1,3-dipolar cycloadditions: Part 121 'Thiobenzophenone S-methylide' (= (diphenylmethylidenesulfonio)methanide), and C,C multiple bonds: Cycloadditions and dipolarophilic reactivities

Thiobenzophenone and diazomethane afford thiadiazoline 1 at -78 degrees. By elimination of N-2 from 1 at -45 degrees (1,, ca. Ih), (diphenylmethyliden esuifonio)methanide (2), which cannot be isolated but is interceptible by d ipolarophiles, is set free. The nucleophilic 1,3-dipole 2 undergoes cycload ditions with electrophilic C,C multiple bonds; the structures of 16 cycload ducts were elucidated. One-step and two-step cycloaddition pathways are dis cussed in the light of the steric course observed for (E)/(Z)-isomeric ethy lene derivatives. Competition experiments with pairs of dipolarophiles at - 45 degrees and HPLC analysis of the adducts provided relative rate constant s of 26 dipolarophiles, involving 2 C dropC, 13 C double bondC, 9 C double bondS, and 2 N double bondN 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 (k(rel)=1 for methyl propiolate and 33 x 10(6) for TCNE). As a consequence of low LU ene rgies, thiones are very active dipolarophiles, and fluorene-9-thione (k(rel ) = 79 x 10(6)) stands at the top.