“Radical-Controlled” Oxidative Polymerization of 4-Phenoxyphenol by a Tyrosinase Model Complex Catalyst to Poly(1,4-phenylene oxide)

Abstract

A new concept, "radical-controlled" oxidative polymerization of phenols catalyzed by a tyrosinase model complex, has been proposed. A µ-η 2 :η 2 -peroxo dicopper(II) species formed by the reaction between the catalyst complex and dioxygen, reacted with phenol to give "controlled" phenoxy radicalcopper(I) intermediate instead of "free" phenoxy radical. The polymerization of 4-phenoxyphenol was performed by the use of the tyrosinase model complexes, (hydrotris(3,5-diphenyl-1-pyrazolyl)borate)copper (Cu(Tpzb)) chloride complex and (1,4,7-R 3-1,4,7-triazacyclononane)copper (Cu(L R ): R ) isopropyl (iPr), cyclohexyl (cHex), n-butyl (nBu)) dichloride complexes. The structures of these complexes were determined by X-ray crystallography, indicating that the order of steric repulsion of the substituents (R) in the Cu(L R ) complexes is cHex > iPr > nBu. Very little of C-C coupling dimers were afforded with the Cu(Tpzb) catalyst in toluene or THF, and with the Cu(L iPr ), Cu(L cHex ), or Cu(L nBu ) catalyst in toluene. The selectivity of para C-O coupling increased with an increase in the steric hindrance of R for the Cu(L R ) catalysts. On the other hand, the formation of C-C dimers was clearly observed in the polymerization catalyzed by a copper/diamine complex or horseradish peroxidase. The selective polymerization almost without the C-C dimer formation produced crystalline poly(1,4-phenylene oxide) having a melting point, although the polymer contained small amounts of 1,2,4-trioxybenzene units (ca. 1-5 unit %). However, the polymers obtained in the cases involving the C-C dimer formation showed no clear melting points. The reaction mechanism of catalytic cycle and oxidative polymerization is also discussed.

Coupling selectivity in the radical-controlled oxidative polymerization of 4-phenoxyphenol catalyzed by (1,4,7-triisopropyl-1,4,7-triazacyclononane)copper(II) complex

“…1). 7,8 From the difference in the structures between the two COC dimers, we thought another COO dimer of o-4Ј might be generated. To examine the formation of o-4Ј, we prepared an authentic sample.…”

“…[7][8][9][10][11][12] By this method, the synthesis of crystalline PPO with a melting point resulting from 4-phenoxyphenol (PPL) was achieved for the first time (Scheme 1). 7,8 The mechanism of the catalytic cycle in this polymerization is proposed (Scheme 2). First, Cu(tacn)X 2 complex (5) reacts with PPL to give phenoxo-copper(II) complex (2), equivalent to phenoxy radical-copper(I) complex (3).…”

“…This mechanism is supported by the fact that the coupling selectivity is evidently influenced by the bulkiness of Nsubstituents of (1,4,7-trisubstituted-1,4,7-triazacyScheme 1 Scheme 2 RADICAL-CONTROLLED OXIDATIVE POLYMERIZATION clononane)copper(II) complexes. 8 This article reports various effects on the coupling selectivity of PPL by the Cu(tacn)X 2 catalyst, including the catalyst amount, the halide ion (X) of Cu(tacn)X 2 , the reaction temperature, the reaction solvent, the added base, and the base amount. In particular, the use of less hindered amines as the base was found to change the coupling selectivity drastically.…”

Scorpionate chemistry at the 50th anniversary

“…In addition to biochemical models, Fujisawa investigated scorpionates as ligands for catalyzed polymerization reactions of phenols by copper (Higashimura et al, 2000) and olefins by manganese (Fujisawa & Nabika, 2013). Second-generation scorpionates are proving to be very useful in both research directions.…”

Synthesis of Poly(binaphthol) via Controlled Oxidative Coupling

“…However, the OCP of the 2,6-unsubstituted phenols affording poly(1,4-phenylene oxide)s was unsuccessful because of the difficult regiocontrol during the phenoxy radical coupling reaction. Recently, a highly regioselective OCP of phenol derivatives using tyrosinase model complexes, such as 10 and 11, was attained by Higashimura et al (Scheme 11.8) [21][22][23].…”