Triarylamine-Mediated Electrochemical [2+2+2] Cycloaddition Forming 3,6-Diarylpyridines

Keywords

diarylpyridines - electrochemical reaction - [2+2+2] cycloaddition - triarylamines

Significance

Triphenylamine (TPA)-mediated electrochemical [2 + 2 + 2] cycloaddition of arylalkynes with nitriles was achieved with benzoic acid as a sacrificial reductant to form 2-alkyl-3,6-diarylpyridines with excellent regio- and chemoselectivity. Under electrochemical conditions, phenylacetylene reacted stepwise with acetonitrile and the second phenylacetylene via int-i and int-ii to give 2-methyl-3,6-phenylpyridine in 74 % yield (eq 1). Representative examples are shown in the graphic. The [2 + 2 + 2] annulation did not proceed with electron-deficient arylacetylenes (e. g., methyl 4-ethynylbenzoate) because of their less-oxidizable properties.

Comment

Cross-annulation with two different acetylenes was achieved by using a combination of oxidizable and less-oxidizable alkyne substrates (eq 2). Thus, for example, phenylacetylene, acetonitrile, and methyl 4-ethynylbenzoate gave 2-methyl-3-(4-methoxycarbonylphenyl)-6-phenylpyridine in 64 % yield under the electrochemical conditions. An induction period was observed in the electrochemical annulation, during which a CT-complex of TPA radical cation/benzoate anion, a key SET mediator, should be generated electrochemically in situ.

Publication History

Article published online:
25 March 2025