Two‐Dimensional Metal‐Organic Layers for Electrochemical Acceptorless Dehydrogenation of N‐Heterocycles

The catalytic acceptorless dehydrogenation (CAD) is an attractive synthetic route to unsaturated compounds because of its high atomic efficiency. Here we report electrochemical acceptorless dehydrogenation of N‐heterocycles to obtain quinoline or indole derivatives using metal‐organic layer (MOL) catalyst. MOL is the two‐dimensional version of metal‐organic frameworks (MOF), and it can be constructed on conductive multi‐walled carbon nanotubes via facile solvothermal synthesis to overcome the conductivity constraint for MOFs in electrocatalysis. TEMPO‐OPO₃^2? was incorporated into the system through a ligand exchange with capping formate on the MOL surface to serve as the active catalytic centers. The hybrid catalyst is efficient in the organic conversion and can be readily recycled and reused.     

Metal‐Free Dehydrogenation of N‐Heterocycles by Ternary h‐BCN Nanosheets with Visible Light

An efficient metal‐free catalytic system has been developed based on hexagonal boron carbon nitride (h‐BCN) nanosheets for the dehydrogenation of N‐heterocycles with visible light; hydrogen gas is released in the process, and thus no proton acceptor is needed. This acceptorless dehydrogenation of hydroquinolines, hydroisoquinolines, and indolines to the corresponding aromatic N‐heterocycles occurred in excellent yield under visible‐light irradiation at ambient temperature. With h‐BCN as the photocatalyst and water as the solvent, this environmentally benign protocol shows broad substitution tolerance and high efficiency.     

Acceptorless Dehydrogenation of N‐Heterocycles by Merging Visible‐Light Photoredox Catalysis and Cobalt Catalysis

Herein, the first acceptorless dehydrogenation of tetrahydroquinolines (THQs), indolines, and other related N‐heterocycles, by merging visible‐light photoredox catalysis and cobalt catalysis at ambient temperature, is described. The potential applications to organic transformations and hydrogen‐storage materials are demonstrated. Primary mechanistic investigations indicate that the catalytic cycle occurs predominantly by an oxidative quenching pathway.     

Spontaneous Metal‐Free Transfer Hydrogenation of Iminoboranes with Ammonia Borane and Amine Boranes

The first spontaneous, metal‐free, room‐temperature hydrogen transfer from ammonia borane to an iminoborane is reported. Mechanistic studies of the reaction indicate a concerted transfer of H⁺ and H^? from donor to acceptor with an activation energy far below those of comparable concerted transfer hydrogenations. This reaction was employed in the syntheses and isolation of new B,N‐disubstituted aminoboranes, a comparatively rare subset within the aminoborane family. This successful transfer hydrogenation to a highly dehydrogenated BN system may serve as a starting point for the design of new systems capable of reversible dehydrogenation/rehydrogenation.     

A Concerted Transfer Hydrogenolysis: 1,3,2‐Diazaphospholene‐Catalyzed Hydrogenation of NN Bond with Ammonia–Borane

1,3,2‐diazaphospholenes catalyze metal‐free transfer hydrogenation of a N?N double bond using ammonia–borane under mild reaction conditions, thus allowing access to various hydrazine derivatives. Kinetic and computational studies revealed that the rate‐determining step involves simultaneous breakage of the B? H and N? H bonds of ammonia–borane. The reaction is therefore viewed as a concerted type of hydrogenolysis.     

Ordered Porous Nitrogen‐Doped Carbon Matrix with Atomically Dispersed Cobalt Sites as an Efficient Catalyst for Dehydrogenation and Transfer Hydrogenation of N‐Heterocycles

Single‐atom catalysts (SACs) have been explored widely as potential substitutes for homogeneous catalysts. Isolated cobalt single‐atom sites were stabilized on an ordered porous nitrogen‐doped carbon matrix (ISAS‐Co/OPNC). ISAS‐Co/OPNC is a highly efficient catalyst for acceptorless dehydrogenation of N‐heterocycles to release H₂. ISAS‐Co/OPNC also exhibits excellent catalytic activity for the reverse transfer hydrogenation (or hydrogenation) of N‐heterocycles to store H₂, using formic acid or external hydrogen as a hydrogen source. The catalytic performance of ISAS‐Co/OPNC in both reactions surpasses previously reported homogeneous and heterogeneous precious‐metal catalysts. The reaction mechanisms are systematically investigated using first‐principles calculations and it is suggested that the Eley–Rideal mechanism is dominant.     

Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnI and FeII PNP Pincer Complexes

Herein, we describe an efficient coupling of alcohols and amines catalyzed by well‐defined isoelectronic hydride Mn^I and Fe^II complexes, which are stabilized by a PNP ligand based on the 2,6‐diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth‐abundant non‐precious metal catalysts, and is based on the acceptorless alcohol dehydrogenation concept. A range of alcohols and amines including both aromatic and aliphatic substrates were efficiently converted in good to excellent isolated yields. Although in the case of Mn selectively imines were obtained, with Fe—exclusively monoalkylated amines were formed. These reactions proceed under base‐free conditions and required the addition of molecular sieves.     

Catalytic Dehydrogenative Cyclization of o‐Teraryls under pH‐Neutral and Oxidant‐Free Conditions

A cobaloxime‐catalyzed acceptorless dehydrogenative cyclization of o‐teraryls was developed. In stark contrast to the established methods such as the Scholl or Mallory reactions, this method does not require any strong acids or oxidants, and shows high atom economy and a broad substrate scope. It operates at near room temperature with light as the source of energy. Acid‐ or oxidant‐sensitive functional groups, such as 4‐methoxyphenyl, unprotected benzyl alcohol, silyl ether, and thiophene groups are tolerated. Remarkably, aryls with electron‐withdrawing groups, and electron‐poor heteroarenes, such as pyridine and pyrimidine, can also react. Preliminary mechanistic study reveals that hydrogen gas is released during the reaction, and both light and the cobalt catalyst are important for the dehydrogenation step.     

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes,gem‐Difluoroalkenes,and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value‐added fluorinated organic compounds. Recent progress has been mainly focused on the transition‐metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B?H activation of N‐heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom‐economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem‐difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition‐metal free, and the regioselectivity obtained is complementary to the reported transition‐metal‐catalysis in many cases.     

Towards the design of novel boron‐ and nitrogen‐substituted ammonia‐borane and bifunctional arene ruthenium catalysts for hydrogen storage

Electronic‐structure density functional theory calculations have been performed to construct the potential energy surface for H₂ release from ammonia‐borane, with a novel bifunctional cationic ruthenium catalyst based on the sterically bulky β‐diketiminato ligand (Schreiber et al., ACS Catal. 2012, 2, 2505). The focus is on identifying both a suitable substitution pattern for ammonia‐borane optimized for chemical hydrogen storage and allowing for low‐energy dehydrogenation. The interaction of ammonia‐borane, and related substituted ammonia‐boranes, with a bifunctional η⁶‐arene ruthenium catalyst and associated variants is investigated for dehydrogenation. Interestingly, in a number of cases, hydride‐proton transfer from the substituted ammonia‐borane to the catalyst undergoes a barrier‐less process in the gas phase, with rapid formation of hydrogenated catalyst in the gas phase. Amongst the catalysts considered, N,N‐difluoro ammonia‐borane and N‐phenyl ammonia‐borane systems resulted in negative activation energy barriers. However, these types of ammonia‐boranes are inherently thermodynamically unstable and undergo barrierless decay in the gas phase. Apart from N,N‐difluoro ammonia‐borane, the interaction between different types of catalyst and ammonia borane was modeled in the solvent phase, revealing free‐energy barriers slightly higher than those in the gas phase. Amongst the various potential candidate Ru‐complexes screened, few are found to differ in terms of efficiency for the dehydrogenation (rate‐limiting) step. To model dehydrogenation more accurately, a selection of explicit protic solvent molecules was considered, with the goal of lowering energy barriers for H‐H recombination. It was found that primary (1°), 2°, and 3° alcohols are the most suitable to enhance reaction rate. © 2014 Wiley Periodicals, Inc.     

Manganese‐Catalyzed Direct Olefination of Methyl‐Substituted Heteroarenes with Primary Alcohols

Herein, we present the first catalytic direct olefination of methyl‐substituted heteroarenes with primary alcohols through an acceptorless dehydrogenative coupling. The reaction is catalyzed by a complex of the earth‐abundant transition metal manganese that is stabilized by a bench‐stable NNN pincer ligand derived from 2‐hydrazinylpyridine. The reaction is environmentally benign, producing only hydrogen and water as byproducts. A large number of E‐disubstituted olefins were selectively obtained with high efficiency.     

Carbocatalysis in Liquid‐Phase Reactions

There is broad interest in metal‐free catalysis for sustainable chemistry. Carbocatalysis is a “green” option for catalytic transformations in the gas phase as well as in the liquid phase. This is evident by the numerous reports on gas‐phase dehydrogenation and selective oxidation where carbon can be used as a successful alternative to metal oxide systems. Carbocatalysis for liquid‐phase reactions, especially for organic synthesis, is an emerging research discipline and has undergone rapid development in recent years. This Review provides a critical analysis on the state‐of‐the‐art of carbocatalysts for liquid‐phase reactions, with a focus on the underlying mechanisms as well as the advantages and limitations of metal‐free carbocatalysts.     

Bis‐Boron Compounds in Catalysis: Bidentate and Bifunctional Activation

The development of metal‐free catalysts as an alternative to the use of transition metals has gained tremendous interest in the past. In catalysis, Lewis acidity is one of the major principles used for the activation of organic compounds. Improving the reactivity and selectivity of Lewis acids by utilizing bidentate interactions was already proposed 50 years ago. Nevertheless, product inhibition due to strong binding has made applications of bidentate Lewis acids challenging for many years. Recently, bis‐boron compounds have been found to be very effective and several applications in Diels–Alder reactions, carbon dioxide reduction, and ammonia‐borane dehydrogenation were reported. All three transformations are enabled by the catalyst at different stages during the course of the reaction. These new and useful examples illustrate the great potential of the concept.     

B(C6F5)3‐Catalyzed Transfer Hydrogenation of Imines and Related Heteroarenes Using Cyclohexa‐1,4‐dienes as a Dihydrogen Source

The strong boron Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, is shown to abstract a hydride from suitably donor‐substituted cyclohexa‐1,4‐dienes, eventually releasing dihydrogen. This process is coupled with the FLP‐type (FLP=frustrated Lewis pair) hydrogenation of imines and nitrogen‐containing heteroarenes that are catalyzed by the same Lewis acid. The net reaction is a B(C₆F₅)₃‐catalyzed, i.e., transition‐metal‐free, transfer hydrogenation using easy‐to‐access cyclohexa‐1,4‐dienes as reducing agents. Competing reaction pathways with or without the involvement of free dihydrogen are discussed.     

Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N‐protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP‐catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X‐ray crystal analysis, and by quantum‐mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate‐determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.     

Towards a Practical Development of Light‐Driven Acceptorless Alkane Dehydrogenation

The efficient catalytic dehydrogenation of alkanes to olefins is one of the most investigated reactions in organic synthesis. In the coming years, an increased supply of shorter‐chain alkanes from natural and shale gas will offer new opportunities for inexpensive carbon feedstock through such dehydrogenation processes. Existing methods for alkane dehydrogenation using heterogeneous catalysts require harsh reaction conditions and have a lack of selectivity, whereas homogeneous catalysis methods result in significant waste generation. A strong need exists for atom‐efficient alkane dehydrogenations on a useful scale. Herein, we have developed improved acceptorless catalytic systems under optimal light transmittance conditions using trans‐[Rh(PMe₃)₂(CO)Cl] as the catalyst with different additives. Unprecedented catalyst turnover numbers are obtained for the dehydrogenation of cyclic and linear (from C₄) alkanes and liquid organic hydrogen carriers. These reactions proceed with unique conversion, thereby providing a basis for practical alkane dehydrogenations.     

Metal‐Free Nitrile Diboration through Activation by an Electron‐Rich Diborane

Diboration of unsaturated organic compounds is an extremely useful reaction in synthetic chemistry. Herein, we report the first diboration of a nitrile by an electron‐rich diborane, mediated by an electrophilic borane. The reaction is metal‐free, and all of the reagents are readily available.     

Highly Efficient Electrocatalysts for Oxygen Reduction Based on 2D Covalent Organic Polymers Complexed with Non‐precious Metals

A class of 2D covalent organic polymers (COPs) incorporating a metal (such as Fe, Co, Mn) with precisely controlled locations of nitrogen heteroatoms and holes were synthesized from various N‐containing metal–organic complexes (for example, metal–porphyrin complexes) by a nickel‐catalyzed Yamamoto reaction. Subsequent carbonization of the metal‐incorporated COPs led to the formation of COP‐derived graphene analogues, which acted as efficient electrocatalysts for oxygen reduction in both alkaline and acid media with a good stability and free from any methanol‐crossover/CO‐poisoning effects.     

Nanocatalosomes as Plasmonic Bilayer Shells with Interlayer Catalytic Nanospaces for Solar‐Light‐Induced Reactions

Interest and challenges remain in designing and synthesizing catalysts with nature‐like complexity at few‐nm scale to harness unprecedented functionalities by using sustainable solar light. We introduce “nanocatalosomes”—a bio‐inspired bilayer‐vesicular design of nanoreactor with metallic bilayer shell‐in‐shell structure, having numerous controllable confined cavities within few‐nm interlayer space, customizable with different noble metals. The intershell‐confined plasmonically coupled hot‐nanospaces within the few‐nm cavities play a pivotal role in harnessing catalytic effects for various organic transformations, as demonstrated by “acceptorless dehydrogenation”, “Suzuki–Miyaura cross‐coupling” and “alkynyl annulation” affording clean conversions and turnover frequencies (TOFs) at least one order of magnitude higher than state‐of‐the‐art Au‐nanorod‐based plasmonic catalysts. This work paves the way towards next‐generation nanoreactors for chemical transformations with solar energy.     

Reductive Deamination with Hydrosilanes Catalyzed by B(C6F5)3

The strong boron Lewis acid tris(pentafluorophenyl)borane B(C₆F₅)₃ is known to catalyze the dehydrogenative coupling of certain amines and hydrosilanes at elevated temperatures. At higher temperature, the dehydrogenation pathway competes with cleavage of the C?N bond and defunctionalization is obtained. This can be turned into a useful methodology for the transition‐metal‐free reductive deamination of a broad range of amines as well as heterocumulenes such as an isocyanate and an isothiocyanate.