In order to determine the reactive geometries of enone-TiCl 4 complexes in Diels-Alder reactions compounds 1a‚TiCl 4 and 1b‚TiCl 4 were prepared. A crystal structure of 1a‚TiCl 4 confirmed that the titanium in this complex has the desired out-of-plane geometry, with a TisOdCsC 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 1 H and 13 C NMR chemical shift changes upon complex formation indicate that the out-of-plane TiCl 4 in 1a‚TiCl 4 is a stronger Lewis acid than the in-plane TiCl 4 of 1e‚TiCl 4 . These results are further supported by the changes in carbonyl stretching frequencies upon complex formation, with a ∆ν CdO for 1a‚TiCl 4 which is 24 cm -1 larger than that for 1e‚TiCl 4 . Expectations of heightened reactivity for 1a‚TiCl 4 as compared to 1e‚TiCl 4 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.(8) Corcoran, R. C.; Ma, J.