Valence isomerization of cyclohepta-1,3,5-triene and its heteroelement analogues

Abstract

SummaryThe valence isomerization of the all-carbon and heteroelement analogues of cyclohepta-1,3,5-triene into the corresponding bicyclo[4.1.0]hepta-2,4-dienes is reviewed to show the impact of the heteroatom on the stability of both valence isomers. The focus is on the parent systems and their synthetic applications.

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

“…Simple amine inversion is also a facile interconversion pathway between 6a and 6b while N -inversion of aziridines ( 7a and 7b ) have much higher barriers [ 9 ]. The large difference in enthalpy between 6 and 7 is consistent with the computational and experimental comparisons with CH 2 , O, S, and P analogs of N in the equilibrium between 6 and 7 [ 10 ]. Scheme 2 depicts the interconversion pathways between the corresponding N -oxides of 6 ( 8a and 8b ) and 7 ( 9a and 9b ).…”

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

“…Cyclic 8π‐electron molecules in their S 0 states normally adopt non‐planar structures that are non‐aromatic rather than antiaromatic. Cyclooctatetraene (COT) in S 0 adopts a tub‐shaped geometry, avoiding the angle strain at the planar D 4h symmetric structure, and also azepines, oxepines, and thiepines adopt puckered conformations . The resonance energies of the planar structures of azepine and oxepines obtained through extended Hückel MO theory and early Hartree‐Fock computations suggested antiaromatic character, later supported by NICS calculations .…”

“…CASSCF/6-31G(d) including all the π-orbitals in the active space were carried out at TD-PBE0 geometries to evaluate the configurational weights and verify the character of the S 1 state. For molecules containing seven and six members in the central ring the active space used was 16 electrons in 15 orbitals (CASSCF (16,15) and 16 electrons in 14 orbitals (CASSCF (16,14), respectively. For all the molecules investigated, the S 1 state has a single-excitation character, and with regard to the triplet states all molecules with ππ* T 1 states have < S 2 > below 2.0014.…”

“…Cyclooctatetraene (COT) in S 0 adopts a tub-shaped geometry, avoiding the angle strain at the planar D 4h symmetric structure, [12] and also azepines, oxepines, and thiepines adopt puckered conformations. [13][14][15][16][17][18][19] The resonance energies of the planar structures of azepine and oxepines obtained through extended Hückel MO theory and early Hartree-Fock computations suggested antiaromatic character, [20,21] later supported by NICS calculations. [14,22] However, the dibenz[b,f]annelated derivatives were found to have positive resonance energies associated with some aromatic character as their benzene rings keep their Hückel aromaticity in line with Glidewell-Lloyd's extension of Clar's rule.…”

[d]-Carbon–carbon double bond engineering in diazaphosphepines: a pathway to modulate the chemical and electronic structures of heteropines

“…Previous studies have focused on their fundamental characteristics, including their conformational change, chemical reactivity and electronic structure. 33 – 40 With the exception of Group 13-element-containing aromatic derivatives, 41 – 52 these heteropines generally adopt “boat-like” structures and constantly undergo conformational inversion in solution, severely limiting backbone π-conjugation. 52 – 56 …”