E/Z Conformational equilibrium of N-substituted 2H-pyran-2-imines

Abstract

Prevailing Z-conformations were found in N-aryl(or alkyl)substituted 2H-pyran-2-imines 4a-e on the NMR time scale. Conformational assignment was based on 1 H and 13 C NMR chemical shifts as well as on induced shifts by LSR. The barrier of E/Z interconversion ( ) was calculated from variable temperature ≠ ∆ Tc G 1 H-NMR spectra.

The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

“…It is noteworthy that the angle difference Δ exo is smallest for the trans isomer 17c (+1.8 o ) and largest for the cis isomer 17b (+15.0 o ); this closely parallels the analogous structures 10 and 11 (Table 1). In agreement with experimental data [25], the trans isomer 17c is calculated to be the more stable (ΔG rel -0.23 kcal mol -1 ) (Table 5).…”

“…To explore whether similar anomeric effects are found in other heterocyclic rings with exocyclic substituents, we have calculated properties of the pyrone derivatives 17a-d. N,4,6-Triphenypyran-2-imines are known [23,24], and Uncuţa and coworkers, in a detailed NMR study, have shown that in equilibrium mixtures, e.g., 17b,c, the trans (Z) configuration prevails [25].…”

The influence of exocyclic lone pairs on the bonding and geometry of type A mesoionic rings

“…It is noteworthy that the angle difference ∆ exo is smallest for the trans isomer 17c (+1.8 o ) and largest for the cis isomer 17b (+15.0 o ); this closely parallels the analogous structures 10 and 11 (Table 1). In agreement with experimental data [25], the trans isomer 17c is calculated to be the more stable (∆G rel -0.23 kcal mol -1 ) (Table 3).…”

“…To explore whether similar anomeric effects are found in other heterocyclic rings with exocyclic substituents, we have calculated properties of the pyrone derivatives 17a-d. N,4,6-Triphenypyran-2-imines are known [23,24], and Uncuţa and coworkers, in a detailed NMR study, have shown that in equilibrium mixtures, e.g., 17b,c, the trans (Z) con guration prevails [25]. Table 4 reveals that the structural changes in the series 17a-d are similar to the trends in Tables 1 and 2.…”

“…Overall the dicyano derivatives have comparable properties to oxygen and nitrogen analogues and are realistic hyperconjugation-free references structures for studying the in uence of lone pairs. To explore whether similar anomeric effects are found in other heterocyclic rings with exocyclic substituents, we have calculated properties of the pyrone derivatives 17a-d. N,4,6-Triphenypyran-2-imines are known [23,24], and Uncuţa and coworkers, in a detailed NMR study, have shown that in equilibrium mixtures, e.g., 17b,c, the trans (Z) con guration prevails [25].…”