Showing papers on "Cyclopropane published in 2001"

Single-Event Rate Parameters for the Hydrocracking of Cycloalkanes on Pt/US-Y Zeolites

Abstract: A single-event kinetic model is applied to the hydrocracking of cycloalkane model components on two bifunctional Pt/US-Y zeolites over a wide range of experimental conditions (T = 493−573 K, P = 10−50 bar, molar H2-to-hydrocarbon ratio = 50−300). Values for the standard activation entropies of the elementary steps were obtained from transition state theory, while independently determined Henry coefficients were used to describe the physisorption. Values for the composite activation energies, i.e., the sums of the protonation enthalpies of the alkene intermediates and the activation energies of the elementary carbenium ion transformations, were obtained from a regression of the experimental data. The composite activation energies for intra-ring alkyl shifts vary from 21 to 25 kJ/mol, which is higher than those for the corresponding acyclic alkyl shifts, which vary from 10 to 16 kJ/mol. The composite activation energies for cyclic protonated cyclopropane branching reactions range between 27 and 40 kJ/mol an...

Synthetic Methodology for the Construction of Structurally Diverse Cyclopropanes

Abstract: Practical and efficient routes for the stereoselective conversion of homoallylic alchols to diastereomerically pure cis-, trans-1,2-disubstituted, and 1,2,3-trisubstituted cyclopropanes have been developed. The routes are highlighted by olefin metathesis strategies and the stabilization of an intermediate cyclopropylcarbinyl cation by the β-silicon effect. The stereospecificity of the key cyclization step has been rationalized by transition-state models in which the important determinants include (i) a minimization of the steric interactions about the forming cyclopropane bond and (ii) an inversion of stereochemistry at the activated homoallylic alcohol position. The cyclopropane product chirality is ultimately controlled by the choice of homoallylic alcohol starting material. Through this method nonracemic, diasteromerically pure homoallylic alcohols can be converted in two steps to nonracemic, diasteromerically pure cyclopropane structural units. The scope and limitations of this versatile methodology h...

Isodiiodomethane is the methylene transfer agent in cyclopropanation reactions with olefins using ultraviolet photolysis of diiodomethane in solutions: a density functional theory investigation of the reactions of isodiiodomethane, iodomethyl radical, and iodomethyl cation with ethylene.

Abstract: We examine the chemical reactions of the isodiiodomethane (CH2I-I), .CH2I and CH2I(+) species with ethylene using density functional theory computations. The CH2I-I species readily reacts with ethylene to give the cyclopropane product and an I2 product via a one-step reaction with a barrier height of approximately 2.9 kcal/mol. However, the.CH2I and CH2I(+) species have much more difficult pathways (with larger potential barriers) to react with ethylene via a two-step reaction mechanism. Comparison of experimental results to our present calculation results indicates that the CH2I-I photoproduct species is most likely the methylene transfer agent for the cyclopropanation reaction of olefins via ultraviolet photoexcitation of diiodomethane.

Mechanism of Olefin Cyclopropanation by Diazomethane Catalyzed by Palladium Dicarboxylates. A Density Functional Study

Abstract: The reaction of diazomethane with ethylene in the presence of palladium diformate has been studied through density funcional calculations. Several mechanistic paths leading to the formation of cyclopropane have been studied. The results obtained show that the reaction of palladium diformate with diazomethane is more favorable than the reaction with ethylene. The reaction with diazomethane may lead to two different isomeric complexes: a methylene-inserted complex and a palladium−carbene complex. Insertion of methylene is the most favorable process, but the resulting complex is not suitable for cyclopropanation. The reaction with two additional diazomethane molecules makes the formation of the bismethylene-inserted palladium−carbene complex favorable. Attack of ethylene on this palladium−carbene complex leads to the formation of cyclopropane.

Mechanistic Investigations on the Reaction between 1,2-Dioxines and Bulky Stabilized Phosphorus Ylides: An Efficient Route to Closely Related Cyclopropane Stereoisomers

Abstract: The bulky stabilized ylides (2a-d) react with a range of 1,2-dioxines (1a-d) to afford the diversely functionalized cyclopropanes 7 in excellent yield and diastereomeric excess. This is in direct contrast to the situation when nonbulky ester ylides are utilized which results in a completely different cyclopropyl series. Through a combination of isolation, spectroscopic, temperature, and deuterium and additive effects studies, the mechanism of cyclopropane formation from this second pathway can be proposed. Importantly, enolate quenching of the intermediate 1-2lambda(5)-oxaphospholanes 4 prior to collapse results in an equilibrium mixture of intermediates 10 and 11 which have been fully characterized, and their formation is primarily a result of the steric bulk of the stabilized ester ylide. These intermediates (10/11) then collapse further and result in formation of the observed closely related cyclopropyl stereoisomers 7 and 8. Moreover, the addition of LiBr to these reactions allows for the control of which of the two possible cyclopropanation pathways will be dominant. Finally, optimal protocols that demonstrate the potential of this new cyclopropanation methodology for the ready construction of closely related cyclopropyl stereoisomers are presented.

Synthesis of (-)-delobanone.

Abstract: On irradiation in the presence of Fe(CO)(5) under a CO atmosphere, the alkenyl cyclopropane 2 underwent smooth ring expansion to give the sesquiterpene (-)-delobabone 3. The alkenyl cyclopropane 2 was prepared from the enantiomerically enriched epoxide 1.

Lewis acid-promoted cycloaddition reaction of cyclopropanes with allylsilanes.

Abstract: The treatment of cyclopropanes having donor and acceptor substituents at the vicinal positions on the cyclopropane ring with a Lewis acid readily generates a 1,3-zwitterion, which reacted with allylsilanes to produce cycloadducts and allylic products. It was found that the yield of the cycloadduct depends on the steric demand of the alkyl substituents on the silicon atom.

Electron-induced reactions in thin solid films of cyclopropane

Abstract: The electron-induced reactions of cyclopropane in the condensed phase have been induced and monitored simultaneously using a conventional high-resolution electron energy loss (HREEL) spectrometer The reaction rate is fast enough for the reaction to be followed even at the sub-nanoampere currents typical of an HREELS experiment The major initial reaction product is propene, as identified from characteristic vibrational bands Subsequent reactions possibly lead to the formation of longer saturated chains The reaction rate in the multilayer is considerably faster than at monolayer coverage The findings are compared to literature data from recent studies on electron-induced reactions in self-assembled monolayers and adsorbed cyclopropane monolayers The relatively fast reaction suggests that cyclopropane derivatives may be interesting candidates for use in surface patterning by electron beam techniques

Oxiranones: α-lactones or zwitterions? Insights from calculated electron density distribution analysis

Abstract: Electron density distributions for oxiranone and hydroxyoxiranone have been analysed in vacuo [MP2/6-31+G(d,p)] and in water [SCI-PCM/MP2/6-31+G(d,p)//HF/6-31+G(d,p)] and compared with those for cyclopropane, cyclopropanone, and oxirane. Oxiranone possesses a ring critical point in vacuo, and may be considered as an α-lactone with considerable ionic character in the endocyclic Cα–On bond. In water, oxiranone has neither a ring critical point nor a bond critical point for Cα–On, and may be considered as a zwitterion, whose carboxylate group has a net charge of −0.63. Geometrically, however, the molecule still possesses an acute-angled three-membered ring with a CαCOn angle of only 69°. Electronically, hydroxyoxiranone is acyclic and zwitterionic even in vacuo, but geometrically it still looks like an α-lactone.

Carboxylic acid compound having cyclopropane ring

Abstract: A carboxylic acid compound having cyclopropane ring(s) of the formula (I): wherein R is alkyl or alkenyl optionally having one or more 1,2-cyclopropylene in a carbon chain and/or optionally having cyclopropyl at the end of a chain,X is a single bond or alkylene, wherein the total number of carbon less the number of cyclopropane ring is 10-25, and a pharmaceutically acceptable salt thereof are provided. The compound (I) shows an LTP-like potentiation of synaptic transmission, allows slow metabolism in the living body, show a stable LTP-like potentiation of synaptic transmission, and is useful as an agent for LTP-like potentiation of synaptic transmission, a cognition-enhancing drug or an agent for the prophylaxis and treatment of dementia, a learning and memory disorder and a neurotransmitter release disorder.

Carboxylic acid compound cyclopropane ring

Abstract: A carboxylic acid compound having cyclopropane ring(s) of the formula (I): wherein R is alkyl or alkenyl optionally having one or more 1,2-cyclopropylene in a carbon chain and/or optionally having cyclopropyl at the end of a chain, X is a single bond or alkylene, wherein the total number of carbon less the number of cyclopropane ring is 10-25, and a pharmaceutically acceptable salt thereof are provided. The compound (I) shows an LTP-like potentiation of synaptic transmission, allows slow metabolism in the living body, shows a stable LTP-like potentiation of synaptic transmission, and is useful as an agent for LTP-like potentiation of synaptic transmission, a cognition-enhancing drug or an agent for the prophylaxis and treatment of dementia, a learning and memory disorder and a neurotransmitter release disorder.

Rates of Reaction for Cyclopropane and Difluoromethoxydifluoromethane with Hydroxyl Radicals

Abstract: The rates of reaction, Arrhenius preexponential factors, and activation energies of cyclopropane and difluoromethoxydifluoromethane, CHF2OCHF2 (HFOC-134), reacting with hydroxyl radical were measured by a relative method. Using ethane as the reference, eight measurements in the range from 25 to 186 °C yielded an Arrhenius fit of k = (6.82 ± 1.5) × 10-12e-(1335±90)/T cm3 molecule-1 s-1 for the reaction of cyclopropane with hydroxyl radical. Measurements were repeated using CH3CHF2 (FC-152a) instead of ethane as the reference. Both measurements agreed within 3%. Nine measurements in the range from 0 to 191 °C were made of HFOC-134 using CF3CHF2 (FC-125) as the reference. The result was k = (1.29 ± 0.5) × 10-12e-(1895±100)/T cm3 molecule-1 s-1 for HFOC-134. Preexponential factors for these two reactions are shown to be consistent with values predicted from a wide range of observed values for OH abstraction reactions. The rate constant results are compared to previously published values for these same compoun...

Excited- and ground-state versions of the tri-pi-methane rearrangement: mechanistic and exploratory organic photochemistry.

Abstract: The di-π-methane rearrangement with two π-groups bonded to a single carbon leading to π-substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three π-moieties attached to an sp3-hybridized atom in a search for a tri-π-methane rearrangement. Indeed, it was found that such systems do rearrange photochemically to afford cyclopentenes. However, it was also established that vinylcyclopropanes ring-expand to cyclopentenes on direct irradiation. Since both three-ring and five-ring photoproducts often are found to be produced, it was important to establish that the observed photochemistry was really the result of a true single-step tri-π-methane rearrangement and not the consequence of two sequential rearrangements, first to form a vinyl cyclopropane which subsequently ring expanded to the cyclopentene. The general situation has three speciesA, B, and Ccorresponding to tri-π-methane reactant A, vinylcyclopropane photoproduct B, and cycl...