Room-temperature Pd/Ag direct arylation enabled by a radical pathway.

Amy L Mayhugh,Christine K Luscombe

doi:10.3762/bjoc.16.36

Amy L Mayhugh, Christine K Luscombe

Open Access

https://doi.org/10.3762/bjoc.16.36

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Abstract

Direct arylation is an appealing method for preparing π-conjugated materials, avoiding the prefunctionalization required for traditional cross-coupling methods. A major effort in organic electronic materials development is improving the environmental and economic impact of production; direct arylation polymerization (DArP) is an effective method to achieve these goals. Room-temperature polymerization would further improve the cost and energy efficiencies required to prepare these materials. Reported herein is new mechanistic work studying the underlying mechanism of room temperature direct arylation between iodobenzene and indole. Results indicate that room-temperature, Pd/Ag-catalyzed direct arylation systems are radical-mediated. This is in contrast to the commonly proposed two-electron mechanisms for direct arylation and appears to extend to other substrates such as benzo[b]thiophene and pentafluorobenzene.

Highlights

Introduction πConjugated polymers are of significant interest as they have the potential to combine the mechanical flexibility and affordability of synthetic polymers with the optical and electronic properties of semiconductors
We investigated the mechanism of room-temperature direct arylation and direct arylation polymerization (DArP) by extending the method reported by Larrosa to synthesize a conjugated polymer, polyindole (PIn)
While low molecular weight contributes to the large signals from the chain ends, there are a few factors that indicate branching is occurring: firstly, the large variety in C3-H signals; secondly, the relative integration of the C2-H/C3-H signals to the N-CH2-R signals show that for every indole unit, there is less than one H on the pyrrole ring

Summary

Introduction

Introduction πConjugated polymers are of significant interest as they have the potential to combine the mechanical flexibility and affordability of synthetic polymers with the optical and electronic properties of semiconductors. Known DArP conditions have limited monomers that can produce polymers with both high molecular weight and regioregularity, and elevated reaction temperatures are typically required [3]. An indole/iodoarene direct arylation method reported by Larrosa is notable: proceeding in high yields, at room temperature, and with no reported regioselectivity issues (Scheme 1) [12]. We investigated the mechanism of room-temperature direct arylation and DArP by extending the method reported by Larrosa to synthesize a conjugated polymer, polyindole (PIn).

Results

Conclusion