Titanium/magnesium transmetallation

A transmetallation Irom magnesium to titanium occurs first. 

Deprotonation of the ketone educt with iV-(bromomagnesio)-2,2,6,6-tetramethyl-piperidide affords the f(7)-enolate selectively. Addition of the magnesium E(0)-enolate having a trimethylsiloxy group affords anti adducts (entry 6, [73]), and is postulated to occur via chelated path d (Scheme 5.11). Transmetalation of the tert-butyldimethylsiloxy-protected magnesium f fOj-enolate affords a titanium enolate that cannot chelate, and adds to aldehydes via path b (entry 7, [73]). In this case, only benzaldehyde afforded selectivity lower than 95%. 

The transmetallation of alkali enolates 164 (M = Li, Na, K) with metal salts (M Y L, ) is a general method for the preparation of a large variety of enolates 165, provided that is less electropositive than M. It is particularly suitable for such enolates 165 whose reactivity and/or selectivity is tuned by additional ligands L. Thus, a variety of magnesium, boron, aluminum, siUcon, tin, titanium, zirconium, and zinc enolates become readily available (Scheme 2.48) [2c,d]. Usually, the configuration of the enolates is maintained during the transmetallation, but cis-tmns isomerization in the transmetallated enolates occur occasionally. Individual examples will be discussed with their applications in asymmetric syntheses. 

---