A general method for the enantioselective hydrogenation of β-keto esters using monodentate binaphthophosphepine ligands

17 monodentate phosphepine ligands with a 4,5-dihydro-3H-dinaphtbo[2,1-c;l',2'-e]phosphepine structural motif have been synthesized and tested in the asymmetric hydrogenation of various beta-keto esters. By variation of the substituents of the aryl group on the phosphorus atom a fine tuning of the selectivity of the catalytic system is possible. Quantitative yield and enantioselectivities up to 95% ee have been achieved for the hydrogenation of methyl acetoacetate (7a), methyl 3-oxovalerate (7b) and ethyl 4-phenyl-3-oxo-propionate (7d) using 4-(4-methoxy-phenyl)-4,5-dihydro-3H-dinaphtho-[2,1-c;l',2'-e]phosphepine (4g) as ligand. Best enantioselectivities were obtained at comparably high temperatures (100-120 degrees C), which had the advantage of increased reaction rates.