NEW CLASS OF CHIRAL DIPHOSPHINE LIGANDS FOR HIGHLY EFFICIENT TRANSITION-METAL-CATALYZED STEREOSELECTIVE REACTIONS - THE BIS(DIPHENYLPHOSPHINO) 5-MEMBERED BIHETEROARYLS
T. Benincori et al., NEW CLASS OF CHIRAL DIPHOSPHINE LIGANDS FOR HIGHLY EFFICIENT TRANSITION-METAL-CATALYZED STEREOSELECTIVE REACTIONS - THE BIS(DIPHENYLPHOSPHINO) 5-MEMBERED BIHETEROARYLS, Journal of organic chemistry, 61(18), 1996, pp. 6244-6251
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
.