2-Isocyanato-2-alkenoates-Synthesis and Reactions

reaction of 2-dimethylphosphono-1,3-benzodithiole 3a with conjugated 1,4-diketones for the preparation of 2,2'-bis(I,3dithiole) donors.[41 We have now used the Wittig-Horner reaction of 3 b, which was synthesized by a similar method as that for 3a,15] to prepare 1. In this case, a cyclopentadiene adduct ofp-benzoquinone 4 was used as a diketone and after introduction of the 1,3-dithiole rings the cyclopentadiene was removed by a retro-Diels-Alder reaction, since the direct reaction of p-benzoquinone with 3 b gave a complex mixture of products without formation of 1. Thus, a solution of 3b in dry THF was treated with 1.1 equivalents of n-butyllithium at -78 "C under nitrogen, and after stirring for 5 min, 0.43 equivalent of 4 was added. The solution was then stirred at -78 "C for a further 10 min and allowed to warm to room temperature, whereupon the bis(l.3-dithiole) derivative 5 was obtained in 27% yield. Thermolysis of 5 at 200 "C under reduced pressure furnished, via a retro-Diels-Alder reaction, dark red crystals of 1 in 54% yield.161 The benzo derivatives 6a, b were prepared analogously by reaction of the Diels-Alder adducts of 1,4naphthoquinone and the corresponding methyl compound, respectively with cyclopentadiene. The dibenzo derivative 7 could be synthesized in 74% yield by direct Wittig-Horner reaction of 3b with 9,lO-anthraquinone. The dibenzotetrathiafulvalene (TTF) analogue 2 a could be prepared by a retro-Diels-Alder reaction of 8, which was obtained in 73% yield by reaction of 3a and 4. 9a, b and 10 were prepared similarly. The decomposition temperatures of the new donors are listed in Table 1 . The TTF analogues 1 and 6a, b are air sensitive, whereas the dibenzo analogues 2a, 7,9a, b, and 10 are stable towards air. The oxidation potentials of the donors (Table 1) were measured by cyclic voltammetry. The values for the donors with quinoid structures are lower than those of TTF mea-