The first regioselective metalation and functionalization of unprotected 4-halobenzoic acids

By treatment with s-BuLi, s-BuLi/TMEDA, or t-BuLi at similar to-78 degreesC, 4-fluoro- and 4-chlorobenzoic acids (1a,b) are metalated preferentially in the position adjacent to the carboxylate. A complete reversal in regioselectivity is observed for 1a when treated with LTMP; a sequential process involving a rapid intraaggregate lithiation through a quasi dianion complex QUADAC is postulated to explain the unusual reactivity of Me2S2 and I-2.