Formation of a Stable Rhodium(I) Dihydride Complex and Its Reactions with Prochiral Substrates of Asymmetric Hydrogenation

Abstract

A new P-chirogenic diphosphine, (S,S)-R,R′-bis(tert-butylmethylphosphino)-o-xylene (1), was synthesized in two steps from optically active secondary phosphine-borane 4 via the corresponding diphosphine-borane 3. The Rh(I) complex (2) of 1 was characterized by X-ray study, which revealed a significantly distorted C 2 -symmetry. The catalytic hydrogenation of four representative prochiral substrates gave modest enantioselection. Unlike all previously studied Rh(I) complexes of chiral diphosphines, 2 reacts with dihydrogen at -70 °C, producing two diastereomers of a solvate dihydride, 5a,b, in a ratio 1:0.07. The formation of 5a,b is almost quantitative; the hydrogen is not eliminated from 5. At temperatures above 20 °C, further transformation of 5 takes place, eventually yielding a bridging binuclear complex 6. The reaction of 5 with methyl Z-(R)-acetamidocinnamate ( 7) is slow at -80 °C, and gradual accumulation of a mixture of monohydride intermediates 15a-d could be monitored by 1 H NMR spectroscopy. When 5 was reacted with dimethyl 1-benzoyloxyethenephosphonate (10), two monohydride intermediates, 19a,b, formed cleanly in a ratio 1:0.04, which corresponded to the 92% ee observed after reductive elimination and quenching the reaction mixture.

Asymmetric hydrogenation of α,β-unsaturated phosphonates with Rh-BisP* and Rh-MiniPHOS catalysts: Scope and mechanism of the reaction

“…Operation of either unsaturated or dihydride pathway is possible on low-temperature hydrogenation of a catalyst-substrate complex. temperature reactions of solvate dihydride 4 produced by hydrogenation of the Rh-BisP* catalysts with methyl (Z)-␣-acetamidocinnamate and other related substrates (31)(32)(33)(34)(35)(36)(37). These experiments simulated the dihydride mechanism of the asymmetric hydrogenation and demonstrated the possibility of an almost perfect enantioselection occurring after the hydrogenactivation step.…”

“…In addition we studied the mechanism of this hydrogenation, in conjunction with our continuing mechanistic study on rhodium-catalyzed asymmetric hydrogenations of prochiral alkenes (31)(32)(33)(34)(35)(36)(37)(38).…”

Optically active 1,1′-di-tert-butyl-2,2′-dibenzophosphetenyl: a highly strained P-stereogenic diphosphine ligand

“…The use of ligand 12 resulted in 76% enantioselectivity. 17 The X-ray analysis of the Rh-complex of 12 shows that it is considerably rigid but not C 2 -symmetric. The observed fairly good selectivity can be interpreted by considering these asymmetric environments around the rhodium metal centre.…”

Recent Developments in Catalytic Asymmetric Hydrogenation Employing P‐Chirogenic Diphosphine Ligands

“…Our group [13,15,21,22,57] and others [12,16,18±20] have carried out an extensive series of hydrogenation reactions of dehydroamino acids using the rhodium complexes of P-chirogenic ligands, achieving enantiomeric excesses up to 99.9% (BisP* 3), [13,57] 99% (MiniPHOS 4, [15,57] TangPHOS 5, [16] Juge ¬×s ligand 7 [18] and Nagel×s ligand 8 [19] ), 92% (Brown×s ligand 9), [20] and 84% (BIPNOR 2), [12] as shown in Scheme 8.…”