DABCO-promoted photocatalytic C–H functionalization of aldehydes

Abstract

Herein we present a direct application of DABCO, an inexpensive and broadly accessible organic base, as a hydrogen atom transfer (HAT) abstractor in a photocatalytic strategy for aldehyde C–H activation. The acyl radicals generated in this step were arylated with aryl bromides through a well stablished nickel cross-coupling methodology, leading to a variety of interesting aryl ketones in good yields. We also performed computational calculations to shine light in the HAT step energetics and determined an optimized geometry for the transition state, showing that the hydrogen atom transfer between aldehydes and DABCO is a mildly endergonic, yet sufficiently fast step. The same calculations were performed with quinuclidine, for comparison of both catalysts and the differences are discussed.

The versatility of DABCO as a reagent in organic synthesis: a review

“…126 Scheme 69 Photocatalytic C-C cross-coupling reaction via aldehyde C-H bond cleavage using DABCO. 134 Scheme 70 The use of DABCO-derived cationic catalysts under photoredox catalysis conditions in the cleavage of inactivated C-H bond for the addition of alkyl radical to electron-deficient alkenes. 135 of aliphatic bicyclic amines are efficient in the abstraction of hydrogen atom.…”

“…According to the hypothesised mechanism, it was involved in the Ni-catalyzed oxidative C-C cross-coupling that involved the breaking of the aldehyde C-H bond and the production of acyl radicals (Scheme 69). 134 Scheme 70 represents a novel class of redox-organocatalysts based on the synthetically accessible DABCO fragment. The quaternization of one of nitrogen atoms of DABCO results in significantly increased hydrogen atom abstracting activity, thereby allowing to achieve CH-functionalization of alkanes using electron-deficient alkenes.…”

“…According to the hypothesised mechanism, it was involved in the Ni-catalyzed oxidative C–C cross-coupling that involved the breaking of the aldehyde C–H bond and the production of acyl radicals (Scheme 69). 134…”

Photocatalytic 1,2-oxo-alkylation reaction of styrenes with diazoacetates

“…Based on the above data and previous research, [9][10][11][12] we consider that this reaction proceeds via initial reductive quenching of the photoexcited state of the photocatalyst with DABCO (E Ox = 0.69 V vs. SCE). 16 Subsequent proton-coupled electron transfer with ethyl diazoacetate furnishes a radical intermediate 18 that can engage in a radical addition to styrene to give a benzylic radical 19. In parallel, the TBHP anion can undergo electron transfer with the DABCO radical cation to generate a TBHP radical.…”