Showing papers on "Metathesis published in 1996"

Synthesis and Applications of RuCl2(CHR‘)(PR3)2: The Influence of the Alkylidene Moiety on Metathesis Activity

Abstract: The reactions of RuCl2(PPh3)3 with a number of diazoalkanes were surveyed, and alkylidene transfer to give RuCl2(CHR)(PPh3)2 (R = Me (1), Et (2)) and RuCl2(CH-p-C6H4X)(PPh3)2 (X = H (3), NMe2 (4), OMe (5), Me (6), F (7), Cl (8), NO2 (9)) was observed for alkyl diazoalkanes RCHN2 and various para-substituted aryl diazoalkanes p-C6H4XCHN2. Kinetic studies on the living ring-opening metathesis polymerization (ROMP) of norbornene using complexes 3−9 as catalysts have shown that initiation is in all cases faster than propagation (ki/kp = 9 for 3) and that the electronic effect of X on the metathesis activity of 3−9 is relatively small. Phosphine exchange in 3−9 with tricyclohexylphosphine leads to RuCl2(CH-p-C6H4X)(PCy3)2 10−16, which are efficient catalysts for ROMP of cyclooctene (PDI = 1.51−1.63) and 1,5-cyclooctadiene (PDI = 1.56−1.67). The crystal structure of RuCl2(CH-p-C6H4Cl)(PCy3)2 (15) indicated a distorted square-pyramidal geometry, in which the two phosphines are trans to each other, and the alkyli...

Application of Ring-Closing Metathesis to the Synthesis of Rigidified Amino Acids and Peptides

Abstract: Ruthenium complexes 1a and 1b have been applied to the ring-closing metathesis (RCM) reactions of a number of dienic substrates. The substrate scope includes rings of 6 to 20 members. In addressing macrocyclic peptides, a class of tetrapeptide disulfides inspired the synthesis of the carbon−carbon bond analogs. Replacement of cysteine residues with allylglycines resulted in the acyclic precursors which were subjected to RCM to afford the corresponding macrocycles. In addition, several macrocycles were prepared which were not based upon disulfide-bridge-containing species found in nature. The method was found to function on dienic peptides which were either dissolved in organic solvents or bound to solid supports.

High metathesis activity ruthenium and osmium metal carbene complexes

Abstract: Ruthenium and osmium carbene compounds that are stable in the presence of a variety of functional groups and can be used to catalyze olefin metathesis reactions on unstrained cyclic and acyclic olefins are disclosed. Also disclosed are methods of making the carbene compounds. The carbene compounds are of the formula where M is Os or Ru; R 1 is hydrogen; R is selected from the group consisting of hydrogen, substituted or unsubstituted alkyl, and substituted or unsubstituted aryl; X and X 1 are independently selected from any anionic ligand; and L and L 1 are independently selected from any neutral electron donor. The ruthenium and osmium carbene compounds of the present invention may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, using cumulated olefins, and in a one-pot method using diazo compounds and neutral electron donors. The ruthenium and osmium carbene compounds of the present invention may be used to catalyze olefin metathesis reactions including, but not limited to, ROMP, RCM, depolymerization of unsaturated polymers, synthesis of telechelic polymers, and olefin synthesis.

Tandem Ring Opening−Ring Closing Metathesis of Cyclic Olefins

Abstract: Ruthenium alkylidene 1 has been utilized in the tandem ring opening−ring closing metathesis of cyclic olefins. This reaction produces a bicyclic molecule from a cyclic olefin. Reactivity is dependent upon strain, and thus ring size, in the substrate molecules. Competing oligomerization is observed in substrates having low ring strain; this process is inhibited by increasing dilution of the reaction or by adding alkyl substitution to the acyclic olefins.

Synthesis of Refractory Ceramics via Rapid Metathesis Reactions between Solid-State Precursors

Abstract: Chemical exchange (metathesis) reactions are used in many syntheses of important solids. While metathesis reactions in the liquid and gas phase are well-known, metathesis reactions from solid-state precursors have received much less attention. This review details work on the synthesis of refractory ceramics via rapid metathesis reactions between solid metal halides and alkali (or alkaline earth) metal main group compounds (e.g., Li3N or MgB2). The discussion includes thermodynamic considerations in choosing appropriate precursor couples. Through a careful choice of precursors, rapid, highly exothermic reactions can reach high temperatures (>1000 °C) on very short time scales (<1 s). The products are often crystalline and single phase with crystallite sizes varying from tens of angstroms to a few microns, depending on the refractory nature of the material and the reaction conditions (i.e., scale and the use of inert additives). The utility of this metathesis route is demonstrated by metal nitride, boron ni...

Synthesis of α-Norbornenylpoly(ethylene oxide) Macromonomers and Their Ring-Opening Metathesis Polymerization

Abstract: This work focuses on the synthesis and the ring-opening metathesis polymerization of α-norbornenylpoly(ethylene oxide) macromonomers. Although obtained in 100% yield and with a narrow distribution of molar masses, the polymacromonomers exhibit degrees of polymerization that are systematically higher than expected. The reasons for this feature are thoroughly discussed.

A Concise Total Synthesis of Dactylol via Ring Closing Metathesis

Abstract: A straightforward total synthesis of the cyclooctenoid sesquiterpene dactylol (1) and of 3a-epi-dactylol (13) has been achieved in six synthetic operations. The unusual rearranged bicyclo[6.3.0]undecane isoprenoid skeleton of these target molecules has been formed via an initial three-component coupling triggered by 1,4-addition of a methylcopper reagent (MeLi, CuI, Bu(3)P) to cyclopentenone, followed by trapping of the enolate formed with 2,2-dimethyl-4-pentenal as the electrophile. The aldol 8 thus obtained was elaborated into the trans-disubstituted cyclopentanone derivative 10 which reacted with a methallylcerium reagent to afford a mixture of the tertiary alcohols 11a and 12a. Separation and O-silylation of these diastereoisomers, ring-closing metathesis (RCM) of the resulting dienes 11b and 12b to form the cyclooctene ring using Schrocks molybdenum carbene 5 as a precatalyst, and a final deprotection afforded the title compound and its epimer in excellent yields. This approach clearly surpasses previous ones in terms of efficiency, flexibility, accessibility of the substrates, number of steps, atom economy, and overall yield.

Olefin metathesis coupling using ruthenium and osmium carbene complexes

Abstract: Disclosed are ruthenium and osmium carbene compounds which are stable in the presence of a variety of functional groups and which can be used to catalyze olefin metathesis reactions on unstrained cyclic and acyclic olefins. Specifically, the present invention relates to carbene compounds of the formula ##STR1## wherein: M is Os or Ru; R and R 1 are independently selected from hydrogen and functional groups C 2 -C 20 alkenyl, C 2 -C 20 alkynyl, C 1 -C 20 alkyl, aryl, C 1 -C 20 carboxylate, C 2 -C 20 alkoxy, C 2 -C 20 alkenyloxy, C 2 -C 20 alkynyloxy, aryloxy, C 2 -C 20 alkoxycarbonyl, C 1 -C 20 alkylthio, C 1 -C 20 alkylsulfonyl or C 1 -C 20 alkylsulfinyl; each optionally substituted with C 1 -C 5 alkyl, a halogen, C 1 -C 5 alkoxy or with a phenyl group optionally substituted with a halogen, C 1 -C 5 alkyl or C 1 -C 5 alkoxy; X and X 1 are independently selected from any anionic ligand; and L and L 1 are each trialkyl phosphine ligands where at least one of the alkyl groups on the phosphine is a secondary alkyl or a cycloalkyl. A broad array of metathesis reactions are enabled including ring-opening metathesis polymerization of cyclic olefins, ring closing metathesis of acyclic dienes, cross metathesis involving at least one acyclic or unstrained cyclic olefin, and depolymerization of unsaturated polymers.

Catalytic and Enantioselective Route to Medium-Ring Heterocycles. Asymmetric Zirconium-Catalyzed Ethylmagnesation of Seven- and Eight-Membered Rings

Abstract: A variety of medium-ring heterocycles, prepared efficiently by the Ru-catalyzed diene metathesis method, undergo asymmetric catalytic ethylmagnesation to afford nonracemic unsaturated alcohols and ...

Synthesis of ABA triblock copolymers of norbornenes and 7-oxanorbornenes via living ring-opening metathesis polymerization using well-defined, bimetallic ruthenium catalysts

Abstract: Norbornene and 7-oxanorbornene derivatives were polymerized by ring-opening metathesis polymerization (ROMP) in a living manner by employing three new well-defined, bimetallic ruthenium catalysts (...

Reactions of Si-Si σ-Bonds with Bis(t-alkyl isocyanide)palladium(0) Complexes. Synthesis and Reactions of Cyclic Bis(organosilyl)palladium Complexes

Abstract: Four- to seven-membered cyclic bis(organosilyl)palladium(II) complexes were synthesized by oxidative addition of cyclic disilanes onto bis(t-butyl isocyanide)palladium(0) as well as by intramolecular metathesis of the two Si–Si bonds of bis(disilane)s with the palladium(0) complex. The 4-membered bis(organosilyl)bis(t-butyl isocyanide)palladium complex prepared from 2,2-bis(1,1,2,2-tetramethyl-2-phenyldisilanyl)-1,3-dithiane was characterized by a single-crystal X-ray method, which showed an unusually short cross-ring Si–Si distance in the 4-membered ring. The reactivities of the cyclic bis(organosilyl)bis(t-butyl isocyanide)palladium complexes toward phosphines, cyclic disilanes, and alkynes were also examined. Cyclic organosilicon compounds were synthesized by bis-silylation of alkynes with cyclic bis(organosilyl)palladium intermediates generated by the intramolecular metathesis. The bis-silylation was also applicable for electron-deficient alkenes: α,β-unsaturated ketones gave 7-membered 1,4-bis-silyla...

Living Ring-Opening Metathesis Polymerizations of 3,4-Disubstituted Cyclobutenes and Synthesis of Polybutadienes with Protic Functionalities

Abstract: The compounds bis(phenylmethyl) cis-cyclobutene-3,4-dicarboxylate (1) and cis-3,4-bis(2-oxa-3-phenylpropyl)cyclobutene (2) were polymerized by ring-opening metathesis polymerizations using Mo(CHC(CH3)2R1)(NC6H3-2,6-diisopropyl)(OR2)2 (R1 = Ph, R2 = C(CH3)3, I; R1 = CH3, R2 = C(CF3)2CH3, II) as the initiator. For I as initiator, no chain termination or chain transfer processes were observed during the lifetime of the polymerization, showing that the polymerizations of 1 and 2 by I are living. Block copolymers containing 1 and 2 and diethyl cis-cyclobutene-3,4-dicarboxylate (5) were synthesized. The double bonds of the poly-2 and poly-3 ranged from 70% trans to 92% cis depending on the initiator and monomer used. Poly-1 and poly-2 were treated with iodotrimethylsilane and converted to the acid-functionalized and alcohol-functionalized poly-4 and poly-6, respectively. Block copolymers were treated with iodotrimethylsilane and converted to materials with blocks bearing protic functionalities and blocks bearin...

Acyclic diene metathesis (ADMET) polymerization using a well‐defined ruthenium based metathesis catalyst

Abstract: A series of α,ω-dienes have been exposed to Ru(Cl 2 )(CH=Ph)(PCy 3 ) 2 , [Ru] 3, metathesis catalyst under acyclic diene metathesis (ADMET) conditions, and the products were compared with those obtained previously using Schrock's well defined Mo and W alkylidenes. High molecular weight polymers can be synthesized using [Ru] 3 if lower monomer to catalyst ratios (with respect to [Mo] 1 and [W] 2 are used. Monomer structure plays a role since 1,9-decadiene produces molecular weights typical of ADMET chemistry, whereas 1,5-hexadiene produces only oligomers. This decrease in reactivity is attributed to a manifestation of intramolecular π-complexation of the tethered olefin in 1,5-hexadiene polymerizations. Substituents at the 2-position of α,ω-dienes also affects polymerization in that only dimers can be produced from 2-methyl-1,5-hexadiene, a result similar to the same reaction performed with [W] 2. Diallyl ether oligomerizes effectively in the presence of [Ru] 3 with a ratio of cyclics to oligomers comparable to results obtained with [Mo] 1 and [W] 2.

Synthesis of α-norbornenyl polystyrene macromonomers and their ring-opening metathesis polymerization

Abstract: Polystyrene macromonomers, fitted with a norbornenyl unsaturation in α position, have been obtained from a norbornene based carbanionic initiator that has been purposely designed to this end. The advantages of this synthetic scheme over that based on the deactivation of living PS chains are emphasized. One of the macromonomers prepared has been subsequently polymerized by ring-opening metathesis, using a Schrock complex as initiator. The characterization of the resulting polymacromonomers showed that the metathesis process occurred under “living” conditions.

Determination of the tacticity of ring‐opened metathesis polymers of norbornene and norbornadiene by 13C NMR spectroscopy of their hydrogenated derivatives

Abstract: The 125 MHz 13 C nuclear magnetic resonance (NMR) spectra of the hydrogenated derivatives of ring-opened metathesis polymers (ROMP) of bicydo[2.2.1]hept-2-ene (norbornene) and bicydo[2.2.1]hepta-2,5-diene (norbornadiene) prepared using a range of initiators, have been analysed in detail. The signals due to the methylene carbon atoms in the enchained cyclopentyl rings of these polymers show fine structure which is assigned to m and r ring dyads and mm, mr/rm and rr ring triads, enabling the tacticity of the hydrogenated polymer, and thus of the polyolefin precursor, to be determined for high-cis or high-trans polymers. Novel methods for making high-cis polymers are described and solvent-dependent variations in the stereospecificity of the propagation reaction have been observed.

Ring-Opening Metathesis Polymerizations in Carbon Dioxide

Abstract: The polymerization of norbornene was successfully initiated with Ru(H2O)6(tos)2 in a carbon dioxide medium. The resultant poly(norbornene) was prepared with yields and molecular weights comparable to those obtained in other solvent systems. It was also found that the cis/trans ratio of the polymer microstructure could be controlled by the addition of various amounts of methanol to the reaction mixture. A detailed account of the polymerizations is described and believed to be the first example of a ring-opening metathesis polymerization in carbon dioxide.