B(C6F5)3-Catalyzed Ring-Opening Reaction of Bicyclo[1.1.0]butanes with Silyl Ketene Imines.

Nucleophilic addition to strained rings is an efficient approach for the synthesis of polysubstituted cyclobutanes, which are privileged structural scaffolds in many pharmaceuticals. In this work, we have developed a simple, efficient, transition-metal-free, and scalable route for the synthesis of trisubstituted methylenecyclobutanes via ring opening of bicyclo[1.1.0]butanes with silyl ketene imines, using B(C6F5)3 as the catalyst under mild reaction conditions. In this fashion, 24 new potential bioisosteres containing trisubstituted methylene cyclobutanes were obtained in good to excellent yields (up to 99% yield). Furthermore, example reactions were performed on a gram scale, and a proposed mechanism is discussed.