Light promoted aqueous phase amine synthesis via three-component coupling reactions

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Synthesis of amino-diamondoid pharmacophores via photocatalytic C-H aminoalkylation

We report a direct C-H aminoalkylation reaction using two light-activated H-atom transfer catalyst systems that enable the introduction of protected amines to native adamantane scaffolds with C-C bond formation. The scope of adamantane and imine reaction partners is broad and deprotection provides versatile amine and amino acid building blocks. Using readily available chiral imines, the enantioselective synthesis of the saxagliptin core and rimantadine derivatives is also described.

Visible light induced 3-position-selective addition of arylpropiolic acids with ethers via C(sp3)-H functionalization

Although the 2-position-selective decarboxylative coupling or addition of arylpropiolic acids with cyclic ethers has been intensively investigated, selective functionalization of arylpropiolic acids at the 3-position is still a big challenge. Herein, an intriguing and mild method for visible light induced regioselective addition of arylpropiolic acids by attacking exclusively at the 3-position with cyclic/acyclic ethers was developed. A variety of 3,3-bis-substituted acrylic acids were successfully obtained in moderate to excellent yields. A plausible reaction mechanism involving an energy transfer induced radical addition in the presence of visible light and photocatalyst was proposed.

Radical Addition to N-Tosylimines via C-H Activation Induced by Decatungstate Photocatalyst

A method for the radical addition toward N-tosylimines based on light promoted C-H activation by tetrabutylammonium decatungstate is described. The reaction proceeds under irradiation using 400 nm light emitting diodes in conventional glassware. The method can be applied for alkylations (cycloalkanes, ethers, dimethylformamide) and acylation (primary aliphatic aldehydes) of both aromatic and aliphatic N-tosylimines.

Site-Specific Functionalization of 1,3-Dioxolane with Imines: A Radical Chain Approach to Masked α-Amino Aldehydes

A thiol-promoted site-specific addition of 1,3-dioxolane to imines through a radical chain process is described. This process represents a metal-free and redox-neutral way to convert inexpensive materials to a broad range of protected α-amino aldehydes in good to excellent yields using only a catalytic amount of radical precursor. Control experiments revealed that both the thiol and a small amount of oxygen from air are indispensable to the success of this reaction.

Transition metal-catalyzed C-H bond functionalization in multicomponent reactions: a tool toward molecular diversity

Transition metal-catalyzed C-H bond functionalization has numerous applications in organic synthesis as a powerful type of bond transformation. In particular, the combination of C-H functionalization with other types of chemical transformations in the manner of multicomponent reactions is an even more beneficial tool in the synthesis of small organic molecules because such reactions provide a platform for the rapid generation of high molecular diversity and complexity by making use of the advantages of both latent C-H bond transformation and the multicomponent reaction. Herein, we provide a review highlighting the research advances in the multicomponent reactions built upon transition metal-catalyzed C-H bond functionalization. The content spans from the reactions featuring the functionalization of C(sp³)-H, C(sp²)-H and C(sp)-H bonds over the last decade.

Direct α-heteroarylation of amides (α to nitrogen) and ethers through a benzaldehyde-mediated photoredox reaction

A photoredox reaction for cross-dehydrogenative coupling (CDC) was developed to Cα-arylate amides (α to nitrogen) and ethers using a variety of five- and six-membered electron-deficient heteroarenes. A unique decomposition mechanism of ammonium persulfate enhanced by photoexcited benzaldehydes was revealed. This benzaldehyde-mediated photoredox reaction proceeded smoothly with household 23 W CFL bulbs as the energy source under metal-free conditions, allowing the construction of new Csp2 -Csp2 and Csp3 -Csp2 bonds and access to important pharmacophores of broad utility using commercially available reagents.

UV-light induced domino type reactions: synthesis and photophysical properties of unreported nitrogen ring junction quinazolines

An expedient method for the synthesis of 5,6-dihydrobenzo[h][1,2,4]triazolo[5,1-b]quinazolines by UV light has been developed.

Radical addition to iminium ions and cationic heterocycles

Carbon-centered radicals represent highly useful reactive intermediates in organic synthesis. Their nucleophilic character is reflected by fast additions to electron deficient C=X double bonds as present in iminium ions or cationic heterocycles. This review covers diverse reactions of preformed or in situ-generated cationic substrates with various types of C-radicals, including alkyl, alkoxyalkyl, trifluoromethyl, aryl, acyl, carbamoyl, and alkoxycarbonyl species. Despite its high reactivity, the strong interaction of the radical's SOMO with the LUMO of the cation frequently results in a high regioselectivity. Intra- and intermolecular processes such as the Minisci reaction, the Porta reaction, and the Knabe rearrangement will be discussed along with transition metal and photoredox catalysis or electrochemical methods to generate the odd-electron species.