Visible Light‐Induced Selective Generation of Radicals from Organoborates by Photoredox Catalysis

Abstract

A new strategy for the generation of carbon‐centered radicals via oxidation of alkyl‐, allyl‐, benzyl‐ and arylborates by visible‐light‐driven single electron transfer (SET) photoredox catalysis has been established. The generated radicals smoothly react with TEMPO and electron‐deficient alkenes to afford CO and CC coupling products, respectively. In this radical initiating system, cyclic organo(triol)borates turn out to be useful radical precursors.

Visible‐Light Photocatalysis as an Enabling Tool for the Functionalization of Unactivated C(sp3)‐Substrates

“…[69] In contrast with organoboron-based compounds, this class of precursors offers convenient access to primary radical intermediates due to their significantly lower oxidation potential (E 1/2 ox > +1.2 V vs. SCE for primary alkyltrifluoroborates [70] vs. E 1/2 ox = +0.34 to +0.61 V vs. SCE for primary alkylsilicates) [69] Importantly, the precursors can be readily prepared from inexpensive trimethoxysilane derivatives and catechols and are stable upon storage at room temperature. The authors demonstrated the utility of these substrates in a large number of examples based on Giesetype radical additions to electron-deficient alkenes.…”

Single-Electron Transmetalation: An Enabling Technology for Secondary Alkylboron Cross-Coupling

“…Nevertheless, encouragement was found in cases where thermodynamically unfavorable SETs occur, particularly within the context of an irreversible oxidation or reduction. 22,24 Thus, only a small portion of the oxidation wave need overlap with the potential of the photocatalyst for the reaction to proceed, driven forward by irreversible C−B bond fragmentation.…”

The Cleavage of a CC Bond in Cyclobutylanilines by Visible‐Light Photoredox Catalysis: Development of a [4+2] Annulation Method

“…Although thermodynamically unfavorable, such SET processes have been reported. [25] Stern-Volmer quenching studies revealed that cyclobutylaniline 1a quenches the photoexcited 4a while alkynes 2a and 2h showed little quenching (see SI).…”