Synthesis and characterization of in-plane and out-of-plane enone Lewis acid complexes: Implications for Diels-Alder reactions

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 Ti-O=C-C dihedral angle of 57.6 degrees, 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 H-1 and C-13 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 Delta nu(C-O) 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-place reactive geometry for titanium-based Lewis acid-mediated Diels-Aider reactions may be in error.