C-enolate form

Spectroscopic and crystallographic studies of Reformatsky reagents derived from a-halo esters showed that the enoiate is present in the C-enolate form and in ether solvents they form dimers. Enolates derived from a-halo ketones prefer the O-metal enoiate form. It is assumed, based on theoretical calculation, that the zinc enoiate dimers are dissociated by the action of the carbonyl compound and converted to the corresponding O-zinc enolates. Subsequently, the reaction goes through six-membered chairlike transition state. 

Scheme 4 Suitable 1,3-dione candidates for the Biginelli reaction, (a) Linear forms (b) cyclic forms (c) enol forms and (d) ketones with electron-withdrawing group on the p-position...

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