Evaluation of calix[4]arene-based chiral diphosphite ligands in Rh-catalyzed asymmetric hydrogenation of simple dehydroamino acid derivatives

The calix[4]arene framework was readily modified to generate a number of chiral BINOL-based diphosphite ligands (3) capable of forming in situ Rh-complexes which catalyzed the asymmetric hydrogenation of model substrates methyl-(Z)-2-(acetamido)acrylate (1a) and methyl-(Z)-2-(acetamido)cinnamate (1b). The (S,S)-catalyst generated the (R)-product. Upper rim (R-1) and 1,3-O-alkylation (R-2) substitution on the calixarene strongly influenced catalyst activity and chiral induction. Optimum results were obtained when RI was -C(CH3)(3) and R-2 was -CH2CH2CH3 (3b). Under optimized conditions, 3b hydrogenated la and 1b in 98 and 96% ee, respectively. Overall, better catalyst performance was observed for "locked" cone-conformers of 3, with higher activity evident for the less sterically hindered 1a (TOF up to 1300h(-1) at P(H-2) = 5 atm). (C) 2010 Elsevier B.V. All rights reserved.