Elucidation of the Enantioselection Mechanism of Transition Metal-Catalyzed Asymmetric Reactions

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This study is directed toward the elucidation of the enantioselection mechanism of rhodium-catalyzed asymmetric hydrogenation that is one of the representative asymmetric catalyses. P-Chirogenic, electron-rich diphosphines such as BisP^* and MiniPHOS were used in this study because of their simple molecular structure and very high enantioinduction ability in rhodium-catalyzed asymmetric hydrogenations of α- or β-and dehydroamino acid derivatives, enamides, and enol esters. One of the most important findings is the detection and characterization of the rhodium dihydride complexes bearing diphosphine ligands. The dihydride complexes reacted readily with the alkene substrates even at -100 - -90 ℃ to give the corresponding monohydride intermediates, and subsequent reductive elimination occurred at about -50 ℃. These results are interpreted by considering a dihydride mechanism. It is concluded that the enantioselection is determined at the stage of the migratory insertion of the Rh-H to the C-C double bond. The main factors to control the enantioselection are 1. Trans effect of by the coordinated phosphorus atom, 2. The orbital interaction between of the Rh-H and the C-C double bond, 3. Formation of the chelate cycle, 4. The steric interaction between the bulky group at the phosphorus and the chelate cycle, 5. Electronic effect of the substituent at the olefic group. It is also concluded that the enantioselection of these Rh-catalyzed asymmetric hydrogenations is governed by multi stereo regulation factors. A new quadrant rule to correlate between the stereochemistry of the hydrogenation products and the structure of the catalyst structures has been proposed. A new, highly efficient chiral diphosphine ligand has been designed and synthesized based on these research results.