Showing papers on "Norbornadiene published in 2003"

A chiral chelating diene as a new type of chiral ligand for transition metal catalysts: its preparation and use for the rhodium-catalyzed asymmetric 1,4-addition.

Abstract: As a new type of chiral ligand, a C2-symmetric norbornadiene derivative (1R,4R)-2,5-dibenzylbicyclo[2.2.1]hepta-2,5-diene (1) was prepared and used for the rhodium-catalyzed asymmetric addition of organoboron and -tin reagents to α,β-unsaturated ketones, which gave high yields of the 1,4-addition products with up to 99% enantioselectivity.

Probing the stereoselectivity of the ruthenium-catalyzed ring-opening metathesis polymerization of norbornene and norbornadiene diesters

Abstract: The ring-opening metathesis polymerization (ROMP) of two optically active 2,3-dicarboalkoxynorbornadienes derived from (S)-(−)-2-methyl-1-butanol and (R)-(−)-2-butanol were carried out in the presence of the [RuCl2(p-cymene)]2 catalyst precursor activated by trimethylsilyldiazomethane (TMSD). 1H and 13C NMR analyses showed that a high degree of stereoregularity was achieved, and homonuclear proton−proton COSY spectroscopy indicated that the major trans fractions of the polymers were most likely isotactic, while the minor cis fractions were syndiotactic. Ring-opened metathesis polymers were also made from 2,3-dicarbomethoxynorbornadiene and exo,exo-2,3-dicarbomethoxy-5-norbornene. They were hydrogenated into the corresponding polynorbornanes using diimide. The NMR spectra of the reduced materials confirmed that the unsaturated parent polymers had an all-trans highly isotactic microstructure. A tentative mechanism involving arene loss, carbene formation, and monomer chelation is proposed to account for the ...

Easily Accessible Ring Opening Metathesis and Atom Transfer Radical Polymerization Catalysts based on Arene, Norbornadiene and Cyclooctadiene Ruthenium Complexes Bearing Schiff Base Ligands

Abstract: A group of four mixed ligand ruthenium(II)-Schiff base complexes has been synthesized and characterized. These complexes are easily accessible from [RuCl2(p-cymene)]2, [RuCl2(NBD)]n, [RuCl2(COD)]x and salicylaldimine salts. They have been found to serve as good catalyst precursors for ring-opening metathesis polymerization (ROMP) and atom transfer radical polymerization (ATRP) of different vinyl monomers. The catalytic activity and the control over the produced polymer can be dramatically improved after the addition of additives such as Et2AlCl and (n-Bu)2NH.

The Synthesis, Characterisation, and Reactivity of Some Polydentate Phosphinoamine Ligands with Benzene-1,3-diyl and Pyridine-2,6-diyl Backbones

Abstract: The polydentate phosphinoamines 1,3-{(Ph2P)(2)N}(2)C6H4 and 2,6-{(Ph2P)(2)N}(2)C5H3N have been prepared in a single step from the reaction of the amines 1,3-(NH2)(2)C6H4 or 2,6-(NH2)(2)C5H3N with Ph2PCl in presence of Et3N (1:4:4 molar ratio) in CH2Cl2. Reaction of 1,3-{(Ph2P)(2)N}(2)C6H4 or 2,6-{(Ph2P)(2)N}(2)C5H3N with elemental sulfur or selenium in CH2Cl2 affords the corresponding tetrasulfide or tetraselenide, respectively, in good yield. The complexes [1,3-{Mo(CO)(4)(Ph2P)(2)N}(2)(C6H4)] and [2,6-{Mo(CO)(4)(Ph2P)(2)N}(2)(C5H3N)] were prepared from the reaction reaction of these phosphinoamines with [Mo(CO)(4)(nbd)] (nbd = norbornadiene) in toluene, and the structure of the latter complex has been determined by single-crystal X-ray diffraction analysis.

CpRuCl(PPh3)2/MeI Catalyst System for the [2+2] Cycloaddition of Norbornenes with Disubstituted Alkynes

Abstract: CpRu(PPh 3 ) 2 I generated in situ by mixing Cp-Ru(PPh 3 ) 2 Cl and methyl iodide catalyzed the [2+2] cycloaddition ol norbornene (or norbornadiene) and disubtituted alkynes to afford functionalized cyclobutenes.

Novel chiral 1,5-diaza-3,7-diphosphacyclooctane ligands and their transition metal complexes

Abstract: Reaction of bis(hydroxymethyl)phenylphosphine with (R)- or (S)-α-methylbenzylamine leads to the novel cyclic chiral bisphosphine ligands 1,5-(R,R)- and 1,5-(S,S)-bis(α-methylbenzyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane (1r or 1s). Novel chiral chelate complexes of PtII (2r, 3r and 3s), PdII (4s, 5s) and ReI (6r) have been obtained by reaction of 1r or 1s with [MCl2(cod)] (M = Pt, Pd; cod = 1,5-cyclooctadiene) and [{ReBr(CO)3(thf)}2]. Compounds 1–6 were characterised by multinuclear NMR (1H, 13C, 31P) and IR spectroscopy. The former technique revealed the presence of two inequivalent phosphorus atoms in 6r. The molecular structure of 1r confirmed the absolute configuration of the chiral centers and a chair-chair conformation of the heterocycle with equatorial orientation of the substituents on phosphorus and axial on nitrogen atoms. Compound 1s was used to form a palladium catalyst for the co-polymerisation of carbon monoxide and norbornadiene. The structures of the co-polymers obtained were characterised by GPC, 1H and 13C NMR spectroscopy and elemental analysis.

Synthesis and supramolecular properties of trimethylene-bridged clips.

Abstract: The novel trimethylene-bridged clips 3 and 4 have been synthesized by using repetitive stereoselective Diels-Alder reactions of the benzo- and naphthobismethylenenorbornenes 8 and 19 as dienes and norbornadiene 9 as bisdienophile, and subsequent dehydrogenation of the primary cyclobisadducts 10 and 20 by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Clips 3 and 4 serve as receptors for a variety of electron-deficient neutral and cationic aromatic substrates, comparable to the molecular tweezers 1 and 2. The thermodynamic parameters of the complex formation, K(a) and DeltaG, were determined by (1)H NMR titration experiments and, in the case of the highly stable complex TCNB 32@4, by the use of isothermal titration microcalorimetry. The finding that clip 4 forms more stable complexes than 3 can be explained by the larger van der Waals contact surfaces of the naphthalene sidewalls in 4 compared to the corresponding benzene systems in 3. In the complexes with 4 as receptor, the plane of each aromatic substrate molecule is calculated to be oriented almost parallel to the naphthalene sidewalls. However, in the complexes of tweezers 2, the substrate is usually oriented parallel to the central naphthalene spacer unit. Due to the more open topology of 4, most complexes were calculated to consist of two or more equilibrating noncovalent conformers.

A new ligand system based on a bipyridine-functionalized calix[4]arene backbone leading to mono- and bimetallic complexes.

Abstract: The synthesis of a new ligand system for mono- and bimetallic complexes based on a calixarene is described. Ligand BBPC (3, bis(bipyridine)-calix[4]arene) is obtained in three steps in 40% overall yield by first brominating one of the methyl groups of the 4,4'-dimethyl-2,2'-bipyridine in two steps and subsequently reacting it with p-tert-butylcalix[4]arene under basic conditions. Reaction of BBPC (3) with 2 equiv of [Rh(NBD)(2)]BF(4) or [Rh(NBD)(CH(3)CN)(2)]BF(4) (NBD = norbornadiene) produces the bimetallic compound BBPC[Rh(NBD)BF(4)](2) (4). Treatment of the ligand with PdCl(2)(CH(3)CN)(2) leads to the isolation of the bimetallic complex BBPC[PdCl(2)](2) (5). When the nickel precursor NiBr(2)(DME) (DME = dimethoxyethane) is reacted with BBPC, the bimetallic complex BBPC[NiBr(2)](2) (6) is isolated which, upon crystallization from methanol, gives the mononuclear bis(bipyridine) complex BBPC[NiBr(OMe)] (7). Full characterization includes X-ray structural studies of complexes 4, 5, and 7. The bimetallic compounds 4 and 5 show metal to metal distances of 4.334 A (for 4) and 3.224 A (for 5). For all three complexes, unique molecular packing arrangements were found, based on hydrophobic/hydrophilic interactions.

Photoinduced intramolecular electron transfer and triplet energy transfer in a steroid-linked norbornadiene-carbazole dyad.

Abstract: Bichromophoric compound 3β-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17β-yl-[2-(N-carbazolyl)acetate] (NBD-S-CZ) was synthesized and its photochemistry was examined by fluorescence quenching, flash photolysis, and chemically induced dynamic nuclear polarization (CIDNP) methods. Fluorescence quenching measurements show that intramolecular electron transfer from the singlet excited state of the carbazole to the norbornadiene group in NBD-S-CZ occurs with an efficiency (ΦSET) of about 14 % and rate constant (kSET) of about 1.6×107 s−1. Phosphorescence and flash photolysis studies reveal that intramolecular triplet energy transfer and electron transfer from the triplet carbazole to the norbornadiene group proceed with an efficiency (ΦTET + ΦTT) of about 52 % and rate constant (kTET + kTT) of about 3.3×105 s−1. Upon selective excitation of the carbazole chromophore, nuclear polarization is detected for protons of the norbornadiene group (emission) and its quadricyclane isomer (enhanced absorption); this suggests that the isomerization of the norbornadiene group to the quadricyclane proceeds by a radical-ion pair recombination mechanism in addition to intramolecular triplet sensitization. The long-distance intramolecular triplet energy transfer and electron transfers starting both from the singlet and triplet excited states are proposed to proceed by a through-bond mechanism.

Influence of the temperature on the enantioselectivity of the cobalt catalysed homo Diels–Alder reactions

Abstract: Depending on the variation of the temperature and on the nature of the substituent of alk-1-ynes 2, the enantioselectivity of the deltacyclene formation in the cobalt catalysed [2+2+2] cycloaddition of norbornadiene 1 and acetylenes 2, can be achieved with up to 98%. The best enantioselectivities observed around 14 °C are in agreement with the Isoinversion principle (IIP).

Cationic polymerization of norbornadiene

Abstract: The polymerization of norbornadiene (NBD) initiated by the 2-chloro-2,4,4-trimethylpentane/titanium tetrachloride system was investigated. Efforts were made to develop conditions for the living polymerization of NBD by the use of proton trap and electron donor in the −35 to −60 °C range however this objective was only partially attained. The molecular weights increased linearly with conversion, and the rate was first-order in confirmed monomer concentration up to approximately 25%; however, chain transfer became operational beyond this range. The microstructure of polynorbornadiene (PNBD) was investigated by high-resolution 1H and 13C NMR spectroscopy. According to these techniques, the chain consisted of about equal amounts of exo/exo and exo/endo connected tricyclic repeat units. The head and tail groups were identified and quantitated, and this led to absolute molecular weight determination by integration. Molecular weights obtained by this method and by gel permeation chromatography (relative to polyisobutylene standards) were in good agreement. NMR spectroscopy indicated the presence of small but still identifiable amounts of branching units and their structures. The plot of the glass-transition temperature against the reciprocal of the number-average molecular weight was linear and yielded a glass-transition temperature of 323 °C for the infinite molecular weight polymer. According to thermogravimetric analysis, PNBD was stable up to approximately 250 °C and showed a 5% weight loss at approximately 335 °C. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 732–739, 2003

Hydroformylation studies using Nafion supported rhodium based homogeneous catalysts

Abstract: A series of rhodium complexes of the form [Rh(L)2NBD]BF4 where L represents a phosphine ligand and [Rh(L–L)NBD]BF4 where (L–L) represents a diphosphine ligand and NBD = norbornadiene have been prepared. The complexes have been incorporated into Nafion by ion-exchange and the supported catalysts thus obtained have been compared with their homogeneous precursors for their activity as catalysts for the hydroformylation of hex-1-ene. The overall yield and the ratio of normal heptanal to branched products are reduced in all cases by introduction of the catalyst into Nafion. The modification of Nafion to produce a polymeric phosphine and its use as a ligand for hydroformylation using rhodium is also described.

Synthesis of New Photoresponsive Polymers Containing Trifluoromethyl-Substituted Norbornadiene Moieties

Abstract: Polymers containing trifluoromethyl (CF3)-substituted donor−acceptor NBD moieties in the side chain or in the main chain were synthesized by substitution or addition of a CF3−NBD−carboxylic acid derivative or by polycondensation of a CF3−NBD−dicarboxylic acid derivative. The photoisomerization of the NBD moieties in these polymers proceeded very smoothly to give the corresponding QC moieties. Furthermore, the NBD moieties exhibited efficient fatigue resistance.

Preparation of nano-sized silica with solar energy conversion and storage function by immobilization of norbornadiene moieties to hyperbranched poly(amido amine)-grafted nano-sized silica

Abstract: The immobilization of norbornadiene (NBD) moieties onto nano-sized silica surface by the direct condensation of the surface terminal amino groups of hyperbranched poly(amido amine) (PAMAM)-grafted silica with 3-phenyl-2,5-norbornadiene-2-carboxylic acid is examined. It is found that the immobilization of NBD moieties onto the silica successfully proceeds in the presence of N,N′-dicyclohexylcarbodiimide (DCC) as a condensing agent. The immobilized NBD moieties onto the silica surface increases with increasing amount of amino groups of hyperbranched PAMAM-grafted silica, but the percentage of amino groups used for the immobilization of NBD moieties is decreased. The immobilized NBD moieties on the silica surface are readily isomerized to quadricyclane (QD) by photo-irradiation in n-hexane. Stored thermal energy by QD-immobilized silica is released as thermal energy by heating. The stored thermal energy of QD-immobilized silica increases with progress of the photo-irradiation time and became constant, about ...

Photochemical reaction of N,N-dimethyl-4-chloroaniline with dienes: new synthetic paths via a phenyl cation.

Abstract: The irradiation of N,N-dimethyl-4-chloroaniline in the presence of open-chain dienes in acetonitrile leads to addition of the aminophenyl and chloro groups across one of the double bonds; transannular cyclization takes place with cyclic dienes, leading to an arylnortricyclene from norbornadiene and to 1-arylbicyclo[3.3.0]octanes from 1,5-cyclooctadiene. The reaction proceeds by photoheterolysis of the chloroaniline to yield the 4-aminophenyl cation and addition to a C=C double bond. The chemistry of the adduct cation depends on structure and medium, involving ion pairs in MeCN and solvated ions in CF 3 CH 2 OH. In the latter solvent, formation of ethers from open-chain alkenes is accompanied by Wagner-Meerwein hydride shift. In acetonitrile, the cation from cyclooctadiene partitions between deprotonation and Ritter addition, while the one from norbornadiene is reduced; both cations undergo nucleophilic addition in trifluoroethanol. The relevance of these cationic reactions under unusually mild conditions is discussed.

Synthesis and photochemical reaction of polystyrenes with pendant donor–acceptor type norbornadienes containing carbamoyl chromophores

Abstract: The donor–acceptor type norbornadiene (D–A NBD) carboxylic acids containing carbamoyl groups such as dipropylcarbamoyl, methylphenylcarbamoyl, propylcarbamoyl, and phenylcarbamoyl groups were synthesized in good yields. Benzyl esters of the D–A NBD carboxylic acids were also prepared by the reaction of D–A NBD carboxylic acids with benzyl bromide as reference compounds for the polymers. Polystyrenes with pendant D–A NBDs were prepared with 100% degree of substitution by the reaction of D–A NBD carboxylic acids and poly[( p -chloromethyl)styrene] at 70 °C for 6 h using 1,8-diazabicyclo[5.4.0]undecene-7 as a base in N -methylpyrrolidone. The photochemical valence isomerizations of the reference compounds and all the NBD polymers were examined upon UV-irradiation in THF solution or in the film state, and it was found that the polymer containing phenylcarbamoyl group had especially high photoreactivity. In addition, the rate of the photochemical reaction of the NBD polymers increased efficiently by the addition of 4,4′-bis(diethylamino)benzophenone as photosensitizer in the film state, and all the NBD groups of the polymers isomerized to the QC group in as little as only 20 s. The stored thermal energy of the irradiated polymers was also evaluated by DSC to be 32–52 kJ/mol.

Synthesis and characterization of two novel star blocks: tCum[poly(isobutylene‐b‐norbornadiene)]3 and tCum[poly(norbornadiene‐b‐isobutylene)]3

Abstract: Two structurally closely related three-arm star blocks were synthesized and characterized: tCum(PIB-b-PNBD)3 and tCum(PNBD-b-PIB)3 [where tCum (tricumyl) stands for the phenyl-1,3,5-tris(-2-propyl) fragment and PIB and PNBD are polyisobutylene and polynorbornadiene, respectively]. The syntheses were accomplished in two stages: (1) the preparation of the first (or inner) block fitted with appropriate chlorine termini capable of initiating the polymerization of the second (or outer) block with TiCl4 and (2) the mediation of the polymerization of the second block. Therefore, the synthesis of tCum(PIB-b-PNBD)3 was effected with tCum(PIB-Clt)3 [where Clt is tert-chlorine and number-average molecular weight (Mn) = 102,000 g/mol] by the use of TiCl4 and 30/70 CH3Cl/CHCl3 solvent mixtures at −35 °C. PNBD homopolymer contamination formed by chain transfer was removed by selective precipitation. According to gel permeation chromatography, the Mn's of the star blocks were 107,300–109,200 g/mol. NMR spectroscopy (750 MHz) was used to determine structures and molecular weights. Differential scanning calorimetry (DSC) indicated two glass-transition temperatures (Tg's), one each for the PIB (−65 °C) and PNBD (232 °C) phases. Thermogravimetric analysis thermograms showed 5% weight losses at 293 °C in air and at 352 °C in N2. The synthesis of tCum(PNBD-b-PIB)3 was achieved by the initiation of isobutylene polymerization with tCum(PNBD-Clsec)3 (where Clsec is sec-chlorine and Mn = 2900 g/mol) by the use of TiCl4 in CH3Cl at −60 °C. DSC for this star block (Mn = 14,200 g/mol) also showed two Tg's, that is, at −67 and 228 °C for the PIB and PNBD segments, respectively. It is of interest that the Clsec terminus of PNBD, , readily initiated isobutylene polymerization. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 740–751, 2003

Addition of Bromotrichloromethane and Tetrachloromethane to cis -Cyclooctene, Cyclohexene, and Norbornadiene in the Presence of Palladium(II) Complexes

Abstract: Addition of bromotrichloromethane and tetrachloromethane to alkenes in the presence of palladium(II) complexes begins with single-electron transfer from the palladium atom to polyhalomethane molecule, followed by fragmentation of the radical anion thus formed and generation of trichloromethyl radical. Halogen transfer to a carbon-centered radical arising from addition of trichloromethyl radical at the double bond occurs both from the polyhalomethane molecule and from halide palladium complex.

Modeling the adsorption of norbornadiene on the Si(001) surface: The predominance of non-[2+2]-cycloaddition products

Abstract: Norbornadiene (NBE) chemisorbs to a Si(001) surface in a flagpolelike structure that has potential as an anchor point for nanoscale molecular devices to the surface. Its bindings to the reconstructed Si(001)-(2×1) surface and a partially depassivated Si(001)-(2×1)-H surfaces are modeled by slab-based density functional theory using the PW91 density functional. This method is shown to quantitatively and qualitatively reproduce many known properties of bulk silicon, the silicon surface reconstruction, and the gas-phase NBE molecule. Four strongly bound adsorbate configurations are found, with the C–C bonds located either above a Si–Si dimer row or trough, oriented either parallel or perpendicular to each other. The calculated binding energies are 96, 85, 81, and 72 kcal mol−1 for the perpendicular row, perpendicular trough, parallel row, and parallel trough configurations, respectively, evaluated at quarter-monolayer coverage on the bare surface, with hydrogen passivation of the surrounding sites having lit...