Modern Developments in Aryl Radical Chemistry

Abstract

This review summarizes recent advances in the field of aryl radical chemistry. In particular, modern developments of the well-known Meerwein, Pschorr, and Gomberg-Bachmann reactions are presented along with new applications in natural product syntheses. Among the methods for the generation of aryl radicals, tin hydrides play a predominant role, but more and more attractive and promising alternatives are beginning to emerge.

A case of chain propagation: α-aminoalkyl radicals as initiators for aryl radical chemistry

“…Aryl radicals are versatile synthetic intermediates for the assembly of sp 2 C-C and C-Y (Y ¼ heteroatom) bonds. 1 Reactions like the Meerwein cyclization and arylation, 2 the Pschorr cyclization 3 and the Gomber-Bachmann biphenyl synthesis 4 are text-book examples of aryl radical reactivity and are still used in the assembly of high-value materials. 5 Historically, aryl radicals have been generated by SET (singleelectron transfer) reduction of aryl diazonium, 6 iodonium 7 and sulfonium salts 8 or oxidation of benzoates 9 (Scheme 1A).…”

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

“…The most commonly encountered type is C-centred radical addition (often an aryl radical) to an aromatic or heteroaromatic ring ortho to the point of attachment of the tether. In Pschorr and related processes re-aromatisation follows with production of phenanthrene-type derivatives [1–2]. Spiro-cyclisations in which tethered radicals add to the ipso-C-atoms of the rings are less common, although minor spiro - products not infrequently accompany the main ortho - ones in Pschorr syntheses [3–5].…”

How the Structural Elucidation of the Natural Product Stephanosporin Led to New Developments in Aryl Radical and Medicinal Chemistry

“…Alongside the Meerwein arylation, the Gomberg-Bachmann reaction is the second member of the group of wellknown radical arylation reactions that are suitable for use in C-C bond-formation processes. 6 Although many studies had been conducted to enlarge the substrate scope and to improve the conditions for Gomberg-Bachmann reactions, 15,16,97 its synthetic applications remained comparably limited as a result of two general disadvantages. First, the addition of an aryl radical to most substituted benzenes proceeds with rate constants of around k = 1 × 10 6 M -1 s -1 , which is an order of magnitude slower than the addition to nonactivated alkenes (e.g., allyl acetate) and as much as two orders of magnitude slower than addition to the preferred group of activated alkenes (e.g., acrylic esters).…”

“…For closer insights into the expanding fields of aryl radicals in natural product synthesis, 16 photocatalyzed reactions 17 or the functionalization of surfaces, 18 we refer the reader to a number of more comprehensive review articles and book chapters.…”