Transformation reactions zirconacycles

Cleavage of the (3,(3-carbon—carbon bonds of zirconacydopentanes and zirconacyclopen-tenes is very often observed. This cleavage reaction is useful for the preparation of various zirconacycles. As examples, various transformations of zirconacyclopentenes involving (3,(3-carbon—carbon bond cleavage are shown in Eq. 2.69 [13,48],... 

Further transformations of such zirconacycles are possible before the organic product is removed from the metal center. Buchwald followed the insertion of an alkyne into a zirconaaziridine with the insertion of CO to obtain substituted pyrroles (in a single pot at ambient temperature) in moderate to good yields. A diverse array of substituents can be introduced by varying the amine and alkyne examples are shown in Table 4. The reaction is tolerant not only of various alkyls and aryls, but also of thiophenyl, furyl, and pyrrole substituents, and can create 2,2 -bipyrroles (see Table 4). It does, however, require high CO pressure (as much as 1,500 psi can be necessary) [56]. 

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... 

Norton et al. investigated cyclization of zirconocene and imine to form several types of zirconaaziridine 1-16. The rich reaction chemistiy of 1-16 includes cyclization with alkene or alkyne to afford azazirconacyclopentene 1-17 and cyclization with isocyanate or aldehyde to form azaoxazirconacycle 1-18 and 1-19. These functionalized zirconacycles could be further transformed into zirco-naoxazolidione, aUylic amine, and a-amino amide (Scheme 1.12) [43, 44]. 

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