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

Abstract

AbstractSuzuki‐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.