B-alkylcatecholborane-mediated radical reactions

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 beta-alkylcatecholboranes to enones and enals proceeds very efficiently under oxygen intiation. Inter- and intramolecular addition to other radical traps such as alpha,beta-unsaturated esters, amides and sulfones requires the use of PTOC-OMe {1-[(methoxycarbonyl)oxy]pyridine2(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-alkylcate-cholboranes 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).