1.2- Dithiete cycloadditions

Tile a-dithione 158 (dithiete 159) underwent a [4 + 2] cycloaddition with fra s-(l,2-dimethoxy)ethylene to give 197 in addition to reactions with HCN and CH2N2, which yielded 198 and 199, respectively (74JA3502). 

The a-dithionc 91, generated by photolysis of 92, is transformed into the dithiin 93 (41%) in the absence of a trapping agent. The conversion was proposed to proceed by a [4 + 2] cycloaddition of 91 with its dithiete tautomer 94 leading to the dithietane 95, which was followed by loss of S2 (85JOC1550). Such [4 + 2] dimerizations are often encountered in the chemistry of 1,2-dithietes as discussed later. 

Cycloadditions in which 1,2-dithietes acted formally as dienes are among the most typical reactions of 1,2-dithietes. The dithiete 144 is highly reactive and capable of reactions even with simple alkenes and alkynes (60JA1515 61JA3434,61JA3438). Thus, 144 reacted with acetylene to form 191 and 192 with the initial formation of 193, and with tetramethylethylene to give 194. Other [4 + 2] cycloadditions of 144 involved those with ethylene, cyclohexene, trans-stilbene, ethyl vinyl ether, butyl vinyl sulfide, 3-hexyne, and DMAD. 

Dithiete 753, when heated, undergoes cycloaddition through its tautomeric dithione 754 to produce 1,4-dithiin 755 which gives the thiophene 756 by extrusion of sulfur. Heating 753 with alkynes affords the thiophenes 757 (Scheme 118) <1999JOC8489>. 2,3,4,5-Tetra(2-chlorotetrafluoroethyl)thiophene 758 can be prepared in a similar way. Irradiation of 758 produces the Dewar thiophene 759 <2000JFC(102)323>. 

Typical for disubstituted 1,2-dithietes is their valence isomerization, which results in the formation of 1,2-dithiones. The equilibrium favors the 1,2-dithiete with electron-withdrawing substituents such as CF3. The reaction with 2,3-dimethylbut-2-ene to give a hexasubstituted 2,3-dihydro-l,4-dithiine proceeds, however, as a (4 + 2) cycloaddition via the 1,2-dithione. 

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