Electrochemical reduction benzo thiophene

Reductions in aqueous THF-TBABF4 of benzo[b]thiophene [55], benzofurans, and 1-methylindole [56] have been shown to yield the 2,3-dihydro compounds this is probably a direct electrochemical reduction, whereas the further reduction involving ring opening of the hetero ring seems to be mediated by TBA(Hg). 

Benzo[fr]thiophene 1,1-dioxide and its derivatives are reduced to the corresponding 2,3-dihydro derivatives by catalytic hydrogenation in good yields (Scheme 94). In this reduction, the sulfonyl moiety remains unchanged [13,189, 238, 239]. However, reduction of benzo[ ]thiophene 1,1-dioxide with LiAlH4 affords deoxygenated 2,3-dihydrobenzo[ ]thiophene in 79% yield [233]. Benzo[fr]thiophene 1,1-dioxide was also reduced to the 2,3-dihydro derivative by an electrochemical method (Scheme 95) [240]. 

This section will discuss both electrochemical reductions and electrochemical oxidations of the thiophene and benzo[Z)]thiophene systems. 

The electrolytic reduction of benzo[ii]thiophene has been described by Pacut and Kariv-Miller (equation 121). Because the two products can be easily separated, this electrochemical Birch reduction is a very good preparation of 2,3-dihydrobenzo[i]thiophene. 

---