Photoinduced Intermolecular Radical Hydroalkylation of Olefins via Ligated Boryl Radicals-Mediated Halogen Atom Transfer.

Light-mediated Halogen-Atom Transfer (XAT) has become a significant methodology in contemporary synthesis. Unlike α-aminoalkyl and silyl radicals, ligated boryl radicals (LBRs) have not been extensively explored as halogen atom abstractors. In this study, we introduce NHC-ligated boranes as optimal radical chain carriers for the intermolecular reductive radical hydroalkylation and hydroarylation of electron-deficient olefins by using direct UV-A light irradiation. DFT analysis allowed us to rationalize the critical role of the NHC ligand in facilitating efficient chain propagation.

C-H Functionalization of Heterocycles with Triplet Carbenes by means of an Unexpected 1,2-Alkyl Radical Migration.

The C-H functionalization of indole heterocycles constitutes a key strategy to leverage the synthesis of endogenous signaling molecules such as tryptamine or tryptophol. Herein, we report on the photocatalytic reaction of ethyl diazoacetate with indole, which shows an unusual solvent dependency. While C2-functionalization occurs under protic conditions, the use of aprotic solvents leads to a complete reversal of selectivity and exclusive C3-functionalization occurs. To rationalize for this unexpected reactivity switch, we have conducted detailed theoretical and experimental studies, which suggest the participation of a triplet carbene intermediate that undergoes initial C2-functionalization. A distinct cationic [1,2]-alkyl radical migration then leads to formation of C3-functionalized indole. We conclude with the application of this photocatalytic reaction to access oxidized tryptophol derivatives including gram-scale synthesis and derivatization reactions.

Pyridine N-oxides as HAT reagents for photochemical C-H functionalization of electron-deficient heteroarenes.

Pyridine N-oxides have only recently marked their presence in the photocatalysis field, mainly serving as oxypyridinium salt precursors. Herein, their unique reactivity as a hydrogen atom transfer reagent in photochemical, Minisci-type alkylation of electron-deficient heteroarenes is unveiled. The formation of an EDA complex between a heterocyclic substrate and N-oxide precludes the need for a photocatalyst. The developed method allows for a broad range of radical precursors to be used, namely alkanes, alkenes, amides, and ethers, for efficient alkylation of azines.

Photocatalytic Alkylation of Pyrroles and Indoles with α-Diazo Esters.

This article describes the photoalkylation of electron-rich aromatic compounds with diazo esters. C-2-alkylated indoles and pyrroles are obtained with good yields even though the photocatalyst loading is as low as 0.075 mol %. For EWG-substituted substrates, the addition of a catalytic amount of N,N-dimethyl-4-methoxyaniline is required. Both EWG-EWG- and EWG-EDG-substituted diazo esters are suitable as alkylating agents. The reaction selectivity and mechanistic experiments suggest that carbenes/carbenoid intermediates are not involved in the reaction pathway.

Recent developments in photochemical reactions of diazo compounds.

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.