Abstract
Interest in the structural dependence of the tautomerism between cycloheptatriene derivatives and their norcaradiene tautomers dates back to the proposal of the norcaradiene structure for “Buchner's ester”. Today, the tautomerism is recognized as a typical example of electrocyclic reactions. Very recently, the spectroscopic detection of the elusive parent norcaradiene (C7H8) has been reported. In the meantime, various factors have been found to affect the equilibrium between the cycloheptatriene and the norcaradiene form. The stabilization of the norcaradiene form by a π-acceptor group on C (7) is well recognized, both experimentally and theoretically, but there still remains the possibility that a π-donor group might also stabilize the norcaradiene form. Recently, steric factors have been shown to shift the equilibrium to the norcaradiene side. So far, thirteen systems have been subjected to structural analysis by X-ray, microwave, or electron-diffraction method ; however, the prediction of the structure - norcaradiene or cycloheptatriene-in crystal appears difficult for the equilibrating systems in solution. As reactions of interest the endoperoxide formation from cycloheptatriene-norcaradiene systems and singlet oxygen, stereochemistry of the Berson-Willcott rearrangement, and solvolytic reactions are reviewed.
