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The kinetic resolution of Z and E olefins by [2+2] cycloaddition with ketenes allows the isolation of pure E olefin, as well as the synthesis of pure cis-cyclobutanones, starting from Z/E mixtures. A computational rationale for this kinetic difference is reported. The obtained difference of energy of activation matches with the experimental results. [ABSTRACT FROM AUTHOR]

The mechanism of the Pauson-Khand reaction has been studied by mass spectrometry and it has been found, through ion-molecule reaction with 13CO, that the carbon monoxide incorporated into the product cyclopentenone is one that has been retained within the complex. Theoretical and kinetic calculations support this finding, which provides a complementary explanation for the effect of Pauson-Khand promoters. [ABSTRACT FROM AUTHOR]

Theoretical calculations at the DFT level indicate that the reaction of a nitrone with a silyl ketene acetal proceeds, contrary to previous suggestions, through a classical 1,3-dipolar cycloaddition, followed by a silyl group transfer step to give the open-chain product. Introduction of a diffuse basis set is necessary to describe properly nitrones. The influence of a Lewis acid has been studied. © 2005 Wiley Periodicals, Inc. J Comput Chem 27: 157–162, 2006 [ABSTRACT FROM AUTHOR]

Studies the cyclization reactions of the 4-pentenyl, 5-hexenyl, 6-heptenyl and 7-octenyl fluorinated and ether radicals through Hartree-Fock density functional method. Factors for the determination of the regiochemistry and the reactivity of the reactions; Enthalpic effect of the reaction; Intermolecular radical additions to double bonds.

The procedure uses the high difference in reactivity and thus allows the access to pure (E) alkenes, often difficult or impossible to isolate from the mixtures or expensive in trade. [ABSTRACT FROM AUTHOR]

Abstract: The 1,3-dipolar cycloadditions of cyclic nitrones to five-membered unsaturated lactones are studied. Special attention was focused on a single and double asymmetric induction when one or both components were chiral. The energies of the cycloaddition reactions are investigated through application of molecular orbital calculations at the B3LYP/6-31+G(d) theory level. A study of the different reactants’ approaches and their conformational aspects revealed the stereochemical preferences of these reactions. The results of these calculations correspond well with the previously reported experimental data. [Copyright &y& Elsevier]

Abstract: The 1,3-dipolar cycloaddition of cyclic nitrones with electron-poor and electron-rich cyclic dipolarophiles (α,β-unsaturated lactones and vinyl ethers) is studied. The energies of the cycloaddition reactions have been investigated through molecular orbital calculations at the B3LYP/6-31+G(d) level theory. Different reaction channels and reactants approaches, effective in regio- and stereochemical preferences are discussed. The results were compared with experimental data to find a good agreement. [Copyright &y& Elsevier]

Abstract: We have performed a detailed study of the cyclization of carbonyl groups on unactivated alkynyl-quinolines. Two cyclization modes are accessible: the exo vs. endo mode. The influence of the substituents of the alkynyl-quinolines have been studied using two examples giving different reactivity: the phenyl and the CH2OMe substituent. From an experimental point of view, the reaction occurs in basic medium and the first step of the reaction has been shown to be the formation of the hemi-acetal. Two mechanisms have been investigated with and without deprotonation of the hemi-acetal prior to the carbon–oxygen bond formation and the proton’s transfer. The influence of the solvent effects has been studied using supermolecular and continuum/supermolecular techniques. The mechanism without deprotonation of the hemi-acetal presents high barriers and the inclusion of solvent effects both with the continuum method and the supermolecular approach stresses strongly this tendency especially when phenyl substituents are used. On the other hand, the base mechanism exhibits lower barriers but solvent effects also increase these barriers, which remain quite reasonable around 15kcalmol−1. Whatever the methods used, the exo mode of reaction appears to be the kinetic pathway whereas the endo mode appears to be under thermodynamic control. [Copyright &y& Elsevier]