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 three derivatives carrying chloro (BCD), bromo (BBD) or nitro (BND) substituents in the p-position of both phenyl rings is reported. The results obtained by cyclic voltammetry in MeCN indicated that DPD is reduced 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 increasing concentration of the acid with only little consumption of substrate, 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 nitro derivative, BND, was reduced to a dianion stable on the voltammetric timescale in two one-electron transfers separated by 55-60 mV. During constant current coulometry in the absence of acid DPD, BCD and BBD rearranged to the corresponding 2,6-diaryl-1,4-dithiafulvenes in 0.1-0.2 F processes. Evidence is presented that these rearrangements do not involve the radical anions of the 1,4-dithiins, but, rather, are catalyzed by base generated during the electrolysis. The rearrangements were shown to take place also by addition of Bu(4)NOH to MeCN solutions 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-dithiin 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 involves ring closure by intramolecular nucleophilic attack of the thiolate at the triple bond to yield the 1,4-dithiafulvene anion followed by protonation and thus regeneration of the base catalyst. In the case of BND the thiolate ion is further cleaved to p-O2N-C6H4-C = C-S-, which upon protonation dimerizes and undergoes ring closure to the 1,4-dithiafulvene.