Photoinduced Halogen-Atom Transfer (XAT) by N-heterocyclic carbene boryl radicals for C(sp3)−C(sp3) bond formation

Abstract

In the realm of radical chemistry, halogen-atom transfer (XAT) is emerging as a powerful activation strategy in synthetic endeavors to engage otherwise recalcitrant organic halides. Although toxic tin radicals have been in the spotlight as halo-gen abstractors for decades, the era of photocatalysis has brought to light more sustainable options. Herein, we present a comprehensive study on the use of ligated boryl radicals to enable C(sp3)−C(sp3) bond forming reactions via XAT from alkyl halides. Two strategies have been developed using either direct light activation with near UV or photoredox conditions with visible light; both pivoting on N-heterocyclic carbene (NHC) boryl radicals. The present XAT strategy is very mild and accommodates a broad scope of alkyl halides, including medicinally-relevant compounds and biologically-active molecules. The key role of NHC boryl radicals in the operative reaction mechanism has been elucidated through a combination of ex-perimental, spectroscopic and computational studies.

Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes**

“…Ye [19d] and Wu [19f] recently reported the straightforward single‐electron oxidation of amine borane by excited photosensitizer to generate a boron‐centered radical. Furthermore, halogen bonding between a N‐heterocyclic carbene (NHC) borane and alkyl halide was believed to facilitate NHC borane radical formation [21,19h] . However, boron radicals derived from phosphine boranes remain relatively less studied [22] .…”

Photoinduced Intermolecular Radical Hydroalkylation of Olefins via Ligated Boryl Radicals-Mediated Halogen Atom Transfer

“…Our studies commenced with the radical hydroalkylation of dibutyl maleate ( 2a ) in acetonitrile using 2 equiv of iodocyclohexane ( 1a ) as organic halide to form dibutyl 2-cyclohexylsuccinate ( 3 ), upon exposure to 390 nm for 12 h (Table ). A diverse set of easily accessible ligated boranes ( B1 – 5 , 1 equiv) was evaluated under those reaction conditions, and we found that only NHC-ligated borane B1 promoted the targeted reactivity (Table , entry 1: 75% 1 H NMR yield). While other ligated boranes ( B2 – 5 ) did not afford 3 (entry 2), we noticed that when the pyridine-ligated borane B2 was used, 2a was quantitatively reduced to dibutyl succinate.…”