Zirconacyclopentadienes synthesis

Based on their interest in medium-size ring synthesis, Takahashi and co-workers used a copper(l) salt to mediate the [4+ 4]-reaction of zirconacyclopentadienes 82a, 82b and 82d and l,2-bis(bromomethyl)arenes (Scheme 31).85 The reaction works with substoichiometric amounts of copper(l) chloride (10 mol%), but in general higher yields and faster reaction times are observed with 2 equiv. 

For a long time, zirconacyclopentadiene was considered to be inert towards carbon-carbon bond formation. However, through transmetalation to Cu, Ni, Zn, Li, and Al, various kinds of carbon—carbon bond-formation methodologies have been widely developed. One major advantage of this chemistry is that zirconacyclopentadienes can be conveniently prepared in situ, in high yields and with excellent selectivities, from two different alkynes. Combination of the selective formation of zirconacyclopentadienes and the chemistry of Cu, Ni, Zn, Li, or Al provides useful tools in organic synthesis. In the near future, it can be expected that more metals will be used for the transmetalation reactions of zirconacycle derivatives and some catalytic reactions will hopefully be developed. 

The Zr-based route to main group heteroles is a convenient alternative to the method using l,4-dilithio-2,3-butadienes (see Section II.B.l.a), the more so as these dilithium reagents are easily available from zirconacyclopentadienes. Thus, the synthesis of 1,1-dichloro-2,3,4,5-tetramethylsilole (27) was readily performed from 70 by use of the I2In-BuLi/SiCLt sequence of reagents40 (see Section II.B.l, equation 8)... 

Studies in the chemistry of group 14 metalloles over the past few years have been numerous. Mention should be made of the synthesis of the C-unsubstituted l//-silole the structures of the product itself, as well as of its tautomeric forms and dimer, have been established. Progress has also been made in developing new synthetic routes. By way of a transmetallation reaction from zirconacyclopentadiene, the synthesis of group 14 hete-rocyclopentadiene derivatives was considerably facilitated. On the other hand, though the method is limited to 3,4-diphenylsiloles, a further noticeable contribution was a general and versatile synthesis of the corresponding 2,5-difunctional derivatives. 

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