The Search for Homoaromatic Semibullvalenes. 5.¹1,5-Dimethyl-2,4,6,8-semibullvalenetetracarboxylic Dianhydride

Abstract

The synthesis and study of a novel bisannelated semibullvalene that is calculated to be a neutral homoaromatic ground state molecule is presented. Through variable-temperature X-ray, 13C solution-phase NMR, and 13C solid-state CP-MAS NMR studies and by a modified Saunders' isotopic perturbation method, 1,5-dimethyl-2,4,6,8-semibullvalenetetracarboxylic dianhydride is shown to be a highly fluxional molecule that is not homoaromatic in the solution or solid states. The barrier to the Cope rearrangement of this dianhydride, in solution, is estimated (from 125 MHz 13C NMR data) to be less than 14 kJ mol-1.