The reaction of benzenesulfonyl azides with oxabicyclic alkenes to form aziridines, reported by Chen et al (J. Org. Chem. 2019, 84, 18, 11863-11872), could proceed via initial [3+2] cycloaddition to form triazoline intermediates followed by dinitrogen cleavage or via initial dinitrogen cleavage of the benzenesulfonyl azide to afford a nitrene intermediate followed by insertion of this species into the olefinic bond of the oxabicyclic alkene. Calculations at the DFT M06-2X/6-311G+(d,p) level show that the initial [3+2] cycloaddition has barriers of 17.3 kcal/mol (endo) and 10.2 kcal/mol (exo) while the initial nitrogen extrusion step has a barrier of 38.9 kcal/mol. The rate-determining step along the former pathway is the dinitrogen cleavage from triazoline cycloadducts which has barriers of 32.3 kcal/mol (endo) and 38.6 kcal/mol (exo) and that along the latter pathway is dinitrogen cleavage from benzenesulfonyl azide with an activation of barrier of 38.9 kcal/mol. The [3+2] addition of benzenesulfonyl azide with oxabicyclic alkene to afford endo and exo triazoline intermediates is kinetically favored over the dinitrogen cleavage from benzenesulfonyl azide by 21.6 and 28.1 kcal/mol for endo and exo pathway respectively. Thus, the preferred pathway for the reaction of oxabicyclic alkene with benzenesulfonyl azide is via initial [3+2] addition followed by dinitrogen cleavage, contrary to the proposal by Chen et al. The lower activation barrier for the dinitrogen extrusion step leading to endo aziridine compared to exo isomer means that the endo product will be formed as the major product, confirming the experimental observation. The position of substituents on the benzene group of the benzenesulfonyl azide greatly affects the endo / exo diastereoselectivity.
Keywords: Aziridines, oxabicyclic alkene, benzenesulfonyl azides, triazoline, dinitrogen extrusion, cycloaddition.
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Akuamoah, D. A.; Tia, R.; Adei, E. Beilstein Arch. 2020, 202057. doi:10.3762/bxiv.2020.57.v1
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