Huisgen-type alkyne-azide cycloaddition

Huisgen-type 1,3-dipolar cycloaddition of azides to alkynes... 

In 1893, Arthur Michael observed the formation of a 1,2,3-triazole derivative in the reaction of diazobenzolimid (i.e. Ph-Ns) with dimethyl acetylenedicarboxylate Michael - a future Harvard professor - worked with R. Bunsen and A.W. von Hofmann. Numerous cycloadditions of organic azides and HN3 to alkynes and alkenes were described in the sequel. In the general definition and classification azides belong to the 1,3-dipoles of propargyl-allenyl type (R. Huisgen, I960). 1,3-Dipolar cycloadditions share the 6 r-electron balance with Diels-Alder reactions - and the wide synthetic apphcation. Albert Padwa edited monographs on 1,3-dipolar cycloaddition chemistry in 1984 and 2003 -substantial chapters on azides were included. ... 

Rate constants ( 2) for cycloadditions of phenyl azide to substituted ethylenes and acetylenes stretch over seven magnirndes high values were observed for enamines, moderate k2 for the acrylic ester type, and the rate minimum was found for common alkenes and alkynes (Huisgen, Szeimies, Mobius, 1967). hi the PMO treatment of concerted cycloadditions, Reiner Sustmann found the key to the understanding of substituent effects e.g., a plot of k2 for the cycloadditions of Ph-Ns versus the ionization potential of substituted ethylenes and acetylenes furnished a degenerate U shape (1971). Such a plot is a distinguishing feature for each 1,3-dipole and reflects the specific mix of nucleophilic and electrophilic activity, modified by steric effects. 

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