Synthesis and Characterization of In-Plane and Out-of-Plane Enone−Lewis Acid Complexes: Implications for Diels−Alder Reactions¹

Abstract

In order to determine the reactive geometries of enone−TiCl4 complexes in Diels−Alder reactions compounds 1a·TiCl4 and 1b·TiCl4 were prepared. A crystal structure of 1a·TiCl4 confirmed that the titanium in this complex has the desired out-of-plane geometry, with a TiOCC dihedral angle of 57.6°, a value remarkably similar to dihedral angles found in simple carbocyclic compounds. Difference NOE studies support a similar geometry in solution. The magnitudes of key 1H and 13C NMR chemical shift changes upon complex formation indicate that the out-of-plane TiCl4 in 1a·TiCl4 is a stronger Lewis acid than the in-plane TiCl4 of 1e·TiCl4. These results are further supported by the changes in carbonyl stretching frequencies upon complex formation, with a ΔνCO for 1a·TiCl4 which is 24 cm-1 larger than that for 1e·TiCl4. Expectations of heightened reactivity for 1a·TiCl4 as compared to 1e·TiCl4 have been confirmed; the former complex undergoes Diels−Alder reactions with cyclopentadiene 15 times more rapidly than the latter, despite being disfavored on a thermodynamic basis. These results suggest that the commonly held assumption of an in-plane reactive geometry for titanium-based Lewis acid-mediated Diels−Alder reactions may be in error.