Showing papers on "Cyclopropane published in 1996"

Breaking of the C−C Bond of Cyclobutanones by Rhodium(I) and Its Extension to Catalytic Synthetic Reactions

Abstract: A study of rhodium(I)-catalyzed synthetic transformations involving selective breaking of the C−C bond α to the carbonyl group of cyclobutanones is described. Decarbonylation took place on treatment of a cyclobutanone with an equimolar amount of (Ph3P)3RhCl at reflux in toluene to afford the corresponding cyclopropane. The formation of the cyclopropane suggests that Rh(I) undergoes an insertion into the bond between the carbonyl carbon and the α-carbon in the initial step. Catalytic decarbonylation of cyclobutanone was also achieved. The mode and rate of the reaction depended greatly on the ligands of the rhodium(I) complex. When a cyclobutanone bearing a hydrogen atom at the 3-position was used, appropriate choice of the catalyst system led to the selective formation of either a cyclopropane or an alkene. Breaking of the carbon−carbon bond was next combined with hydrogenolysis. When cyclobutanone was treated under hydrogen pressure with a catalytic amount of a rhodium(I) complex having a bidentate diphos...

Microscopic structure of tetrahedral amorphous carbon

Abstract: Simulations are reported of a highly tetrahedral amorphous carbon network at a density of 3 g/cc using Car-Parrinello first principles molecular dynamics. The simulated structure consists of 65% fourfold and 35% threefold coordinated carbon sites, in good agreement with experiment. The structure is also in good agreement with recent neutron diffraction data. An unexpected observation was the presence of small carbon rings in the structure containing as few as three atoms. These carbon triangles and quadrilaterals, which resemble the organic compounds cyclopropane and cyclobutane, give the network a topology unique among tetrahedrally bonded amorphous materials.

Conformational Restriction by Repulsion between Adjacent Substituents on a Cyclopropane Ring: Design and Enantioselective Synthesis of 1-Phenyl-2-(1-aminoalkyl)-N,N-diethylcyclopropanecarboxamides as Potent NMDA Receptor Antagonists

Abstract: Adjacent substituents on a cyclopropane ring mutually exert steric repulsion quite significantly, because they are fixed in eclipsed conformation to each other. Based on this structural feature of the cyclopropane ring, conformationally restricted analogs of milnacipran (1), namely 1-phenyl-2-(1-aminoalkyl)-N,N-diethylcyclopropanecarboxamides (2, 3, ent-2, and ent-3) were designed as potent NMDA receptor antagonists and were synthesized highly enantioselectively. Reaction of (R)-epichlorohydrin [(R)-5] and phenylacetonitrile (6) in the presence of NaNH2 in benzene gave a chiral cyclopropane derivative that was isolated as lactone 4 with 96% ee in 67% yield, after alkaline hydrolysis of the cyano group. The nucleophilic addition reaction of Grignard reagents to aldehyde 10, which was readily prepared from 4, proceeded highly selectively from the si-face to afford addition products 11 in high yields. Although hydride reduction of the corresponding ketone 15b, prepared from 11b, with L-Selectride also procee...

σ* resonances in electron impact‐induced vibrational excitation of n‐propane, cyclopropane, ethylene oxide, cyclopentane, and cyclohexane

Abstract: Electron‐energy‐loss spectra in the range of vibrational excitation, and excitation functions for selected vibrational peaks, were measured for the title compounds. Angular distributions of the vibrationally inelastic peaks were measured for n‐propane and cyclopropane. The results in n‐propane are similar to the published results in ethane, only one very broad band is observed in all channels, with gradual onset at about 3 eV and a maximum around 8 eV. In contrast, narrower resonances emerge in all cyclic compounds. The effect is most pronounced in cyclopropane, where two resonances appear, at 2.6 and 5.5 eV. The latter is exceptional in several respects. It is narrow and thus relatively long lived for a shape resonance of this energy. It causes ring stretch excitation with very high selectivity and pronounced angular distribution, which is reproduced very well by the theory of Read and Andrick, revealing dominance of a partial wave with an unusually high angular momentum, l=3, m=3, and unambiguously identifying the resonance as a2′. The resonances in ethylene oxide are similar, but somewhat broader and shifted to 3 and 4.8 eV, respectively. Resonances in cyclopentane and cyclohexane resemble loosely the cyclopropane case. It is concluded that the major cause of the dramatic differences in spectral appearance between linear and cyclic alkanes are not major changes of resonant energies, but decrease of their (lifetime‐determined) width, caused by higher symmetry, rigidity, and consequently larger contribution of partial waves with high l to the scattering. This implies that the vertical electron affinity of linear alkanes is not around −8 eV, as could be assumed from the position of the peak in the attachment spectra, but higher, around −3 eV.

Enantioselective total syntheses of cyclopropane amino acids: Natural products and protein methanologs

Abstract: The syntheses of (−)-allo-coronamic acid, (−)-allo-norcoronamic acid, (−)-(Z)-2,3-methanohomoserine, (−)-(Z)-2,3-methanomethionine, and (2S,3R)-Cbz-cyclo-Asp-OMe have been achieved in 45–68% overall yields from suitable intermediates derived from homochiral aminopentenoates which were obtained, in turn, from D-glyceraldehyde. The key synthetic step involves the quantitative and highly diastereoselective cyclopropanation of such precursors. The factors dealing with the control of stereoselectivity are highlighted and the main features in side-chain functionalization to the respective target molecules are discussed.

The hydrogen-bond C–H donor and π-acceptor characteristics of three-membered rings

Abstract: Crystallographic results, retrieved from the Cambridge Structural Database, show that the C--H protons of cyclopropane, aziridine and oxirane form C—H⋯O (particularly C—H⋯O—C) hydrogen bonds. The frequency of formation and geometrical characteristics of these bonds indicate a bond-strength ordering: Csp1—H⋯O > C(ring)—H⋯O ≃ Csp2—H⋯O > Csp3—H⋯O, which is in excellent agreement with the well known ethylenic properties of C(ring)—H and with residual δ+ charges calculated for these systems. There is some evidence to suggest that C=C—H in cyclopropene, known to be a highly acidic H, forms stronger hydrogen bonds than C—H in saturated three-membered rings. Crystallographic data have also been used to provide geometrical evidence for the formation of O,N—H⋯π(ring) bonding to three-membered rings, proposed on the basis of spectroscopic data [Joris, Schleyer & Gleiter (1968). J. Am. Chem. Soc. 90, 327–336]. The two modes of H⋯π(ring) binding suggested there, viz. `edge-on' approach of H to a ring C—C bond and `face-on' approach towards the ring centroid, are found to be dominant in crystallographic observations of this novel hydrogen bond.

Generation and Cycloaddition Behavior of Spirocyclic Carbonyl Ylides. Application to the Synthesis of the Pterosin Family of Sesquiterpenes

Abstract: The Rh(II)-catalyzed reaction of 1-acetyl-1-(diazoacetyl)cyclopropane and ethyl 3-(1-acetyl-cyclopropyl)-2-diazo-3-oxopropiolate with various dipolarophiles afforded dipolar cycloadducts in good yield. The reaction involves the formation of a rhodium carbenoid and subsequent transannular cyclization of the electrophilic carbon onto the adjacent keto group to generate a five-membered cyclic carbonyl ylide which undergoes a subsequent dipolar cycloaddition reaction. The regiochemical results encountered can be rationalized on the basis of FMO considerations. For carbonyl ylides, the HOMO dipole is dominant for reactions with electron deficient dipolarophiles, while the LUMO becomes important for cycloaddition to more electron rich species. A short synthesis of several members of the pterosin family of sesquiterpenes is described in which the key step involves a dipolar cycloaddition using a carbonyl ylide. The Rh(II)-catalyzed reaction of 1-acetyl-1-(diazoacetyl)cyclopropane with cyclopentenone afforded a d...

Gas Phase Kinetics of Neutral Transition Metal Atoms: Reactions of Hf, Ta, Ir, Pt, and Au with Alkanes and Alkenes

Abstract: The chemical kinetics of the ground-state, neutral transition metal atoms Hf, Ta, Ir, Pt, and Au with alkanes and alkenes is surveyed. Laser-induced fluorescence measures effective bimolecular rate constants at 300 K in 0.5−1.1 Torr of He buffer gas. Pt(5d96s1) is thus far the only ground-state neutral atom in the entire transition metal block that reacts with methane. It also reacts rapidly with linear alkanes, cyclopropane, and the alkenes. Au(5d106s1) reacts only with the largest alkenes studied, and then very slowly. In spite of their 6s2 ground-state configurations, Hf(5d26s2), Ta(5d36s2), and Ir(5d76s2) react rapidly with alkenes including ethene; these atoms do not react with methane and the linear alkanes. An attempt is made to interpret the pattern of reactivity in terms of simple chemical bonding concepts, including electron configuration and spin, guided by earlier extensive electronic structure calculations for the 4d series. It appears that the lanthanide contraction of the 6s orbital strongl...

Theoretical Study on the Mechanism of the Superacid-Catalyzed Unimolecular Isomerization of n-Butane and 1-Butene

Abstract: Owing to the practical interest of the acid-catalyzed isomerization of n-butane and 1-butene, the mechanism of the isomerization and scrambling reactions of the n-butyl cation has been studied theoretically using ab initio methods which include electron correlation and extended basis sets. It has been found that the protonated cyclopropane ring does not appear as a common intermediate for carbon scrambling and branching isomerization of the n-butyl cation, since it is a transition state and not a minimum on the potential energy surface. The transition states for both reactions have been determined and the activation energies calculated. These values are in very good agreement with those obtained experimentally.

Photoinduced Electron Transfer Reactions of alpha-Cyclopropyl- and alpha-Epoxy Ketones. Tandem Fragmentation-Cyclization to Bi-, Tri-, and Spirocyclic Ketones.

Abstract: Reductive photoinduced electron transfer (PET) reactions have been performed with various bicyclic alpha-cyclopropyl-substituted ketones and tertiary amines. The reaction resulted in a regioselective cleavage of one cyclopropyl bond under formation of an exocyclic radical with an endocyclic enolate unit. In the case of bicyclic ketones with an unsaturated side chain, various bicyclic, spirocyclic, and tricyclic products are accessible via radical cyclization, depending on the position of the alkenyl substituent. In addition to triethylamine, N-silylated amines have also been used as electron donors, leading to a variety of compounds, among them are silylated fragmentation products, indicating that a proton is transferred from not only the amine radical cation but also the cationic silyl group. The intramolecular Paterno-Buchi reaction has also been studied for cyclopropane derivatives of the jasmone type leading to tetracyclic oxetanes. Finally, alpha-epoxy-substituted ketones have been investigated under PET conditions, yielding ring-opened products.

Cationic cyclopropanation by antibody catalysis

Abstract: Reactions involving highly reactive carbocations play a central role in many important chemical processes, such as cyclization reactions. However, the potential for controlling the pathways of such reactions to obtain energetically disfavoured (but desirable) products has been hard to realize because of the difficulties inherent in controlling the conformation and chemical environment of the carbocation intermediates. Antibody catalysts, with their high specificity and binding energies, can provide the degree of conformational and chemical control necessary for directing such reactions. Here we show how antibody catalysis can guide cationic cyclization reactions selectively to form products (in high yield) that would otherwise be highly disfavoured. Most notable is the formation of a strained bicyclic compound containing a rare cyclopropane group. To explain our results, we propose a common reaction scheme in which the key step is the formation of a highly reactive protonated cyclopropane intermediate; subtle structural modifications to the substrate (the compound on which the catalytic antibody acts) lead to dramatic differences in the structure of the final product.

Evidence for concert in the thermal unimolecular vinylcyclopropane to cyclopentene sigmatropic 1,3-shift

Abstract: Gas phase pyrolysis of cis-2,3-dideuterio-trans-(1‘-tert-butyl-2‘-(Z)-deuteriovinyl)cyclopropane at 290 °C gives trans,trans-3,4,5-trideuterio-1-tert-butylcyclopentene as the major 1,3-shift product with greater than 90% stereospecificity in an orbital symmetry “allowed” suprafacial-inversion sense after correction for the geometric isomerization of starting material and the other materials present. The isotopic substitution at the critical sites of rearrangement eliminates steric or electronic influences of substituents on a biradical pathway as a source of the suprafacial-inversion stereochemistry observed with more highly substituted derivatives. The stereochemical results coupled with a normal deuterium kinetic isotope effect (kH/2kD = 1.14 at 311.6 °C) at the exo-methylene carbon are best interpreted in terms of a concerted pathway for rearrangement. A less likely alternative is a stereospecific disrotatory ring opening to a biradical followed by rate-determining closure to a five-membered ring. Acco...

Stereoselective Synthesis of Cyclopropanes via Homoallylic Participation

Abstract: A facile method for synthesizing trans-disubstituted cyclopropanes is described. γ,γ-Dimethyl homoallyl alcohols, upon conversion to the corresponding triflate, undergo smooth cyclization to give cyclopropanes in a stereoselective manner (trans) and in a stereospecific manner (inversion of configuration at the chiral center). Various functionalities could be introduced at the α-position of the newly formed cyclopropane ring by treating the cyclization mixture with hetero as well as carbon nucleophiles. Alternatively, treatment of the reaction mixture with triethylamine effects the clean elimination of a triflic acid to give high yields of the cyclopropanes with a 2-propenyl group, which are convertible to the corresponding cyclopropyl methyl ketones by ozonolysis in high yields. Also described is an application of the method to a short-step synthesis of a cyclopropane-containing eicosanoid, related to marine prostanoid biosynthesis.

Redox Chemistry Associated with the Complexation of Vanadium(V) and Tungsten(VI) by meso-Octaethylporphyrinogen: Formation and Cleavage of Cyclopropane Units Functioning as Shuttles of Two Electrons

Abstract: Oxidized forms of meso-octaethylporphyrinogen containing mono- and bis(cyclopropane) units, which function as shuttles of two electrons, form from the reaction of high-valent, early transition metal halides, namely [p-MeC6H4N⋮VCl3] (2) and WOCl4, with meso-octaethylporphyrinogen−lithium derivatives, [Et8N4Li4(THF)4] (1). The reaction of 2 with 1 occurs in a 2:1 ratio since the intermediate vanadium(V)−porphyrinogen complex undergoes a fast one-electron oxidation by 2 leading to [p-MeC6H4N⋮V{Et8N4(Δ)}] (3), which contains a vanadium(IV)−nitrene fragment bound to the two-electron, oxidized form of the porphyrinogen having a cyclopropane unit. This latter can be reduced using Li metal to a vanadium(IV)−porphyrinogen complex, [p-MeC6H4N⋮V(Et8N4)Li2(THF)4}] (5), which was also synthesized from 1 and [(p-MeC6H4N⋮VCl2)2] (4). In the reaction of WOCl4, we found that the tungsten(VI)−porphyrinogen complex forms first and is able to oxidize the excess of 1, to give [Et8N4(Δ)Li2(THF)2] (8) or the [Et8N4(Δ)2Li(THF)]+...

Cyclization versus Pd−H Elimination−Readdition: Skeletal Rearrangement of the Products of Pd−C6F5 Addition to 1,4-Pentadienes

Abstract: The reactions of [Pd(C6F5)Br(CH3CN)2] with 1,4-pentadiene or 3-methyl-1,4-pentadiene at low temperature afford, after insertion of one double bond into the Pd−C6F5 bond, two types of η1-η2-pentenylpalladium derivatives: the 4.5-membered palladacycles [Pd2(μ-Br)2(5-Pf-3-R-1,2,4-η1-η2-pentenyl)2] (Pf = C6F5; R = H (2a), Me (2b)) and the 5.5-membered metallacycles [Pd2(μ-Br)2(5-Pf-3-R-1,2,5-η1-η2-pentenyl)2] (R = H (3a), Me (3b)). Both enyls isomerize to η3-allylic derivatives following two competitive pathways, i.e. (i) Pd migration or (ii) cyclopropane formation and reopening, which lead to the isomeric η3-allylpalladium complexes [Pd2(μ-Br)2(5-Pf-3-R-1-3-η3-pentenyl)2] (R = H (5a), Me (5b)) and [Pd2(μ-Br)2(4-Pf-3-(CH2R)-1-3-η3-butenyl)2] (R = H (6a), Me (6b)), respectively. After the cyclopropane formation−reopening process, [Pd2(μ-Br)2(3-CH2Pf-1,2,5-η1-η2-pentenyl)2] (8) is also formed which isomerizes to 6b. The reaction of a mixture of the η1-η2-pentenyl complexes at low temperature with CO/NaOMe give...

Highly Stereocontrolled Cyclopropanation by the 1,3-Elimination of a Bis(tributylstannyl)propanol Derivative.

Abstract: The highly stereoselective ring closure of γ-hydroxystannyl derivative was realized. The aldol reaction of methyl bis(tributylstannyl)propionate (2) with aldehyde 5 proceeds stereoselectively to give (γ-hydroxypropyl)stannane 6, and the cyclopropanation reaction of aldol product 6 proceeds smoothly in a highly stereoselective manner presumably via a W-shape transition state. The stannyl group on the cyclopropane ring can be converted into various electrophiles with a retention of configuration. As a result, various stereocontrolled 1,2,3-trisubstituted cyclopropanes can be obtained in high yields.

Gas Phase Atomic Hydrogen Induced Carbon−Carbon Bond Activation in Cyclopropane on the Ni(100) Surface

Abstract: Carbon−carbon bond activation in adsorbed cyclopropane is observed following exposure to gas phase atomic hydrogen on the Ni(100) surface for temperatures as low as 100 K. Exposure to either gas phase atomic hydrogen or deuterium results in formation of adsorbed propyl. In both cases subsequent reaction between adsorbed propyl and coadsorbed hydrogen/deuterium produces propane at 121 K. The activation of a single C−C bond in adsorbed cyclopropane dominates as indicated by the fact that propane is the only product observed. No multiple C−C bond activation which would result in methane or ethane formation was ever observed. These reactions and their mechanisms have been investigated using temperature-programmed reaction (TPR) and vibrational spectroscopy using high-resolution electron energy loss spectroscopy (HREELS). The reactivities of hydrogen and deuterium were indistinguishable during these experiments so we have used the generic term hydrogen or gas phase atomic hydrogen to describe the reactions of ...