Site-selectivity of protonation in gaseous toluene

The favoured protonation sites leading to isomeric toluenium ions have been assayed, envisaging the mildest possible route for proton transfer to naked toluene.     

Site of protonation in the reaction of gaseous Brønsted acids with halobenzene derivatives

The site of protonation in the reaction of gaseous Brønsted acids with chlorobenzene and fluorobenzene derivatives has been examined using deuterium labelling of chlorobenzene and deuterated reagent gases (D₂ and N₂/D₂). In the protonation of chlorobenzene by [H₃]⁺ ～30% of the [MH? HCl]⁺ fragment ions arise from a chlorine-protonated species while ～70% arise from a ring-protonated species in which complete hydrogen scrambling has occurred. In the reaction of [N₂H]⁺ with chlorobenzene ～75% of the fragment ions arise from the chlorine-protonated form with ～25% arising from the ring-protonated form of [MH]⁺. By contrast fluorobenzene fragments almost entirely from the fluorine-protonated form. Similar results are obtained for dihalobenzenes. The mechanistic implications are discussed.     

The ring current in cyclopropane

Mapping of induced current density using the coupled Hartree-Fock “ipsocentric” (CTOCD-DZ/ 6-31G**//RHF/6-31G**) method shows a ring current arising in the σ framework of cyclopropane that is intense, diatropic and annular, and hence is consistent with the numerous literature attributions of σ-aromaticity to this molecule. Localised orbital analysis shows that this current can be attributed to the set of three carbon–carbon bonds. In a similar analysis, the four carbon–carbon bonds of (planar constrained) cyclobutane are found to give rise to a central paratropic current. The maps are consistent with the view that cyclopropane is strongly σ-aromatic, cyclobutane is weakly σ-anti-aromatic, and the larger cycloalkane systems are essentially σ-non-aromatic. Contribution to the Fernando Bernardi Memorial Issue.     

Effect of protonation exothermicity on the reaction of gaseous Brønsted acids with halobenzene derivatives

The protonation of haloaromatics by [N₂H]⁺ and [CO₂H]⁺ has been studied by chemical ionization mass spectrometry. In general, the fragmentation reactions following protonation by [CO₂H]⁺ are similar to those observed following protonation by [CH₅]⁺, while the fragmentation reactions induced by protonation by [N₂H]⁺ are intermediate between those observed on reaction with [CH₅]⁺ and with [H₃]⁺. These results are consistent with the conclusion that the fragmentation mode is determined by the protonation exothermicity since the proton affinity of CO₂ is the same as that of CH₄ while the proton affinity of N₂ is intermediate between that of CH₄ and H₂.     

The stereospecific ring protonation of diindenyliron

Reactions of protonic acids (HCl, CF₃COOH) with diindenyliron give the h⁵-indenyl-h⁶-indeneiron(II) monocation, [Fe(C₉H₇)(C₉H₈)]⁺, which can be isolated as a hexafluorophosphate salt. Two deuterium labeling experiments, the reaction of Fe(C₉H₇)₂ and DCl and the reaction 1,1′,3,3′-tetradeuteriobis(indenyl)iron with HCl, confirm that the products in this reaction have been formed by stereospecific addition of the proton to the indenyl ring. Because of the wealth of data on metal protonations, including data on the protonation of ferrocene at the iron atom, preference is indicated here for endo ring protonation which is proposed to occur via an intermediate protonated metal species. Protonation of Fe(C₅H₅)(C₉H₇) is also reported. Deprotonation with n-butyllithium regenerates diindenyliron. However, data on these reactions suggest preferential loss of an exo-proton, which is perhaps realistic in view of the known attack of nucleophiles on the exo position of coordinated hydrocarbon rings.     

The site of protonation in amino-substituted pteridines

SCF -LCAO -MO -CI calculations made on 2- and 4-amino-, and 2,4-diaminopteridines indicate that the most favorable site where the protonation can take place is N-1.     

The protonation of chlorins (dihydroporphyrins)

Octaethylchlorin forms a violet mono-cation and a deep blue di-cation in two discrete stages on spectrophotometric titration with trifluoroacetic acid in chloroform solution, and similar results are obtained with chlorophyll derivatives although the colour changes are much less pronounced.     

1-Chloro-1-(trichlorovinyl)cyclopropane : Molecular structure and conformation in the gaseous phase as determined by electron diffraction

A gas phase electron diffraction investigation of 1-chloro-1-(trichlorovinyl)cyclopropane at 65° showed the existence of one conformer having a perpendicular arrangement of the cyclopropyl- and the trichlorovinylgroup, with torsional angle τ = 91(3)° relative to τ = 180° for the antiperiplanar conformer (CCl-CClanti arrangement). Both the experimental data and a molecular mechanics calculation indicate the lower barrier toward the antiperiplanar form,less than 40 kJ · mol^-1. The second barrier was by molecular mechanics calculated to 60 kJ · mol^-1.     

Angle-resolved photoelectron spectroscopy of cyclopropane

The angular distribution parameter, β, determined for the valence orbitals (IP < 18 eV) of cyclopropane in the 10–30 eV photon energy range using dispersed polarized synchrotron radiation. The energy dependence of β for photoelectron energies between, 2 and 10 eV above threshold was found to be similar to those found previously for other σ orbitals. The effects of Jahn-Teller splitting on β for the 3e′ orbital were found to be small but definitely present. The overall shape and magnitude of the β(hv) curve are, however, sufficiently for the different Jahn-Teller components that, for purposes of orbital assignments using β(hv) curves the shape and magnitude of the curves can be considered associated only with the initial state. Resonance photoionization features at a photon ener of ≈ 18 eV were observed in the 3e′ and 3a′₁ orbitals and tentatively assigned to autoionization.     

1,2-dimagnesium derivatives of cyclopropane

cis-1,2-Dibromomagnesiocyclopropane (cis-$\text{4}$) was isolated from the reaction of both trans- and cis-1,2-dibromocyclopropane ($\text{1}$) with magnesium. The corresponding dialkylmagnesium species ($\text{7}$), an oligomer of 2-magnesabicyclobutane, was obtained from cis-$\text{4}$ by precipitation in THF; it forms a soluble complex 8 with HgBr₂.     

The protonation thermodynamics of cyclodextrin-containing polymers for drug inclusion

A poly(amido-amine), PAA, bearing β-CD units in the side chain was synthesized by a polyaddition reaction of 1,4-bis-acryloyl-piperazine with 6-monodeoxy-6-monoamino-β-cyclodextrin (β-CD-NH₂). Unlike the simple β-CD-NH₂ with a greater basicity constant (log K = 8.60), the polymer revealed an unusual polyelectrolyte behaviour with a lower basicity constant (log K° = 6.29) of the tertiary nitrogen atom, that is strongly dependent on the degree of protonation α of the whole macromolecule. It follows the modified Henderson–Hasselbalch equation with n = 1.75, in a wide α-range. The greater (?46.1 kJ/mol) and the lower (?27.6 kJ/mol) enthalpy (ΔH°) changes of the compounds were in line with the protonation of a primary or a tertiary nitrogen atom. The calorimetric data suggested that the PAA protonation destroyed a packing structure formed by two rigid β-CD side chains interacting head-to-head. The UV spectrophotometric data showed that the PAA exhibits affinity towards the l-ascorbic acid at low pH (pH 2.46) with an isosbestic point at 241 nm and a slight blue shift of the maximum absorption of the ascorbic acid (244 nm) on PAA additions.     

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.     

The photoinduced triplet of flavins and its protonation states

The photogenerated triplet states of riboflavin and flavin mononucleotide (FMN) have been examined by time-resolved electron paramagnetic resonance (EPR) spectroscopy at low temperature (T = 80 K). Because of the high time resolution of the utilized EPR instrumentation, the triplets are for the first time observed in the nonequilibrated electron-spin polarized state and not in their equilibrated forms with the population of the triplet sublevels governed by Boltzmann distribution. The electron-spin polarization pattern directly reflects the anisotropy of the intersystem crossing from the excited singlet-state precursor. Spectral analysis of the resulting enhanced absorptive and emissive EPR signals yields the zero-field splitting parameters, |D| and |E|, and the zero-field populations of the triplet at high accuracy. These parameters are sensitive probes for the protonation state of the flavin's isoalloxazine ring, as becomes evident by a comparison of the spectra recorded at different pH values of the solvent. The three protonation states of the flavins can furthermore be distinguished by the kinetics of the transient EPR signals, which are dominated by spin-lattice relaxation. The fastest decays are observed for the protonated FMN and riboflavin triplets, followed by the deprotonated flavin triplets. Slow decays are measured for the triplet states of neutral FMN and riboflavin. Because proton transfer is found to be slow on the time scale of spin-polarized triplet detection by transient EPR, the pH-dependent spin-relaxation and zero-field splitting parameters offer a novel approach to probe the protonation state of flavins in their singlet ground state through the characterization of their triplet-state properties.     

Synthesis of cyclopropane analogs of citral

Conclusions Depanding on thre ratio of the reactant, cital diethyl acetal reacts with dichlorocarbene at either the terminal or both double bonds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 709–711, March, 1977.     

Thermodynamics of liquid propane + cyclopropane

The vapour pressure differences between a mixture of (propane + cyclopropane) and cyclopropane and between propane and cyclopropane have been measured simultaneously with the absolute vapour pressure of cyclopropane. This was done at 13 temperatures between 175 K and 210 K, as a function of composition. The mixtures show small positive deviations from Raoult's law. The excess molar Gibbs energy (G_m^E) has been calculated from the vapour pressure data fitted to the equation G_m^E/(RTx₁x₂ = (−0.2490±0.0072)+(81.8±1.4)/T. The estimated value of the excess molar enthalpy (H_m^E) for the equimolar composition, in the same temperature range is 170 ± 4 J mol⁻¹. The results were interpreted using Deiters' equation of state.     

Aryl vinyl cyclopropane Cope rearrangements

While the divinyl cyclopropane Cope rearrangement is well-known, and has been broadly applied in synthesis, examples of the aryl vinyl cyclopropane Cope rearrangement are less common and generally limited in scope or reaction yield. The aryl vinyl cyclopropane Cope rearrangement gives access to the benzocycloheptene scaffold, which is present in a variety of naturally occurring and medicinally relevant products. Herein we report a method to obtain either of two regioisomeric benzocycloheptene products via an aryl vinyl cyclopropane Cope rearrangement, featuring additive-controlled regioselectivity. Mechanistic studies indicate a dynamic equilibration of cyclopropane stereoisomers, followed by rearrangement of the cis diastereomer.