Showing papers on "Norbornene published in 2000"

Synthesis of Norbornenyl Polymers with Bioactive Oligopeptides by Ring-Opening Metathesis Polymerization

Abstract: Synthetic norbornenyl polymers with pendent cell adhesive sequences glycine-arginine-glycine-aspartic acid (GRGD) and serine-arginine-asparagine (SRN) were synthesized by ring-opening metathesis polymerization (ROMP) using newly developed ruthenium initiators. Initially, simpler polymers with pendent glycine, alanine, or penta(ethylene glycol) (EO_5) units attached directly or through ethyl and propyl spacers to various norbornenyl backbones were synthesized using Ru CHPh(Cl)_2(PCy_3)_2 (1) as the initiator. The molecular weights, PDI's, polymerization times, yields, and glass transition temperatures were compared for these polymers. As a result of this comparison, poly(5-norbornene-2-carboxyl) was chosen as the backbone for the more complex oligopeptide containing polymers, and norbornene monomers with pendent EO_5 (21), GRGD (24), and SRN (25) units were made. Monomers 21 and 24 were copolymerized to form a poly(norbornene) containing 9.2 mol % GRGD (26a) using 1 as the initiator. However, incorporating larger amounts of GRGD resulted in extremely low yields of polymers that exhibited bimodal molecular weight distributions. Homopolymers and copolymers with larger amounts of GRGD and SRN were synthesized in good yields (32−92%) with monomodal molecular weight distributions using the newly developed, more active, 2,3-dihydroimidazolylidene initiators, Ru CHPh(Cl)_2(PCy_3)(DHIMes) (2) and Ru CH−CH C(CH_3)_2(Cl)_2(PCp_3)(DHIMes) (3). In this way, EO_5 containing copolymers with 49 mol % GRGD (26b), 53 mol % SRN (27b), or 32 mol % GRGD and 21 mol % SRN (28a) were synthesized, as well as copolymer 28b with 53 mol % GRGD and 47 mol % SRN. To alter the presentation of the GRGD, an EO_5 containing copolymer with a propyl spacer between the GRGD and the backbone (30) was also synthesized.

Molecular dynamics simulation study of Norbornene-POSS polymers

Abstract: Abstract Atomistic molecular dynamics simulations have been used to delineate the effects of introducing polyhedral oligomeric silsesquioxane (POSS) moieties substituted by cyclopentyl and cyclohexyl rings as pendant groups on polynorbornene. Simulations were also performed on polynorbornene for comparison. Calculated volume–temperature behavior and X-ray scattering profiles matched well with experimental results. Most importantly, the effects of incorporating the POSS moieties into the polymer have been identified via simulations. These were judged on the basis of the increase in the glass transition temperature, retardation of the chain dynamics and improvements in the calculated elastic tensile, bulk and shear moduli of the POSS containing polymers compared to the norbornene homopolymer. The most important conclusion from the study is that aggregation of the POSS moieties is not required for the beneficial effects to be realized. Indeed, the simulations show that there is no tendency for aggregation to occur among the POSS moieties if they are well dispersed to begin with over the time scale of the simulation. Packing features are delineated with the aid of intermolecular site–site radial distribution functions. In addition, the mean squared displacement of the POSS moieties in the polymer matrix was found to be very small at all temperatures leading to a slowing of the segmental dynamics of the polymer chain, and thereby imparting the macroscopically observed stiffness. It is reasoned that the chief source of reinforcement arises from the POSS moieties behaving as strong anchor points in the polymeric matrix. This has more to do with the ponderous nature of these moieties versus any specific intermolecular interactions.

Ru(II)-Catalyzed Cycloadditions of 1,6-Heptadiynes with Alkenes: New Synthetic Potential of Ruthenacyclopentatrienes as Biscarbenoids in Tandem Cyclopropanation of Bicycloalkenes and Heteroatom-Assisted Cyclocotrimerization of 1,6-Heptadiynes with Heterocyclic Alkenes

Abstract: The ruthenium(II)-catalyzed tandem cycloaddition of 1,6-heptadiynes with bicyclic alkenes, such as bicyclo[3.2.1]heptenones and norbornene derivatives, furnishes the 1:2 adducts between the diynes and two molecules of the bicycloalkenes together with common [2 + 2 + 2] cyclocotrimerization products. The structure of a representative tandem 1:2 adduct between dimethyl dipropargylmalonate and 2,4-dimethylbicyclo[3.2.1]oct-6-en-3-one was unequivocally determined by X-ray analysis and was concluded to involve an unusual 1,2-dicyclopropylcyclopentene skeleton. On the basis of the spectroscopic analogy, the previously communicated structures of the tandem cycloadducts between the diynes and norbornene derivatives were corrected. The formation of the tandem double-cyclopropanation products from the diynes is chemical evidence of a bis-carbenoid hybrid structure, 1,3,5-metallacyclopentatriene, of the corresponding 2,4-metallacyclopentadiene intermediates. The selectivity for the formation of the tandem cyclopropa...

Fishing for Catalysts: Mechanism‐Based Probes for Active Species in Solution

Abstract: A combination of derivatization with charged substrates and electrospray-ionization mass spectrometry is used to fish out the active species in a catalytic reaction. The observed species in the mass spectrometer corresponds to the resting state of the catalyst. Data for the ring-opening metathesis polymerization (ROMP) of norbornene by (Cy3P)2Cl2Ru=CHPh (Cy=cyclohexyl) are used to illustrate the method.

Rhodium and Iridium β-Diiminate Complexes – Olefin Hydrogenation Step by Step

Abstract: The bulky β-diiminate ligands [(2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2)]– (X = Me, LMe; X = Cl, LCl) have been found to be effective in stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14- complex LMeRh(COE) (COE = cyclooctene) has a three-coordinate T-shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N2, MeCN, olefins), by the intramolecular coordination of a chlorine atom in LClRh(COE), or by an agostic interaction in LMeRh(norbornene). In solution at room temperature, LMeRh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2,3-dimethylbutene complex actually prefers the allyl hydride structure. Rhodium(I) complexes of LMe and LCl catalyze olefin hydrogenation; hydrogenation of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky β-diiminate ligands allow direct observation of a number of reactive intermediates or their iridium analogues, including an olefin–dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, hydrogenation according to this cycle appears to follow a 14-e/16-e path, in contrast to the more usual 16-e/18-e paths.

Asymmetric hydrocyanation of olefins catalyzed by chiral diphosphite–nickel complexes

Abstract: Chiral aryl diphosphite ligands derived from binaphthol were found to be effective in the nickel-catalyzed hydrocyanation of a variety of olefins. Enantioselective hydrocyanations of styrene, 4-substituted styrenes and norbornene were achieved with excellent regioselectivity and moderate enantioselectivity. The hydrocyanation of vinyl acetate gave 72.9% ee. The catalytic activity and the enantioselectivity of the Ni(0)–diphosphite complexes were found to be highly dependent on the structures of the ligands.

Characteristics of RuCl2(CHPh)(PCy3)2 as a Catalyst for Ring-Opening Metathesis Polymerization

Abstract: Ring-opened metathesis polymers and copolymers have been formed from norbornene, norbornadiene, a range of their derivatives, and cyclopentene using RuCl2(CHPh)(PCy3)2, as catalyst. 13C NMR analysis of the polymers illustrate a rather selective propagation reaction. The catalyst is highly regioselective, and the polymers are generally high trans with a strong isotactic bias. However, polymers from diene monomers tend to be less isotactic than those from the corresponding monoenes, and in the case of 7-methylnorbornadiene the polymer has an overall syndiotactic bias. A rate enhancing effect, noted previously, due to an oxygen atom proximate to the monomer double bond, is less pronounced than with other initiators. Catalyst activity, in the case of certain diene monomers, was shown to be monomer dependent and rate enhancements were also achieved using phenol as solvent. The results are interpreted in terms of the steric and electronic properties of both the catalyst and the monomers.

Dramatic solvent effects on ring-opening metathesis polymerization of cycloalkenes

Abstract: A series of metathesis polymers and copolymers have been formed and their structures were analysed by 13 C NMR spectroscopy. Noble metal and non-noble metal salt catalysts are distinguished by their behaviour in various solvents. Thus, in phenolic solvents, the former class produce alternating copolymers from cyclopentene and norbornene, while the latter are unaffected and produce random copolymers. In contrast, ether solvents have the effect of markedly increasing the cis content of polymers from the latter catalysts while the former are unaffected. The tacticity of various polymers are correlated through their hydrogenated derivatives and found to depend on the type of monomer as well as the catalysts.

Nickel (salen) / methylaluminoxane catalyzed polymerization of norbornene

Abstract: Nickel (salen) in conjunction with methylaluminoxane (MAO) catalyzes the polymerization of norbornene to poly (2,3-bicyclo [2.2.1] hept-2-ene) [poly (norbornene)]. Methylaluminoxane (MAO) was used as cocatalyst as such (MAO-I) and after distilling off free trimethylaluminium (MAO-II). The catalyst system was very active in chlorobenzene at room temperature. It was also found that MAO-II gave higher activity as compared to MAO-I. The samples of polynorbornene were soluble in 1,2,4-trichlorobenzene. The poly (norbomene)s were characterized by intrinsic viscosity and thermal properties.

Homogeneous metallocene/MAO‐catalyzed polymerizations of polar norbornene derivatives: copolymerizations using ethene, and terpolymerizations using ethene and norbornene

Abstract: Terpolymerizations of norbornene derivatives containing different functional substituents were carried out with ethene and norbornene using the homogeneous catalyst system iPr[CpInd]ZrCl2/MAO. The norbornene derivative 5-norbornene-2-methanol and 5-norbornene-2-carboxylic acid were prereacted with triisobutylalmuninium to prevent the deactivation of the catalyst. 13 CNMR studies revealed the composition of the polymer. The incorporation rate was 5-12 mol-% at a content of 50 mol-% of the norbornene derivative in the monomer feed-stock. IR-GPC coupled experiments confirmed the homogeneous composition of the polymer. In addition, we investigated the ethene copolymerization and the ethene/norbornene terpolymerization using the trialkylsilyl protected norbornene derivated such at 5-norbornene-2-methyeleneoxytriethylsilane and 5-norbornene-2-methyeleneoxy-tert-butyldimethylsilane. These norbornene derivatives reveal an incorporation rates of 5-6 mol-% in the polymer at a content of 20 mol-% in the monomer feed-stock.

Norbornene-constrained cyclic peptides with hairpin architecture: design, synthesis, conformation, and membrane ion transport

Abstract: A novel family of hairpin cyclic peptides has been designed on the basis of the use of norbornene units as the bridging ligands. The design is flexible with respect to the choice of an amino acid, the ring size, and the nature of the second bridging ligand as illustrated here with the preparation of a large number of norborneno cyclic peptides containing a variety of amino acids in ring sizes varying from 12- to 29-membered, with the choice of the second bridging ligand being a rigid norbornene (11, 13a,b), an adamantane unit (7a,b and 8), or a flexible cystine residue (4a,b and 10). The presence of built-in handles (as protected COOH groups) permits the attachment of a variety of subunits as shown here with the ligation of Leu-Leu, Val-Val, or Aib-Aib pendants in 4b, 7b, and 13b, respectively. This novel class of constrained cyclic peptides are demonstrated to adopt beta-sheet- or hairpin-like conformation as shown by (1)H NMR and CD spectra. Membrane ion-transport studies have shown that the norborneno cyclic peptides 4b and 7b containing Leu-Leu or Val-Val pendants symmetrically placed on the exterior of the ring show high efficiency and selectivity in the transport of specifically monovalent cations. This property can be attributed to the hairpin-like architecture induced by the norbornene unit since the bis-adamantano peptide 15 containing two pairs of Leu-Leu pendants on the exterior is able to transport both monovalent (Na(+), K(+)) and divalent (Mg(2+)/Ca(2+)) cations.

Low-energy pathway for pauson-khand reactions: synthesis and reactivity of dicobalt hexacarbonyl complexes of chiral ynamines

Abstract: A family of dicobalt hexacarbonyl complexes of 1-(dialkylamino)-2-(trimethysilyl)acetylenes (3a−f) derived both from achiral and chiral amines [a, morpholine; b, (S)-2-methoxymethylpyrrolidine; c, (2R,5R)-2,5-bis(methoxymethyl)pyrrolidine; d, (±)-trans-2,5-bis(benzyloxymethyl)pyrrolidine; e, (2R,5R)-2,5-dimethylpyrrolidine; f, (S)-(α-methylbenzyl)benzylamine] has been prepared by a one-stage process from dichloroacetylene. The methanolysis at room temperature of these complexes (MeOH/K2CO3) induces the selective cleavage of the carbon−silicon bond, leading to the thermally unstable terminal ynamine complexes 12a−f. The Pauson−Khand reaction of 12a−f with strained olefins (norbornadiene and norbornene) takes place at unprecedentedly low temperatures (−35 °C) in the absence of chemical promoters. Diastereoselectivities of up to 94:6 are recorded in these reactions with the ynamine complexes derived from C2 symmetrical chiral auxiliaries (12c,d). The high reactivity depicted by terminal ynamines in the Pauso...

Norbornene open-ring polymers, products of hydrogenation thereof and processes for the production of both

Abstract: A norbornene open-ring polymer having a weight-average molecular weight of 500 to 1,000,000 in terms of polystyrene and a melting point of 140°C or above is produced by polymerizing a norbornene monomer mixture comprising a polycyclic norbornene monomer by using a polymerization catalyst containing as the main component a compound of a Group 6 transition metal of the periodic table bearing at least one imido group and at least two substituents (A) selected from the group consisting of alkoxy, aryloxy, alkylamido and arylamido; and hydrogenation of this polymer gives a product of hydrogenation thereof having a melting point of 140°C or above.

Ring-Opening Metathesis Polymerization via Ruthenium Complexes of Chelating Diphosphines

Abstract: High activity and low polymer polydispersity are found in ring-opening metathesis polymerization (ROMP) of norbornene by ruthenium complexes containing chelating diphosphine ligands. Treatment of compounds with general formula RuCl 2 (PP)(PPh 3 ) (PP = dppb (3b), binap (3c), dcypb (3d)) or [RuCl 2 (PP)] 2 (PP = dppb (4b)) with PhCHN 2 generates carbene complexes of the type RuCl 2 (PP)-(CHPh), which exhibit high ROMP activity without halide or phosphine abstraction. Investigations into the role of dissociated PPh 3 have shown that free phosphine acts as a poison, dramatically retarding the rate of polymerization via 3 relative to catalysts derived from 4.

Polynorbornene with Cross-Linkable Side Chains via Ring-Opening Metathesis Polymerization

Abstract: The ring-opening metathesis polymerization (ROMP) of 5-(methyl methacryloyl isocyanate)bicyclo[2.2.1]hept-2-ene [i.e., norbornene containing methacryloyl isocyanate (NBMMAI)] having cyclic olefin and terminal double bond was investigated in order to produce novel polymers with cross-linkable side chains under ligand exchange with MMA by using {RuCl2(CHPh)[P(C6H11)3]2} (II) as catalyst. Results of infrared spectra, 1H NMR, and 13C NMR showed that poly(NBMMAI) contains polynorbornene backbone ring structure and methacryloyl side chains. The 1H NMR spectrum of poly(NBMMAI) showed new vinyl protons as a broad signal between 5.15 and 5.37 ppm in the ratio of 0.4 and 2.8, corresponding to the vinyl protons of the cis and trans double bond of the ring-opened polymer, respectively. Gel formation was still observed in the polymerization of NBMMAI using ruthenium catalyst II with 20 mol % p-methoxyphenol (MEHQ). An increase in bulkiness of the ligand reduced the cross metathesis reaction and led to reduced activity...

Enantioselective Heck-type hydroarylation of norbornene with phenyl iodide catalyzed by palladium/quinolinyl-oxazolines

Abstract: Enantioselective Heck-type hydroarylation of norbornene with phenyl iodide catalyzed by the palladium complex of quinolinyl-oxazolines is described. Using DMSO as the solvent and HCOOH/Et 3 N as the hydride source, exo -2-phenybicyclo[2.2.1]heptane is produced in reasonable yields and up to 74% ee.

Incorporation of Conformationally Constrained β‐Amino Acids into Peptides

Abstract: The use of norbornene units to induce the formation of β-sheet and β-turn type structures in peptides is discussed. The norbornene unit is readily prepared by a desymmetrization reaction and is easily incorporated into a peptide chain. Depending upon the exact nature of the norbornene unit, it is possible to form structures which resemble parallel β-sheets, antiparallel β-sheets or β-turns. Similar peptide analogues incorporating a cis-2-amino-cyclopropane carboxylic acid unit can also be prepared. As an illustration of the application of this chemistry, a short, asymmetric synthesis of conformationally constrained metalloprotease inhibitors is presented. Copyright © 2000 European Peptide Society and John Wiley & Sons, Ltd.