Dedicated to Siegfried Hünig on the occasion of his 80th birthday Thiobenzophenone and diazomethane afford thiadiazoline 1 at À 788. By elimination of N 2 from 1 at À 458 (t 1/2 ca. 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 À 458 and HPLC analysis of the adducts provided relative rate constants of 26 dipolarophiles, involving 2 CC, 13 CC, 9 CS, and 2 NN bonds. In accordance with Sustmanns 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 Â 10 6 for TCNE). As a consequence of low LU energies, thiones are very active dipolarophiles, and fluorene-9-thione (k rel 79 Â 10 6 ) stands at the top.1. Introduction. ± Thiobenzophenone (3) and diazomethane furnish 4,4,5,5-tetraphenyl-1,3-dithiolane (4) as a 2 : 1 product at 08, as observed by two groups in 1930 and 1931 [2] [3]. Schönberg and his school extended the reaction to many pairs of thiones and diazoalkanes, but the mechanistic pathway remained unclear [4]. The key to the mechanism was offered by the observation that 3 and CH 2 N 2 react by a 1 : 1 stoichiometry at À 788 to produce 2,5-dihydro-2,2-diphenyl-1,3,4-thiadiazole (1) [5] [6]. The latter loses N 2 at À 458 with a half-life of 56 min in THF and affords (diphenylmethylidenesulfonio)methanide (2) in a 1,3-dipolar cycloreversion. The methanide 2 is not isolable, but is easily intercepted in situ by suitable dipolarophiles; with a second equiv. of 3 in that role, the 1,3-dithiolane 4 is formed in 95% yield (Scheme 1).The first-order rate constant k 1 for the conversion 1 3 2 increases faster with rising temperature than k 2 , which refers to the cycloaddition of CH 2 N 2 with 3. At À 788, k 2 b k 1 is found, but the ratio switches to k 2 ( k 1 at 08, i.e., the initial cycloaddition becomes rate-determining, and thiadiazoline 1 assumes the role of a transient intermediate. Thus, the Schönberg reaction consists of two 1,3-dipolar cycloadditions linked by a 1,3-dipolar cycloreversion (Scheme 1) [6].
