Issue 28, 2022

NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

Abstract

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B–B bond. We report herein that bis-NHC adducts of the type (NHC)2·B2(OR)4, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC–BR2˙, exemplified by Me2ImMe·Bneop˙ 1a (Me2ImMe = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B–B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me2ImMe·B2pin23 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me2ImMe)2·Bpin]+I7, demonstrated the fate of the second boryl moiety of B2pin2. Interestingly, Me2ImMe NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me2ImMe·B2pin23 to aryl iodides and bromides is proposed based on these experimental observations.

Graphical abstract: NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

Supplementary files

Article information

Article type
Edge Article
Submitted
12 Apr. 2022
Accepted
07 Jūn. 2022
First published
07 Jūn. 2022
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2022,13, 8321-8333

NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

L. Kuehn, L. Zapf, L. Werner, M. Stang, S. Würtemberger-Pietsch, I. Krummenacher, H. Braunschweig, E. Lacôte, T. B. Marder and U. Radius, Chem. Sci., 2022, 13, 8321 DOI: 10.1039/D2SC02096C

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