Asymmetric Transfer and Pressure Hydrogenation with Earth‐Abundant Transition Metal Catalysts

The asymmetric transfer and pressure hydrogenation of various unsaturated substrates provides a succinct pathway to important chiral intermediates and products such as chiral alcohols, amines, and alkanes. The use of earth‐abundant transition metals such as Fe, Co, Ni, and Cu in hydrogenation reactions provides an attractive alternative to traditionally used metals such as Ru, Rh, Ir, and Pd because they are comparatively inexpensive, less toxic, and as their name suggests, more abundant in nature. Earth‐abundant transition metal‐catalyzed asymmetric hydrogenation is rapidly becoming an important area of research. This review summarizes advances in the asymmetric hydrogenation of unsaturated bonds (ketones, imines, and alkenes) with earth‐abundant transition metals.     

An Alkene‐Promoted Borane‐Catalyzed Highly Stereoselective Hydrogenation of Alkynes to Give Z‐ and E‐Alkenes

The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Z‐alkenes, the hydrogenation of alkynes to give E‐alkenes is still not well resolved. In particular, selective preparation of both Z‐ and E‐alkenes by the same catalytic hydrogenation system using molecular H₂ has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C₆F₅)₂‐catalyzed metal‐free hydrogenation of alkynes was adopted. Significantly, both Z‐ and E‐alkenes can be furnished by hydrogenation with molecular H₂ in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H₂ play an essential role in promoting the hydrogenolysis reaction.     

Katalytische Hydroaminierung von Alkenen und Alkinen

Nitrogen containing molecules are among the most important classes of chemical subbtances. They play an outstanding role as biologically active compounds as well as industrial chemicals. In the past these facts lead to the development of a large number of methods for the synthesis of amines, imines and enamines. While most of these well established methods employ ketones/altlehydes or nitro compounds as starting materials, the direct conversion of alkenes or alkynes into nitrogen containing compounds is not very common. This is surprising because the direct addition of amines or ammonia to alkynes and alkenes (hydroamination) represents the shortest and economically most interesting way for the synthesis of amines, imines and enamines. A general method for this transfomation would produce the desired nitrogen containing compounds in a single step without the production of waste. To solve this synthetic problem, several more or less successful attempts have been made in the past. In this review we describe established as well as new and promising methods for the catalytic hydroamination of alkenes and alkynes. The catalysts mentioned are alkali metals, alkali metal hydrides and amides, lanthanoid complexes, carly and late transition metal complexes as well as zeolithes. For each type of catalyst the reaction mechanism is described. Further, the scope and limitations of each procedure is discussed.     

Consecutive Intermolecular Reductive Hydroamination: Cooperative Transition‐Metal and Chiral Brønsted Acid Catalysis

Enantiomerically pure chiral amines are of increasing importance and commercial value in the fine chemical, pharmaceutical, and agrochemical industries. Here, we describe the straightforward synthesis of chiral amines by combining the atom‐economic and environmentally friendly hydroamination of alkynes with an enantioselective hydrogenation of in situ generated imines by using inexpensive hydrogen. By following this novel approach, a wide range of terminal alkynes can be reductively hydroaminated with primary amines including alkyl‐, and arylalkynes as well as aryl and heteroaryl amines. Excellent yields and selectivities up to 94 % ee and 96 % isolated yield were obtained.     

Migratory Hydrogenation of Terminal Alkynes by Base/Cobalt Relay Catalysis

Migratory functionalization of alkenes has emerged as a powerful strategy to achieve functionalization at a distal position to the original reactive site on a hydrocarbon chain. However, an analogous protocol for alkyne substrates is yet to be developed. Herein, a base and cobalt relay catalytic process for the selective synthesis of (Z)‐2‐alkenes and conjugated E alkenes by migratory hydrogenation of terminal alkynes is disclosed. Mechanistic studies support a relay catalytic process involving a sequential base‐catalyzed isomerization of terminal alkynes and cobalt‐catalyzed hydrogenation of either 2‐alkynes or conjugated diene intermediates. Notably, this practical non‐noble metal catalytic system enables efficient control of the chemo‐, regio‐, and stereoselectivity of this transformation.     

Chemoselective Synthesis of Substituted Imines,Secondary Amines,and β‐Amino Carbonyl Compounds from Nitroaromatics through Cascade Reactions on Gold Catalysts

Substituted imines, α,β‐unsaturated imines, substituted secondary amines, and β‐amino carbonyl compounds have been synthesized by means of new cascade reactions with mono‐ or bifunctional gold‐based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, α,β‐unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO₂ system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three‐step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO₂ can also be used as a bifunctional catalyst to obtain substituted β‐amino carbonyl compounds from nitroaromatics and α,β‐unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold‐catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.     

Metal Nanoparticles Supported on Biomass‐Derived Hierarchical Porous Heteroatom‐Doped Carbon from Bamboo Shoots: Design,Synthesis and Applications

Heteroatom‐doped porous carbon derived from biomass have recently received increasing attention due to their unique properties such as high electrical conductivity, large specific surface area, high porosity, and easy availability, which are appealing materials for versatile applications in catalysis, energy, separation and adsorption, and life sciences as well. On the basis of our previous work in this field, we summarized in this account our recent progress on design, synthesis of metal (e. g., Pd, Co) nanoparticles supported heteroatom‐doped hierarchical porous carbon material derived from bamboo shoots and their applications for important organic transformations, including chemoselective semihydrogenation of alkynes, hydrosilylation of alkynes, cascade synthesis of benzofurans from terminal alkynes and iodophenols, selective hydrogenation of functionalized nitroarenes to form anilines, imines, and formamides. Finally, the current state and future challenges in this field are discussed. We hope this account could shed light on the rational design of novel non‐noble metal based heterogeneous catalysts derived from biomass for efficient and sustainable organic transformations.     

Fast and Selective Semihydrogenation of Alkynes by Palladium Nanoparticles Sandwiched in Metal–Organic Frameworks

The semihydrogenation of alkynes into alkenes rather than alkanes is of great importance in the chemical industry. Unfortunately, state‐of‐the‐art heterogeneous catalysts hardly achieve high turnover frequencies (TOFs) simultaneously with almost full conversion, excellent selectivity, and good stability. Here, we used metal–organic frameworks (MOFs) containing Zr metal nodes (“UiO”) with tunable wettability and electron‐withdrawing ability as activity accelerators for the semihydrogenation of alkynes catalyzed by sandwiched palladium nanoparticles (Pd NPs). Impressively, the porous hydrophobic UiO support not only leads to an enrichment of phenylacetylene around the Pd NPs but also renders the Pd surfaces more electron‐deficient, which leads to a remarkable catalysis performance, including an exceptionally high TOF of 13835 h^?1, 100 % phenylacetylene conversion 93.1 % selectivity towards styrene, and no activity decay after successive catalytic cycles. The strategy of using molecularly tailored supports is universal for boosting the selective semihydrogenation of various terminal and internal alkynes.     

Nickel‐Catalyzed Synthesis of N‐Aryl‐1,2‐dihydropyridines by [2+2+2] Cycloaddition of Imines with Alkynes through T‐Shaped 14‐Electron Aza‐Nickelacycle Key Intermediates

Despite there being a straightforward approach for the synthesis of 1,2‐dihydropyridines, the transition‐metal‐catalyzed [2+2+2] cycloaddition reaction of imines with alkynes has been achieved only with imines containing an N‐sulfonyl or ‐pyridyl group. Considering the importance of 1,2‐dihydropyridines as useful intermediates in the preparation of a wide range of valuable organic molecules, it would be very worthwhile to provide novel strategies to expand the scope of imines. Herein we report a successful expansion of the scope of imines in nickel‐catalyzed [2+2+2] cycloaddition reactions with alkynes. In the presence of a nickel(0)/PCy₃ catalyst, a reaction with N‐benzylidene‐P,P‐diphenylphosphinic amide was developed. Moreover, an application of N‐aryl imines to the reaction was also achieved by adopting N‐heterocyclic carbene ligands. The isolation of an (η²‐N‐aryl imine)nickel(0) complex containing a 14‐electron nickel(0) center and a T‐shaped 14‐electron five‐membered aza‐nickelacycle is shown. These would be considered as key intermediates of the reaction. The structure of these complexes was unambiguously determined by NMR spectroscopy and X‐ray analyses.     

Iron‐Catalyzed Mild and Selective Hydrogenative Cross‐Coupling of Nitriles and Amines To Form Secondary Aldimines

The first example of a base‐metal‐catalyzed homogeneous hydrogenative coupling of nitriles and amines to selectively form secondary cross‐imines is reported. The reaction is catalyzed under mild conditions by a well‐defined (iPr‐PNP)Fe(H)Br(CO) pincer pre‐catalyst and catalytic t BuOK.     

Three characteristic reactions of alkynes with metal compounds in organic synthesis

Alkynes have two sets of mutually orthogonal π‐bonds that are different from the π‐bonds of alkenes. These π‐bonds are able to bond with transition metal compounds. Alkynes easily bond with the various kinds of compounds having a π‐bond such as carbon monoxide, alkenes, other alkynes and nitriles in the presence of the transition metal compounds. The most representative reaction of alkynes is called the Pauson–Khand reaction. The Pauson–Khand reactions include the cyclization of alkynes with alkenes and carbon monoxide in the presence of cobalt carbonyls. Similar Pauson–Khand reactions also proceed in the presence of other transition metal compounds. These reactions are the first type of characteristic reaction of alkynes. Other various kinds of cyclizations with alkynes also proceed in the presence of the transition metal compounds. These reactions are the second type of characteristic reaction of alkynes. These include cyclooligomerizations and cycloadditions. The cyclooligomerizations include mainly cyclotrimerizations and cyclotetramerizations, and the cycloadditions are [2 + 2], [2 + 2 + 1], [2 + 2 + 2], [3 + 2], [4 + 2], etc., type cycloadditions. Alkynes are fairly reactive because of the high s character of their σ‐bonds. Therefore, simple coupling reactions with alkynes also proceed besides the cyclizations. The coupling reactions are the third type of characteristic reactions of alkynes in the presence of, mainly, the transition metal compounds. These reactions include carbonylations, dioxycarbonylations, Sonogashira reactions, coupling reactions with aldehydes, ketones, alkynes, alkenes and allyl compounds. Copyright © 2008 John Wiley & Sons, Ltd.     

Advances in chemoselective and/or stereoselective semihydrogenation of alkynes

The chemoselective semihydrogenation of alkynes is one of the most important reactions in synthetic organic chemistry. During the last decade or so, a multitude of innovative catalytic methods have been developed to address the selective hydrogenation of alkynes. This digest focuses on the recent developments in chemoselective semihydrogenation of alkynes to alkenes.     

Unprecedented hydrogen transfer from water to alkenes and alkynes mediated by TiIII and late transition metals

We describe how alkenes and alkynes can be hydrogenated under mild conditions by hydrogen transfer from water mediated by titanocene(III) and a substoichiometric quantity of one of the late transition metals usually employed as hydrogenation catalysts. This process proceeds presumably by H-atom transfer from TiIII-coordinated water to the late transition metal partner (depicted in the drawing above), a mechanism in support of which we provide theoretical and experimental evidence.     

Lithium‐Aluminate‐Catalyzed Hydrophosphination Applications

Synthesized, isolated, and characterized by X‐ray crystallography and NMR spectroscopic studies, lithium phosphidoaluminate iBu₃AlPPh₂Li(THF)₃ has been tested as a catalyst for hydrophosphination of alkynes, alkenes, and carbodiimides. Based on the collective evidence of stoichiometric reactions, NMR monitoring studies, kinetic analysis, and DFT calculations, a mechanism involving deprotonation, alkyne insertion, and protonolysis is proposed for the [iBu₃AlHLi]₂ aluminate catalyzed hydrophosphination of alkynes with diphenylphosphine. This study enhances further the development of transition‐metal‐free, atom‐economical homogeneous catalysis using common sustainable main‐group metals.     

B(C6F5)3‐Catalyzed sp3 C—Si Bond Forming Consecutive Reactions

Transition metal‐catalyzed hydrosilylation is one of the most widely utilized reduction methods as an alternative to hydrogenation in academia and industry. One feature distinct from hydrogenation would be able to install sp³ C—Si bond(s) onto substrates skeleton via hydrosilylation of alkenes. Recently, B(C₆F₅)₃ with hydrosilanes has been demonstrated to be an efficient, metal‐free catalyst system for the consecutive transformation of heteroatom‐containing substrates accompanied by the formation of sp³ C—Si bond(s), which has not been realized thus far under the transition metal‐catalyzed hydrosilylative conditions. In this review, I outline the B(C₆F₅)₃‐mediated consecutive hydrosilylations of heteroarenes containing quinolines, pyridines, and furans, and of conjugated nitriles/imines to provide a new family of compounds having sp³ C—Si bond(s) with high chemo‐, regio‐ and/or stereoselectivities. The silylative cascade conversion of unactivated N‐aryl piperidines to sila‐N‐heterocycles catalyzed by B(C₆F₅)₃ involving consecutive dehydrogenation, hydrosilylation, and intramolecular C(sp²)—H silylation, is presented in another section. Chemical selectivity and mechanism of the boron catalysis focused on the sp³ C—Si bond formation are highlighted.     

Stereoselective Synthesis of Trisubstituted Alkenes through Sequential Iron‐Catalyzed Reductive anti‐Carbozincation of Terminal Alkynes and Base‐Metal‐Catalyzed Negishi Cross‐Coupling

The stereoselective synthesis of trisubstituted alkenes is challenging. Here, we show that an iron‐catalyzed anti‐selective carbozincation of terminal alkynes can be combined with a base‐metal‐catalyzed cross‐coupling to prepare trisubstituted alkenes in a one‐pot reaction and with high regio‐ and stereocontrol. Cu‐, Ni‐, and Co‐based catalytic systems are developed for the coupling of sp‐, sp²‐, and sp³‐hybridized carbon electrophiles, respectively. The method encompasses a large substrate scope, as various alkynyl, aryl, alkenyl, acyl, and alkyl halides are suitable coupling partners. Compared with conventional carbometalation reactions of alkynes, the current method avoids pre‐made organometallic reagents and has a distinct stereoselectivity.     

Rigid pyridyl substituted NHC ligands,their Pd(0) complexes and their application in selective transfer semihydrogenation of alkynes

The synthesis of an air‐stable series of Pd⁰ complexes with unsymmetric bidentate N‐pyridine N‐heterocyclic carbene ligands has been described. The carbenes were generated by synthesis of the silver(I) complexes from the imidazolium salts, followed by transmetallation of the C‐N ligands to obtain the target electron‐rich zerovalent palladium compounds. The bidentate coordination behaviour of the ligands was confirmed by ¹H, ¹³C NMR and X‐ray spectroscopy. The complexes are active precatalysts for the selective transfer semihydrogenation of alkynes to Z‐alkenes, with selectivities up to 99%. Copyright © 2011 John Wiley & Sons, Ltd.     

2,4‐Dinitrophenol‐Catalyzed α‐C(sp3)−H and C(sp)−H Bond Functionalization of Cyclic Amines and Alkynes: Highly Regio‐/Diastereoselective Synthesis of α‐Alkynyl‐3‐Amino‐2‐Oxindoles

A transition‐metal‐ and oxidant‐free DNP (2,4‐dinitrophenol)‐catalyzed atom‐economical regio‐ and diastereoselective synthesis of monofunctionalized α‐alkynyl‐3‐amino‐2‐oxindole derivatives by C?H bond functionalization of cyclic amines and alkynes with indoline‐2,3‐diones has been developed. This cascade event sequentially involves the reductive amination of indoline‐2,3‐dione by imine formation and cross coupling between C(sp³)?H and C(sp)?H of the cyclic amines and alkynes. This reaction offers an efficient and attractive pathway to different types of α‐alkynyl‐3‐amino‐2‐oxindole derivatives in good yields with a wide tolerance of functional groups. The salient feature of this methodology is that it completely suppresses the homocoupling of alkynes. To the best of our knowledge, this is the first example of a DNP‐catalyzed metal‐free direct C(sp³)?H and C(sp)?H bond functionalization providing biologically active α‐alkynyl‐3‐amino‐2‐oxindole scaffolds.     

Lewis Acid Mediated Vinyl‐Transfer Reaction of Alkynes to N‐Alkylimines by Using the N‐Alkyl Residue as a Sacrificial Hydrogen Donor

A variety of N‐alkyl‐α,α‐dichloroaldimines were vinylated by terminal acetylenes in the presence of Lewis acids such as In(OTf)₃ or BF₃ ? OEt₂ and hexafluoroisopropanol (HFIP) as an additive. The reaction proceeds at ambient temperature and leads to geometrically pure allylic β,β‐dichloroamines. This approach is complementary to previously reported transition‐metal‐catalyzed vinyl‐transfer methods, which are not applicable to aliphatic imines and are restricted to imines that contain an electron‐withdrawing nitrogen substituent. In the present approach, terminal alkynes were used as a source of the vinyl residue, and the N‐alkyl moiety of the imine acts as a sacrificial hydrogen donor. The additional advantage of this methodology is the fact that no external toxic or hazardous reducing agents or molecular hydrogen has to be used. This new methodology nicely combines a C(sp²)? C(sp) bond formation, hydride transfer, and an unusual cleavage of an unactivated C? N bond, thereby giving rise to functionalized primary allylic amines. A detailed experimental study supported by DFT calculations of the mechanism has been done.     

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes

The selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)-based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization-reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd−C₄ structure and, meanwhile, facilitate the desorption of alkenes by forming the Si−O−C structure with zeolite framework, as validated by the state-of-the-art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H₂), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite-encapsulated carbon-free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite-confined Pd-carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.