Stereoelectronic and dynamical effects dictate nitrogen inversion during valence isomerism in benzene imine
Nilangshu Mandal1, Ankita Das1, Chandralekha Hajra1
1School of Chemical Sciences, Indian Association for the Cultivation of Science 2A and 2B Raja S. C. Mullick Road, Jadavpur Kolkata 700032 West Bengal India spad@iacs.res.in +91-33-24734971.
Abstract:
Benzene imine (1) ⇌ 1H-azepine (2) isomerization occurs through sequential valence and endo-exo 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 (S T) ∼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 1a → 2a 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.
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