The aminophosphine-phosphinites and related ligands: synthesis, coordination chemistry and enantioselective catalysis1Dedicated to the memory of Professor Francis Petit1

Origin of Stereoinduction by Chiral Aminophosphane Phosphinite Ligands in Enantioselective Catalysis: Asymmetric Hydroformylation

The origin of stereoinduction by chiral aminophosphane phosphinite (AMPP) ligands in asymmetric hydroformylation was investigated with a theoretical approach. The roles of the stereogenic center at the aminophosphane phosphorus atom (NP*) and of the chirality of the backbone were analyzed by considering three experimentally tested cases: 1) P-stereogenic yielding high ee, 2) P-nonstereogenic yielding low ee, and 3) P-stereogenic yielding low ee. We succeeded in reproducing the experimentally observed trends for the three studied AMPP ligands. Our results indicated that alkene insertion into the rhodium-hydride bond is the selectivity-determining step, and not alkene coordination. Additional calculations on model systems revealed that the different nonbonding weak-type interactions of styrene with the substituents of the NP* stereogenic center in an axial position is responsible for stereodifferentiation. The chirality of the AMPP backbone plays a secondary role. The rationalization of the stereochemical outcome is not straightforward, because two competitive equatorial/axial reaction paths, showing opposite asymmetric induction, must be considered. Construction of stereochemical models and evaluation of stereoinduction for novel ligand systems suggested that two prerequisites are required to improve the performance of AMPP-type ligands in asymmetric hydroformylation: 1) combination of stereorecognition and stereohindrance by substituents at the NP* atom, and 2) more rigid backbones.

In situ Chiral Modification of Nickel Catalysts for Enantioselective Hydrogenation of Methyl Acetoacetate

Chiral modifications of SiO2-supported and Raney nickel catalysts were studied. The catalysts were employed in asymmetric hydrogenation of methyl acetoacetate (MAA) to (R)- and (S)-enantiomers of methyl 3-hydroxybutanoate. The effects of modification parameters such as type and concentration of modifier; presence of a co-modifier and other additives, pH of modification solution on the enantioselectivity of MAA hydrogenation were discussed. Characteristic features of the in situ modification of Ni/SiO2 were also evaluated and the results obtained were compared with the conventional (premodification) approach. Parameters for the conventional and in situ methods were optimised in a series of experiments for both types of catalysts. The in situ modified Ni/SiO2 was found specifically suitable for repeated use due to virtually no decrease in selectivity and activity.

Unsymmetric bidentate ligands in metal-catalyzed carbonylation of alkenes

Characteristic features of unsymmetric bidentate ligands, in which the two coordination atoms are not equivalent, are reviewed with a focus on their use in metal-catalyzed olefin carbonylations. High enantioselectivity for a variety of substrates was achieved using ligand 9 in Rh-catalyzed hydroformylation. Unsymmetric ligand 21e shows high productivity accompanied by high regio- and enantioselectivities in the Pd-catalyzed alternating copolymerization of 1-alkene with CO. The advantages of electronic unsymmetry are demonstrated especially in the spectroscopic observation of single steps involved in catalytic cycles.