Allylzirconocene

Allylzirconocene derivatives are reactive toward aldehydes, presumably because of the availability of six-centered transition states permitting an anti-selective synthesis of homo-allyl alcohols (Scheme 1.27) [115]. 

The development of a new route to allylzirconocene derivatives has prompted further investigations in this area [76—79]. Its elegant application to the synthesis of (—)-macro-necine is noteworthy [116]. More recent investigations have led to an assortment of interesting transformations, as shown in Scheme 1.28 [117,118],... 

Closely related to the reaction of allylzirconocene derivatives with aldehydes are various carbonyl addition reactions of 1,3-dienezirconocene complexes [119], including additions to even ketones, esters, and nitriles (Scheme 1.29). 

Scheme 1.28. Addition of Y,y-bis(ethoxy)allylzirconocene derivatives to aldehydes.

Virtually all known acyclic carbozirconation reactions involve activation through dynamic polarization or ate complexation as shown in Scheme 1.7. Some of the representative examples are shown in Scheme 1.43. The reaction of allylzirconocene derivatives [151] shares similar regiochemical features with the Zr-catalyzed allylalumination [22,152] and appears to be mechanistically closely related. 

The most extensively studied application of carbenoid insertion into organozirconocene species is the insertion of allyl carbenoids into zirconacycles and subsequent elaboration of the thus formed allylzirconocenes with electrophiles (Schemes 3.24—3.26) [48,53,55-60],... 

Scheme 3.28. Convergent synthesis and elaboration of functionalized allylzirconocenes.

Allylzirconation of alkynes with allylzirconocene chloride reagents (obtained by hydrozir-conation of allenes) takes place in the presence of a catalytic amount of methylaluminox-ane (MAO) [67,68]. MAO presumably abstracts chloride to form an allylzirconocene cation, which coordinates to the alkyne triple bond. The subsequent migratory insertion is regioselective, as it is found that the new bond is mainly formed between the a-carbon of the allylzirconium species and the internal carbon of a terminal alkyne (Scheme 8.33). 

Elimination reactions are typically found in transition metal chemistry. The conversion of allylic ethers into allylzirconocenes [18] is a well-known zirconium-mediated process of this type. In contrast, the y-elimination reactions are less frequently encountered, and only a few examples of y-elimination involving zirconocenes have been reported. When studying isonitrile insertion into zirconacycles, Whitby and coworkers observed an interesting reaction leading to a cyclopropane derivative [19]. The overall transformation is depicted in Scheme 12. Treatment of the diene 18 with preformed butene-zirconocene gave... 

The reaction of allylzirconocene with aUcynes in presence of MAO, which begin with an ate complexation see Ate Complexes) of the alkyne, appears to be mechanistically closely related to the above example (equation 52). ... 

Scheme 4.19. MAO-catalyzed allylmetalations of alkynes with allylzirconocenes.

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