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Heteroleptic complexes, formed selectively by using a 1 : 1 combination of a sigma-donor and a pi-acceptor ligand, are involved in Rh- and Pd-catalysed reactions.

A small library of chiral BINOL-derived monodentate phosphites containing either a carboxylic acid (-COOH, A1-A3) or a tertiary amine (-NMe 2 , B1-B4) was synthesized. The ligand combinations were screened in the enantioselective rhodium-catalyzed hydrogenation of methyl 2-acetamidoacrylate. The heterocombinations of the amine-derived phosphite (R)-B2 and of a carboxylic-phosphite [(R)-A1, (R)-A2, (R)-A3] displayed a slightly higher level of enantioselectivity compared to the corresponding homocombinations [up to 90 % ee using (R)-A1/(R)-B2]. The nature and extent of the interaction between the acidic and basic ligands in the rho-

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