Crystal zirconium enolate

After almost half century of intensive, fundamental, and fruitful investigations of enolate structures, there is now clear evidence indicating that enolates of groups 1, 2, and 13 metals - lithium and boron being the most relevant ones - exist as the O-bound tautomers 1 the same holds in general for silicon, tin, titanium, and zirconium enolates [4]. Numerous crystal structure analyses and spectroscopic data confirmed type metalla tautomer 1 to be the rule for enolates of the alkali metals, magnesium, boron, and silicon [5]. 

Scheme 3.8 Selected structures of O-bound titanium and zirconium enolates, confirmed by crystal structure analyses.

Titanium bis-enolate 19, readily available by transmetallation of the lithium enolate of acetaldehyde with dicyclopentadienyl titanium dichloride (Cp2TiCl2), was characterized by a crystal structure analysis [53, 54]. Mono-enolates of titanium and zirconium 20 were obtained analogously from Cp2Ti(Me)Cl and... 

Cp2Zr(Me)Cl, respectively. Their NMR spectra clearly reveal that the O-bond character of the enolate, indicated by the carbon-carbon double bond, is maintained in solution (Scheme 3.8) [53]. Crystal structures were also obtained for O-bound zirconium acetophenone enolate 21 [55], titanium ketone enolate 22, derived from/) r -methylacetophenone, and amide enolate 23 [56]. Whereas the latter readily added to benzaldehyde, the ketone enolate 22 (X = Ph) failed to undergo an aldol addition. This difference in reactivity was explained - based on a computational study - by a higher electron density at the methylene carbon atom in the amide compared to the ketone enolate [56]. 

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See also in sourсe #XX -- [ ]

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