Metallation reactions XXII: Regioselective metallation of (trifluoromethyl)(alkylthio) benzenes

Abstract

The metallation reactions of (trifluoromethyl)(alkylthio)benzenes with organolithium reagents and with the butyllithium/potassium tert-butoxide superbasic mixture are here described. The results, according to the theoretical calculations of energy minima, show the monometallation regiochemistry is directed by the sulphur atom. On the other side, the bimetallation, that can be performed only on methylthio derivatives, depends on the organometallic reagent employed. Using butyllithium the stronger coordinative power of sulphur prevails and products metallated in ortho and alpha positions to this atom are mainly formed. With the more basic sec-butyllithium and with the superbase, because of mutual competition between the thioalkyl and trifluoromethyl groups, mixtures of products, coming from metallation in the ortho position to the trifluoromethyl group and in the alpha position of the thiomethylic group, are obtained. In addition, products coming from substitution ortho,alpha to the thiomethyl group are also formed. All mono- and bimetallated intermediates showed to be good synthons for the synthesis of fluoro- and (trifluoromethyl)-substituted benzothiophenes and (trifluoromethyl)benzenes substituted on the thioalkylic chain and/or on the ring.