Half-Cleavage of a Zirconacyclopentadiene

1
Half-Cleavage of a Zirconacyclopentadiene
Matthias Zeller a a Youngstown State University,
Department of Chemistry, 1 University Plaza,
Youngstown, Ohio 44555-3663, USA, e-mail
mzeller_at_cc.ysu.edu .
Introduction Zirconacyclopentadienes which can
be easily prepared from two alkynes are useful
intermediates for organic synthesis. Various
metallacyclopentadienes containing main group
elements such as Ge and Sn have been prepared by
the direct reaction of zirconacyclopentadienes
with metal halides such as GeCl4 and SnCl4.
However, the direct reaction with Silicon halides
does not give silacyclopentadienes derivatives.
In order to synthesize these from the
zirconacyclopentadienes, diiododienes are
prepared by the iodination of the
zirconacyclopentadienes and then the reaction of
their lithiated dienes with silylhalides provides
the silacyclopentadienes.
To achieve full conversion to the diiodinated
diienes either a large excess of iodine and long
reaction times are used or the reactions are
catalyzed with copper(I) chloride. 1,2 With no
catalysis or only two equivalents of I2 the
monoiodides are isolated as the major product
after workup. 1
Vinylzirconocenes, the intermediates of this
reaction, are itself highly valuable starting
materials for the selective CC bond formation as
e.g. nickel catalyzed cyclizations and
multicomponent couplings 3 or in the synthesis of
?-silyl substituted ?,?-unsubstituted ketones.4
While this compounds are generally synthesized
via hydrozirconation or carbozirconation, a
direct isolation of the half-cleaved
zirconacyclopentadienes would offer an elegant
and straightforward synthetic pathway.
Results When the sterically strained bicyclic
zirconacyclopentadiene 1 is reacted with two
equivalents of iodine in the presence of
copper(I) chloride, the diiodinated product 3
forms in the range of 10 hours. However, when
reacted only for two hours, the monoiodinated
product 2 can be isolated as the only product by
aqueous workup with Na2S2O3 and extraction with
ether as large yellow crystals. As a solid 2 is
indefinitely stable in air.
Structure of 2 in the solid state.
The rather unexpected stability of 1 can be
partly traced back to the sterically protecting
cyclohexyl backbone. Nevertheless, it can be
expected, that this class of compounds are stable
enough to be either isolated or to be used as
reactive intermediates. They are easily
synthesized in a one pot synthesis from readily
available starting alkynes, and, with having two
functional groups of opposite polarity, this
class of compounds offers new attractive
synthetic pathways for organic synthesis.

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