Unstoichiometric Suzuki-Miyaura Polycondensation through Intramolecular Pd-Catalyst Transfer for the Synthesis of Linear and Cyclic Aromatic Polymers

Abstract

This account deals with the unstoichiometric Suzuki-Miyaura polycondensation of excess dibromoarylene 5 with arylenediboronic acid (ester) 6 in the presence of t-Bu₃PPd precatalyst 7 and CsF/18-crown-6 as a base. Since t-Bu₃PPd(0), generated from 7 with the assistance of base, has a propensity for intramolecular catalyst transfer on the π-electron face of aromatics, successive reaction of 5 with 2 equivalents of 6 takes place through intramolecular catalyst transfer of t-Bu₃PPd(0) on 5, affording high-molecular-weight polymer, even though an excess of 5 is used. High-molecular-weight poly(tetraalkoxy-substituted stilbene), polyphenylene, polyfluorene, poly-(thiophene-alt-phenylene), poly(fluorene-alt-benzthiadiazole), poly(phenylene-alt-benzothiadiazle), poly(cyclopentadithiophene-alt-benzothiadiazole), and polyphenylene containing C≡C, N=N, CH₂, CO, N-Bu, O, SO₂ in the backbone have been obtained under this unstoichiometric condition. The obtained polymers have a boronic acid (ester) moiety at both ends, which can be converted to other functional groups by means of Suzuki-Miyaura coupling reaction with another reagent, oxidation, bromination, and so on. In the synthesis of poly(tetraalkoxy-substituted stilbene), the abnormal unstoichiometric polycondensation, affording high-molecular-weight polymer with a boronic acid ester moiety at both ends, can be switched to normal unstoichiometric polycondensation, affording low-molecular-weight polymer with bromine at both ends, simply by the addition of unsubstituted stilbene, which promotes intermolecular catalyst transfer due to catalyst trapping. When at least one of the two monomers has a kinked structure, such as m-dibromophenylene or m-phenylenediboronic acid (ester), many kinds of cyclic polymers can be selectively obtained even under unstoichiometric conditions.