Cobalt-promoted B-H and C-H activation: Facile B-C coupling of carboranedithiolate and cyclopentadienyl

Article abstract of DOI:10.1021/ja303334t

We report the one-pot reactions of the 16e^- half-sandwich complex CpCoS₂C₂B₁₀H₁₀ (1), methyl propiolate, and 3e^--donor ligands, which lead to selective B-functionalization at carborane with cyclopentadienyl as a functional group at ambient temperature in good yields. Metal-promoted activations of both a B-H bond of the carborane and a C-H bond of the Cp unit take place sequentially in the cooperation of organic ligands. The reaction requires a 3e^--donor ligand and an activated alkyne and is therefore suitable for a broad range of substrates. This investigation provides a simple and efficient synthetic route to B-functionalized carborane derivatives.

Full text of DOI:10.1021/ja303334t

Communication
pubs.acs.org/JACS
Cobalt-Promoted B−H and C−H Activation: Facile B−C Coupling of
Carboranedithiolate and Cyclopentadienyl
Rui Zhang, Lin Zhu, Guifeng Liu, Huimin Dai, Zhenzhong Lu, Jianbo Zhao, and Hong Yan*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
Jiangsu 210093, China
S
* Supporting Information
carborane cage and the Cp ligand was generated at ambient
temperature (Schemes 1 and 2). As an example, the addition of
ABSTRACT: We report the one-pot reactions of the 16e−
half-sandwich complex CpCoS2C2B10H10 (1), methyl
propiolate, and 3e−-donor ligands, which lead to selective
Scheme 1. Reaction of 1, 2, and 3a at Ambient Temperature
Leading to 4a, in Contrast to the Reactions of 1 with 2 or 3a
Leading to A or B, Respectively
B-functionalization at carborane with cyclopentadienyl as a
functional group at ambient temperature in good yields.
Metal-promoted activations of both a B−H bond of the
carborane and a C−H bond of the Cp unit take place
sequentially in the cooperation of organic ligands. The
reaction requires a 3e−-donor ligand and an activated
alkyne and is therefore suitable for a broad range of sub-
strates. This investigation provides a simple and efficient
synthetic route to B-functionalized carborane derivatives.
or decades, considerable effort has been devoted to develop-
ing new synthetic methods to functionalize polyhedral
F
boranes because of the important applications1−3(actual and
potential) of their derivatives in medicine, catalysis, and
materials. In the functionalization of polyhedral boranes,
however, selective and straightforward boron substitution has
been proven difficult,4 although significant advances in metal-
promoted or -catalyzed hydroboration of polyhedral boranes
have been achieved.5−8 Metal-catalyzed carborane−alkyne
hydroboration reactions that provide alkenylcarborane deriva-
tives have been developed,6a and metal-promoted stepwise and
selective hydroboration of o-carborane with selected substrates
has also been reported.7,8 Of course, in the absence of a metal, an
alternative route to selective B-substituted o-carborane deriva-
tives could be obtained from carborane cage reconstruction by an
insertion reaction, but this way is limited by the availability of
borane reagents with suitable functional groups.9
3a to a mixture of 1 and 2 in dicloromethane led to adduct 4a in a
good yield. In sharp contrast, the reaction of 1 with either 2 or 3a
alone gave rise to the hydroborated product A8a or B, respectively
(Scheme 1).
The solid-state structure of 4a demonstrates that the
carborane cage is linked to the Cp ring through a covalent B−C
bond with a length of 1.569(6) Å (Figure 1). The substituted
Metal-promoted C(sp2)−H activation,10 including that of
cyclopentadienyl (Cp)11 and its subsequent functionalization,
has been extensively investigated. Compounds with Cp and a
carborane conjugated through either a bridging atom (i.e., C, Si,
P, B)12 or even a direct C−C bond13 have been described, but the
direct coupling of the two units through a B−C bond is rare
because of the bulkiness of the carborane cage.14 Here we report
how carboranedithiolate and metal-bound Cp interact at a metal
in the presence of a range of ligands and how the metal facilitates
elaborate steric and electronic arrangements to realize the
coupling of the two units via a stable B−C bond.
Figure 1. Molecular structure of 4a.
Cp ring remains coordinated to cobalt in an η5-mode. Com-
pound 3a has lost one hydrogen atom and contributes 3e− to
metal, while the alkyne has been reduced to an olefinic unit in
a Z configuration and is connected to one S atom rather than to a
In this study, a third reagent was introduced to the reaction
of the cobalt-centered o-carboranedithiolato compound CpCo-
S2C2B10H10 (1) with methyl propiolate (2), and as a result, an
unexpected product containing B−C coupling between the
Received: April 7, 2012
Published: June 15, 2012
© 2012 American Chemical Society
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Communication
B atom as in A.8a Clearly, the breakage of the rigid S-chelating
binding in 1 is essential for constructing the intramolecular
coupling of the carboranedithiolate and cyclopentadienyl moie-
3f could also lead to the analogous products 4e and 4f with an allyl
unit (3e− donor) [SI-Figure 4 in the Supporting Information (SI)],
parallel to the chelating mode of 3a and 3b. To our surprise,
cyclopentadiene (3g) also produced a B−C-coupled complex (4g)
containing an unusual in situ-generated 3e−-donor o-carboranedi-
thio ligand that bears a moiety formed by a [4 + 2] Diels−
Alder cycloaddition (Figure 3). The chelating ability of the newly
1
ties. Consistent with the solid-state structure, the H NMR
spectrum displays four well-separated singlets at 6.01, 5.92, 5.24,
and 4.97 ppm for the substituted Cp group, in contrast to one
singlet at 5.26 ppm in 1.15 The 13C resonances of the Cp ring
appear at 91.0, 84.7, 84.3, and 83.8 ppm for the four C−H bonds
compared with 81.8 ppm in 1,15 but the characteristic broad C
resonance of the B−C bond is remarkably low-field-shifted to
127.0 ppm. The 11B signal of the B−C bond (2.1 ppm) is low-
field-shifted by 7−11 ppm in comparison with those in 1,15
further supporting the formation of a B−C bond.
4a is unique. The small ligand 3a plays a vital role in the
formation of adduct 4a. To provide a better understanding of the
role of the organic ligand, other small molecules were examined
(Scheme 2). As expected, the similar 3b was able to lead to 4b
Scheme 2. Three-Component Reactions of 1, 2, and 3a−3g
and the Ligand Substitution Reactions Leading to 4a or 4b
from 4e−g upon Addition of 3a or 3b, Respectively
Figure 3. Molecular structure of 4g.
generated bulky dimercapto ligand is weaker because of the electron-
deficiency of the adjacent carborane cage, allowing it to be replaced
by the stronger chelating ligands 3a and 3b to yield 4a and 4b
quantitatively. Similarly, 4e and 4f can be readily converted to 4a or
4b upon addition of 3a or 3b, but 4c remains intact, demonstrating
the binding ability of these organic ligands (see Scheme 2 and
section III in the SI). It should be noted that in control experiments
substrates 3c−g did not react with 1. Furthermore, the reaction
should be applicable to many other 3e−-donor ligands and activated
alkynes containing an electron-withdrawing group to yield ana-
logous products.
Both B−H and C−H activation as well as hydrogen transfer
play key roles in the formation of the B−C-coupled products.
Therefore, a series of deuterium-labeling experiments were
performed with the deuterated reagents d5-1 [(d5-Cp)-
CoS2 C2 B1 0 H1 0 ], d1-2 [DCC−CO2 Me], d1-3a
[(CH2)4NCS2D], and d1-3c [(CH3)2CHCO2D] to find out
the origin and destination of the hydrogen transfer (see reactions
14−21 in the SI). As an example, the reaction of d5-1, d1-2, and
containing a B−C bond. However, the sodium salts of 3a and 3b
did not react, indicating the importance of an acidic hydrogen
atom. The weakly coordinating 3c and 3d also gave rise to a
product with the B−C bond formation (4c), but the ligand did
not appear in the structure. Instead, the newly generated olefinic
unit was utilized to donate 3e− to metal and yield the stable five-
membered ring Co1−C3−C4−C5−O1 (Figure 2). Here, 3c and
1
d1-3a led to d5-4a, demonstrating that the H NMR signal at
7.64 ppm for C8−H arises from the terminal alkyne and that the
signal at 6.12 ppm for C9−H of the olefinic unit migrates from
the activated B3−H of the carborane (see Figure 1 and reaction
16 in the SI). Those two hydrogen atoms from 3a and the Cp
ligand that were not detected by NMR spectroscopy formed H2,
as confirmed by GC analysis (see section V in the SI).
On the basis of these experimental results, a plausible mech-
anism for the generation of 4a is summarized in Scheme 3. The
B−H activation at B3 of the carborane and hydrogen transfer to
the alkyne should occur as the earlier steps (I−III) after alkyne
insertion into one Co−S bond, as previously reported7,8 and
revealed by the capture experiments (see section V in the SI). It
should noted that the iridium analogue of intermediate III is
rather stable,7g suggesting that the proposed cobalt complex is
reasonable. The chelating coordination of 3a would induce a
change in the binding mode of Cp from η5 to η1 (IV),16 and
subsequent α-H elimination (V),11a,17 H2 release, Co−B and
Co−C cleavage, B−C bond formation, and Cp switching back to
η5 coordination16 would lead to 4a.
Figure 2. Molecular structure of 4c.
3d might be considered as Brønsted acid catalysts. The 4e−-donor
chelating ligands such as 2,2-bipyridine and 1,10-phenanthroline did
not lead to similar products, reflecting the requirement of electron
accounting of the metal center. Interestingly, allenes such as 3e and
Concerning the formation of 4g (Scheme 3), as for 3a, the
coordination of CpH to the metal could lead to ring slippage of
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Communication
Scheme 3. Proposed Mechanisms for the Formation of 4a and 4g
AUTHOR INFORMATION
Corresponding Author
hyan1965@nju.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Kin Shing Chan and Prof. Weiping Ding for their
helpful suggestions. This work was supported by the National
Natural Science Foundation of China (20925104 and
21021062), the National Basic Research Program of China
(2010CB923303), and the Natural Science Foundation of Jiangsu
Province (BK2010052). This paper is dedicated to Professor
Thomas P. Fehlner on the occasion of his 75th birthday
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, spectroscopic data, crystallographic data
(CIF), and detailed mechanisms. This material is available free of
charge via the Internet at http://pubs.acs.org.
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