Thienyl MIDA Boronate Esters as Highly Effective Monomers for Suzuki–Miyaura Polymerization Reactions

Abstract

The synthesis of highly regioregular poly­(3-hexylthiophene-2,5-diyl), rr-P3HT, by Suzuki–Miyaura polymerization is reported. The key N-methyliminodiacetic acid (MIDA) boronate ester thienyl monomer was synthesized using a one-pot multigram scale procedure, in high purity, and in good isolated yield (80%) by direct electrophilic borylation. Conditions for the hydrolysis of the MIDA protecting group and the polymerization reaction were investigated. The optimal procedure gave rr-P3HT with >98% HT couplings, excellent isolated yields (up to 94%), and polymer molecular weights up to M n = 18.7 kDa and M w = 42.7 kDa. The performance of the MIDA containing monomer was compared to that of the pinacol boronate ester under identical polymerization conditions, with the latter producing lower molecular weight polymers in reduced yield.

Towards the generalized iterative synthesis of small molecules

“…137 This mechanistic divergence is consistent with extensive reaction kinetics, kinetic isotope effects, 18 O labelling, and computational data. In-situ slow release of MIDA boronates has been advantageous for many reactions including couplings of unstable heteroarylboronates, 138,139 polymerization reactions, 140–141 asymmetric methodologies, 142 the synthesis of organic photovoltaics, 143 and a one-pot homologation of boronic acids. 144 …”

“…4–5 MIDA boronates have been employed independent of iteration on a wide range of other small molecule synthesis applications as well. 140–141, 143, 150, 180, 186–193 …”

Maximising the hydrogen evolution activity in organic photocatalysts by co-polymerisation

“…Attempts to synthesise P17 via Suzuki-Miyaura polycondensation resulted in chloroform-soluble oligomeric products rather than polymers, probably due to the commonly observed hydrolytic deboronation under polycondensation conditions. 67 All polymers precipitated during the reaction and are insoluble in organic solvents and water. Aer ltration and washing with water and methanol, all polymers were puried via Soxhlet extraction with chloroform.…”

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

“…For example, the hydrolysis of BMIDA in 1 is more rapid than that in 3 and thus optimal hydrolysis conditions for 1 required K 3 PO 4 and not KOH (in basic THF/water mixtures). This is exemplified by the Suzuki–Miyaura polymerization of 1 using KOH as base (under otherwise identical conditions) giving significantly poorer outcomes due to BMIDA hydrolysis being too fast and protodeboronation of the thienylboronic acid becoming a significant factor (P3HT produced from 1 with K 3 PO 4 as base yield = 90%, M n / M w = 13.8/24.1 kDa; 12 with KOH as base yield = 47%, M n / M w = 10.0/14.1 kDa). Nevertheless, excluding the change in base, the polymerization of 1 and 3 both proceed under otherwise identical conditions.…”