The mechanism of asymmetric hydrogenation catalyzed by a new effective catalyst, viz., a rhodium complex of (S,S)-1,2-bis(tert-butylmethylphosphino)ethane (BisP*), has been studied by multinuclear NMR. Hydrogenation of the precatalyst [Rh(BisP*)(nbd)]BF4 (8) at −20 °C in deuteriomethanol affords solvate complex [Rh(BisP*)(CD3OD)2]BF4 (9), which is, in turn, hydrogenated at −90 °C producing equilibrium amounts (20% at −95 °C) of [RhH2(BisP*)(CD3OD)2] (10)the first observable dihydride of a Rh(I) complex with a diphosphine ligand. Dihydride 10 is in equilibrium with 9 and dihydrogen, which was studied in the temperature interval from −100 to −50 °C, yielding thermodynamic parameters ΔH = −6.3 ± 0.2 kcal M-1 and ΔS = −23.7 ± 0.7 cal M-1 K-1. The hydrogenation of 9 is stereoselective:  two isomers 10a and 10b are produced in a ratio 10:1. Use of HD for the hydrogenation of 9 yields the isomers with deuterium cis and trans to the phosphine in a ratio 1.3 (±0.1):1. The thermodynamic parameters of the equilibrium between 9, 10 d , and HD are ΔH = −10.0 ± 0.4 kcal M-1 and ΔS = −20.3 ± 1 cal M-1 K-1. Dihydride 10 reacts with the substrate 12 at −90 °C, yielding the monohydride intermediate 17a. The same product is obtained when 13 is hydrogenated at −80 °C. At temperatures above −50 °C monohydride intermediate 17a undergoes reductive elimination, affording the hydrogenation product 15 in equilibrium with the product−catalyst complex 16 in which the catalyst is η6-coordinated to the phenyl ring of the product. The experimental data require that the dihydride mechanism is operating in the case of asymmetric hydrogenation catalyzed by 9. This, in turn, suggests that the enantioselective step is the migratory insertion in a dihydride intermediate 18.

The asymmetric hydrogenation of aryl- and alkyl-substituted enamides catalyzed by Rh-BisP complex affords optically active amides with very high ee values. The Rh-MiniPHOS catalyst gives somewhat less satisfactory results. Hydrogenation of the aryl-substituted enamides with (S,S)-BisP-Rh catalyst gives R-amides, whereas the t-Bu- and 1-adamantyl-substituted enamides give S-products with 99% ee. Reaction of [Rh(BisP)(CD(3)OD)(2)]BF(4) (11) with CH(2)=C(C(6)H(5))NHCOCH(3) (5) gives two diastereomers of the catalyst-substrate complex (12a,b), which interconvert reversibly by both intra- and intermolecular pathways as shown by EXSY data. Only one isomer in equilibrium with solvate complex 11 was detected for each of the catalyst-substrate complexes 17 and 18 obtained from CH(2)=C(t-Bu)NHCOCH(3) (6) or CH(2)=C(1-adamantyl)NHCOCH(3) (7). Hydrogenation of these equilibrium mixtures at -100 degrees C gave monohydride intermediates 19 and 20, respectively. In these monohydrides the Rh atom is bound to the beta-carbon. A new effect of the significant decrease of ee was found for the asymmetric hydrogenation of CH(2)=C(C(6)H(4)OCH(3)-o)NHCOCH(3) (21), when H(2) was substituted for HD or D(2).

A new class of chiral C2‐symmetric bis(trialkyl)phosphine ligands has been prepared and used in Rh(I)‐catalyzed asymmetric hydrogenation reactions. The ligands, 1,2‐bis(alkylmethylphosphino)ethanes 1a‐g(abbreviated as BisP*, alkyl = t‐butyl, 1‐adamantyl, 1‐methylcyclohexyl, 1,1‐diethylpropyl, cyclopentyl, cyclohexyl, isopropyl) and 1,2‐bis(alkylmethylphosphino)methanes 2a‐d(abbreviated as MiniPHOS, alkyl = t‐butyl, cyclohexyl, isopropyl, phenyl) are prepared by a simple synthetic approach based on the air‐stable phosphine–boranes. These new ligands give the corresponding Rh(I) complexes, which are effective catalytic precursors for the asymmetric hydrogenation of a representative series of dehydroamino acids and itaconic acid derivatives. Enantioselectivities observed in these hydrogenations are universally high and in many cases exceed 99%. X‐Ray characterization of four precatalysts, study of the pressure effects, deuteration experiments, and characterization of the wide series of intermediates in the catalytic cycle are used for the discussion of the possible correlation between the structure of the catalysts and the outcome of the catalytic asymmetric hydrogenation.

Excellent enantioselectivities up to 99.7% were achieved in the hydrogenation of (E)-beta-(acylamino)acrylates by the use of Rh(I)-complexes of electron-rich diphosphines, t-Bu-BisP and t-Bu-MiniPHOS. Low-temperature NMR experiments testify that monohydrides with beta-carbon atom of the substrate bound to rhodium are involved in the catalytic cycle.

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.