Showing papers on "Cyclopropane published in 1991"

Synthesis of cycloprop[c]indol-5-ones from 4-diazo-3-[n-(2-propenyl)amido]cyclohexadien-1-ones. Exploration of copper(I) and copper(II) complexes as catalysts

Abstract: The cyclization of 4-diazo-3-[N-(2-propenyl)amido]cyclohexadienones to cycloprop[c]indol-5-ones under the influence of copper(I) and copper(II) compounds has been investigated. Catalysis is observed with copper(I) triflate, the carbon monoxide complexe of copper(I) triflate, and the carbon monoxide complexes of trifluoropentanedionato- and hexafluoropentanedionatocopper(I). The best results, essentially quantitative conversion, are achieved with a catalyst solution containing trifluoropentanedionatocopper(I) carbonyl and 1 equiv of n-butalymine. No significant enantioselectivity is observed with a chiral salicyliminatocopper(II), [(trifluoroacetyl)camphorato]copper(I) carbonyl, or a trifluoropentanedionatocopper(I) carbonyl solution containing (S)-α-naphthylethylamine. A mechanistic interpretation involving reductive dediazonization, exo-trig radical cyclization, and cyclopropane formation by the resulting intermediate is proposed

Enantioselective cyclopropane syntheses using the chiral carbene complexes (SFe)- and (RFe)-C5H5(CO)(PR3)Fe:CHCH3+. A mechanistic analysis of the carbene transfer reaction

Abstract: The enantiomerically pure acyl complexes were converted to the corresponding enantiomerically pure carbene complexes (S Fe )- and (R Fe )-C 5 H 5 (CO)(PR 3 )Fe=CHCH 3 + by using standard techniques. Enantioselective ethylidene transfer from these complexes to styrene, vinyl acetate, and isopropenyl acetate gave methylcyclopropanes in high optical yields. A mechanistic analysis of the transfer reaction is presented by using the stereochemical results obtained coupled with deuterium labeling and relative reactivity studies. It is concluded that the most likely mechanism for carbene transfer involves reaction of the olefin with the minor but more reactive synclinal isomer of C 5 H 5 (CO)(PR 3 )Fe=CHCH 3 + followed by backside attack of the developing electrophilic center at Cγ on the Fe-Cα bond. A rationale is offered for the differing diastereoselectivities of ethylidene transfer from C 5 H 5 (CO)(PR 3 )Fe=CHCH 3 + versus C 5 H 5 (CO) 2 Fe=CHCH 3 + to various olefins

Some aspects of total scattering cross sections of electrons for simple hydrocarbon molecules

Abstract: Total electron scattering cross sections for C2H4, C2H6, C3H6 (propene and cyclopropane) and C3H8 have been measured in the energy range between 4 and 500 eV. Measured results show a remarkable isomer effect and dependence on the geometrical dimension of the molecule.

Use of radical ring opening for introduction of alkyl and substituted alkyl groups with stereochemical control: a synthetic application of cyclopropylcarbinyl radicals

Abstract: Cyclopropylcarbinols, which are accessible by a number of routes, can be converted into the corresponding radicals. These radicals undergo peripheral ring-opening of the cyclopropyl substructure to afford substituted cycloalkenes. The whole sequence represents a general method for attaching alkyl, and substituted alkyl, groups to an existing cyclic structure, and it can often be carried out with predictable stereo- and regiochemical control. Reaction conditions for the ring-opening depend on the substitution pattern of the cyclopropane: where the non-bridgehead carbon of the cyclopropane carries a strongly electron-withdrawing group, the ring-opening can be done at the reflux temperature of benzene. However, in the absence of such electron-withdrawing groups, a low temperature is best used in order to suppress ring expansion

The regio- and stereoselectivities of the reaction of allyl acetates and silyl ketene acetals catalyzed by palladium(0) complexes: a new route to cyclopropane derivatives

Abstract: Pd(0) complexes of chelating phosphines catalyzed the coupling of allyl acetates and ketene silyl acetals to yield α-allylated carboxylic acid esters. Unexpectedly alkylation of the central carbon atom (C2) of the allyl groups was also observed with concomitant formation of cyclopropane derivatives. In both cases the silyl enolate attacked the allyl group from the side opposite Pd. The yield of the reaction was sensitive to the nature of the ligand coordinated with palladium. The 1,1'-bis(diphenylphosphino) ferrocene-Pd complex was the most effective catalyst

Synthesis of 1,4-keto esters and 1,4-diketones via palladium-catalyzed acylation of siloxycyclopropanes. Synthetic and mechanistic studies

Abstract: The reaction of a variety of siloxycyclopropanes with acid chlorides in the presence of a catalytic amount of a palladium/phosphine complex gives 1,4-dicarbonyl compounds to good yield. 1-Alkoxy-1-(trialkylsiloxy)-cyclopropanes react with both aromatic and aliphatic acid chlorides in chloroform to give 1,4-keto esters. Synthesis of 1,4-diketones by the acylation of 1-alkyl- or 1-aryl-1-siloxycyclopropanes has been achieved by carrying out the reaction if HMPA under 10−20 atm of carbon monoxide. Kinetics studies and product analysis revealed the unique mechanism of this reaction, which involves rate-determining cleavage of the strained cyclopropane carbon−carbon bond with a coordinatively unsaturated acylpalladium chloride complex. Ab initio calculations of hydroxylated cyclopropane model compounds showed that the unique reactivities of the siloxycyclopropanes may be correlated with the molecular orbital properties of these compounds rather than their ground-state structural properties

Remote control of stereogenicity transfer by ring-generated anisotropic orbital overlap. Stereochemistry of hydrogen shift in the intramolecular reverse ene reaction of a cis-2-alkyl-1-alkenylcyclopropane

Abstract: The thermal rearrangement of cis-2-(2-propyl)-1 (E)-propenylcyclopropane at temperatures near 230°C in the gas phase occurs with activation parameters of E a =35.5±0.6 kcal/mol and log A=12.05±0.5 (A, s −1 ). The optically active isotopically doubly labeled analogue (cis-2(S)-(2(S)-propyl-1-d 3 )-1(S)-(1(E)-propenyl-2-d)cyclopropane 5 was synthesized in 12 steps from dicyclopentadiene. Pyrolysis of 5 gave only 2-methyl-octa-2(Z),5(Z)-diene-1-d 3 -7(S)-d, with high stereospecificity at each of the three sites of sterogenicity. This result is the one predicted if the reaction is controlled by optimal overlap of the reacting C―H and π bond orbitals with the C s symmetric component of the degenerate 3E' highest occupied orbital of the cyclopropane ring

Asymmetric synthesis of cis and trans 2-methyl and 2-ethyl 1-amino cyclopropanecarboxylic acids

Abstract: A new four step asymmetric synthesis of 2-methyl and 2-ethyl 1-amino cyclopropane carboxylic acids resulted from the cycloaddition of diazomethane to the corresponding chirally derivatized dehydro-aminoacid.

Obtention de cyclopropanes gem-dicarboxylate par cyclocondensation de carbanions bromo- et chloromalonate sur des accepteurs de Michaël

Abstract: Cyclopropane gem-dicarboxylates are prepared through cyclocondensation of chloro or bromomalonate carbanion and electrophilic alkenes. The carbanion can be formed in THF by deprotonation of the mon...

Ab initio study of some methylene and silylene insertion reactions

Abstract: The theoretical barriers and reaction energetics have been determined, using ab initio calculations at the MP4/6-3IG(d) level, with 3-2IG* geometries, for the insertions of methylene and silylene into the X-Y bonds of ethane, methylsilane, disilane, and cyclopropane and into the X-H bonds of disilane and ethane. A correlation was observed between barrier heights and lengths of substrate bonds. The largest barriers were observed for methylene and silylene inserting into the C-C bonds of ethane. The barriers became successively lower for the C-Si and Si-Si bonds. Steric interactions thus appear to be a major factor in determining barrier heights. The observed barriers for silylene and methylene inserting into the strained c-c bonds of cyclopropane were approximately 40 kcalfmol less than those for the analogous unstrained compounds.

Intramolecular cyclopropanation-ring fragmentation leading to spirocyclic ring construction: a stereoselective synthesis of .beta.-chamigrene

Abstract: The transannular cyclopropanation of a keto carbene generated by Rh 2 (OAc) 4 catalysis on a bicyclic dihydropyran nucleus provided a key oxatricyclic ketone intermediate for the synthesis of the [6.6] spirocyclic ring construction. Selective fragmentation of the cyclopropane followed by hydrolytic cleavage of the C−O bond provided the spirocyclic skeleton. Functional group manipulations to adjust oxidation states led to a total synthesis of (±)-β-chamigrene in 14 steps without the use of protection/deprotection schemes

Inter- and intra-molecular reactions of indol-2-yl carbenes and related species. Preparation of 1,1a,2,8b-tetrahydroazirino[2′,3′ : 3,4]pyrrolo[1,2-a]indoles

Abstract: The intramolecular cycloaddition of 2-indolylcarbenes, or their precursors, has been successfully extended to the CN bond of O-methyl oximes. Thus the sodium salts of the tosylhydrazones 10, prepared from the corresponding indole-2-carbaldehydes, decomposed to the azirinopyrroloindoles 11. The N-methoxyaziridine 11a exists as a single invertomer at nitrogen. Several 1-(2-alkynyl)indole-2-carbaldehydes were converted into the corresponding Eschenmoser hydrazones by reaction with trans-1-amino-2,3-diphenylaziridine. No addition of the resulting carbene onto the triple bond occurred upon thermolysis. The only products that were isolated corresponded to attack of the carbene on the solvent. In one case the carbene could be trapped bimolecularly to give an indolyl substituted cyclopropane derivative.

Oxidative fragmentation of catharanthine by dichlorodicyanoquinone

Abstract: Oxidation of catharanthine by DDQ leads to formation of products resulting from fragmentation of the C16-C21 bond as well as C3 and C5 dehydrogenation. Among the products are compounds containing a cyclopropane ring formed by bonding between C14 and C16. Cyclopropane ring formation can be also be observed from the intermediate generated by Poiter-Polonovski fragmentation of catharanthine N-oxide

Investigation of the stereochemistry of iron-carbon.alpha. bond cleavage when phenylcyclopropane is generated by .gamma.-ionization of stereospecifically deuterated C5H5(CO)2FeCHDCHDCH(OCH3)C6H5 complexes. A transition-state model for transfer of the carbene ligand from C5H5(CO)2Fe:CHR+ to alkenes

Abstract: C 5 H 5 (CO) 2 FeCH 2 CH 2 CH(OCH 3 )C 6 H 5 , 4, and stereospecifically deuterium labeled threo-d 2 -C 5 H 5 -(CO) 2 FeCHDCHDCH(OCH 3 )C 6 H 5 , 7a,b and erythro-d 2 -C 5 H 5 (CO) 2 FeCHDCHDCH(OCH 3 )C 6 H 5 , 8a,b were synthesized. Treatment of compound 4 with trimethylsilyl triflate results in ionization of the γ-methoxy group and formation of phenylcyclopropane in good yields. Ionization of 7a,b gives a 1:1 mixture of cis-2, cis-3-dideuterio- and trans-2, trans-3-dideuterio-r-1-phenylcyclopropane, while ionization of 8a, b gives cis-2, trans-3-dideuterio-r-1-phenylcyclopropane. These results established that the cyclopropane ring is formed by backside attack of electrophilic Cγ on Cα with net inversion of stereochemistry at Cα. These reactions serve as models for the reactions of carbene complexes C 5 H 5 (CO) 2 Fe=CHR + with alkenes to give cyclopropanes and suggest that in the transfer reactions Fe-cα is cleaved with inversion

Comparison of the magnetic anisotropy of the cyclopropane and cyclobutane ring systems as probed by proton NMR spectroscopy of spiro[cycloalkanefluorenes]

Abstract: The fluorene and spirocyclopentylfluorene were used as model compounds so that the «special» magnetic anisotropy effects of cyclopropane and cyclobutane rings could be estimated. The typical behavior of cyclopropane was confirmed: upfield shifits of protons located above and below the plane of the ring. A small downfield shift was found for the spirocyclobutanes; however, it is not large enough to force the conclusion that cyclobutane has a group magnetic anisotropy similar but opposite in direction to cyclopropane. Proton chemical shifts in this series of compounds cannot be used as an argument for the antiaromaticity of cyclobutane

Addition of Ketocarbenes to Alkenes, Alkynes and Aromatic Systems

Abstract: The reactions of keto-carbenes (1) or -carbenoids with CC π-bonds1−7 offer a direct entry to highly functionalized cyclopropanes (2),8 which, due to their lability, are readily transformed to a range of products (Scheme 1). The cyclopropanation is often highly stereoselective and further reaction can lead to products of defined stereochemistry, controlled by the compact arrangement of functionality within the cyclopropane ring. Although free carbenes undergo additions to alkenes, these reactions are unselective and competing carbene rearrangements tend to occur. Consequently, metal-stabilized ketocarbenoids, generally formed by catalyzed decomposition of diazo compounds, are of much greater synthetic utility and will be the emphasis of this chapter.

Theoretical studies on the ring-opening reactions of bicycloalkyl radicals: homolytic .beta.-scission in bicyclo[1.1.0]but-2-yl radical

Abstract: Ab initio UHF and CASSCF calculations have been performed on the two modes of ring opening in bicyclo [1.1.0] but-2-yl radical (8), involving homolytic β scission of a C-C bond. Although the fusion bond cleavage in 8 to give cyclobutenyl radical is formally an orbital-symmetry-forbidden electrocyclic process, the activation energy is calculated to be only ∼3 kcal/mol. This ring opening is predicted to take place via a highly nonsymmetric transition structure, which allows the mixing of the SOMO (a' symmetry) and LUMO (a″ symmetry) of 8, and the coupling between the crossing states (2A' and 2A″), resulting in an adiabatic reaction on the ground-state potential surface. The alternative β-scission mode in 8, involving cleavage of an outer cyclopropane bond to afford cyclopropenylmethyl radical, is predicted to have an activation energy of ∼26 kcal/mol.

The prediction and calculation of properties of atoms in molecules

Abstract: An outline of the manner in which the theory of atoms in molecules evolved from the study and characterization of the accurate molecular charge distributions made available by the work of Professor Hall and Professor Roothaan is presented. The contribution of Professor Hall to the application of the theory of catastrophes to the study of the changes in the gradient vector field of the charge distribution is also discussed. This is followed by the application of the theory of atoms in molecules to the determination of the molecular structure and of the atomic contributions to the molecular properties of cyclopropane. The recovery of the experimentally observed group additivity and transferability of the energy and polarizability in hydrocarbon molecules, and of the accompanying specific constitutive effects exhibited by these properties in the case of cyclopropane, demonstrate that the atoms of theory are the atoms of chemistry.

The chemistry of tetrakis(trifluoromethyl)cyclopentadienone

Abstract: Reactivity of tetrakis (trifluoromethyl) cyclopentadienone, which include nucleophilic addition, reaction with proton containing nucleophiles, cyclopropane formation, one-electron reduction leading to a stable radical anion salt and finally chemical reduction with silanes to yield siloxydienes which, upon deprotonation, afford new tetrakis (trifluoromethyl) cyclopentadienyl derivatives