Catalytic Enantioselective Chlorination and Bromination of β-Keto Esters

The Ti(TADDOLato) complexes dichloro[(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanolato(2−)-O,O′]titanium ((R)-1a) and dichloro[(4R,5R)-2,2-dimethyl-α,α,α′,α′-tetra(naphthalen-1-yl)-1,3-dioxolane-4,5-dimethanolato(2−)-O,O′]titanium ((R)-1b) are efficient catalysts for the electrophilic enantioselective chlorination and bromination of β-keto esters with N-chlorosuccinimide (NCS) and N-bromosuccinimide (NBS), respectively. With 5 mol-% of catalyst at room temperature an enantioselectivity of up to 88% ee could be obtained for the chlorination reaction. Under comparable conditions, bromination reactions are slower and less stereoselective.