Reaction intermediates of a dihydropyridine derivative of 2′,3′-dideoxycytidine related to AIDS dementia studied by laser flash photolysis

A derivative of 2′,3′-dideoxycytidine (ddC), carrying one 1,4-dihydro-1-methyl-3-pyridinyl-carbonyl group at the cytidine exocyclic amino group and one at the ribose 5-hydroxyl group, was exposed in water-ethanol solution to high-power pulsed laser emission at a wavelength of 354.7 nm. Measurement of the transient absorption spectra with nanosecond time resolution shows that the photoejection of an electron occurs due to stepwise two-photon absorption by the dihydropyridine via its fluorescent state. The spectrum of the cation radical formed in this reaction was determined, together with that of the neutral radical appearing following deprotonation of the cation. The electron is apparently abstracted only from the pyridine group attached to the ribose, since a comparative study on ddC carrying a dihydropyridine only at the exocyclic amino group showed no evidence for photoionization in the same experimental conditions.     

Triplet excitation dynamics of β-carotene studied in three solvents by ns flash photolysis spectroscopy

Upon anthracene-sensitizing, triplet excitation dynamics of β-carotene(β-Car) were studied in nhexane, in methanol, and in acetonitrile, respectively, by ns flash photolysis spectroscopy. In n-hexane,only the bleaching of the ground state absorption(GSB) and the excitation triplet(~3Car*) absorption were observed, and there were no cationic species detected. In both methanol and acetonitrile, similar excitation dynamics were observed, i.e.,~3Car* having a similar lifetime to that in n-hexane, and the immediate generation of the cation dehydrodimer(~#[Car]2~+) upon excitation following transformation into the radical cation Car*~+, since Car*~+ has much longer lifetime in acetonitrile than in methanol. The results prove that both solvent and carotenoid structure determine the triplet excitation mechanism.     

Facile cleavage of C–C bond: conversion of pyrane derivative to 1,3-oxazin derivative

A cascade reaction that involves a unique C–C bond cleavage has been discovered. This protocol affords an unusual and facile method for the synthesis of 1,3-oxazin derivatives under mild conditions.     

Steady-state and laser flash photolysis studies on photochemical formation of 4-tert-butyl-4′-methoxydibenzoylmethane from its derivative via the Norrish Type II reaction in solution

A photochemical formation process of avobenzone (AB; 4-tert-butyl-4′-methoxydibenzoylmethane) from 1,1-(4-tert-butybenzoyl)(4′-methoxybenzoyl)butane (PrAB) is studied by steady-state and laser flash photolysis in solution. The quantum yield of the formation via the triplet state of PrAB is determined to be 0.23 in degassed acetonitrile at 295 K. The Arrhenius plots of the decay rate of triplet PrAB show that photoelimination proceeds with an activation energy of 6.0 kcal mol^?1 and the frequency factor of 4.6 × 10¹⁰ s^?1.     

Hydrogen bond CH…O in a methanolic solution of hydrogen chloride

The existence of a hydrogen bond in which a methyl group of the (MeOH)₂H⁺ ion acts as a proton donor is examined. The fundamental vibration frequencies of this ion were calculated for different numbers and strengths of CH…O bonds. The atomic charges in neutral ((MeOH) _n ,n=1–4) and protonated ((MeOH) _m H⁺,m=2–6) associates of methanol molecules were also calculated. The experimentally observed decrease in the v(CH) vibration frequencies of the (MeOH)₂H⁺ ion to 2890 cm⁻¹ and 2760 cm⁻¹ is attributable to the fact that each methyl group of the ion is involved in formation of two CH…O bonds with strength of −12.5 kJ mol⁻¹. The proton-donating ability of the CH bond depends on the charge on its H atom; however, it does not correlate with the dipole moment of this bond. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 306–312, February, 1999.     

Oxidation-induced ortho-selective C–H bond functionalization of 2-naphthylamine derivative

Selective C–H bond functionalization has been emerged as a versatile strategy for the construction of new chemical bonds. In the past decades, the directing group(DG)-assisted C–H bond activation has been developed as one of the most efficient methods for selective C–H functionalization. Although a great progress has been made by utilizing this traditional method, developing new strategy for selective C–H bond functionalization is still highly demanded. Hence, a novel oxidation-induced C–H bond functionalization method was demonstrated in this work. By this new method, ortho-C(sp2)–H chlorination of N-substituted 2-naphthylamine was realized in a highly selective manner.     

Photodissociation of pyrene dimer radical cation during the pulse radiolysis–laser flash photolysis combined method

Photodissociation of pyrene (Py) dimer radical cation (Py ₂ ^?+ ) giving pyrene radical cation (Py^?+) and Py and subsequent regeneration of Py ₂ ^?+ by association of Py^?+ and Py were directly observed during the pulse radiolysis–laser flash photolysis combined method at room temperature. When Py ₂ ^?+ was excited at the local excitation band with the 532-nm laser flash, the rapid growth and decay of monomeric Py^?+ were observed at 460 nm. The dissociation of Py ₂ ^?+ proceeded via a one-photon process to give the ground-state Py^?+(D₀) and Py in the quantum yield (Φ_diss) of (2.9 ± 0.9) × 10^?3. It was shown that Py^?+ decayed with a time constant of several tens of nanoseconds, indicating that the association of Py^?+ with Py regenerating Py ₂ ^?+ proceeds at a diffusion-controlled rate. The photodissociation proceeded from the lowest excited state of Py ₂ ^?+ , even when Py ₂ ^?+ was excited to the higher excited state. The difference between the Φ_diss value of Py ₂ ^?+ and that previously reported for naphthalene dimer radical cation (Np ₂ ^?+ ) is discussed.     

On the kinetics of polymer degradation. X—Laser flash photolysis of poly-α-methylstyrene in chloroform solution

The essential step in the photolysis of poly-α-methylstyrene in chloroform solution is the interaction of singlet excited repeating units with solvent molecules (in about 10⁻⁹s) leading to CT-complexes (decay constant 3·1 × 10⁶ s⁻¹, maxima of the optical absorption spectrum at 320 and 520–540 nm). Lateral macroradicals are formed upon the decomposition of the CT-complexes rather than via attack by solvent radicals. Oxygen does not interfere with the formation of macroradicals at concentrations up to oxygen-saturation at p_O2 = 1 bar but it reacts with macroradicals, thus preventing main-chain scission.     

Cis—trans isomerization of styrenes via the triplet pathway: the perpendicular triplet state observed by laser flash photolysis

The kinetics of the cis—trans photoisomerization of 1-phenylcyclohexene via the triplet state, studied by either nanosecond pulse radiolysis or laser flash photolysis in the presence of sensitizers, reveal that the triplet species involved in the isomerization mechanism has a lifetime of 55 ns in fluid solution at room temperature. A transient absorption decaying with the same 55 ns lifetime, and therefore assigned to this triplet species, was observed in the 320 – 345 nm region. Quite similar triplet—triplet absorptions were observed with 1-phenylcycloheptene, 1-phenylpropene and styrene itself. From the experimental results and from considerations of the energy surfaces of the excited states of styrene, the observed triplet species is identified as the perpendicular (or “phantom”) triplet state of the styrene moiety.     

Cobalt-catalyzed C–N,C–O,C–S bond formation: synthesis of heterocycles

In recent decades, a large number of reports related to the synthesis of N-, O- and S-containing heterocycles have appeared owing to a wide variety of their biological activity. Traditional approaches require expensive or highly specialized equipment or would be of limited use to the synthetic organic chemist due to their highly inconvenient approaches. New strategies have been developed for the preparation of heterocycles in the last decades. Metal and non-metal catalysts are used in organic reactions with high activity. These synthetic strategies are becoming important and highly rewarding protocols in organic synthesis. In this review article, the synthesis of heterocycles is presented with the application of cobalt metal as a catalyst. It describes the formation of different sized heterocyclic rings containing different heteroatoms.

     

Solubilization of cholesterol in aqueous solution by two β-cyclodextrin dimers and a negatively charged β-cyclodextrin derivative

Two βCD dimers (linked by succinic acid, 2, or ethylenediaminetetraacetic acid, EDTA, 3, bridges) and a negatively charged monomer derivative of βCD, 1, have been synthesized and their ability to solubilize cholesterol in aqueous solution was studied. The three compounds exhibit a great capacity in solubilizing cholesterol as, for instance, concentrations up to 6 mM of cholesterol were measured in the presence of 25 mM of 3. The phase-solubility diagrams of the two dimers exhibit A _L type profiles while the monomer 1 follows an A _P isotherm. The cholesterol/dimer complexes have 1:1 stoicheiometries while monomer 1 forms two complexes with molar ratios of 1:1 and 1:2 (cholesterol/1). The equilibrium constants are K _1:1 = (5.9 ± 0.3) × 10⁴ M^?1 and K _1:1 = (8.8 ± 0.2) × 10⁴ M^?1 for 2 and 3, respectively, and K _1:1 = 73 ± 19 M^?1 and K _1:2 = 204 ± 65 M^?1 for 1. The comparison of K _1:1(3) with the product K _1:1 × K _1:2 (1) reveals that a chelate effect in binding the cholesterol by 3 exists. The structure of the cholesterol/3 complex was studied by ROESY experiments and by molecular dynamics simulations.     

Homolytic dissociation in hydrogen-bonding liquids: energetics of the phenol O–H bond in methanol and the water O–H bond in water

The energetics of the phenol O–H bond in methanol and the water O–H bond in liquid water were investigated by microsolvation modelling and statistical mechanics Monte Carlo simulations. The microsolvation approach was based on density functional theory calculations. Optimised structures for clusters of phenol and the phenoxy radical with one and two methanol molecules are reported. By analysing the differential solvation of phenol and the phenoxy radical in methanol, we predict that the phenol O–H homolytic bond dissociation enthalpy in solution is 24.3±11 kJ/mol above the gas-phase value. The analysis of the water O–H bond dissociation by microsolvation was based on optimised structures of OH–(H₂O)_1–6 and –(H₂O)_1–7 clusters. Microsolvation modelling and statistical mechanics simulations predict that the HO–H bond dissociation enthalpies in the gas phase and in liquid water are very similar. Our results stress the importance of estimating the differences between the solvation enthalpies of the radical species and the parent molecule and the limitations of local models based on microsolvation.Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail     

Challenges in C–C bond formation through direct transformations of sp2 C–H bonds

In the past several decades, organic chemists have made significant contributions to approach direct C–H transformations. However, limited substrate scope and reaction classifications, harsh condition, and the use of noble and toxic late transition-metal catalysts typically with high loadings clogged its applications. This article summarizes our recent efforts to explore new reaction types and develop new catalytic systems in this field, which may open a new channel to consider organic synthesis from easily available chemicals.     

Synthesis of C–B–O five membered ring by hydroboration of allylalcohols in standard conditions

The hydroboration of protected allylalcohols by two different groups gives no linear structures of siloxypropylboranyls as predicted but gives the C–B–O five membered ring 1,2-H-oxaborolanes with good yield. A mechanism is also proposed to explain this cyclisation.     

O–H···C hydrogen bond in the methane–water complex

Quantum chemical calculations were performed at different levels of theory (SCF, DFT, MP2, and CCSD(T)) to determine the geometry and electronic structure of the HOH···CH₄ complex formed by water and methane molecules, in which water is a proton donor and methane carbon (sp³) is an acceptor. The charge distribution on the atoms of the complex was analyzed by the CHelpG method and Hirshfeld population analysis; both methods revealed the transfer of electron charge from methane to water. According to the natural bond orbital (NBO) analysis data, the charge transfer upon complexation is caused by the interaction between the σ orbital of the axial С–H bond of methane directed along the line of the O–H···C hydrogen bridge and the antibonding σ* orbital of the О–H bond of the water molecule. Topological analysis of electron density in the HOH···CH₄ complex by the AIM method showed that the parameters of the critical point of the bond between hydrogen and acceptor (carbon atom) for the O–H···C interaction are typical for Н-bonded systems (the magnitude of electron density at the critical point of the bond, the sign and value of the Laplacian). It was concluded that the intermolecular interaction in the complex can be defined as an Н bond of O–H···σ(С–H) type, whose energy was found to be 0.9 kcal/mol in MP2/aug-cc-pVQZ calculations including the basis set superposition error (BSSE).