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

Diazo compounds are among the most important building blocks in organic synthesis. Generally, their photoinitiated, thermal, or transition metal catalyzed decomposition yields the corresponding carbenes or metal carbenoids, which subsequently undergo various transformations. Modern chemistry of diazo compounds is dominated by transition metal catalysis, leaving both catalyzed and non-catalyzed phototransformations of these reagents behind. Recently, photoinitiated reactions of diazo compounds have experienced a revival of interest due to the increased understanding of modern photochemistry. This mini-review covers recently published (year >2000) reports on the reactivity of diazo compounds under light irradiation.

Recent developments in photochemical reactions of diazo compounds

Inorganic sulfites as the sulfur dioxide surrogates in sulfonylation reactions

Lightening Diazo Compounds

Der “Persistent Radical Effect” in der organischen Chemie

The Cu-catalyzed three-component reaction between quinolines, diazo compounds, and alkenes has been established for direct construction of indolizine derivatives via quinolinium ylides. This methodology is distinguished by the use of a commercially inexpensive catalyst and readily available starting materials, wide substrate scope, and operational simplicity.

In Situ Generation of Quinolinium Ylides from Diazo Compounds: Copper-Catalyzed Synthesis of Indolizine.

Bu4NI-Catalyzed Cross-Coupling between Sulfonyl Hydrazides and Diazo Compounds To Construct β-Carbonyl Sulfones Using Molecular Oxygen.

A novel in situ generation of nitrilium from a nitrile ylide and the subsequent Mumm rearrangement of carboxylic acid, nitrile, and diazo compounds gave various unsymmetrical diacylglycine esters in moderate to high yields This copper-catalyzed cascade reaction enables one-pot generation of two C–N bonds, one CO bond, and one C–H bond, with nitrogen as the only byproduct The reaction has a broad functional-group tolerance, is rapid, easily scales up to the 100 mmol scale, and is insensitive to air and moisture

In situ generation of nitrilium from nitrile ylide and the subsequent Mumm rearrangement: copper-catalyzed synthesis of unsymmetrical diacylglycine esters

A novel Bu4NI-catalyzed pyrazole formation reaction is well described via sequential [3 + 2] cycloaddition and oxidative dehydrogenation reactions using TBHP as the primary oxidant. In comparison with previous cases toward pyrazoles from alkenes and diazo compounds, alkenes without a pre-organized leaving group were applied in this transformation. In addition, this methodology was distinguished by its broad substrate scope, commercially available inexpensive starting materials, high atom economy and operational simplicity.

[3 + 2] cycloaddition and subsequent oxidative dehydrogenation between alkenes and diazo compounds: a simple and direct approach to pyrazoles using TBAI/TBHP.

Liang Ma

Papers

In Situ Generation of Quinolinium Ylides from Diazo Compounds: Copper-Catalyzed Synthesis of Indolizine.

Bu4NI-Catalyzed Cross-Coupling between Sulfonyl Hydrazides and Diazo Compounds To Construct β-Carbonyl Sulfones Using Molecular Oxygen.

In situ generation of nitrilium from nitrile ylide and the subsequent Mumm rearrangement: copper-catalyzed synthesis of unsymmetrical diacylglycine esters

[3 + 2] cycloaddition and subsequent oxidative dehydrogenation between alkenes and diazo compounds: a simple and direct approach to pyrazoles using TBAI/TBHP.

Radical-carbene coupling reaction: Mn-catalyzed synthesis of indoles from aromatic amines and diazo compounds