Alkyl Formates as Transfer Hydroalkylation Reagents and Their Use in the Catalytic Conversion of Imines to Alkylamines

Easily accessible via a simple esterification of alcohols with formic acid, alkyl formates are used as a novel class of transfer hydroalkylation reagents, CO₂ acting as a traceless linker. As a proof-of-concept, their reactivity in the transfer hydroalkylation of imines is investigated, using a ruthenium-based catalyst and LiI as promoter to cleave the C-O σ-bond of the formate scaffold. Providing tertiary amines, the reaction displays a divergent regioselectivity compared to previously reported transfer hydroalkylation strategies.

The Nickel Age in Synthetic Dual Photocatalysis: A Bright Trip Toward Materials Science

Recently, the field of dual photocatalysis has grown rapidly, to become one of the most powerful tools for the functionalization of organic molecules under mild conditions. In particular, the merging of Earth-abundant nickel-based catalytic systems with visible-light-activated photoredox catalysts has allowed the development of a number of unique green synthetic approaches. This goes in the direction of ensuring an effective and sustainable chemical production, while safeguarding human health and environment. Importantly, this relatively new branch of catalysis has inspired an interdisciplinary stream of research that spans from inorganic and organic chemistry to materials science, thus establishing itself as one dominant trend in modern organic synthesis. This Review aims at illustrating the milestones on the timeline evolution of the photocatalytic systems used, with a critical analysis toward novel applications based on the use of photoactive two-dimensional carbon-based nanostructures. Lastly, forward-looking opportunities within this intriguing research field are discussed.

Covalent Organic Frameworks Enabling Site Isolation of Viologen-Derived Electron-Transfer Mediators for Stable Photocatalytic Hydrogen Evolution

Electron transfer is the rate-limiting step in photocatalytic water splitting. Viologen and its derivatives are able to act as electron-transfer mediators (ETMs) to facilitate the rapid electron transfer from photosensitizers to active sites. Nevertheless, the electron-transfer ability often suffers from the formation of a stable dipole structure through the coupling between cationic-radical-containing viologen-derived ETMs, by which the electron-transfer process becomes restricted. Herein, cyclic diquats, a kind of viologen-derived ETM, are integrated into a 2,2'-bipyridine-based covalent organic framework (COF) through a post-quaternization reaction. The content and distribution of embedded diquat-ETMs are elaborately controlled, leading to the favorable site-isolated arrangement. The resulting materials integrate the photosensitizing units and ETMs into one system, exhibiting the enhanced hydrogen evolution rate (34600 μmol h^-1  g^-1 ) and sustained performances when compared to a single-module COF and a COF/ETM mixture. The integration strategy applied in a 2D COF platform promotes the consecutive electron transfer in photochemical processes through the multi-component cooperation.

Photoactive electron donor-acceptor complex platform for Ni-mediated C(sp3)-C(sp2) bond formation

A dual photochemical/nickel-mediated decarboxylative strategy for the assembly of C(sp3)-C(sp2) linkages is disclosed. Under light irradiation at 390 nm, commercially available and inexpensive Hantzsch ester (HE) functions as a potent organic photoreductant to deliver catalytically active Ni(0) species through single-electron transfer (SET) manifolds. As part of its dual role, the Hantzsch ester effects a decarboxylative-based radical generation through electron donor-acceptor (EDA) complex activation. This homogeneous, net-reductive platform bypasses the need for exogenous photocatalysts, stoichiometric metal reductants, and additives. Under this cross-electrophile paradigm, the coupling of diverse C(sp3)-centered radical architectures (including primary, secondary, stabilized benzylic, α-oxy, and α-amino systems) with (hetero)aryl bromides has been accomplished. The protocol proceeds under mild reaction conditions in the presence of sensitive functional groups and pharmaceutically relevant cores.

Using the Thiyl Radical for Aliphatic Hydrogen-Atom Transfer: Thiolation of Unactivated C-H Bonds

A metal- and catalyst-free thiyl-radical-mediated activation of alkanes is described. Tetrafluoropyridinyl disulfide is used to perform thiolation of the C-H bonds under irradiation with 400 nm light-emitting diodes. The key C-H activation step is believed to proceed via hydrogen-atom abstraction effected by the fluorinated thiyl radical. Secondary, tertiary, and heteroatom-substituted C-H bonds can be involved in the thiolation reaction. The resulting sulfides have wide potential as photoredox-active radical precursors in reactions with alkenes and heteroarenes.

Photoredox-Catalyzed Addition of Carbamoyl Radicals to Olefins: A 1,4-Dihydropyridine Approach

Functionalization with C1-building blocks are key synthetic methods in organic synthesis. The low reactivity of the most abundant C1 -molecule, carbon dioxide, makes alternative carboxylation reactions with CO2 -surrogates especially important. We report a photoredox-catalyzed protocol for alkene carbamoylations. Readily accessible 4-carboxamido-Hantzsch esters serve as convenient starting materials that generate carbamoyl radicals upon visible light-mediated single-electron transfer. Addition to various alkenes proceeded with high levels of regio- and chemoselectivity.

Dual Benzophenone/Copper-Photocatalyzed Giese-Type Alkylation of C(sp3 )-H Bonds

Photocatalyzed Giese-type alkylations of C(sp³ )-H bonds are very attractive reactions in the context of atom-economy in C-C bond formation. The main limitation of such reactions is that when using highly polymerizable olefin acceptors, such as unsubstituted acrylates, acrylonitrile, or methyl vinyl ketone, radical polymerization often becomes the dominant or exclusive reaction pathway. Herein, we report that the polymerization of such olefins is strongly limited or suppressed when combining the photocatalytic activity of benzophenone (BP) with a catalytic amount of Cu(OAc)₂ . Under mild and operationally simple conditions, the Giese adducts resulting from the C(sp³ )-H functionalization of amines, alcohols, ethers, and cycloalkanes could be synthesized. Preliminary mechanistic studies have revealed that the reaction does not proceed through a radical chain, but through a dual BP/Cu photocatalytic process, in which both Cu^II and low-valent Cu^I/0 species, generated in situ by reduction by the BP ketyl radical, may react with α-keto or α-cyano intermediate radicals, thus preventing polymerization.

Visible-Light-Driven Intermolecular Reductive Ene-Yne Coupling by Iridium/Cobalt Dual Catalysis for C(sp3 )-C(sp2 ) Bond Formation

A new methodology to form C(sp3 )-C(sp2 ) bonds by visible-light-driven intermolecular reductive ene-yne coupling has been successfully developed. The process relies on the ability of the Hantzsch ester to contribute in both SET and HAT processes through a unified cobalt and iridium catalytic system. This procedure avoids the use of stoichiometric amounts of reducing metallic reagents, which is translated into high functional-group tolerance and atom economy.

Photochemical C-H Hydroxyalkylation of Quinolines and Isoquinolines

We report herein a visible light-mediated C-H hydroxyalkylation of quinolines and isoquinolines that proceeds via a radical path. The process exploits the excited-state reactivity of 4-acyl-1,4-dihydropyridines, which can readily generate acyl radicals upon blue light absorption. By avoiding the need for external oxidants, this radical-generating strategy enables a departure from the classical, oxidative Minisci-type pattern and unlocks a unique reactivity, leading to hydroxyalkylated heteroarenes. Mechanistic investigations provide evidence that a radical-mediated spin-center shift is the key step of the process. The method's mild reaction conditions and high functional group tolerance accounted for the late-stage functionalization of active pharmaceutical ingredients and natural products.

Photochemical Asymmetric Nickel-Catalyzed Acyl Cross-Coupling

Photochemical enantioselective nickel-catalyzed cross-coupling reactions are difficult to implement. We report a visible-light-mediated strategy that successfully couples symmetrical anhydrides and 4-alkyl dihydropyridines (DHPs) to afford enantioenriched α-substituted ketones under mild conditions. The chemistry does not require exogenous photocatalysts. It is triggered by the direct excitation of DHPs, which act as a radical source and as a reductant, facilitating the turnover of the chiral catalytic nickel complex.

Carboamination of Unactivated Alkenes through Three-Component Radical Conjugate Addition

Two-component Giese type radical additions are highly practical and established reactions. Herein, three-component radical conjugate additions of unactivated alkenes to Michael acceptors are reported. Amidyl radicals, oxidatively generated from α-amido oxy acids using redox catalysis, act as the third reaction component which add to the unactivated alkenes. The adduct radicals engage in Giese type additions to Michael acceptors to provide, after reduction, the three-component products in an overall alkene carboamination reaction. Transformations which can be conducted under practical mild conditions feature high functional group tolerance and broad substrate scope.