Boron: A key element in radical reactions

Abstract

Boron derivatives are becoming key reagents in radical chemistry. Here, we describe reactions where an organoboron derivative is used as a radical initiator, a chain-transfer reagent, and a radical precursor. For instance, B-alkylcatecholboranes, easily prepared by hydroboration of alkenes, represent a very efficient source of primary, secondary, and tertiary alkyl radicals. Their very high sensitivity toward oxygen-and heteroatom-centered radicals makes them particularly attractive for the development of radical chain processes such as conjugate addition, allylation, alkenylation, and alkynylation. Boron derivatives have also been used to develop an attractive new procedure for the reduction of radicals with alcohols and water. The selected examples presented here demonstrate that boron-containing reagents can efficiently replace tin derivatives in a wide range of radical reactions.

A Mild meta‐Selective C–H Alkylation of Catechol Mono‐Ethers

“…When 4a was exposed to the same conditions as those in Table , 5‐isopropylguaiacol ( 9 ) was isolated in a 71 % yield (Scheme ). Yet, it is our belief that this method should not be limited to trialkylboranes as evidence shows that under similar mild conditions, other B ‐alkylborane derivatives could also be used to generate alkyl radicals, including B ‐alkylcatecholboranes, B ‐alkylboracyclanes, and B ‐alkyldiphenylboranes …”

Radical Polymerization of Alkyl 2-Cyanoacrylates

“…The initiating radical was generated from the auto-oxidation of the alkylboron compound with air to give the alkyl borane peroxide and butyl radicals [61]. Homopolymerizations were carried out using 1.8 wt% of TBBO in THF at room temperature.…”

Alkylboranes in Conventional and Controlled Radical Polymerization

“…Owing to the Lewis acid character of boron atom, the neutral organoborane readily reacted with a heteroatom‐centered radical to form a Lewis acid–base complex, further providing an alkyl radical by β‐fragmentation . The autoxidation of organoboranes was a classic example to give an alkyl radical, which made trialkylborane as a universal radical initiator under a wide range of reaction temperature and solvents …”