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 de...

Borrowing Hydrogen-Mediated N‑Alkylation Reactionsby a Well-Defined Homogeneous Nickel Catalyst

A simple metal-ligand cooperative approach for the dehydrogenative functionalization of alcohols to various substituted quinolines and quinazolin-4(3H)-ones under relatively mild reaction conditions (≤90 °C) is reported. Simple and easy-to-prepare air-stable Cu(II) complexes featuring redox-active azo-aromatic scaffolds, 2-arylazo-(1,10-phenanthroline) (L1,2), are used as catalyst. A wide variety of substituted quinolines and quinazolin-4(3H)-ones were synthesized in moderate to good isolated yields via dehydrogenative coupling reactions of various inexpensive and easily available starting materials under aerobic conditions. A few control experiments and deuterium labeling studies were carried out to understand the mechanism of the dehydrogenative coupling reactions, which indicate that both copper and the coordinated azo-aromatic ligand participate in a cooperative manner during the catalytic cycle.

Metal-Ligand Cooperative Approach To Achieve Dehydrogenative Functionalization of Alcohols to Quinolines and Quinazolin-4(3 H )-ones under Mild Aerobic Conditions.

Synthetic and medicinal perspective of quinolines as antiviral agents.

A simple and efficient approach of C-S cross-coupling of a wide variety of (hetero)aryl thiols and (hetero)aryl halides under mild conditions, mostly at room temperature, catalyzed by well-defined singlet diradical Ni(II) catalysts bearing redox noninnocent ligands is reported. Taking advantage of ligand centered redox events, the high-energetic Ni(0)/Ni(II) or Ni(I)/Ni(III) redox steps were avoided in the catalytic cycle. The cooperative participation of both nickel and the coordinated ligands during oxidative addition/reductive elimination steps allowed us to perform the catalytic reactions under mild conditions.

Achieving Nickel Catalyzed C-S Cross-Coupling under Mild Conditions Using Metal-Ligand Cooperativity.

Direct synthesis of N-heterocycles via the acceptorless dehydrogenative coupling is very challenging and scarcely reported under 3d transition-metal catalysis. Here, we have developed an efficient Mn(I)-catalyzed sustainable synthesis of various quinoxalines from 1,2-diaminobenzenes and 1,2-diols via the acceptorless dehydrogenative coupling reaction. Further, this strategy was successfully applied for the unprecedented synthesis of quinazolines by the reaction of 2-aminobenzyl alcohol with primary amides. The present protocol provides an atom-economical and sustainable route for the synthesis of various quinoxaline and quinazoline derivatives by employing an earth-abundant manganese salt and simple phosphine-free NNN-tridentate ligand.

Manganese(I)-Catalyzed Sustainable Synthesis of Quinoxaline and Quinazoline Derivatives with the Liberation of Dihydrogen

Simple, straightforward, and atom economic methods for the synthesis of quinolines, 2-aminoquinolines, and quinazolines via biomimetic dehydrogenative condensation/coupling reactions, catalyzed by well-defined inexpensive and easy to prepare singlet diradical Ni(II)-catalysts featuring two antiferromagnetically coupled singlet diradical diamine type ligands are described. Various polysubstituted quinolines, 2-aminoquinolines, and quinazolines were synthesized in moderate to good yields from different low-cost and readily accessible starting materials. Several control experiments were carried out to get insight into the reaction mechanism which shows that the nickel and the coordinated diamine ligands participate in a synergistic way during the dehydrogenation of alcohols.

Seemika Banerjee

Papers

Dehydrogenative Synthesis of Quinolines, 2-Aminoquinolines, and Quinazolines Using Singlet Diradical Ni(II)-Catalysts