NEW CLASS OF CHIRAL DIPHOSPHINE LIGANDS FOR HIGHLY EFFICIENT TRANSITION-METAL-CATALYZED STEREOSELECTIVE REACTIONS - THE BIS(DIPHENYLPHOSPHINO) 5-MEMBERED BIHETEROARYLS

The synthesis and application of three examples of a new class of chir al (Ct) atropisomeric diphosphines characterized by two interconnected five-membered heteroaromatic rings, with hindered rotation around the interanular bond, are described. Optically pure (+)- and no)-4,4',6,6 '-tetramethyl-3,3'-dibenzo[b]thiophene (tetraMe-bitianp)(1a) and the p arent unsubstituted system (+)- anti (-)-bitianp (1b) were synthesized . They were found to be optically stable at 100 degrees C and were suc cessfully employed as ligands in the Ru(II)-catalyzed hydrogenation of alpha- and beta-oxo esters to the corresponding alpha- and beta-hydro xy esters and in the hydrogenation of olefinic substrates., The optica l and chemical yields were comparable with those reported for the same Ru(II)-binap-catalyzed reactions carried out under the same experimen tal conditions. The 2,2'-bis(diphenylphosphino)-3,3'-bibenzo[b]furan ( 1c), the oxygenated analogue of bitianp, was found to be configuration ally unstable at room temperature. Complete structural X-ray elucidati on of the Pd complexes of 1a-e is reported. The advantages of these bi heteroaromatic ligands over the classical biaryl systems are discussed .