Hybridization in cyclopropane,cyclobutane and cubane

The general procedure derived by Del Re [6] for determining the hybrid orbitals of a molecule from the overlap matrix has been applied to cyclopropane, cyclobutane and cubane, for which the concept of hybridization is of a peculiar interest. The results are in good agreement with experimental facts and results of other theoretical procedures. The approximation which describes these molecules just in terms of localized bonds appears to be rather limited and such a phenomenon seems general for methylenic compounds. The problem of bent bonds is equally discussed.

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Zusammenfassung Die von Del Re [6] vorgeschlagene, von der Matrix der Überlappungsintegrale ausgehende Methode zur Bestimmung der Hybrid-Orbitale eines Moleküls wurde auf Cyclopropan, Cyclobutan und Cuban angewandt. Bei diesen Molekülen beansprucht die Frage der Hybridisierung besonderes Interesse. Die Ergebnisse stimmen gut mit den experimentellen Befunden und den Schlußfolgerungen anderer Theorien überein. Die Beschreibung dieser Moleküle durch lokalisierte Bindungen zwischen atomaren Hybrid-Orbitalen ist nur eine grobe Näherung; das scheint allgemein für Verbindungen mit Methylengruppen zu gelten. Das Problem der gekrümmten Bindungen wird diskutiert.

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Résumé La méthode générale proposée par Del Re [6] pour déterminer les orbitales hybrides d'une molécule à partir de la matrice des intégrales de recouvrement a été appliquée au cyclopropane, au cyclobutane et au cubane, pour lesquels le concept d'hybridation présente un intérêt particulier. Les résultats sont en bon accord avec les faits expérimentaux et avec les conclusions auxquelles conduisent les autres méthodes théoriques. L'approximation qui consiste à décrire ces molécules en termes de liaisons localisées entre orbitales atomiques hybrides paraît limitée et ce phénomène semble général pour les composés à groupements méthylènes. Le problème des liaisons courbes est également discuté.

     

The kinetics of hydroxyl radical reactions with cyclopropane and cyclobutane

A laser flash photolysis/resonance fluorescence investigation has been carried out to study the kinetics of the overall reactions OH + cyclopropane (1) and OH + cyclobutane (2) in the temperature range 298–490 K and at 298 K, respectively. The following kinetic parameters have been determined: k₁ =(3.9 ±0.6) 10⁻¹²exp- (2.2 ± 0.1)kcal mol⁻¹/RT molecule⁻¹cm³s⁻¹, k₂(298 K) = (17.5 ± 1.5)10⁻¹³molecule⁻¹ cm³s⁻¹.     

Rate constants of the reactions of OH radicals with cyclopropane and cyclobutane

The kinetics of the reactions of hydroxy radicals with cyclopropane and cyclobutane has been investigated in the temperature range of 298–492 K with laser flash photolysis/resonance fluorescence technique. The temperature dependence of the rate constants is given by k₁ = (1.17 ± 0.15) × 10^?16 T^3/2 exp[?(1037 ± 87) kcal mol^?1/RT] cm³ molecule^?1 s¹ and k₂ = (5.06 ± 0.57) × 10^?16 T^3/2 exp[?(228 ± 78) kcal mol^?1/RT] cm³ molecule^?1 s^?1 for the reactions OH + cyclopropane → products (1) and OH + cyclobutane → products (2), respectively. Kinetic data available for OH + cycloalkane reactions were analyzed in terms of structure-reactivity correlations involving kinetic and energetic parameters.     

Approaches to dihydrooxazine ring systems and application in the synthesis of bioactive natural products

This review describes selected approaches to the synthesis of 4H-5,6-dihydro-1,2-oxazines and application of these methods to the synthesis of natural products containing this heterocyclic ring system. The focus is on applications and on interesting aspects of chemistry related to this subclass of natural products, as well as newer methods for the synthesis of oxazines.     

Comments on the electronic properties of cyclopropane,cyclobutane, and cyclohexane

Spectral quantities of cyclopropane, cyclobutane, cyclohexane, and of several derivatives, have been calculated by a semiempirical all-valence electron SCF-CI MO method. In cyclopropane, HOMO is practically localized in the carbon-frame, and LVMO is purely so. In cyclobutane, these two MO's are based on C-H bonds, while cyclohexane holds an intermediate position. Despite the overall similarity-experimental and computed-of the spectra of these molecules, assignments are non-parallel. Like cyclopropane, cyclobutane can extend conjugation, but to a diminished degree; cyclohexane behaves in this respect like an acyclic alkane. An interpretation of this gradation, in terms of the nature of high-lying MO's, is proposed.     

Experimental and theoretical study on cation radicals of cyclopropane,cyclobutane and cyclopentane

Cation radicals of cycloalkanes have been produced for the first time in γ-irradiated rigid solutions using fluoromethane as a matrix. Observed ESR spectra are analyzed by ab initio MO calculations; molecular geometries of the cations are optimized for several low-lying doublet states by the energy gradient method. Based on the optimized geometry thus determined the hyperfine splitting constants are calculated by the pseudo-orbital theory. Large Jahn-Teller distortions are calculated for the carbon rings and the H-C-H frames of the cation radicals. The distortions are consistent with the nodal picture of singly occupied orbitals. The calculated hfs constants are sensitive to the change in geometry due to a large contribution of spin-delocalization. The average of the calculated proton hfs constants is compatible with the observed ESR spectra which indicates that all the protons of the individual cations have become equivalent owing to the dynamic Jahn-Teller distortion.     

Electron-impact induced fragmentation of some natural products containing bis-2,2-dimethylchromene and bis-2,2-dimethylchroman ring systems

Derivatives of eriostoic and eriostemoic acids (II and III) and their tetrahydro analogs (VI and IX) which contain the bis-2,2-dimethylchromene and the bis-2,2-dimethylchroman ring systems respectively, fragment subsequent to electron-impact in characteristic fashion. The former yield mass spectra dominated by the loss of a methyl radical from their molecular ions while the latter exhibit preferential fragmentation of the heterocyclic ring system. In particular the hydrogen transfer process accompanying elemination of C₄H₇ from the parent ion of bis-2,2-dimethylchromans was investigated by deuterium labeling studies. Fragment ions containing the bis-2,2-dimethylchroman ring undergo further fragmentation by the loss of C₄H₈ rather than C₄H₇. An unusual elimination of CH₅ (accompanied by the appropriate metastable ion) from the molecular ions of the 3,5-dinitrobenzoate esters (XIV and XV) was observed in their mass spectra.     

Nitrile anion cyclization with epoxysilanes followed by brook rearrangement/ring opening of cyclopropane nitriles/alkylation

The reaction of delta-silyl-gamma,delta-epoxypentanenitrile derivatives with a base and an alkylating agent affords delta-siloxy-gamma,delta-unsaturated pentanenitrile derivatives via a tandem process that involves the formation of the cyclopropane derivative by epoxy nitrile cyclization followed by Brook rearrangement and an anion-induced cleavage of the cyclopropane ring.     

Dynamics of strained ring systems as studied by ESR and saturation transfer sensitive spectroscopic techniques

Effective spin-lattice relaxation rates for the cyclopentyl-1-carboxylic acid radical in X-irradiated 1,1-cyclopentanedicarboxylic acid measured by saturation-recovery and electron spin echo techniques are found to be determined by van Vleck and Raman mechanisms. Analysis of ESR, ELDOR, ENDOR, and passage spectra permitted characterization of the temperature dependence of the ring inversion frequency as τ_c^?1 = 3.35 × 10¹¹ exp(?1540/T), implying a barrier to ring inversion of 3 kcal. The magnetic resonance and double resonance spectra at low temperatures are characterized by nearly isotropic beta hyperfine couplings of 56 and 112 MHz which are averaged by the dynamic ring puckering to yield a value of 84 MHz at ambient temperatures. The gamma couplings are not resolved in the ENDOR spectra but from the linewidth of the ENDOR line at the free proton frequency a value of 3 MHz is established as the upper limit for these couplings.     

A sequential stereocontrolled cyclopropane ring formation and semi-pinacol rearrangement

Treatment of an unsaturated 2,3-epoxy alcohol with SnBr₄ leads to a stereoselective formation of a cyclopropane ring, and an α-ketol unit as part of a subsequent ring expansion.     

Selective D ring side chain tritiation of steroidal natural products

This paper describes the tritiation of several steroid natural products at their D ring side chain, taking advantage of selective catalytic tritium reductions.

     

Synthesis and some transformations of bicyclic γ-lactones with a fused cyclopropane ring

Reductive cyclization of 4-hydroxymethyl-5,5-dimethyl(or pentamethylene)-2,5-dihydrofuran-2-ones by the action of sodium tetrahydridoborate gave bicyclic compounds in which the lactone ring is fused to a cyclopropane ring. Hydrolysis of the products with aqueous sodium hydroxide resulted in the formation of the corresponding disodium cyclopropane-1,1-dicarboxylates, which reacted with alkyl halides to produce the diesters. Acid hydrolysis of the fused systems was accompanied by opening of the cyclopropane ring with formation of 4-chloromethyl-5,5-dimethyl-2-oxotetrahydrofuran-3-carboxylic acid.     

Expeditious total syntheses of natural allenic products via aromatic ring umpolung

Concise diastereoselective syntheses of the marine (+/-)-panacene and terrestrial (+/-)-desbromopanacene have been achieved in a few steps based on a concept of "aromatic ring umpolung". The synthetic avenue to the (+/-)-panacene involved a novel oxymercuration strategy to produce the correct configuration of the bromoallene moiety.     

Electronic structure and spectra of fused n/m ring systems with one or two bridgehead nitrogen atoms

Electronic structures and absorption spectra of the quinolizinium cation, all its monoaza-derivatives and the pyridazo(1.2-a)pyridazinium dication, have been computed by using the Pariser-Parr-Pople method, including all singlet mono-excited electronic configurations. The assignment of the observed bands of the quinolizinium cation has been made by comparing its observed transition energies and intensities with those of naphthalene, quinoline and isoquinoline. Discussion is given on reactivity and band interpretation.

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Zusammenfassung Die Elektronenstrukturen und Absorptionsspektren des Quinolizinium Kations und aller seiner Monoazaderivate und des Pyridazo(1,2-a)-pyridazinium Dikations, werden nach der Pariser-Parr-Pople Methode berechnet, wobei alle einzeln angeregten Konfigurationen berücksichtigt werden. Die Banden des Quinolizinium Kations werden durch einen Vergleich der beobachteten Übergangsenergien und Intensitäten mit denen des Naphthalins, Quinolins und Isoquinolins zugeordnet. Reaktivität und Bandeninterpretation werden diskutiert.

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Résumé La structure électronique et les spectres d'absorption du cation quinolizinium, de tous ses monoazadérivés et du dication pyridazo(1,2-a)pyridazinium ont été calculés par la méthode de Pariser-Parr-Pople, avec intéraction de toutes les configurations singulets mono-excitées. L'attribution des bandes d'absorption du cation quinolizinium a été faite en comparant ses énergies de transition et intensités observées avec celles du naphthaléne, quinoléine et isoquinoléine. On a aussi discuté la reactivité et l'interprétation des bandes.