Mechanism of Asymmetric Hydrogenation of β-Dehydroamino Acids Catalyzed by Rhodium Complexes: Large-Scale Experimental and Computational Study

The mechanism of asymmetric hydrogenation of five representative beta-dehydroamino acids catalyzed by rhodium complexes of (R)-(tert-butylmethylphosphino) (di-tert-butylphosphino)methane (trichickenfootphos, TCFP) and (R,R)-1,2-bis(tert-butylmethylphosphino)benzene (BenzP*) was studied through a combination of extensive NMR experiments and state-of-the-art DFT computations in order to reveal the crucial factors governing the sense and order of enantioselectivity in this industrially important reaction. The binding mode of the substrate with a Rh(I) catalyst was found to be highly dependent on the nature of the rhodium complex and the substrate. Thus, no substrate binding was detected for [Rh((R,R)-BenzP*)S-2]+SbF6- (5) and (E)-3-acetylamino-2-butenoate (2a) even at 173 K. [Rh((R)-TCFP) S2]+BF4- (3) exhibited weak reversible binding with 2a in the temperature interval 173-253 K with the formation of complex 4a, whereas at ambient temperature, slow isomerization of 2a to (Z)-3-acetylamino-2-butenoate (2b) took place. The investigations with a total of 10 combinations of the catalysts and substrates demonstrated various binding modes that did not affect significantly the enantioselectivities observed in corresponding catalytic reactions and in low temperature hydrogenations of the catalyst-substrate complexes. The monohydride intermediate 10 formed quantitatively when the equilibrium mixture of 2a, 3, and 4a was hydrogenated at 173 K. Its molecular structure including relative sfereochemiStry was determined by NMR experiments. These results together with the stereochemichal outcome of the low-temperature hydrogenation (99.2% ee, R) and DFT calculations led to the reasonable reaction pathway of the asymmetric hydrogenation of 2a catalyzed by 3. The conceivable catalytic pathways were computed for five combinations of the BenzP*-Rh catalyst and prochiral beta-dehydroamino acids 2a,b and 21-23. In most cases, it was found that the pathways involving the hydrogenation of Rh(I) square planar chelate complexes are usually higher in free energy than the pathways with the hydrogen activation prior to the chelate formation. Computed differences in the free energies of the transition states for the double bond coordination stage of the R and S pathways reasonably well reproduce the optical yields observed experimentally in the corresponding catalytic reactions and in the low temperature hydrogenation experiments. To explain extremely high ee's (>99% ee) in some of the hydrogenations, it is necessary to analyze in more detail the participation of the solvent in the enantiodetermining step.