Synthesis of Tetrasubstituted Stannacyclohexadienes

1
Synthesis of Tetrasubstituted Stannacyclohexadiene
s
Matthias Zeller a,b, Gregory C. Fu a a
Massachusetts Institute of Technology, Department
of Chemistry, Dreyfus Building, Cambridge, MA
02139-4307 USA, e-mail gcf_at_mit.edu b new
address Youngstown State University, Department
of Chemistry, 1 University Plaza, Youngstown,
Ohio 44555-3663, USA, e-mail mzeller_at_cc.ysu.edu.
Introduction Stannacyclohexadienes are excellent
starting materials for a wide range of six
membered aromatic heterocycles isolelectronic to
benzene. 1 Examples are the higher homologs of
pyridine such as phospha-, arsa- and
stibabenzenes, which have been long regarded as
exotic compounds or laboratory curiosities.
Recently, however, a remarkable change took
place, when it was shown that some of the species
such as phosphabenzenes function as versatile
ligands in coordination chemistry. 2 When the tin
moiety of stannacyclohexadienes is exchanged by a
negatively charged B-R unit anionic
boratabenzenes are formed. Those are able to
replace cyclopentadienyl ligands in transition
metal complexes and the bis(boratabenzene)zirconiu
m dichlorides are as active catalysts for the
polymerization as their pure carbacyclic
counterparts. 3
The unsubstituted stannacyclohexadiene itself is
easily formed by the reaction of 1,4-pentadiyne,
4 but this method provides no general access to
substituted derivatives.
Asymmetric Stannacyclohexadienes
With no general synthetic pathway towards
asymmetric stannacyclohexadienes known today, we
decided to switch from the usual C5 Sn1
approach to a new C4 C1Sn1 route. In initial
tries, we used dilithium salts of several
1,4-butadiene dianions and 1,2-dihalostannaethanes
, but none of the desired products formed.
Instead five membered symmetric stannapentadiene
derivatives formed. Transmetallation from lithium
to copper resulted in no improvement, with both
the lithium and the copper reagents reacting very
fast even at 78C.
Less reactive zirconacyclopentadienes can be
prepared from non terminal alkynes using the
Negishi reagent Cp2Zr(butene). In recent reports
Takahashi has demonstrated, that this compound
can be reacted with both alkyl- and
stannylhalides. When the reactions are catalyzed
by copper(I)chloride, good yields of the coupled
products have been isolated. 5 When the
zirconacyclopentadienes derived from non-terminal
alkynes are reacted with the appropriate
1,2-dihalostannaethanes stannacyclohexadienes 1
to 4 are isolated
Discussion
straight forward one pot synthesis readily available starting materials for zirconacyclopentadiene (alkynes, Cp2ZrCl2) tetrasubstituted zirconacyclopentadienes can be made regioselective annelated stannacycles easy to make first synthesis of a tetraalkyl or tetraaryl stannacyclohexadiene, easy access to electron rich and electron poor heterocycles zirconacyclopentadienes have to be tetrasubstituted, reaction fails with H or SnR3 substituents 1,2-dihalostannaethanes are highly toxic and sensitizers the synthesis of 1,2-dihalostannaethanes is either tedious (ISnMe2CH2I) or involves diazomethane (ClSnMe2CH2Cl)
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