Radical Addition to 1,4‐Benzoquinones: Addition at O‐ versus C‐Atom.

Abstract

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Formal Glycosylation of Quinones with exo-Glycals Enabled by Iron-Mediated Oxidative Radical–Polar Crossover

“…Several other observations could account for a cationic scenario. First, in contrast to the related radical-based process involving 1,4quinones, [25][26][27]29,30 no 1,4-addition product was observed regardless of the reaction conditions or quinone used.…”

“…Based on previous mechanistic studies on MHAT reactions to alkenes 20,21 and our own work, 19 we indeed assumed that a fast, irreversible transfer of a hydrogen atom from highly reactive transient Fe(III)−H species to the carbohydrate olefin afforded the tertiary pseudoanomeric radical intermediate A (Scheme 7a). However, it was not clear if the key C−O bond-forming event proceeded through a radical process 27,29,30,45 (Scheme 7a, pathway A) or a cationic process (Scheme 7a, pathway B). Both pathways would lead to the same intermediates�the rather stable phenoxy radical D and finally to the phenolic O-ketoside product, possibly through a concerted proton-coupled electron transfer from an iron(II) ethanol complex.…”

“…Radical addition to 1,4-quinones may also lead to different regioselectivities as shown by the group of Renaud. 27 Addition of bulky secondary or tertiary alkyl radicals generated from B-alkylcatecholboranes onto 1,4benzoquinones afforded the formation of the corresponding phenol ethers, whereas less-hindered alkyl residues provide the classical 1,4-addition products (Scheme 2c).…”

Tin-Free Radical Reactions Mediated by Organoboron Compounds

“…The addition of B-alkylcatecholboranes to quinones has recently been investigated [85]. Good yield of the expected conjugate addition product are obtained with primary and most secondary radicals (Scheme 34, Eq.…”