N-Demethylation of trialkylamines is a useful transformation, but typically requires harsh reaction conditions and stepwise procedures, as well as judicious protection of labile functional groups. Herein we report a mild, catalytic approach for the demethylation of trialkylamines by utilizing photoinduced nickel catalysis wherein C(sp2)-bromides serve as hydrogen-atom transfer (HAT) reagents. This method achieves direct demethylation of trialkylamines with wide functional group compatibility, making it highly suitable for late-stage derivatization of complex molecules. Mechanistic investigations provide evidence that C(sp2) radicals generated via photoinduced Ni-C(sp2) bond homolysis are involved in hydrogen atom abstraction from trialkylamines. Utilizing steric control of the C(sp2)-bromides, our HAT approach achieves demethylation with excellent site selectivity in the presence of benzyl-substituted amines, which is complementary to the selectivity of classical approaches that afford debenzylation product instead.

Photoinduced Nickel-Catalyzed Selective N-Demethylation of Trialkylamines Using C(sp2)-Bromides as HAT Reagents.

The merger of photocatalysis and transition-metal catalysis has recently emerged as an adaptable platform for the development of innovative and environmentally benign synthetic methodologies. In contrast to classical transformation by Pd complexes, photoredox Pd catalysis operates through a radical pathway in the absence of a radical initiator. Using the synergistic merger of photoredox and Pd catalysis, we have developed a highly efficient, regioselective, and general meta-oxygenation protocol for diverse arenes under mild reaction conditions. The protocol showcases the meta-oxygenation of phenylacetic acids and biphenyl carboxylic acids/alcohols and is also amenable for a series of sulfonyls and phosphonyl-tethered arenes, irrespective of the nature and position of the substituents. Unlike thermal C–H acetoxylation which operates through the PdII/PdIV catalytic cycle, this metallaphotocatalytic C–H activation involves PdII/PdIII/PdIV intermediacy. The radical nature of the protocol is established through radical quenching experiments and EPR analysis of the reaction mixture. Furthermore, the catalytic path of this photoinduced transformation is established through control reactions, absorption spectroscopy, luminescence quenching, and kinetic studies.

https://pubs.acs.org/doi/pdf/10.1021/jacsau.3c00231

Photoinduced meta-Selective C–H Oxygenation of Arenes

We report the use of dual nickel photocatalysis in the synthesis of O-aryl carbamates from aryl iodides or bromides, amines, and carbon dioxide. The reaction proceeded in visible light, at ambient carbon dioxide pressure, and without stoichiometric activating reagents. Mechanistic analysis is consistent with a Ni(I–III) cycle, where the active species is generated by the photocatalyst. The rate-limiting steps were the photocatalyst-mediated reduction of Ni(II) to Ni(I) and subsequent oxidative addition of the aryl halide. The physical properties of the photocatalyst were critical for promoting formation of O-aryl carbamates over various byproducts. Nine new phthalonitrile photocatalysts were synthesized, which exhibited properties that were vital to achieve high selectivity and activity.

Dual Nickel Photocatalysis for O-Aryl Carbamate Synthesis from Carbon Dioxide

Abstract This report describes the synthesis of a new NNSe pincer ligand and its mono‐ and dinuclear palladium(II) pincer complexes. In the absence of a base, a dinuclear palladium pincer complex ( C1 ) was isolated, while in the presence of Et 3 N base a mononuclear palladium pincer complex ( C2 ) was obtained. The new ligand and complexes were characterized using techniques like 1 H, 13 C{ 1 H} nuclear magnetic resonance (NMR), fourier transform infrared (FTIR), high‐resolution mass spectrometry (HRMS), ultraviolet‐visible (UV‐Visible), and cyclic voltammetry. Both the complexes showed pincer coordination mode with a distorted square planar geometry. The complex C1 has two pincer ligands attached through a Pd−Pd bond in a dinuclear pincer fashion. The air and moisture‐insensitive, thermally robust palladium pincer complexes were used as the catalyst for decarboxylative direct C−H heteroarylation of (hetero)arenes. Among the complexes, dinuclear pincer complex C1 showed better catalytic activity. A variety of (hetero)arenes were successfully activated (43–87 % yield) using only 2.5 mol % of catalyst loading under mild reaction conditions. The PPh 3 and Hg poisoning experiments suggested a homogeneous nature of catalysis. A plausible reaction pathway was proposed for the dinuclear palladium pincer complex catalyzed decarboxylative C−H bond activation reaction of (hetero)arenes. 

Mono and Dinuclear Palladium Pincer Complexes of NNSe Ligand as a Catalyst for Decarboxylative Direct C-H Heteroarylation of (Hetero)arenes.

Sunlight Assisted Photocatalytic Valorization of Lignin Using Recyclable Light Harvesters

Amide bonds are incredibly important fundamental entity which connect the amino acids in proteins and exist as a prevalent structural motif in biomolecules. Herein, we have exploited the concept of cross-electrophile coupling by merging the photo-redox and transition metal catalysis to construct carbamides from superabundant (hetero)aryl halides along with commercially feasible carbamoyl chlorides. The success of this protocol relies on the prior formation of NiII-aryl halide intermediate, which involves in a photoexcited Ni-halide homolysis event by energy transfer from aryl bromide and single electron transfer from aryl chloride to assist in generation of the vital carbamoyl radical. The breadth of application of this technique is significantly demonstrated both in inter- as well as intramolecular fashion for the synthesis of a plethora of (hetero)aryl carbamides with diverse functionalities, and biologically important benzolactams.

Photo-Excited Nickel-Catalyzed Silyl-Radical-Mediated Direct Activation of Carbamoyl Chlorides To Access (Hetero)aryl Car-bamides.

Transition metal pincer complexes: A series of potential catalysts in C H activation reactions

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Photo-Excited Nickel-Catalyzed Silyl-Radical-Mediated Direct Activation of Carbamoyl Chlorides To Access (Hetero)aryl Car-bamides.

Transition metal pincer complexes: A series of potential catalysts in C H activation reactions