Catalytic hydroformylation of (1S,5S)-(-)- and (1R,5R)-(+)-beta-pinene: Stereoselective synthesis and spectroscopic characterization of (1S,2R,5S)-, (1S,2S,5S)-, (1R,2R,5R)- and (1R,2S,5R)-10-formylpinane

(1S,5S)-(-)- and (1R,5R)-(+)-beta-pinene have been hydroformylated in toluene to give (1S,2R,5S)- and (1R,2S,5R)-10-formylpinane with up to 95% diastereoselectivity using bimetallic CoRh(CO)(7) as a catalyst; the latter was generated in situ from preformed Co2Rh2(CO)(12) or a stoichiometric mixture of either [Rh-4(CO)(12)] or [Rh-6(CO)(16)] and [Co-2(CO)(8)]. At 70-125 degrees C and under 60 arm of syngas, the yields of hydroformylated products do not exceed 30% because of the concomitant isomerization of beta- to alpha-pinene. In all cases the catalyst is recovered as a mixture of soluble cobalt carbonyl derivatives and a crystalline precipitate that contains most of the rhodium, mainly as [Rh-6(CO)(16)]. Comparable yields and diastereoselectivities were obtained from reactions in tetrahydrofuran with a mixture of [Rh-4(CO)(12)] and [N(PPh(3))(2)]Cl as the catalyst precursor. The corresponding (1S,2S,5S)- and (1R,2R,5R)-10-formylpinanes, along with the corresponding alcohols, were obtained diastereoselectively by use of bimetallic Co-Rh or homometallic Rh carbonyl catalysts modified with bis(diphenylphosphine)ethane (dppe). When unidentate phosphines such as triphenylphosphine were used in place of dppe, as the ligand/metal (L/M) ratio was raised the diastereoselectivity of both the hetero- and the homo-metallic catalytic system fell progressively, and was completely lost for L/M greater than or equal to 4. However, a further increase in L/M to ca. 70-100 allows chemio- and diastereo-selective synthesis of both the (1S,2S,5S)- and (1R,2R,5R)-10-formylpinane. The (1S,2R,5S)-, (1S,2S,5S)-, (1R,2R,5R)- and (1R,2S,5R)-10-formylpinane diastereomers were isolated by distillation under reduced pressure and fully characterized by IR, UV, H-1 and C-13 NMR and circular dicroism (CD) spectroscopy, and mass spectrometry. The possible factors favouring the diastereoselective hydroformylation of beta-pinenes under the conditions used are discussed.