Enolic Group Research Articles

Carbon−Carbon Bond-Forming Reductive Elimination from Arylpalladium Complexes Containing Functionalized Alkyl Groups. Influence of Ligand Steric and Electronic Properties on Structure, Stability, and Reactivity

A series of arylpalladium alkyl complexes of the formula [(DPPBz)Pd(Ar)(R)] (DPPBz = 1,2-bis(diphenylphosphino)benzene; R = methyl, benzyl, enolate, cyanoalkyl, trifluoroalkyl, or malonate) has been prepared to reveal the influence of steric and electronic parameters on structure, stability, and reactivity. Arylpalladium enolate and cyanoalkyl complexes ligated by EtPh2P, 1,1‘-bis(diisopropylphosphino)ferrocene (DiPrPF), and BINAP were prepared to evaluate the effect of the ancillary ligand. The coordination modes of the enolate and cyanoalkyl complexes were determined by spectroscopic methods, in combination with X-ray crystallography. In the absence of steric effects, the C-bound isomer was favored electronically if the enolate or cyanoalkyl group was located trans to a phosphine, and the O-bound isomer was favored if the enolate was located trans to an aryl group. The thermodynamic stability of the enolate and cyanoalkyl complexes was controlled by the steric properties of the enolate or cyanoalkyl gro...

5-(4-Hydroxy-3-methoxyphenyl)-1-phenylpent-4-ene-1,3-dione

In the crystalline state, the title mol­ecule, C18H16O4, exists in the enolized form and displays a strong intramolecular hydrogen bond. The molecular structure, as well as theoretical calcu­lations, support the hypothesis that the conjugation in the mol­ecule is a determining factor for the position of the enol H atom. The packing of the mol­ecules is dominated by interactions between 3-methoxy-4-hydroxyphenyl- and enol groups, thus forming planar molecular chains. These interactions include normal O—H⋯O bonds, as well as weak C—H⋯O interactions and van der Waals forces.

Excited‐State Behavior and Photoionization of 1,8‐Acridinedione Dyes in Micelles—Comparison with NADH Oxidation

The photophysics and photochemistry of 1,8-acridinedione dyes, which are analogues of reduced nicotinamide adenine dinucleotide (NADH), are studied in anionic and cationic micelles. Acridinedione dyes (ADDs) are solubilized in micelles at the micelle-water interface and are in equilibrium between the aqueous and micellar phase. The binding of the ADDs with micelles is attributed to hydrophobic interactions and the binding constants are determined with steady-state and time-resolved techniques. Nanosecond laser flash photolysis studies are carried out in aqueous, anionic, and cationic micellar solutions. The ADD undergoes photoionization in the excited state to give a solvated electron. The solvated electron reacts with the ADD to give an anion radical. In anionic micelles, the yield of the solvated electron increases because of the efficient separation of the cation radical and the electron. Cation radicals derived from the photooxidation of ADDs are involved in keto-enol tautomerization. Under acidic conditions, an enol radical cation of the acridinedione dye is formed from the keto form of the cation radical by intramolecular hydrogen atom transfer. In cationic micelles, due to electrostatic attraction, the electron cannot escape from the micelle and recombination of the cation radical and the electron results in the formation of a triplet state. For the first time, a solvated electron is observed in the laser flash photolysis of ADDs in anionic micelles. The photoionization of ADDs depends on the excitation wavelength and is biphotonic at 355 nm and monophotonic at 248 nm. From the results with this NADH model compound, the sequential electron-proton-electron transfer oxidation of NADH is confirmed and the nature of the intermediates involved in the oxidation is unraveled; these intermediates are found to depend on the pH value of the medium.

Intramolecular Hydrogen Bonding and Double H-Atom Transfer in Peroxy and Alkoxy Radicals from Isoprene

Quantum mechanical calculations were used to determine the structure and stability of second-generation peroxy and alkoxy radicals formed in the atmospheric degradation of isoprene (2-methyl-1,3-butadiene). Certain of these radicals exhibit a novel hydrogen bonding motif, consisting of two intramolecular hydrogen bonds. The hydrogen bonds are donated in series, with an enol group donating a hydrogen bond to a −CH2OH group, which donates in turn to the oxygen radical center. This hydrogen bonding motif opens a new reaction pathway: the simultaneous transfer of two H-atoms across the hydrogen bonds with a barrier of only ∼5 kcal/mol in the alkoxy radicals, but ∼20 kcal/mol in the peroxy radicals. Rate constants for these reactions were calculated, and the effects of tunneling on the rate constant were examined. All species and reactions were analyzed at the B3LYP/6-311G(2df,2p) level of theory; the transition states for the double H-atom transfer reactions were also studied using the MPW1K functional and t...

Electrochemical studies of isolapachol with emphasis on oxygen interaction with its radical anions

Cyclovoltammetric studies, in aprotic medium (DMF+0.1 mol l −1 TBAP or DMSO+0.1 mol l −1 TEAP), on glassy carbon and/or platinum electrodes were performed with isolapachol [2-hydroxy-3-(3-methyl-1-butenyl)-1,4-naphthoquinone], in the absence and presence of oxygen, which were aimed at investigating its electrochemical reduction mechanism and possible oxygen interaction with its radical anion. The electrochemical behaviour is complex and similar to that observed for lapachol, and the first peak is related to the semiquinone formation, although complicated by the occurrence of self-protonation mechanisms and hydrogen-bonded intermediates formation. The observed positive shift in the potential of the first wave of isolapachol, in comparison to lapachol, is related to the higher acidity of the enolic group. The cyclovoltammetric results obtained in the presence of O 2 clearly indicate the consumption of the semiquinone anion-radicals of isolapachol by oxygen, in an EC type reaction, generating the deprotonated form of isolapachol and HOO •. The observed generation of the superoxide radical, after electron transfer can be related to the mechanism of biological action and toxicity of isolapachol.

Nickel‐Based Thick Film Ethanol Sensor

AbstractThick films ethanol sensors were prepared. The composition of the electrode was measured qualitatively to consist of Ni, O, Al, Mg and Pb. Among these composing elements, it was found that only the nickel could sense the ethanol. Both the infrared (IR) spectrum and cyclic voltammetry (CV) tests were employed to investigate its derivatives in the sensing reaction. It was found that the derivatives presented as adsorbed enol group and β‐ketoaldehyde as enols. Besides, both the redox between Ni(II) and Ni(III), and solid‐state electrochemical reactions in the electrode were found to occur in this system simultaneously. A linear dependence of response current vs. ethanol concentration with a detection limit of 37 ppm was obtained and the response time was estimated to be 140 seconds in this sensing system.

Antioxidant properties of galantamine hydrobromide.

The antioxidant properties of galantamine hydrobromide ((4alpha,6beta)-4a,5,9,10,11,12-hexahydro-3-methoxy-11-methyl-6H-benzofuro[3a,3,2-ef][2]benzazepin-6-ol hydrobromide) were studied in vitro, using luminol-dependent chemiluminescence and spectrophotometry. It was found that this compound was a scavenger of reactive oxygen species (ROS). By comparing the antioxidant effects of galantamine ((4alpha,6beta)-4a,5,9,10,11,12-hexahydro-3-methoxy-11-methyl-6H-benzofuro[3a,3,2-ef][2]benzazepin-6-ol), galantamine hydrobromide, narwedine (4a,5,9,10,11,12-hexahydro-3-methoxy-11-methyl-6H-benzofuro[3a,3,2-ef][2]benzazepin-6-one), and narwedine hydrobromide it was found that the antioxidant activity depended on the enolic OH group in the molecule. The presence of a quaternary nitrogen in the compound increased the strength of the scavenging effect. It is proposed that the antioxidant properties observed in vitro may contribute to the therapeutical effect of galantamine hydrobromide on patients with brain degeneration.

Mechanisms of electron-transfer oxidation of NADH analogues and chemiluminescence. Detection of the keto and enol radical cations.

The radical cation of an NADH analogue (BNAH: 1-benzyl-1,4-dihydronicotinamide) has been successfully detected as the transient absorption and ESR spectra in the thermal electron transfer from BNAH to Fe(bpy)(3)(3+) (bpy = 2,2'-bipyridine) and Ru(bpy)(3)(3+). The ESR spectra of the radical cations of BNAH and the dideuterated compound (BNAH-4,4'-d(2)) indicate that the observed radical cation is the keto form rather than the enol form in the tautomerization. The deprotonation rate and the kinetic isotope effects of the keto form of BNAH(*)(+) were determined from the kinetic analysis of the electron-transfer reactions. In the case of electron transfer from BNAH to Ru(bpy)(3)(3+), the chemiluminescence due to Ru(bpy)(3)(2+) was observed in the second electron-transfer step from BNA(*), produced by the deprotonation of the keto form of BNAH(*)(+), to Ru(bpy)(3)(3+). The observation of chemiluminescence due to Ru(bpy)(3)(2+) provides compelling evidence that the Marcus inverted region is observed even for such an intermolecular electron-transfer reaction. When BNAH is replaced by 4-tert-butylated BNAH (4-t-BuBNAH), no chemiluminescence due to Ru(bpy)(3)(2+) has been observed in the electron transfer from 4-t-BuBNAH to Ru(bpy)(3)(3+). This is ascribed to the facile C-C bond cleavage in 4-t-BuBNAH(*)(+). In the laser flash photolysis of a deaerated MeCN solution of BNAH and CHBr(3), the transient absorption spectrum of the enol form of BNAH(*)(+) was detected instead of the keto form of BNAH(*)(+), and the enol form was tautomerized to the keto form. The rate of intramolecular proton transfer in the enol form to produce the keto form of BNAH(*)(+) was determined from the decay of the absorption band due to the enol form and the rise in the absorption band due to the keto form. The kinetic isotope effects were observed for the intramolecular proton-transfer process in the keto form to produce the enol form.

Calopins and Cyclocalopins − Bitter Principles fromBoletus calopus and Related Mushrooms

Boletus calopus and closely related mushrooms of the Boletus section Calopodes are characterized by their bitter taste, which is caused mainly by O-acetylcyclocalopin A (6a), a member of a series of unique δ-lactone derivatives. Besides the simple lactone calopin (1b) and its O-acetyl derivative 1a, the more complex cyclocalopins 6−12 have been isolated and their structures elucidated. Cyclocalopin D (10a) is an unprecedented α-glucoside in which the sugar residue is bound to an enolic hydroxy group. The chemical conversion of 6b into 1a established the same stereochemistry for the calopin and cyclocalopin types of metabolites. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Chlorophyll a Radical Ions: A Density Functional Study

The chlorophyll a radical cation, Chl a + . , and the corresponding radical anion, Chl a - . , have been investigated applying density functional methods. Furthermore, the C 1 0 epimer Chl a' and O 1 Chl a enol radical cations have been studied, which have been proposed to play a role in photosystem I of oxygenic photosynthesis. Isotropic hyperfine coupling constants (hfcs) and related spin density distributions calculated at the B3LYP/ EPR-II//BLYP/DZVP(D) level of theory are reported. For Chl a + . and Chl a - . , good agreement is observed between calculated and experimental hfcs. Whereas no larger differences in the hfcs between Chl a + . and (Chl a') + . are predicted to occur, the enol forms of Chl a + . give rise to significantly altered hyperfine coupling constants in comparison with the respective keto form. In addition to the chlorophyll systems, the epimer and O 1 enol of bacteriochlorophyll a have also been calculated. From a comparison with experimental data it is concluded that the enol forms do not occur in photosynthetic reaction centers.

Bioluminescence reaction catalyzed by membrane-bound luciferase in the “firefly squid,” Watasenia scintillans

The small Japanese “firefly squid,” Watasenia scintillans, emits a bluish luminescence from dermal photogenic organs distributed along the ventral aspects of the head, mantle, funnel, arms and eyes. The brightest light is emitted by a cluster of three tiny organs located at the tip of each of the fourth pair of arms. Studies of extracts of the arm organs show that the light is due to a luciferin–luciferase reaction in which the luciferase is membrane-bound. The other components of the reaction are coelenterazine disulfate (luciferin), ATP, Mg 2+, and molecular oxygen. Based on the results, a reaction scheme is proposed which involves a rapid base/luciferase-catalyzed enolization of the keto group of the C-3 carbon of luciferin, followed by an adenylation of the enol group by ATP. The AMP serves as a recognition moiety for docking the substrate molecule to a luciferase bound to membrane, after which AMP is cleaved and a four-membered dioxetanone intermediate is formed by the addition of molecular oxygen. The intermediate then spontaneously decomposes to yield CO 2 and coelenteramide disulfate (oxyluciferin) in the excited state, which serves as the light emitter in the reaction.

Conformation-dependent reaction thermochemistry: study of lactones and lactone enolates in the gas phase.

Gas-phase acidities (Delta H degrees (acid)) of lactones with ring sizes from four to seven have been measured on a Fourier transform ion cyclotron resonance mass spectrometer. Electron affinities (EAs) of the corresponding lactone enolate radicals were measured on a continuous-wave ion cyclotron resonance mass spectrometer, and the bond dissociation energies (BDEs) of the alpha C-H bonds were derived. In order of increasing ring size, Delta H degrees (acid) = 368.7 +/- 2., 369.4 +/- 2.2, 367.3 +/- 2.2, and 368.3 +/- 2.2 kcal/mol and BDE = 99.4 +/- 2.3, 94.8 +/- 2.3, 89.2 +/- 2.3, and 92.8 +/- 2.4 kcal/mol for beta-propiolactone, gamma-butyrolactone, delta-valerolactone, and epsilon-caprolactone, respectively. For their corresponding enolate radicals, EA = 44.1 +/- 0.3, 38.8 +/- 0.3, 35.3 +/- 0.3, and 37.9 +/- 0.6 kcal/mol. All of these lactones are considerably more acidic than methyl acetate, consistent with a dipole repulsion model. Both BDEs and EAs show a strong dependence on ring size, whereas Delta H degrees (acid) does not. These findings are discussed, taking into account differential electronic effects and differential strain between the reactant and product species in each reaction.

Orthorhombic form of 6-phenylhex-5-ene-2,4-dione

The title compound, C12H12O2, crystallizes in at least two polymorphic forms, one monoclinic and the other ortho­rhombic. In the orthorhombic polymorph, the enol group is asymmetric with the H atom bonded to the O atom closest to the most electronegative substituent, and the mol­ecule displays the longest possible conjugation. The molecular packing is dominated by weak C—H⋯O interactions.

Ter-body intermediates in the gas phase: reaction of ionized enols with tert-butanol

In the gas phase, the CH 2CHOH ·+ enol radical cation 1 as well as its higher homologues CH 3CHCHOH ·+ 2 and (CH 3) 2CCHOH ·+ 3, undergo exactly the same sequence of reactions with tert-butanol, leading to the losses of isobutene, water and water plus alkene. Fourier transform ion cyclotron resonance (FT-ICR) experiments using labeled reactants as well as ab initio calculations show that independent pathways can be proposed to explain the observed reactivity. For ion 1, taken as the simplest model, the first step of the reaction is formation of a proton bound complex which gives, by a simple exothermic proton transfer, the ter-body intermediate [CH 2CHO ·, H 2O, C(CH 3) 3 +]. This complex, which was shown to possess a significant lifetime, is the key intermediate which undergoes three reactions. First, it can collapse to yield tert-butylvinyl ether with elimination of water. Second, by a regiospecific proton transfer, this complex can isomerize into three different ter-body complexes formed of water, isobutene and ionized enol. Within one of these complexes, which does not interconvert with the others, elimination of isobutene leads to the formation of a solvated enol ion. Within the others, a cycloaddition–cycloreversion process can proceed to yield the ionized enol 3 (loss of water and ethylene channel).

Engineering reactions in crystalline solids: photochemical generation of secondary and tertiary enol radical pairs from crystalline ketodiesters.

The photochemical decarbonylation of several crystalline 1,3-acetonedicarboxylates has been analyzed in solution and in the solid state. It is shown that the efficiency of the solid-state reaction depends on the stability of the intermediate acyl-alkyl and alkyl-alkyl radical pairs. Reactions proceeding through tertiary enol radicals are more efficient than reactions proceeding through secondary enol radical centers. Solid-state reactions that require the intermediacy of primary enol radicals do not occur. It is also shown that the selectivity of product formation in crystals depends on the structure of the reactant solid phase.

Radical cation ester cleavage in solution: mechanism of the mesolytic O-CO bond scission.

A wide range of enol carbonate, carbamate, and ester radical cations is characterized in solution by cyclic voltammetry and EPR spectroscopy. Preparative transformations using one-electron oxidants or anodic oxidation yield benzofurans after O-CO bond cleavage. Mechanistic investigations and direct detection of radical intermediates reveal that all enol radical cations undergo exclusively O-CO bond cleavage to provide alpha-carbonyl cations and acyl (or alternatively, alkoxyacyl and aminoacyl) radicals, respectively. The kinetics of the mesolytic fragmentation and the influence of nucleophilic additives have been determined using fast-scan cyclic voltammetry.

An estimation of strength of intramolecular hydrogen bonds — ab initio and AIM studies

Intramolecular H-bonds existing for derivatives of malonaldehyde have been studied by using the MP2/6-311++G ∗∗ level of theory. The nature of these interactions, known as resonance assisted hydrogen bonds, has been discussed. The topological properties of the electron density distributions for O–H⋯O intramolecular bridges have been analyzed in terms of the Bader theory of atoms in molecules (AIM). The results of calculations show that the Q-parameter describing the degree of π-electron delocalization within the OC–CC–O–H keto–enol group correlates with the strength of the H-bond. Correlations between the H-bond strength and topological parameters have been also studied.

Catalytic Roles of Water Protropic Species in the Tautomerization of Excited 6-Hydroxyquinoline: Migration of Hydrated Proton Clusters

Fluorescence kinetic constants are measured as functions of wavelength, pH, hydrogen isotope, and temperature to understand proton transfers involved in the excited-state tautomerization of aqueous 6-hydroxyquinoline. Not only the detailed mechanism but also the individual catalytic roles of H2O, H3O+, and OH- in the tautomerization reaction have been determined. The variation of the relative contribution with pH is determined for each reaction pathway as well. Solvent structural dynamics is found to be important in the deprotonation of the enol group and in the migration of a hydrated proton cluster.

SYNTHESIS AND CHARACTERIZATION OF NEW Cu(II) COMPLEXES DERIVED FROM BENZILIC AND MANDELIC HYDRAZONES

Two new sets of Cu(II) complexes with newly synthesised benzilic and mandelic hydrazone derivatives were prepared in the mole ratios 1:1 and 1:2 (Cu:L). The structure of the complexes was identified on the basis of the data obtained from elemental and thermal analyses as well as IR, UV-visible, ESR spectra, and X-ray diffraction. The results indicate that the ligands are tightly bound to the metal ion through the phenolic OH oxygen, the azomethine nitrogen, and the enolic OH oxygen in case of the 1:1 complexes while for the 1:2 complexes the enolic OH group did not participate in bonding. The complexes have elongated octahedral as well as square-planar symmetries.

Electron-transfer-induced tautomerization in methylindanones: electronic control of the tunneling rate for enolization.

The radical cations generated from 4-methyl- and 4,7-dimethylindanone, as well as their deuterated isotopomers, isolated in Argon matrices, were found to undergo enolization to the corresponding enol radical cations at rates that differ by orders of magnitude. It is shown by quantum chemical calculations that the effect of the remote methyl group in the 4-position is of purely electronic nature in that it stabilizes the unreactive pi-radical relative to the reactive sigma-radical state of the 7-methylindanone radical cation. The observed kinetic behavior of the two compounds can be reproduced satisfactorily on the basis of calculated height and width of the thermal barrier for enolization, using the Bell model for quantum mechanical tunneling. High-level calculations on the methylacrolein radical cation show that barriers for enolization in radical cations are overestimated by B3LYP/6-31G.