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Peroxy Acid Epoxidation of Acyclic Allylic Alcohols. Competition between s-trans and s-cis Peroxy Acid Conformers
No full textRB3LYP calculations, reported here, indicate that peroxy acid s-cis conformer is more stable than its s-trans counterpart, in agreement with experimental data. Difference in stability is the highest in the gas phase, but it falls considerably on going from the gas phase to moderately polar solvent. In the case of peroxy formic acid, the enthalpy (free energy) difference is about 3.4 (2.5) kcal/mol, respectively, in the gas phase but decreases to 1.2 (0.6) kcal/mol in dichloromethane solution.
Introduction of an alkyl or aryl substituent on the peroxy acid, that is, on passing to peroxy acetic, peroxy benzoic (PBA), and m-chloroperoxy benzoic acid (MCPBA), adds a further significant (1.0-1.5 kcal/mol) favor to the s-cis isomer. RB3LYP/6-31+G(2d,p) calculations on the epoxidation of 2-propenol with peroxy formic and peroxy benzoic acids, respectively, suggest that the less stable peroxy acid s-trans conformer can compete with the more stable s-cis form in epoxidation reaction of these substrates. Transition structures arising from s-trans peroxy acids (“trans” TSs) retain both the well-established, for “cis” TS, perpendicular orientation of the O-H peroxy acid bond relative to the CdC bond and the one-step oxirane ring formation. These TSs collapse to the final epoxide via a 1,2-H shift at variance with the 1,4-H transfer of the classical Bartlett’s “cis” mechanism. The “trans” reaction pathways have a higher barrier in the gas phase than the “cis” reaction channels, but in moderately polar solvents they become competitive. In fact, the “trans”
TSs are always significantly more stabilized than their “cis” counterparts by solvation effects. Calculations also suggest that going from peroxy formic to peroxy benzoic acid should slightly disfavor the “trans” route relative to the “cis” one, reflecting, in an attenuated way, the decrease in the peroxy acid s-trans/s-cis conformer ratio. The predicted behavior for MCPBA parallels that of PBA acid
Hydrogen bonding effects in the epoxidation of propenol with dioxiranes. A DFT computational study
No full textPotential-energy surfaces for the epoxidns. of CH2:CHCH2OH (I) with dioxirane (II) and dimethyldioxirane (III) were studied at the B3LYP/6-31G* level. Seven transition structures (TSs) were located for epoxidn. with II. The four chem. more significant TSs were located also for the reaction with III. Geometries and energies of 2 of them clearly show that stabilizing H-bonding interactions can be at work, and that they involve both dioxirane O atoms. Calcns. indicate that the electron-attracting effect of the allylic hydroxy group has a relatively small rate-retarding effect
Structure-Activity Relationship in Dihydropteroate Synthase Inhibition by Sulfanilamides. Comparison with the Antibacterial Activity.
No full textStructure-Activity Relationship in Dihydropteroate Synthase Inhibition by Sulfanilamides. Comparison with the Antibacterial Activity
Facial selectivity in 1,3-dipolar cycloadditions to cis-3,4-dimethylcyclobutene. An experimental and computational study
No full textFacial selectivity in 1,3-dipolar cycloaddn. of diazomethane (2a), 3,4-dihydroisoquinoline N-oxide (2b), pyrroline N-oxide (2c), 5,5-dimethylpyrroline N-oxide (2d) and several nitrile oxides with cis-3,4-dimethylcyclobutene (1) has been investigated. The stereochem. of the cycloaddn. of 2a, 2b-2d and encumbered nitrile oxides (2i and 2j) is controlled by steric interactions with dominant formation of the anti diastereoisome
New paradigms for the peroxy acid epoxidation of CC double bonds: the role of the peroxy acid s-trans conformer and of the 1,2-H transfer in the epoxidation of cyclic allylic alcohols
No full textRB3LYP calculations, on reaction of performic acid with cyclic allylic alcohols, demonstrate that
the less stable s-trans conformer of peroxy acids can be involved in epoxidations of CdC bonds.
Transition structures (TSs) arising from s-trans performic acid retain some of the well-established
characteristics of the TSs of the s-cis isomer such as the perpendicular orientation of the O-H
peroxy acid bond relative to the CdC bond and a one-step oxirane ring formation. These TSs are
very asynchronous but collapse directly (without formation of any intermediate) to the final epoxideperoxy
acid complex via a 1,2-H shift. Thus, our findings challenge the traditional mechanism of
peroxy acid epoxidation of CdC bonds by demonstrating that the involvement of the s-trans isomer
opens an alternative one-step reaction channel characterized by a 1,2-H transfer. This novel reaction
pathway can even overcome, in the case of the reaction of cyclic allylic alcohols in moderately polar
solvents (e.g., in dichloromethane), the classical Bartlett’s mechanism that is based on the s-cis
peroxy acid form and that features a 1,4-H shift. However, the latter mechanism remains strongly
favored for the epoxidation of normal alkenes
DFT computational study of the epoxidation of olefins with dioxiranes
No full textTransition structures (TSs) of the reactions of dioxirane and dimethyldioxirane with ethylene, propene, cis-2-butene and trans-2-butene were located with the B3LYP/6-31G* method. The TSs of the reactions of ethylene and cis-2-butene exhibit a sym. spiro butterfly structure with synchronous formation of the two new C-O bonds and with substantial alignment of π bond axis with the breaking O-O bond
Facial selectivity in the reactions of 1,3-dipoles with cis- and trans-3,4-dimethyl-1-methoxycarbonylcyclobutenes
No full textAn Ab-initio theoretical study of the electrochemical grafting process of alkynil(aryl)iodonium salts on glassy carbon surfaces
No full textThe behaviour of alkynil(aryl)iodonium salts upon electrochemical reduction, on glassy carbon electrodes, is here studied by comparing both theoretical and experimental results. In particular, experimental results are obtained by means of cyclic voltammetry and chronocoulometric measurements as well as by collecting XPS spectra, while the standard electrode potential and different dissociation paths (I-C, arylic, compared to the I-C, alkynil, bond cleavage) have been characterized theoretically at the B3LYP/3-21g** level of the theory (the solvation free Gibbs energy of the cation and of the neutral radical species have been calculated by using the CPCM method)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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