Mechanism of asymmetric hydrogenation catalyzed by a rhodium complex of (S,S)-1,2-bis(tert-butylmethylphosphino)ethane.: Dihydride mechanism of asymmetric hydrogenation

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 degrees C in deuteriomethanol affords solvate complex [Rh(BisP*)(CD3OD)(2)]BF4 (9), which is, in turn, hydrogenated at -90 degrees C producing equilibrium amounts (20% at -95 degrees 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 degrees C, yielding thermodynamic parameters Delta H = -6.3 +/- 0.2 kcal M-1 and Delta S = -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 I-ID fur 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 IID are Delta H = -10.0 +/- 0.4 kcal M-1 and Delta S = -20.3 +/- 1 cal M-1 K-1. Dihydride 10 reacts with the substrate 12 at -90 degrees C, yielding the monohydride intermediate 17a. The same product is obtained when 13 is hydrogenated at -80 degrees C. At temperatures above -50 degrees 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 eta(6)-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.