Showing papers on "Metathesis published in 2001"

New Insights into the Mechanism of Ruthenium-Catalyzed Olefin Metathesis Reactions

Abstract: Over the past two decades, olefin metathesis has emerged as a mild and efficient method for the formation of carbon−carbon double bonds. In particular, (PCy_3)_2(Cl)_2RuCHPh (1)^2 has found extensive use in organic and polymer chemistry due to its high reactivity with olefins in the presence of a diverse array of functional groups. Recently, a new family of ruthenium-based olefin metathesis catalysts have been prepared by the substitution of a single PCy_3 ligand of 1 with an N-heterocyclic carbene. These new alkylidenes, particularly [Figure 1], exhibit dramatically increased activity over the parent system in ring-opening metathesis polymerization, ring-closing metathesis,4a and cross metathesis reactions. The mechanism of olefin metathesis reactions catalyzed by 1 has received intense investigation in our group and others and early studies established that phosphine dissociation is a crucial step along the reaction coordinate. As such, it has been suggested that the high activity of 2 and its analogues is due to their increased ability to promote this critical phosphine dissociation step. We report herein a detailed mechanistic study of phosphine exchange and initiation kinetics in alkylidenes 1 and 2. This study provides new and surprising evidence concerning the origin of the large activity differences between these two catalysts.

Comparative investigation of ruthenium-based metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands.

Abstract: Exchange of one PCy3 unit of the classical Grubbs catalyst 1 by N-heterocyclic carbene (NHC) ligands leads to "second-generation" metathesis catalysts of superior reactivity and increased stability. Several complexes of this type have been prepared and fully characterized, six of them by X-ray crystallography. These include the unique chelate complexes 13 and 14 in which the NHC- and the Ru-CR entities are tethered to form a metallacycle. A particularly favorable design feature is that the reactivity of such catalysts can be easily adjusted by changing the electronic and steric properties of the NHC ligands. The catalytic activity also strongly depends on the solvent used; NMR investigations provide a tentative explanation of this effect. Applications of the "second-generation" catalysts to ring closing alkene metathesis and intramolecular enyne cycloisomerization reactions provide insights into their catalytic performance. From these comparative studies it is deduced that no single catalyst is optimal for different types of applications. The search for the most reactive catalyst for a specific transformation is facilitated by IR thermography allowing a rapid and semi-quantitative ranking among a given set of catalysts.

Catalytic asymmetric olefin metathesis.

Abstract: This paper provides a survey of the first examples of efficient catalytic enantioselective olefin metathesis reactions. Mo-catalyzed asymmetric ring-closing (ARCM) and ring-opening (AROM) reactions allow access to myriad optically enriched compounds that are otherwise difficult to access.

Indenylidene complexes of ruthenium: optimized synthesis, structure elucidation, and performance as catalysts for olefin metathesis--application to the synthesis of the ADE-ring system of nakadomarin A.

Abstract: An optimized and large scale adaptable synthesis of the ruthenium phenylindenylidene complex 3 is described which employs commercially available diphenyl propargyl alcohol 5 as a stable and convenient carbene source. Previous ambiguities as to the actual structure of the complex have been ruled out by a full analysis of its NMR spectra. A series of applications to ring closing metathesis (RCM) reactions shows that complex 3 is as good as or even superior to the classical Grubbs carbene 1 in terms of yield, reaction rate, and tolerance towards polar functional groups. Complex 3 turns out to be the catalyst of choice for the synthesis of the enantiopure core segment 77 of the marine alkaloid nakadomarin A 60 comprising the ADE rings of this target. Together with a series of other examples, this particular application illustrates that catalyst 3 is particularly well suited for the cyclization of medium-sized rings by RCM. Other key steps en route to nakadomarin A are a highly selective intramolecular Michael addition setting the quaternary center at the juncture of the A and D rings and a Takai-Nozaki olefination of aldehyde 73 with CH2I2, Ti(OiPr)4 and activated zinc dust.

Alkyne metathesis: development of a novel molybdenum-based catalyst system and its application to the total synthesis of epothilone A and C.

Abstract: Sterically hindered molybdenum(III) amido complexes of the general type [Mo{(tBu)(Ar)N} 3 ] (1), upon treatment with CH 2 Cl 2 or other halogen donors, have been converted into highly effective catalysts for all kinds of alkyne metathesis reactions. Although the actual nature of the propagating species formed in situ is still elusive, halogen transfer to the Mo center of 1 plays a decisive role in the activation of such precatalysts. It was possible to isolate and characterize by X-ray crystallography some of the resulting molybdenum halide derivatives such as 15, 16 and 20 which themselves were shown to be catalytically active. Numerous applications illustrate the performance of the catalytic system 1/CH 2 Cl 2 which operates under mild conditions and tolerates an array of polar functional groups. The wide scope allows the method to be implemented into the total synthesis of sensitive and polyfunctional natural products. Most notable among them is a concise entry into the potent anticancer agents epothilone A (86) and C (88). The macrolide core of these targets is forged by ring closing alkyne metathesis (RCAM) of diyne 113, followed by Lindlar hydrogenation of cycloalkyne 114 thus formed. Since this strategy opens a stereoselective entry into (Z)-alkene 115, the approach is inherently more efficient than previous syntheses based on conventional RCM.

Olefin metathesis in supercritical carbon dioxide.

Abstract: Liquid or supercritical carbon dioxide (scCO2) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy3 or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO2, complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO2 provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO2 as a protective medium for basic amine functions. The latter phenomenon is expl...

Olefin metathesis involving ruthenium enoic carbene complexes.

Abstract: Olefin metathesis has become a valuable reaction in organic synthesis, as has been demonstrated by its frequent use as the key bond constructions for total syntheses of many natural products. With the recent discovery of highly active catalyst 1, trisubstituted and functionalized alkenes have been synthesized efficiently by cross-metathesis (CM), further expanding the substrate scope for this reaction. With these successes in hand, unprecedented metathesis reactions were explored. There have been no previous reports of the dimerization of α,β-unsaturated carbonyl compounds by a metathesis mechanism. Molybdenum- and tungsten-based catalysts form metallocyclobutane with acrylates, but they are inactive due to carbonyl oxygen chelation. Our group reported the synthesis of ester carbene 4 by a nonmetathesis route and showed that 4 was extremely reactive. In fact, ester carbene 4 was the first carbene to ring-open cyclohexene but did not react in a catalytic fashion.4 The nontrivial synthesis, lack of stability, and the ineffective catalytic activity of ester carbene 4 has limited its uses in organic synthesis.

Formation of Covalently Attached Polymer Overlayers on Si(111) Surfaces Using Ring-Opening Metathesis Polymerization Methods

Abstract: We describe a method for growing uniform, covalently attached polymer onto crystalline Si(111) surfaces H-Terminated Si was first chlorinated, and the surface-bound chlorine was then replaced by a terminal olefin using a Grignard reaction A ruthenium ring-opening metathesis polymerization catalyst was then crossed onto the terminal olefin, and the resulting surface was subsequently immersed into a solution of monomer to produce the desired surface-attached polymer The method provides a direct linkage between the polymer and the Si without the presence of an electrically defective oxide layer Growth of the polymeric layer could be controlled by varying the concentration of monomer in solution, and polynorbornene films between 09 and 5500 nm in thickness were produced through the use of 001−244 M solutions of norbornene

Total Synthesis of (−)‐Salicylihalamide

Abstract: A concise total synthesis of the potent cytotoxic marine natural products salicylihalamide A and B (la, b) is reported. Key steps of our approach were the asymmetric hydrogenation reactions of β-keto esters 18 and 32 catalyzed by [((S)-BINAP)Ru-Cl 2 ] 2 .NEt 3 and the cyclization of the macrolide core by ring closing olefin metathesis (RCM) using the "second-generation" ruthenium carbene complex 24 as the catalyst which bears an imidazol-2-ylidene ligand. The E/Z ratio obtained in this macrocyclization reaction was determined by the protecting groups at the remote phenolic OH group of the cyclization precursor. The elaboration of the resulting cycloalkene 37 into the final target involved a CrCl 2 -mediated synthesis of vinyliodide 49 which, after deprotection, did undergo a copper-catalyzed cross-coupling process with the (Z,Z)-configurated carboxamide 42 to form the labile enamide moiety of 1. Compound 42 was derived from a palladium-catalyzed Negishi coupling between butynylzinc chloride and 3-iodoacrylate 39 followed by a Lindlar reduction of enyne 40 thus obtained and a final aminolysis of the ester group.

Immobilization of Olefin Metathesis Catalysts on Monolithic Sol–Gel: Practical, Efficient, and Easily Recyclable Catalysts for Organic and Combinatorial Synthesis

Abstract: Glass-bound Ru-based catalysts! Ru-containing glass pellets efficiently promote olefin metathesis reactions and are easily employed in syntheses of compound libraries. These robust catalysts are active in air and commercially available solvents, can be recycled up to 16 times, and removed from reaction mixtures with a simple pair of tweezers (minimal solvent waste).

Highly active metathesis catalysts generated in situ from inexpensive and air stable precursors

Abstract: The invention provides a process for the in-situ generation of a metathesis active catalyst of the formula: comprising contacting an NHC carbene with a dimer of the formula [(arene)MX1X]2 and an alkyne of the formula RC≡CR1 or wherein M is ruthenium or osmium; X and X1 are the same or different and are each independently an anionic ligand; NHC is any N-heterocyclic carbene ligand; R, R1 and R2 are each independently hydrogen or a substituted or unsubstituted substituent selected from the group consisting of C1-C20 alkyl, C2-C20 alkenyl, C2-C20 alkynyl, aryl, C1-C20 carboxylate, C1-C20 alkoxy, C2-C20 alkenyloxy, C2-C20 alkynyloxy, aryloxy, C2-C20 alkoxycarbonyl, C1-C20 alkylthio, C1-C20 alkylsulfonyl and C1-C20 alkylsulfinyl; and R3 is OH. Preferably, the NHC is an s-IMES or IMES carbene ligand; the arene is preferably p-cymene and the alkyne is preferably acetylene. The invention further provides a process for ROMP and RCM reactions using the metathesis active catalysts.

Monolithic Materials: New High-Performance Supports for Permanently Immobilized Metathesis Catalysts.

Abstract: Polymerchemistry and materials sciences have seen the introductionof new trends by metathesis-based techniques, such as ring-opening metathesis polymerization (ROMP) or acyclic dienepolymerization (ADMET). Complementary, cross-metathesisand ring-closing metathesis find ample application in organicchemistry.

Recyclable metathesis catalysts

Abstract: Highly active, recoverable and recyclable transition metal-based metathesis catalysts and their organometallic complexes including dendrimeric complexes are disclosed, including a Ru complex bearing a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and styrenyl ether ligand. The heterocyclic ligand significantly enhances the catalytic activity, and the styrenyl ether allows for the easy recovery of the Ru complex. Derivatized catalysts capable of being immobilized on substrate surfaces are also disclosed. The present catalysts can be used to catalyze ring-closing metathesis (RCM), ring-opening (ROM) and cross metatheses (CM) reactions, and promote the efficient formation of various trisubstituted olefins at ambient temperature in high yield.

A Readily Available and User-Friendly Chiral Catalyst for Efficient Enantioselective Olefin Metathesis.

Abstract: No glove box required! A new chiral catalyst for olefin metathesis is prepared from commercially available materials and is equally or more effective and selective than the previous systems. The new user-friendly catalyst can be prepared in situ (see scheme, Ar=aryl, Tf=trifluoromethanesulfonyl), isolation not needed, outside of a glove box, and delivers reactivity and enantioselectivity levels formerly available only through multiple catalysts.

Total Synthesis of (−)-Cylindrocyclophanes A and F Exploiting the Reversible Nature of the Olefin Cross Metathesis Reaction

Abstract: Efficient total syntheses of the C2-symmetric (−)-cylindrocyclophanes A and F (1a and 1f) have been achieved. The initial strategy featured the use of a common advanced intermediate to assemble in stepwise fashion the required macrocycle of 1f, exploiting in turn a Myers reductive coupling followed by ring-closing metathesis. In a second-generation strategy, a remarkable cross olefin metathesis dimerization cascade was discovered and exploited to assemble the requisite [7,7]-paracyclophane macrocycles of both 1a and 1f from dienyl monomers. The successful syntheses also featured the effective use of the Danheiser annulation to construct substrates for both the Myers reductive coupling and the metathesis dimerizations strategies. Finally, the Kowalski two-step chain homologation of esters to siloxyalkynes proved superior over the original one-step protocol.

Formation of macrocycles via ring-closing olefin metathesis.

Abstract: The enhanced metathesis activity of 1,3-dimesityl-4,5-dihydroimidazole-2-ylidene ruthenium carbene catalyst 3 significantly increases the feasibility of synthesizing macrocyclic compounds. Catalyst 3 exhibits sufficient activity in RCM to dimerize α,β-unsaturated ester substrates and afford the corresponding head-to-tail (E,E)-dimeric (and trimeric) macrocycles. The dimerization appears to be under thermodynamic control with the product mixture dependent not only on the electronic and steric nature of the substrate but also on concentration.

First ruthenium complexes with a chelating arene carbene ligand as catalytic precursors for alkene metathesis and cycloisomerisation

Abstract: Electron-rich carbene precursors 2 and 3, containing the imidazolidin-2-ylidene moiety with one (2) and two (3) pendent N-(2,4,6-trimethylbenzyl) groups, on reaction with [RuCl2(arene)]2 lead to ruthenium(II) complexes 5 and 6 containing the chelating 8-electron mixed arene–carbene ligand; the X-ray diffraction crystal structure of RuCl2{η1-CN[CH2( η6-2,4,6-Me3C6H2)]CH2CH2N(CH2CH2OMe)} 6, was established. These complexes are precursors of the unstable ruthenium-allenylidene intermediates 7 and 8, but are active catalysts either for selective catalytic alkene metathesis or cycloisomerization, depending on the nature of the 1,6-diene.

Synthesis of polyethylene hybrid copolymers containing polyhedral oligomeric silsesquioxane prepared with ring‐opening metathesis copolymerization

Abstract: Ring-opening metathesis copolymerizations of cyclooctene and the polyhedral oligomeric silsesquioxane (POSS) monomer 1-[2-(5-norbornen-2-yl)ethyl]-3,5,7,9,11,13,15-heptacyclopentylpentacyclo[9.5.1.13,9.15,15.17,13] octasiloxane (POSS–norbornylene) were performed with Grubbs's catalyst, RuCl2(CHPh)(PCy3)2. Random copolymers were formed and fully characterized with POSS loadings as high as 55 wt %. Diimide reduction of these copolymers afforded polyethylene–POSS random copolymers. Thermogravimetric analysis of the polyethylene–POSS copolymers under air showed a 70 °C improvement, relative to a polyethylene control sample of similar molecular weight, in the onset of decomposition temperature based on 5% mass loss. The homopolymer of POSS–norbornylene was also synthesized. This polymer had a rigid backbone according to 1H NMR evidence of broad olefinic signals. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2920–2928, 2001

σ-Bond Metathesis of Alkanes on a Silica-Supported Tantalum(V) Alkyl Alkylidene Complex: First Evidence for Alkane Cross-Metathesis

Abstract: C-C activation of unactivated alkanes on a silica-supported neopentyl neopentylidene TaV complex [Eq. (1)] affords the alkane cross-metathesis products tBuCH2 R and an alkane-metathesis catalyst. Since the activity and product distributions are similar to those obtained with a silica-supported TaIII hydride, these results are a first step in understanding alkane σ-bond metathesis on metal hydrides.

Synthesis of Poly(macromonomer)s by Repeating Ring-Opening Metathesis Polymerization (ROMP) with Mo(CHCMe2Ph)(NAr)(OR)2 Initiators

Abstract: Various poly(macromonomer)s of ring-opened poly(norbornene) backbone containing ring-opened poly(norbornene) derivatives in the side chain have been prepared efficiently by repeating the ring-opening metathesis polymerizations (ROMP) using well-defined molybdenum initiator of the type, Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OR)2 [OR = OCMe3, OCMe(CF3)2]. The key steps for synthesis of the macromonomer are both exclusive end-capping and quantitative esterification of hydroxy group at the polymer chain end with norbornene carboxylic acid chloride, and the use of Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)[OCMe(CF3)2]2 was found to be an effective initiator in order for this polymerization to proceed to the complete conversion. The present synthetic approach should be useful for preparing various poly(macromonomer)s containing functional groups, especially amphiphilic poly(macromonomer) architectures precisely.

Siloxyalkyne–Alkene Metathesis: Rapid Access to Highly Functionalized Enones

Abstract: Mechanistically intriguing participation of siloxyalkynes occurs in the intramolecular Ru-catalyzed metathesis with terminal alkenes (see scheme; Ms=methanesulfonyl). Combined with efficient protodesilylation, this process resulted in the development of a new method for the synthesis of highly functionalized enones starting from readily accessible acyclic precursors. Heterocyclic and polycyclic compounds were prepared efficiently, which illustrates the generality of this novel method.

ROMP and RCM catalysed by (R3P)2Cl2RuCHPh immobilised on a mesoporous support

Abstract: Heterogeneous hybrid catalysts with respect to the metathesis of olefins have been prepared by ligand exchange reaction between Ru-alkylidene complexes (Grubbs’ type) and a phosphinated mesoporous matrix (P-MCM-41). The host/guest interaction is studied by FT-Raman, thermogravimetric analysis and N2 adsorption. Catalytic tests in olefin metathesis reveal ROMP activity towards norbornene, even in aquatic environment, and a very high activity for the RCM of diallylamine and diethyldiallylmalonate.