Polymerization of 2-chloro-3-substituted thiophenes proceeded with a stoichiometric amount of magnesium amide, TMPMgCl·LiCl, or a combination of a Grignard reagent and a catalytic amount of secondary amine in the presence of a nickel catalyst. Although the nickel-catalyzed polymerization with NiCl(2)dppe, which exhibited high catalytic activity in the reaction of bromothiophenes, was less effective, use of a nickel catalyst bearing N-heterocyclic carbene as a ligand was found to induce polymerization with controlled molecular weight and molecular weight distribution.

Revisiting Murahashi coupling, we found that it effectively allows polymerization of lithiated (hetero)arenes by nickel(II)-catalyzed polycondensation. Deprotonative polymerization of 2-chloro-3-substituted thiophene with n-butyllithium gave head-to-tail-type poly(3-substituted thiophene). Poly(1,4-arylene)s were obtained by the reaction of the corresponding dibromides through lithium-bromine exchange. A lithiated thiophene derivative obtained via deprotonative halogen dance also underwent polymerization to afford a bromo-substituted polythiophene.

Iterative growth of thiophene oligomers by single-step extensions has been realized by regioselective metalation of 3-substituted thiophenes with the Knochel-Hauser base (TMPMgCl·LiCl) and coupling with bromothiophene using a nickel catalyst. Treatment of 3-hexylthiophene with TMPMgCl·LiCl induces metalation at the 5-position selectively. Subsequent addition of 2-bromo-3-hexylthiophene and a nickel catalyst leads to the corresponding bithiophene. The obtained bithiophene is converted to the terthiophene and then to the quaterthiophene by repeating the similar protocol. A concise synthesis of MK-1 and MK-2, which are organic dye molecules bearing an oligothiophene moiety that are used in photovoltaic cells, has been achieved.

Palladium-catalyzed C-H arylation of electron-enriched heteroarenes with aryl bromides and aryl chlorides proceeds in the presence of LiO-t-Bu as a base. The reaction allows one-pot synthesis of differently substituted 2,5-diarylthiazole with the same catalyst system by switching the solvent and the amount of base.

Poly(3-hexylthiophene) was synthesized by the polycondensation of 2-bromo-3-hexylthiophene with a nickel catalyst and (2,2,6,6-tetramethylpiperidin-1-yl)magnesium. The polymerization proceeded at room temperature in a highly regioregular manner.Oligothiophenes and polythiophenes attract considerable attention in materials science. Regioregular polythiophene, which involves head-to-tail (HT) repeating unit such as poly(3-hexylthiophene) (HT-P3HT) (1) (Chart 1) is of particular interest since the compound shows remarkable physical properties materials such as organic TFTs, thin-film organic solar cells, and conductive polymers.1 Debrominative polycondensation of 2,5-dibromo-3-hexylthiophene (2a) with a Grignard reagent in the presence of a transition-metal catalyst is a method of choice for the preparation of 1.1d,2 A regiochemical error in the halogenmetal exchange causes incomplete conversion to the polymer 1. Although use of the 5-iodinated derivative 2b is a solution to such problem, 3 preparation of 2b requires multistep procedures as well as inferior atom efficiency. On the other hand, the dehydrobrominative method to afford 1 with 2-bromo-3-hexylthiophene (3) is a potentially atom-economic pathway. Deprotonation of 3 with lithium amide and following metal exchange with zinc 1d or magnesium 2a2f,3 also gives 1 although extremely low temperature is necessary. A palladium-catalyzed direct polycondensation in the presence of an alkali metal carbonate has recently been shown to lead to polymer, 4 however, high temperature is necessary to afford the polythiophene in a reasonable yield with high degree of polymerization. It is thus highly intriguing to develop milder dehydrobrominative polymerization (around room temperature), which would be a goal for the efficient preparation of regioregular polythiophenes. We envisaged that our recent efforts on the development of transition-metal-catalyzed CH functionalization of heteroaromatic compounds 5,6 can be applied to the synthesis of HT-P3HT (1) and herein disclose that the employment of Knochel-Hauser base 7 and a nickel-catalyst is a practical dehydrobrominative polycondensation method toward highly regioregular poly(3-substituted thiophene)s at room temperature with high atom efficiency.We first examined palladium-catalyzed polymerizaton of 3a in the presence of lithium t-butoxide. 8 The polymerization occurred at 50°C to afford the corresponding polymer 1 with M n of 7600. In contrast to the above undesired results, it was found that the use of (2,2,6,6-tetramethylpiperidin-1-yl)magnesium chloride¢LiCl (4), which was developed by Knochel for the proton abstraction of various sp 2 CH bonds, 7 induced polymerization highly efficiently. When the reaction was carried out with 0.5 mol % [NiCl 2 (dppe)] as catalyst, the corresponding HT-P3HT (1) was obtained in quantitative yield in contrast to debrominative polymerization with a Grignard reagent which does not reach complete conversion due to regiochemical error in the halogenmetal exchange. The obtained polymer 1 exh...