R. Huisgen, Xing-Cheng Li, H. Giera
May 16, 2001
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0
Influential Citations
49
Citations
Journal
Helvetica Chimica Acta
Abstract
Thiobenzophenone and diazomethane afford thiadiazoline 1 at −78°. By elimination of N2 from 1 at −45° (t1/2ca. 1 h), (diphenylmethylidenesulfonio)methanide (2), which cannot be isolated but is interceptible by dipolarophiles, is set free. The nucleophilic 1,3-dipole 2 undergoes cycloadditions with electrophilic C,C multiple bonds; the structures of 16 cycloadducts were elucidated. One-step and two-step cycloaddition pathways are discussed in the light of the steric course observed for (E)/(Z)-isomeric ethylene derivatives. Competition experiments with pairs of dipolarophiles at −45° and HPLC analysis of the adducts provided relative rate constants of 26 dipolarophiles, involving 2 C≡C, 13 C=C, 9 C=S, and 2 N=N bonds. In accordance with Sustmann`s reactivity model of concerted cycloadditions, 2 shows the highest selectivity of all known 1,3-dipoles, i.e., the largest spread of rate constants (krel=1 for methyl propiolate and 33×106 for TCNE). As a consequence of low LU energies, thiones are very active dipolarophiles, and fluorene-9-thione (krel=79×106) stands at the top.