Herein, we report our study on the design and development
of a
novel photocarboxylation method. We have used an organic photoredox
catalyst (PC, 4CzIPN) and differently substituted dihydropyridines
(DHPs) in combination with an organic base (1,5,7-triazabicyclodec-5-ene,
TBD) to access a proton-coupled electron transfer (PCET) based manifold.
In depth mechanistic investigations merging experimental analysis
(NMR, IR, cyclic voltammetry) and density-functional theory (DFT)
calculations reveal the key activity of a H-bonding complex between
the DHP and the base. The thermodynamic and kinetic benefits of the
PCET mechanism allowed the implementation of a redox-neutral fixation
process leading to synthetically relevant carboxylic acids (18 examples
with isolated yields up to 75%) under very mild reaction conditions.
Finally, diverse product manipulations were performed to demonstrate
the synthetic versatility of the obtained products.