Reactions of Acylzirconocene Complexes

Recent chemical accomplishments based on acylzirconocene chloride complexes indicate the potential utility of this reagent, not only as a nucleophilic donor of an unmasked acyl group but also as a characteristic dichotomous reagent in carbon—carbon bond-forming reactions. Although there have been many reports on the reactions of acylmetal complexes, the ready availability, stability, and notable reactivity of acylzirconocene complexes — especially acylzirconocene chloride complexes — merits their recognition as useful reagents. Further research on the reactivity of acylzirconocene species is anticipated to lead to the discovery of new synthetic applications. 

Clear formation of ketene—zirconocene complexes upon treatment of acylzirconocene chlorides with a hindered amide base indicates that the carbonyl group of the acylzirconocene chloride possesses usual carbonyl polarization (Scheme 5.10). However, these zirconocene—ketene complexes are exceptionally inert due to the formation of strongly bound dimers [13a], Conversion of the dimer to zirconocene—ketene—alkylaluminum complexes by treating with alkylaluminum and reaction with excess acetylene in toluene at 25 °C has been reported to give a cyclic enolate in quantitative yield. Although the ketene—zirconocene—alkylaluminum complex reacts cleanly with acetylene, it does not react with ethylene or substituted acetylenes [13b]. Thus, the complex has met with limited success as a reagent in organic synthesis. 

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. 

Reaction of saturated acylzirconocene chlorides with (CH3)2Cu(CN)Li2 gives the secondary alcohol (73%), and D20 work-up of the reaction mixture gives the a-deuterio alcohol. This observation suggests the formation of a ketone—zirconocene complex (Scheme 5.40 see also Section 5.3.2.1). Thus, for the reaction of a,p-unsaturated acylzirconocene chlorides with R2Cu(CN)Li2, initial formation of an unsaturated ketone—zirconocene complex followed by 1,3-rearrangement of the zirconocene moiety to an oxazirconacyclopentene, which is a ketone carbanion equivalent, has been proposed (Scheme 5.41). 

The sequential double migratory insertion of CO into acydic and cydic diorganozircono-cene complexes through acylzirconocene and ketone—zirconocene species provides a convenient procedure for preparing acyclic and cyclic ketones (Scheme 5.6) [8], Thus, the bi-cydic enones from enynes can be obtained through CO insertion into zirconacyclopen-tenes followed by a subsequent rearrangement (Scheme 5.7). The scope and limitations of this procedure have been described in detail elsewhere [8d]. This procedure provides a complementary version of the well-known Pauson Khand reaction [9]. 

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