Photocatalytic Carboxylation of C−N Bonds in Cyclic Amines with CO₂ by Consecutive Visible‐Light‐Induced Electron Transfer

Abstract

Visible‐light photocatalytic carboxylation with CO2 is highly important. However, it still remains challenging for reluctant substrates with low reduction potentials. Herein, we report a novel photocatalytic carboxylation of C−N bonds in cyclic amines with CO2 via consecutive photo‐induced electron transfer (ConPET). It is also the first photocatalytic reductive ring‐opening reaction of azetidines, pyrrolidines and piperidines. This strategy is practical to transform a variety of easily available cyclic amines to valuable β‐, γ‐, δ‐ and ϵ‐amino acids in moderate‐to‐excellent yields. Moreover, the method also features mild and transition‐metal‐free conditions, high selectivity, good functional‐group tolerance, facile scalability and product derivations. Mechanistic studies indicate that the ConPET might be the key to generating highly reactive photocatalysts, which enable the reductive activation of cyclic amines to generate carbon radicals and carbanions as the key intermediates.

Generation of Thioxanthone Hydrogen Anion by Double Photoreduction and Uses for Catalytic Photoreductions

“…3 The redox potential of a photoredox catalyst is a crucial parameter that can be broadened by consecutively absorbing two photons and accumulating multiple-photon energy (Figures 1b,c). 4 The consecutive photoinduced electron transfer (ConPET) process 5 allows the energydemanding redox transformations, such as dehalogenation and further functionalization, 4,6 alkene oxidation, 7 carboxylation, 8 arene oxidation, 9 Birch reduction, 10 and N−O bond cleavage, 11 under mild conditions. 12 While excited-state lifetime is another important feature of photoredox catalysts, open-shell intermediates, such as radical anions 13 or radical cations 14 generated by one-electron transfer of precatalysts, often have significantly shorter excited-state lifetimes because of fast nonradiative decay.…”