Visible‐Light‐Induced Manganese‐Catalyzed Reactions: Present Approach and Future Prospects

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Near-Infrared-Light-Induced Iron(I) Dimer-Enabled Radical Cascade Reactions of Fluoroalkyl Bromides for the Synthesis of Ring-Fused Quinazolinones

The use of an earth-abundant and inexpensive iron complex as a catalyst, coupled with near-infrared (NIR) light as the energy source, for radical reactions with alkyl halides has been far less developed. In this study, we report NIR light-mediated iron(I) dimer-catalyzed radical cascade reactions of fluoroalkyl bromides for the synthesis of ring-fused quinazolinones bearing a difluoromethyl group. In this process, the 3-bromo-1,10-phenanthroline ligand facilitates the reactivity of [CpFe(CO)2]2, thereby improving the efficiency of the reaction.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Transition-metal free C-N bond formation from alkyl iodides and diazonium salts via halogen-atom transfer

Construction of C-N bond continues to be one part of the most significant goals in organic chemistry because of the universal applications of amines in pharmaceuticals, materials and agrochemicals. However, E2 elimination through classic SN2 substitution of alkyl halides lead to generation of alkenes as major side-products. Thus, formation of a challenging C(sp3)-N bond especially on tertiary carbon center remains highly desirable. Herein, we present a practical alternative to prepare primary, secondary and tertiary alkyl amines with high efficiency between alkyl iodides and easily accessible diazonium salts. This robust transformation only employs Cs2CO3 promoting halogen-atom transfer (XAT) process under transition-metal-free reaction conditions, thus providing a rapid method to assemble diverse C(sp3)-N bonds. Moreover, diazonium salts served as alkyl radical initiator and amination reagent in the reaction. Mechanism studies suggest this reaction undergo through halogen-atom transfer process to generate active alkyl radical which couples with diazonium cations to furnish final products.

Manganese-Catalyzed Chemoselective Hydrosilylation of Nitroarenes: Sustainable Route to Aromatic Amines

Herein we report efficient catalytic hydrosilylations of nitroarenes to form the corresponding aromatic amines using a well-defined manganese(II)-NNO pincer complex with a low catalyst loading (1 mol %) under solvent-free conditions. This base-metal-catalyzed hydrosilylation is an easy and sustainable alternative to classical hydrogenation. A large variety of nitroarenes bearing various functionalities were selectively transformed into the corresponding aromatic amines in good yields. The potential utility of the present catalytic protocol was demonstrated by the preparation of commercial drug molecules.

Visible-light-induced Mn(0)-catalyzed direct C-3 mono-, di- and perfluoroalkylation reactions of 2H-indazoles

A general and efficient method for visible-light-driven fluoroalkylation, such as difluoromethylphosphonation, difluoroacetamidation, monofluoromethylation, difluoromethylation, and perfluoroalkyalation, of 2H-indazoles using an inexpensive Mn₂(CO)₁₀ photocatalyst has been developed. The present methodology affords a new series of C-3 fluoroalkylated 2H-indazole derivatives with wide functional group tolerance in good to excellent yields. Difluoromethylenated indiazoles are also prepared from difluoroester derivatives. Our mechanistic investigations support a radical pathway for the reaction.

Mn2(CO)10 and UV light: a promising combination for regioselective alkene hydrosilylation at low temperature

The low temperature regioselective hydrosilylation of various alkenes with (1,1,1,3,5,5,5-heptamethyltrisiloxane) MD^HM is described using Mn₂(CO)₁₀ under UV irradiation with Mn loadings as low as 1 mol%, in the absence of additives and with excellent selectivity and yields. The generation of a manganese radical allowed the anti-Markovnikov hydrosilylation products to be selectively obtained in yields up to 99%.

Visible Light-Induced Transition Metal Catalysis

In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

Rapid Construction of Polysubstituted “Caged” Oxa-Bishomocubane Framework from Vinylidenecyclopropanes through a Sequential Dual Catalysis of Copper(I) and Visible-Light-Induced Photosensitization

This context describes a sequential dual catalytic transformation involving copper(I)-catalyzed cyclization/isomerization/migration-dimerization and visible-light photo-induced intramolecular [2+2] cycloaddition of vinylidenecyclopropanes for the rapid construction of polysubstituted “caged” oxa-bishomocubane products. The reaction...

Manganese-Mediated Direct Functionalization of Hantzsch Esters with Alkyl Iodides via an Aromatization-Dearomatization Strategy

We report, for the first time, manganese-mediated direct functionalization of the Hantzsch esters with readily accessible alkyl iodides through an aromatization-dearomatization strategy. Applying this protocol, a library of valuable 4-alkyl-1,4-dihydropyridines were facilely afforded in good yields. This simple and practical reaction proceeds under visible-light irradiation at room temperature and displays high functional-group compatibility. Additionally, the method is applicable for gram-scale synthesis and late-stage functionalization of complex molecules.