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Showing papers in "Journal of the American Chemical Society in 1987"



Journal ArticleDOI
TL;DR: The use of 3A or 4A molecular sieves substantially increases the scope of the titanium(IV)-catalyzed asymmetric epoxidation of primary allylic alcohols as mentioned in this paper.
Abstract: The use of 3A or 4A molecular sieves (zeolites) substantially increases the scope of the titanium(IV)-catalyzed asymmetric epoxidation of primary allylic alcohols. Whereas without molecular sieves epoxidations employing only 5 to 10 mol % Ti(O-i-Pr)/sub 4/ generally lead to low conversion or low enantioselectivity, in the presence of molecular sieves such reactions generally lead to high conversion (>95%) and high enantioselectivity (90-95% ee). The epoxidations of 20 primary allylic alcohols are described. Especially noteworthy are the epoxidations of cinnamyl alcohol, 2-tetradecyl-2-propen-1-ol, allyl alcohol, and crotyl alcohol - compounds which heretofore had been considered difficult substrates for asymmetric epoxidation. In the case of allyl alcohol, the use of cumene hydroperoxide substantially increases both the reaction rate and the conversion, even in the absence of molecular sieves. In general, enantioselectivities are slightly depressed (by 1-5% ee) relative employing 50-100 mol % Ti(O-i-Pr)/sub 4/. The epoxidation of low molecular weight allylic alcohols is especially facilitated and, in conjunction with in situ derivatization, provides for the synthesis of many epoxy alcohol synthons which were previously difficult to obtain. The kinetic resolution of four secondary allylic alcohols with 10 mol % Ti(O-i-Pr)/sub 4/ is also described. The role of molecular sieves in the reaction andmore » the effects of variation in reaction stoichiometry, oxidant, and tartrate are discussed.« less

1,713 citations



Journal ArticleDOI
TL;DR: The preparation of CdS sols with a mean diameter between 40 and 60 A and a relatively narrow size distribution is described in this paper, where the colloids could be separated as solids, which in turn could be redissolved to give solutions of some 10/sup -2/M.
Abstract: The preparation of CdS sols with a mean diameter between 40 and 60 A and a relatively narrow size distribution is described. The colloids could be separated as solids, which in turn could be redissolved to give solutions of some 10/sup -2/ M. Activation of the particles by a cadmium hydroxide precipitate yielded fluorescing samples with a quantum yield exceeding 50%. The blue or green fluorescence occurred close to the band gap energy, which depended on the size of the particles. Violet fluorescing samples of activated ZnS-CdS co-colloids were also prepared. Photoanodic corrosion measurements showed that the activated CdS colloid was 2000 times more stable than the nonactivated one. Under laser illumination the particles became a little larger which is ascribed to an acceleration of Ostwald ripening due to local heating. Extremely intense laser light decomposed the particles into Cd + S.

1,214 citations



Journal ArticleDOI
TL;DR: Rietveld analysis of time-of-flight pulsed neutron diffraction of powders shows a nearly isotropic shrinkage of the structures of both the rutile and anatase polymorphs of TiO/sub 2/ upon cooling from 295 to 15 K.
Abstract: Rietveld analysis of time-of-flight pulsed neutron diffraction of powders shows a nearly isotropic shrinkage of the structures of both the rutile and anatase polymorphs of TiO/sub 2/ upon cooling from 295 to 15 K and no change in the sense of the distortion of the TiO/sub 6/ octahedra (two long and four short Ti-O distances in both): rutile at 295 (first) and 15 K (second), a/sub 0/ 4.593 08 (4), 4.586 66 (4), c/sub 0/, 2.958 89 (3), 2.954 07 (3), x/sub oxygen/, 0.304 76 (6), 0.304 69 (6), Ti-O(4x), 1.9486 (3), 1.9459 (3), Ti-O(2X), 1.9796 (4), 1.9764 (4); anatase at 295 and 15 K, a/sub 0/, 3.784 79 (3), c/sub 0/, 9.502 26 (12), 0.504 65 (12), x/sub oxygen/, 0.16686 (5), 0.166 75 (4), Ti-O(4x), 1.9338 (1), 1.9322 (1), Ti-O(2X), 1.9799 (5), 1.9788 (4): all distances (A) referenced to silicon (a = 5.430 88 A). Both tight-binding calculations on the crystalline solids and molecular mechanics computations on the oxide lattice alone lead to a model in which the balance of attractive Ti-O and repulsive O-O interactions control the details of the overall structures. The relative bond lengths around metal centers in some other systems are predicted.

1,047 citations


Journal ArticleDOI
TL;DR: In this paper, the adsorption of methanethiol and dimethyl disulfide on an Au(111) surface under UHV conditions was studied and it was found that, under these conditions, the disulfides bond is dissociated to give a stable surface thiolate.
Abstract: Studies of the adsorption of methanethiol and dimethyl disulfide on an Au(111) surface under UHV conditions are described. Both adsorbates bind strongly, with the bonding of the disulfide being greatly favored. It is found that, under these conditions, the disulfide bond is dissociated to give a stable surface thiolate. Adsorption of methanethiol does not involve cleavage of the S-H bond. The implications of these results for solution adsorption experiments and the thermodynamics characterizing monolayer formation are discussed.

902 citations


Journal ArticleDOI
TL;DR: In this paper, a normalized difference edge analysis is used to quantitatively determine the oxidation states of the copper sites in type 2 copper-depleted (T2D) and native forms of the multicopper oxidase, Rhus vernicifera laccase.
Abstract: Cu X-ray absorption edge features of 19 Cu(I) and 40 Cu(II) model complexes have been systematically studied and correlated with oxidation state and geometry. Studies of Cu(I) model complexes with different coordination number reveal that an 8983-8984-eV peak (assigned as the Cu 1s ..-->.. 4p transition) can be correlated in energy, shape, and intensity with ligation and site geometry of the cuprous ion. These Cu(I) edge features have been qualitatively interpreted with ligand field concepts. Alternatively, no Cu(II) complex exhibits a peak below 8985.0 eV. The limited intensity observed in the 8983-8985-eV region for some Cu(II) complexes is associated with the tail of an absorption peak at approx. 8986 eV which is affected by the covalency of the equatorial ligands. These models studies allow accurate calibration of a normalized difference edge procedure which is used for the quantitative determination of Cu(I) content in copper complexes of mixed oxidation state composition. This normalized difference edge analysis is then used to quantitatively determine the oxidation states of the copper sites in type 2 copper-depleted (T2D) and native forms of the multicopper oxidase, Rhus vernicifera laccase. The type 3 site of the T2D laccase is found to be fully reduced and stable tomore » oxidation by O/sub 2/ or by 25-fold protein equivalents of ferricyanide, but it can be oxidized by reaction with peroxide. The increase in intensity of the 330-nm absorption feature which results from peroxide titration of T2D laccase is found to correlate linearly with the percent of oxidation of the binuclear copper site.« less

892 citations


Journal ArticleDOI
TL;DR: In this article, the spontaneous organization of structurally complex organic disulfides on polycrystalline gold substrates has been shown to achieve very high coverages, approaching that equivalent to the bulk phase densities of the adsorbate molecules.
Abstract: This paper shows that stable, oriented, polyfunctional organic monolayers can be prepared by the spontaneous organization of structurally complex organic disulfides on polycrystalline gold substrates. Chemisorption proceeds to very high coverages, approaching that equivalent to the bulk-phase densities of the adsorbate molecules. The bonding to the surface is also highly specific, inasmuch as the chemisorption of the disulfide moiety is favored greatly over a wide range of other functionality. This latter feature allows the ready preparation of a broad variety of organic surfaces with well-defined microscopic and macroscopic characteristics. Several representative examples of monolayer films are described, their chemical and thermal properties explored, and their structures characterized by several techniques including infrared and photoelectron spectroscopies.

821 citations


Journal ArticleDOI
TL;DR: The PASADENA effect as mentioned in this paper is a method for transient high-sensitivity proton spin-labelling by molecular addition of dihydrogen, which can be converted to magnetization observable by NMR.
Abstract: The PASADENA effect is a method for transient high-sensitivity proton spin-labelling by molecular addition of dihydrogen. When the parahydrogen mole fraction differs from the high-temperature limit of 1/4, this population difference constitutes a form of spin order which can be converted to magnetization observable by NMR. Large NMR signals are observed, if subsequent to the hydrogen addition, the two protons experience magnetic inequivalence and spin-spin coupling and if observation is made before spin-lattice relaxation restores the equilibrium spin order. The analogous effect for D2 is also possible. The kinetic mechanisms of the homogeneous hydrogenation catalysts which permit the realization of the PASADENA effect have been the target of the experimental applications. The enhancement of the NMR transitions has facilitated the determination of true molecular rate constants. Ordinarily, the activity of a catalyst is assessed by dividing the observed rate by the total catalyst concentration. However, the question as to whether most of the catalytic rate is due to a tiny fraction of active species or a large fraction with a relatively low molecular rate is not clearly addressed by such an analysis. This ambiguity is entirely avoided in the PASADENA studies, since only active catalyst molecules can contribute to the enhanced signals from which all kinetic inferences are made. The sensitivity enhancement has also led to the identification of a novel intermediate in the mechanism for the Rh(DIPHOS)+ catalyzed hydrogenation of styrene. The rate of conversion of this species into product and starting material has been studied using two-dimensional NMR. The dramatically improved sensitivity should make it possible to observe key catalytic intermediates which do not build up in sufficient quantity to allow detection by conventional NMR arising from Curie-Law magnetization. The study of surface sites which bind pairwise with H2 is also a potentially fruitful area for future experimental work. The ambient temperature NMR spectroscopy of surfaces is not often feasible due to sensitivity limitations. Simulations have been performed using typical shift and coupling parameters in an effort to characterize the enhanced lineshapes which can be expected. The inverse of the PASADENA effect has also been proposed, whereby the spin order of a molecule containing hydrogen is probed by measuring the branching ratio to ortho and para dihydrogen. This RAYMOND phenomenon (radiowave application yields modulated ortho number desorbed) has the potential for measuring precursor NMR with extraordinary sensitivity, since it finesses the need for detection of radiowaves.

775 citations


Journal ArticleDOI
TL;DR: In this paper, the authors described a new method for the catalytic enantioselective reduction of ketones to chiral secondary alcohols, where the stoichiometric reagent in the reduction is borane and the catalyst is a chiral oxazaborolidine such as 1 (0.05-0.1 molfmol of ketone).
Abstract: We have recently described a new method for the catalytic enantioselective reduction of ketones to chiral secondary alcohols.' The stoichiometric reagent in the reduction is borane (usually 0.6 molfmol of ketone), and the catalyst is a chiral oxazaborolidine such as 1 (0.05-0.1 molfmol of ketone). Excellent enantioselectivities, easy recoverability of the chiral catalyst predecessor, near quantitative yields, short reaction times (a few minutes at 23 \"C), and predictability of the absolute configuration of the product contribute to the outstanding utility of this (CBS') process. This paper reports several subsequent developments in this area with respect to improved practicality and important applications. In contrast to 1 which is both air and moisture sensitive, the B-methylated oxazaborolidine 2 can be stored in closed containers at room temperature and weighed or transferred in air. Catalyst 2 is also much more easily prepared than 1. Reaction of (S)-

Journal ArticleDOI
TL;DR: In this article, a structural analysis of hydride and hydrocarbyl derivatives of permethylscandocene has been carried out, and the possibility of involvement of the π-bond system in electrophilic activation of aromatic and vinylic C-H bonds has been examined.
Abstract: The authors report herein synthetic and structural studies of hydride and hydrocarbyl derivatives of permethylscandocene, together with investigations of their reactivities with H/sub 2/ and hydrocarbons. Experiments designed to probe the mechanism of these processes have been carried out, and the possibility of involvement of the ..pi.. system in electrophilic activation of aromatic and vinylic C-H bonds has been examined. A picture of the transition state for such sigma-bond metathesis reactions is developed, which accounts for the relative reactivities of sp-, sp/sup 2/-, and sp/sup 3/-hybridized C-H bonds with Cp*/sub 2/Sc-R (R = H, alkyl, aryl).





Journal ArticleDOI
TL;DR: The adsorption of formaldehyde on different oxides (silica, pure and fluorided alumina, magnesia, titania, thoria, zirconia, and iron oxide) has been studied by FT-IR spectroscopy in the temperature range 170-570 K as mentioned in this paper.
Abstract: The adsorption of formaldehyde on different oxides (silica, pure and fluorided alumina, magnesia, titania, thoria, zirconia, and iron oxide) has been studied by FT-IR spectroscopy in the temperature range 170-570 K. The following adsorbed species have been identified and characterized spectroscopically: (i) physisorbed HCHO, (ii) coordinated HCHO, (iii) dioxymethylene, (iv) polyoxymethylene, (v) formate ions, and (vi) methoxy groups. On silica at 170 K formaldehyde physisorbs on surface OH groups and, by warming, polymerizes producing linear polyoxymethylene. On ionic oxides at about 250 K dioxymethylene is always observed, generally together with variable amounts of the linear polymer that has been isolated on magnesia at 170 K. Heating up to or above room temperature results in the disproportionation of dioxymethylene into formate and methoxide groups, probably via a Cannizzaro-type mechanism. Such a route probably parallels an oxidative route, involving direct oxidation of dioxymethylene into formates, as observed on iron oxide.

Journal ArticleDOI
TL;DR: Les deux composes cristallisent dans le systeme monoclinique, groupe C2/C as discussed by the authors, are cristalisent.
Abstract: Les deux composes cristallisent dans le systeme monoclinique, groupe C2/C. Comportement magnetique

Journal ArticleDOI
TL;DR: In this article, a series of molecules was synthesized containing a 1,4-dimethoxynaphthalene donor (D) and an 1,1-dicyanoethylene acceptor (a) interconnected by five different, rigid, nonconjugated bridges.
Abstract: A series of molecules 1 was synthesized containing a 1,4-dimethoxynaphthalene donor (D) an a 1,1-dicyanoethylene acceptor (a) interconnected by five different, rigid, nonconjugated bridges. The length of the bridges varies with increments of two sigma-bonds from four in 1(4) to 12 sigma-bonds in 1(12), to provide donor-acceptor center-to-center separations (R/sub c/) ranging from 7.0-14.9 A. In solvents of medium and high polarity, excitation of the donor D is followed by rapid intramolecular electron transfer. The rate constant (k/sub et/) shows only small dependence upon the solvent polarity (a factor of 2-3 between benzene and acetonitrile, for example) but decreases with increasing separation ranging from >10/sup 11/ s/sup -1/ for a four-bond separation to approx. =4 x 10/sup 8/ s/sup -1/ for a 12-bond separation. In saturated hydrocarbon solvents photoinduced electron transfer is not observed for 10 and 12-bond separations, while it is not significantly decreased for the shorter homologues. Therefore the absence of electron transfer at 10- and 12-bond separations in saturated hydrocarbon solvents is attributed to a thermodynamic rather than to a kinetic effect. In solvents where electron transfer is thermodynamically feasible, its rate is considerably greater than that found from various other experimental studies where either different bridgesmore » were used or intermolecular electron transfer was studied. Through-bond interaction involving sigma/..pi.. interaction between the bridge and the donor-acceptor pair is proposed to explain the very high electron transfer rates observed in 1; this is qualitatively correlated with independent information about this coupling derived from both theory and experiment (photoelectron spectroscopy).« less

Journal ArticleDOI
TL;DR: In this article, the piezoelectric quartz crystal microbalance (QCM) is used to investigate the ion populations and solvent content of thin films of polyaniline (PA) on electrode surfaces as functions of redox state and pH.
Abstract: The piezoelectric quartz crystal microbalance (QCM) is used to investigate the ion populations and solvent content of thin films of polyaniline (PA) on electrode surfaces as functions of redox state and pH. The data are shown to be consistent with a model for PA in which the initial oxidation at a potential of ca. 0.2 V vs. SCE to give the conductive form of the polymer creates charged sites by oxidation of amine moieties along the polymer chains. Higher potentials cause further (pH dependent) oxidation with cumulative removal of exactly one electron per ring to produce a polymer containing imine groups which can hydrolyze to form quinone and quinone/imine types of structures. The initial oxidation process is accompanied by proton expulsion at low pH values, indicating partial protonation of the PA amine nitrogens in the reduced form with loss of these protons on oxidation. In addition to the proton expulsion, anion insertion also takes place during oxidation under all conditions that were studied. Data from such measurements as a function of pH allow estimation of the pK/sub a/ values of the reduced (insulating) and oxidized (conducting) forms of PA, which are given as ca. -0.3 and 3. respectively. Experiments inmore » strong (10 M) acid solutions reveal changes in solvent content during oxidation which are observed to influence the electrochemical response of PA. The ability to measure both electrochemical and gravimetric data simultaneously is shown to greatly constrain possible models for systems such as PA, leading to less ambiguity in their description.« less




Journal ArticleDOI
TL;DR: In this paper, the photocatalytic action of colloidal or suspended semicon-ductor particles is based on the generation of electrons and positive holes which rapidly move to the surface of the particles and initiate redox processes.
Abstract: Colloidal solutions of CdS containing colloidal TiO, or ZnO were illuminated with visible light. The fluorescence of CdS (decay time -50 ns) was quenched by Ti02, several Ti02 particles being required per CdS particle. The rate of photoanodic corrosion in aerated solution was drastically increased in the presence of Ti02 In deaerated CdS solutions containing methanol and Cd2+ ions, cadmium metal was formed when Ti02 was present. Methyl viologen was reduced with a quantum yield of close to one, while it reacted about ten times more slowly in the absence of Ti02 These effects are explained in terms of improved charge separation by rapid electron injection from illuminated CdS into the conduction band of attached Ti02 particles. Electron injection into ZnO was less efficient and occurred only in the case of Q-CdS particles (very small particles having a greater band gap). The injected electrons caused a blue shift of the absorption threshold of ZnO. The photocatalytic action of colloidal or suspended semicon- ductor particles is based on the generation of electrons and positive holes which rapidly move to the surface of the particles and initiate redox processes. The efficiency of charge separation is often increased by contacting the semiconductor particle with a metal or another semiconductor. Typical examples are platinized ti- tanium dioxide' and cadmium sulfide2 as well as Ru02-covered Ti02.3 Serpone et al. reported a few years ago that H2 was formed from H2S on CdS powder illuminated with visible light in aqueous solution and that the yield was slightly increased in the presence of Ti02 powder! The effect was explained by an improved charge separation due to electron transfer from the illuminated CdS particles into the conduction band of the Ti02 particles. The increase in yield was only 20%, Le., little above the increase which could be explained by more efficient light absorption of CdS due to the increased internal light scattering by the TiO, additive. In the present paper, experiments with transparent colloidal solutions of CdS containing colloidal Ti02 or ZnO as additives are described. Efficient electron injection from the excited CdS part of the "sandwich" colloids to the Ti02 or ZnO part was observed with three methods of observation: (1) With use of a CdS colloid that fluoresces with a high quantum yield, the quenching of the fluorescence by added TiO, or ZnO was studied. (2) Redox processes, such as the reduction of excess Cd2+ ions and of methyl viologen and the photoanodic dissolution of CdS, were initiated by visible light illumination and the influence of added Ti02 investigated. (3) In the case of ZnO as additive, the electron injection was accompanied by the typical changes in the absorption spectrum of ZnO which have recently been observed in other experiments on the deposition of excess electrons on small semiconductor particles.s-6

Journal ArticleDOI
TL;DR: In this article, the structures of the rotamers about the C-O bonds of formic acid, methyl formate, acetic acid, and methyl acetate were calculated by using the 6-31G* basis set and complete geometrical relaxation.
Abstract: The structures of the rotamers about the C-O bonds of formic acid, methyl formate, acetic acid, and methyl acetate were calculated by using the 6-31G* basis set and complete geometrical relaxation Large basis sets (6-311+G**) and correction for electron correlation were needed in order to obtain calculated barriers that were in good agreement with the available experimental data The factors that control the geometry at a carbonyl group are considered, and it is shown that an analysis in terms of bond path angles leads to a direct connection with electronegativity The nature of the interaction between an amino group and a carbonyl, as in an amide, is examined and shown not to involve charge transfer from the nitrogen to the carbonyl oxygen, but rather it involves charge transfer between carbon and nitrogen The origin of the rotational barrier in esters and of the difference in energy between the E and Z conformers is discussed

Journal ArticleDOI
TL;DR: In this paper, the authors report their success in the preparation of two polymers which are soluble in water in the doped and undoped states, which is a first for organic-solvent-soluble conducting polymers.
Abstract: It has only been within the last few months that organic-solvent-soluble conducting polymers have been developed. Previously, solubility in a mixture of arsenic trifluoride/pentafluoride was the singular path available to processability of conducting polymers. In this paper they report their success in the preparation of two polymers which are soluble in water in the doped and undoped statesexclamation Sodium poly(3-thiophene-..beta..-ethanesulfonate) (P3-ETSNa) and sodium poly(3-(thiophene-delta-butanesulfonate) (P3-BTSNa) and their respective conjugate acids are water soluble. The latter are of interest because, upon oxidation, they can lose a proton concomitant with electron loss to produce self-doped polymers; i.e., doped polymers where the counterion is attached to the polymer via a covalent bond.

Journal ArticleDOI
TL;DR: In the presence of rhodium carbonyle or palladium, hydrogenation de composes acetyleniques ou ethylenique in presence of complexes de rhodanium carbonyles ou de palladium.
Abstract: Hydrogenation de composes acetyleniques ou ethyleniques en presence de complexes de rhodium carbonyle ou de palladium


Journal ArticleDOI
TL;DR: In this article, the authors show that electrostatic interactions have a dramatic influence on the kinetics for charge separation and hydrogen production in aqueous systems (pH 9.8) of 20-nm diameter TiO/sub 2/modified SiO/Sub 2/ colloids in various combinations with an electron relay, a photosensitizer, and a Pt catalyst.
Abstract: Laser flash photolysis and steady-state photolysis studies show that electrostatic interactions have a dramatic influence on the kinetics for charge separation and hydrogen production in aqueous systems (pH 9.8) of 20-nm diameter TiO/sub 2/-modified SiO/sub 2/ colloids in various combinations with an electron relay, a photosensitizer, and a Pt catalyst. Either direct excitation of the semiconductor or the photosensitizer Ru(bpy)/sub 3//sup 2 +/ (bpy = 2,2'-bipyridine), electrostatically adsorbed to the colloid, initiate electron transfer to either the zwitterionic electron relay, N,N'-bis(3-sulfonatopropyl)-4,4'-bipyridinium (PVS/sup 0/), or N,N'-bis(3-sulfonatopropyl)-2,2'-bipyridinium (DQS/sup 0/), or methyl viologen (MV/sup 2 +/). The rates and quantum yields for the formation of the radical PVS/sup -/ anion in both the TiO/sub 2/-SiO/sub 2//PVS/sup 0/ and the TiO/sub 2/-SiO/sub 2//Ru(bpy)/sub 3//sup 2 +//PVS/sup 0/ systems decline with increasing ionic strength. The rate and quantum yields for H/sub 2/ production in both the TiO/sub 2/-SiO/sub 2//DQS/sup 0//Pt and the TiO/sub 2/-SiO/sub 2//Ru(bpy)/sub 3//sup 2 +//DQS/sup 0//Pt systems also show a similar ionic strength dependence.

Journal ArticleDOI
TL;DR: Etude cinetique du transfert d'electron entre la perhydrodiethano-1,4:6,9 pyridazino [1,2]-pyridazine et son cation dans l'acetonitrile as mentioned in this paper.
Abstract: Etude cinetique du transfert d'electron entre la perhydrodiethano-1,4:6,9 pyridazino [1,2-a] pyridazine et son cation dans l'acetonitrile