Diastereoselective Hydroformylation of 2‐Substituted Allylic o‐DPPB‐Esters—On the Origin of 1,2‐Asymmetric Induction

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Abstract
2‐Substituted secondary alcohol o‐DPPB esters (o‐DPPB=ortho‐diphenylphosphanylbenzoyl) have been prepared and their o‐DPPB‐directed diastereoselective hydroformylation examined. It was found that the diastereoselectivity increased as a function of the steric demand of the substituents both at the stereogenic center and in the alkene 2‐position. Hydrolytic cleavage of the o‐DPPB group afforded—via the lactols 29—the corresponding lactones 30, the relative configurations of the vicinal stereogenic centers of which were ascertainable by 2D‐NOESY spectroscopy. In addition, a crystal structure analysis of the hydroformylation product 2 d provided further confirmation of the relative configuration. Replacement of the ester carbonyl group of the o‐DPPB by a methylene unit resulted in significantly worse diastereoselectivity in the course of the hydroformylation (34→35), which indicates a decisive role for the ester carbonyl function. All the experimental observations were combined in a model of the origin of the 1,2‐asymmetric induction during the title reaction. The key feature is the consideration of diastereomeric trigonal‐bipyramidal rhodium‐hydrido‐olefin complexes I and II, capable on the basis of the Hammond postulate of acting as good models for the transition states of the selectivity‐determining hydrometalation step. Investigation of these complexes by force‐field methods indicated good correlation between theoretically predicted and experimentally determined diastereoselectivities.