Spectroscopic studies on the TiCl₄-promoted reaction of allylsilanes with aldehydes and α,β-enones

The complexation of n-heptanal and 4-(n-heptyl)benzaldehyde with TiCl₄ was studied spectroscopically. It was observed that the nature of the complex formed was dependent upon the stoichiometry of TiCl₄ present. The 2:1 (Lewis base /TiCl₄) complex was preferred at sub-stoichiometric quantities of TiCl₄ while the 1:1 complex formed preferentially when excess TiCl₄ was present. Reaction of the pre-complexed aldehyde with allyltrimethylsilane led to the formation of a titanium alkoxide, the structure of which was independently established by metathesis of the corresponding silyl ether. No metathesis of the allylsilane occurred before addition. The reaction of allyltrimethylsilane with TiCl₄-complexed enones was also studied. A titanium enolate was observed spectroscopically as the intermediate in this reaction. No metathesis of the allylsilane occurred before addition to the complexed enone.