Reactivity of several deactivated 3-aminobenzo[b]thiophenes in the Buchwald-Hartwig C-N coupling. Scope and limitations

Di(hetero)arylamines were prepared in moderate to high yields by Buchwald-Hartwig C-N coupling of bromobenzenes bearing electron-withdrawing groups and of a bromobiphenyl with several methyl 3-aminobenzo[b]thiophene-2-carboxylates, using the coupling conditions for heteroaromatic amines [Pd(OAc)2, Xantphos, Cs2CO3 in dioxane, 120 °C]. The use of these aminobenzo[b]thiophenes as coupling components avoids the step of changing the amino group into a bromine atom, like we have done before to perform C-N couplings using the corresponding 3-bromobenzo[b]thiophenes. Nevertheless, the couplings using the methyl 3-aminobenzo[b]thiophene-2-carboxylates were only successful with bromobenzenes bearing electron-withdrawing groups and a bromobiphenyl or with electron deficient rings such us as bromopyridines. Using the latter compounds, different substituted 6H-benzothieno[3,2-d]pyrido[1,2-a]pyrimid-6-ones were obtained by C-N coupling followed by an intramolecular cyclization. These tetracyclic compounds may have interesting biological activity like it was already demonstrated by us for the non substituted derivative.