- Nickel-catalysed chemoselective C-3 alkylation of indoles with alcohols through a borrowing hydrogen method
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An inexpensive, air-stable, isolable nickel catalyst is reported that can perform chemoselective C3-alkylation of indoles with a variety of alcohols following "borrowing hydrogen". A one-pot, cascade C3-alkylation starting from 2-aminophenyl ethyl alcohols, and thus obviating the need for pre-synthesized indoles, further adds to the broad scope of this method. The reaction is radical-mediated, and is significantly different from other examples, often dictated by metal-ligand bifunctionality. This journal is
- Adhikari, Debashis,Bains, Amreen K.,Biswas, Ayanangshu
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supporting information
p. 15442 - 15445
(2020/12/25)
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- Homogeneous Nickel-Catalyzed Sustainable Synthesis of Quinoline and Quinoxaline under Aerobic Conditions
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Dehydrogenative coupling-based reactions have emerged as an efficient route toward the synthesis of a plethora of heterocyclic rings. Herein, we report an efficacious, nickel-catalyzed synthesis of two important heterocycles such as quinoline and quinoxaline. The catalyst is molecularly defined, is phosphine-free, and can operate at a mild reaction temperature of 80 °C. Both the heterocycles can be easily assembled via double dehydrogenative coupling, starting from 2-aminobenzyl alcohol/1-phenylethanol and diamine/diol, respectively, in a shorter span of reaction time. This environmentally benign synthetic protocol employing an inexpensive catalyst can rival many other transition-metal systems that have been developed for the fabrication of two putative heterocycles. Mechanistically, the dehydrogenation of secondary alcohol follows clean pseudo-first-order kinetics and exhibits a sizable kinetic isotope effect. Intriguingly, this catalyst provides an example of storing the trapped hydrogen in the ligand backbone, avoiding metal-hydride formation. Easy regeneration of the oxidized form of the catalyst under aerobic/O2 oxidation makes this protocol eco-friendly and easy to handle.
- Bains, Amreen K.,Singh, Vikramjeet,Adhikari, Debashis
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p. 14971 - 14979
(2020/11/30)
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- Borrowing Hydrogen-Mediated N-Alkylation Reactions by a Well-Defined Homogeneous Nickel Catalyst
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We report herein a well-defined and bench-stable azo-phenolate ligand-coordinated nickel catalyst which can efficiently execute N-alkylation of a variety of anilines by alcohol. We demonstrate that the redox-active azo ligand can store hydrogen generated during alcohol oxidation and redelivers the same to an in-situ-generated imine bond to result in N-alkylation of amines. The reaction has wide scope, and a large array of alcohols can directly couple to a variety of anilines. Mechanistic studies including deuterium labeling to the substrate establishes the borrowing hydrogen method from alcohols and pinpoints the crucial role of the redox-active azo moiety present on the ligand backbone. Isolation of the ketyl intermediate in its trapped form with a radical quencher and higher kH/kD for the alcohol oxidation step suggest altogether a hydrogen-atom transfer (HAT) to the reduced azo backbone to pave alcohol oxidation as opposed to the conventional metal-ligand bifunctional mechanism. This example clearly demonstrates that an inexpensive base metal catalyst can accomplish an important coupling reaction with the help of a redox-active ligand backbone.
- Bains, Amreen K.,Kundu, Abhishek,Yadav, Sudha,Adhikari, Debashis
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p. 9051 - 9059
(2019/10/02)
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- New metal chelates with sterically hindered azo ligands: Synthesis and physicochemical properties
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Metal chelates based on 3,5-di-(tert-butyl)-2-hydroxyazobenzene are synthesized and their physicochemical properties were studied. Four alternative structures and isomeric transformations between these structures were suggested on the basis of NMR data, electric dipole and magnetic moments, X-ray diffraction and quantum-chemical data. The change in stereochemistry of a complex through introduction of tert-butyl substituents was found to give unusual (for metal chelates with azo ligands (MN2O2)) dome-shaped isomers with the trans-planar coordination core at a vertex or with a distorted cis-structure.
- Kogan,Lyubchenko,Shcherbakov,Ionov,Tkachev,Shilov,Aldoshin
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p. 533 - 540
(2008/10/09)
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