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Pure Appl. Chem., 2006, Vol. 78, No. 2, pp. 303-310

Rhodium-catalyzed asymmetric reactions with a dynamic library of chiral tropos phosphorus ligands

Cesare Gennari1, Chiara Monti1 and Umberto Piarulli2

1 Dipartimento di Chimica Organica e Industriale, Centro di Eccellenza C.I.S.I., Università degli Studi di Milano, Istituto di Scienze e Tecnologie Molecolari (ISTM) del CNR, Via G. Venezian 21, 20133 Milano, Italy
2 Dipartimento di Scienze Chimiche e Ambientali, Università degli Studi dell'Insubria, Via Valleggio 11, 22100 Como, Italy

Abstract: Nineteen chiral tropos phosphorus ligands, based on a flexible (tropos) biphenol unit and a chiral P-bound alcohol (11 phosphites) or secondary amine (8 phosphoramidites), were screened, individually and as a combination of two, in various Rh-catalyzed asymmetric reactions. In the Rh-catalyzed asymmetric conjugate addition of phenylboronic acid to cyclic enones, enantiomeric excesses (ee's) up to 95 % were obtained with a 1:1 mixture of a phosphite [derived from (1R,2S)-2-(1-methyl-1-phenylethyl)cyclohexanol] and a phosphoramidite [derived from (S,S)-2,5-diphenylpyrrolidine]. In the mixed Rh precatalyst, which was characterized via 31P-NMR, the biphenol-derived phosphite is free to rotate (tropos) while the biphenol-derived phosphoramidite shows a temperature-dependent tropos/atropos behavior (coalescence temperature = 310 K). The ligands were also screened in the hydrogenation of dehydro-α-amino acids and dehydro-β-amino acids. Ee's up to 98% were obtained for the dehydro-α-amino acids, using the combination of a phosphite [derived from (1R,2S)-2-phenyl-1-cyclohexanol] and a phosphoramidite [derived from (S,S)-bis(α-methylbenzyl)amine]. The reaction was optimized by lowering the phosphite/phosphoramidite ratio to 0.25:1.75 with a resulting improvement of the product ee.