The 1,4-dicarbonyl motif is a privileged scaffold in downstream heterocycle synthesis, via Paal-Knorr routes to pyrrols, furans and thiophenes. Classic approaches to these versatile structures are predicated on the venerable Stetter reaction. Reactivity “Umpolung” of an aldehyde, using an NHC catalyst or cyanide allows for subsequent attack of an α,β-unsaturated carbonyl compound, yielding the desired molecules. However, employment of aldehydes as acceptor moiety remains elusive, due to competitive engagement with the catalyst and a resulting low chemoselectivity leading to product mixtures. This problem becomes particularly acute in intermolecular reactions. A retrosynthetic reinterpretation of the Stetter reaction enabled the selective synthesis of 1,4-ketoaldehydes, utilizing α,β-unsaturated aldehydes and α-ketocarboxylic acids in a photo-iminium catalysed reaction. As a key intermediate a close photoactive ion pair is postulated, which undergoes facile single electron transfer (SET) and fast decarboxylation prior to radical-radical recombination. Consequently, the obtained products are formed in a formal “radical-Umpolung” reaction and are versatile synthetic building blocks in organic synthesis.

Radical Stetter reaction enabled by merger of ion-pair photocatalysis with radical umpolung

2019 Fall National Meeting

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