A DFT and ab initio study of conjugated and semi-conjugated mesoionic rings and their covalent isomers

Influence of Endo- and Exocyclic Heteroatoms on Stabilities and 1,3-Dipolar Cycloaddition Reactivities of Mesoionic Azomethine Ylides and Imines

“…22 Our observations are in agreement with recent DFT studies of similar compounds, where bonding, charges, and magnetic analyses confirmed the absence of aromaticity. 23 Although it is known that multiple resonance forms are required to properly describe mesoionic compounds, 23b one has to be chosen for drawing purposes. We propose that resonance structure I , which shows a C═Y double bond and highlights the 1,3-dipolar nature of the ring systems, is more consistent with the actual geometries of AY and should be used.…”

“…22 Schleyer previously devised a general homodesmotic equation to assess the aromaticity of five-membered heterocycles (eq 1), 24 which was recently applied to some heterocyclic betaines. 23c, 25 When applied to the most commonly used resonance structures of AY 1–12 (e.g., 1 in eq 2), this scheme predicted that most of them are anti-aromatic (negative Δ E , Table S4). In addition, it predicted that the most reactive dipoles (see below) are also the most aromatic.…”

The relative stability of mesoionic and N-heterocyclic carbene tautomers: a computational study of substituent effects

“…This distortion is almost certainly attributable to an anomeric effect due to the exocyclic hetero substituent; an effect already recognized in mesoionic rings. 6,8 This provides further evidence of the hybrid NHC-mesoionic character of the anions 8g and 8h, which show some features characteristic of both species.…”

“…The geometries of the mesoionic rings 7a-d are in accord with known X-ray crystal structures of mesoionic rings and previous DFT calculations. 8 Notable features are the long C 3 -N 4 bonds (1.43-1.46 Å) and the large N 2 -C 3 -X angles (133-135 o ), both of which are attributable to an anomeric effect associated with the exocyclic substituent X. 6,8 As expected from solid state studies, the N 1 -C 5 -N 4 angles in the carbenes 8 (av.…”

Formation of N‐Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2‐(Imidazolium‐1‐yl)phenolates

“…In 2013, based on a matrix‐connectivity approach, supported by calculations, and in an extension of earlier theories, a comprehensive classification system of heterocyclic mesomeric betaines was developed by Ramsden and co‐workers.…”