Acceptorless dehydrogenation of amines to nitriles catalyzed by N-heterocyclic carbene-nitrogen-phosphine chelated bimetallic ruthenium (II) complex

Ru-CNP Complex-Catalyzed Hydrogen Transfer/Annulation Reaction of 2-Nitrobenzylalcohol via an Outer-Sphere Mechanism

The development of efficient catalysts plays a central role in advancing chemical reactions. In this study, a ruthenium complex modified with a N-heterocyclic carbene-imine-phosphine ligand (CNP) was employed to enhance the hydrogen transfer/annulation process of 2-nitrobenzyl alcohols with various primary or secondary alcohols. The NMR and HRMS analyses of the reaction solution revealed the in situ formation of [fac-RuH(CO)(PPh3)(κ3-CN(H)P)]Cl through the transfer hydrogenation of the imine moiety within the CNP ligand under the reaction conditions. This species, a bifunctional Noyori-type ruthenium complex featuring facial coordination with a CN(H)P ligand, served as a key catalytic intermediate. By leveraging the outer-sphere mechanism facilitated by [fac-RuH(CO)(PPh3)(κ3-CN(H)P)]Cl, the synthesis of 75 quinolines from 2-nitrobenzyl alcohols and a wide range of alcohols has been achieved with yields as high as 95%.

Acceptorless oxidant-free dehydrogenation of amines catalyzed by Ru-hydride complexes of amide-acid/ester ligands

Traditional dehydrogenation of amines involves the transfer of hydrogen molecule(s) from a substrate to an acceptor. In acceptorless dehydrogenation, hydrogen gas is liberated without an oxidant, providing an efficient synthetic method. Acceptorless dehydrogenation of primary amines to nitriles without using an oxidant or hydrogen acceptor is significant yet challenging. Herein, we present efficient Ru-based catalysts capable of carrying out such a transformation with hydrogen gas as the only by-product. A new class of air and moisture-stable ruthenium-hydride complexes (1-4) of amide-acid/ester-based ligands have been synthesized and characterized. Crystal structures of two representative complexes, 2 and 3, illustrate the bidentate N-O coordination mode of the ligands. At the same time, additional binding sites are occupied by one hydride, one CO, and two PPh3 co-ligands. The catalytic behavior of these complexes is explored towards the oxidant-free, acceptorless, and selective dehydrogenation of primary and secondary amines affording nitriles and imines, respectively. Among four Ru(II) complexes, complex 2 showed the best catalytic activity for the dehydrogenation of amines. A wide variety of both primary and secondary amines were utilized to explore the substrate scope. The catalytic system tolerated both electron-withdrawing and electron-releasing substituents on amine substrates. Various control experiments and mechanistic studies were carried out to support the dehydrogenation of amines by using complex 2 as a representative catalyst.

Ruthenium-Catalyzed Synthesis of 2-Pyrazolines via Acceptorless Dehydrogenative Coupling of Allylic Alcohols with Hydrazines

Described herein is the synthesis of 2-pyrazolines via acceptorless dehydrogenative coupling of allylic alcohols with hydrazines based on a Ru3(CO)12/NHC-phosphine-phosphine ligand L catalytic system. The reaction not only exhibits low catalyst loading (only 0.3 mol %), wide substrate scope, good to excellent yields, and high selectivity but also omits the use of sacrificial hydrogen acceptor with only H2 and H2O as byproducts, making the reaction green and atom-economical.

Cobalt-Catalyzed Acceptorless Dehydrogenation of Primary Amines to Nitriles

The direct double dehydrogenation from primary amines to nitriles without an oxidant or hydrogen acceptor is both intriguing and challenging. In this paper, we describe a non-noble metal catalyst capable of realizing such a transformation with high efficiency. A cobalt-centered N,N-bidentate complex was designed and employed as a metal-ligand cooperative dehydrogenation catalyst. Detailed kinetic studies, control experiments, and DFT calculations revealed the crucial hydride transfer, proton transfer, and hydrogen evolution processes. Finally, a tandem outer-sphere/inner-sphere mechanism was proposed for the dehydrogenation of amines to nitriles through an imine intermediate.

Acceptorless Dehydrogenation of Aliphatics, Amines, and Alcohols with Homogeneous Catalytic Systems

The dehydrogenation of saturated substrates is fundamentally essential for producing value-added unsaturated organic molecules both in academia and industry. In recent years, homogeneously catalyzed acceptorless C–C, C–N, and C–O bond desaturations have attracted increasing attention due to high atom economy, environmentally benign nature, and wide availability of the starting materials. This short review discusses the acceptorless dehydrogenation of aliphatics, alcohols, and amines by homogeneous catalytic systems based on two categories of reaction mechanisms: thermal transition-metal-catalyzed two-electron pathway and photoredox catalyzed or electrochemically driven one-electron pathway.

1 Introduction

2 Catalytic Acceptorless Dehydrogenation of Aliphatics

3 Catalytic Acceptorless Dehydrogenation of Amines

4 Catalytic Acceptorless Dehydrogenation of Alcohols

5 Conclusion

Ruthenium-Catalyzed Divergent Acceptorless Dehydrogenative Coupling of 1,3-Diols with Arylhydrazines: Synthesis of Pyrazoles and 2-Pyrazolines

Herein, the divergent transformations of 1,3-diols with arylhydrazines via acceptorless dehydrogenative coupling reactions to selectively synthesize pyrazoles and 2-pyrazolines were reported, which were based on Ru₃(CO)₁₂/NHC-phosphine-phosphine catalytic systems. The reactions featured low catalyst loading, high selectivity, wide substrate scope, and good yields, with only water and hydrogen gas (H₂) as the byproducts.