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

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posted on 28.05.2008, 00:00 by David W. Norman, Charles A. Carraz, David J. Hyett, Paul G. Pringle, Joseph B. Sweeney, A. Guy Orpen, Hirrahataya Phetmung, Richard L. Wingad
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 LA) or (S)-10,10′-bi-9-phenanthrol (to give LB). Racemic 10,10′-bi-9-phenanthrol is synthesized in three steps from phenanthrene in 44% overall yield. The complexes [PdCl2(LA,B)] (1a,b), [PtCl2(LA,B)] (2a,b), [Rh(cod)(LA,B)]BF4 (3a,b) and [Rh(LA,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 L1a (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)(L1a)]BF4 (5) and homoligand complexes [Rh(cod)(PMePh2)2]BF4 (6) and [Rh(cod)(L1a)2]BF4 (7) in the ratio 2:1:1. The same mixture of 57 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 57 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)(L1a)(S)2]BF4 (5′), [Rh(S)2(PMePh2)2]BF4 (6′) and [Rh(S)2(L1a)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(L1a)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 L2a and L2b, (b) catalysts based on methyl monophosphonites L1a and L1b, and (c) catalysts derived from mixture M. For the cinnamate and acrylate substrates studied, the catalysts derived from the phosphino/phosphonite bidentates LA,B generally give superior enantioselectivities to the analogous diphosphonites L2a and L2b; 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(L2a)(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.

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