Photoinduced Single‐Electron Transfer as an Enabling Principle in the Radical Borylation of Alkenes with NHC–Borane

Abstract

A photoinduced SET process enables the direct B−H bond activation of NHC–boranes. In contrast to common hydrogen atom transfer (HAT) strategies, this photoinduced reaction simply takes advantage of the beneficial redox potentials of NHC–boranes, thus obviating the need for extra radical initiators. The resulting NHC–boryl radical was used for the borylation of a wide range of α‐trifluoromethylalkenes and alkenes with diverse electronic and structural features, providing facile access to highly functionalized borylated molecules. Labeling and photoquenching experiments provide insight into the mechanism of this photoinduced SET pathway.

Electrochemical Hydroboration of Alkynes

“…Very recently, the photocatalyzed formation of boryl radicals using iridium catalysts has been reported (Scheme 1). This new method was applied to the addition of NHC‐boron residues on fluorinated olefins and aryls, according to an addition/fluoride elimination sequence [9] and to the hydroboration of imines, [10] alkenes [11] and electron poor aromatic derivatives [12] . Importantly, in contrast to the formation of boryl radicals from NHC‐borane, their formation from diboron compounds remains scarce [13] .…”

New Radical Borylation Pathways for Organoboron Synthesis Enabled by Photoredox Catalysis

“…. The direct formation of NHC‐boryl radicals and their employment in photoredox catalysis via single‐electron transfer, however, has remained elusive . 2) The precise control of chemo‐, regio‐, and stereoselectivity remains a formidable challenge when multiple reactive species, such as free radicals and radical anions/cations, coexist in a reaction mixture.…”

Boryl Radical‐Mediated C−H Activation of Inactivated Alkanes for the Synthesis of Internal Alkynes

“…Xie and Zhu disclosed an elegant inverse hydroboration of imines under photoredox conditions, providing a practical protocol towards medically valuable aminoboranes compounds [8] . Very recently, Xiang, Chen and Yang reported an photoinduced Single‐Electron Transfer (SET) process for radical borylation of alkenes with NHC‐borane [9] . Despite these exciting development in amine‐, phosphine‐, and NHC‐ligated boryl radical chemistry, however, investigations engaging electron‐deficient boron radicals are still quite limited [10]…”