New class of chiral diphosphine ligands for highly efficient transition metal-catalyzed stereoselective reactions: The bis(diphenylphosphino) five-membered biheteroaryls

The synthesis and application of three examples of a new class of chiral (Ct) atropisomeric diphosphines characterized by two interconnected five-membered heteroaromatic rings, with hindered rotation around the interanular bond, are described. Optically pure (+)- and (-)-2,2'-bis(diphenylphosphino)-4,4',6,6'-tetramethyl-3,3'-dibenzo[b]thiophene (tetraMe-bitianp)(1a) and the parent unsubstituted system (+)- anti (-)-bitianp (1b) were synthesized. They were found to be optically stable at 100 degrees C and were successfully employed as ligands in the Ru(II)-catalyzed hydrogenation of alpha- and beta-oxo esters to the corresponding alpha- and beta-hydroxy esters and in the hydrogenation of olefinic substrates., The optical and chemical yields were comparable with those reported for the same Ru(II)-binap-catalyzed reactions carried out under the same experimental conditions. The 2,2'-bis(diphenylphosphino)-3,3'-bibenzo[b]furan (1c), the oxygenated analogue of bitianp, was found to be configurationally unstable at room temperature. Complete structural X-ray elucidation of the Pd complexes of 1a-e is reported. The advantages of these biheteroaromatic ligands over the classical biaryl systems are discussed.