Bidentates versus Monodentates in Asymmetric Hydrogenation Catalysis: Synergic Effects on Rate and Allosteric Effects on Enantioselectivity

C₁-Symmetric phosphino/phosphonite ligands are prepared by the reactions of Ph₂P(CH₂)₂P(NMe₂)₂ with (S)-1,1′-bi-2-naphthol (to give L_A) or (S)-10,10′-bi-9-phenanthrol (to give L_B). Racemic 10,10′-bi-9-phenanthrol is synthesized in three steps from phenanthrene in 44% overall yield. The complexes [PdCl₂(L_A,B)] (1a,b), [PtCl₂(L_A,B)] (2a,b), [Rh(cod)(L_A,B)]BF₄ (3a,b) and [Rh(L_A,B)₂]BF₄ (4a,b) are reported and the crystal structure of 1a has been determined. A ³¹P NMR study shows that M, a 1:1 mixture of the monodentates, PMePh₂ and methyl monophosphonite L_1a (based on (S)-1,1′-bi-2-naphthol), reacts with 1 equiv of [Rh(cod)₂]BF₄ to give the heteroligand complex [Rh(cod)(PMePh₂)(L_1a)]BF₄ (5) and homoligand complexes [Rh(cod)(PMePh₂)₂]BF₄ (6) and [Rh(cod)(L_1a)₂]BF₄ (7) in the ratio 2:1:1. The same mixture of 5−7 is obtained upon mixing the isolated homoligand complexes 6 and 7 although the equilibrium is only established rapidly in the presence of an excess of PMePh₂. The predominant species 5 is a monodentate ligand complex analogue of the chelate 3a. When the mixture of 5–7 is exposed to 5 atm H₂ for 1 h (the conditions used for catalyst preactivation in the asymmetric hydrogenation studies), the products are identified as the solvento species [Rh(PMePh₂)(L_1a)(S)₂]BF₄ (5′), [Rh(S)₂(PMePh₂)₂]BF₄ (6′) and [Rh(S)₂(L_1a)₂]BF₄ (7′) and are formed in the same 2:1:1 ratio. The reaction of M with 0.5 equiv of [Rh(cod)₂]BF₄ gives exclusively the heteroligand complex cis-[Rh(PMePh₂)₂(L_1a)₂]BF₄ (8), an analogue of 4a. The asymmetric hydrogenation of dehydroamino acid derivatives catalyzed by 3a,b is reported, and the enantioselectivities are compared with those obtained with (a) chelate catalysts derived from analogous diphosphonite ligands L_2a and L_2b, (b) catalysts based on methyl monophosphonites L_1a and L_1b, and (c) catalysts derived from mixture M. For the cinnamate and acrylate substrates studied, the catalysts derived from the phosphino/phosphonite bidentates L_A,B generally give superior enantioselectivities to the analogous diphosphonites L_2a and L_2b; these results are rationalized in terms of δ/λ-chelate conformations and allosteric effects of the substrates. The rate of hydrogenation of acrylate substrate A with heterochelate 3a is significantly faster than with the homochelate analogues [Rh(L_2a)(cod)]BF₄ and [Rh(dppe)(cod)]BF₄. A synergic effect on the rate is also observed with the monodentate analogues: the rate of hydrogenation with the mixture containing predominantly heteroligand complex 5 is faster than with the monophosphine complex 6 or monophosphonite complex 7. Thus the hydrogenation catalysis carried out with M and [Rh(cod)₂]BF₄ is controlled by the dominant and most efficient heteroligand complex 5. In this study, the heterodiphos chelate 3a is shown to be more efficient and gives the opposite sense of optical induction to the heteromonophos analogue 5.