B‐Alkylcatecholborane‐Mediated Radical Reactions

Abstract

B‐Alkylcatecholboranes, easily prepared by hydroboration of alkenes, represent a very efficient source of primary, secondary and tertiary alkyl radicals. The very high sensitivity towards oxygen‐and heteroatom‐centered radicals makes them particularly attractive for the development of radical chain processes. Very mild conditions for radical hydroxylation of organoboranes using either TEMPO (= 2,2,6,6‐tetramethylpiperidine‐N‐oxyl) or oxygen have been developed. Conjugate addition of β‐alkylcatecholboranes to enones and enals proceeds very efficiently under oxygen intiation. Inter‐ and intramolecular addition to other radical traps such as α,β‐unsaturated esters, amides and sulfones requires the use of PTOC‐OMe {1‐[(methoxycarbonyl)oxy]pyridine‐2(1H)‐thione} as a chain transfer reagent. Finally, a very general method for the radical hydroallylation of olefins using allyl sulfones as radical traps is presented. This reaction takes advantage of the very efficient reaction of B‐alkylcatecholboranes with the benzenesulfonyl radical. The diversity of the chemistry presented in this microreview demontrates that catecholborane represents a very good alternative to tin derivatives for radical reactions involving alkyl radicals. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Boron: A key element in radical reactions

“…B-Alkylcatecholboranes, easily prepared from olefins via hydroboration with catecholborane, offer the possibility to generate selectively primary, secondary, and tertiary alkyl radicals from a suitable alkene [20]. Indeed, reaction of the B-alkylcatecholborane with a heteroatom-centered radical leads in an irreversible manner to the alkyl radical.…”

Tin-Free Radical Reactions Mediated by Organoboron Compounds

“…However, the most attractive feature of B-alkylcatecholboranes is the possibility to generate selectively one alkyl radical from an olefin; a possibility that trialkylboranes do not offer since no selective cleavage of the desired alkyl group is observed (vide supra). Indeed, reaction of B-alkylcatecholborane with a heteroatom centered radical leads in an irreversible manner to the alkyl radical since cleavage of the "wrong" B -O bond is a reversible process that finally leads to the irreversible formation of an alkyl radical (Scheme 31) [9].…”

“…The simple formation of carbonheteroatom bonds via a radical process is not treated in this review since it has been treated in previous review articles [3,9].…”

A Tin‐Free, Radical Photocatalyzed Addition to Vinyl Sulfones

“…Thus, cycloalkyl radicals were obtained starting from cyclohexene by one-pot hydroboration with catecholborane followed by irradiation mediated by a Barton carbonate as the chain carrier. [20] Moreover, various substituted alkyl radicals were generated both from (diacyl-A C H T U N G T R E N N U N G oxyiodo)arenes by photolysis [21] and from tetramethylammonium phenyltrialkylborates by an electron transfer reaction with benzophenone triplet. [22] While in the cases above radicals are generated from halides, an appealing and atom-economical way to generate substituted alkyl radicals involves C À H activation, a process that has been demonstrated to be viable by having recourse to a photocatalyst.…”