Palladium catalyzed/counter ion tuned selective methylation of o-carboranes

Article abstract of DOI:10.1016/j.jorganchem.2019.120956

The palladium catalyzed/counter ion tuned selective methylation of 9-amide-o-carboranes on B(4) and B(12) has been developed, and a series of o-carborane derivatives decorated with various groups have been synthesized with moderate yields. The in situ for

Full text of DOI:10.1016/j.jorganchem.2019.120956

Journal of Organometallic Chemistry 902 (2019) 120956
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Palladium catalyzed/counter ion tuned selective methylation of
o-carboranes
,
Ke Cao a *, Cai-Yan Zhang a, Tao-Tao Xu a, Ji Wu a, Li-Fang Ding a, Linhai Jiang b,
Junxiao Yang a
a State Key Laboratory of Environment-friendly Energy Materials & School of Material Science and Engineering, Southwest University of Science and
Technology, 59 Qinglong Road, Mianyang, Sichuan, PR China
b Instrumental Analysis Center, Shenzhen University, Xili Campus, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2019
Received in revised form
20 September 2019
Accepted 25 September 2019
Available online 26 September 2019
The palladium catalyzed/counter ion tuned selective methylation of 9-amide-o-carboranes on B(4) and
B(12) has been developed, and a series of o-carborane derivatives decorated with various groups have
been synthesized with moderate yields. The in situ formed palladium species via counter ion exchange
was proposed for the tunable BeH activation.
© 2019 Elsevier B.V. All rights reserved.
Keywords:
o-carborane
BeH activation
Methylation
Selectivity
1. Introduction
For direct alkylation of BeH bonds in o-carborane, the AlCl3
promoted Friedel-Crafts reaction has been the well known trans-
Carboranes are a class of carbon-boron molecular clusters with
three dimensional aromaticity analogues to benzene, which have
proved to be important building blocks in functional materials
[1e5], coordination chemistry [6e13] as well as pharmaceuticals
[14e21]. Therefore, developing methodologies for selective func-
tionalization of cage carbon and cage boron has attracting much
interest from chemists. However, as the 10 BeH bonds of o-car-
borane are not fully equal and have slight difference in reactivity
[22,23], which makes the selective boron functionalization more
difficult and complicated. In recent years, by utilizing the transition
metal catalyzed BeH activation for regioselective boron function-
alization of o-carborane, especially for arylation, has achieved
much advancement [24e46]. This succinct synthetic strategy has
proved to be a powerful tool in controlling the selectivity in BeH
functionalization. However, the selective alkylation of o-carbor-
ane via BeH activation is still an intricate subject as the alkyl ha-
formation for decades [47e49]. However, the insurmountable
drawback is the uncontrollable selectivity and multiple alkylation.
Recently, Xie and coworkers have developed a selective nucleo-
philic alkylation of BeH bond with Grignard reagents by adjusting
the electron density and steric effect of BeH vertex via introducing
electron-withdrawing groups on cage carbon, which has proved to
be an effective strategy for regioselective alkylation of o-carboranes
on B(4), B(3,6) and B(9) [50,51] (Scheme 1). In connection with our
recent work on amide reversed/directed selective arylation on B(4)
based on the polarized B(4)-H and B(5)-H bonds [42], we envi-
sioned to exploring a selective alkylation of o-carboranes via BeH
activation.
2. Material and methods
2.1. Generals
lides are liable to b-H elimination prior to oxidative addition to
transition metal, especially for palladium catalysts.
1a-1n were synthesized according to literature methods
[52e56]. Other materials were purchased from Acros, J&K and
Aladdin, and used as received unless otherwise specified. All re-
actions under standard conditions were monitored by thin-layer
chromatography (TLC) on gel F254 plates. The silica gel (200e300
* Corresponding author.
E-mail address: caoke@swust.edu.cn (K. Cao).
https://doi.org/10.1016/j.jorganchem.2019.120956
0022-328X/© 2019 Elsevier B.V. All rights reserved.

2
K. Cao et al. / Journal of Organometallic Chemistry 902 (2019) 120956
Scheme 1. Strategies for selective alkylation of o-carboranes.
meshes) was used for column chromatography, and the distillation
range of petroleum ether was 60e90 ꢀC. 1H, 13C{1H}, 11B{1H} and 11
residue was purified by column chromatography on 200e300 mesh
silica gel with petroleum ether/EtOAc ¼ 4:1 as eluent. All products
were characterized by 1H, 13C{1H}, 11B{1H} and 11B NMR spectra
(refer to electronic supporting information).
B
NMR spectra were recorded on the Bruker 600 MHz instruments.
All 1H NMR and 13C{1H} NMR spectral data were reported in ppm
relative to tetramethylsilane (TMS) as internal standard, 11B NMR
and 11B{1H} NMR spectra data were referenced to external
BF3$Et2O. HRMS data were measured with ESI techniques.
3. Results and discussion
To initiate our research, 9-benzamide-o-carborane and iodo-
methane were selected as model substrates to screen conditions,
and the results were summarized in Table 1. Just as we expected,
the B(4) methylation could take place in the presence of 10 mol%
Pd(OAc)2 and 3 equivalents of AgOAc in CH2Cl2 for 12 h (entry 1).
Further studies indicated that toluene is more favorable for this
transformation (entries 2e3), and the methylation could be
completed in 3 h with Pd(MeCN)2Cl2 (entries 4e6). Moreover, we
found that 2 equivalents of MeI are enough to fulfill this selective
methylation and generated the expected 2a with 64% yield (entry
7). On the other hand, the yield was reduced distinctly along with
decreasing the loading amount of AgOAc (entry 8). Furthermore,
we found that this transformation could proceed smoothly at 25 ꢀC,
and gave the 2a with 60% yield after 12 h (entry 9). To our surprise,
when AgBF4 and AgOTf were examined, the regioselectivity was
reversed, the B(4) methylation was almost inhibited and the B(12)
with more electrophilicity was methylated with 21% and 69%
yields, respectively (entries 10e11). It is worth noting that the
electrophilic multi-methylation was not found even with excess
amount of MeI, and the methylation could not took place in the
absence of Pd(MeCN)2Cl2 (entry 12). These results indicated the
selective methylation would not be a AgOTf promoted Friedel-
Crafts reaction.
2.2. General procedure for synthesis of 2a-2n
To a 10 mL dried flask was sequentially added 9-amide-o-car-
borane (0.1 mmol), toluene (1 mL), iodomethane (12.5 mL, 0.2 mmol),
Pd(MeCN)2Cl2 (2.6 mg, 0.01 mmol) and AgOAc (50.1 mg, 0.3 mmol)
under an argon atmosphere. After the reaction mixture was stirred at
25 ꢀC for 12 h, the reaction mixture was filtered through a short silica
gel column using ethyl acetate as eluent. After evaporation of the
solvent, the residue was purified by column chromatography on
200e300 mesh silica gel with petroleum ether/EtOAc ¼ 4:1 as
eluent. All products were characterized by 1H, 13C{1H}, 11B{1H} and
11B NMR spectra (refer to electronic supporting information).
2.3. General procedure for synthesis of 3a-3l
To a 10 mL dried flask was sequentially added 9-amide-o-car-
borane (0.1 mmol), toluene (1 mL), iodomethane (12.5 mL,
0.2 mmol), Pd(MeCN)2Cl2 (2.6 mg, 0.01 mmol) and AgOTf (77.1 mg,
0.3 mmol) under an argon atmosphere. After the reaction mixture
was stirred at 40 ꢀC for 0.5 h, the reaction mixture was cooled to
room temperature and filtered through a short silica gel column
using ethyl acetate as eluent. After evaporation of the solvent, the

K. Cao et al. / Journal of Organometallic Chemistry 902 (2019) 120956
3
Table 1
Optimized conditions for the selective methylation of 9-benzamide-o-carborane.a,b
with moderate yields (2l-2n).
Subsequently, the scope of 9-amide-o-carboranes was explored
for B(12) methylation. As shown in Table 3, we can see that the
methylation could proceed smoothly for amides substituted with
kinds of groups, and generated the corresponding products with
moderate yields (3a-3j), the exact structure of 3 g was also
confirmed by X-ray crystallographic analysis [57] (Fig. 1). On the
other hand, the cage carbon substituted 9-benzamide-o-carboranes
were also compatible with this transformation despite with slight
lower yields (3k-3l).
Entry
MeI (eq.)
Catalyst
Oxidant
t/h
Yield/2a
Yield/3a
1
2
3
4
5
6
7
8
3
3
3
3
3
3
2
2
2
2
2
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(PPh3)2Cl2
Pd(MeCN)2Cl2
Pd(MeCN)2Cl2
Pd(MeCN)2Cl2
Pd(MeCN)2Cl2
Pd(MeCN)2Cl2
Pd(MeCN)2Cl2
e
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgBF4
AgOTf
AgOTf
12
12
12
12
12
3
3
3
12
3
0.5
12
22c
40d
63
38
e
e
e
e
e
e
e
e
e
e
21
69
e
To further expanding the scope of alkyl iodide, the iodoethane,
2-iodopropane and n-iodobutane were subjected to the standard
conditions for B(4) and B(12) methylation, respectively. However,
54
64
43e
60f
4g
the expected products were not found due to the b-H elimination of
alkyl iodide in the presence of palladium. This result further proved
that the selective methylation is arise from the palladium catalyzed
BeH activation other than a Friedel-Crafts reaction.
Additionally, the B(4) methylated products could be further
methylated on B(12), and gave the 4,12-dimethyl-9-amide-o-car-
boranes with moderate yields (Scheme 2). This stepwise trans-
formation offers an efficient approach for selective methylation of
BeH bonds with different reactivity.
Based on the Mulliken charge calculation and the selective B(4)
arylation of 9-amide-o-carboranes [42], a plausible mechanism for
tuned selective methylation on B(4) and B(12) was proposed
(Scheme 3). For B(4) methylation, the reaction is most likely initi-
ated by the ion exchange to form catalytic active species
PdL2(OAc)2, which would activate the more polarized B(4)-H bond
by forming intermediate A, after oxidative addition/reductive
elimination would generate the B(4) methylated products 2 [58].
On the other hand, the stronger electron withdrawing effect of TfOꢁ
would increase the electrophilicity of the in situ formed PdL2(OTf)2,
which would favorable to activate the B(12)-H bond with much
more negative charge via intermediate B, after oxidative addition/
reductive elimination would gave the B(12) methylated products 3.
9
10
11
12
5g
e
a
All reactions were carried out using 0.1 mmol 1a, 10 mol% catalyst, 3 equivalents
of oxidant and 1 mL toluene at 40 ꢀC under an argon atmosphere.
b
Isolated yields. cReacted in CH2Cl2.
d
Reacted in Et2O. e2 equivalents of AgOAc.
f
Reaction conducted at 25 ꢀC.
The yield is determined by GC analysis.
g
Based on the optimized conditions for selective B(4) methyl-
ation (Table 1, entry 9), the scope of 9-amide-o-carboranes was
then examined. As can be seen from Table 2, either the amides
substituted with electron donating groups or electron withdrawing
groups were all compatible with this selective methylation, and
gave the corresponding products with moderate yields (2a-2k),
meanwhile, the exact structure of 2c was further confirmed by X-
ray crystallographic analysis [57] (Fig. 1). Additionally, when the
cage carbon substituted 9-benzamide-o-carboranes were subjected
to the standard conditions, the expected products were afforded
Table 2
Selective B(4) methylation of 9-amide-o-carboranesa,b
.
aAll reactions were carried out on a 0.1 mmol scale in 1 mL of toluene at 25 ꢀC for 12 h under an argon atmosphere.
bIsolated yields.

4
K. Cao et al. / Journal of Organometallic Chemistry 902 (2019) 120956
Fig. 1. Crystal structure of 2c (left) and 3 g (right).
Table 3
Selective B(12) methylation of 9-amide-o-carboranesa,b
.
aAll reactions were carried out on a 0.1 mmol scale in 1 mL of toluene at 40 ꢀC for 0.5 h under an argon atmosphere.
bIsolated yields.
4. Conclusions
In conclusion, we have developed
a tunable selective
methylation of 9-amide-o-carboranes on B(4) and B(12) via BeH
activation. A series of o-carborane derivatives decorated with
various groups have been synthesized with moderate yields. A
plausible mechanism involving in situ formed palladium species
catalyzed B(4)-H and B(12)-H activation was proposed. This work
provides an efficient protocol for the selective methylation of o-
carboranes via BeH activation, and has important value in the
design and synthesis of o-carborane derivatives with tunable
selectivity.
Scheme 2. Selective B(12) methylation of 2b and 2 h.
Acknowledgment
This work is supported by National Natural Science Foundation
of China (21602182), Longshan academic talent research support-
ing program of SWUST (17LZX324, 18LZX305, 18LZXT02) and the
Project of State Key Laboratory of Environment-friendly Energy
Materials, SWUST (17fksy0102, 18fksy0206).
Appendix A. Supplementary data
Scheme 3. Proposed mechanism for selective B(4)-H and B(12)-H activation.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2019.120956.

K. Cao et al. / Journal of Organometallic Chemistry 902 (2019) 120956
carboranyl amines, J. Am. Chem. Soc. 138 (2016) 12727e12730.
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