Synthesis of cyclic aromatic polymer containing thiophene or pyridine by means of unstoichiometric Suzuki-Miyaura cyclic polycondensation: Effect of the position of bromine of heteroarylenes on cyclic polycondensation

Suzuki-Miyaura cyclic polycondensation of 1.3 equivalents of thiophene dibromide or pyridine dibromide with 1.0 equivalent of phenylenediboronic acid ester was investigated in the presence of t-Bu3PPd G2 precatalyst, which generates t-Bu3PPd(0), and CsF/18-crown-6 as a base. Polycondensation of 2,5-dibromothiophene and 5,5 '-dibromo-2,2 '-bithiophene with pinacol meta-phenylenediboronate (2) yielded corresponding cyclic polymers. On the other hand, polycondensation of 3,4-dibromothiophene with para-phenylenediboronate (5) gave linear polymer with bromothiophene at both ends via conventional unstoichiometric polycondensation involving excess dibromo monomer, implying that intramolecular catalyst transfer did not proceed effectively on 3,4-dibromothiophene. A model reaction of 3,4-dibromothiophene with phenylboronic acid indeed gave monosubstituted thiophene preferentially via intermolecular catalyst transfer. In the polycondensation of excess pyridine dibromide with 5, the use of 2,6-dibromopyridine gave linear polymer, whereas the use of 3,5-dibromopyridine yielded cyclic polymer. Thus, the position of bromine in heteroarylenes determines whether cyclic polymer or linear polymer is formed, in contrast to the case of unstoichiometric Suzuki-Miyaura cyclic polycondensation with dibromophenylenes.