A solution-phase route to a tetraethynylated (cyclobutadiene)cyclopentadienylcobalt complex with a para-(1,3,2,4)-substitution pattern

Article abstract of DOI:10.1039/b109848a

The synthesis of a tetraethynylated, CpCo-stabilized cyclobutadiene complex via a metalation/carbonylation/alkynylation route is reported; this synthetic sequence allows, for the first time, the solution phase synthesis of a tetraethynylcyclobutadiene (cy

Full text of DOI:10.1039/b109848a

A solution-phase route to a tetraethynylated
(cyclobutadiene)cyclopentadienylcobalt complex with a
para-(1,3,2,4)-substitution pattern†
Matthew Laskoski, Jason G. M. Morton, Mark D. Smith and Uwe H. F. Bunz*
Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South
Carolina 29208, USA, E-mail: Bunz@mail.chem.sc.edu
Received (in Cambridge, UK) 1st October 2001, Accepted 22nd October 2001
First published as an Advance Article on the web 23rd November 2001
The synthesis of a tetraethynylated, CpCo-stabilized cyclo-
butadiene complex via a metalation/carbonylation/alkynyla-
tion route is reported; this synthetic sequence allows, for the
first time, the solution phase synthesis of a tetraethynylcy-
clobutadiene(cyclopentadienyl)cobalt derivative, and sorts
the four alkyne substituents into two para-related groups.
are valuable due to their robustness, ease of handling and their
potential to form super-polycyclic3,8 organometallic materials.
However, the synthesis of these complexes is tedious, requiring
a gas-phase flash vacuum pyrolysis step that bottlenecks the
amount of tetraethynyl available.3 These species invariably
display a ortho,ortho-substituent pattern of their four sub-
stituents, a witness to their genesis by a Bergman-type
rearrangement. To expand the chemistry of tetraethynylcyclo-
butadiene(cyclopentadienyl)cobalt complexes, a solution-phase
synthesis with control of their regiochemistry would be
advantageous.9
We have a long-standing commitment in carbon rich organome-
tallics,1 concentrating our efforts on synthesis, reactions, and
properties of multiply alkynylated cyclobutadiene complexes as
building blocks for nano-scale carbon-rich2 objects. Tetra-
ethynylcyclobutadiene(cyclopentadienyl)cobalt complexes3–7
Reaction of the ethynylated dioxane 1 with CpCo(CO)2
followed by desilylation furnishes a mixture of the two isomeric
acetals 2a and 2b in a 3.5+1 ratio in a combined yield of 95%
(Scheme 1).9 BuLi cleanly metalates 2b at the cyclobutadiene
ring and leaves the Cp-ring intact. We attribute the facile
† Electronic supplementary information (ESI) available: experimental,
including details of preparation and spectroscopic characterization of all
new compounds. See http://www.rsc.org/suppdata/cc/b1/b109848a/
Scheme 1
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Chem. Commun., 2001, 2590–2591
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b109848a

metalation of the four-ring to the enhanced s-character of the C–
H-bonds and the ortho-metalating power of the two acetal
groups, that drive this deprotonation reaction selectively.10
Workup of the formed anion with DMF provided the
complex 3 in 78% yield after chromatography. Ohira’s
reagent11 converts the aldehyde group of 3 into an alkyne that is
further protected by lithiation and reaction with triethyl-
chlorosilane to give 4. Repetition of the metalation/carbonyla-
tion/alkynylation process with 4 as substrate furnishes the para-
diyne 5, in which the protecting group is lost during the
K2CO3-promoted Ohira reaction. Pd-catalyzed arylation trans-
forms 5 into 6 (80% yield). In the last steps of the reaction
sequence, the acetal groups are cleaved off 6, and a double
Ohira reaction converts the intermediate dialdehyde into the
title compound 7 in 54% yield.
As a proof of this concept we explored the chemistry of the
major isomer 2a and have transformed it in an analogous
reaction sequence into complex 9, in which two free alkyne
groups and two free aldehyde functionalities reside on the
cyclobutadiene complex. This material is surprisingly stable
and can be manipulated in air at ambient temperature. The
dialdehyde 9 is an attractive building block for further
elaboration of linear, cyclic and star-shaped organometallic
cyclobutadiene architectures.
The tetrayne 7 is stable and can be handled under ambient
laboratory conditions without decomposition. Due to the
unusual tetragonal symmetry of the ligand we performed a
single crystal X-ray structure determination. Suitable needle-
shaped specimen were grown from dichloromethane.‡ Fig. 1
shows the ORTEP of 7; bond lengths and bond angles are in
excellent agreement with literature reported values for alkyny-
lated cyclobutadiene complexes.1,3,4 The four-membered ring is
square with angles in the range of 89–91°, and, typical for
cyclobutadiene complexes, the alkyne groups are bent away on
average by 10° from the side of the cyclopentadienylcobalt
fragment. This feature is observed in other tetraalkynylated
cyclobutadiene complexes and is attributed to electronic
effects.4
In conclusion, we have developed a solution-phase route to
tetraethynylated, CpCo-stabilized cyclobutadiene complexes.
We have executed a stepwise solution-phase synthesis for the
hitherto unknown para-substituted complex 7, and could show
with the transformation of 2a into 9 that it will work similarly in
the case of the corresponding ortho-substituted substrate 2a.
With this methodology, almost complete control over the
regiochemistry of the substitution of cyclopentadienylcobalt-
stabilized cyclobutadiene complexes is achieved. In future we
will report upon the use of these building blocks for novel
organometallic oligomers, polymers, cycles and polycycles.
U. H. F. B. and M. L. thank the National Science Foundation
(CAREER CHE 9981765, 2000–2003). U. H. F. B. is a Camille
Dreyfus Teacher–Scholar (2000–2004).
Notes and references
‡ CCDC reference number 171473. See http://www.rsc.org/suppdata/cc/b1/
b109848a/ for crystallographic data in CIF or other electronic format.
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Fig. 1 ORTEP of the tetraethynylated cyclobutadiene complex 7. Bond
lengths and bond angles are in excellent agreement with literature reported
values for multiply alkynylated cyclobutadiene complexes.
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