Showing papers on "Double bond published in 1994"

Evidence for resonance-assisted hydrogen bonding. 4. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H--O system by crystal structure correlation methods

Abstract: All cases of strong (2.50≤d(O--O)≤2.65 A) and very strong (d(O--O) <2.50 A) O-H---O hydrogen bonds whose geometries are known from accurate neutron or X-ray diffraction studies are reviewed and classified in chemical classes belonging to three fundamental types: (A) -O-H---O-, or negative charge assisted hydrogen bonding, (-)CAHB; (B)=O---H + --O=, or positive charge assisted hydrogen bonding, (+)CAHB; and (C) -O-H---O=, where the two oxygens are interconnected by a system of π-conjugated double bonds, or resonance-assisted hydrogen bonding, RAHB

Detection of conjugated dienes by second derivative ultraviolet spectrophotometry

Abstract: Publisher Summary This chapter discusses the detection of conjugated dienes by second derivative ultraviolet spectrophotometry. Conjugated dienes (CD) refers to two double bonds separated by a single bond. This structure is unusual in polyunsaturated fatty acids (PUFA). It is generally accepted that the occurrence of conjugated dienes in lipids means autoxidation of lipids. In fact, because of the divinylmethane structure, PUFA are particularly susceptible to hydrogen abstraction by free radical attack, becoming themselves free radical intermediates. This results in the rearrangement of the double bond to conjugated dienes and, in the presence of O 2 , the formation of fatty acid hydroperoxides. The conjugated diene moiety is a strong chromophore that can be detected spectrophotometrically. When present in fatty acids they show a characteristic absorption in the UV region at around 234 nm. However, detection and quantitation of conjugated dienes in mixtures of peroxidized and nonperoxidized lipids, by means of simple UV spectrophotometry, is complicated by the end absorption exhibited by naturally occurring and nonperoxidized lipids.

Chemical Transformations and Facile Disproportionation of Sulfur Dioxide on Transition Metal Complexes

Abstract: Aside from its renown as a source of acid precipitation, sulfur dioxide is remarkable in possessing physicochemical and coordination properties that are more diverse than those of any other small molecule. SO{sub 2} is amphoteric, behaving as a Lewis acid or base, mild oxidant or reductant, or oxygen donor or acceptor. It is an excellent nonaqueous solvent when liquefied at -10{degrees}C and coordinates to many types of compounds, including metal complexes at both metal and ligand sites, strong Lewis acids, and virtually all nucleophiles, even halide ion. SO{sub 2} can bind strongly to low-valent metals like CO or NO or completely reversibly like O{sub 2} or H{sub 2}. The diversity of metal-SO{sub 2} bonding geometries is unmatched and has been reviewed. This Account will focus on the reactivity of SO{sub 2} e.g. SO double bond cleavage. 72 refs., 7 figs., 1 tab.

Thiol-reactive fluorescent probes for protein labelling

Abstract: Cyclisation of N-alkylmaleamic acids mediated by acetic anhydride in dimethylacetamide in the presence of traces of cobalt naphthenate has been used for efficient assembly of a range of fluorescent maleimide reagents. The fluorescence responses of these reagents to addition of thiol across the maleimide double bond, and to hydrolysis of the maleimide ring, are described.

Synthetic Chemistry with Fullerenes. Photooxygenation of Olefins

Abstract: Under irradiation with visible or UV (>290 nm) light in the presence of molecular oxygen and a minute amount of fullerenes, olefins and dienes undergo ene and Diels-Alder reactions with singlet oxygen to give photooxygenation products. The regio-and stereoselectives of the photooxygenation of {beta}-myrcene, (+)-pulegone, 4-methylpent-3-en-2-ol, and (+)-limonene were very similiar to those observed in known singlet oxygen reactions, indicating that the fullerene-sensitized reaction generates free singlet oxygen. The efficiency of fullerenes and conventional sensitizers was qualitively examined by using the Diels-Alder reaction between {sup 1}O{sub 2} and furan-2-carboxylic acid as a probe. Among those examined, C{sub 70} was found to be the most effective. The reaction was the fastest and completed with as little as 0.0001 equiv of C{sub 70}. C{sub 60} and hematoporphyrin were found to be of similiar efficiency. The methanofullerene 13, which lacks one olefinic conjunction in the C{sub 60} core, was as good as C{sub 60} itself, but the aminofullerene 14, lacking six double bonds, was quite inferior. The fullerene carboxylic acid 15, which was previously shown to show considerable biochemical activity, was found to be capable of generating singlet oxygen in aqueous DMSO. 25 refs., 1 tab.

Synthesis of optically active cis- and trans-1,2-disubstituted cyclopropane derivatives by the Simmons-Smith reaction of allyl alcohol derivatives derived from (R)-2,3-O-isopropylideneglyceraldehyde

Abstract: The Simmons-Smith reactions of Z- and E-allyl alcohol derivatives 6 derived from (R)-2,3-O-isopropylideneglyceraldehyde (6) were used for the synthesis of optically active cis- and trans-1,2-disubstituted cyclopropane derivatives. Reaction of 6 with diethyl zinc and diiodomethane gave cyclopropane derivatives 7 in 84% to quantitative yields with 35 to ≃100% des. Identical facial selectivities toward the double bonds, 1re-2si for Z-6 and 1re-2re for E-6, were observed in the cyclopropanations. The diastereoselectivity was dependent on the protecting group on the terminal allylic oxygen (R of 6, TBDPS>MOM>Bn) and on the stereochemistry of the double bond (Z >E). For TBDPS ethers Z-and E-6c, cis- and trans-7c were obtained as single distereomers, respectively. It was clearly demonstrated that the stereoselectivity of the cyclopropantion is controlled by the directing effect of the allylic oxygen (O-1) of the dioxolane ring, which coordinates to the reagent. The terminal allylic oxygen (O-2) lowered the diastereoselectivity. This reaction was applied to the synthesis of optically active cyclopropane analogs of γ-aminobutyric acid (GABA) 18,22, and ent-22

Influence of Ru precursor, support and solvent in the hydrogenation of citral over ruthenium catalysts

Abstract: A series of ruthenium supported catalysts were prepared and investigated in the liquid-phase hydrogenation of citral. The mechanism of the reaction was found to be dependent on the Ru precursor, support and solvent used. On the sample prepared from RuCl3 the acetals of citronellal were formed with the subsequent hydrogenation of the isolated C=C double bond. On the samples obtained from precursors which do not contain chloride ions, citronellal was the main reaction product. Hydrogenation of the isolated C=C double bond was negligible. Using cyclohexane as solvent, a large amount of isopulegol was also obtained. The results of the hydrogenation of citral have been correlated to the presence of acid sites and incompletely reduced ruthenium which favour cyclization and acetals formation from citronellal. On the basis of the reported results, a reaction scheme for hydrogenation of citral over ruthenium catalysts is proposed.

Structure and fundamental vibrations of phenoxyl radical

Abstract: The geometry and harmonic force field of the phenoxyl radical are determined by ab initio calculations of the electronic structure. All the π electrons are involved in strong nondynamical correlations, which are treated in a complete π active space wave function. Results from using multiconfigurational self‐consistent‐field orbitals in the active space are found to be closely approximated by use of the more economical unrestricted Hartree–Fock natural orbitals. Large polarized basis sets are also required to obtain qualitatively correct results. Properties of the carbonyl chromophore are particularly interesting. Compared to closed‐shell analogs, the equilibrium geometry indicates the CO bond in phenoxyl to be nearly as short as a typical double bond, whereas the scaled quantum‐mechanical force field shows it to have strength intermediate between those of typical single and double bonds. The calculated fundamental vibrational frequencies generally confirm current interpretations of the major features of the observed experimental resonance Raman spectra, while leading to reassignments of some minor features. Some limited theoretical characterization of valence excited states provides qualitative explanation of the observed intensity pattern. Calculations carried out at lower levels of theory consistently find the CO bond to be too long and weak, leading to misassignment of the observed vibrational spectrum.

Regioselective copolymerization of 5-vinyl-2-norbornene with ethylene using zirconocene-methylaluminoxane catalysts: a facile route to functional polyolefins

Abstract: Copolymerization of ethylene with 5-vinyl-2-norbornene (VNB) using the dicyclopentadienylzirconium dichloride/methylaluminoxane (zirconocene/MAO) catalyst system has been shown to occur with regioselective insertion of the endocyclic double bond of VNB into the copolymer chain, leaving the exocyclic vinyl double bond as a pendant unsaturation. The pendant vinyl group in the copolymer can be readily converted to the hydroxy/epoxy group with complete conversion under mild reaction conditions, giving rise to the corresponding functionalized copolymers. The copolymers (parent and functionalized) have been characterized using IR, NMR, TGA, DSC, and intrinsic viscosity measurements

Electrochemically Induced Isomerization of a Fulleroid to a Methanofullerene

Abstract: Though six electrons can be reversibly added to C{sub 60}, the influence of the {pi}-electron count on cyclic voltammetry (CV) has not yet been investigated in detail. Saturation of a double bond of C{sub 60} in methanofullerenes causes a negative shift of about 50-150 mV for the first three waves. Here we report the synthesis and electrochemical investigation on a p-(methoxycarbonyl)-phenyl-substituted fulleroid (2) and its methanofullerene isomer (3). Further, by a combination of CV and UV - vis spectrophotometry, we show that (1) 2 and 3 have only very slightly different electron acceptor character and (2) 2 is converted to 3 upon addition of a third electron in 2. 13 refs., 2 figs.

Extrusion of Alkenes from Rhenium(V) Diolates: Energetics and Mechanism

Abstract: The kinetics of extrusion of ethylene, 2-butene, and bicyclo[2.2.2]oct-2-ene from Cp * Re(O)(OCH(R)CH-(R)O) (Cp * =η-C 5 (CH 3 ) 5 ) are measured, as are the kinetics of oxidation of norbornene, norbornadiene, and trans-cyclooctene by Cp * ReO 3 . Activation parameters calculated from the data show that strain in the double bond has a large effect on the enthalpy of activation for alkene oxidation but little or no effect on the enthalpy of activation for alkene extrusion. Entropies of activation for alkene extrusion are less than zero. A secondary deuterium isotope effect of 1.3 at 99.5 o C is measured for extrusion of ethylene-d 4 . Taken together, this data is inconsistent with a concerted mechanism for interaction of alkenes with Cp * ReO 3 but consistent with a stepwise mechanism with a metallaoxetane intermediate

Ferrocenyl diphosphines as ligands for homogeneous catalysts

Abstract: Compounds of the formula I, in which R1 is C1-C8-alkyl, phenyl or phenyl which is substituted by 1 to 3 C1-C4-alkyl or C1-C4-alkoxy; R2 and R3 are identical and are C1-C12-alkyl, C5-C12-cycloalkyl, C5-C12-cycloalkyl which is substituted by C1-C4-alkyl or C1-C4-alkoxy, or phenyl which is substituted by one to three C1-C4-alkyl, C1-C4-alkoxy, -SiR4R5R6, halogen, -SO 3M, -CO2M, -PO3M, -NR7R8 or -[ NR7R8R9]X ; or R2 and R3 are different and are C1-C12-alkyl, C5-C12-cycloalkyl, C5-C12-cycloalkyl which is substituted by C1-C4-alkyl or C1-C4-alkoxy, phenyl or phenyl which is substituted by one to three C1-C4-alkyl, C1-C4-alkoxy, -SiR4R5R6, halogen, - SO3M, -CO2M, -PO3M2, -NR7R8 or -[ NR7R8R9]X ; R4, R5 and R6, independently of one another, are C1-C12-alkyl or phenyl, R7 and R8 are H, C1-C12-alkyl or phenyl, or R7 and R8 together are tetramethylene, pentamethylene or 3-oxa-1,5-pentylene, R9 is H or C1-C4-alkyl, M is H or an alkali metal, X is the anion of a monobasic acid, and is a stereogenic carbon atom, in the form of their racemates and diastereomers or mixtures of diastereomers. Rhodium and iridium complexes containing these ligands are suitable as homogeneous, enantioselective catalysts for the hydrogenation of prochiral compounds containing carbon-carbon or carbon-heteroatom double bonds.

Conformational dynamics and relaxation in bulk polybutadienes: A molecular dynamics simulation study

Abstract: Conformational dynamics and relaxation of the torsional angle autocorrelation (ACF) function in bulk polybutadienes (PBD) have been studied over a wide temperature range via molecular dynamics simulations. All trans, all cis and 70 mol % trans were the configurations of the systems studied. Companion studies of these systems in the phantom state were also carried out to illuminate the effects of packing in the bulk. Conformational transitions at the –CH2–CH= bond alpha to the double bond and at the –CH2–CH2– beta bond were monitored. The activation energies for the overall transition rates for the α bond and for the β bond were found to correspond in both bulk and phantom states to one barrier height in their respective torsional potentials. A considerable degree of correlation was found between second neighbor bonds. At the α bond in trans‐PBD correlated conformational changes across the double bond predominated but changes across the β bond when in the trans conformation were common also. In cis‐PBD, du...

Preparation of Alkyl-Substituted Indoles in the Benzene Portion. Part 12. Enantiospecific Synthesis of Hapalindole O.

Abstract: An important chiral ketone derivative, (3S, 4R)-3-methyl-4-pivaloyloxy-3-vinylcyclohexan-1-one (16) was prepared from (R)-(-)-carvone (10) using a stereo-controlled conjugate addition of the vinyl group to (R)-3-methyl-6-(1-methylethylidene)-4-pivaloyloxy-2-cyclohexen-1-one (13). The first enantiospecific total synthesis of a terrestrial blue-green alga constituent, hapalindole O (1) was accomplished by condensation of this ketone 16 with α, α-dimethyl-1-(p-toluenesulfonyl)-1H-indole-4-methanol (6) to construct the fundamental carbon framework of the hapalindole 21, followed by introduction of the nitrogen function, stereoselective reduction of the tetra-substituted double bond with lithium aluminum hydride, and subsequent isothiocyanate formation.

Stereochemical studies on the addition of allylstannanes to aldehydes. The SE' component

Abstract: Compounds (l)-1 and (u)-1 have revealed both the position of the silicon electrofuge and the relative orientation of the double bonds in the allylmetalaldehyde addition. Cyclization was found to proceed with high selectivity via an anti S E ' pathway regardless of the proximal/distal ratio