Homoheteroaromaticity: the case study of azepine and dibenzazepineElectronic supplementary information (ESI) available: absolute chemical shieldings calculated at the B3LYP/6-311++G**//B3LYP/6-311++G** computational level for compounds 1b, 2b, 4b and 5b. See http://www.rsc.org/suppdata/ob/b3/b314742h/

Abstract

Geometrical and energetic DFT calculations as well as GIAO and NICS chemical shifts have been calculated for 1H-azepine and 5H-dibenz[b,f]azepine and their cations. The last compound has been studied experimentally by 1H and 13C NMR in neutral and acidic conditions establishing that the cation corresponds to an N-protonated structure. The conclusion is that the neutral molecules are antiaromatic while the cations are aromatic (homoheteroaromaticity).

Computational NMR Spectroscopy

“…Besides heterocycles, we have discussed the cases of benzene versus cyclohexatriene [86], compounds related to the Mills-Nixon effect, dehydroannulenes, polyacenes, s-indacene, and M€ obius rings [87], as well as the structure of homotropylium cation [63] and that of the cation obtained by protonation of 5 [29]. We have devoted several papers to the aromaticity of azoles (related to those of Scheme 2.4) and to the effect of their protonation (azolium salts) [88][89][90][91].…”

Heterocumulenes reactions with organometallic reagents: XV. Quantum-chemical investigatiom of skeleton rearrangements of 2-methyl-6-methoxy-3H-azepine originating from valence tautomerism

“…The azepine structures find also interesting applications in the organic synthesis [5]. Besides these heterocycles are convenient models for further investigation of aromaticity and antiaromaticity [6][7][8], for the study of valence tautomerism [8][9][10][11][12][13][14][15][16][17][18], sigmatropic rearrangements [16,17,19], various stereochemical problems (transannular effect of the unshared electron pair of the heteroatom, formation of intramolecular hydrogen bonds) [2,13,16,20], for quantum-chemical calculations [6, 11-13, 20, 21]. The heightened interest to the seven-membered azaheterocycles stimulates the search for and development of new and at the same time simpler, cheaper and more general procedures for their preparation, in particular, those that lead to new derivatives, for the range of available representatives of this class compounds is still narrow.…”

“…The azepine molecules (according to XRD and NMR data existing mainly in the boat conformation) are known [2] to be fairly labile that shows in slow stereochemical transformations, sigmatropic rearrangements with the migration both of hydrogen and heteroatomic substituents (e.g., SR, OR) [6,19], and in various electrocyclic…”

“…For instance, previously unknown and difficultly available by another route 2-methyl-6-methoxy-3H-azepine (Ia) and 7-methyl-2-(methyl-sulfanyl)-3-methoxy-4,5-dihydro-3H-azepine (II) were synthesized from α-lithiated methoxyallene, isopropyl isothiocyanate, and methyl iodide along Scheme 1 [23]. In the absence of the deprotonating base 1-aza-1,3,4-triene III and the product of its [1,5]-sigmatropic rearrangement, 2-aza-1,3,5-triene IV, undergo thermally induced intramolecular cyclization reactions with the formation of pyrrole V and 2,3-dihydropyridine VI [24].The azepine molecules (according to XRD and NMR data existing mainly in the boat conformation) are known [2] to be fairly labile that shows in slow stereochemical transformations, sigmatropic rearrangements with the migration both of hydrogen and heteroatomic substituents (e.g., SR, OR) [6,19], and in various electrocyclic …”

“…The most probable typical concerted reactions were analyzed of the formation of valence isomers, fused three-/six-and four-/five-membered carbo-and heterocycles, azabicyclo[4.1.0]hepta-2,4-dienes (azanorcaradienes) and azabicyclo[3.2.0]hepta-3,6-dienes respectively. Azepines (azacycloheptatrienes) and their hydrogenated derivatives were recently described in many synthetic, theoretical, and medical publications [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], first of all owing to the biological activity of these substances [2][3][4]. Azepine derivatives find wide and versatile application in pharmacology, mainly as psychotropic, antidepressant and anticonvulsant drugs, and also as antiviral, antibacterial, and anticancer pharmaceuticals [4].…”