The S _RN 1 Reaction

Abstract

Since its discovery, the radical nucleophilic substitution, or S RN 1 reaction, has been widely used to achieve new CC or C‐heteroatom bonds. The mechanism involves a chain propagation cycle with radicals and radical anions as intermediates. The initiation step is an electron transfer to a substrate to form its radical anion, which fragments to afford a radical and the anion of the leaving group. The radical thus formed couples with a nucleophile to give a new radical anion which, in the next propagation step, transfers its extra electron to the substrate, closing the cycle. Here we discuss S RN 1 reactions of aromatic substrates with carbanions and heteroaromatic nucleophiles as well as ring‐closure reactions to obtain heterocycles by different approaches. S RN 1 reactions of vinylic halides with nucleophiles and the competing mechanisms are also examined. Reactions of aliphatic substrates that do not react or are poorly reactive toward polar nucleophilic substitutions (cycloalkyl, bridgehead, neopentyl, and perfluoroalkyl halides) are discussed in terms of mechanistic requirements to follow an S RN 1 process. In addition, the reactions of alkyl substrates with electron‐withdrawing groups also are presented. Every year, new synthetic strategies and mechanistic studies involving S RN 1 reactions are reported, which constitute a useful tool in the field of organic chemistry.

Excited State Anions in Organic Transformations

“…[135] Theinitiation of the BHAS reaction was also reported by other photoactivation modes,f or example, through PET from an iridium sensitizer to R-X, or upon excitation with light of an in situ formed photosensitive complex between KO t Bu and phenanthroline. [136,137] Nonnucleophilic bases are commonly employed to avoid the competing S RN 1r eaction pathway.L ight-mediated substitutions following the S RN 1r eaction with organic anions as nucleophiles have been studied extensively and were the subject of recent reviews [4,134,[138][139][140][141][142] and thus will not be further discussed herein.…”

(Fluoro)alkylation of alkenes promoted by photolysis of alkylzirconocenes

“…These mild conditions are quite different from the challenge S N 1 or S N 2 substitution of difluoroalkyl halides, because of the difficulty in forming a difluoroalkyl carbocation (S N 1) or the repulsion of the lone pairs of fluorine atoms to the carbon nucleophile (S N 2). 18 This difference implies a novel mechanism, which is discussed below. Encouraged by these results, we tested a series of reaction parameters (for details, see the ESI†), and found that the use of 3.0 equiv.…”

Photoangeregte Anionen in organischen Reaktionen

“…Lichtvermittelte Substitutionsreaktionen, die nach dem S RN 1-Mechanismus verlaufen, wurden weitreichend untersucht und sind Gegenstand von Buchkapiteln und neueren Aufsätzen, weshalb wir hier nicht weiter auf sie eingehen. [4,134,[138][139][140][141][142]…”