Evidence that a 1,2-dihydroxycyclohexadienide anion is stabilized by aromatic "negative hyperconjugation" is described. It complements an earlier inference of "positive" hyperconjugative aromaticity for the cyclohexadienyl cation. The anion is a reactive intermediate in the dehydration of benzene cis-1,2-dihydrodiol to phenol. Rate constants for 3-substituted benzene cis-dihydrodiols are correlated by σ(-) values with ρ = 3.2. Solvent isotope effects for the reactions are k(H(2)O)/k(D(2)O) = 1.2-1.8. These measurements are consistent with reaction via a carbanion intermediate or a concerted reaction with a "carbanion-like" transition state. These and other experimental results confirm that the reaction proceeds by a stepwise mechanism, with a change in rate-determining step from proton transfer to the loss of hydroxide ion from the intermediate. Hydrogen isotope exchange accompanying dehydration of the parent benzene cis-1,2-dihydrodiol was not found, and thus, the proton transfer step is subject to internal return. A rate constant of ~10(11) s(-1), corresponding to rotational relaxation of the aqueous solvent, is assigned to loss of hydroxide ion from the intermediate. The rate constant for internal return therefore falls in the range 10(11)-10(12) s(-1). From these limiting values and the measured rate constant for hydroxide-catalyzed dehydration, a pK(a) of 30.8 ± 0.5 was determined for formation of the anion. Although loss of hydroxide ion is hugely exothermic, a concerted reaction is not enforced by the instability of the intermediate. Stabilization by negative hyperconjugation is proposed for 1,2-dihydroxycyclohexadienide and similar anions, and this proposal is supported by additional experimental evidence and by computational results, including evidence for a diatropic ("aromatic") ring current in 3,3-difluorocyclohexadienyl anion. 相似文献
A study of scavenging effects of iso-propanol, n-propanol, n-butanol and acetone on H• radicals, precursors for nonfaradaic products of anodic contact glow discharge electrolysis (CGDE) in aqueous media, has
been carried out. A kinetic analysis of the competing reactions of H• with different species in the system leads to a primary yield of 9.8 mol (mol electron)−1 each of H• and OH• radicals in the liquid-phase reaction zone of anodic CGDE. The method is considerably more convenient to apply than those
reported earlier based on the use of OH• scavengers. 相似文献
Enzymatic cis-dihydroxylation of benzo[b]thiophene, benzo[b]furan and several methyl substituted derivatives was found to occur in both the carbocyclic and heterocyclic rings. Relative and absolute configurations and enantiopurities of the resulting dihydrodiols were determined. Hydrogenation of the alkene bond in carbocyclic cis-dihydrodiols and ring-opening epimerization/reduction reactions of heterocyclic cis/trans-dihydrodiols were also studied. The relatively stable heterocyclic dihydrodiols of benzo[b]thiophene and benzo[b]furan showed a strong preference for the trans configuration in aqueous solutions. The 2,3-dihydrodiol metabolite of benzo[b]thiophene was utilized as a precursor in the chemoenzymatic synthesis of the unstable arene oxide, benzo[b]thiophene 2,3-oxide. 相似文献
Fourth order derivative gravity in 3+1 dimensions is perturbatively renormalizable and is shown to describe a unitary theory of gravitons in a limited coupling parameter space. The running gravitational constant which includes graviton contribution is computed. Generically, gravitational Newton?s constant vanishes at short distances in this perturbatively renormalizable and unitary theory. 相似文献
cis-Dihydrodiols of anthracene and benz[a]anthracene, and acetonide derivatives of the cis-dihydrodiols of benzene, fluorobenzene, biphenyl and phenanthrene have been identified as substrates for dioxygenase enzymes, yielding the corresponding enantiopure arene bioproducts, bis(cis-dihydrodiol)s and cis-diol acetonides respectively. 相似文献
Toluene- and naphthalene-dioxygenase-catalysed oxidation of six bicyclic disulfide substrates, using whole cells of Pseudomonas putida, gave the corresponding monosulfoxides with high ee values and enantiocomplementarity, in most cases. Two alcohol-sulfoxide diastereoisomers, formed from the reaction of the (R)-1,3-benzodithiole-1-oxide metabolite with n-butyllithium and benzaldehyde, were separated and stereochemically assigned. Treatment, of enantiopure (1R,3R)-benzo-1,3-dithiole-1,3-dioxide, obtained by chemoenzymatic synthesis, with alkyllithium reagents, resulted in a novel ring-opening reaction which proceeded with inversion of configuration to yield a series of acyclic disulfoxides. 相似文献
The bioreductive enzymes typically upregulated in hypoxic tumor cells can be targeted for developing diagnostic and drug delivery applications. In this study, a new fluorescent probe 4?(6?nitro?1,3?dioxo?1H?benzo[de]isoquinolin?2(3H)?yl)benzaldehyde (NIB) based on a nitronaphthalimide skeleton that could respond to nitroreductase (NTR) overexpressed in hypoxic tumors is designed and its application in imaging tumor hypoxia is demonstrated. The docking studies revealed favourable interactions of NIB with the binding pocket of NTR-Escherichia coli. NIB, which is synthesized through a simple and single step imidation of 4?nitro?1,8?naphthalic anhydride displayed excellent reducible capacity under hypoxic conditions as evidenced from cyclic voltammetry investigations. The fluorescence measurements confirmed the formation of identical products (NIB-red) during chemical as well as NTR?aided enzymatic reduction in the presence of NADH. The potential fluorescence imaging of hypoxia based on NTR-mediated reduction of NIB is confirmed using in-vitro cell culture experiments using human breast cancer (MCF?7) cells, which displayed a significant change in the fluorescence colour and intensity at low NIB concentration within a short incubation period in hypoxic conditions.
Densities and surface tensions were measured for the binary liquid mixtures formed by benzonitrile, chlorobenzene, benzyl chloride and benzyl alcohol with benzene, at 298.15, 303.15 and 313.15 K and atmospheric pressure, over the whole composition range. The Prigogine?CFlory?CPatterson model (PFP), the Ramaswamy and Anbananthan (RS) model, the model derived by Glinski, the Sanchez equation, the Goldsack relation and Eberhart??s model were used to predict the associational behavior of weakly interacting liquids. The measured properties were fitted to the Redlich?CKister polynomial equation to estimate the binary coefficients and standard errors. Furthermore, the McAllister multi-body interaction model was also used to correlate the binary solution properties. These non-associated and associated models were compared and tested for different systems, showing that considering the association processes yields better agreement between theory and experiment as compared to non-associated processes. 相似文献