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.

[reaction: see text] We report a one-step phosphine-catalyzed annulation between aldehydes and ethyl allenoate to form 6-substituted 2-pyrones. The mechanistic rationale for this reaction requires explicit discussion of the E/Z-isomerism of the zwitterionic intermediate formed by the addition of a phosphine to the allenoate. Sterically demanding trialkylphosphines facilitate the shift of equilibrium toward the E-isomeric zwitterion and lead to the formation of 6-substituted 2-pyrones. Various aromatic as well as aliphatic aldehydes undergo the transformation in moderate to excellent yield.

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)

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The development of the 1,3,5-triazepane-2,6-dione system as a novel, conformationally restricted, and readily accessible class of dipeptidomimetics is reported. The synthesis of the densely functionalized 1,3,5-triazepane-2,6-dione skeleton was achieved in only four steps from a variety of simple linear dipeptide precursors. To extend the practical value of 1,3,5-triazepane-2,6-diones, a general polymer-assisted solution-phase synthesis approach amenable to library production in a multiparallel format was developed. The conformational preferences of the 1,3,5-triazepane-2,6-dione skeleton were investigated in detail by NMR spectroscopy and X-ray diffraction. The ring exhibits a characteristic folded conformation which was compared to that of related dipeptide-derived scaffolds including the more planar 2,5-diketopiperazine (DKP). Molecular and structural diversity was increased further through post-cyclization appending operations at urea nitrogens. Preliminary biological screens of a small collection of 1,3,5-triazepane-2,6-diones revealed inhibitors of the underexplored malaria liver stage and suggest strong potential for this dipeptide-derived scaffold to interfere with and to modulate biological pathways.

In contrast to the situation observed in the crystal state, the urea moiety in N-Boc-N'-carbamoyl-gem-diaminoalkyl derivatives (single-residue ureidopeptides) 1-4 exclusively assumes a cis-trans conformation in solution. When R(3) = H, the resulting structure can be further stabilized by an intramolecular hydrogen bond that closes an eight-membered pseudocycle. The root-mean-square deviation calculated for heavy atoms between a peptide gamma-turn and the folded conformation that we propose to call urea turn is 0.60 A. [structure: see text]

An efficient formal anti-Markovnikov addition of HX (X = Cl, Br, I, SR and SeR) to olefins under mild reaction conditions is described. The procedure is based on the hydroboration of alkenes with catecholborane. The conversion of the intermediate B-alkylcatecholboranes to the corresponding halides, sulfides and selenides is based on a common process, i.e., generation of a radical from the alkylborane followed by abstraction of a heteroatom from an aromatic sulfonyl reagent. The efficiency of these radical reactions is remarkable. The mildness of the reaction conditions is well illustrated by the preparation of iodoalkanes. Despite the notorious reactivity of iodoalkanes under radical reaction conditions, no product degradation was observed.

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