Borenium Salt Catalysts Readily Reduce Imines

Canadian chemists have learned how to generate powerful metal-free catalysts for important imine reduction reactions by starting from boron-based Lewis acid-base complexes (J. Am. Chem. Soc., DOI: 10.1021/ja307374j). Patrick Eisenberger, Adrian M. Bailey, and Cathleen M. Crudden of Queen’s University, in Kingston, Ontario, coupled B(C₆F₅)₃ or [(C₆H₅)₃C][B(C₆F₅)₄] with 1,4-diazabicyclo[2.2.2]octane and pinacolborane. The resulting catalytic salts consist of a highly electrophilic borenium cation paired with a fluorophenylborate anion (one shown). Crudden’s team found that the catalytic borenium complexes activate the C=N bond of a broad range of imines and use the hydrogen from pina­colborane to reduce the C=N bond to form secondary amines under mild reaction conditions. In another paper just published, Jeffrey M. Farrell, Jillian A. Hatnean, and Douglas W. Stephan of the University of Toronto report a related borenium catalyst that activates and uses H₂ for imine reductions (J. Am. Chem. Soc., DOI: 10.1021/ja307995f). These two reports are the first examples of borenium catalysts being used for metal-free reductions, Crudden says. Both groups provide new strategies for avoiding the use of stoichiometric boro­hydride salts, but Crudden’s version offers a unique mechanism that also avoids the need to use H₂ gas.