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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-bromo3-hexylthiophene (3) is a potentially atom-economic pathway. Deprotonation of 3 with lithium amide and following metal exchange with zinc1d or magnesium2a2f,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 compounds5,6 can be applied to the synthesis of HT-P3HT (1) and herein disclose that the employment of Knochel-Hauser base7 and a nickel-catalyst is a practical dehydrobrominative polycondensation method toward highly regioregular poly(3substituted 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 Mn 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 sp2 CH bonds,7 induced polymerization highly efficiently. When the reaction was carried out with 0.5mol% [NiCl2(dppe)] as catalyst, the corresponding HTP3HT (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 exhibited extremely high HT selectivity. DPPP as a ligand of the nickel catalyst also was as effective as DPPE, while DPPB or tricyclohexylphosphine (PCy3) was found less effective. The average molecular weight of the obtained polymer was found to be controlled by the amount of the employed nickel catalyst. The number of the polymer chain showed relatively good correspondence to the amount of nickel catalyst and the Mn value was found to increase by lowering the catalyst loading. The use of arylnickel complex as a catalyst also resulted in smooth polymerization with excellent HT selectivity and it was confirmed by 1HNMR that the phenyl end group was at the terminal of the polymer chain.2e These results are summarized in Table 1. It should be pointed out that the reaction proceeded at room temperature to afford the corresponding polymer in a quantitative yield. Comparing the preceding polymer syntheses with CH functionalization conducted at elevated temperature4 or at extremely low temperature for proton abstraction with lithium amide followed by lithiummagnesium exchange,3 the present reaction conditions proceed at room temperature throughout the

Nickel-catalyzed Dehydrobrominative Polycondensation for the Practical Preparation of Regioregular Poly(3-substituted thiophene)s