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

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)center dot B-2(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 Me(2)Im(Me)center dot Bneop 1a (Me(2)Im(Me) = 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 Me(2)Im(Me)center dot B(2)pin(2)3 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, [(Me(2)Im(Me))(2)center dot Bpin](+)I(-)7, demonstrated the fate of the second boryl moiety of B(2)pin(2). Interestingly, Me(2)Im(Me) NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me(2)Im(Me)center dot B(2)pin(2)3 to aryl iodides and bromides is proposed based on these experimental observations.

Automated summary

This article reports a new borylation methodology that utilizes the homolytic cleavage of the B-B bond to convert bis-NHC adducts into boryl radicals, which can be used for the transformation of substituted aryl iodides and bromides into aryl boronate esters.