Chelate metallocycle, structure

The aldol reactions of the titanium Z-enolates proceeded smoothly with various aldehydes precomplexed with titanium chloride at -78° C. The diastereose-lectivity is high to excellent, with the single exception of benzaldehyde. The high degree of diastereoselection associated with this current asymmetric anti-aldol process can be rationalized by a Zimmerman-Traxler type of six-membered chairlike transition state Al9fl (Scheme 2.2r). The model is based on the assumptions that the titanium enolate is a seven-membered metallocycle with a chairlike conformation, and a second titanium metal is involved in the transition state, where it is chelated to indanolyloxy oxygen as well as to the aldehyde carbonyl in a six-membered chairlike transition-state structure. 

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