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991.
[reaction: see text]. A parallel array of isoxazoline diamides was prepared using an ionic liquid [bmim][BF4] as the phase where a three-step procedure (Schotten-Baumann, 1,3-dipolar cycloaddition, ester amidation with Me3Al) was carried out. At the end, selective extraction of the final products with diethyl ether allowed simple isolation of the 16 components of the array (Syncore technology). 相似文献
992.
Two new poly(ethylene glycol) supported manganese porphyrins have been prepared and their catalytic activity and recyclability were investigated for the epoxidation of alkenes using H2O2 and PhIO as stoichiometric oxidants. 相似文献
993.
We report on the precise location of Cl atoms chemisorbed on a Cu(0 0 1) surface and the interlayer relaxations of the metal surface. Previous studies have shown that chlorine dissociates on Cu(0 0 1) to form a c(2 × 2) chemisorbed layer with Cl atoms occupying four-fold hollow sites. A Cu-Cl interlayer spacing of 1.60 Å and a slightly expanded Cu-Cu first interlayer spacing of 1.85 Å (1.807 Å for bulk Cu) was determined by LEED. The resulting Cu-Cl bond length, 2.41 Å, is very similar to the SEXAFS value of 2.37 Å. Contradictory results were obtained by angle-resolved photoemission extended fine structure: while confirming the Cu-Cl interlayer spacing of 1.60 Å, no first Cu-Cu interlayer relaxation has been observed. On the other hand, a small corrugation of the second Cu layer was pointed out. We carried out a detailed structural determination of the Cu(0 0 1)-c(2 × 2)-Cl system using surface X-ray diffraction technique with synchrotron radiation. We find a Cu-Cl interlayer spacing of 1.584(5) Å and confirm the expansion of the first Cu-Cu interlayer, with an average spacing of 1.840(5) Å. In addition, we observe a small corrugation of the second Cu layer, with Cu atoms just below Cl atoms more tightly bound to the surface layer, and even a second Cu-Cu interlayer expansion. 相似文献
994.
Maurizio Speranza 《Tetrahedron letters》1980,21(20):1983-1986
Tritium atom distribution in the anisole formed from the attack of a nucleogenic tritiated phenylium ion on gaseous methanol provides the first experimental evidence for a gas-phase automerization of phenylium ion. 相似文献
995.
Francesco Paolo Colonna Giuseppe Distefano Maurizio Guerra Derek Jones 《Journal of Electron Spectroscopy and Related Phenomena》1980,18(3):309-328
The photoelectron He(I) and He(II) spectra of 4H-pyran-4-one (1), 4H-thiopyran-4-one (2), 4H-pyran-4-thione (3) and 4H-thiopyran-4-thione (4) are reported. The assignments are based on experimental evidence, taking into account the results of theoretical calculations. The outermost orbital sequences proposed (X = O, S) are nX (σ), 3b1 (π), 1a2(π) for 1, 2 and 4; and 3b1(π), nX(σ) and 1a2(π) for 3. The shifts of the core-ionization energy values for 1–4 are ascribed to a drift of π-charge from the intracyclic heteroatom towards the carbonyl or thiocarbonyl group. Low-energy shake-up satellites (up to 25% with respect to the main line) are observed in the various energy regions of the XPS spectra. They are qualitatively reproduced by CNDO/2 calculations. The most important satellites derive from the transition in the ion corresponding to the 3b1 (π) → 4b1(π*) transition in the UV-visible spectra of the neutral molecules. Charge rearrangements accompanying this transition lead to charge depletion of the core-ionized atom. 相似文献
996.
Two possible pathways have been investigated for the cis—trans conversion in diimide N-oxide, viz. a rotation around the NN bond and an in-plane inversion. Calculations have been performed for the ground and lowest excited states by means of a Cl treatment including the configurations found to be the most important in large Cl calculations performed in a few points. It is shown that the rotational mechanism is favoured in the excited states and is probable also in the thermal reaction. Such investigation has been repeated in parallel by using other techniques of lower accuracy: (1) Cl limited to single excitations (EHP method); (2) rigid excitation to virtual orbitals; (3) the half electron methods. Deficiencies of such methods have been brought out and analyzed. In particular methods (2) and (3) lead to an erroneus interpretation of the actual mechanism of the photochemical conversion by rotation. 相似文献
997.
Ienco A Calhorda MJ Reinhold J Reineri F Bianchini C Peruzzini M Vizza F Mealli C 《Journal of the American Chemical Society》2004,126(38):11954-11965
The dicationic complex [(triphos)Rh(mu-S)(2)Rh(triphos)](2+), 1 (modeled as 1c) [triphos = CH(3)C(CH(2)PPh(2))(3)], is known to activate two dihydrogen molecules and produce the bis(mu-hydrosulfido) product [(triphos)(H)Rh(mu-SH)(2)Rh(H)(triphos)](2+), 2 (modeled as 2b), from which 1 is reversibly obtained. The possible steps of the process have been investigated with DFT calculations. It has been found that each d(6) metal ion in 1c, with local square pyramidal geometry, is able to anchor one H(2) molecule in the side-on coordination. The step is followed by heterolytic splitting of the H-H bond over one adjacent and polarized Rh-S linkage. The process may be completed before the second H(2) molecule is added. Alternatively, both H(2) molecules are trapped by the Rh(2)S(2) core before being split in two distinct steps. Since the ambiguity could not be solved by calculations, (31)P and (1)H NMR experiments, including para-hydrogen techniques, have been performed to identify the actual pathway. In no case is there experimental evidence for any Rh-(eta(2)-H(2)) adduct, probably due to its very short lifetime. Conversely, (1)H NMR analysis of the hydride region indicates only one reaction intermediate which corresponds to the monohydride-mu-hydrosulfide complex [(triphos)Rh(H)(mu-SH)(mu-S)Rh(triphos)](2+) (3) (model 5a). This excludes the second hypothesized pathway. From an energetic viewpoint the computational results support the feasibility of the whole process. In fact, the highest energy for H(2) activation is 8.6 kcal mol(-1), while a larger but still surmountable barrier of 34.6 kcal mol(-1) is in line with the reversibility of the process. 相似文献
998.
Guizzardi B Mella M Fagnoni M Albini A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(7):1549-1555
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 CdoublebondC . 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. 相似文献
999.
Alagona G Ghio C Persico M Tomasi S 《Journal of the American Chemical Society》2003,125(33):10027-10039
Chiral oxazaborolidines, known as CBS catalysts after the work of Corey, Bakshi and Shibata, are used for the stereoselective reduction of prochiral ketones to secondary chiral alcohols. Due to their relative low cost, ease of use, and high selectivity, their popularity has remarkably grown in the last 15 years. Oxazaborolidine-catalyzed reductions have been much studied, both experimentally and computationally, by means of semiempirical methods. Though, a more accurate high level quantum mechanical study on the complete system, capable of elucidating reliably the origins of stereoselectivity, is still lacking. Therefore, the acetophenone (PhMK) reduction with Corey's oxazaborolidine has been modeled for the first time with ab initio and DFT-B3LYP calculations on the complete system as well as with AM1. Calculations on the complexation of BH(3) to CBS, which can occur only in a cis fashion with respect to the hydrogen on the stereogenic C-4 carbon atom, have allowed us to confirm the great rigidity of Corey's catalyst, possibly determining its excellent enantioselectivity. Acetophenone-CBS-BH(3) complexes were characterized at various levels of theory, and it was found that the picture obtained depends heavily on the method adopted. A computational strategy for identifying the hydride transfer transition states of the competing pathways was developed and tested, using a model system for which the transition state geometry was already known. The application of the TS search method to the reduction of acetophenone allowed the characterization of the TS's for the competing pathways in this reaction, making it possible to predict with good quantitative accuracy the stereochemical outcome of the reaction at all the levels of theory adopted. The characterization of the intermediate oxazadiboretane products confirmed that the highly exothermic hydride transfer provides the thermodynamical drive for the reaction. 相似文献
1000.
Guldi DM Holzinger M Hirsch A Georgakilas V Prato M 《Chemical communications (Cambridge, England)》2003,(10):1130-1131
A variety of single-wall carbon nanotube samples were tested in steady-state and time-resolved emission experiments, where it was found that chemical functionalization and different degrees of oxidation affect the emission behavior. 相似文献