C-H Insertion from Isolable Copper Benzylidenes

Despite the utility of copper catalysts for the insertion of carbene moieties into C-H bonds, the copper carbene intermediate often invoked in these transformations has not been isolated. Herein, we describe the synthesis and structural characterization of a series of copper benzylidenes utilizing the sterically encumbered dipyrrin ligand (^EmL)H. These isolated copper carbenes demonstrate intramolecular insertion into the primary C(sp³)-H bond of the ligand (^EmL)H and intermolecular insertion into ethereal and allylic C-H bonds. The copper carbenes isolated are best described as Cu(I) carbene adducts akin to canonical Fischer carbenes, given their diamagnetic ground state and electrophilic carbene reactivity. Furthermore, the insertion chemistry can be rendered catalytic utilizing a more sterically exposed dipyrrin ligand (^ArFL)H. The ability to isolate and observe stoichiometric C-H insertion and olefin cyclopropanation from well-characterized copper benzylidenes illuminates their viability as catalytic intermediates and their participation in potential catalyst deactivation pathways.