An improved protocol for the synthesis of [(ν4-C 4R4)Co(ν5-C5H5)] complexes

Article abstract of DOI:10.1021/om200662g

The reaction of bulky alkynes C₂R₂ with (ν⁵-C₅H₅)Co(CO)(dimethyl fumarate) under microwave irradiation provides complexes of the type [(ν⁴-C ₄R₄)Co(ν⁵-C₅H₅)] in good to excellent yields. This protocol represents a significant improvement over those reported previously. In particular, the formation of insertion products such as cyclopentadienones or cyclohexadienes can be avoided. In addition, because of the exceptional stability of (ν⁵-C ₅H₅)Co(CO)(dimethyl fumarate), the reactions can be carried out in crude solvents. The easy access to [(ν⁴-C ₄R₄)Co(ν⁵-C₅H₅)] complexes stimulated a study of their reactivity, notably under cross-coupling conditions.

Full text of DOI:10.1021/om200662g

Article
pubs.acs.org/Organometallics
An Improved Protocol for the Synthesis of [(η4-C4R4)Co(η5-C5H5)]
Complexes
Guillaume Bertrand,†,‡ Ludovic Tortech,†,‡ Denis Fichou,†,‡ Max Malacria,† Corinne Aubert,*,†
and Vincent Gandon*,§
†UPMC, IPCM, UMR CNRS 7201, 4 place Jussieu, 75005 Paris, France
‡CEA-Saclay, Organic Nanostructures and Semiconductors Group, SPCSI/IRAMIS, F-91191 Gif-sur-Yvette, France
§Universite
́
Paris-Sud 11, ICMMO, UMR CNRS 8182, F-91405 Orsay, France
S
* Supporting Information
ABSTRACT: The reaction of bulky alkynes C2R2 with (η5-C5H5)Co(CO)(dimethyl fumarate) under microwave irradiation
provides complexes of the type [(η4-C4R4)Co(η5-C5H5)] in good to excellent yields. This protocol represents a significant
improvement over those reported previously. In particular, the formation of insertion products such as cyclopentadienones or
cyclohexadienes can be avoided. In addition, because of the exceptional stability of (η5-C5H5)Co(CO)(dimethyl fumarate), the
reactions can be carried out in crude solvents. The easy access to [(η4-C4R4)Co(η5-C5H5)] complexes stimulated a study of their
reactivity, notably under cross-coupling conditions.
insertion of ethene when using CpCo(C2H4)2.9 One way to
INTRODUCTION
■
Sandwich cobalt complexes of the type [(η4-C4R4)Co-
(η5-C5H5)] (also referred to as CpCoCb) are useful frame-
works and building blocks for many scientific disciplines. For
instance, they are constituents of ligands used in palladium-
catalyzed asymmetric transformations,1 of electronic devices,2
of molecular gears,3 and of complex molecular assemblies.4
CpCoCb's also give rise to polymers with interesting
properties.5 These complexes are usually air-stable and easy
to purify using common chromatography techniques. They are
typically synthesized by formal [2 + 2] cycloaddition of two
alkynes mediated by CpCoL2, L being a labile ligand.6 In the
majority of cases, the commercially available complex CpCo-
(CO)2 is employed. Alternatively, CpCo(C2H4)2 or other
active [CpCo] sources can be used. The mechanism of
CpCoCb formation involves the substitution of L by the two
alkyne units, oxidative coupling to give a cobaltacyclopenta-
diene, and reductive elimination (Scheme 1).7
prevent insertion of the precatalyst’s ligands is to use derivatives
of CpCo(PPh3)2; however, the product must be separated from
large quantities of PPh3.10 Finally, insertion of a third alkyne
unit is also possible in some cases, giving rise to [2 + 2 + 2]
cycloadducts.11 While the latter selectivity problem can be
circumvented by using diarylalkynes or alkynes bearing bulky
substituents, it remains a challenge to avoid the formation of
the other types of insertion products.
Recently, we have synthesized a cobalt complex of formula
(η5-C5H5)Co(CO)(dimethyl fumarate).12 This species proved
to be a highly efficient catalyst for various types of
cycloaddition reactions.13 It is also much easier to handle
than CpCo(CO)2 or CpCo(C2H4)2 because it is air- and
moisture-stable and does not even require the use of distilled or
degassed solvents. During our preliminary screening, we
noticed that the reaction of this complex under microwave
irradiation in the presence of diphenylacetylene provided the
CpCoCb complex 1 in 83% yield, contaminated by 9% of the
Pauson−Khand type product 2 (Scheme 2). Of particular
The insertion of L into the coordinatively unsaturated
cobaltacyclopentadiene often leads to undesired side products,
precluding high yields. For instance, cyclopentadienones are
likely to be formed when using CpCo(CO)2 (Pauson−Khand
type reaction).6,8 Likewise, cyclohexadienes can be isolated after
Received: July 22, 2011
Published: December 15, 2011
© 2011 American Chemical Society
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Scheme 1. Key Steps in the Formation of CpCoCb and Other Insertion Products
Scheme 2. Cobalt-Mediated Cyclodimerization of
Diphenylacetylene
Table 2. Cobalt-Mediated Dimerization of Symmetrical
Diphenylalkynes
Table 1. Solvent Screening
a
entry
solvent
xylenes
T (°C)
yield (%)
1
2
3
4
5
6
7
8
9
10
175
150
150
150
150
170
200
200
130
100
88 (59)
99 (66)
98 (65)
99 (66)
51 (34)
90 (60)
82 (55)
THF
EtOH
THF/EtOH (14/1)
MeCN
pyridine
DMF
b
DMSO
CH2Cl2
Et2O
0
81 (54)
27 (18)
a
Isolated yields relative to the alkyne (limiting reagent); yields
a
b
Isolated yields relative to the alkyne (limiting reagent); yields based
based on cobalt are given in parentheses. The best yield reported
so far in the literature is given in brackets. Decomposition. No
reaction.
b
c
d
on cobalt are given in parentheses. Degradation.
Thus, by keeping the incorporation of CO to a minimum
extent, and because dimethyl fumarate does not insert,
CpCo(CO)(dimethyl fumarate) appeared to be an expedient
interest, under the same conditions, 1 and 2 were isolated in 52%
and 40% yields, respectively, when using CpCo(CO)2.6h On the
other hand, CpCo(C2H4)2 led to the insertion product 3b.14
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choice for the selective construction of CpCoCb's. We describe
herein an experimental procedure which allows the rapid and
efficient synthesis of such complexes. The ease by which
CpCoCb's are made available by this strategy also prompted us
to study their reactivity.
in high yields when using alkynes bearing phenyls para- or
meta-substituted by methoxy, amino, halo, and alkyl groups
(entries 1−6).17,18 Although carboxylic acid and azide sub-
stituents led to decomposition (entries 7 and 10), an ester, and
even a pinacol boronic ester, could be used successfully (entries
8 and 9).
It is also worthy of note that the experimental procedure
used herein proved to be compatible with thienyl-substituted
alkynes (Chart 1). Again, in contrast with a previously
described protocol, no Pauson−Khand product was formed
and the yield was significantly improved.6h
The exceptional stability of CpCoCbs was exploited for the
formation of the tetracid derivative 14 (Scheme 3, which could
not be formed directly (see Table 2, entry 7). In spite of a
prolonged time in a refluxing basic medium, the desired
product was isolated quantitatively by saponification of 10.17
Likewise, the reduction of 10 by LiAlH4 furnished 15
quantitatively.
The straightforward preparation of the tetrabromide 7 and of
the tetraiodide 8 (see Table 1, entries 4 and 5) prompted us to
carry out cross-coupling reactions (Scheme 4). Gratifyingly, the
tetra-Heck, the tetra-Suzuki,15 and the tetra-Sonogashira
coupling reactions proved successful, giving rise to the new
polyconjugated complexes 16−18.19
Interestingly, the synthesis of 17 is also possible from the
borylated derivative 11 and phenyl iodide, with a better yield of
99% (Scheme 5).20
RESULTS AND DISCUSSION
■
We started our investigation by looking for experimental
conditions that would improve the selectivity for the
CpCoCb products. The 83% yield in 1 shown in Scheme 2
could be slightly increased to 88% by using a slight excess of
CpCo(CO)(dimethyl fumarate) (Co/alkyne ratio of 1.5/2
instead of 1/2; see Table 1, entry 1). Keeping this Co/alkyne
ratio, excellent yields of 99% and 98% were obtained in THF
and EtOH, respectively (entries 2 and 3). Using a 14/1
mixture of THF and EtOH, compound 1 was also isolated in
99% yield.15 However, the rise of the temperature within the
microwave vial is much faster in this polar mixture than in
THF alone (20 s vs 180 s), allowing us to maintain the
microwave power between 60 and 80 W instead of 150 W
(entry 4). In the other solvents, the yields were lower (entries
5−10).
Using the THF/EtOH medium, the cyclodimerization of
symmetrical diarylalkynes was investigated next (Table 2).
Electron-donating and -withdrawing groups could be employed,
provided no substituents are present at the ortho position
(entries 11 and 12).16 The expected complexes were isolated
The case of unsymmetrical diphenylalkynes was also
investigated (Table 3). As before, all substituents were
tolerated, except CO2H, which led to decomposition products
(entry 6). Although the yields were moderate with borylated
groups, they were excellent with other substituents, including
NO2 (entry 2) and SiMe3 (entry 8). However, in line with
previous reports,6 a low regioselectivity was observed and the
mixtures could not be separated.
a
Chart 1. CpCoCb Derived from Thienylalkynes
Unsymmetrical diarylalkynes bearing pyridyl, thienyl, and
ferrocenyl groups were also tested (Table 4). This time, better
regioselectivities could be obtained. In particular, with phenyl
and 2-thienylcarbaldehyde groups at the starting alkyne
1
(entry 3), a 10/1 ratio was observed by H NMR. Only one
regioisomer of 32 was isolated when using ferrocenyl-2-
a
Isolated yields relative to the alkyne (limiting reagent); yields based
on cobalt are given in parentheses.
thienylcarbaldehyde acetylene as the starting material, albeit
Scheme 3. Saponification and Reduction of 10
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Scheme 4. Cross-Coupling Reactions Involving 7 and 8
Scheme 5. Cross-Coupling Reactions between 11 and Phenyl Iodide
in moderate yield (entry 5).21 Unfortunately, regioselectivity
remained an issue in the other cases.
Finally, the case of diynes was briefly studied (Scheme 6).
When they bear aryl or silyl groups at the alkyne termini, these
Clearly the ring strain in the 4/5 bicyclic product 33 is detri-
mental to its formation and favors the competitive [2 + 2 + 1]
cycloaddition process.
substrates are also prone to formal CpCo-mediated [2 + 2] and
CONCLUSION
■
[2 + 2 + 1] cycloadditions.22 We found that the selectivity
An expedient method for the synthesis of CpCoCb's has been
devised. It is highly chemoselective in the sense that the
formation of insertion products can be minimized. The desired
products can be obtained in good to excellent yields, which will
favor their use as building blocks, notably in materials science.
The experimental procedure is very convenient to carry out in
crude solvents without taking any specific precautions. We are
currently using this original synthetic pathway to synthesize
between [2 + 2] and [2 + 2 + 1] cycloadditions was critically
dependent on the nature of the tether. While the three-carbon
linker led virtually exclusively to the Pauson−Khand type
complex in moderate yield,23 the desired CpCoCb became the
major component of the mixture with a four-carbon tether.
With a five-carbon tether, the CpCoCb was isolated in 93%
yield, accompanied by 4% of the Pauson−Khand product.
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Table 3. Cobalt-Mediated Dimerization of Unsymmetrical Diphenylalkynes
a
b
1
Isolated yields relative to the alkyne (limiting reagent); yields based on cobalt are given in parentheses. Determined by H NMR, unattributed.
Decomposition.
c
a
Table 4. Cobalt-Mediated Dimerization of Unsymmetrical
Diarylalkynes
Scheme 6. Cyclodimerization of Diynes
a
Isolated yields relative to the alkyne (limiting reagent); yields based
on cobalt are given in parentheses.
novel CpCoCb derivatives bearing π-conjugated peripheral
arms on the Cb moiety in view of applications in the field of
organic devices such as field-effect transistors, photovoltaic
solar cells, and magnetic heterostructures.24 In particular we
have recently shown that thin solid films of CpCoCb's
substituted by oligothiophenes of various lengths are efficient
electron-donating materials (i.e., p-type semiconductors),
leading to high-performance organic solar cells.25
EXPERIMENTAL SECTION
■
General Procedure for the [2 + 2] Cycloadditions. In a 2−5
mL Biotage microwave vial, 2 equiv of diarylacetylene and 1.5 equiv of
(η5-C5H5)Co(CO)(dimethyl fumarate) were dissolved in 4.6 mL of
THF and 0.4 mL of EtOH. The mixture was heated at 150 °C for
a
Isolated yields relative to the alkyne (limiting reagent); yields based on
b
cobalt are given in parentheses. Determined by 1H NMR, unattributed.
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30 min in a Biotage initiator 2.0 microwave (P = 60−75 W). The
crude material was then concentrated under reduced pressure. The
remaining solid was washed with 25 mL of EtOH to dissolve all
organic impurities. The residue was paper-filtered using CH2Cl2.
The solution was concentrated under reduced pressure to afford the
desired product.
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ASSOCIATED CONTENT
■
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S
* Supporting Information
Text, figures, tables, and CIF files giving experimental details,
characterization data, crystal data and refinement tables, and
X-ray crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
P.; McAdam, C. J.; Courtney, D.; Ortin, Y.; Muller-Bunz, H.; Manning,
̈
■
A. R.; McGlinchey, M. J.; Simpson, J. J. Organomet. Chem. 2011, 696,
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Corresponding Author
*E-mail: corinne.aubert@upmc.fr (C.A.); vincent.gandon@
u-psud.fr (V.G.).
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VCH: Weinheim, Germany, 2009; Vol. 1, pp 367−406.
ACKNOWLEDGMENTS
■
G.B. is thankful to the MERS for a Ph.D. grant. We thank
Xavier Dorland for the synthesis of some compounds.
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