Reaction of carbenes with bicyclo[2.1.0]pentane

Bicyclo[2.1.0]pentane reacts with phenylcarbene and dicarbomethoxycarbene by simple insertion at the cyclobutane methylene position. By contrast, difluorocarbene reacts by two bond cleavage to give 1,1-difluoro-1,5-hexadiene.     

Understanding the puzzling chemistry of bicyclo[2.1.0]pentane

Three thermal reactions of bicyclo[2.1.0]pentane have been studied by CASPT2-g3 and CASSCF electronic structure calculations. They are isomerization to cyclopentene, isomerization to 1,4-pentadiene, and cycloaddition to fumaronitrile. All three of these reactions exhibit unusual features that have prompted mechanistic debate. The present computational results provide a basis for understanding the experimental observations.     

Synthesis of novel 2-azabicyclo[2.2.0]- and [2.1.1]hexanols

Methyl- and phenyl-substituted N-(ethoxycarbonyl)-2-azabicyclo[2.2.0]hex-5-enes 6 were reacted with NBS in wet DMSO to afford bromohydrins. Mixtures of unrearranged 6-exo-bromo-5-endo-hydroxy-2-azabicyclo[2.2.0]hexanes 7a,b and rearranged 5-anti-bromo-6-anti-hydroxy-2-azabicyclo[2.1.1]hexanes 8a,b were formed stereoselectively from the parent alkene 6a and 4-methyl alkene 6b. The 5-methyl alkene 6c affords only unrearranged bromohydrin 7c and dibromohydrin 9. By contrast, solely rearranged 3-endo-substituted-2-azabicyclo[2.1.1]hexane bromohydrins 8d-f result from additions to 3-endo-methyl alkene 6d, 3-endo-4-dimethyl alkene 6e, and 3-endo-phenyl alkene 6f. As an alternative route to bromohydrins, the parent 5,6-exo-epoxide 10a and 5-endo-methyl-5,6-exo-epoxide 10b were ring opened with bromine/triphenylphosphine to afford unrearranged 5-endo-bromo-6-exo-hydroxy-2-azabicyclo[2.2.0]hexanes 11a,b, while the 3-endo-methyl epoxide 10c afforded solely the rearranged 5-anti-bromo-6-anti-hydroxy-3-exo-methyl-2-azabicyclo[2.1.1]hexane isomer 8g. Tributyltin hydride reduction of bromohydrins 7a,b and 11a afforded novel 2-azabicyclo[2.2.0]hexan-5-ols 13a,b and -6-ol 14, and bromohydrins 8a,b, 8d-g afforded new 2-azabicyclo[2.1.1]-hexan-5-ols 15a,b and 15d-g.     

The electronic structure of bicyclo [2.1.1] hexane

Non empirical SCF-LCAO-MO calculations have been performed on bicyclo [2.1.1] hexane using a molecule optimized minimal basis set of gaussian functions. The electronic structure, the nature of the molecular orbitals and of the bonds are analyzed. The most notable results lie in the strong interaction between the two bridgehead sites and between two of the CH bonds of the methylene bridges. Interactions between fragment orbitals are considered. The relations with the molecular properties are discussed, especially with respect to molecular strain.

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Zusammenfassung Nichtempirische SCF-LCAO-MO-Rechnungen wurden für das Bicyclo [2.1.1]-Hexan durchgeführt, wobei ein minimaler am Äthylen optimierter Basissatz von Gaußfunktionen verwendet wurde. Die Elektronenstruktur sowie die Art der Molekülorbitale und der Bindungen wird untersucht. Als bemerkenswerte Resultate erscheinen die starke Wechselwirkung zwischen den beiden Brückenkopf-Gruppen sowie zwischen den beiden CH-Bindungen der Methylenbrücken. Die Wechselwirkungen zwischen den Orbitalen von Molekülbruchstücken werden untersucht. Die Beziehungen zu den molekularen Eigenschaften werden diskutiert, insbesondere im Hinblick auf molekulare Ringspannung.

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Résumé Une étude théorique ab initia SCF-LCAO-MO du bicyclo [2.1.1] hexane a été effectuée en utilisant une base minimale de fonctions gaussiennes optimisée sur une molécule. La structure électronique, la nature des orbitales moléculaires et des liaisons ont été analysées. Le résultat le plus marquant est l'existence d'une forte interaction entre les deux sites en tête de pont et aussi entre deux des liaisons C-H des ponts méthylène. Par ailleurs on peut mettre en évidence des interactions entre «orbitales de fragments». Les relations entre structure électronique et propriétés moléculaires (spécialement la tension de cycle) sont discutées.

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Equipe de Recherche Associée au C.N.R.S. N 265.     

1-bar|t-butyl-5-isopropylidenebicyclo[2.1.0]pentane

The title compound, synthesized by a carbenoid cyclization-rearrangement route, is the most stable member of the bicyclo[2.1.0]pentane series yet prepared.     

Synthesis and ring opening reactions of a 2-silabicyclo[2.1.0]pentane

Methyl 2-silabicyclo[2.1.0]pentane-1-carboxylate, obtained by a photochemical intramolecular cyclopropanation reaction of an [small alpha]-allylsilyl-[small alpha]-diazoacetate, undergoes ring opening reactions under different conditions leading to methyl 2-[diisopropyl(methoxy)silylmethyl]cyclopropane-1-carboxylate, a 1-sila-4-cyclopentene-2-carboxylate or an allyl(methoxysilyl)ketene.     

Conversion of bicyclo[2.1.0]pentane into 2,3-dioxabicyclo[2.2.1]heptane via t-butyl peroxymercuriation

Bicyclo[2.1.0]pentane has been converted in 45% yield into 2,3-dioxabicyclo-[2.2.1]heptane by the sequence t-butyl peroxymercuriation, iododemercuriation, epimerisation of the resultant 1-t-butylperoxy-3-iodocyclopentane, and reaction of the trans isomer with silver trifluoroacetate.     

Simple synthesis of endo-2-bromo-5-thiabicyclo[2.1.1]hexane

A brief synthesis of endo-2-bromo-5-thiabicyclo [2.1.1]hexane ( 9 ) has been developed involving conversion of 3-cyclopentenol ( 6 ) to 3-thioacetoxycyclopentene ( 7 ), bromination of this giving trans-1,2-dibromo-4-thioacetoxycyclopentane ( 8 ), and treatment of the latter with base. Compound 9 is oxidized to its S-oxide 10 and S, S-dioxide 11 . Comparative ¹³C and ¹H NMR data are given for 9–11 .     

Gas and liquid phase kinetics of the unimolecular isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane: The radical stabilization energy of an α-bromine atom

The kinetics of the gas phase isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane to 2-chloro-5-bromohexa-1,5-diene have been measured in a static system over the temperature range of 135–215°C, with a variation in the total pressure from 0.6 to 400 torr. For these conditions the rate constants are well represented by the Arrhenius equation: where θ = 2.303RT kcal/mole. Transition state estimates for the biradical mechanism for the isomerization of bicyclo[2.2.0]hexanes are shown to be in good agreement with these Arrhenius parameters. By comparison of the activation energy with that for the isomerization of bicyclo[2.2.0]hexane and 1,4-dichlorobicyclo[2.2.0]hexane, the radical stabilization energy of an α-bromine atom is shown to be 1.0 ± 1.8 kcal/mole. Rates are also reported in the liquid phase at temperatures of 155°C and 175°C with diphenyl ether, nitrobenzene, and dimethylsulfoxide as solvents. The observed rate constants are all faster (by a factor of 1.1–1.7) than those measured in the gas phase and display no correlation between rate and solvent polarity.