Reaction of 1,2-dithiete with alkenes and alkynes: Experimental and theoretical study

Reactions of 3,4-bis(methoxycarbonyl)-1,2-dithiete (1) with various alkenes or alkynes formed 2,3-dihydro-1,4-dithiins or thiophenes, respectively. The reactions with alkenes were stereospecific, which indicates the concerted reaction between 1,2-bis(methoxycarbonyl)ethane-1,2-dithione (13), the valence isomer of the 1,2-dithiete, and the dienophiles. Theoretical study confirmed the reactions of 13 with alkenes and alkynes are of reverse electron demand hetero Diels-Alder type. The MO calculations showed the 1,2-dithiete 1 was 5.8 kcal mol(-1) more stable than the corresponding ethane-1,2-dithione 13, and the tautomerization energy between the 1,2-dithiete and the ethane-1,2-dithione was also calculated to be 28.5 kcal mol(-1)from the 1,2-dithiete, which suggests the tautomerization from 1,2-dithiete 1 to ethane-1,2-dithione 13 is possible at least at high temperature. Reaction of 3,4,7,8-tetrakis(methoxycarbonyl)-1,2,5, (2) or (Z,Z,Z,Z)-3,4,7,8,1 1,12,15,16-octakis-(methoxycarbonyl)-1,2,5,6,9,10,13,14-octathiacyclohexadeca-3,7,11,12-tetraene (3) with ethyl vinyl ether also formed the 2,3-dihydro-1,4-dithiin derivative, which is the same compound obtained by the reaction of 1,2-dithiete 1 with the ether. 1,2,5,6-Tetrathiocin 2 and 16-membered cyclic compound 3 also reacted with diphenylacetylene to give the thiophene derivative.