Benzene Oxide‐Oxepin Valence Tautomerism

Abstract

Abstract Benzene oxide and the potential 8π‐electron system oxepin exist in valence‐tautomeric equilibrium with each other, to which both components contribute to approximately the same extent. NMR spectroscopic measurements show that the equilibrium is rapidly established (activation energies of the forward and reverse reactions 9.1 and 7.2 kcal mole −1 , respectively). The present knowledge of the properties of oxepin justifies its classification as a “heterotropilidene”. Benzene oxide‐oxepin represents a system having fluctuating bonds, the equilibrium of which can be displaced from one extreme to the other by means of suitable substituents. The oxide component determines the reactions of the system with most agents. With 1,6‐oxido[10]annulene, which is formally a 2,7‐bridged oxepin, the oxepin character is completely suppressed by the formation of a delocalized 10π‐electron system extending over the C 10 perimeter. The existence and aromatic character of 1,6‐oxido[10]‐annulene give rise to the conception of a homologous series of oxygen bridged annulenes (1,6; 8,13‐bisoxido[14]annulene, 1,6; 8,17; 10,15‐trisoxido[18]annulene etc.), which, like the parent acenes, possess a (4n + 2)π‐electron system. Molecular models demonstrate that a considerable flattening of the C 4n+2 perimeter is achievable in the case of a syn or all‐syn arrangement of the oxygen bridges, and that the requirement for aromaticity is thus satisfied. This is confirmed in a striking manner by the synthesis and properties of syn‐1,6; 8,13‐bisoxido[14]annulene.