Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

The Persistent Radical Effect in Organic Synthesis

The development of intermolecular alkene aminopyridylation has great potential for quickly increasing molecular complexity with two valuable groups. Here we report a strategy for the photocatalytic aminopyridylation of alkenes using a variety of N-aminopyridinium salts as both aminating and pyridylating reagents. Using Eosin Y as a photocatalyst, amino and pyridyl groups are simultaneously incorporated into alkenes, affording synthetically useful aminoethyl pyridine derivatives under mild reaction conditions. Remarkably, the C4-regioselectivity in radical trapping with N-aminopyridinium salt can be controlled by electrostatic interaction between the pyridinium nitrogen and sulfonyl group of β-amino radical. This transformation is characterized by a broad substrate scope, good functional group compatibility, and the utility of this transformation was further demonstrated by late-stage functionalization of complex biorelevant molecules. Combining experiments and DFT calculations on the mechanism of the reaction is investigated to propose a complete mechanism and regioselectivity.

/pdf/visible-light-induced-alkene-aminopyridylation-using-n-58y5e1f8yi.pdf

Visible light induced alkene aminopyridylation using N-aminopyridinium salts as bifunctional reagents.

this perspective gives an overview on recent findings in the emerging area of c-radical borylation using diborons as radical trapping reagents. aryl, vinyl and alkyl boronic esters can be accessed via such an approach under mild conditions. these processes are complementary to established transition metal catalysed cross coupling reactions. radical borylations can be conducted in the absence of a transition metal but some processes require transition metals as catalysts. it will be shown that various readily available c-radical precursors can be used to run these borylations. for a better understanding of the chemistry, mechanistic discussions are also presented and an outlook on this topic will be provided at the end of the article.

/pdf/new-avenues-for-c-b-bond-formation-via-radical-intermediates-4kvja5lgtj.pdf

new avenues for c–b bond formation via radical intermediates

This review describes recent advances in difluoroalkylation reactions using different substrates. Generally speaking, RCF2 radical is generally proposed in the mechanism of these reactions. At present, the most used difluoroalkylation reagent is RCF2X, which can be reduced by metal, photocatalyst or bases to RCF2 radical. Various substrates such as alkenes, alkynes, amines and so on could well be applied to these difluoroalkylation reactions.

Promising reagents for difluoroalkylation

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine.

A metal-free and photo-free method for the perfluoroalkylative pyridylation of alkenes has been developed via a combination of computational and experimental studies. Density functional theory calculations and control experiments indicate that the homolysis of Rf−X (X = Br, I) bonds by the 4-cyanopyridine-boryl radicals in situ generated from 4-cyanopyridine and B2pin2 is the key step. Sequential addition of Rf radicals to alkenes and the selective cross-coupling of the resulting alkyl radicals and 4-cyanopyridine-boryl radicals gives alkene difunctionalization products with a quaternary carbon center. This method exhibits a broad substrate scope and good functional group compatibility.

/pdf/perfluoroalkylative-pyridylation-of-alkenes-via-4-1xkhj2wajg.pdf

Perfluoroalkylative pyridylation of alkenes via 4-cyanopyridine-boryl radicals

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Perfluoroalkylative pyridylation of alkenes via 4-cyanopyridine-boryl radicals