`ThiobenzophenoneS-Methylide' (=(Diphenylmethylidenesulfonio)methanide), and C,C Multiple Bonds: Cycloadditions and Dipolarophilic Reactivities

Abstract

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].

Studies on the Reactions of Thiocarbonyl S‐Methanides with Hetaryl Thioketones

“…-Thiocarbonyl ylides belong to the class of the so-called Scentered 1,3-dipoles, and they are widely applied for the preparation of S-heterocycles with diverse sizes of the formed ring [1]. The reactive thiocarbonyl S-methanides, generated via thermal N 2 -elimination from 2,5-dihydro-1,3,4-thiadiazoles, easily react with diverse dipolarophiles, and special attention is focused on their reactions with 'superdipolarophilic' thioketones [2]. These reactions leading to 1,3-dithiolane derivatives (Schönberg reaction [3]) are of interest not only as a method for the preparation of these products, but also for studies on organic reaction mechanisms.…”

“…These results confirm that hetaryl thioketones belong to the group of 'superdipolarophiles' in reactions with thiocarbonyl S-methanides [2]. According to…”

Synthesis of Ferrocenyl Thioketones and their Reactions with Diphenyldiazomethane

“…1 In the last three decades, aromatic thioketones and non-enolizable aliphatic thioketones have extensively been explored as versatile building blocks for the synthesis of more complex sulfur-containing compounds. Cycloaddition reactions leading to five-and six-membered S-heterocycles are of special importance, as thioketones are recognized as superdipolarophilic 2 and superdienophilic agents. 3 Moreover, thioketones are unique substrates for the preparation of differently substituted alkenes via the so-called 'two-fold extrusion reaction' with diazo compounds.…”

Pyrazole, pyrazolo[1,2-c]-1,3,4-thiadiazole and thiadiazepine derivatives from thiosemicarbazides

“…In addition, 2 is a useful reagent for synthesis of cyanocarbon acids, spiro compounds and novel heterocycles. [15][16][17][18][19][20][21] Recently, it has been reported that 4-phenyl-and 4-benzylthiosemicarbazides 1a,b reacted with 2 in ethyl acetate with charge-transfer complex (CT) formation, ultimately giving a mixture of thiadiazepine and thiadiazole derivatives 3-5 ( Figure 1). 22 …”