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Combined experimental and theoretical studies of regio- and stereoselectivity in reactions of ?-isoxazolyl- and ?-imidazolyl enamines with nitrile oxides.

ABSTRACT: Reactions of ?-azolyl enamines and nitrile oxides were studied by both experimental and theoretical methods. (E)-?-(4-Nitroimidazol-5-yl), (5-nitroimidazol-4-yl) and isoxazol-5-yl enamines smoothly react regioselectively at room temperature in dioxane solution with aryl, pyridyl, and cyclohexylhydroxamoyl chlorides without a catalyst or a base to form 4-azolylisoxazoles as the only products in good yields. The intermediate 4,5-dihydroisoxazolines were isolated as trans isomers during the reaction of (E)-?-imidazol-4-yl enamines with aryl and cyclohexylhydroxamoyl chlorides. Stepwise and concerted pathways for the reaction of ?-azolyl enamines with hydroxamoyl chlorides were considered and studied at the B3LYP/Def2-TZVP level of theory combined with D3BJ dispersion correction. The reactions of benzonitrile oxide with both E- and Z-imidazolyl enamines have been shown to proceed stereoselectively to form trans- and cis-isoxazolines, respectively. The preference of E-isomers over Z-isomers, driven by the higher stability of the former, apparently controls the stereoselectivity of the investigated cycloaddition reaction with benzonitril? oxide. Based on the reactivity of azolyl enamines towards hydroxamoyl chlorides, a novel, effective catalyst-free method was elaborated to prepare 4-azolyl-5-substituted isoxazoles that are otherwise difficult to obtain.

SUBMITTER: Efimov IV

PROVIDER: S-EPMC5238569 | biostudies-literature | 2016

REPOSITORIES: biostudies-literature

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Combined experimental and theoretical studies of regio- and stereoselectivity in reactions of β-isoxazolyl- and β-imidazolyl enamines with nitrile oxides.

Efimov Ilya V IV Shafikov Marsel Z MZ Beliaev Nikolai A NA Volkova Natalia N NN Beryozkina Tetyana V TV Dehaen Wim W Fan Zhijin Z Grishko Viktoria V VV Lubec Gert G Slepukhin Pavel A PA Bakulev Vasiliy A VA

Beilstein journal of organic chemistry 20161115

Reactions of β-azolyl enamines and nitrile oxides were studied by both experimental and theoretical methods. (<i>E</i>)-β-(4-Nitroimidazol-5-yl), (5-nitroimidazol-4-yl) and isoxazol-5-yl enamines smoothly react regioselectively at room temperature in dioxane solution with aryl, pyridyl, and cyclohexylhydroxamoyl chlorides without a catalyst or a base to form 4-azolylisoxazoles as the only products in good yields. The intermediate 4,5-dihydroisoxazolines were isolated as <i>trans</i> isomers duri ...[more]

PMID: 28144307

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Project description:This work presents the study of a series of electrocyclic reactions with the main aim of obtaining new insights into the reaction process along IRCs. The energy variation of the different reaction paths as well as the different transition states have been calculated. These trends are according to the experimental data. The natural bond orbitals have been obtained and the second order perturbational theory analysis has been carried out to determine the main charge transfers due to delocalization. Bond reactivity indexes have been used to describe the reactivity mechanism in a local way. These reactivity indexes are also based on NBOs and this has made it possible to connect the results of the indexes with the previous analysis. To determine quantitatively the bond structure, we used the quantum theory of atoms in molecules and we have hereby completed the information obtained from the NBO analysis. Finally, we used the Hirshfeld population analysis as an approximation to understand how the load density changes in the different reaction pathways, and we have connected these variations with the information obtained from the bond structure. The results has found that the reaction path with the lowest energy barrier Transition State Inward Conrotatory (TSIC) or Transition State Outward Conrotatory (TSOC) is determined by two magnitudes: the charge donations by delocalisation of the substituents (which we obtained from the Second Order Perturbational Theory Analysis of the NBOs) and in the case that these donations were very similar, the non-covalent interactions dominated (which we studied by means of the interaction energies of the Hirshfeld charges). Additionality, the most important factor influencing the lower energy reaction path was the interaction of lone pairs of the substituents with the σ∗(C-C) bond that is broken at the opening of the cycle. The alignment of these lone pairs with the C-C bond favours charge donation between them and, as can be seen in the discussion, this alignment varies depending on whether the structure is TSIC and TSOC.

Dataset Information

Combined experimental and theoretical studies of regio- and stereoselectivity in reactions of ?-isoxazolyl- and ?-imidazolyl enamines with nitrile oxides.

Publications

Combined experimental and theoretical studies of regio- and stereoselectivity in reactions of β-isoxazolyl- and β-imidazolyl enamines with nitrile oxides.

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