Hydroformylation of optically pure monoterpenes catalyzed by dinuclear thiolato-bridged rhodium complexes

The low-pressure hydroformylation of di-or trifunctionalized cyclic monoterpenes was carried out with the dinuclear rhodium complexes [Rh-2(mu-(SBu)-Bu-1)(2)(CO)(2)L-2] under 5-13 bar (1 bar = 10(5) Pa). The exocyclic ethylenic double bond of R-limonene, S-carvone, S-perillyl alcohol, or (1R,4R)-isolimonene was rather easily and selectively carbonylated. Attempts to induce high diastereoselectivities gave a 22% diastereoisomeric excess (d.e.) under the best conditions. On the other hand, cyclization following the hydroformylation of (1R,2S,5R)-isopulegol allowed a good 64% d.e. This reaction does not require the presence of chiral phosphorus ligands. The asymmetric induction is directed by the substrate configuration. Under slightly higher pressures (13-20 bar), (-)-beta-pinene and (-)-camphene were selectively carbonylated and d.e. values ranging from 26 to 85% were obtained with classical phosphite or diphosphine ligands. Finally, at 100 bar, (-)-alpha-pinene and (-)-myrtenol were transformed with moderate yields and selectivities but high d.e. of up to 100%.