This review offers an overview of recent synthetic strategies employing photoredox catalysis and electrochemistry in the framework of multicomponent reactions.

Transition metal-catalysed processes have been widely used for the functionalization of inert CÀ H bonds. Strategies for the functionalization of the benzylic CÀ H position having a relatively weak CÀ H bond (bond dissociation energy~80-90 kcal/mol) differ from the inert aliphatic and aromatic CÀ H positions with stronger CÀ H bonds. The recent advances in the direct activation of the benzylic position through the generation of C(sp 3) radicals have demonstrated the potential of electrochemistry and photochemistry as a means for constructing new chemical bonds. This review will cover the recent progress of benzylic CÀ H functionalization through organic radical strategies employing photochemistry and electrochemistry as sustainable tools. In addition, the mechanistic details of the typical reactions have been included which, in turn, will help the researchers to look at this promising area from a different perspective towards new discoveries and often hidden opportunities. 2.1. C(sp 3)À C Bond Formation 2.2. C(sp 2)À O and C(sp 3)À O Bond Formation 2.3. C(sp 3)À N Bond Formation 2.4. C(sp 3)À S Bond Formation 2.5. C(sp 3)À Halogen Bond Formation 3. Electrochemical Benzylic C(sp 3)À H Functionalization 3.1. C(sp 3)À C Bond Formation 3.2. C(sp 3)À O Bond Formation 3.3. C(sp 3)À N Bond Formation 3.4. C(sp 3)À P Bond Formation 3.5. C(sp 3)À Halogen Bond Formation 4. Benzylic C(sp 3)À H Functionalization via Synthetic Photoelectrochemistry 5. Conclusions and Outlook

Despite their prevalence in organic synthesis, the application of boronic acids (BAs) as alkyl radical precursors in visible-light-assisted photocatalyzed reactions has been limited by their high oxidation potential. This study demonstrates the prominent ability of amide solvents, namely, N,N-dimethylacetamide, to participate in hydrogen-bonding interactions with BAs, thus enabling the modulation of their oxidation potential toward the generation of alkyl radicals. The developed protocol is simple and robust and demonstrates broad applicability for alkylation, allylation, and elimination reactions in batch and continuous flow. The application toward dehydroalanine allows the synthesis of unnatural amino acids. Furthermore, the chemoselective generation of radical species from BAs in the presence of boronic ester-containing molecules is now feasible, endorsing plausible boron-selective (bio-) orthogonal modifications.

Summary Multicomponent reactions (MCRs) are ideal platforms for the generation of a wide variety of organic scaffolds in a convergent and atom-economical manner. Many strategies for the generation of highly substituted and diverse structures have been developed and among these, the Petasis reaction represents a viable reaction manifold for the synthesis of substituted amines via coupling of an amine, an aldehyde and a boronic acid (BA). Despite its synthetic utility, the inherent drawbacks associated with the traditional two-electron Petasis reaction have stimulated continuous research towards more facile and tolerant methodologies. In this regard, we present the use of free alkyl BAs as effective radical precursors in this MCR through a single-electron transfer mechanism under mild reaction conditions. We have further demonstrated its applicability to photo-flow reactors, facilitating the reaction scale-up for the rapid assembly of complex molecular structures.

A high-yielding fast spirocyclization of easily available indol ynones has been developed by applying N-iodosuccinimide. The formation of the desired product occurs in an atom-economical way, under mild conditions, instantly after the addition of the reagent. The expected 1,2-rearrangement was not observed. The procedure represents a metal free spirocyclization of indoles with an opportunity for further functionalizations.