Régiospécificité et stéréosélectivité de la cycloaddition de l'oxyde de benzonitrile sur les pyrazolines-3

Pursuing our study on the cycloaddition of benzonitrile oxide with Δ₃-pyrazolines (1,2), we can now affirm that this reaction is regiospecific and stereoselective. The behaviour in acidic medium was studied. We have then discussed the process of epimerization and degradation observed according to the C₄ substituent of the bicyclic molecule and the solvent used.     

Position of the equilibrium during the migration of double bonds in the pyrazoline ring

Data characterizing the position of the equilibrium between Δ²- and Δ¹-pyrazolines with alkyl substituents in different positions, which is established under the influence of potassium tertbutoxide in tert-butyl alcohol at 90°C, were obtained. 3-Alkyl-Δ²-pyrazolines are thermodynamically more stable than their isomers with different positions of the double bond, so that the fraction of the latter in equilibrium mixtures does not exceed 1–2%, and they are practically completely isomerized to the 3-Alkyl-Δ²-substituted derivatives. If the position of the side chains excludes the possibility of the formation of 3-alkyl-Δ²-pyrazolines by migration of the double bond (4-alkyl- and 5,5-dialkyl-substituted compounds), the fraction of Δ¹-pyrazolines in the equilibrium rises appreciably and reaches 12% for 3,3-diethyl-Δ¹-pyrazoline.     

Competition entre les réactivités dipolarophiles et nucléophiles des pyrazolines-2 dans l'action de l'oxyde de benzonitrile. Synthèse,identification, comportement chimique des tétrahydropyrazolo-[4,5-b] oxadiazoles-1,2,4

The reaction of benzonitrile oxide with different series of Δ²-pyrazolines provides evidence for the three reactive centers of compounds i.e., the dipolarophile reactivity of the C = N-bond, and the presence of the two the other nucleophilic sites: the N₁ nitrogen atom and the C₃ carbon atom for unsubstituted pyrazolines at these corresponding positions. Furthermore the regiospecificity of the cycloaddition reaction leading to new series of condensed heterocycles is well demonstrated.     

Un nouvel exemple de transposition: l'épimérisation en C(3) d'hydroxy-3-δ1-pyrazolines dérivées de sucres

A Novel Example of Reversible Ring Opening: The Epimerization at C(3) of Sugar 3-Hydroxy-Δ¹-pyrazolines Reaction of 1 (either geometrical isomer) with hydrazine followed by in situ Ag₂O oxidation led to two pairs of interconverting isomers 4 ? 5 and 6 ? 7 . By the same treatment, (Z)- 10 and (or) (E)- 10 gave the pair 11 ? 12 . Acetylation of 4 ? 5 led to a non interconverting mixture of 8 and 9 . This fact, and the lack of incorporation of ¹⁸O when the epimerization took place in the presence of H₂¹⁸O indicated that the most probable mechanism consisted in a reversible ring opening ( D ? E ? F ). The kinetic parameters of these reactions are given and structural assignments proposed for the new compounds.     

Analyse conformationnelle de pyrazolines-1 substituées

Conformational analysis of 1-pyrazolines bearing different substituents on carbons 3 and 4 has been achieved by NMR. The use of the Karplus equation to explain the values of the coupling constants ³J(H₄? H₅) allows us to ‘check’ the existence of an equilibrium between two envelope conformations. The folding angle of the envelope, calculated on a model compound which is held in only one conformation, wad found to be near 36°. The ratio of the two conformers at room temperature is calculated for the various pyrazolines studied.     

Etude en Résonance Magnétique Nucléaire du Carbone-13 de N-Aryl Pyrazolines-2 Diversement Substituées. Effets de Substituant et Etude Conformationnelle

The ¹³C chemical shifts of 1-phenyl-2-pyrazoline and 25 N-nitrophenyl-2-pyrazolines, with different substituents on the heterocyclic ring, have been assigned. These values are discussed as a function of the steric and electronic nature of the various substituents. The steric effects produced conformational modifications, both in the nitrophenyl and 2-pyrazoline moieties, which explain some of the chemical shifts.     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

Fluorescent Molecular Logic Gates Driven by Temperature and by Protons in Solution and on Solid

Temperature-driven fluorescent NOT logic is demonstrated by exploiting predissociation in a 1,3,5-trisubstituted Δ²-pyrazoline on its own and when grafted onto silica microparticles. Related Δ²-pyrazolines become proton-driven YES and NOT logic gates on the basis of fluorescent photoinduced electron transfer (PET) switches. Additional PASS 1 and YES+PASS 1 logic gates on silica are also demonstrated within the same family. Beside these small-molecule systems, a polymeric molecular thermometer based on a benzofurazan-derivatized N-isopropylacrylamide copolymer is attached to silica to produce temperature-driven fluorescent YES logic.     

4,8-Substituted homotropilidenes from cycloheptatrienes

A versatile synthesis for 4,8-mono-, di-, tri- and tetrasubstituted homotropilidenes (bicyclo[5.1.0]octa - 2,5 - dienes) is described. The C₈ unit of the homotropilidene framework is built up from cycloheptatriene (C₇) and diazoalkane (c₁) subunits. Diazoalkane addition to Diels-Alder adducts of cycloheptatrienes and 1,2,4 - triazoline - 3,5 - diones, photochemical ring contraction of the Δ¹-pyrazolines formed, and degradation of the urazole unit gives the azo compounds 8. The latter yield homotropilidenes in a smooth[_σ2_s+_σ2_s+_σ2_s]cycloreversion. The ability of the homotropilidenes to undergo a Cope rearrangement depends on the pattern of substitution.     

Synthesis of Isomeric Δ2-Pyrazolines From 2′-Hydroxy-5′-chlorochalcones and its Derivatives

Suitably substituted 2′-hydroxy-5′-chlorochalcones with hydrazine hydrochloride in refluxing DMF afforded isomeric Δ²-pyrazolines, which were characterized on the basis of chemical and spectral data.     

Thermolysis of hexasubstituted‐4,5‐dihydro‐3H‐pyrazoles: Kinetics and activation parameters

A kinetics study of the thermolysis of a series of hexasubstituted‐4,5‐dihydro‐3H‐pyrazoles (pyrazolines 1a: 3,3,4,4‐tetramethyl‐5‐phenyl‐5‐acetoxy; 1b: cis‐3,5‐diphenyl‐3,3,4‐trimethyl‐5‐acetoxy; 1c: cis‐3,5‐diphenyl‐3,4,4‐trimethyl‐5‐methoxy; 1d: 3,3,5‐triphenyl‐4,4‐dimethyl‐5‐acetoxy), which produced the corresponding hexasubstituted cyclopropanes 2a–d in quantitative yields was carried out. The first order rate constants (k₁) for thermal decomposition and activation parameters were determined. The relative reactivity series was found to be 1d >> 1b ∼ 1c > 1a. The activation parameters for thermolysis were found to be: for 1a ΔH‡ = 39.8 kcal/mol, ΔS‡ = 14 eu, k_150° = 6.8 × 10⁻⁵ s⁻¹; for 1b ΔH‡ = 33.5 kcal/mol, ΔS ‡ = 0.2 eu, k_150° = 1.7 × 10⁻⁴s⁻¹; for 1c ΔH‡ = 32.7 kcal/mol, ΔS‡ = −1.8 eu, k_150° = 1.2 × 10⁻⁴s⁻¹; for 1d ΔH‡ = 30.1 kcal/mol, ΔS‡ = −1.6 eu, k_150° = 8.8 × 10⁻³s⁻¹. The effect of variation of C3 substituents on the activation parameters for thermolysis paralleled the trend reported for acyclic analogs. The results are consistent with the formation of a (singlet) 1,3‐diradical intermediate with subsequent closure to yield the cyclopropanes. The mechanism of diradical formation appears to involve N₂‐C₃ bond cleavage as the rate determining step rather than simultaneous two bond scission. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:299–302, 2000     

The distonic ion ˙CH2CH2CO+ and its formation from ionized cyclopentanone

Collisional activation decomposition (CAD) spectra are interpreted as indicating that formation of ˙CH₂CH₂CO⁺ (1) from ionized cyclopentanone, succinic anhydride arid butyrolactone is important at 70 eV electron energy. However, photoionization appearance energy measurements and CAD spectra demonstrate that CH₃CH?C?O^+· (3) is formed from ionized cyclopentanone near threshold. Ab initio molecular orbital calculations place ΔH _f (1) about 36 kJ mol^?1 above ΔH _f (3).     

The absolute rate constant for the metathesis t-C4H9• + DI → C4H9D + I• and the heat of formation of the t-butyl radical

The Arrhenius parameters for the title reaction have been measured in a very-low-pressure pyrolysis apparatus in the temperature range 644–722 K and are given by log k₂ (M^?1 · sec^?1) = 9.68 - 2.12/θ, where θ = 2.303RT in kcal/mol. Together with the published Arrhenius parameters for the reverse reaction from iodination studies, they result in a standard heat of formation of the t-butyl radical of 8.4 kcal/mol, accepting S⁰(C₄H₉·) = 72.2 e.u. at 300 K from other kinetic data, and thus confirm the accepted value for ΔH_f⁰(t-C₄H₉·), at variance with recent investigations which yielded significantly higher values. This value for ΔH_f⁰(t-C₄H₉·) results in a bond-dissociation energy (BDE) for isobutane of 92.7 kcal/mol.     

Photochemisches Verhalten von 3,4-Diarylsydonen. Vorläufige Mitteilung

3, 4-Diphenyl-sydnone ( 1a ) and 3-p-tolyl-4-phenyl-sydnone ( 1b ) on irradiation with a mercury high-pressure lamp in pyrex vessels yield 2,4,5-triphenyl- ( 3a ) and 2-p-tolyl-4,5-diphenyl-1,2,3-triazole ( 3b ), respectively. Irradiation of 1a and 1b in the presence of dimethyl acetylene-dicarboxylate or indene leads to the formation of the pyrazoles 4a and 4b and the Δ²-pyrazolines 5a and 5b , respectively. Reactive intermediates of the photoreaction of 1a and 1b seem to be the diazirines 2a and 2b , which open to give the 1,3-dipolar structures 8a and 8b .     

Thermolysis of 3‐alkyl‐3‐methyl‐1,2‐dioxetanes: activation parameters and chemiexcitation yields

3‐Methyl‐3‐(3‐pentyl)‐1,2‐dioxetane 1 and 3‐methyl‐3‐(2,2‐dimethyl‐1‐propyl)‐1,2‐dioxetane 2 were synthesized in low yield by the α‐bromohydroperoxide method. The activation parameters were determined by the chemiluminescence method (for 1 ΔH‡ = 25.0 ± 0.3 kcal/mol, ΔS‡ = −1.0 entropy unit (e.u.), ΔG‡ = 25.3 kcal/mol, k₁ (60°C) = 4.6 × 10⁻⁴s⁻¹; for 2 ΔH‡ = 24.2 ± 0.2 kcal/mol, ΔS‡ = −2.0 e.u., ΔG‡ = 24.9 kcal/mol, k₁ (60°C) = 9.2 × 10⁻⁴s⁻¹. Thermolysis of 1–2 produced excited carbonyl fragments (direct production of high yields of triplets relative to excited singlets) (chemiexcitation yields for 1: ϕ^T = 0.02, ϕ^S ≤ 0.0005; for 2: ϕ^T = 0.02, ϕ^S ≤ 0.0004). The results are discussed in relation to a diradical‐like mechanism. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:176–179, 2001     

Pressure effects on cation migration in 2,5‐di‐tert‐butyl‐1,4‐benzoquinone radical anion

Pressure effects on the two‐site jumping of sodium and potassium cations in a 2,5‐di‐tert‐butyl‐1,4‐benzoquinone ion pair have been studied using a high‐pressure EPR technique. The rate constants of the intramolecular and intermolecular migrations for Na⁺ and K⁺ were determined from an EPR spectral simulation. The migration rates were found to be accelerated by increasing the external pressure. Using the pressure dependence of the migration rates, we estimated the activation volumes of the intramolecular (ΔV₁^?) and intermolecular (ΔV₂^?) processes for the Na⁺ and K⁺ migrations: ΔV₁^? = ?5.3 cm³ mol^?1 and ΔV₂^? = ?29 cm³ mol^?1 for Na⁺, and ΔV₁^? = ?8.3 cm³ mol^?1 and ΔV₂^? = ?0.85 cm³ mol^?1 for K⁺. Based on the results, the mechanisms for the two‐site jumping of Na⁺ and K⁺ are discussed in terms of volume. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 397–401, 2001     

Thermolysis of 3‐alkyl‐3‐methyl‐1,2‐dioxetanes: Activation parameters and chemiexcitation yields

3‐Methyl‐3‐(3‐pentyl)‐1,2‐dioxetane 1 and 3‐methyl‐3‐(2,2‐dimethyl‐1‐propyl)‐1,2‐dioxetane 2 were synthesized in low yield by the α‐bromohydroperoxide method. The activation parameters were determined by the chemiluminescence method (for 1 ΔH‡ = 25.0 ± 0.3 kcal/mol, ΔS‡ = −1.0 entropy unit (e.u.), ΔG‡ = 25.3 kcal/mol, k₁ (60°C) = 4.6 × 10⁻⁴s⁻¹; for 2 ΔH‡ = 24.2 ± 0.2 kcal/mol, ΔS‡ = −2.0 e.u., ΔG‡ = 24.9 kcal/mol, k₁ (60°C) = 9.2 × 10⁻⁴s⁻¹. Thermolysis of 1–2 produced excited carbonyl fragments (direct production of high yields of triplets relative to excited singlets) (chemiexcitation yields for 1: ϕ^T = 0.02, ϕ ≤ 0.0005; for 2: ϕ^T = 0.02, ϕ^S ≤ 0.0004). The results are discussed in relation to a diradical‐like mechanism. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:459–462, 2001     

Thermochemical and spectrophotometric study of the complex formation of nickel(II) ions with sulfur containing α,ω-aminopyridines

The complexation in aqueous solution of ligands of the type o-Py? (CH₂)_(n-1) S(CH₂)_mNH₂ (n and m: 2 or 3) with Ni^II ions has been investigated. The thermodynamic functions ΔG, ΔH, and ΔS of the complex formation have been determined at 25°C in 0.5 M KNO₃ solution by means of potentiometric and of direct calorimetric titrations. They revealed that all ligands, except o-Py? (CH₂)₂S(CH₂)₃NH₂ for which no appreciable complexation with Ni^II could be stated, act as tridentates towards Ni^II. Information on the stereochemistry of the Ni^II complexes was obtained from the electronic spectra. The NiL²⁺ and NiL₂²⁺ complexes both have distorted octahedral structures. The NiL₂²⁺ species were also compared to the corresponding NiL₂(NO₃)₂ compounds in the solid state.     

The anisotropic effect of functional groups in H NMR spectra is the molecular response property of spatial NICS—the frozen conformational equilibria of 9-arylfluorenes

Rotation about the single bond adjoining the aryl and fluorene moieties in 9-arylfluorenes can be frozen out on the NMR timescale if methyl groups are located at either one or both of the ortho positions of the aryl substituent. In the ground-state of these rotamers, the planes of the aryl and fluorene moieties are perpendicular to each other and the methyl substituents are consequently positioned either above the fluorene moiety or in-plane with it; thus, the methyl protons are either shielded or deshielded, respectively, due to the ring current effect of the fluorene moiety. This anisotropic effect on the ¹H chemical shifts of the methyl protons has been quantified on the basis of through-space NMR shieldings (TSNMRS) and subsequently Δδ_calcd compared with the experimentally observed chemical shift differences, Δδ_exp. In this context, the experimental anisotropic effects of functional groups in the ¹H NMR have proven to quantitatively be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). Differences between Δδ_calcd and Δδ_exp were, for the first time, also quantified as arising from steric compression.     

Lewis acid catalyzed reactions of donor-acceptor cyclopropanes with 1- and 2-pyrazolines: formation of substituted 2-pyrazolines and 1,2-diazabicyclo[3.3.0]octanes

The reaction of 2-substituted cyclopropane-1,1-dicarboxylates with 1- and 2-pyrazolines is efficiently catalyzed by scandium or ytterbium triflates to give N-substituted 2-pyrazolines or 1,2-diazabicyclo[3.3.0]octanes. The reactions of 2-pyrazolines give diazabicyclooctanes as the major products. In contrast, the reactions starting from 1-pyrazolines predominantly give N-substituted 2-pyrazolines, which become the major compounds obtained if an equimolar amount of GaCl₃ is used. A possible reaction mechanism is suggested.