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Highly Enantioselective Hydrogenation of b-Acyloxy and b-Acylamino a,b-Unsaturated Phosphonates Catalyzed by Rhodium Phosphane-Phosphite Complexes

IIQ

M. Ángeles Chávez, Sergio Vargas, Andrés Suárez, Eleuterio Álvarez, Antonio Pizzano Advanced Synthesis and Catalysis 2011, Vol. 353, 2775-2794

The enantioselective hydrogenation of b-(acyloxy)- and b-(acylamino)vinylphosphonates with rhodium catalysts based on chiral phosphane-phosphite ligands has been studied. In the case of the b-(acyloxy)vinylphosphonates, the reaction also produces an achiral phosphonate resulting from the elimination of the benzoate group. High ligand modularity has led to a highly chemo- and enantioselective catalyst for both types of substrates, which afford a good range of b-acyloxy- and b-acylaminophosphonates with enantioselectivities between 90 and 99% ee. Most interestingly, the configuration of the hydrogenation products indicates the same stereochemical sense for the reduction of both types of substrates, which is opposite to that observed before for a- (acyloxy)vinylphosphonates. This observation has been rationalized by assuming the formation of a b-alkyl intermediate during the catalytic cycle, which also explains the formation of the elimination product in the hydrogenation of the b-(acyloxy)-vinylphosphonates.

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