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1 H‐NMR Spectra of Cyclohexa‐1,4‐dienes and Cyclohexenes Annellated to Bicyclo[2.2.1]hept‐2‐enes. The Inter‐Ring Homoallylic H,H Coupling Constants as Stereochemical Probes
Author(s) -
Mahaim Cyril,
Carrupt PierreAlain,
Vogel Pierre
Publication year - 1985
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19850680814
Subject(s) - bicyclic molecule , chemistry , allylic rearrangement , vicinal , coupling constant , ring (chemistry) , methylene , stereochemistry , chemical shift , nmr spectra database , ene reaction , spectral line , double bond , crystallography , computational chemistry , medicinal chemistry , physics , organic chemistry , catalysis , particle physics , astronomy
The 360‐MHz‐ 1 H‐NMR spectra of cyclohexa‐1,4‐dienes and cyclohexenes annellated to bicyclo[2.2.1]hept‐2‐enes and 7‐oxabicyclo[2.2.1]hept‐2‐enes show inter‐ring homoallylic coupling constants between the bridgehead protons of the bicyclo[2.2.1]heptenes and the exo ‐protons of the allylic methylene groups (0.8 ± 0.15 Hz for bicyclo[2.2.1]hept‐2‐enes; 0.8–1.4 Hz for 7‐oxabicyclo[2.2.1]hept‐2‐enes). Contrastingly, the corresponding coupling between the bridgehead protons and the endo ‐protons is absent. The observed values are compared with those calculated by the INDO and CNDO/2 methods and discussed in the light of the bicyclo[2.2.1]hept‐2‐ene bond π‐anisotropy. Vicinal as well as intra‐ring homoallylic coupling constants are consistent with a small puckering of the cyclohexa‐1,4‐diene rings toward the endo ‐face. The allylic exo ‐methylene protons are more deshielded than the endo ‐protons independent of the nature of the substituents, the nature of the bridges, and the degree of unsaturation of the annellated systems. These results constitute a probe for the configuration of cyclohexa‐1,4‐dienes and cyclohexenes annellated to these bicyclic skeletons.