Practical considerations and known limitations

The main starting materials used in this area of chemistry are Cp2TiCl2 and Cp2ZrCl2, both of which are commercially available and may be handled in air. All the other metallocene complexes described are moisture and, in most cases, oxygen sensitive, and therefore all reactions must be carried out under anhydrous and oxygen-free conditions. Hydroxyl groups are not tolerated in 

The use of titanocene and zirconocene r - n complexes in organic synthesis will now be described in an order which reflects the first formed Ti-complex. As this is intended to be an illustrative rather than an exhaustive account of the field, the reader is directed towards several excellent reviews which provide access to much more of the primary literature.  

Both of these processes tolerate terminal alkynes as the first component. 

An alternative route to the methyl vinyl zirconocenes 11 is based on the reaction between a vinyllithium 15 and zirconocene methyl chloride. This approach has been used to form several ri -alkyne complexes.  

The co-cyclisation of substituted enynes is often highly diastereoselective. Substituents next to the alkene component exert complete control over the adjacent ring junction stereochemistry for example, 21 and 22 are formed as single isomers. In some cases the zirconacycles must be given time to equilibrate thermally to the more stable isomer in order to achieve high levels of diastereocontrol (the formation of zirconacyclopentenes is a reversible process). 

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