Sigmatropic and Haptrotropic Shifts In Complexes of Cyclopental[ℓ]phenanthrene

Abstract

<p>In a calculational study at the unrestricted Hartree-Fock (UHF) level of theory, it has been predicted that strategically incorporating an aromatic ring onto indenyltrimethylsilane would lower the barrier for [1,5]-sigmatropic shifts of silicon around the five-membered ring through retention of aromatic character in the transition state and in the intermediate iso-indene. In this thesis, the synthesis and dynamic behavior of the tricyclic system angular trimethylsilylbenzindene, 50, and also the tetracycle trimethylsilylcyclopental[ℓ]phenanthrene, 53, are reported. Incorporation of one aromatic ring onto the indenyl ligand does lower the barrier for [1,5]-silatropic shifts to 21.9 ± 0.5 kcal/mol, compared to 24 kcal/mol for the indenyl system. Addition of a second aromatic ring further lowers the barrier to 17.6 ± 0.2 kcal/mol, in good agreement with calculations. The intermediate iso-indenes for both systems have been trapped as their Diels-Alder adducts with tetracyanoethylene. In an attempt to study the migration of transition metals over the surface of cyclopenta[ℓ]phenanthrene, 55, a series of complexes have been synthesized in which the metals are bonded to the 5-membered ring in an η⁵-fashion. It seemed reasonable that the ligand might behave as a "super-indenyl" in that a transition state in a ligand substitution reaction would be stabilized by two aromatic rings compared to the single aromatic ring in the indenyl system. The experimental data revealed that haptotropic shifts, in which a metal migrates from a five-membered ring to a six-membered ring have a high barrier and are not observed. Moreover, it was not possible to substitute ligands such as phosphines for carbonyls or ethylene. One can conclude that the ligand is behaving as a cyclopentadienyl ligand rather than an indenyl unit. We hypothesized that we might be able to induce haptotropic shifts in complexes in which the transition-metal is bound to a six-membered ring of cyclopental[ℓ]phenanthrene, by deprotonating such complexes. However, attempts to bond transition-metals to a six-membered ring of the ligand were unsuccessful. When the ligand was not carefully dried, the result was to hydrogenate the double bond of the five-membered ring and yield a complex with Cr(CO)₃ attached to a terminal six-membered ring. This molecule, (cyclopental[ℓ]phenanthrene)Cr(CO)₃, 88, was characterized by X-ray crystallography. In an attempt to bond Cr(CO)₃ to a six-membered ring by using carefully dried ligand, we isolated the Diels-Alder dimer, 94, of the ligand cyclopental[ℓ]phenanthrene. It was also possible to synthesize this molecule by heating it under reflux in n-butyl ether, and we obtained an X-ray crystal structure of the molecule. Of course, it is well-known that cyclopentadiene dimerizes at relatively low temperature; in contrast, indene itself does not dimerize, but instead yields a styrene-type polymer. Theoretical studies on indene itself does not dimerize, but instead yields a styrene-type polymer. Theoretical studies on indene and cyclopental[ℓ]phenanthrene revealed that both have a high barrier (= 43 kcal/mol for cyclopental[ℓ]phenanthrene and 46 kcal/mol for indene) to hydrogen migration, but that the iso-indene type structure for cyclopental[ℓ]phenanthrene is only 4.3 kcal/mol above the ground state, while the intermediate structure for cyclopental[ℓ]phenanthrene is only 4.3 kcal/mol above the ground state, while the intermediate for indene is 9.0 kcal/mol above the ground state.</p>