Results 1 - 10 of 1473
Results 1 - 10 of 1473. Search took: 0.022 seconds
|Sort by: date | relevance|
[en] The published data on the use of 1,3-dipolar cycloaddition reactions of nitrile oxides in the synthesis of natural compounds and their analogues are systematised and reviewed. The bibliography includes 145 references.
[en] Elimination reactions of (E)-2,4-(NO2)2C6H2CH=NOC6H3-2-X-4-NO2 (1a-e) promoted by R3N/R3NH+ in 70 mol % MeCN(aq) have been studied kinetically. The reactions are second-order and exhibit Bronsted β = 0.80-0.84 and |βlg| = 0.39-0.42, respectively. For all leaving groups and bases employed in this study, the β and |βlg| values remained almost the same. The results can be described by a negligible pxy interaction coefficient, pxy = ∂β/pKlg = ∂βlg/pKBH ∼ 0, which describes the interaction between the base catalyst and the leaving group. The negligible pxy interaction coefficient is consistent with the (E1cb)irr mechanism. Change of the base-solvent system from R3N/MeCN to R3N/R3NH+-70 mol % MeCN(aq) changed the reaction mechanism from E2 to (E1cb)irr. Noteworthy was the relative insensitivity of the transition state structure to the reaction mechanism change
[en] Elimination reactions of (E)-2,4,6-(NO_2)_3C_6H_2CH=NOC(O)C_6H_4X (3) promoted by R_2NH/R_2NH_2"+ in 70 mol% MeCN (aq) have been studied. The reactions produced elimination products and exhibited second-order kinetics. The β and |β_l_g| values remained nearly the same for all leaving groups and bases. The results can be described by the negligible p_x_y interaction coefficient, p_x_y = ∂β/∂pK_l_g = ∂β_l_g/∂pK_B_H ≈ 0, which provides a strong support for the (E1cb)_i_r_r mechanism. For eliminations from (E)-ArCH=NOC(O)C_6H_4X (1, 3) and (E)-2,4,6-(NO_2)_3C_6H_2CH=NOAr' (2, 3), the change of the β-aryl group (Ar) from 2,4-dinitrophenyl (1) to 2,4,6-trinitrophenyl (3) increased the rate by 270-fold without appreciable change in the transition state structure. On the other hand, the leaving group (OAr') variation from benzoate (3) to 4-nitrophenoxy (2) induced a change in reaction mechanism from (E1cb)_i_r_r to E2. These results have been attributed to the cyclic transition state for the nitrile-forming eliminations involving the benzoate leaving group.
[en] The concise synthesis of a novel chiral diyne substrate for the assembly of chiral naphthylpyridines was described and different conditions for the cobalt-catalyzed co-cyclotrimerization with nitriles investigated. The products are novel naphthylpyridines possessing configurationally stable biaryl axes. Graphical abstract: .
[en] The published data on the chemistry of intermolecular 1,3-dipolar cycloaddition of nitrile oxides to different types of alkene derivatives are systematised. Various aspects of stereo- and regiochemistry of this reaction are considered. The bibliography includes 182 references.
[en] We have studied the nitrile-forming elimination reactions from 1 promoted by R2NH in MeCN. The reaction proceeded by (E1cb)irr mechanism. Change of the β-aryl group from 2,4-dinitrophenyl to a more strongly electron-withdrawing 2,4,6-trinitrophenyl increased the reaction rate by 470-fold, shifted the transition state toward more reactant-like, and changed the reaction mechanism from E2 to (E1cb)irr. To the best of our knowledge, this is the first example of nitrile-forming elimination reaction that proceeds by the (E1cb)irr mechanism in MeCN. Noteworthy is the carbanion stabilizing ability of the 2,4,6-trinitrophenyl group in aprotic solvent. Nitrile-forming elimination reactions of (E)-benzaldoxime derivatives have been extensively investigated under various conditions. The reactions proceeded by the E2 mechanism in MeCN despite the fact that the reactants have syn stereochemistry, poor leaving, and sp2 hybridized β-carbon atom, all of which favor E1cb- or E1cb-like transition state. Moreover, the transition state structures were relatively insensitive to the variation of the reactant structures. The results have been attributed to the poor anion solvating ability of MeCN, which favors E2 transition state with maximum charge dispersal. For eliminations from strongly activated (E)-2,4-(NO2)2C6H3CH=NOC(O)C6H4X, a change in the reaction mechanism from E2 to (E1cb)irr was observed as the base-solvent was changed from R2NH in MeCN to R2NH/R2NH2+ in 70 mol % MeCN(aq). A combination of a strong electron-withdrawing β-aryl group and anion-solvating protic solvent was required for the mechanistic change