Mechanism of the Chelation Controlled Addition of CH₅TiCl₃ to α-Alkoxy Carbonyl Compounds. A Theoretical Study

The mechanism of the chelation controlled addition of CH₃TiCl₃ to α-alkoxy aldehydes has been studied using quantum chemical ab initio methods. The geometries of the CH₃TiCl₃ complexes of α-alkoxy aldehydes have been optimized at the MP2 level of theory employing effective core potentials with valence double-ζ basis sets at titanium and polarized double-ζ all electron basis sets at the ligands. IGLO-calculations of the ¹³C chemical shifts have been carried out at the MP2 optimized geometries with large all electron basis sets. The calculations indicate that the rearrangement between the isomeric octahedral CH₃TiCl₃-complexes occurs via a dissociation-association mechanism with pentacoordinated intermediates. The barrier for the methyl shift from titanium to the carbonyl group of the complexed aldehyde is calculated to be ca. 19 kcal/mol at the MP2 level of theory. The calculations also give a possible explanation for the different mechanism of the analogous reaction of CH₃TiCl₃ with α-alkoxy ketones.