Synergistic catalysis: enantioselective cyclopropanation of alkylidene benzoxazoles by Pd(ii) and secondary amine catalysis. Scope, limitations and mechanistic insight†
The reaction of alkyl-benzoxazoles with enals has been reported using a synergistic approach. It consists of the activation of enals with a secondary amine catalyst and the activation of benzoxazoles with a metal Lewis acid. This approach has also been applied to the synthesis of cyclopropanes with excellent results. We demonstrated the applicability of this reaction in a cascade reaction (cyclopropanation + ring opening) that circumvents some of the limitations of the simple Michael addition between alkyl-benzoxazoles and enals. Finally, mechanistic studies have been reported to explain the stereoselectivity of the cyclopropanation reaction that renders an unusual cis conformation.