Direct Spectroscopic Characterization of a Transitory Dirhodium Donor-Acceptor Carbene Complex

摘要

A multitude of organic transformations catalyzed by dirhodium(Ⅱ) (Rh_2) complexes are thought to proceed via the intermediacy of highly reactive, electrophilic carbenoid intermediates that have eluded direct observation. Herein, we report the generation of a metastable Rh_2-carbenoid intermediate supported by a donor-acceptor carbene fragment. This intermediate is stable for a period of ～20 hours in chloroform solution at 0℃, allowing for an exploration of its physical and chemical properties. The Rh=C bond, characterized by vibrational and nuclear magnetic resonance spectroscopy, extended x-ray absorption fine structure analysis, and quantum-chemical calculations, has weak a and π components. This intermediate performs stoichiometric cyclopropanation and C-H functionalization reactions to give products that are identical to those obtained from analogous Rh_2 catalysis.