hydroformylation of alkenes by use of rhodium complex catalysts
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.