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-dim ethanolato(2 -)-O,O']titanium ((R)-1a) and dichloro[(4R,5R)-2,2-dimethyl-α,α,α',α'-tetra(naphthalen-1-yl)-1,3-dioxol ane-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.