Visible-Light-Initiated Manganese Catalysis for C−H Alkylation of Heteroarenes: Applications and Mechanistic Studies

Manganese-Catalyzed C(sp2)-H Alkylation of Indolines and Arenes with Unactivated Alkyl Bromides

Selective C(sp 2 ) - H bond alkylation of indoline, carbazole and (2-pyridinyl)arenes with unactivated alkyl bromides is achieved using MnBr 2 catalyst in the absence of an external ligand. The alkylation uses a simple LiHMDS base and avoids the necessity of Grignard reagent, unlike other Mn-catalyzed C - H functionalization. This reaction proceeded either through a five- or a less-favored six-membered metallacycle, and tolerated diverse functionalities, including alkenyl, alkynyl, silyl, aryl ether, pyrrolyl, indolyl, carbazolyl and alkyl bearing fatty alcohol and polycyclic-steroid moieties. Alkylation follows a single electron transfer (SET) pathway involving 1e oxidative addition of alkyl bromide and a rate-limiting C-H metalation.

Visible-Light-Induced Palladium-Catalyzed Dehydrogenative Carbonylation of Amines to Oxalamides

The palladium-catalyzed oxidative carbonylation of amines toward the synthesis of oxalamides has been established around 30 years ago and it usually needs the presence of (over)stoichiometric amounts of oxidant. In this work, the first transformation of this type in which the oxidant was replaced by visible light is described. The new approach uses a simple robust Pd complex, which can even be partially recycled. A mechanistic reason is provided and supported by control experiments and EPR studies, showing that Pd^I was formed and Pd⁰ was the active species. Both nitrogen- and the intermediate acyl radical can be detected. Moreover, the formation of hydrogen was confirmed by gas GC.

Light- and Manganese-Initiated Borylation of Aryl Diazonium Salts: Mechanistic Insight on the Ultrafast Time-Scale Revealed by Time-Resolved Spectroscopic Analysis

Manganese-mediated borylation of aryl/heteroaryl diazonium salts emerges as a general and versatile synthetic methodology for the synthesis of the corresponding boronate esters. The reaction proved an ideal testing ground for delineating the Mn species responsible for the photochemical reaction processes, that is, involving either Mn radical or Mn cationic species, which is dependent on the presence of a suitably strong oxidant. Our findings are important for a plethora of processes employing Mn-containing carbonyl species as initiators and/or catalysts, which have considerable potential in synthetic applications.

Regioselective Hydroalkylation and Arylalkylation of Alkynes by Photoredox/Nickel Dual Catalysis: Application and Mechanism

Alkynes are an important class of organic molecules due to their utility as versatile building blocks in synthesis. Although efforts have been devoted to the difunctionalization of alkynes, general and practical strategies for the direct hydroalkylation and alkylarylation of terminal alkynes under mild reaction conditions are less explored. Herein, we report a photoredox/nickel dual-catalyzed anti-Markovnikov-type hydroalkylation of terminal alkynes as well as a one-pot arylalkylation of alkynes with alkyl carboxylic acids and aryl bromides via a three-component cross-coupling. The results indicate that the transformations proceed via a new mechanism involving a single-electron transfer with subsequent energy-transfer activation pathways. Moreover, steady-state and time-resolved fluorescence-spectroscopy measurements, density functional theory (DFT) calculations, and wavefunction analysis have been performed to give an insight into the catalytic cycle.

Generation and Reactivity of Amidyl Radicals: Manganese-Mediated Atom-Transfer Reaction

A simple and efficient protocol to generate amidyl radicals from amine functionalities through a manganese-mediated atom-transfer reaction has been developed. This approach employs an earth-abundant and inexpensive manganese complex, Mn₂ (CO)₁₀ , as the catalyst and visible light as the energy input. Using this strategy, site-selective chlorination of unactivated C(sp³ )-H bonds of aliphatic amines and intramolecular/intermolecular chloroaminations of unactivated alkenes were readily realized under mild reaction conditions, thus providing efficient access to a range of synthetically valuable alkyl chlorides, chlorinated pyrrolidines, and vicinal chloroamine derivatives. These practical reactions exhibit a broad substrate scope and tolerate a wide array of functional groups, and complex molecules including various marketed drug derivatives.

Continuous Visible-Light Photoflow Approach for a Manganese-Catalyzed (Het)Arene C-H Arylation

Manganese photocatalysts enabled versatile room-temperature C-H arylation reactions by means of continuous visible-light photoflow, thus allowing for efficient C-H arylations in 30 minutes with ample scope. The robustness of the manganese-catalyzed photoflow strategy was shown by visible light-induced gram-scale synthesis, clearly outperforming the batch performance.

A Desulfurative Strategy for the Generation of Alkyl Radicals Enabled by Visible-Light Photoredox Catalysis

Herein, we present a new desulfurative method for generating primary, secondary, and tertiary alkyl radicals through visible-light photoredox catalysis. A process that involves the generation of N-centered radicals from sulfinamide intermediates, followed by subsequent fragmentation, is critical to forming the corresponding alkyl radical species. This strategy has been successfully applied to conjugate addition reactions that features mild reaction conditions, broad substrate scope (>60 examples), and good functional-group tolerance.