DFT Mechanistic Study on the Reaction of Benzenesulfonyl Azideswith Oxabicyclic Alkenes

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The reaction of benzenesulfonyl azides with oxabicyclic alkenes to form aziridines, reported by Chen et al (J. Org. Chem.2019, 84, 18, 11863-11872), couldproceed via initial [3+2] cycloaddition to form triazoline intermediates followed by dinitrogen cleavage or via initial dinitrogen cleavage of the benzenesulfonyl azideto afford a nitrene intermediate followed by insertion of this species into the olefinic bond of the oxabicyclic alkene. Calculations at the DFTM06-2X/6-311G+(d,p) level showthat 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 isthe dinitrogen cleavage from triazoline cycloadducts which hasbarriers of 32.3 kcal/mol (endo) and 38.6 kcal/mol (exo) and that along the latter pathway isdinitrogen 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 endoandexotriazoline intermediatesis kinetically favoredover the dinitrogen cleavage from benzenesulfonyl azide by 21.6 and 28.1 kcal/mol for endoand exopathway 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 endoaziridine compared toexoisomer means that the endoproduct will be formed as the major product, confirming the 2experimental observation.The position of substituents on the benzene group of the benzenesulfonyl azide greatly affectsthe endo/ exodiastereoselectivity. Keywords: Aziridines, oxabicyclic alkene, benzenesulfonyl azides, triazoline, dinitrogen extrusion, cycloaddition
An article published by Beilstein-Institut and also available at doi: https://doi.org/10.3762/bxiv.2020.57.v1