Cu(I)-Catalyzed Cross-Coupling of Diazo Compounds with Terminal Alkynes: An Efficient Access to Allenes.

Cu(I)-catalyzed reaction of diazo compounds generates a Cu(I)-carbene intermediate that undergoes diverse transformations. In the past few years, the diazo compounds (or their precursor N-tosylhydrazones) have been established as cross-coupling partners under transition-metal catalysis, affording various organic compounds. Particularly the breakthrough has been made in allene synthesis by Cu(I)-catalyzed carbene coupling with terminal alkynes. Moreover, the Cu(I)-catalyzed coupling reaction of diazo compounds with terminal alkynes generates allene intermediate that undergoes tandem cyclization/coupling to afford cyclic compounds. This review article summarizes the most recent developments in allene synthesis based on the Cu(I)-carbene coupling reactions and the utilization of allene intermediates in tandem reactions.