Ruthenium in Catalysis

Table of Contents

1. Frontmatter

2. Cyclometalated Ruthenium Alkylidene Complexes: A Powerful Family of Z-Selective Olefin Metathesis Catalysts

   Vanessa M. Marx, Lauren E. Rosebrugh, Myles B. Herbert, Robert H. Grubbs

   Abstract

   The past 5 years have witnessed an enormous growth in the field of Z-selective olefin metathesis. The development of a new class of cyclometalated ruthenium-based catalysts has extended the utility of olefin metathesis to the synthesis of useful Z-olefin-containing small molecules, polymers, and natural products. This review highlights the recent advances in the area of Z-selective olefin metathesis employing cyclometalated ruthenium alkylidene catalysts, with particular focus on its applications and mechanistic basis. A deeper understanding of structure–activity relationships should aid in the future design of even more active and selective olefin metathesis catalysts.

3. Hydrogenation of Polar Bonds Catalysed by Ruthenium-Pincer Complexes

   Ekambaram Balaraman, David Milstein

   Abstract

   Catalytic hydrogenation of polar bonds using molecular hydrogen is an important, atom-economical synthetic reaction. Classical reduction methods of polar bond often require reactive metal-hydride reagents in stoichiometric amount and produce copious waste. Hydrogenation of carbonyl compounds in particular provides ‘green’ approaches to synthetically important building blocks, such as alcohols and amines. We have designed and synthesized several ruthenium-based pincer catalysts for unprecedented hydrogenation reactions including: (1) amides to alcohols and amines, (2) biomass-derived di-esters to 1,2-diols and (3) CO₂ and CO derivatives to methanol. These atom-economical reactions operate under neutral, homogeneous conditions, at mild temperatures, mild hydrogen pressures, and can operate in absence of solvent with no generation of waste. The postulated mechanisms involve metal–ligand cooperation (MLC) by aromatization–dearomatization of the heteroaromatic pincer core.

4. Ruthenium-Catalyzed Hydrogen Generation from Alcohols and Formic Acid, Including Ru-Pincer-Type Complexes

   Pamela G. Alsabeh, Dörthe Mellmann, Henrik Junge, Matthias Beller

   Abstract

   The current feedstock for global energy demands is fossil fuels, which are not renewable and therefore have a limited lifetime as an energy supply. Renewable feedstocks such as biologically derived substrates, formic acid, and alcohols have been proposed as alternative energy sources, which can be used to produce hydrogen gas as one of the most simple chemical energy carriers. The dehydrogenation reaction is thus a necessary key step to establish a potential “hydrogen economy.” The following review chapter highlights recent advances in the areas of alcohol and formic acid dehydrogenation focusing on ruthenium-catalyzed processes. Although alcohol dehydrogenation has been studied extensively for its organic synthetic aspects, significantly fewer systems have directed efforts towards efficient hydrogen generation; those examples detailing TON and TOF values for gas evolution are described. Not only are ruthenium complexes bearing simple monodentate ligands successful as catalysts for conversion of challenging alcohols, but also those featuring pincer-type ligands. In addition, various ruthenium-catalyzed formic acid dehydrogenation methods have been developed. These protocols are performed mainly in the presence of amine or base to generate hydrogen but also include the absence of base, use of ionic liquids, continuous flow systems as well as hydrogen storage processes. In all of the abovementioned examples, ruthenium catalysts demonstrate high activity at relatively low loadings as well as long-term stability.

5. Ruthenium-Catalyzed Amide-Bond Formation

   Pascale Crochet, Victorio Cadierno

   Abstract

   The amide functionality is one of the most important functional groups in organic and biological chemistry. Classical synthetic strategies of amides involve the stoichiometric, and poor atom efficient, reaction of amines with carboxylic acid derivatives. Transition-metal-catalyzed reactions have emerged in recent years as more atom-economical and powerful tools for preparing amides, opening previously unavailable routes from substrates other than the carboxylic acids and their derivatives. Ruthenium-based catalysts have been at the heart of these advances, and this chapter pretends to give an overview of the field. Among others, the following ruthenium-catalyzed synthetic approaches of amides will be discussed: the hydration of nitriles, the hydrolytic amidation of nitriles with amines, the rearrangement of aldoximes, the coupling of aldehydes with hydroxylamine, and the dehydrogenative amidation of alcohols, aldehydes, and esters.

6. Ruthenium(II)-Catalysed sp2 C–H Bond Functionalization by C–C Bond Formation

   Bin Li, Pierre H. Dixneuf

   Abstract

   The selective catalytic activation/functionalization of sp² C–H bonds is expected to improve synthesis methods by better step number and atom economy. This chapter describes the recent achievements of ruthenium(II) catalysed transformations of sp² C–H bonds for cross-coupled C–C bond formation. First arylation and heteroarylation with aromatic halides of a variety of (hetero)arenes, that are directed at ortho position by heterocycle or imine groups, are presented. The role of carboxylate partners is shown for Ru(II) catalysts that are able to operate profitably in water and to selectively produce diarylated or monoarylated products. The alkylation of (hetero)arenes with primary and secondary alkylhalides, and by hydroarylation of alkene C=C bonds is presented. The recent access to functional alkenes via oxidative dehydrogenative functionalization of C–H bonds with alkenes first, and then with alkynes, is shown to be catalysed by a Ru(II) species associated with a silver salt in the presence of an oxidant such as Cu(OAc)₂. Finally the catalytic oxidative annulations with alkynes to rapidly form a variety of heterocycles are described by initial activation of C–H followed by that of N–H or O–H bonds and by formation of a second C–C bond on reaction with C=O, C=N, and sp³ C–H bonds. Most catalytic cycles leading from C–H to C–C bond are discussed.

7. sp3 C–H Bond Functionalization with Ruthenium Catalysts

   Christian Bruneau

   Abstract

   The selective formation of carbon–carbon bond by functionalization of an sp³C–H bond is still a challenge in organic synthesis. There are already examples involving transition metal catalysis. In this chapter we review the use of ruthenium(0) and ruthenium(II) catalysts for the formation of carbon–carbon bonds based on creation of reactive sites by sp³C–H bond activation. We show that in most cases, regioselective sp³C–H bond activation is induced either from functional substrates bearing a directing group, which strongly coordinates the metal centre, or by selective C–H bond activation at the α-carbon of a heteroatom accompanied by hydrogen transfer processes and transient creation of reactive functional groups.

8. Catalytic Transformations of Alkynes via Ruthenium Vinylidene and Allenylidene Intermediates

   Jesús A. Varela, Carlos González-Rodríguez, Carlos Saá

   Abstract

   Vinylidenes are high-energy tautomers of terminal alkynes and they can be stabilized by coordination with transition metals. The resulting metal-vinylidene species have interesting chemical properties that make their reactivity different to that of the free and metal π-coordinated alkynes: the carbon α to the metal is electrophilic whereas the β carbon is nucleophilic. Ruthenium is one of the most commonly used transition metals to stabilize vinylidenes and the resulting species can undergo a range of useful transformations. The most remarkable transformations are the regioselective anti-Markovnikov addition of different nucleophiles to catalytic ruthenium vinylidenes and the participation of the π system of catalytic ruthenium vinylidenes in pericyclic reactions. Ruthenium vinylidenes have also been employed as precatalysts in ring closing metathesis (RCM) or ring opening metathesis polymerization (ROMP).

   Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metal-allenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Cα and Cγ, while Cβ is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the π system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy.

9. C–C Bond Formation on Activation of Alkynes and Alkenes with (C5R5)Ru Catalysts

   Sylvie Dérien

   Abstract

   Electron-rich ruthenium(II) catalysts of type (C₅R₅)XRuL _n are used to perform selective carbon–carbon bond formation by combination of simple substrates such as the coupling of functional alkynes and alkenes with a variety of unsaturated molecules (alkynes, diynes, alkenes, dienes) or non-unsaturated molecules such as alcohols or water, often with atom economy. Various selective transformations are developed and can provide access to high multifunctional molecules. These reactions often proceed via an initial oxidative coupling leading to a ruthenacycle intermediate.

10. Organometallic Ruthenium Nanoparticles and Catalysis

    Karine Philippot, Pascal Lignier, Bruno Chaudret

    Abstract

    Due to a high number of possible applications in various domains, metal nanoparticles are nowadays the subject of an extensive development. This interest in metal nanoparticles is related to their electronic properties at the frontier between those of molecular species and bulk compounds which are induced by their nanometric size. Regarding the field of catalysis, the growing attention for metal nanoparticles also results from the high proportion of surface atoms present in the upper layer of the metallic core which gives rise to numerous potential active sites. Thus, nanocatalysis (which involves the use of catalysts with at least one dimension at the nanoscale) has emerged in the field of modern catalysis as a domain on the borderline between homogeneous and heterogeneous catalysis. Present developments aim at multifunctionality which can be achieved by the proper design of complex nanostructures also named nanohybrids. In nanohybrid the term “hybrid” refers to the appropriate association between a metal core and a stabilizing shell such as a polymer, a ligand, an ionic liquid, or even a support (inorganic materials, carbon black, carbon nanotubes, etc.…). This association can be considered as crucial to tune the surface properties of nanostructures and consequently their catalytic performance. The main expectation for the scientific community is that precisely designed nanoparticles (in terms of size, shape, and composition including surface ligands) should offer the benefits of both homogeneous and heterogeneous catalysts, namely high efficiency and better selectivity.

    In that context, we have been developing an efficient and versatile synthesis method using common tools from organometallic chemistry to produce well-controlled nanostructures which have been proved to be of interest for application in catalysis. A high number of studies have been focused on ruthenium nanosystems due to the use of a very convenient organometallic precursor, namely [Ru(COD)(COT)], as the metal source. This Ru complex is easily decomposed under dihydrogen atmosphere at room temperature. In addition, it is a complex of choice to prepare “naked” ruthenium nanostructures since the olefinic ligands present in the coordination sphere of ruthenium are hydrogenated into alkanes which exhibit no interaction with the metal surface. As a consequence, the metallic surface of the obtained nanoparticles is only covered by hydrides and the stabilizer which was deliberately added. This is highly convenient for studying the influence of the stabilizer on the morphology of the nanoparticles as well as their surface chemistry and related catalytic performance.

    This chapter gives an overview of our experience in the preparation of ruthenium nanoparticles of controlled size and surface state. Insights are given on the study of their surface chemistry by using simple techniques, mainly IR and NMR, both in solution and in solid state, as well as model hydrogenation reactions. We also discuss the performances of the Ru nanoparticles in catalysis which have been investigated both in solution (in organic or aqueous phases) and after their deposition on a support (alumina, silica, or carbon supports).

11. Visible-Light-Induced Redox Reactions by Ruthenium Photoredox Catalyst

    Takashi Koike, Munetaka Akita

    Abstract

    Photoredox catalysis by well-known ruthenium(II) polypyridine complexes is a versatile tool for redox reactions in synthetic organic chemistry, because they can effectively catalyze single-electron-transfer (SET) processes by irradiation with visible light. These favorable properties of the catalysts provide a new strategy for efficient and selective radical reactions. Salts of tris(2,2′-bipyridine)ruthenium(II), [Ru(bpy)₃]²⁺, were first reported in 1936. Since then, a number of works related to artificial photosynthesis and photofunctional materials have been reported, but only limited efforts had been devoted to synthetic organic chemistry. Remarkably, since 2008, this photocatalytic system has gained importance in redox reactions. In this chapter, we will present a concise review of seminal works on ruthenium photoredox catalysis around 2008, which will be followed by our recent research topics on trifluoromethylation of alkenes by photoredox catalysis.

12. Backmatter