Current Organic Chemistry - Volume 10, Issue 2, 2006

Applications of organo-ruthenium derivatives in homogeneous catalysis have disclosed one of the cornerstones in the spectacular development of Organometallic Chemistry during the last decade. The special ability of ruthenium complexes to form a wide range of inorganic and organometallic complexes, featuring a wide scope of oxidation states, several coordination geometries and reactivity patterns, has triggered a growing interest. Nowadays, they may be considered, in a wide number of chemical transformations, as very competitive alternatives to other classical transition metal catalysts such as palladium, rhodium, platinum etc. giving rise, indeed, to a very appealing field of research. The present issue of Current Organic Chemistry entitled "Ruthenium Catalyzed Processes" deals with a series of nine articles from expert authors in the field illustrating recent developments of the usefulness in organic synthesis. The articles show the state of the art of specific aspects as well as complement important issues not covered by recent reviews or monographs on their catalytic applications The contributions highlight the progress of the catalytic activity in novel C-C bond forming reactions including allylic activation (C. Bruneau and co-workers), propargylic alcohol substitutions, (Y. Nishibayashi and S. Uemura) and other type involving Lewis acid catalysis (J. Faller and J. Parr). In their article J. Gimeno and co-workers summarize the chemistry of bis-allyl ruthenium(IV) derivatives and their catalytic applications in, among other reactions, polymerization of olefins, transfer hydrogenation of ketones and isomerization of allylic alcohols in water. Two articles dealing with Ring-closing metathesis (D.E. Fogg and J.C. Conrad) and applications of novel N-heterocyclic carbene-ruthenium complexes (A.F. Noels and co-workers) give further details on the well-known catalytic applications of carbene ruthenium complexes. The catalytic activity of ruthenium complexes in reactions involving transfer of atoms through radical additions such as Kharash reactions (K. Severin) and asymmetric atom-transfer reactions (C. Bonaccorsi and A. Mezzetti), illustrates the ability to catalyze this relatively less studied type of processes. Finely, the article from K. Kaneda and co-workers reviews the catalytic activity of immobilized-ruthenium complexes showing the promising applications of heterogeneous ruthenium catalysts in the development of a sustainable chemistry. I thank all the authors for their valuable reviews which I hope will be helpful to many organic chemists interested in innovative and non-classical chemical transformations.

Recent developments in the chemistry of η3-allylruthenium complexes (synthesis and reactivity) are described. Among different possible preparations, their straightforward formation via oxidative addition of allylic substrates occurs either at ruthenium(0) or ruthenium(II) centres. Subsequent reaction with an electrophile or a nucleophile is the basis of their ambiphilic involvement in catalysis. In this review, we focus on catalytic substitution of allylic substrates by C, N, O and S-nucleophiles, and show that selected ligands make possible stereospecific, as well as regio- and enantioselective substitutions.

Our recent studies on novel carbon-carbon bond forming reactions using propargylic alcohols catalyzed only by thiolate-bridged diruthenium complexes are reviewed from the viewpoint of organic synthesis. The reactions are substitutions of hydroxyl group of the alcohols with a variety of carbon-centered nucleophiles such as simple ketones, cyclic 1,3-dicarbonyl compounds, aromatic and heteroaromatic compounds, phenols, naphthols, and alkenes. The asymmetric version of propargylic alkylation with acetone is also described. All of the reactions are considered to proceed via ruthenium-allenylidene complexes as key intermediates.

Recent progress in the Lewis acid catalysis of organic reactions by ruthenium complexes. The review focuses on reactions in which coordinatively unsaturated ruthenium species function as conventional Lewis acids, rather than those involving oxidation or reduction, such as in hydrogenations. Particular emphasis is placed on the development of asymmetric catalysts.

The present review reports on the chemistry of the bis(allyl)-ruthenium(IV) complexes [{Ru(η3:η3-C10H16)(μ- Cl)Cl}2] (C10H16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) and [Ru(η:η2:η-C12H18)Cl2] (C12H18 = dodeca-2,6,10-triene- 1,12-diyl). Stoichiometric reactions allowing the preparation of a variety of organoruthenium(IV) and (II) derivatives, as well as the involvement of these species in a series of catalytic organic transformations are presented.

Recent advances in ruthenium-catalyzed ring closing metathesis are discussed, in context of both substrate and catalyst parameters. As well as thermodynamic (substrate) constraints on ring-closing, root causes and effects of non-ideal catalytic performance are examined. Key substrate parameters are outlined, with a particular focus on the balance between oligomerization and ring-closing in RCM macrocyclization reactions. Advances in catalyst design are examined from a mechanistic viewpoint, including initiation requirements, catalyst deactivation, and opportunities resulting from incorporation of pseudohalide ligands. An overview of methods for reducing ruthenium residues in organic products to ppm levels is presented.

New imidazolium and imidazolinium salts were synthesized and their ability to act as stable N-heterocyclic carbene (NHC) ligand precursors was investigated in various ruthenium-catalyzed processes. Thus, 1,3- diarylimidazol(in)ium chlorides bearing the phenyl, 1-naphthyl, 4-biphenyl, 3,5-dimethylphenyl, 2-tolyl, 2,6- dimethylphenyl, 2,4,6-trimethylphenyl (mesityl), and 2,6-diisopropylphenyl substituents were prepared. They were combined with the [RuCl2(p-cymene)]2 dimer and potassium tert-butoxide or sodium hydride to generate the corresponding ruthenium-arene complexes [RuCl2(p-cymene)(NHC)] in situ. The catalytic activity of all these species was investigated in the photoinduced ring-opening metathesis polymerization (ROMP) of norbornene and cyclooctene. Results from this study showed that the C4-C5 double bond in the imidazole ring of the NHC ligands was not crucial to achieve high catalytic efficiencies. The presence or the absence of alkyl groups on the ortho positions of the phenyl rings had a more pronounced influence. Blocking all the ortho positions was a requisite for obtaining efficient catalysts. Failure to do so probably resulted in the ortho-metallation of the carbene ligand, thereby altering the coordination sphere of the ruthenium active centers. Catalytic screenings were also carried out with the various imidazol(in)ium salts to evaluate their ability at promoting the cyclopropanation of styrene and cyclooctene with ethyl diazoacetate. Under the experimental conditions adopted, the exact nature of the N,N'-diaryl groups had very little influence on the outcome of these reactions. The imidazolium salts were further probed as catalyst modifiers for the Atom Transfer Radical Addition (ATRA) of carbon tetrachloride to styrene. Some species displayed a dual activity and promoted both olefin metathesis and ATRA.

Recent advances in the development of ruthenium catalysts for the atom transfer radical addition of polyhalogenated compounds to olefins ('Kharasch reaction') are described. Three classes of homogeneous catalysts are discussed: halfsandwich complexes with η6-arene, η5-cyclopentadienyl or η5-carborane ligands, ruthenium complexes with alkylidene- or vinylidene ligands and polynuclear complexes. Furthermore, first attempts to use immobilized ruthenium complexes as heterogeneous catalysts for the Kharasch reaction are summarized.

This account describes the application of ruthenium complexes containing chiral tetradentate ligands PNNP, featuring a P 2N2 ligand set as catalysts for enantioselective reactions involving atom-transfer between the metal complex and a non-coordinated molecule. The five-coordinate 16-electron [RuCl(PNNP)]+ species and their octahedral analogues [RuCl(L)(PNNP)]+ have been used in the asymmetric epoxidation and cyclopropanation of olefins, in which oxene or carbene are transferred from ruthenium to the noncoordinated substrate. The [RuCl(PNNP)]+ catalysts cyclopropanate styrenes and 1-octene in the presence of ethyl diazoacetate with excellent cis- and enantioselectivity. By means of anion optimization and electronic tuning of the PNNP ligand, we achieved the highly cis-selective cyclopropanation of 1-octene, which is, to the best of out knowledge, the first example for a terminal aliphatic olefin. A different mode of enantioselective atom transfer has been observed in the hydroxylation and electrophilic fluorination of 1,3-dicarbonyl compounds, in which the oxene or F+-transfer agent attacks a ruthenium-bound substrate. This mechanism is supported by stoichiometric reactions with the isolated enolato complexes formed upon reaction with activated ruthenium species obtained by double chloride abstraction from [RuCl2(PNNP)] with Et3OPF6. Nucleophilic fluorination of activated haloalkanes is also reported.

This article reviews a novel approach to design heterogeneous Ru catalysts using hydroxyapatites and hydrotalcites, and their excellent catalytic performances for aerobic alcohol oxidations, carbon-carbon bond formations, and one-pot syntheses. The catalytic systems using the above heterogeneous Ru catalysts offer significant benefits in achieving simple and clean organic syntheses. Furthermore, the present preparation method for the immobilization of metal species is much simpler than previous synthetic methods of the solid-supported transition metal catalysts, which can allow a strong protocol to create various nanostructured and functionalized heterogeneous metal catalysts.