Ketenes Finally Give In To Cycloadditions

In a chemical first, ketenes have been incorporated into nickel-catalyzed cycloaddition reactions, somehow avoiding decarbonylation of the ketene that has thwarted past attempts (J. Am. Chem. Soc., DOI: 10.1021/ja2007627). Nearly every conceivable unsaturated substrate—alkynes, dienes, carbon dioxide, nitriles, isocyanates, and carbonyls—has been used in transition-metal-catalyzed cycloadditions. But ketenes had been notoriously absent from that list. The reason is not because ketenes are unreactive with typical cycloaddition catalysts, Janis Louie of the University of Utah and coworkers note, but because of the tendency of ketenes to shed their carbonyl groups to form alkenes and unreactive metal-CO complexes. “In view of these pitfalls, we were surprised and delighted to discover that nickel phosphine catalysts mediate the cycloaddition of ketenes and diynes to afford cyclohexadienones in good yields,” they write. The reaction, shown generically, works with an assortment of ketenes and diynes. The team is still hashing out the details of how the reaction avoids decarbonylation, but it stems from selection of the phosphine ligand.