Thiobenzophenone .S -methylide

Over the past two decades, important contributions to the chemistry of thiocarbonyl ylides were made by Huisgen et al. (27). By carrying out the reaction of thiobenzophenone with diazomethane at low temperature, formation of 2,5-dihydro-l,3,4-thiadiazole (15) with subsequent elimination of N2 was established as the route to the reactive thiobenzophenone (S)-methylide (16) (17,28). In the absence of intercepting reagents, 16 undergoes electrocyclization to give 17 or head-to-head dimerization to yield 1,4-dithiane 18 (Scheme 5.3). 

Numerous examples involving the preparation of tetrahydrothiophenes via [3 + 2] cycloaddition of thiocarbonyl ylides with electron-poor alkenes have been reported. Thiobenzophenone (S)-methylide (16), generated from 2,5-dihydro-1,3,4-thiadiazole (15) and analogous compounds, react with maleic anhydride, N-substituted maleic imide, maleates, fumarates, and fumaronitrile at —45°C (28,91,93,98,128,129). Similar reactions with adamantanethione (S)-methylide (52) and 2,2,4,4-tetramethyl-3-thioxocyclobutanone (S)-methylide (69) occur at ca. +45°C and, generally, the products of type 70 were obtained in high yield (36,94,97,130) (Scheme 5.25). Reaction with ( )- and (Z)-configured dipolaro-philes stereospecifically afford trans and cis configured adducts. 

Like many other 1,3-dipoles (e.g., nitrile ylides, imines, and oxides) (7), thiocarbonyl ylides undergo head-to-head dimerization to give sterically crowded 1,4-dithianes. The first reported example involves the formation of 2,2,3,3-tetraphenyl-l,4-dithiane (18) from thiobenzophenone (S)-methylide (16) (17,28) (cf. Scheme 5.3). Other (S)-methylides are known to form analogous 1,4-dithianes (e.g., thiofluorenone (S)-methylide yields 172) (17). The (S)-methylides of 4,4-dimethyl-2-phenyl-l,3-thiazole-5(47/)-thione (105) and methyl dithiobenzoate (60,104) dimerize to give compounds 173 and 174, respectively. 

A quantitative estimate of the high reactivity of thiones is given by the fact that the cycloadditions of thiobenzophenone S -methylide [formed in situ by extrusion of N2 from intermediate 116 (R = Ph)] to thiofluorenone and thiobenzophenone proceed 7.8 x 107 and 1.15 x 106 times faster than that to methyl propiolate. Even thiofluorenone is 2.4 times more reactive than tetracyanoethylene (TCNE), the superb C=C dipolarophile449. 

A-Sulfinylamines (R—N=S=0) are known to function as reactive dienophiles and dipolarophiles, and some examples of [3 + 2] cycloaddition with thiocarbonyl ylides have been reported (176). For example, the reaction of thiobenzophenone (5)-methylide (16) with both A-phenyl and N-tosylsulfinylamines occurs regiose-lectively to give 1,3,4-dithiazolidine 3-oxides (135). In the case of thiocarbonyl ylide 69, reaction with N-phenyl sulfinylamine selectively afforded the analogous product 136 (R = Ph). However, the corresponding reaction with Al-tosyl sulfinylamine resulted in a mixture of the N,S-adduct (136) (R =Tos) and the 0,S-adduct 137. Formation of a mixture of products is compatible with a stepwise reaction via a zwitterionic intermediate. 

The adamantane 5-methylide 10 reacts with C=S dipolarophiles to give the [3+2] cycloadducts. For example, from 10 and thiobenzophenone, the isomeric [3+2] cycloadducts 11 and 12 are obtained in 50 % and 42 % yields, respectively . 

The relative rate constants for cycloaddition of thiocarbonyl -methylides with dipolarophiles show that C=S dipolarophiles are very efficient. For example, thiofluorenone exceeds tetracyanoethylene and thiobenzophenone reacts 3000 times faster than dimethyl acetylene-dicarboxylate. ... 

---