A carbon-13 NMR study of the arene-olefin valence tautomerism of 1,6-methano[10]annulenes in the solid phase

High-resolution 13C NMR techniques were used in order to investigate the "arene-olefin" valence tautomerism between 11,l I-disubstituted 1,6-methano( IO)annulenes and their respective bisnorcaradienes in the solid phase. It was found that the bridgehead carbon resonances of the dimethyl and of the methylcyano derivatives shift with temperature in a manner that suggests the occurrence of an exchange process proceeding fast on the NMR time scale. In all the cases, the crystal packing forces were found to affect considerably the thermodynamic parameters of the kinetic processes. Although it was not possible to reach a low enough temperature so as to observe the resolved resonances of the aromatic and of the olefinic tautomers, the present solid-state NMR results were combined with previous solution NMR studies in order to obtain the free energies involved in the tautomeric processes. Moreover, a good correlation could be established between the solid-state NMR and the X-ray diffraction results of the compounds. Finally, some of the implications that the present work might have for the understanding of valence tautomerism in 1,6-methano( lO)annulenes are briefly discussed.