HYDROFORMYLATION OF ALPHA-PINENE AND BETA-PINENE CATALYZED BY RHODIUM AND COBALT CARBONYLS

Alpha- and beta-Pinene were hydroformylated to their respective aldehydes and alcohols at 60-130-degrees-C and 600 psi of CO/H-2. Employing Rh6(CO)16 as a catalyst precursor the main product is 10-formylpinane, 2. In contrast, mononuclear rhodium complexes containing phosphine ligands typically give 3-formylpinane, 1, as the major product. 3-Formylpinane, 1, was also the main product upon employing either Co2 (CO)8 or phosphine-modified Rh6(CO)16. The turnover frequency drops sharply as the total rhodium concentration increases upon using Rh6(CO)16, to catalyze alpha-pinene hydroformylation at 130-degrees-C and 600 psi of H-2/CO (1:1). This shows that cluster fragmentation occurs to generate the active catalytic species of lower nuclearity. Furthermore, the product distribution at equivalent alpha-pinene conversions remains the same at different total rhodium concentrations under Rh6(CO)16 catalysis, suggesting there is only (predominantly) one catalytically active species operating in the reaction. Hydroformylation with triphenylphosphite-modified Rh6(CO)16 was initially faster but the catalyst life is short due to hydrogenolysis of triphenylphosphite which produces phenol. Hydrogenation of alpha- and beta-pinene (Pd/C, 1 atm H-2, ambient temperature) generated the two pinane isomers, 10 and 11, which result from the hydrogen addition to the two faces of the double bond. The only hydrogenation byproduct detected during Rh6(CO)16-catalyzed hydroformylations was 11 where hydrogen addition occurred to the face of the double bond which was cis to the C(CH3)2 bridge. Use of the mixed metal carbonyl systems: Co2(CO)8/Rh6(CO)16, Co2(CO)8/Ru3(CO)12, Rh6(CO)16/RU3(CO)12/PPh3 and Co2(CO)12/Rh6(CO)16/PPh3 in toluene at 100-degrees-C and 600 psi H-2/CO=1:1 did not lead to any synergistic rate enhancements in the hydroformylation of alpha-pinene.