Investigation of Pd–Phosphine Ligands Suitable for Suzuki–Miyaura Polymerization through Intramolecular Catalyst Transfer

Abstract

The suitability of four Pd–phosphine ligands, di(tert-butyl)-4-methoxyphenylphosphine (^tBu₂PC₆H₄OMe), di(tert-butyl)-3,4,5-trimethoxyphenylphosphine (^tBu₂PC₆H₂(OMe)₃), di(1-adamantyl)-4-methoxyphenylphosphine (Ad₂PC₆H₄OMe), and di(1-adamantyl)-4-dimethylaminophenylphosphine (Ad₂PC₆H₄NMe₂), for Suzuki–Miyaura nonstoichiometric polycondensation and catalyst-transfer condensation polymerization (CTCP) was investigated. These four phosphine ligands were synthesized by the reaction of the corresponding phenyl lithium with di(tert-butyl)chlorophosphine or di(1-adamantyl)chlorophosphine. To avoid oxidation, we quickly converted the as-obtained phosphines into the corresponding tetrafluoroboric acid salts and purified them by recrystallization. We first conducted a Suzuki–Miyaura coupling reaction of 2.0 equiv of phenylene dibromide 1 with 1.0 equiv of pinacol phenylboronate in the presence of Pd(OAc)₂/synthesized ligands and CsF/18-crown-6 as a base at room temperature. The use of Ad₂PC₆H₄OMe and Ad₂PC₆H₄NMe₂ resulted in the exclusive formation of the disubstituted products. Accordingly, nonstoichiometric Suzuki–Miyaura polycondensation of 1.3 equiv of 1 with 1.0 equiv of phenylenediboronate was conducted in the presence of Pd(OAc)₂/Ad₂PC₆H₄OMe or Ad₂PC₆H₄NMe₂. High-molecular-weight poly(p-phenylene) was obtained with Ad₂PC₆H₄OMe, whereas the molecular weight was low when Ad₂PC₆H₄NMe₂ was used. We next investigated CTCP of fluorene AB monomer 5 with a Pd–Ad₂PC₆H₄NMe₂ initiator. The polymerization of 5 yielded a high-molecular-weight polymer with a narrow dispersity. The matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra showed peaks due to the polymer containing the initiator unit, indicating that the polymerization proceeded through the CTCP mechanism.