A,p-unsaturated acylzirconocene chlorides

Reactivity of a,P-Unsaturated Acylzirconocene Chlorides toward Nucleophiles... 

Reactions of a,(3-unsaturated acylzirconocene chlorides with stable carbon nucleophiles (sodium salts of dimethyl malonate and malononitrile) at 0°C in THF afford the Michael addition products in good yields (Scheme 5.38). Direct treatment of the reaction mixture with allyl bromide in the presence of a catalytic amount of Cul -2LiCl (10 mol%) in THF at 0 °C gives the allylic ketone in a one-pot reaction. This sequential transformation implies the electronic nature of a,P-unsaturated acylzirconocene chloride to be of type E as shown in Scheme 5.37. 

A higher-order cyanocuprate, R2Cu(CN)Li2, reacts with a,P-unsaturated acylzirconocene chlorides at — 78 °C in THF to afford saturated ketones without giving a Michael-type product (Scheme 5.39). Treatment of the reaction mixture with D20 gives the [Pg.174]

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). 

Allyl tosylate is also an excellent reactant (entry 2, Table 5.8), while allyl acetate is not under Cu(I)-catalyzed conditions. Not only saturated acylzirconocene chlorides, but also a,P-unsaturated acylzirconocene chlorides give coupling products in good yields (entry 8, Table 5.8). Prenyl bromide also reacts with acylzirconocene chlorides under identical conditions to give a mixture of regioisomers (Scheme 5.30). However, a longer reaction time (10 h) is required for the completion of the reaction as compared to the reaction of allyl bromide (1 h). 

Scheme 5.38. Michael reaction of a,P-unsaturated acylzirconocene chlorides.

Scheme 5.41. Reaction of an a,P-unsaturated acylzirconocene chloride with higher-order cyanocuprate.

There are three possible active sites in a,(3-unsaturated acylzirconocene chlorides with respect to nucleophiles, namely the (3-unsaturated carbon, the acyl carbon, and the Zr—chlorine bond. The reactions of a, (3-unsaturated acylzirconocene chlorides with nucleophiles indicate bimodal reactivity (nucleophilic or electrophilic) at both the acyl and P-carbons (Scheme 5.37) [40],... 

In the Pd-catalyzed cross-coupling reactions of acylzirconocene chlorides with allylic halides and/or acetates (Section 5.4.4.4), the isolation of the expected p,y-unsaturated ketone is hampered by the formation of the a, P-un saturated ketone, which arises from isomerization of the p,y-double bond. This undesirable formation of the a,p-unsaturated ketone can be avoided by the use of a Cu(I) catalyst instead of a Pd catalyst [35]. Most Cu(I) salts, with the exception of CuBr SMe2, can be used as efficient catalysts Thus the reactions of acylzirconocene chlorides with allyl compounds (Table 5.8 and Scheme 5 30) or propargyl halides (Table 5.9) in the presence of a catalytic amount (10 mol%) of Cu(I) in DMF or THF are completed within 1 h at 0°C to give thr. , , , , , ... 

Acylzirconocene chlorides 60 can be prepared through sequential treatment of alkynes with zirconene hydrochloride and carbon monoxide [84]. This method affords a variety of a,p-unsaturated selenol esters in good yields at room temperature. Diaryl diselenides can also be used in place of ArSeBr [85a]. a,p-Unsaturated tellurol esters can be prepared by the same protocol shown in Scheme 16 [85b]. 

The acylzirconocene chloride depicted in the scheme below served as an acyl group donor, which reacted with m-unsaturated a,p-enones under Pd-Me2Zn-catalyzed conditions to lead to bicyclo[3.3.0] compounds in which tetrahydrofuran rings were embedded <020L4061>. 

Metal catalyzed cross-coupling reactions with acylzirconocene chlorides and various organic halides or acetates can give rise to unsymmetrical ketones or a mixture of p,y-and Q , -unsaturated ketones (Scheme 22). The advantage of the use of a Cu catalyst instead of a Pd catalyst is to avoid the undesirable formation of a ,/3-unsaturated ketone. ... 

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