Zirconacyclopentenes reactions with alkenes

Zirconocene or titanocene mediated intramolecular cyclization reactions of enynes followed by CO insertion into their corresponding five-membered metallacycles led to the formation of bicyclic cyclopentenones (Eqs.40,41) [37,38]. Intermolecular coupling of alkynes, alkenes, and CO mediated by zirconocene or titanocene affording cyclopentenone derivatives have also been achieved (Eq. 39) [18, 39, 40]. It is noteworthy that, in order to obtain the desired cyclopentenones from the reaction of zirconacyclopentenes with CO, termination of the reaction mixture with I2 is necessary. Alcohols are normally formed if the reaction mixture is treated with aqueous acid. However, in case of titanacy-clopentenes, quenching with 3 N HCl gave cyclopentenones exclusively [18]. 

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