The Suzuki-Miyaura cross-coupling reaction is highly desirable for the fabrication of π-conjugated polymers due to its wide functional group tolerance, high regioselectivity, and easy removal of low-toxicity byproducts.(1) However it remains underexplored as a synthetic tool for producing polythiophenes due to the difficulty of preparing the borylated monomers in high yields and purities, and due to side reactions during the Suzuki-Miyaura polymerization,(2) i.e. monomer protodeboronation, that leads to low molecular weights and poor yields of polymer. Modern boron protecting groups such as N-methyliminodiacetic acid (MIDA) boronate esters have proven to be effective cross-coupling partners even for borylated heterocycles highly prone to deboronation, such as thiophenes.(3) Highly pure bi-functional heteroaryl MIDA boronate esters, e.g. 5-bromo-4-hexylthien-2-yl-MIDA boronate ester 1, are now accessible in excellent yield by direct C-H electrophilic borylation.(4) Using 1 we recently demonstrated the efficient synthesis of highly regioregular (> 98%) poly(3-hexylthiophene), rr-P3HT, up to Mn = 19 kDa, and 99% yield via the Suzuki-Miyaura polymerization of this AB-type monomer.(5) The key to effective polymerization was utilizing slow hydrolysis of the MIDA protecting group in 1 to gradually generate the active transmetallating agent. In contrast, classic boron based monomers, such as thienyl pinacol boronate esters, produced shorter chain P3HT in lower yield under identical polymerization conditions. The production of other thienyl MIDA boronate esters has been extended to generate AA-type monomers for the syntheses of a range of D-A copolymers. The mechanistic aspects effecting chain termination, molecular weight, and regioregularity will be also discussed.
References (1) Suzuki, A. Angew. Chem. Int. Ed. 2011, 50, 6722-6737. (2) Sakamoto, S.; Rehahn, M.; Schlüter, A.D. In Design and synthesis of conjugated polymers; Leclerc, M.; Morin, J.-F., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2010; 45-90. (3) Burke, M.D.; Gillis, E.P. Aldrichimica Acta 2009, 42, 17-27. (4) Bagutski, V.; Del Grosso, A.; Ayuso Carrillo, J.; Cade, I.A.; Helm, M.D.; Lawson, J.R.; Singleton, P.J.; Solomon, S.A.; Marcelli, T.; Ingleson, M.J. J. Am. Chem. Soc. 2013, 135, 474-487. (5) Ayuso Carrillo, J.; Ingleson, M.J.; Turner, M.L. Macromolecules 2015, accepted.