Investigating the Mechanism of Ni-Catalyzed Coupling of Photoredox-Generated Alkyl Radicals and Aryl Bromides: A Computational Study.

Photoredox catalysis has emerged as an alternative to classical cross-coupling reactions, promoting new reactivities. Recently, the use of widely abundant alcohols and aryl bromides as coupling reagents was demonstrated to promote efficient coupling through the Ir/Ni dual photoredox catalytic cycle. However, the mechanism underlying this transformation is still unexplored, and here we report a comprehensive computational study of the catalytic cycle. We have shown that nickel catalysts can promote this reactivity very efficiently through DFT calculations. Two different mechanistic scenarios were explored, suggesting that two catalytic cycles operate simultaneously depending on the concentration of the alkyl radical.

On the Role of N-Heterocyclic Carbene Salts in Alkyl Radical Generation from Alkyl Alcohols: A Computational Study.

Formation of carbon-carbon bonds through cross-coupling reactions using readily available substrates, like alcohols, is crucial for modern organic chemistry. Recently, direct alkyl alcohol functionalization has been achieved by the use of N-Heterocyclic Carbene (NHC) salts via in situ formation of an alcohol-NHC adduct and its activation by a photoredox catalyst to generate carbon-centered alkyl radicals. Experimentally, only electron deficient NHC activators work but the reasons of this behavior remain underexplored. Herein, a DFT computational study of the mechanism of alcohol activation using up to seven NHC salts is performed to shed light into the influence of their electronic properties in the alkyl radical formation. This study demonstrates that four reaction steps are involved in the transformation and characterizes how the electronic properties of the NHC salt affect each step. A fine balance of the NHC electron-richness is proved to be determinant for this transformation.