THE ELECTROCHEMICAL REDUCTION AND A NOVEL BASE-CATALYZED REARRANGEMENT OF 2,5-DIARYL-1,4-DITHIINS

The electrochemical reduction of 2,5-diphenyl-1,4-dithiin (DPD) and th ree derivatives carrying chloro (BCD), bromo (BBD) or nitro (BND) subs tituents in the p-position of both phenyl rings is reported. The resul ts obtained by cyclic voltammetry in MeCN indicated that DPD is reduce d in a two-electron process to a dihydro derivative. However, analysis by constant current coulometry in the presence of acetic acid showed that the apparent number of electrons transferred increased with incre asing concentration of the acid with only little consumption of substr ate, most likely due to catalytic reduction of the acid (protons). The two halogen derivatives, BCD and BBD, initially underwent cleavage of the C-X bonds (X = Cl or Br) resulting in the formation of DPD. The n itro derivative, BND, was reduced to a dianion stable on the voltammet ric timescale in two one-electron transfers separated by 55-60 mV. Dur ing constant current coulometry in the absence of acid DPD, BCD and BB D rearranged to the corresponding 2,6-diaryl-1,4-dithiafulvenes in 0.1 -0.2 F processes. Evidence is presented that these rearrangements do n ot involve the radical anions of the 1,4-dithiins, but, rather, are ca talyzed by base generated during the electrolysis. The rearrangements were shown to take place also by addition of Bu(4)NOH to MeCN solution s of the dithiins with yields ranging from 38% (DPD) to 93% (BBD). For DPD, BCD and BBD a mechanism including deprotonation of the 1,4-dithi in followed by ring opening to yield the thiolate intermediate, Ar-C = C-S-C(Ar) = CH-S-, as the first part is proposed. The second part inv olves ring closure by intramolecular nucleophilic attack of the thiola te at the triple bond to yield the 1,4-dithiafulvene anion followed by protonation and thus regeneration of the base catalyst. In the case o f BND the thiolate ion is further cleaved to p-O2N-C6H4-C = C-S-, whic h upon protonation dimerizes and undergoes ring closure to the 1,4-dit hiafulvene.