Do aromatic transition states lower barriers to silatropic shifts? A synthetic, NMR spectroscopic, and computational study

In a previous calculational study at the semi-empirical unrestricted Hartree-Fock (UHF) level of theory, we predicted that strategically incorporating an aromatic ring onto indenyltrimethylsilane would lower the barrier for [1,5]-silatropic shifts around the five-membered ring through retention of aromatic character in the transition state and in the intermediate iso-indene. We now report the synthesis and dynamic behavior of the tricyclic system angular trimethylsilylbenzindene, and also the tetracycle trimethylsilylcyclopenta[l]phenanthrene. Incorporation of one aromatic ring onto the indenyl ligand does lower the ΔG^≠ value for [1,5]-silatropic shifts to 21.9 ± 0.5 kcal mol^-1, compared to 24 kcal mol^-1 for the indenyl system. Addition of a second aromatic ring further lowers the barrier to 17.6 ± 0.2 kcal mol^-1, in good agreement with calculations. The intermediate iso-indenes for both systems have been trapped as their Diels-Alder adducts with tetracyanoethylene.