Showing papers on "Metathesis published in 2006"

Olefin-metathesis catalysts for the preparation of molecules and materials (Nobel Lecture)

Abstract: This is a story of our exploration of the olefin-metathesis reaction, a reaction that has been the major emphasis of my independent research. As with all stories of scientific discovery, there are three components: the discoveries, the resulting applications, and, perhaps the most important of all, the people involved. Starting from observations made from seemingly unrelated work, our investigations into the fundamental chemistry of this transformation have been an exciting journey, with major advances often resulting from complete surprises, mistakes, and simple intuition. Ultimately, these efforts have contributed to olefin metathesis becoming the indispensable synthetic tool that it is today.

Multiple Metal–Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture)

Abstract: It’s my great priviledge to be here today, in a position I never thought possible. I hope the story that I will tell you will give you some idea what I have contributed to the area for which the Nobel Prize in Chemistry was awarded this year. The story begins thirty two years ago in 1973, the year the Nobel Prize was shared by G. Wilkinson and E. O. Fischer. Wilkinson’s Nobel Lecture1 concerned the nature of a single bond between a transition metal and a carbon atom in an alkyl group, and emphasized the fact that the metal-carbon bond is not inherently weak. E. O. Fischer in his Nobel Lecture2 summarized the extensive chemistry of transition metal “carbene” complexes3,4 that contain a metal-carbon double bond discovered by him and his group in 1964 (Fig 1).5 He also reported new “carbyne” complexes that contain a metal-carbon triple bond.6 It was clear that metal-carbon single bonds were of great importance in the emerging area of homogeneous catalysis. However, no catalytic reactions involving species that contain metal-carbon double or triple bonds were known. When I went to the Central Research Department of E. I. DuPont de Nemours and Company in 1972, transition metal organometallic chemistry and homogeneous catalysis were of great interest as a consequence of their huge potential in organic chemistry and therefore in industry. In the early 1970’s inorganic chemists knew that many transition metal species containing a metal-carbon bond are subject to various modes of decomposition that are much more rapid than in a non-transition metal species such as Zn(CH2CH3)2 or Al(CH2CH3)3. The most common of these involves transfer of a hydrogen, from an ethyl group (MCH2CH3) for example, to the metal to

Catalytic Alkane Metathesis by Tandem Alkane Dehydrogenation-Olefin Metathesis

Abstract: With petroleum supplies dwindling, there is increasing interest in selective methods for transforming other carbon feedstocks into hydrocarbons suitable for transportation fuel. We report the development of highly productive, well-defined, tandem catalytic systems for the metathesis of n-alkanes. Each system comprises one molecular catalyst (a "pincer"-ligated iridium complex) that effects alkane dehydrogenation and olefin hydrogenation, plus a second catalyst (molecular or solid-phase) for olefin metathesis. The systems all show complete selectivity for linear (n-alkane) product. We report one example that achieves selectivity with respect to the distribution of product molecular weights, in which n-decane is the predominant high-molecular-weight product of the metathesis of two moles of n-hexane.

Shape-persistent macrocycles: structures and synthetic approaches from arylene and ethynylene building blocks.

Abstract: Shape-persistent arylene ethynylene macrocycles have attracted much attention in supramolecular chemistry and materials science because of their unique structures and novel properties. In this Review we describe recent examples of macrocycle synthesis by cross-coupling (Sonogashira: aryl acetylene macrocycle or Glaser: aryl diacetylene macrocycle) and dynamic covalent chemistry. The primary disadvantage of the coupling methods is the kinetically determined product distribution, since a significant portion of oligomers grow beyond the length of the cyclic targets ("overshooting"). Better results have been obtained recently by a dynamic covalent approach involving reversible metathesis reactions that afford macrocycles in one step. Mechanistic studies demonstrate that macrocycle formation is thermodynamically controlled by this route. Remaining synthetic challenges include the efficient preparation of site-specifically functionalized structures and larger, more complex two- and three-dimensional molecules.

Highly active water-soluble olefin metathesis catalyst.

Abstract: A novel water-soluble ruthenium olefin metathesis catalyst supported by a poly(ethylene glycol) conjugated saturated 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligand is reported. The catalyst displays improved activity in ring-opening metathesis polymerization, ring-closing metathesis, and cross-metathesis reactions in aqueous media.

Highly active chiral ruthenium catalysts for asymmetric ring-closing olefin metathesis.

Abstract: The synthesis of olefin metathesis catalysts containing chiral, monodentate N-heterocyclic carbenes and their application to asymmetric ring-closing metathesis (ARCM) are reported. These catalysts retain the high levels of reactivity found in the related achiral variants (1a and 1b). Using the parent chiral catalysts 2a and 2b and derivatives that contain steric bulk in the meta positions of the N-bound aryl rings (catalysts 3−5), five- through seven-membered rings were formed in up to 92% ee. The addition of sodium iodide to catalysts 2a−4a (to form 2b−4b in situ) caused a dramatic increase in enantioselectivity for many substrates. Catalyst 5a, which gave high enantiomeric excesses for certain substrates without the addition of NaI, could be used in loadings of ≤1 mol %. Mechanistic explanations for the large sodium iodide effect as well as possible mechanistic pathways leading to the observed products are discussed.

Quantitative Structure−Activity Relationships of Ruthenium Catalysts for Olefin Metathesis

Abstract: A quantitative structure−activity relationship (QSAR) model is presented in which both the independent and dependent (response) variables are derived from density functional theory (DFT) calculations on a large set of 14-electron complexes, LCl2RuCH2, with different dative ligands, L. The multivariate model thus correlates the properties of the 14-electron complexes with a calculated measure of activity, with modest computational cost, and reproduces the experimental order of activity for the Grubbs ruthenium catalysts for olefin metathesis. The accuracy and applicability of the model is to a large extent due to the use of highly specific geometric and electronic molecular descriptors which establish a direct connection between activity and chemically meaningful donor ligand properties. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (+4) metallacyclobutane intermediate relative to the ruthenium−carbene structures dominating the rest of the re...

PtCl2-Catalyzed Rearrangement of Methylenecyclopropanes

Abstract: Alkylidenecyclopropanes readily convert into cyclobutene derivatives on treatment with catalytic amounts of PtCl2. The reaction is strongly accelerated when performed under an atmosphere of CO (1 atm). The resulting cyclobutenes are isolated in good to excellent yields for substrates bearing aliphatic as well as aromatic substituents R on their olefinic site. If the substituent R, however, is a very electron-rich arene, the cyclobutenes initially formed react further to give dimeric products with a previously unknown 1,2,2a,7a-tetrahydrospiro[cyclobuta[a]indene-7,1‘-cyclobutane skeleton. A mechanism accounting for these experimental observations as well as for a deuterium-labeling experiment is proposed which implies reactive intermediates at the nonclassical cation/carbene interface. Furthermore it is shown that the PtCl2-catalyzed cyclobutene formation can be geared with subsequent ring-opening/ring-closing metathesis (ROM/RCM) events. Finally, a convenient “one pot” method for the preparation of the al...

Rate acceleration in olefin metathesis through a fluorine-ruthenium interaction.

Abstract: The synthesis, structure, and performance of new ruthenium-based olefin metathesis catalysts, featuring fluorinated NHC ligands are presented. The introduction of halogen atoms into the N-heterocyclic carbene ligand profoundly alters the catalytic activity and can afford a more efficient catalyst. Structural investigations suggest that a fluorine−ruthenium interaction is responsible for this increased activity.

Ring‐Closing Metathesis as a Basis for the Construction of Aromatic Compounds

Abstract: The use of metathesis, especially in the context of ring-closing metathesis (RCM) to form five- and six-membered rings, is widespread in organic chemistry today. However, there are surprisingly few examples of the reaction being used to form aromatic compounds. The central place of aromatic compounds in both medicinal chemistry and natural products synthesis, coupled with the efficiency and functional group tolerance of RCM catalysts, means that there is now an interesting opportunity to use RCM for the synthesis of arenes. Although the formation of an aromatic compound was viewed in many early examples as an undesirable degradation product, several rationally designed methods towards the preparation of aromatic compounds by RCM have recently been developed.

Selective construction of carbocyclic eight-membered rings by ring-closing metathesis of acyclic precursors.

Abstract: Although the efficient preparation of cyclooctanoids has remained a long-standing objective, a real breakthrough was possible only relatively recently with the development of more specific reagents and by the development of new reactions. The direct construction of carbocyclic eight-membered rings from acyclic precursors is still a challenge, mainly because of unfavorable entropic and enthalpic factors that preclude ring formation. This Minireview describes the utilization of ring-closing metathesis (RCM) as a new approach to synthesize cyclooctanoids.

Surface versus Molecular Siloxy Ligands in Well‐Defined Olefin Metathesis Catalysts: [{(RO)3SiO}Mo(NAr)(CHtBu)(CH2tBu)]

Abstract: Keywords: carbene ligands ; heterogeneous catalysis ; metathesis ; molybdenum Reference EPFL-ARTICLE-204411doi:10.1002/anie.200503205View record in Web of Science Record created on 2015-01-08, modified on 2017-12-03

Advanced Fine-Tuning of Grubbs/Hoveyda Olefin Metathesis Catalysts: A Further Step toward an Optimum Balance between Antinomic Properties

Abstract: A 95% yield in the cross metathesis of acrylonitrile with a model olefin is achieved at 25 °C with only 3 mol % of the new air-stable ruthenium catalyst 1f shown in the enclosed structural diagram. Even more remarkable are the performances of its boosted version 1g incorporating an electron-withdrawing group. Both these new enhanced versions of Hoveyda catalyst are readily available from Grubbs second generation precatalyst upon reaction with a styrenyl ether the end group of which has been functionalized by an ester function. The latter acts as a weakly coordinating ligand allowing to reach a desirable balance between antinomic properties such as the catalyst's stability, a high activity for challenging substrates, and a high initiation rate.

A new concept for the noncovalent binding of a ruthenium-based olefin metathesis catalyst to polymeric phases: preparation of a catalyst on Raschig rings.

Abstract: A new concept for noncovalent immobilization of a ruthenium olefin metathesis catalyst is presented. The 2-isopropoxybenzylidene ligand of a Hoveyda−Grubbs carbene is further modified by an additional amino group (7) and immobilization is achieved by treatment with sulfonated polystyrene forming the corresponding ammonium salt. In this novel strategy for the immobilization of ruthenium-based metathesis catalysts, the amino group plays a two-fold role, being first an active anchor for immobilization and second, after protonation, activating the catalysts (electron donating to electron withdrawing activity switch). The polymeric support was prepared by precipitation polymerization which led to small bead sizes (0.2−2 μm) and large surface areas. Compared to commercial resins this tailor-made phase showed superior properties in immobilization of complex 7. This concept of immobilization was applied to glass−polymer composite megaporous Raschig rings. Ru catalyst 7 on Raschig rings was used under batch condit...

Facile One-Pot Synthesis of Brush Polymers through Tandem Catalysis Using Grubbs' Catalyst for Both Ring-Opening Metathesis and Atom Transfer Radical Polymerizations

Abstract: Using Grubbs' catalyst Cl2(PCy3)2Ru=CHPh for tandem catalysis, poly(methyl methacrylate)-based brush polymers were prepared by one-pot sequential ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP) via an inimer having both an alpha-bromoisobutyrate ATRP initiator functionality and an exo-norbornenyl ROMP monomer functionality. The surface morphologies and aggregation behaviors of these nanoscopic single molecules were studied by tapping-mode atomic force microscopy measurements on mica.

Ruthenium metallacycles derived from 14-electron complexes. New insights into olefin metathesis intermediates.

Abstract: Ruthenium(IV) metallacycles derived from both ethylene and propene are reported. The propene-derived metallacycles represent the first observed examples of substituted ruthenacyclobutanes and offer new insight into the preferred stereochemical orientation about metathesis intermediates. In addition, a metallacycle possessing an unsymmetrical N-heterocyclic carbene (NHC) ligand was prepared and investigated to ascertain the dynamics of the NHC relative to the metallacycle ring. Metallacycles investigated were found to possess exchange cross-peaks between the α- and β-positions in the 2D NMR, indicating a dynamic structure. The implications of these results to the mechanism of ruthenium-catalyzed olefin metathesis are discussed.

Search for solutions to the reactivity and selectivity problems in enyne metathesis.

Abstract: Enyne metathesis is a powerful carbon-carbon bond-forming reaction to generate 1,3-dienes from an alkyne and an alkene. Different from the diene and diyne metathesis, the enyne metathesis suffers from both regio- and stereoselectivity problems, yet there is no general solution to these problems. This Account briefly describes the evolution of various new strategies and substrate platforms from these laboratories to address the reactivity and selectivity issues in the enyne metathesis processes.

Allenylidene-to-indenylidene rearrangement in arene-ruthenium complexes: a key step to highly active catalysts for olefin metathesis reactions.

Abstract: The allenylidene−ruthenium complexes [(η6-arene)RuCl(CCCR2)(PR‘3)]OTf (R2 = Ph; fluorene, Ph, Me; PR‘3 = PCy3, PiPr3, PPh3) (OTf = CF3SO3) on protonation with HOTf at −40 °C are completely transformed into alkenylcarbyne complexes [(η6-p-cymene)RuCl(⋮CCHCR2)(PR3)](OTf)2. At −20 °C the latter undergo intramolecular rearrangement of the allenylidene ligand, with release of HOTf, into the indenylidene group in derivatives [(η6-arene)RuCl(indenylidene)(PR3)]OTf. The in situ-prepared indenylidene−ruthenium complexes are efficient catalyst precursors for ring-opening metathesis polymerization of cyclooctene and cyclopentene, reaching turnover frequencies of nearly 300 s-1 at room temperature. Isolation of these derivatives improves catalytic activity for the ring-closing metathesis of a variety of dienes and enynes. A mechanism based on the initial release of arene ligand and the in situ generation of the active catalytic species RuCl(OTf)(CH2)(PR3) is proposed.

Total synthesis of (+)-cylindramide A.

Abstract: A total synthesis of cylindramide A has been completed in 19 steps. The key step of the synthesis is a tandem ring-opening−ring-closing−cross metathesis that converts a readily available norbornene into an advanced intermediate.