Predicted Reversal in N-Methylazepine/N-Methyl-7-azanorcaradiene Equilibrium upon Formation of Their N-Oxides

Abstract

Density functional calculations and up to five different basis sets have been applied to the exploration of the structural, enthalpy and free energy changes upon conversion of the azepine to the corresponding N-oxide. Although it is well known that azepines are typically much more stable than their 7-azanorcaradiene valence isomers, the stabilities are reversed for the corresponding N-oxides. Structural, thermochemical as well as nucleus-independent chemical shift (NICS) criteria are employed to probe the potential aromaticity, antiaromaticity and nonaromaticity of N-methylazepine, its 7-azanorcaradiene valence isomer. For the sake of comparison, analogous studies are performed on N-methylpyrrole and its N-oxide.

The Bond Dissociation Energy of the N–O Bond

“…This also provides a plausible explanation for why attempted alkylation of pyridine- N -oxide results in C2-alkylation. , Another example of a weakened N–O bond involving resonance stabilization has recently been reported for the unknown nonaromatic N -methylpyrrole- N -oxide. This N-oxide, if it could exist with such a weak N–O bond, has a calculated N–O BDE of only 13–18 kcal/mol since N–O bond dissociation yields the aromatic N -methylpyrrole with a resonance energy of 24–27 kcal/mol …”

“…This Noxide, if it could exist with such a weak N−O bond, has a calculated N−O BDE of only 13−18 kcal/mol since N−O bond dissociation yields the aromatic N-methylpyrrole with a resonance energy of 24−27 kcal/mol. 27 It is also of interest to compare the relative N−O BDE of pyridine-N-oxide to that of simple trisubstituted amine oxides. Intuitively, it would seem that these two classes of amine oxides should have different N−O BDEs because of the aromatic nature of the former.…”

Strained Small Nitrogen Heterocycles‐Azabicyclobutanes and Azirines

“…Let us now estimate this quantity with renewed confidence in our analysis of a more “normal” species. From our earlier quantum chemical calculations [62] the enthalpy of hydrogenation of its N‐methyl derivative to form N‐methylpyrrolidine was suggested to be 103 kJ/mol. N‐methylation is expected to have a small effect on the value of enthalpy of hydrogenation and so the enthalpy of hydrogenation of 2,3‐dihydropyrrole to form pyrrolidine is taken as 103 kJ/mol as well.…”

“…We could have chosen the value for its (Z)isomer where we note the CÀ C double bond in cyclopropene is (Z). Arbitrarily we have preferences for more stable isomers and since the (Z)-/(E)-enthalpy of formation difference for the 2pentenes is but 4 kJ/mol, [60][61][62] nothing is conceptually, or even chemically, more than minimally affected by our choice of isomer.…”

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

“…8 Ragyanszki and co-workers studied the oxidation of the anti-aromatic N -methyl-1 H -azepine to the non-aromatic N -oxide of azepine. 9 1 H -Azepine, and its derivatives are also known to undergo rapid dimerization via (6 + 4)π exo -cycloaddition. 10 …”