Selective Catalytic B-H Arylation of o-Carboranyl Aldehydes by a Transient Directing Strategy

Article abstract of DOI:10.1021/jacs.7b07160

Carboranyl aldehydes are among the most useful synthons in derivatization of carboranes. However, compared to the utilization of carboranyl carboxylic acids in selective B-H bond functionalizations, the synthetic application of carboranyl aldehydes is limited due to the weakly coordinating nature of the aldehyde group. Herein, the direct arylation of o-carboranyl aldehydes has been developed via Pd-catalyzed cage B-H bond functionalization. With the help of glycine to generate a directing group (DG) in situ, a series of cage B(4,5)-diarylated- and B(4)-monoarylated-o-carboranyl aldehydes were obtained in good to excellent yields with high selectivity. A wide range of functional groups are tolerated. The aldehyde group in the B-H arylated products could be readily removed or transformed into o-carboranyl methanol. A plausible catalytic cycle for B-H arylation was proposed based on control experiments and stoichiometric reactions, including the isolation of a key bicyclic palladium complex.

Full text of DOI:10.1021/jacs.7b07160

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Article
Selective Catalytic B-H Arylation of o-Carboanyl
Aldehydes by a Transient Directing Strategy
Xiaolei Zhang, Hongning Zheng, Jie Li, Fei Xu, Jing Zhao, and Hong Yan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07160 • Publication Date (Web): 06 Aug 2017
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Journal of the American Chemical Society
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Selective Catalytic B−H Arylation of oꢀCarboanyl Aldehydes
by a Transient Directing Strategy
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Xiaolei Zhang,†* Hongning Zheng,† Jie Li,† Fei Xu,† Jing Zhao,‡ and Hong Yan‡*
†School of Pharmaceutical Sciences, School of Biotechnology, Jiangnan University, Wuxi, Jiangsu 214122, P. R. China
‡State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
Jiangsu 210093, P. R. China
ABSTRACT: Carboranyl aldehydes are among the most useful synthons in derivatization of carboranes. However, compared to
the utilization of carboranyl carboxylic acids in selective B−H bond functionalizations, the synthetic application of carboranyl aldeꢀ
hydes is limited due to the weakly coordinating nature of aldehyde group. Herein, the direct arylation of oꢀcarboranyl aldehydes has
been developed via Pdꢀcatalyzed cage B−H bond functionalization. With the help of glycine to generate a directing group (DG) in
situ, a series of cage B(4,5)ꢀdiarylatedꢀ and B(4)ꢀmonoarylatedꢀoꢀcarboranyl aldehydes were obtained in good to excellent yields
with high selectivity. A wide range of functional groups are tolerated. The aldehyde group in the B−H arylated products could be
readily removed or transformed into oꢀcarboranyl methanol. A plausible catalytic cycle for B−H arylation was proposed based on
control experiments and stoichiometric reactions, incluing the isolation of a key bicyclic palladium complex.
INTRODUCTION
oped for the siteꢀselective functionalization of inert C−H
bonds in aldehydes and ketones.11a−11d This strategy avoids the
additional steps for the installation and removal of the DGs.
Carboranes have been widely used for decades as attractive
building blocks for the construction of unique ligands,1 funcꢀ
tional materials2 and tunable pharmacophores3 owing to their
useful properties such as high stability, enriched boron content
and delocalized threeꢀdimensional aromaticity.4 In order to
broaden these applications, efficient methods for cage vertex
(CH and BH) modifications are required to achieve vertex
selectivity and diverse functionality. In the derivatization of
carboranes, transitionalꢀmetal (TM) catalyzed B−H functionalꢀ
ization of carboranes has drawn increasing interests5−7 since it
provides an efficient tool for direct boronꢀcarbon and boronꢀ
heteroatom bond constructions. However, there are two major
challenges in this approach: the inert character of B−H bonds
and site selectivity among multiple B−H bonds with similar
chemical environment. To these points, directing group (DG)
strategies have been employed to promote the reactivity and
site selectivity in B−H functionalization. Up to date, carboxꢀ
ylic acid group (−COOH)6 has been utilized as traceless DG in
the B−H functionalization of oꢀcarboranes (Scheme 1b). Apart
from carboxylic acid, suitable DGs, robust for the B−H activaꢀ
tion/functionalization and easily removable or transformable
into diverse functional groups, are to be explored.
Scheme 1. Transitionalꢀmetal Catalyzed B−H Functionalization of oꢀ
Carboranes.
Carboranyl aldehydes have been exploited as valuable
synthons for the synthesis of diversely decorated carboranes
for material and biomedical applications.8 Considering their
functional diversity8 and facile accessibility,8b carboranyl alꢀ
dehydes may be competent functional substrates for cage B−H
functionalization. However, in contrast to the superior directꢀ
ing power of carboxylic acids,9 the carbonyl groups in aldeꢀ
hydes or ketones are less coordinative. The utility of these
weakly coordinating DGs was limited in C−H functionalizaꢀ
tion10 and elusive in B−H functionalization. To solve this
problem, transient directing groups (DGs)11 that can bind reꢀ
versibly to the substrate and the metal center have been develꢀ
For example, Mo and Dong reported a Rh(I)ꢀcatalyzed αꢀ
C(sp3)−H alkylation of ketones with olefins using a catalytic
transient DG.11d Recently, Yu and coꢀworkers described the
catalytic C(sp3)−H functionalization11a of oꢀalkyl benzaldehdes
and ketones as well as C(sp2)−H functionalization11c of benꢀ
zaldehydes using transient DGs. In a very recent report, Ge
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and coworkers reported the palladiumꢀcatalyzed C(sp3)−H
was used as the solvents (entry 7), whereas the use of H2O as
1
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8
arylation of βꢀC−H bonds of aliphatic aldehydes with transient
an additive led to no significant improvement (entry 8). We
were pleased to find that the use of 1.0 equiv. of trifluoroacetic
acid (TFA) as an additive can improve the yield to 55% (entry
9). Adjusting the stoichiometric amount of glycine led to the
desired B(4,5)ꢀdiarylated product 3a in 75% isolated yield
without the formation of B(4)ꢀmonoarylated product (entry
10). The N2 protection was necessary and decomposition of oꢀ
carboranyl aldehydes was observed when the reactions were
sealed under air (entry 11). Other amino acids with side chains
(Lꢀalanine, Lꢀvaline, entry 12ꢀ13) did not have a marked effect
on the efficiency of this transformation. Replacing glycine
with 1.0 equiv. of oꢀaminophenol afforded 3a in 55% yield
(entry 14). Removing the Pd(OAc)2 catalyst completely
stopped the reaction (entry 15).
DGs.11b
Despite of these major advances, the utilizaiton of the tranꢀ
sient directing strategy into the selective B−H functionalizaꢀ
tion of carboranes has not been reported so far. Herein, we
describe the development of a Pdꢀcatalyzed direct and siteꢀ
selective arylation of cage B−H bonds of oꢀcarboranyl aldeꢀ
hydes with aryl iodides, delivering B(4,5)ꢀdiarylatedꢀ and
B(4)ꢀmonoarylatedꢀoꢀcarboranyl aldehydes in good to excelꢀ
lent yields (Scheme 1c). The aldehyde group is stable to tolerꢀ
ate the B−H activation/functionalization conditions and can be
removed or transformed into other functional groups. In
mechanistic studies, direct B−H activation has been observed
at the B(4) site in Pdꢀoꢀcarboranylꢀimino complex.
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RESULTS AND DISCUSSION
Table 1. Optimization of B(4,5)−H Diarylation of oꢀCarboranyl Aldeꢀ
hyde 1a.a
H
Pd(OAc)2 (10 mol %)
O
amino acid
O
+
H
AgTFA (3.0 equiv.)
additive
solvent [0.2 M]
N2, 80 oC, 36h
Ph
I
2a
3.0 equiv.
Ph
1a
Figure 1. Molecular structure of 3a (ellipsoids at 30% probability and
H atoms omitted for clarity). Selected bond distances [Å]: C1–C2
1.712(2), B4–B5 1.812(3), B4–C25 1.571(3), B5–C19 1.576(2), C31–
O1 1.1741(19).
3a
yield
(%)b
n.d.
n.d.
messy
27
12
n.d.
41
30
55
75
53
entry
amino acid
solvent
additive
1
2
3
4
5
6
7
8
9
10
11d
12
13
14
15g
0.5 eq glycine
0.5 eq glycine
0.5 eq glycine
0.5 eq glycine
0.3 eq glycine
no glycine
0.5 eq glycine
0.5 eq glycine
0.5 eq glycine
1.0 eq glycine
1.0 eq glycine
1.0 eq Lꢀalanine
1.0 eq Lꢀvaline
oꢀaminophenole
1.0 eq glycine
DCE
Toluene
AcOH
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
/
/
/
/
/
/
Table 2. Substrate Scope of Aryl Iodide Coupling Partners.a,b
Ar
H
Pd(OAc)2 (10 mol %)
AgTFA (3.0 equiv.)
Glycine (1.0 equiv.)
O
O
+
Ar
I
H
Ar
TFA (1.0 equiv.)
HFIP [0.2 M]
N2, 80 oC, 36h
3.0 equiv.
AcOHc
Ph
R
Ph
2aꢀ2v
3aꢀ3v
1a
3.0 eq H2O
1.0 eq TFA
1.0 eq TFA
1.0 eq TFA
1.0 eq TFA
1.0 eq TFA
1.0 eq TFA
1.0 eq TFA
R = H
Me
3a, 75%
3b, 64%
R = Br
CHO
3h, 70%
3i, 77%
OMe 3c, 55%
C(O)Me 3j,
CO2Me 3k,
83%
84%
86%
74%
t-Bu
Ph
F
3d,
3e,
3f,
56%
65%
84%
65
O
CF3
3l,
54
NO2
NHAc
3m,
3n, 75%
55f
0
Ph
R
Cl
3g, 85%
3aꢀ3n
aReaction conditions: 1a (0.1 mmol), iodobenzene (2a) (3.0 equiv.),
Pd(OAc)2 (10 mol%), amino acid (0.3ꢀ1.0 equiv.), AgTFA (3.0 equiv.),
solvent (0.5 mL), 80 oC, N2 atmosphere, 36 h. bIsolated yield.
R
R
O
R
O
R = Me
F
R
3o, 65%
3p, 79%
3q, 77%
cHFIP/AcOH (0.5 mL, v/v = 7/3) was used as solvent. Sealed under
d
e
f
Ph
CO2Me
NO2
air atmosphere. 1.0 eq of oꢀaminophenol used. Isolated after workup
with 3N HCl. No Pd(OAc)2. DCE: 1,2ꢀdichloroethane. HFIP: hexꢀ
Ph
g
3s,
3r,
71%
n.d.
70%
R = F
R = Me 3t,
3oꢀ3t
afluoroisopropanol. TFA: trifluoroacetic acid. n.d. = not detected.
F
Cl
At the outset of our studies, we chose 1ꢀCHOꢀ2ꢀPhꢀoꢀ
C2B10H10 1a as the model substrate and iodobenzene 2a as the
arylation reagent. To generate the DG in situ, amino acids
were employed to form imine linkages with oꢀcarboranyl alꢀ
dehydes. After extensive experimental trials, we obtained the
desired B(4,5)ꢀdiarylated product 3a in 27% yield when the
reaction was conducted in hexafluoroisopropanol (HFIP) with
10 mol% of Pd(OAc)2, 0.5 equiv. of glycine, 3.0 equiv. of 2a
and 3.0 equiv. silver trifluoroacetate (AgTFA) under N2 at 80
oC. (Table 1, entry 4 and Figure 1). The yield was slightly
increased to 41% when a 7 : 3 mixture of HFIP and AcOH
O
O
Cl
F
Ph
Ph
3u,
68%
3v,
82%
aReaction conditions: 1a (0.1 mmol), Ar−I (0.3 mmol), Pd(OAc)2 (10
mol%), glycine (1.0 equiv.), AgTFA (3.0 equiv.), TFA (1.0 equiv.),
HFIP (0.5 mL), 80 oC, N2 atmosphere, 36 h. bIsolated yield. n.d.
detected.
= not
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The substrate scope of aryl iodides 2 was further investigatꢀ
substitution was next explored using methyl 4ꢀiodobenzoate
2k as the arylation reagent (Table 3). For aryl substituents,
products 4b and 4c were obtained in 87% and 85% isolated
yields, whereas 1ꢀCHOꢀoꢀC2B10H11 (1d) afforded an inseparaꢀ
ble mixture (for details, see SIꢀScheme S3). For this case, the
vertex substitution of the byproducts most probably occurred
at the B(3)/B(6) position since all of the B−H bonds at
B(3)/B(4)/B(5)/B(6) positions possess the closest proximity to
the DGs.13 We postulated that the steric effect of the substituꢀ
ents at the cage carbon atom may contribute to the B(4)/B(5)ꢀ
selectivity. As expected, when R = methyl, benzyl, isopropyl
and diphenylmethyl groups, products 4eꢀ4h were isolated as
single regioꢀisomers in good yields (74ꢀ83%). Compounds 3
and 4 were fully characterized by 1H, 11B, and 13C NMR specꢀ
troscopy, infrared (IR) spectrum and high resolution mass
spectrometry (HRMS). The structures of 3a and 3k were furꢀ
ther confirmed by singleꢀcrystal Xꢀray analysis (Figures 1 and
2).
The aldehyde group in oꢀcarboranyl aldehyde can be readily
removed or transformed into other functional groups. As
demonstrated in Scheme 2a, B(4,5)ꢀdiarylated oꢀcarboranyl
aldehyde 3a could be quantitatively reduced to oꢀcarboranyl
methanol 5 in the presence of NaBH4 at ambient temperature.
It is noteworthy that carboranyl methanols are also valuable
synthons14 in the derivation of carboranes. In addition,
KMnO4ꢀmediated oxidation of 3a, followed by in situ decarꢀ
boxylation led to removal of the aldehyde group, as shown in
6. Furthermore, when the reaction was scaled up to 1.0 mmol,
the B(4,5)ꢀdiarylated product 3k was isolated in 72% yield
with a catalyst loading of 5 mol% (Scheme 2b).
1
ed. In general, electronꢀwithdrawing group on the phenyl ring
offered higher yields of 3 than did electronꢀdonating substituꢀ
ents (Table 2). Remarkably, this reaction was well compatible
with many functional groups at the paraꢀ and metaꢀpositions
of the aryl iodide coupling partners, such as ꢀCHO (3i), ꢀ
C(O)Me (3j), ꢀCO2Me (3k), ꢀNHAc (3n), ꢀNO2 (3m and 3r),
furnishing the desired products in good to excellent yields. It
is worth noting that although orthoꢀC−H functionalization of
benzaldehydes using transient DGs has been recently pubꢀ
lished,11c orthoꢀC−H functionalization of 4ꢀiodobenzaldehyde
(2i) was not observed in current B−H functionalization system.
Furthermore, halogen (fluoro, chloro or bromo)ꢀsubstituted
phenyl iodides were also applicable to generate the diarylated
products (3fꢀ3h, 3p, 3s and 3v). In addition, substrate 2s
bearing an orthoꢀfluoro gave the desired product 3s in a slightꢀ
ly decreased yield whereas sterically hindered 2ꢀmethyl iodoꢀ
benzene (2t) failed to deliver the diꢀarylated product (3t). Inꢀ
stead, B(4)ꢀmonoꢀarylated product was isolated in 70% yield.
Isolation of carboranyl aldehyde products proved to be feasiꢀ
ble after a simple workup without further tactics to remove the
DGs.
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3
4
5
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Scheme 2. Further Transformations and Scaleꢀup Reaction.
Figure 2. Molecular structure of 3k (ellipsoids at 50% probability and
H atoms omitted for clarity). Selected bond distances [Å]: C1–C2
1.715(2), B4–C21 1.583(3), B5–C29 1.584(2), C1–C13 1.522(2),
C13–O1 1.1860(19).
a) Further transformations
Ph
OH
Ph
3.0 eq. NaBH4
Ph
Ph
THF, r.t. 3h
quant.
Ph
5
Ph
O
Table 3. Substrate Scope of oꢀCarboranyl Aldehyde Coupling Partꢀ
ners.a,b
Ph
Ph
3a
1.0 eq. KMnO4
1.0 eq. Na2HPO4
H
CO2Me
THF/H2O, r.t. 3h
88%
Ph
6
CO2Me
b) Scale up reaction
H
Pd(OAc)2 (10 mol %)
CO2Me
AgTFA (3.0 equiv.)
Glycine (1.0 equiv.)
O
CO2Me
Pd(OAc)2 (5 mol%)
AgTFA (3.0 equiv.)
glycine (1.0 equiv.)
+
H
O
R
TFA (1.0 equiv.)
HFIP [0.2 M]
N2, 80 oC, 36h
O
R
I
+
MeO2C
3.0 equiv.
O
TFA (1.0 equiv.)
HFIP [0.2 M]
N2, 80 oC, 36h
Ph
1aꢀ1h
R:
2k
4aꢀ4h
I
1a
1.0 mmol
2k
MeO2C
Ph
3.0 equiv.
3k
72%
H
Based on the established progress for B(4,5)ꢀdiarylation, we
also extended our efforts to investigate the catalytic B−H
monoarylation of oꢀcarboranes, which was also very challangꢀ
ing.6d,6h Thus far, stoichiometric Pdꢀmediated B(4)−H monoꢀ
arylation6d and catalytic B(8)/B(9)−H monoarylation have
been achieved,6h of which the latter was afforded in an insepaꢀ
rable mixture of B(8)/B(9)ꢀarylꢀoꢀcarboranes. Herein, stoichiꢀ
ometric control (1.2 equiv.) of aryl iodides and AgTFA in Pdꢀ
catalyzed B(4)−H monoarylation of 1a led to the isolation of
B(4)ꢀmonoarylated product 7a-7q in 55ꢀ94% yields (Table 4).
We found that both electronic and steric factors of aryl iodides
played crucial roles in the formation of 7a-7q. Generally, aryl
OCH3
4c, 85%
CH3
, 87%
4d
4a(3k), 84%
4b
inseperable mixture
CH3
CH3
H3C
4e
, 74%
4f
4h
, 75%
, 76%
4g, 83%
aReaction conditions: 1a-1h (0.1 mmol), 2k (0.3 mmol), Pd(OAc)2
(10 mol%), glycine (1.0 equiv.), AgTFA (3.0 equiv.), TFA (1.0 equiv.),
HFIP (0.5 mL), 80 oC, N2 atmosphere, 36 h. bIsolated yield.
The scope of oꢀcarboranyl aldehydes with different carbon
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Journal of the American Chemical Society
iodides with an electronꢀdonating group react faster than those
Page 4 of 8
1
with an electronꢀwithdrawing group, albeit with decreased
yields. The byproducts were confirmed as the B(4,5)ꢀ
diarylated species. The B(4)ꢀselectivity was incredibly inꢀ
creased when the aryl iodides containing a strong electronꢀ
withdrawing group (ꢀNO2, ꢀCN) were used, delivering 7f, 7g,
7j, 7n-7q in 81ꢀ94% yields. When the reactions were scaled
up to 1.0 mol, comparable isolated yields were obtained for 7f
and 7g. Interestingly, when 2ꢀmethyl iodobenzenes bearing
sterically hindered 2ꢀtolyl group were used, the desired B(4)ꢀ
monoarylated products 7l-7m could been obtained in 65ꢀ70%
yields. Since 2ꢀmethyl iodobenzene (2t) cannot deliver the
B(4,5)ꢀdiarylated compound (3t), the steric effect of the meꢀ
thyl group probably inhibit the reactivity in the second round
sequence of B−H functionalization based on B(4)ꢀ
monoarylated compound 7l. The substrate scope in Table 4
illustrated that both steric and electronic effect can be used to
control the B(4)ꢀmonoselectivity in B−H functionalization of
oꢀcarboranes. Compounds 7a-7q were fully characterized by
NMR, IR spectrometry and HRMS. The molecular structure of
7g was confirmed by singleꢀcrystal Xꢀray analysis (Figure 3).
2
3
4
5
6
7
8
9
Figure 3. Molecular structure of 7g. (ellipsoids at 50% probability
and H atoms omitted for clarity) Selected bond distances [Å]: C1–C2
1.690(3), C1–B4 1.724(4), B4–C6 1.574(4), C1–C10 1.509(6), C10–
O1 1.187(10).
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Scheme 3. Control Experiments.
a) Using catalytic amount of glycine
CO2Me
CO2Me
Pd(OAc)2 (10 mol %)
AgTFA (3.0 equiv.)
O
methyl 4-iodobenzoate (3.0 equiv.)
glycine (cat.)
O
+
TFA (1.0 equiv.)
HFIP [0.2 M]
N2, 80 oC, 36h
O
Ph
MeO2C
Ph
Ph
1a
7c
3k
with 50 mol% glycine: 56%
with 30 mol% glycine: 15%
with 0 mol% glycine: 0%
10%
trace
0%
Table 4. Synthesis of Cage B(4)ꢀmonoarylated oꢀCarboranyl Aldeꢀ
b) Dietherfication of 1a
a,b
Ph
O
hydes 7a-7q
.
Pd(OAc)2 (10 mol %)
TFA (1.0 equiv.)
glycine (0.0-1.0 equiv.)
HFIP [0.2 M]
N2, 80 oC, 24h
O
H
O
Ar
Pd(OAc)2 (5 mol %)
AgTFA (1.2 equiv.)
glycine (1.0 equiv.)
Ph
O
Ph
1a
8
conversion
<5%
20%
51%
Ar
I
+
with 1.0 equiv. glycine:
with 0.5 equiv. glycine:
with 0.0 equiv. glycine:
HFIP [0.2 M]
N2, 60 oC, 24h
R
R
1.2 equiv.
c) Role of AgTFA
7aꢀ7q
2
1
CO2Me
CO2Me
Pd(OAc)2 (50 mol %)
R = Ph, Ar:
no AgTFA
methyl 4-iodobenzoate (3.0 equiv.)
glycine (1.0 equiv.)
O
O
+
TFA (1.0 equiv.)
HFIP [0.2 M]
N2, 80 oC, 36h
O
Ph
MeO2C
Ph
Ph
1a
CO2Me
C(O)Me
, 79%
OMe
7c
3k
13%
17%
7b,
65%
NO2
7c, 71%
7d
7a,
55%
OMe
CN
CHO
7e, 76%
7g, 94%c
7h
7f, 81%
, 55%
(77%)d
(85%)c,d
CH3
OCH3
CO2Me
NO2
c,e
7k, 50%c,e
7i
, 80%
7j,
83%
7l
, 70%
CH3
Figure 4. Molecular structure of 8. (ellipsoids at 30% probability and
H atoms omitted for clarity) Selected bond distances [Å] and angles
[o]: C1–C2 1.687(3), C11–C12 1.697(3), C1–C9 1.504(3), C10–C11
1.501(3), C10–O1 1.354(3), C9–O1 1.329(3), C9–O1–C10 119.2(2).
Br
7m
c,e
, 65%
Ar: 4ꢀCNC6H4, R:
To gain insights into the reaction mechanisms, control exꢀ
periments were conducted. In contrast to the organic phenyl or
aliphatic aldehydes that can use a catalytic transient ligand to
achieve C−H functionalization,11a−11c in the case of oꢀ
carboranyl aldehyde, the use of a catalytic amount (50ꢀ30
mol%) of glycine led to decreased yields of 3k and 7c
(Scheme 3a). When the reaction was conducted without glyꢀ
cine, no desired products were observed and considerable conꢀ
CH3
H3C
CH3
c
7n, 93%c
c
7p, 94%c
7q
, 80%
7o
, 90%
aReaction conditions: 1a (0.1 mmol), Ar−I (1.2 equiv.), Pd(OAc)2
(5 mol%), glycine (1.0 equiv.), AgTFA (1.2 equiv.), HFIP (0.5 mL),
o
b
c
o
d
60 C, N2 atmosphere, 24 h. Isolated yield. heated at 80 C. Reacꢀ
tions conducted at 1.0 mmol scale. eusing 10 mol% of Pd(OAc)2.
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sumption of the starting material 1a was detected. The utilizaꢀ that oꢀaminophenol can also be used as transient directing
1
2
3
4
5
6
7
8
tion of 1.0 equiv. of glycine is not only essential for enabling
B−H functionalization, but plays an important role in stabilizꢀ
ing the oꢀcarboranyl aldehydes under catalytic conditions.
Dietherification was observed when 1a was treated with 10
mol% of Pd(OAc)2 in the presence of 1.0 equiv. TFA at 80 oC
(Scheme 3b). Compound 8 was isolated in moderate yield and
characterized with NMR, HRMS as well as singleꢀcrystal Xꢀ
ray analysis (Figure 4). Utilization of glycine (0.5 to 1.0 equiv.)
significantly inhibited the dietherfication of 1a. In addition,
when 1a was treated with 3.0 equiv. of methyl 4ꢀiodobenzoate
in the presence of 50 mol% of Pd(OAc)2 and 1.0 equiv. of
glycine in HFIP at 80 oC in the absence of AgTFA, both
B(4,5)ꢀdiarylated product 3k and B(4)ꢀmonoarylated 7c were
isolated in 13% and 17% yields, respectively (Scheme 3c).
These outcomes suggest that stoichiometric amount of PdII
does promote cage B−H monoꢀ and diꢀarylation whereas PdII
cannot catalyze this reaction in the absence of AgTFA.
ligand for the palladium catalyzed B−H arylation with oꢀ
carboranyl aldehyde (Table 1, entry 14). Treatment of 1a with
oꢀaminophenol in toluene at 80 C led to the isolation of 9 in
80% yield (Scheme 4b). The reaction of 9 with stoichiometric
amounts (1.0 equiv.) of Pd(OAc)2 and PPh3 at 25 oC gave rise
to the bicyclic palladium complex 10 via direct B−H activaꢀ
tion of the cage B−H bond. Limited examples for Pd(II)ꢀ
mediated B−H activation have been previously documented in
carboranyl based pincers or thioamide complexes.15
o
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 4. Stoichiometric Reactions Toward the Synthesis of Bicyclic
Palldium Complexes and Their Subsequent Arylation.
O
a)
HO
Figure 5. Molecular structures of 9 and 10. (ellipsoids at 50% probaꢀ
bility and H atoms partially omitted for clarity) Selected bond disꢀ
tances [Å] for 9: C1–C2 1.695(4), C1–C13 1.493(4), C13–N1
1.264(4), C2–C14 1.507(4). Selected bond distances [Å] and angles [o]
for 10: C1–C2 1.676(4), C1–C13 1.472(4), C13–N1 1.278(4), N1–
Pd1 2.086(2), O1–Pd1 2.142(2), B4–Pd1 2.042(3), P2–Pd1 2.2485(8),
B4–Pd1–O1 163.20(11), N1–Pd1–P2 171.91(7).
O
AcO
H
H2N
glycine
(1.0 equiv.)
OH
O
O
H
PdII
O
N
N
+
Pd(OAc)2
(1.0 equiv.)
HFIP (0.5 ml)
N2, 40 oC, 3 h
Ph
Ph
1a
Ph
A
, M = C11B10H19NO2
[M+H]+ = 306.25
found: m/z = 306.25
B
, M = C13B10H21NO4Pd
[M-H]- = 468.14
found: m/z = 468.45
ESI-MS (negative mode)
C
B
other
BH
ESI-MS (positive mode)
b)
Complex 10 was fully characterized by multinuclear NMR
spectroscopy, IR spectroscopy and HRMS. The molecular
structure of 10 was determined by a singleꢀcrystal Xꢀray difꢀ
fraction study. The Xꢀray structure of 10 exhibited a four coꢀ
ordinated Pd(II) center with a tortuous planar square configuꢀ
ration (Figure 5, B4–Pd1–O1 163.20(11)o, N1–Pd1–P2
171.91(7)o). The formation of a Pd–B bond (B4–Pd1 2.042(3)
Ph
HO
Ph
Ph
H2N
o-aminophenol
(1.0 equiv.)
OH
O
O
P
H
Pd(OAc)2
(1.0 equiv.)
Pd
N
N
toluene
N2, 80 oC, 12h
PPh3 (1.0 equiv.)
toluene
N2, 25 oC, 12h
Ph
Ph
9, 80%
Ph
10, 75%
1a
c)
CO2Me
CO2Me
Å) was consistent with the fact that direct B−H activation ocꢀ
curred at the B(4) site in oꢀcarboranylꢀimino cyclic palladium
complex. Furthermore, 10 reacted with 3.0 equiv. of methyl 4ꢀ
iodobenzoate under arylation condition to generate the desired
diꢀ and monoꢀarylated products 3k and 7c in 14% and 30%
yield, respectively (Scheme 4c).
methyl 4-iodobenzoate (3.0 equiv.)
O
+
10
O
AgTFA (3.0 equiv.), TFA (1.0 equiv.)
HFIP (0.2 M), 80 oC, 36h
MeO2C
then 3N HCl
Ph
Ph
7c, 30%
3k, 14%
In order to further elucidate the reaction mechanism,
Based on the aforementioned experimental results and relatꢀ
ed literature reports,11b a plausible reaction mechanism for
B(4)–H arylation is proposed in Scheme 5. Condensation beꢀ
tween oꢀcarboranyl aldehyde 1a and glycine leads to an imineꢀ
type intermediate A, followed by ligand exchanging with
Pd(OAc)2 to afford a palladium intermediate B before B−H
activation at the B(4) site to yield intermediate C. Oxidative
addition of the intermediate C with 1.0 equiv. of ArI affords a
Pd(IV) intermediate D, followed by reductive elimination to
generate intermediate E. Then iodide abstraction, protonation
and hydrolysis give rise to the B(4)ꢀmonoarylated product 7
with release of the catalyst and glycine. In the presence of
excess amount of aryl iodides and AgTFA (3.0 equiv.), the
repetition of the similar tandem sequence gives rise to the
B(4,5)ꢀdiarylated product 3 (see SIꢀScheme S12 for detail).
stoichiometric reaction of 1a with Pd(OAc)2 (1.0 equiv.) and
glycine (1.0 equiv.) in HFIP at 40 C lead to the detection of
o
both intermediates A and B by ESIꢀMS analysis (Scheme 4a,
A, m/z = 306.25, [M+H]+, B, X = OAc, m/z = 468.45, [M−H]−)
(for details, see SIꢀFigure S6−S8). Although the observed
intermediates A or B could not be isolated because of their
instability, they were tentatively assigned to be imineꢀtype
intermediates before B−H activation (Scheme 4a). Decompoꢀ
sition of B was observed in solution and the carboranyl related
specie was transformed to oꢀcarboranyl methanol as a known
compound (SIꢀFigure S8). To address the stability issue, oꢀ
aminophenol was selected based on the following consideraꢀ
tions: 1) the presence of benzene ring can form pꢀπ conjugaꢀ
tion with the C=N bond and thus may stabilize the oꢀ
carboranyl imine species; 2) similar to the αꢀamino acid ligand,
the imine moiety and the phenolic hydroxyl group in oꢀ
aminophenol ligand can form a five membered palladacycle
for possible B−H activation; 3) condition screening indicated
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Scheme 5. Proposed Reaction Mechanism for Pdꢀcatalyzed B(4)−H
Arylation of oꢀCarboranyl Aldehyde 1a.
BK20160166). This work was also supported by the Fundamental
1
2
3
Research Funds for the central Universities (No. JUSRP11749).
We also thank the State Key Laboratory of Food Science &
Technology in Jiangnan University for the supports of NMR tests.
4
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9
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11
12
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14
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17
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19
20
21
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29
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41
42
43
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CONCLUSION
In conclusion, we have presented the palladiumꢀcatalyzed
siteꢀselective B−H arylation of oꢀcarboranyl aldehydes using a
transient directing group. Due to steric hinderance, oꢀ
carboranyl aldehydes with arylꢀ or alkylꢀ substituents at the
cage carbon atom tend to functionalize the B(4)/B(5)−H bonds
rather than B(3)/B(6)−H bonds. Both electronꢀrich and elecꢀ
tronꢀdeficient aromatic rings can be efficiently incorporated in
a siteꢀselective manner. This approach gave access to a series
of B(4,5)ꢀdiarylatedꢀ and B(4)ꢀmonoarylatedꢀoꢀcarboranyl
aldehydes, which were difficult to access in previous reports.
Furthermore, the aldehyde group in the products can be conꢀ
veniently removed or modified into other functional groups.
With the help of oꢀaminophenol as the directing ligand, a key
bicyclic palladium intermediate has been isolated and characꢀ
terized, which strongly supports the proposed reaction mechaꢀ
nism. This work could have great potential for broad applicaꢀ
tions in catalysis and materials.
ASSOCIATED CONTENT
The Supporting Information containing experimental details,
compound characterizations and Xꢀray data in CIF format for
3a, 3k, 7g, 8, 9, 10 (CCDC number: 1538449ꢀ1538451,
1561202ꢀ1561204). This material is available free of charge
on the ACS Publications website: http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Email: xlzhang@jiangnan.edu.cn.
*Email: hyan1965@nju.edu.cn.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
We gratefully acknowledge the National Natural Science
Foundation (No. 21601066 and 21603088), the Natural Science
Foundation of Jiangsu Province (No. BK20160159 and
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(14) (a) Kalinin, V. N.; Rys, E. G.; Tyutyunov, A. A.; Starikova, Z.
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