A General Protocol for the Polycondensation of Thienyl N-Methyliminodiacetic Acid Boronate Esters To Form High Molecular Weight Copolymers.

Thienyl di-N-methyliminodiacetic acid (MIDA) boronate esters are readily synthesized by electrophilic C-H borylation producing bench stable crystalline solids in good yield and excellent purity. Optimal conditions for the slow release of the boronic acid using KOH as the base in biphasic THF/water mixtures enables the thienyl MIDA boronate esters to be extremely effective homo-bifunctionalized (AA-type) monomers in Suzuki-Miyaura copolymerizations with dibromo-heteroarenes (BB-type monomers). A single polymerization protocol is applicable for the formation of five alternating thienyl copolymers that are (or are close analogues of) state of the art materials used in organic electronics. The five polymers were produced in excellent yields and with high molecular weights comparable to those produced using Stille copolymerization protocols. Therefore, thienyl di-MIDA boronate esters represent bench stable and low toxicity alternatives to highly toxic di-trimethylstannyl AA-type monomers that are currently ubiquitous in the synthesis of these important alternating copolymers.

Mechanistic studies into amine-mediated electrophilic arene borylation and its application in MIDA boronate synthesis.

Direct electrophilic borylation using Y(2)BCl (Y(2) = Cl(2) or o-catecholato) with equimolar AlCl(3) and a tertiary amine has been applied to a wide range of arenes and heteroarenes. In situ functionalization of the ArBCl(2) products is possible with TMS(2)MIDA, to afford bench-stable and easily isolable MIDA-boronates in moderate to good yields. According to a combined experimental and computational study, the borylation of activated arenes at 20 °C proceeds through an S(E)Ar mechanism with borenium cations, [Y(2)B(amine)](+), the key electrophiles. For catecholato-borocations, two amine dependent reaction pathways were identified: (i) With [CatB(NEt(3))](+), an additional base is necessary to accomplish rapid borylation by deprotonation of the borylated arenium cation (σ complex), which otherwise would rather decompose to the starting materials than liberate the free amine to effect deprotonation. Apart from amines, the additional base may also be the arene itself when it is sufficiently basic (e.g., N-Me-indole). (ii) When the amine component of the borocation is less nucleophilic (e.g., 2,6-lutidine), no additional base is required due to more facile amine dissociation from the boron center in the borylated arenium cation intermediate. Borenium cations do not borylate poorly activated arenes (e.g., toluene) even at high temperatures; instead, the key electrophile in this case involves the product from interaction of AlCl(3) with Y(2)BCl. When an extremely bulky amine is used, borylation again does not proceed via a borenium cation; instead, a number of mechanisms are feasible including via a boron electrophile generated by coordination of AlCl(3) to Y(2)BCl, or by initial (heteroarene)AlCl(3) adduct formation followed by deprotonation and transmetalation.