Issue 3, 2022

Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine

Abstract

Benzene imine (1) ⇌ 1H-azepine (2) isomerization occurs through sequential valence and endoexo isomerism. Quantum chemical and quasiclassical trajectory (QCT) simulations reveal the coupled reaction pathway – ring-expansion followed by N-inversion to the most stable isomer, exo-1H-azepine (Exo-2). Direct-dynamics produce a mixture of endo- and exo-1H-azepine stereoisomers and govern the endo-1H-azepine (Endo-2) ⇌ exo-1H-azepine (Exo-2) ratio. Exo-2 is computationally identified as the most stable product while Endo-2 is fleetingly stable with a survival time (ST) ∼50 fs. N-Methyl substitution exclusively results in an exo-1-methyl-1H-azepine isomer. F-substitution at the N-site increases the barrier for N-inversion and alters the preference by stabilizing Endo-2. Interestingly, the exo-1-fluoro-1H-azepine (minor product) is formed through bifurcation via non-statistical dynamics. A highly concaved Arrhenius plot for 1a2a highlights the influence of heavy-atom tunneling on valence isomerism, particularly at low temperatures. Heavy-atom tunneling also results in a normal N–H(D) secondary KIE above 100 K even though the increase in hybridization from sp2 to sp3 at nitrogen should cause an inverse KIE classically.

Graphical abstract: Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine

Supplementary files

Article information

Article type
Edge Article
Submitted
02 Sep 2021
Accepted
14 Dec 2021
First published
14 Dec 2021
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2022,13, 704-712

Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine

N. Mandal, A. Das, C. Hajra and A. Datta, Chem. Sci., 2022, 13, 704 DOI: 10.1039/D1SC04855D

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