The barriers to trimethylsilyl migrations in indenes and benzindenes: Silatropic shifts via aromatic transition states

The barriers to migration of a trimethylsilyl group around the five-membered ring of indene, and of linear and angular benzindenes, have been calculated at the unrestricted Hartree-Fock (UHF) level of theory by using AMPAC and MOPAC. It is found that the retention of aromatic character in the transition state and in the intermediate isoindene lowers the barrier for the silatropic shifts. A preliminary experimental study on a trimethysilyl derivative of angular benzindene reveals that Me3Si migration is indeed a facile process. However, in the corresponding bis-trimethylsilyl systems, rearrangements which require the intermediacy of geminal Me3Si groups have rather high barriers.