A General Method for the Enantioselective Hydrogenation of β‐Keto Esters using Monodentate Binaphthophosphepine Ligands

Abstract 17 monodentate phosphepine ligands with a 4,5‐dihydro‐3 H ‐dinaphtho[2,1‐ c ;1′,2′‐ e ]phosphepine structural motif have been synthesized and tested in the asymmetric hydrogenation of various β‐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‐methoxyphenyl)‐4,5‐dihydro‐3 H ‐dinaphtho‐[2,1‐c;1′,2′‐ e ]phosphepine ( 4g ) as ligand. Best enantioselectivities were obtained at comparably high temperatures (100–120 °C), which had the advantage of increased reaction rates.