Thiobenzophenone compounds, thiocarbonyl ylide

Numerous examples involving the preparation of tetrahydrothiophenes via [3 + 2] cycloaddition of thiocarbonyl ylides with electron-poor alkenes have been reported. Thiobenzophenone (5)-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 (5)-methylide (52) and 2,2,4,4-tetramethyl-3-thioxocyclobutanone (5)-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 (5)-methylide (16) (17,28) (cf. Scheme 5.3). Other (5)-methylides are known to form analogous 1,4-dithianes (e.g., thiofluorenone (5)-methylide yields 172) (17). The (5)-methylides of 4,4-dimethyl-2-phenyl-l,3-thiazole-5(4//)-thione (105) and methyl dithiobenzoate (60,104) dimerize to give compounds 173 and 174, respectively. 

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