[2+2+2] CYCLOADDITIONS OF ALKYNES WITH HETEROCUMULENES AND NITRILES

摘要

We thank Gary Molander,Tom Rovis,Linda Press,and the other members of the Organic Reactions Editorial Board for useful input at all stages in the preparation of this article. We are also grateful to Wenxing Guo for detailed English translations of several German journal articles. The [2+2+21 cycloaddition of unsaturated systems has proven to be an atom-economical way to create carbocycles and heterocycles rapidly. In 1866,Bcrthelot discovered that acetylene could be thermally converted to benzene.' Over 80 years later,in 1948,nickel complexes were found to catalyze the same trimerization at much lower temperatures.' The first examples of 12+2+2] cycloadditions using heterocumulenes emerged in the 1970s,and the number of publications in this area has increased dramatically over the last decade. The heterocumulenes that have been utilized as substrates include isocyanates,isothiocyanates,carbodiimides,carbon dioxide,carbon disulfide,and ketenes. Although a wide array of 6-membcred heterocycles can be prepared,little is known about how transition-metal catalysts facilitate the cycloaddition reactions. In contrast,reactions of nitriles,a class of substrates believed to undergo cycloadditions with alkynes by a similar mechanism as heterocumulene substrates,have been mechanistically well-explored. Overall,these reactions typically create six-membered heterocycles from two alkynes and a heterocumulene. The types of heterocycles that can be constructed with this chemistry are quite broad(Scheme 1). In addition,these heterocyclic products are useful synthetic building blocks because they are prevalent in a wide array of natural products,pharmacologically important compounds,transition-metal ligands,and organic light-emitting diode materials.