Diastereoselective hydroformylation of 2-substituted allylic o-DPPB-esters-on the origin of 1,2-asymmetric induction |
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Authors: | Breit Bernhard Heckmann Golo Zahn Stephan K |
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Institution: | Albert-Ludwigs-Universit?t Freiburg Institut für Organische Chemie und Biochemie Albertstrasse 21, 79104 Freiburg, Germany. bernhard.breit@organik.chemie.uni-freiburg.de |
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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. |
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Keywords: | asymmetric synthesis catalysis catalyst directing group C?C coupling hydroformylation |
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