2- Methylene-1,3-diselenole

The most convenient route to TSF was reported by Cava in 1987 [64]. The procedure utilizes a novel iodine-promoted coupling of 2-methylene-1,3-diselenole (28). The starting 1,2,3-selenadiazole (29) is easily obtained from the oxidation of the semicarbazone of acetaldehyde with selium dioxide. 

Methylene-1,3-diselenole (108) is obtained in high yield from acetaldehyde semicarbazone using the selenadiazole route (Scheme 25, see also Scheme 49, Section 3.14.10.1.2). Theexocyclic methylene in (108) is quite reactive protonation gives a highly reactive 2-methyl-1,3-diselenolylium ion resulting in polymerization <88JOC3529>. However, the methylene carbon has nucleophilic character due to the contribution from the dipolar mesomeric structure (Scheme 24). 

An interesting reaction is the following in which the parent tetraselenafulvalene (7) (TSF) is obtained from 2-methylene-1,3-diselenole (108) in modest yield. The mechanism probably involves a cycloaddition step (Equation (9)) <87TL5635>. 

Dibenzotetraselenafulvalene can be prepared by this method starting from 1,2,3-benzo-selenadiazole and subsequent coupling of the resulting benzo-l,3-diselenole-2-selone with triphenyl phosphine in 66% yield <83CC295>. Likewise, the parent 2-methylene-1,3-diselenole (27) can be obtained from acetaldehyde semicarbazone after ring closure and subsequent treatment of the 1,2,3-selenadiazole (185) with potassium t-butoxide (selenadiazole route, see Scheme 49) <87TL5635>. 

Vilsmeier-Haack diformylation of 2-methylene-l,3-diselenole 140 gave 2-diformylmethylene-l,3-diselenole 141 (Equation 20) that served as a precursor for the dendralene-type vinylogs of tetrathiafulvalene 1, 2, 39, 142, and 143 by condensation with suitable dithiolium phosphonium salts or phosphonates in the presence of base <2001JOC7757>. 

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