[show abstract][hide abstract] ABSTRACT: BACKGROUND: Friedel-Crafts acetylation is an important route to aromatic ketones, in research laboratories and in industry. The acetyl derivatives of 3,3' -dimethylbiphenyl (3,3' -dmbp) have applications in the field of liquid crystals and polymers and may be oxidized to the dicarboxylic acids and derivatives that are of interest in cancer treatment. FINDINGS: The effect of solvent and temperature on the selectivity of monoacetylation of 3,3'-dmbp by the Perrier addition procedure was studied using stoichiometric amounts of reagents. 4-Ac-3,3' -dmbp was formed almost quantitatively in boiling 1,2-dichloroethane and this is almost twice the yield hitherto reported. Using instead a molar ratio of substrate:AcCl:AlCl3 equal to 1:4:4 or 1:6:6 in boiling 1,2-dichloroethane, acetylation afforded 4,4' - and 4,6' -diacetyl-3,3' -dmbp in a total yield close to 100%. The acetyl derivatives were subsequently converted to the carboxylic acids by hypochlorite oxidation. The relative stabilities of the isomeric products and the corresponding sigma-complexes were studied by DFT calculations and the data indicated that mono- and diacetylation followed different mechanisms. CONCLUSIONS: Friedel-Crafts acetylation of 3,3' -dmbp using the Perrier addition procedure in boiling 1,2-dichloroethane was found to be superior to other recipes. The discrimination against the 6-acetyl derivative during monoacetylation seems to reflect a mechanism including an AcCl:AlCl3 complex or larger agglomerates as the electrophile, whereas the less selective diacetylations of the deactivated 4-Ac-3,3' -dmbp are suggested to include the acetyl cation as the electrophile. The DFT data also showed that complexation of intermediates and products with AlCl3 does not seem to be important in determining the mechanism.
Chemistry Central Journal 06/2012; 6(1):52. · 1.31 Impact Factor
[show abstract][hide abstract] ABSTRACT: The solid state tautomerism of 2-((phenylimino)methyl)naphthalene-1-ol was studied using X-ray measurements and absorption spectroscopy. In the solid state, the keto tautomer predominates. The observed shift in the equilibrium from the enol (dilute solution) to the keto (solid state) forms is explained by the formation of dye aggregates using ab initio quantum chemical calculations.
[show abstract][hide abstract] ABSTRACT: The aim of the current report is to shed light on the tautomerism of 4-((Phenylimino)methyl) naphthalene-1-ol in solution, which was studied by UV-Vis spectroscopy and quantum chemical calculations. It was found that this compound does not have the typical tautomeric behavior of its analog 4-Phenylazo-naphthalen-1-ol. The complicated equilibrium between the enol- and keto-like forms and two kinds of dimers that can exist in solution, is strongly dependent on the proton acceptor/donor abilities of the solvent. Using advanced data treatment quantitative information about the tautomeric and dimeric equilibrium constants was obtained. Copyright # 2007 John Wiley & Sons, Ltd.
[show abstract][hide abstract] ABSTRACT: Shape selective acetylation of 4,4′-dimethylbiphenyl using anhydrous aluminum chloride as catalyst is an effective route for the production of mono- and di-acetyl-4,4′-dimethylbiphenyl. Preparations, characterization and a catalytic study of the Friedel-Crafts acetylation of 4,4′-dimethylbiphenyl, involving use of the Perrier addition procedure are carried out in a range of solvents and under a variety of experimental conditions. The obtained ketones are isolated and identified by various physico-chemical techniques. Mono acetylation of 4,4′-dimethylbiphenyl afforded a mixture of two isomeric acetyl dimethylbiphenyls. In chloroalkane or carbon disulfide solvent, the yields of isomers were in the order: 2->3-; in nitromethane 3-isomer predominated. On the other hand diacetylation of the hydrocarbon gave only the 2,3′-diacetyl isomer. The mono- and di-ketones are converted to the corresponding carboxylic acids. 2-Acetyl-4,4′-dimethylbiphenyl was prepared by indirect multi-step synthetic routes. 3-D molecular modelling supports the positional assignment of the acetyl group with the results obtained from the electronic spectra.
[show abstract][hide abstract] ABSTRACT: Linear (one-photon) and nonlinear (two-photon) spectral properties of 4-N,N-dimethylamino-4′-aminoazobenzene have been investigated under protonation in DMSO/water solvent mixtures. Observed spectral changes have been discussed in terms of ammonium–azonium tautomerism. The corresponding tautomeric constants have been determined for the first time for such kind of tautomeric equilibria. The process of protonation leads to changes of the two-photon absorption properties, suggesting that such process could be used as a tool for change of third-order nonlinear optical activity in solution.
Journal of Photochemistry and Photobiology A-chemistry - J PHOTOCHEM PHOTOBIOL A-CHEM. 01/2006; 181(2):274-282.
[show abstract][hide abstract] ABSTRACT: The electrochemical oxidation of 2,4-dimethyl-3-ethylpyrrole in acetonitrile has been studied using cyclic voltammetry, constant current coulometry, preparative electrolyses and ab initio calculations. The product analysis after the preparative electrolyses was carried out by HPLC combined with UV–vis and electrospray ionization MS detection. The aim of the work was to address some of the unresolved problems in the oxidative oligomerization and polymerization of alkylpyrroles. The title compound was chosen as a model for studies of pyrroles that are more basic than the solvent-supporting electrolyte system and for that reason are forced to serve as the base accepting the protons released during the coupling steps. The voltammograms obtained by cyclic voltammetry at a substrate concentration of 2 mM and voltage scan rates between 0.02 and 2 V s−1 showed a characteristic trace-crossing phenomenon that could be demonstrated by digital simulation to be related to that fact that the deprotonations of the initially formed dimer dication are slow with second order rate constants in the range 103–104 M−1 s−1. The relative stability of the different tautomers of the protonated pyrrole monomer and the corresponding 2,2′-dimer was determined by ab initio calculations at the RHF 6-31G(d) level. The studies also included investigations of the effects resulting from addition of a non-nucleophilic base, 2,6-di-tert-butylpyridine, to the voltammetry solutions. The major product observed after preparative electrolyses was a trimer the structure of which is proposed to include a central 2H-pyrrole unit. Since 2H-pyrroles are stronger bases than the corresponding 1H-pyrroles, the trimer is effectively protected against further oxidation by protonation. Two other trimers were observed as minor or trace products as well as a 1H,2H-dimer and several tetramers, also in trace amounts. In addition to the dimer, the trimers and the tetramers, a number of other minor products could be detected. These could all be traced back to the nucleophilic attack by residual water on the radical cations or dications of the 2,2′-dimer and the trimers. The results obtained by constant current coulometry are in agreement with the formation of a 2H-pyrrole based trimer as the major product.
[show abstract][hide abstract] ABSTRACT: The enol imine a enaminone tautomerization constants, K T , and thermodynamic parameters, ∆H T and ∆S T , of 1-hydroxy-2-naphthaldehyde Schiff bases are determined by UV/vis spectroscopy. Polar solvents shift the equilibrium toward the quinone form (for the unsubstituted derivative 1c, K T) 0.20 (cyclohexane) and K T) 1.49 (ethanol)). Both donor (MeO, NMe 2) and acceptor (CN, NO 2) substituents lead to a decreased K T independent of solvent polarity. In apolar solvents, for all derivatives 1a-1e, the enol imine a enaminone equilibria are endergonic but exothermic. Linear solvation energy relationships allow extrapolation of ∆G T to the gas phase. Density functional theory calculations (B3LYP/6-311+G**) yield good agreement with these extrapolated ∆G T values. Solvent effects on 1c are also successfully reproduced by the calculations. Geometric (O‚‚‚N distance) and energetic criteria (conformer energy differences, homodesmotic reactions) establish the importance of intramolecular hydrogen bonding for the tautomerism of these compounds. The results obtained for 1a-1e are compared with tautomeric properties of the isomeric naphthaldehyde anils 2-4, the monocyclic analogues 5 and 6, the corresponding azo derivatives 7-9, and the N-alkyl derivative 10.
Journal of Physical Chemistry A - J PHYS CHEM A. 01/2004; 108(37).
[show abstract][hide abstract] ABSTRACT: The two-photon absorption (2PA) properties of a series of dyes containing donor and acceptor groups connected via a π-electronic bridge were studied by using a femtosecond Z-scan technique. It was found that the nature of the π-electronic bridge does not substantially affect the measured 2PA intensities. In the case of the substituted azobenzenes the decrease of the strength of the donor and/or acceptor groups leads to decrease of the 2PA intensity, the effect of the acceptor strength being greater.
Physical Chemistry Chemical Physics 01/2003; 5(6):1193-1197. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Photophysical properties of several basically important aromatic azodyes (1-phenylazo-2-naphthol and 2-phenylazo-1-naphthol) and Schiff bases (N-(2-hydroxy-1-naphthylmethylidene) aniline and N-(1-hydroxy-2-naphthylmethylidene) aniline) all containing an intramolecular hydrogen bond were studied by both steady-state and time-resolved fluorescence spectroscopy with temperatures down to 98 K. It was found that the fluorescence results from the quinone form (H-form) only. The enol form (A-form) undergoes rapid excited state intramolecular proton transfer (ESIPT) resulting in the excited H-form. The compounds have relatively low quantum yields at room temperature, which increase considerably at low temperatures. Lifetime data at the different temperatures indicate that a substitution by both acceptor or donor groups on the para position in the phenyl ring decreases the deactivation rate and hence results in increased lifetime.
Journal of Photochemistry and Photobiology A: Chemistry. 01/2002;
[show abstract][hide abstract] ABSTRACT: A UV-Vis spectroscopic study based on the recently developed chemometric approach for quantitative analysis of undefined mixtures is performed on a series of donor and acceptor substituted Schiff bases of 2-hydroxynaphthalde- hydes. In CCl4 solution all compounds preferentially exist as the phenol tautomer independent of the nature of the respective substituent. With increasing polarity the tautomeric equilibrium is shifted towards the quinone form. In CHCl3 and, especially, ethanol a clear distinction between the effect of donors (stabilization of the quinone form) and acceptors (stabilization of the phenol tautomer) is evident. Ab initio calculations including solvent effects via the polarized continuum model of solvation as well as the supermolecule approach are used to rationalize the experimental findings.
Journal of the Chemical Society Perkin Transactions 2 01/2000;
[show abstract][hide abstract] ABSTRACT: The abundance 1H-NMR spectra of some new mono- and diacetyl compounds derived from 2,2′-dimethyl-, 3,3′-dimethyl-, and 4,4′-dimethylbiphenyls were recorded. Unambiguous signal assignment of the methyl groups of the aromatic biphenyl rings were made for the structure elucidation.
[show abstract][hide abstract] ABSTRACT: Friedel-Crafts acetylation of 9H-fluorene is an effective route for the preparation of mono- and diacetyl-9H-fluorenes. Using acetylchloride as the reagent and aluminum chloride as the Lewis acid catalyst the effect of the solvent polarity, the temperature, the reaction time and the mode of addition (Perrier or Bouveault) on the reactivity-selectivity pattern was investigated. The results showed that monoacetylation of 9H-fluorene in chloroalkanes and nitromethane gives mixtures of 2-acetyl- and 4-acetyl-9H-fluorene with the former dominating. In addition to these two isomers, 2,7-diacetyl-9H-fluorene was obtained in 5-11 % yield when carbon disulfide was used as the solvent. Acetylation of 9H-fluorene in dichloroethane and carbon disulfide, using an excess of acetyl chloride and aluminum chloride at reflux temperature, gives 2,7-diacetyl-9H- fluorene exclusively in high yields (> 97%). Attempts to carry out diacetylation in nitromethane on the other hand resulted in the formation of monoacetyl derivatives only. The ketones obtained were isolated and identified by various physico-chemical techniques. The 1 H NMR spectrum of 4-acetyl-9H-fluorene is reported for the first time. Values of the total energies and the Gibbs free energies at 298K for 9H-fluorene, the five isomeric monoacetyl-9H-fluorenes and the four σ- complexes leading to 1-, 2-, 3- and 4-acetyl-9H-fluorene, respectively, were computed at the DFT B3LYP 6-31G** level of theory. The data serve as a basis for the discussion of the reactivity-selectivity pattern observed and it is concluded that the distribution of products is partly kinetically controlled.