The class of nitrogen-containing molecules called diazo compounds has revealed a hidden talent: coupling with certain diazo partners to form carbon-carbon double bonds (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201004923). If the metal-catalyzed process proves applicable to complex substrates, it could complement olefin metathesis and other alkene-forming reactions used to make materials and drugs.
Huw M. L. Davies’ team at Emory University stumbled on the new reactivity by accident. To the same reaction flask, they added both a specialized diazo compound called a donor-acceptor carbenoid and a different, unsubstituted diazo compound to compare their performance in a rhodium-catalyzed C–H functionalization reaction.
“We expected the two diazo compounds to compete,” Davies says. “It turns out the major product was cross-dimerization.” The diazo compounds reacted with each other to make a trisubstituted alkene.
Davies’ graduate student Jørn H. Hansen, intrigued by the chemistry, collaborated with State University of New York, Buffalo, theoretician Jochen Autschbach to learn about the reaction’s selectivity. The team found that boosting the bulkiness of the unsubstituted diazo compound increased the reaction’s stereoselectivity.
Chemists already have powerful alkene-making options, most notably olefin metathesis, Davies points out. The new reaction has the benefit of producing only nitrogen gas as waste, but to prove its worth it “needs the challenge of more elaborate substrates, which we haven’t done yet,” he adds. Still, the reaction is intriguing because it showcases the versatility of donor-acceptor carbenoids, he says, noting that his group has found “numerous examples where donor-acceptor carbenes open up new chemistry possibilities.”
The cross-coupling seems to get around the problem of an unsubstituted diazo compound coupling to itself, which has dampened the compounds’ reputation despite their synthetic usefulness, says organic chemist José Barluenga of the University of Oviedo, in Spain. This work “might be the starting point for a renaissance of diazo compounds,” he adds.