THE SEARCH FOR HOMOAROMATIC SEMIBULLVALENES - 5 - 1,5-DIMETHYL-2,4,6,8-SEMIBULLVALENETETRACARBOXYLIC DIANHYDRIDE

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