Strain induced bond localization in strained aromatic compounds with extended π systems

The influence of small rings annulated to aromatic rings is usually additive. However, in some systems the behavior is nonadditive, for example, in tricarbonyliron‐cyclobutadienobenzene versus tris(tricarbonylironcyclobutadieno)benzene. A study of dimethylenecyclobutabenzene, 1,3‐bis(dimethylenecyclobuta)benzene, tris(dimethylenecyclobuta)benzene, and their bora, imino, and oxo derivatives is presented. The behavior of these systems ranges from an almost perfect additivity (in the bora derivatives) to nonadditive behavior (in the oxo derivatives) where the single ring annulated system shows a negative difference between the bonds endocyclic and exocyclic to the annulated small ring (negative ΔR; anti‐Mills–Nixon distortion), the doubly annulated system shows ΔR=0, and the triply annulated system shows positive ΔR (Mills–Nixon distortion). The geometrical properties of the systems are analyzed in terms of the strain, bond curvature, and electronegativity of the exocyclic substituent. These atomic and σ properties are found to be responsible for the structural properties whereas π factors are of minor or no importance. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1377–1386, 2001     

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ ^{. ?} produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.     

N-oxides in the quinoxaline series

Acyl derivatives of 2-aminoquinoxaline and their 1-N -oxides, as well as methyl derivatives of some of those compounds, are synthesized. IR spectra of these compounds in the solid state, and UV spectra of their solutions, showed that, with respect to the ability of its 2-acylamino derivatives to tautomerize to the imido form, quinoxaline is close to pyrimidine, and below quinoline and pyridine. With respect to the amideimide tautomerism equilibrium position, N-oxides of 2-acylaminoquinoxalines differ little from acylamides of quinoxaline themselves. The action of benzene sulfochloride on 2-aminoquinoxaline-1-N-oxide in pyridine below 0°leads to deoxidation of the N O group, and introduction of the benzene sulfonyloxy group at position 3 in the quinoxaline ring.For Part VIII see [17].     

Synthesis and absorption-fluorescence properties of

1,4-Bis(²-pyrazolin-3-yl)benzene derivatives were synthesized by the reaction of dibenzal-p-diacetylbenzene or 1,4-bis(-dimethylaminopropionyl)benzene with various hydrazines. Some of these derivatives were converted to the corresponding 1,4-bis(3-pyrazolyl)benzene derivatives by oxidative dehydrogenation with sulfur. The absorption and fluorescence spectra of the synthesized compounds were measured, and the fluorescence quantum yields were determined.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 79–83, January, 1980.     

Hexakis(arylisocyano) derivatives of chromium(0) as initiators of vinyl polymerization: Steric effects

The free radical polymerization of methyl methacrylate initiated by the hexakis (arylisocyano)chromium(0)-CCl₄ system has been studied for a number of derivatives in which both chlorine and methyl groups have been introduced into the benzene ring. It has been found that single substituents in the benzene ring, irrespective of position, have little effect on the activity of the system other than that which can be explained by the electronic effects of these substituents. However, where steric overcrowding occurs it has been shown that the rate of initiation is reduced considerably. This confirms the established mechanism of initiation for this type of derivative.     

A systematic shift in the electronic spectra of substituted benzene molecules trapped in helium nanodroplets

Electronic spectra (S1<--S0) have been recorded from five separate substituted benzene derivatives trapped in helium nanodroplets. Each member of the series is found to exhibit a blueshift with respect to the equivalent transition in the gas phase. Taken together with previous results for benzene, the observed shifts show a remarkably good correlation with changes in electron density that occur within each of the aromatic rings as a result of electronic excitation.     

Quantitative Structure-Property Relationship: XIV. Properties of Derivatives of Saturated Hydrocarbons and Benzene,Containing Several Polar Groups

The previously suggested quantitative structure-property relationship was applied to estimate the properties of derivatives of saturated hydrocarbons and benzene, containing several polar groups. The boiling points, logP values (where P is the distribution factor in the octanol-water system), and solvatochromic parameters were calculated for a series of polyhydric alcohols, polyphenols, and isoelectronic chloro derivatives of saturated hydrocarbons and benzene. The calculated values are in good agreement with the experiment. A satisfactory correlation between these properties allows in some cases estimation of properties using a quantitative property-property relationship instead of a structure-property relationship.     

Synthesis and characterisation of tetra-tetrazole macrocycles

The syntheses of tetra-tetrazole macrocycles, containing two bis-tetrazole units linked by a variety of alkyl-chain lengths from four to eight carbons, are described. The crystal structures of three of these derivatives are reported, and the molecular conformation in the solid state is compared to that of the previously reported tetra-tetrazole macrocycle and to other bis- and tris(tetrazole)benzene structures. The macrocycle conformation is influenced by the length of the alkyl-chain linker, the relative orientation of the tetrazole rings on the benzene ring and by intermolecular interactions. In the macrocycles based on 1,2-bis(tetrazole)benzene, the adjacent tetrazole rings on the benzene ring are prevented from becoming co-planar on intramolecular (steric) grounds. In the 1,3- and 1,4-bis(tetrazole)benzene derivatives, there is no such impediment, and a co-planar arrangement is observed where intra- and/or intermolecular stacking interactions exist. Deviations from co-planarity are associated with optimisation of intermolecular interactions between the tetrazole rings and adjacent alkyl chains. In the macrocycle based on 1,4-bis(tetrazole)benzene with four-carbon linkers, an intramolecular stacking interaction exists, which precludes the presence of any cavity. In the macrocycle based on 1,3-bis(tetrazole)benzene with six-carbon linkers, a cavity of 10.8×9.4 Å is observed for each molecule in the solid state, although the packing of adjacent molecules is such that there are no extended channels running through the crystal.     

Mechanistic study of a Pd/C-catalyzed reduction of aryl sulfonates using the Mg-MeOH-NH(4)OAc system

A method for the deoxygenation of phenolic hydroxy groups via aryl triflates or mesylates has been established by using a combination of Pd/C-Mg-MeOH. The addition of NH(4)OAc to the system markedly accelerated the reaction rate and expanded the scope of the reaction. Mechanistic studies suggested that a single-electron transfer process from the Pd(0) center to the benzene ring is involved in the reduction of aryl sulfonates and that NH(4)OAc works as a solubilization reagent of the Mg salt and as an accelerator of the electron transfer, thus enhancing the reaction process. Our method was also applicable to the regioselective deuteration of benzene derivatives with CH(3)OD as the solvent and deuterium source: the original hydroxy group could be efficiently replaced with a deuterium atom.     

Synthesis of masked haloareneboronic acids via iridium-catalyzed aromatic C-H borylation with 1,8-naphthalenediaminatoborane (danBH)

“Masked” areneboronic acids have been prepared by Ir-catalyzed C-H borylation of arenes. A [Ir(OMe)(cod)]₂ complex with a DPPE ligand showed the highest catalytic activity in the C-H borylation of benzene at 80 °C. The reaction system can be applied to substituted arenes, including halogen-substituted arenes. 1,3-Dihalobenzenes undergo the C-H borylation at their 5-positions in a regioselective fashion, affording 3,5-dihaloareneboronic acid derivatives, which serve as useful coupling modules for the convergent dendrimer synthesis.     

Kinetics and products of the reactions of ozone with toluene and its derivatives in acetic anhydride

The reactions of ozone with toluene and its derivatives in acetic anhydride were studied. It was found that competing parallel reactions of ozone with the aromatic ring and substituents occurred in the ozone-arene-acetic anhydride system. The ratio between these reaction paths depended on the arene structure and reaction conditions. The selectivity of the oxidation of toluene and its derivatives at the methyl group varied from 0 to 40%. The fraction of aromatic products decreased as the number of methyl groups at the ring was increased. Tri- and tetramethylbenzenes were oxidized only at the aromatic ring. The stability of an aromatic system increased upon the introduction of electron-acceptor substituents into the benzene ring. Aminotoluenes and hydroxytoluenes were oxidized with ozone mainly at the NH₂ and HO groups; however, as in the case of toluene, the aromatic ring and methyl group became the main directions upon their acylation. The oxidation of the methyl group in acetic anhydride in the presence of sulfuric acid was finished at the step of the formation of the acylated derivatives of benzyl alcohols and benzaldehydes, which are resistant to the action of ozone.     

Crystal structure and reactivity of 2-chloro-3,5-dinitrothiophene and of 2-phenylsulphonyl-3,5-dinitrothiophene with nucleophiles

The geometry obtained by a crystal structure determination of the title compounds, as compared with that of the analogous benzene derivatives, along with results of ab initio calculations, is used to interpret the different S_NAr reactivities in some thiophene and benzene compounds. The smaller rotation of the nitro groups with respect to the aromatic rings observed in thiophene derivatives should be considered a relevant factor in the higher reactivity in the thiophene series.     

Dipole moments of hydroxy- and methoxy-azobenzene derivatives

The dipole moments of some 4-hydroxy and 4-methoxyazobenzene derivatives were measured in benzene solutions. Results show that (a) there are no appreciable mesomeric interactions between the polar groups in the molecule; (b) evidence is obtained for the hindered rotation of the OH and OCH₃, groups; and (c) the difference between the dipole moments of the title compounds and those of the corresponding benzene derivatives can be referred both to the extension of the conjugated system and to the value of the interaction moment. Experimental results are discussed with the aid of CNDO/2 calculations.     

Energies of Intermolecular Interaction and Hydration of Benzene Derivatives Molecules Adsorbed on Active Carbon from an Aqueous Solution

Energy heterogeneity of an adsorbent surface does not affect the isotherm of adsorption from aqueous solutions. Therefore, the balance of the changes in the Gibbs energy resulted from the adsorption of sparingly water-soluble substances shows that, in the whole range of filling of the adsorption phase, the pattern of the adsorption isotherm depends on the difference between the energy of the interaction of adsorbed molecules with each other and the energy of their hydration needed to displace water molecules from the adsorption phase. The standard adsorption energy of molecules of benzene and its derivatives from aqueous solutions and the difference between the energy of their interaction in the adsorption phase and the energy of their hydration are determined by the extrapolation of adsorption isotherms of these substances to the standard conditions ( 0, C 0) and to the conditions of maximal approach of the adsorbed molecules to each other ( 1). The hydration energy of the molecules of benzene and its derivatives is calculated based on the proportionality of this energy to the sum of the concentration potentials of the components in a saturated solution, where the proportionality coefficient is equal to the number of water molecules interacting with one organic molecule. Calculated energies of phenol and aniline hydration are equal to the energies of H-bonds of phenol (OH···OH₂) and amino (NH···OH₂) groups. Hydration energies of phenol and aniline derivatives vary according to the effect of a substituent in the benzene ring on the H-bond energy. Negative values of hydration energy of polar hydrophobic molecules result from their hydrophobic effect on water structure. The interaction energy of hydrated and dehydrated adsorbed molecules, whose benzene ring planes are oriented in parallel to the carbon surface, is found from the calculated hydration energies. For H-bond-forming molecules, this energy is equal to the energy of one H-bond formed upon the surface dimerization. The energy of repulsion between polar hydrophobic molecules of benzene derivatives depends on the vertical component of the dipole moment, and is the higher the larger the polar group volume.     

Difluoromethylation of some C-H acids with chlorodifluoromethane under conditions of phase transfer catalysis (PTC)

Selected C-H acids react with difluorocarbene generated from chlorodifluoromethane with concentrated aqueous solution of sodium hydroxide, and a catalyst, benzyltriethylammonium chloride (TEBAC) in benzene or THF affording C-difluoromethyl substituted derivatives. This process is restricted to C-H acids of pK_a ≅ 16.3-19.1. The observed facts are rationalized.     

Molecular conformation of pyridinic aromatic esters : I. Electronic absorption spectra and dipole moments of the phenylic and pyridin1c esters of the benzoic- and pyridine-carboxylic acids

Nine pyridine aromatic ester derivatives have been prepared, six of which were previously unreported. The electronic absorption spectra have been determined in methanol and cyclohexane solution and band assignment is briefly discussed. The dipole moments were measured in benzene at 25 °C and are compared with values calculated using a vectorial model. Comparison suggests the most probable conformation of derivatives which have the nitrogen either in ortho or meta position in one or both rings of the molecule.     

对硝基二苯乙炔系列衍生物的二阶非线性光学性质的理论研究

在ZINDO方法基础上,按完全态求和(SOS)公式编制了计算分子二阶非线性光学系数β_ijk和β_μ的程序,研究了对硝基二苯乙炔系列衍生物的结构和非线性光学性质,考察了分子共轭链长和给电子基团对β_μ的影响.结果表明,对硝基二苯乙炔系列衍生物上连有推电子基团有利于增大二阶光学非线性,扩大共轭范围也有利于增大二阶光学非线性,但对透明性则影响不大.     

New Fluorescence Domain “Excited Multimer” Formed upon Photoexcitation of Continuously Stacked Diaroylmethanatoboron Difluoride Molecules with Fused π‐Orbitals in Crystals

The crystal‐packing structures of seven derivatives of diaroylmethanatoboron difluoride ( 1 a – gBF₂ ) are characterized by no overlap of the π‐conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π‐orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π‐orbitals of adjacent molecules (type III). The crystal‐packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer‐like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π‐orbitals generates new FL domains, referred to as “excited multimers”, which possess allowed S₀–S₁ electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π‐orbitals in the type III crystals leads to “excited multimer” domains with forbidden S₀–S₁ electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.     

A quantum mechanical study of bioactive 3-chloro-2,5-dihydroxybenzyl alcohol through substitutions

Electronic structures, vibrational and ionization spectra of 3-chloro-2,5-dihydroxybenzyl alcohol (CHBA), a novel bioactive benzene derivative from marine fungi, are presented in this study using quantum mechanical methods such as density functional theory and outer valence Green function method. A number of related benzene derivatives such as chlorobenzene, 3-chlorobenzyl alcohol, hydroquinone and chlorohydroquinone are also studied, in order to assist our understanding of the structure, properties and interactions of CHBA. Vibrational spectra such as infrared (IR) and Raman spectra reveal signatures of the functional group substitutions and their hydrogen bond interactions in CHBA. Solvent effects on the IR spectra of CHBA with polar and non-polar solvents are simulated using the polarizable continuum model (PCM) and cause redshifts of some of the IR spectral frequencies with respect to the gas phase values at both ends of the 400?C4,000?cm^?1 region. The inner-shell ionization spectra, in particular the C?CK spectra of the benzene derivatives, reveal detailed chemical environmental changes of the carbon and oxygen atoms due to the substitutions. The valence ionization energies of the highest occupied molecular orbital (HOMO) and the 3rd HOMO, (HOMO-2) of the benzene derivatives respond significantly to the substitutions, whereas the charge distributions of the HOMO and 2nd HOMO (HOMO-1) do not change significantly from their benzene counterparts. As a result, the 3rd HOMO changes significantly in both ionization energies and the charge distributions, which can serve as a signature of the substitutions among the benzene derivatives.