Catalytic hydroformylation of (1S,5S) -( - ) - and (1R,5R) -( +) -β-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)-( + )-β-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 [Rh4(CO)12] or [Rh6(CO)16] and [Co2(CO)8]. At 70-125°C and under 60 atm of syngas, the yields of hydroformylated products do not exceed 30% because of the concomitant isomerization of β-to α-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 [Rh6(CO)16]. Comparable yields and diastereoselectivities were obtained from reactions in tetrahydrofuran with a mixture of [Rh4(CO)12] and [N(PPh3)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 ≥ 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, 1H and 13C NMR and circular dicroism (CD) spectroscopy, and mass spectrometry. The possible factors favouring the diastereoselective hydroformylation of β-pinenes under the conditions used are discussed.