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

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.