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Evaluation of Asymmetric Hydrogenation Ligands in Asymmetric Hydroformylation Reactions. Highly Enantioselective Ligands Based on Bis-phosphacycles

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posted on 09.10.2006, 00:00 by Alex T. Axtell, Jerzy Klosin, Khalil A. Abboud
An evaluation of 47 phosphorus-based ligands has been conducted in rhodium-catalyzed asymmetric hydroformylation reactions, AHF, at high temperature. Most of the ligands exhibited poor enantio- and regioselectivity as well as low catalytic activity. Two ligands, (R)-Binapine and (S,S,R,R)-TangPhos, were found to give outstanding enantioselectivities in asymmetric hydroformylation of styrene, allyl cyanide, and vinyl acetate. (R)-Binapine gave 94% ee, 94% ee, and 87% ee, whereas (S,S,R,R)-TangPhos gave 90% ee, 93% ee, and 83% ee for hydroformylation products of styrene, allyl cyanide, and vinyl acetate, respectively. Enantioselectivity achieved for the allyl cyanide product with these ligands is the highest ever reported for this substrate. Excess of (S,S,R,R)-TangPhos leads to low enantioselectivities in the AHF of styrene and allyl cyanide due to in situ formation of the ionic complex [[((S,S,R,R)-TangPhos)2]Rh]+[acac]-. The noncoordinating acetylacetonate anion is responsible for this sharp decrease of enantioselectivity in hydroformylation products. X-ray crystal structures of [[((S,S,R,R)-TangPhos)2]Rh]+[acac]- and [(S,S,R,R)-TangPhos]Rh(acac) have been determined and examined. The high success achieved with bis-phosphacycle ligands in asymmetric hydroformylation reactions suggests that this ligand class is unique and highly promising among previously investigated phosphorus-based systems and should be further explored in search of even better ligands for this important reaction.