Regio- and stereoselective hydroformylation of glucal derivatives with rhodium catalysts

The hydroformylation of the differently protected glucal derivatives 3,4,6-tri-O-acetyl-D-glucal, 3,4,6-tri-O-benzyl-D-glucal and 3,4,6-tri-O-methyl-D-glucal was carried out with rhodium catalytic systems, and 2-formyl derivatives were obtained as the main products in yields of 58%, 68%, and 55% respectively, when [Rh2(μ-OMe)2(COD)2]TP(O-o- tBuC6H4)3 was used. The bulky phosphite needs to be used as auxiliary ligand to achieve the hydroformylation of these highly hindered cyclic olefins. A mechanistic cycle is proposed which explains the regio- and stereoselectivity of the reaction. The mononuclear rhodium complex trans[RhCl(CO)(P(O-o-tBuC6H4)3) 2] was isolated at the end of the catalytic reaction by breaking the dinuclear complexes used as catalyst precursors. To confirm the characterization of trans-[RhCl(CO)(P(O-o-tBuC6H4) 3)2], a separate synthesis was carried out which isolated crystals suitable for an X-ray determination.