Unmasked acyl anions

Scheme 5.11. Acylmetal as an unmasked acyl anion donor.

To replace the aforementioned acyl-main group and acyl-transition metal complexes, the natural course of events was to search for a stable and easy-to-handle acyl-metal complex that reacts as an unmasked acyl anion donor. Thus, the salient features of acylzirconocene chlorides as unmasked acyl anion donors remained to be explored. In the following, mostly carbon—carbon bond-forming reactions with carbon electrophiles using acylzirconocene chlorides as acyl group donors are described. 

Synthetic routes to a-ketol through the reactions of an unmasked acyl anion with carbonyl compounds are not numerous. The first practical application of an acylzirconocene chloride as an unmasked acyl anion donor was reported in the reaction with aldehydes in 1998 (Scheme 5.12 and Table 5.1) [19]. 

Although the reactivity of acylzirconocene chlorides towards imine derivatives under Yb(OTf)3/TMSOTf (20 mol%, l l)-catalyzed conditions is not necessarily very high, the direct access to a-amino ketone derivatives indicates the usefulness of acylzirconocene chlorides as unmasked acyl anion donors. 

R)-product. Although there is still room for further improvement of the enantioselectivity, this first example of an enantioselective reaction with a,(S-unsaturated ketones reveals the potential of acylzirconocene chlorides as unmasked acyl anion donors. 

Classically, the acylzirconocene chlorides can be converted into aldehydes, carboxyhc acids, esters, and acyl halides. Recently, the usefulness of acylzirconocenes has been extended. In fact, these unmasked acyl anions directly add to aldehydes and imines affording a-hydroxy or a-amino ketones, respectively. The reaction of acylzirconocene chlorides with imines also proceeds under Bronsted acid-catalyzed conditions, even with aqneons acids. ... 

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