Reliable Suzuki Chemistry for Functionalised Polythiophene Synthesis
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1003-O01-06
Reliable Suzuki Chemistry for Functionalised Polythiophene Synthesis Simon J. Higgins1, Iain Liversedge1, Samer Badriya1, Martin Heeney2, Mark Giles2, and Iain McCulloch2 1 Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, United Kingdom 2 Chilworth Technical Centre, Merck Chemicals, Southampton, SO16 7QD, United Kingdom ABSTRACT Until recently, the synthesis of polythiophenes using Suzuki chemistry has proven difficult because of the ready protodeborylation of thiophene boronates. However, we now report that the new generation of bulky, electronñrich Pd(0)ñphosphane catalysts are effective and reliable for the preparation of regioregular polyalkylthiophenes using Suzuki coupling. Moreover, the monomers can be prepared in high yield by Irñcatalysed borylation, without the need for strong organolithium bases, making this potentially a highly functional group-tolerant approach to polyalkylthiophene derivatives. Perfluoroalkylthiophenes also undergo this reaction. INTRODUCTION Regioregular (head-to-tail) polyñ3ñalkylthiophenes (RP3ATs), first synthesised in the early 1990ís [1], show high field effect mobility as solvent-cast films in organic thin film transistor (OTFT) devices (ca. 0.05 cm2 V-1 s-1) [2]. This is attributed to self-assembly of polymer chains during solvent casting, giving a lamellar structure with organised crystalline domains, and unusually favourable π- π stacking interactions between polymer chains that facilitates inter-chain charge carrier hopping [3]. To date, the best route to RP3ATs is [NiCl2(dppp)]-mediated coupling of the Grignard reagent generated by metallation of 2,5dibromo-3-alkylthiophene with MeMgBr [4]. Polymers with ≥ 98% regioregularity and MW 25,000, MN 20,000 can be made routinely. Appropriately functionalized RP3ATs are of potential interest in applications such as sensing [5]. A problem with R3PATs is that their electrical properties rapidly degrade on air exposure, and it is also of interest to ëtuneí the HOMO energy by introducing electronñ withdrawing groups adjacent to the thiophene ring. Suzuki coupling would be an ideal route to functionalized RP3ATís, since boronates are non-toxic and highly functional groupñtolerant. However, thiophene boronates are notoriously prone to protodeborylation [6], and this has limited the use of Suzuki coupling to generate polythiophenes. A study of Suzuki routes to alternating arylñthiophene polymers, employing copolymerization of aryl dihalides and thiophene bis(boronate)s, showed that deborylation limited the molecular weights achievable, and termination of polymerisation by aryl group transfer from the PPh3 ligands also occurred [6]. The Suzuki polymerization of 1 (see Scheme 1) using a phosphine-free Pd catalyst produced quite high molar mass polymer, but with low regioregularity [7]. In our hands, the yield also proved capricious. More recently conventional Pdñphosphine catalysts have been used to polymerise the more stable boronate 2, but these generally gave either low molar mass, low regioregularity (≤
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