Enolic Group Research Articles

Excited state reactions of acridinedione dyes with onium salts: mechanistic details

Diaryliodonium and triarylsulphonium salts are thermally stable UV photoinitiators for cationic polymerization. Diaryliodonium salts (DPI-1–7) and triarylsulphonium (type-I, type-II and type-III) salts with different substituent have been prepared. Excited state reactions of these onium salts with decahydroacridinedione dyes (ADD-1–6) have been studied. ADD acts as an efficient sensitizer for the decomposition of these onium salts. Photosensitization occurs through electron transfer, which is confirmed by the observation of enol radical cation of ADD and diarylsulphinium radical cation in the laser flash photolysis. The mechanism of photosensitization and the mechanism of photoacid release have been discussed. Both singlet and triplet state of the ADD are involved in the photosensitization of diaryliodonium salts, whereas singlet state of ADD is alone involved in the photosensitization of triarylsulphonium salts. The quantum yield of photoacid generated in direct and the sensitized process is determined and this allows to gauge the practical efficiency of onium salts/ADD combinations as the photoinitiator for the cationic polymerization applications.

Conjugation and hydrogen bonding in a curcumin analogue

The strongly yellow-coloured compound 5-(5-methyl-2,3,4,5-tetra­hydro­furan-2-yl)-1-phenyl­pent-4-ene-1,3-dione, C16H14O3, has different substituents on each side of the dione group. The mol­ecule is essentially planar and the structure indicates a continuous conjugation from the furyl moiety to the carbonyl O atom in the enol group, as well as a strong conjugation within the enol group itself. The enol H atom is bonded to the O atom closest to the furyl group. Weak hydrogen bonds are indicated and comments about the colour of the compound are given.

2,2′-Methylenebis(3-hydroxy-2-cyclohexen-1-one)

In the title compound, C13H16O4, the cyclo­hexene rings adopt a sofa conformation. Adjacent mol­ecules are connected by C—H⋯O intermolecular interactions. Each mol­ecule is characterized by O—H⋯O intramolecular hydrogen bonds. The anti arrangement of the enolic OH group and the carbonyl O atom in the solid state is similar to the anti arrangement of the NH and carbonyl groups in indigo.

The [CH 2CHOH ·+, CH 3CHO] solvated enol radical cation

The bimolecular reaction of the CH 2CHOH ·+ enol ion ( m/z 44) with acetaldehyde gives a strongly dominant product, m/z 45, formed mainly by proton transfer from the ion to the molecule. The abundance of the product coming from a H · abstraction reaction from the neutral, albeit more exothermic, is negligible. In order to explain this result, the long lived [CH 2CHOH ·+, CH 3CHO] solvated ion was generated by reaction of the CH 2CHOH ·+ enol ion with (CH 3CHO) n in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. The structure of this solvated ion was clearly established. Labeling indicates that [CH 2CHOH ·+, CH 3CHO], upon low energy collisions, reacts by H · abstraction more rapidly than by H + transfer to the neutral moiety. This shows that the entropic factors are determinant when the enol ion reacts directly with acetaldehyde.

Stereochemical Aspects of a Two-Step Staudinger Reaction − Asymmetric Synthesis of Chiral Azetidine-2-ones

Reaction of N-trialkylsilylimines with a variety of glycine-derived ketenes produced 1,3-azadienes, which in some cases have been isolated and characterised. A conrotatory ring closure of these compounds gave rise to the formation of the azetidine-2-ones, thus allowing a formal two-step Staudinger reaction. Exclusive trans diastereoselectivity was observed. A less stringent diastereofacial selectivity was obtained. A set of experiments has been performed in order to evaluate the influence of the structural parameters as well as reaction conditions. Ab initio studies at MP2/6−31G* and QCISD(T)/6−311G** levels on model compounds provide a rationalization of the experimental results obtained. From the experimental as well as theoretical data it is clear that the presence of the silyl enol group in the intermediate azadiene is crucial in its stabilisation and plays a fundamental role in the conrotatory ring closure and, therefore, in the formation of the azetidine-2-one ring.

Potential drugs and nondrugs: prediction and identification of important structural features

Using decision trees, a model to discriminate between potential drugs and nondrugs has been developed. Compounds from the Available Chemical Directory and the World Drug Index databases were used as training set; the molecular structures were represented using extended atom types. The error rate on an independent validation data set is 17.4%. The number of false negatives can be reduced by penalizing the misclassification of drugs so that 92 out of 100 potential drugs are correctly recognized. At the same time, 34 out of 100 nondrugs are classified as potential drugs. The predictions of the model can be used to guide the purchase or selection of compounds for biological screening or the design of combinatorial libraries. The visualization of the generated models in the form of colored trees allowed us to identify a few, surprisingly simple features that explain the most significant differences between drugs and nondrugs in the training set: Just by testing the presence of hydroxyl, tertiary or secondary amino, carboxyl, phenol, or enol groups, already three quarters of all drugs could be correctly recognized. The nondrugs, on the other hand, are characterized by their aromatic nature with a low content of functional groups besides halogens. The general applicability of the model is shown by the predictions made for several Organon databases.

Hydrogen-Transferred Radical Cations of NADH Model Compounds. 3. 1,8-Acridinediones

Two different tautomeric forms of radical cations generated from 1,8-acridinediones were characterized by steady-state methods, pulse radiolysis, and laser flash photolysis techniques. In solution the primarily formed keto radical cations tautomerize in less than 1 ns to the corresponding enol radical cations. This process is significantly retarded in cryogenic glasses.

Hydrogen-Transferred Radical Cations of NADH Model Compounds. 2. Sequential Electron−Proton Addition to NAD+

Sequential electron−proton addition to different NAD+ model compounds was studied by pulse radiolysis. Under mildly acidic conditions, the radicals obtained by reduction of the cations are protonated at the carbonyl group. The pKa values of the resulting enol radical cations were determined. The same enol radical cations can also be formed via a deprotonation−reprotonation process from the keto radical cations of the corresponding NADH model compounds.

Hydrogen-Transferred Radical Cations of NADH Model Compounds. 1. Spontaneous Tautomerization

The radical cations generated from three NADH model compounds were investigated with particular emphasis on the possible formation of tautomeric enol radical cations. To a small extent, hydrogen transfer occurs upon ionization of all three compounds in argon matrixes. In the bicyclic seven-membered ring derivative enolization continues in a thermal reaction at 12 K (with highly dispersive kinetics), or upon visible irradiation. These processes are not observed in methylcyclohexane glasses at 77 K, which indicates that a cage effect is preventing the attainment of a reactive conformation in this medium. The assignment of electronic spectra of the primary (keto) cations and of their tautomeric (enol) forms is supported by CASSCF/CASPT2 and TD-B3LYP calculations.

Structure and Energetics of Vinoxide and the X(2A‘ ‘) and A(2A‘) Vinoxy Radicals

Enolate anions are intermediates in many organic reactions that involve carbon-carbon or carbon-oxygen food formation. They also play a key role in the development of stereoselective and stereocontrolled syntheses of complex compounds. Enolate radicals are important intermediates in combustion and photochemical smog cycles. In particular, the vinoxy radical, C{sub 2}H{sub 3}O{sup {sm{underscore}bullet}} is a major product of the reaction of odd oxygen and ethylene. The photoelectron spectrum of binoxide, C{sub 2}H{sub 3}O{sup {minus}}, at 355 nm is reported, showing photodetachment to both the X({sup 2}A{double{underscore}prime}) ground and first excited A({sup 2}A{prime}) states of the vinoxy radical. Both direct interpretations and Franck-Condon simulations of the photoelectron spectrum of this simple enolate anion have been used to obtain insights into the energetics and structures of the anion and the ground and first excited state of the neutral radical. Franck-Condon simulations were generated from ab initio geometry and frequency calculations using the CASSCF method and showed good agreement with the vibrational structure visible in the experimental spectrum. The electron affinity (E.A.{sub exp} = 1.795 {+-} 0.015 eV; E.A.{sub calc} = 1.82 eV) and separation energy of the ground and first excited states (T{sub 0,exp} = 1.015 {+-} 0.015 eV; T{sub 0,calc} = 0.92more » eV) obtained from the ab initio calculations are in good accord with the experimental values.« less

Oxygenation of Saturated Hydrocarbyl Groups in the Dinuclear Ni(III) Bis(μ-oxo) Complexes with the Hydrotris(pyrazolyl)borate Ligands TpR (R = Me3 and Pri2)

Abstract Intramolecular oxygenation of the Me and Pri substituents of TpR to give the carboxylate (–CO2−) and enolate (–CMe=CHO−) groups, respectively, is mediated by the Ni(III)2-bis(μ-oxo) complexes in the presence of an excess amount of H2O2.

The 4,4,4-Trifluoroacetoacetic Acid Keto−Enol System in Aqueous Solution. Generation of the Enol by Hydration of Trifluoroacetylketene

Trifluoroacetylketene was generated in aqueous solution by flash photolysis of 2,2-dimethyl-6-trifluoromethyl-4H-1,3-dioxin-4-one and its spiro analogue, 4-trifluoromethyl-1,5-dioxaspiro[5.5]undec-3-en-2-one, and rates of hydration of the ketene to 4,4,4-trifluoroacetoacetic acid enol as well as subsequent ketonization of the enol were measured in this solvent across the acidity range [H+] = 10-1−10-12 M. Analysis of the rate profile produced by these data provides the acidity constants pQa,E = 1.85 for the carboxylic acid group of the enol and p = 9.95 for its enolic hydroxyl group, which make these groups 2 and 3 orders of magnitude more acidic, respectively, than the corresponding groups in the parent unfluorinated acetoacetic acid enol. Rates of enolization of the keto group of 4,4,4-trifluoroacetoacetic acid were also measured, by bromine scavenging, and these, together with a value of the equilibrium constant for hydration of the keto group to its gem-diol derivative based upon a free energy relatio...

ChemInform Abstract: Enol Radical Cations in Solutions. Part 13. First Example of a Radical Dication Rearrangement. One‐Electron Oxidation of Dihydrobenzofuranyl Cations Leads to Drastic Rate Enhancement in the Oxidative Benzofuran Formation from Enols.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Antioxidant properties of EPC-K1: a study on mechanisms

Scavenging effects of l-ascorbic acid 2-[3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethytridecyl)-2 H-1-benzopyran-6-yl-hydrogen phosphate] potassium salt (EPC-K1) on hydroxyl radicals, alkyl radicals and lipid radicals were studied with ESR spin trapping techniques. The inhibition effects of EPC-K1 on lipid peroxidation were assessed by TBA assay. The kinetics of EPC-K1 reacting with hydroxyl radicals and linoleic acid radicals were studied by pulse radiolysis. The active site of EPC-K1 and the structure-antioxidative activity relationships were discussed. The superoxide radicals scavenging capacity of the brain homogenate of EPC-K1-treated rats was measured. The results revealed that in comparison with Trolox and vitamin C, EPC-K1 showed better overall antioxidative capacity in vitro and in vivo. EPC-K1 was a moderate scavenger on hydroxyl radicals and alkyl radicals, a potent scavenger on lipid radicals, and an effective inhibitor on lipid peroxidation. EPC-K1 could react with hydroxyl radicals with a rate constant of 7.1×10 8 dm 3 mol −1 s −1 and react with linoleic acid radicals with a rate constant of 2.8×10 6 dm 3 mol −1 s −1. The active site of EPC-K1 was the enolic hydroxyl group. After administration of EPC-K1, the ability of rat brain to scavenge superoxide radicals was significantly increased. The potent scavenging effects of EPC-K1 on both hydrophilic and hydrophobic radicals were relevant with its molecular structure, which consisted of both hydrophilic and hydrophobic groups.

Synthesis and characterisation of the first organotin complex of piroxicam. An extended network system via non-hydrogen, hydrogen bonding linkages and C–H · · · π contacts

The organotin complex, [SnBu2(pir)]n, of the potent and widely used anti-inflammatory drug piroxicam, H2pir, was obtained and a crystal structure determination showed that in this complex the ligand is doubly deprotonated at the oxygen and amide nitrogen atoms. There are two similar molecules in the asymmetric unit. The isolated molecules of Sn(1) or Sn(2) are arranged in polymers in a head to tail fashion with a stacking of alternate parallel chains. Extended networks of Sn–O–Sn, C–H· · ·O and C–H· · ·π contacts lead to aggregation and a supramolecular assembly. Real concentration protonation constants for the zwitterionic form (pyridyl group) and the protonated piroxicam (enolic group) were determined spectrophotometrically in pure aqueous solutions of constant ionic strength. It is the first example where piroxicam is proved to act as a doubly deprotonated tridentate ligand.

Zirconium enolate radical cations in solution—characterisation and kinetics of their mesolytic Zr–O bond cleavage †

Six zirconocene enolate radical cations and one hydroquinone biszirconocene radical cation have been generated and characterised in solution by standard and fast scan cyclic voltammetry. The electroanalytical results along with those from product analysis of preparative one-electron oxidations indicate that the sterically congested radical cations undergo a mesolytic Zr–O bond cleavage process. The kinetics of the mesolytic Zr–O bond cleavage yielding zirconocene cations and the lifetime (t½) of the zirconium enolate radical cations in dichloromethane at room temperature are determined.

Rameswaralide, a novel diterpenoid from the soft coral Sinularia dissecta

Rameswaralide ( 4), a novel diterpene having a 5-7-6 tricyclic skeleton has been isolated along with known terpenes, africanene, 4,5- seco-african-4,5-dione, β-elemene and isomandapamate ( 3) from the soft coral Sinularia dissecta from the Mandapam coast, South India. The structure and stereochemistry of rameswaralide ( 4) were determined using spectroscopic methods and confirmed by selective reduction of the unusually stable enol group of 4 to form dihydrorameswaralide ( 5).

Enol Radical Cations in Solution. 13. First Example of a Radical Dication Rearrangement. One-Electron Oxidation of Dihydrobenzofuranyl Cations Leads to Drastic Rate Enhancement in the Oxidative Benzofuran Formation from Enols.

The synthesis and electrochemical investigation of six stable, simple enols E1-E6 are reported that are characterized by electron-releasing substituents in the alpha-position. Oxidative benzofuran formation from these enols is unusually slow because a key intermediate in the reaction, the dihydrobenzofuranyl cation X(+), is substantially stabilized vs rearrangement by the attached electron-releasing substituents. The persistent cations X(+) were characterized by (1)H NMR and cyclic voltammetry, and the kinetics of their rearrangement was followed by UV/vis. Notably, upon one-electron oxidation of X(+) to the radical dication, the formation of the benzofurans B was markedly accelerated by a factor of >10(6).

Metabolites of the angiotensin II antagonist tasosartan: the importance of a second acidic group.

Described in this paper is the synthesis and pharmacological activity of five metabolites of the angiotensin II antagonist tasosartan (1). Of particular interest is the effect of the additional acidic group of the enol metabolite (8) on activity. As suggested by the structural-activity relationship of other angiotensin II antagonist series, a second acidic group can improve receptor binding activity but decrease in vivo activity after oral dosing. The metabolic introduction of a second acidic group in tasosartan bypasses this problem and contributes to the excellent profile of the compound. A molecular modeling study provides a rationale for the role of the enol group of 8 in AT1 receptor binding.

Topological Analysis of the Charge Density in Short Intramolecular O−H···O Hydrogen Bonds. Very Low Temperature X-ray and Neutron Diffraction Study of Benzoylacetone

A study of intramolecular hydrogen bonding in benzoylacetone (1-phenyl-1,3-butadione) has been carried out with 8.4(4)K X-ray data and 20(1)K neutron data. Analysis of the neutron data shows that the hydrogen, between the two oxygens in the keto−enol part of the molecular structure, is asymmetrically placed in a large flat potential well. The charge density obtained from X-ray and neutron data has been analyzed by using multipolar functions and topological methods, which gave evidence of extensive π-delocalization in the keto−enol group. The multipole populations show that there are large formal charges on the oxygens and the enol hydrogen, which impart polar character to the hydrogen bond. This effect is also evident in the Laplacian and in the electrostatic potential calculated from the X-ray data and it is found that the hydrogen position is stabilized by both electrostatic and covalent bonding contributions at each side of the hydrogen atom. The resonance assisted hydrogen bonding model has been refin...