Asymmetric catalysis, part 108 copper catalysts with optically active ligands in the enantioselective Meerwein arylation of activated olefins

Abstract

The copper-catalyzed Meerwein reaction of activated olefins with arenediazonium halides leads to a formal addition of aryl halide to the olefinic double bond. Methyl acrylate, p-tolyldiazonium tetrafluoroborate and tetrabutylammonium chloride in anhydrous acetonitrile were used as a model system, the product being methyl α-chloro- β-tolylpropionate. Applying a newly established methodology, the isolated diazonium tetrafluoroborate and an equivalent amount of tetrabutylammonium chloride were added to the olefin and the copper catalyst, containing optically active ligands, such as 2- and 2,6-pyridinyloxazolines and bisoxazolines. The low optical inductions obtained in the room temperature reaction could be raised to 8.7% ee in the low temperature variant at −40°C. Two phenyl substituents in the neighborhood of the chiral centers in the bisoxazoline ligands of the catalyst and increasing steric bulk in the ester group of the substrate (methyl, ethyl, menthyl) increase the stereoselectivity of the Meerwein arylation. Replacement of the p-tolyldiazonium salt by the mesityldiazonium salt caused the optical induction in the formation of the corresponding mesityl product rise to 19.5% ee. Thus, though considered to be a radical reaction, the Meerwein arylation of activated olefins can be rendered enantioselective by using copper catalysts with optically active ligands.