Chiral Bicyclo[2.2.2]octane-Fused CpRh Complexes: Synthesis and Potential Use in Asymmetric C?H Activation

Article abstract of DOI:10.1002/anie.202010489

A new class of chiral cyclopentadienyl rhodium(I) complexes (CpRh^I) bearing C₂-symmetric chiral bridged-ring-fused Cp ligands was prepared. The complexes were successfully applied to the asymmetric C?H activation reaction of N-methoxybenzamides with quinones, affording a series of chiral hydrophenanthridinones in up to 82 % yield with up to 99 % ee. Interestingly, structure analysis reveals that the side wall of the optimal chiral CpRh^I catalyst is vertically more extended, horizontally less extended, and closer to the metal center in comparison with the classic binaphthyl and spirobiindanyl CpRh^I complexes, and may thus account for its superior catalytic performance.

Full text of DOI:10.1002/anie.202010489

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Accepted Article
Title: Chiral Bicyclo[2.2.2]octane-Fused CpRh Complexes: Synthesis
and Potential Use in Asymmetric C−H Activation
Authors: Guozhu Li, Xiaoqiang Yan, Jijun Jiang, Hao Liang, Chao
Zhou, and Jun Wang
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10.1002/anie.202010489
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Chiral Bicyclo[2.2.2]octane-Fused CpRh Complexes: Synthesis
and Potential Use in Asymmetric C−H Activation
Guozhu Li, Xiaoqiang Yan, Jijun Jiang, Hao Liang, Chao Zhou, and Jun Wang*
Dedicated to Zhaozheng Wang on the occasion of his 100th birthday and Furong Jiang on the occasion of her 98th birthday
Abstract:
A
new class of chiral cyclopentadienyl rhodium(I)
Chiral bridged-ring-containing compounds play an important
role in asymmetric synthesis. They are widely used as chiral
auxiliary reagents, chiral reagents, chiral ligands and chiral
catalysts. However, there are merely few studies on chiral
bridged-ring-based Cp ligands.[19] The earliest example of chiral
bridged-ring-based CpH was documented by Burgstahler et al.
in 1976,[19i] which was derived from camphor. Later, from 1986
to 1991, some other relevant ligands were synthesized by
Halterman,[19a, 19c, 19e, 19f, 19h] Vollhardt,[19e, 19f, 19h] Paquette,[19d, 19g]
complexes (CpRhI) bearing C2-symmetric chiral bridged-ring-fused
Cp ligands was prepared and successfully applied to the asymmetric
C-H activation reaction of N-methoxybenzamides with quinones,
affording a series of chiral hydrophenanthridinones in up to 82%
yield with up to 99% ee. Interestingly, structure analysis reveals that
the side wall of the optimal chiral CpRhI catalyst is vertically more
extended, horizontally less extended, and closer to the metal center
in comparison with the classic binaphthyl and spirobiindanyl CpRhI
complexes, which may account for its superior catalytic performance. and Erker[19b] et al. However, surprisingly, during the past two
decades these Cp ligands have not aroused much interest of
chemists and been rarely studied. Driven by our continuous
Asymmetric synthesis involving the process of transition-metal-
interest in exploring new chiral Cp catalysts[17] and new
catalyzed inert C-H bond cleavage to form C-M bond constitutes
reactions[6, 12a, 12b, 20] for asymmetric C-H activation, recently we
one of the most significant but highly challenging research
dedicated ourselves to developing chiral bridged-ring-based Cp
frontiers in modern organic chemistry.[1] Notably, Group 9
catalysts for highly enantioselective C-H activation. As a result,
cyclopentadienylmetal (CpMIII, M = Co, Rh and Ir)-catalyzed
we present herein the first synthesis and application of chiral
asymmetric C-H activation[2] has achieved great success by
bridged-ring-based CpRh catalyst in asymmetric C-H activation.
using chiral Cp ligands,[3] artificial metalloenzymes,[4] achiral
CpM in combination with chiral carboxylic acids or chiral
sulfonates,[5] or chiral transient directing groups.[6] So far, the use
of CpM catalysts derived from chiral Cp ligands has become one
of the most general and effective methods to realize asymmetric
C-H activation.[3a] Since the pioneering work by Cramer,[7]
diverse chiral Cp ligands have been revealed, including
cyclohexane-fused Cp I,[7-8] binaphthyl Cp II,[9],[10] spirobiindanyl
Cp III,[11],[12] biphenyl Cp IV,[13] piperidine-fused Cp V,[14],[15]
cyclopentane-fused Cp VI,[16] and ferrocenyl Cp VII[17] (Figure 1).
Besides, optically pure planar chiral CpM catalysts bearing
achiral Cp ligands obtained by chiral resolution were also
disclosed.[18] Despite these advances, it remains highly
important to continuously develop new chiral Cp ligands and
build a rich library of CpM catalysts in order to meet the diverse
catalyst needs by various reactions to be studied.
Scheme 1. Synthesis of chiral bridged-ring-containing CpRh(I) complexes.
Our study was commenced with the synthesis of C2-
symmetric bicyclo[2.2.2]octane fused cyclopentadienes that
were initially reported by Vollhardt and Halterman in 1987.[19a, 19c,
19f]
In comparison with other analogs derived from various chiral
bridged-ring-containing natural products, such as camphor,[19b,
19e, 19g-i]
nopol,[19g] and verbenone,[19d] its advantages include: 1)
the starting materials don't rely on chiral pool, which makes the
structure easier to tune; 2) the two faces of the Cp moiety are
homotopic because of its C2 symmetry, avoiding the formation of
Figure 1. Chiral Cp ligands studied in asymmetric C-H activation.
diastereomers when preparing CpM complexes. As depicted in
Scheme 1, the 1,4-cyclohexadienes 2 prepared from Birch
reduction of 1,4-dialkylbenzenes 1 were subjected to the
asymmetric
hydroboration-oxidation
reaction
with
monoisopinocampheylborane (IpcBH2, from (1R)-(+)-α-pinene)
as the chiral reducing reagent to give the C2-symmetric chiral
[*]
G. Li, X. Yan, J. Jiang, H. Liang, C. Zhou, Prof. Dr. J. Wang
Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry
of Education, School of Chemistry, Sun Yat-Sen University,
Guangzhou, 510275, P. R. China
cis-cyclohexane-1,4-diols 3. Then, dimesylation of
3 with
methanesulfonyl chloride gave the esters 4, which were allowed
to react with CpNa in the presence of NaH to provide a mixture
E-mail: wangjun23@mail.sysu.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/anie.202010489
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of annulated cyclopentadienes 5 and 6. After thermolysis at
200 °C, the desired chiral bridged-ring-fused cyclopentadienes 6
were obtained. Then the corresponding CpRh(I) complexes Rh-
1, Rh-2 and Rh-3 were prepared by the classic
thalliation/transmetallation procedure.[9b]
So is the angle between the C3-C4-C2 plane and the C3-C4-C6
plane (entry 2). It suggests the Cp plane of Rh-1 bends the most
significantly away from the metal, implying Rh-1 experiences the
strongest repulsion between its side wall and metal among all
the three complexes. On the other hand, all the Rh-C bond
lengths were measured (entries 3-7). Interestingly, the maximum
difference between the Rh-C bond lengths in Rh-1 is remarkably
larger than those in (R)-Rh-4 and (S)-Rh-5 (entry 8). The extent
of the geometry distortion of the halfsandwich CpRh complex
can reflect the intensity of interaction between the side wall and
the metal center. Thus, the side wall in Rh-1 was expected to be
closer to the metal center than those in (R)-Rh-4 and (S)-Rh-5,
which might great benefit stereocontrol for some reactions.
The structure of the complex Rh-1 was confirmed by single
crystal X-ray crystallographic analysis (Figure 2). Besides, the
steric map of the binding pocket around the rhodium was
generated with SambVca 2.1 tool.[21] For comparison, the single
crystal structures and steric maps of the binaphthyl complex (R)-
Rh-4[9b] and spirobiindanyl complex (S)-Rh-5[11] were also
provided in Figure 2. In addition, the overlay of the crystal
structures of Rh-1 (red), (R)-Rh-4 (blue) and (S)-Rh-5 (green)
were shown in Figure 3. According to the model proposed by
Cramer and coworkers,[7] for Rh-1 the cyclohexyl group in SW
quadrant of the steric map acts as the side wall, which appears
vertically more extended and horizontally less extended
compared with the corresponding side walls in (R)-Rh-4 and (S)-
Rh-5. As a result, the metal center is more accessible for Rh-1,
which may lead to some unique catalytic properties.
Table 1. Selected angles and bond lengths of the chiral CpRhI complexes Rh-
1, (R)-Rh-4 and (S)-Rh-5.
entry
1
parameters
Rh-1[a]
(R)-Rh-4
(S)-Rh-5
the angle between the
C3-C4-C5 plane and
the C3-C4-C7 plane (o)
12.978 (14.013)
2.942
6.258
2
the angle between the
C3-C4-C2 plane and
the C3-C4-C6 plane (o)
13.041 (10.972)
7.811
8.082
3
4
5
6
7
8
Rh-C1 bond length (Å )
Rh-C2 bond length (Å )
Rh-C3 bond length (Å )
Rh-C4 bond length (Å )
Rh-C5 bond length (Å )
2.235 (2.247)
2.241 (2.271)
2.300 (2.312)
2.335 (2.291)
2.211 (2.183)
2.266
2.263
2.216
2.263
2.229
2.240
2.239
2.210
2.282
2.227
maximum difference
between two Rh-C
bond lengths (Å )
0.124 (0.129)
0.050
0.072
[a] As two independent molecules with slightly differed conformations were
found in the lattice, the data in parentheses is for another molecule.
Figure 2. Crystal structures and steric maps of the chiral CpRhI complexes
Rh-1, (R)-Rh-4 and (S)-Rh-5. The steric maps were generated by the
SambVca 2.1 tool (Bondi radii scaled by 1.17, sphere radius 7.0 Å, mesh
spacing 0.1 Å).
Then, the potential utility of these newly developed chiral
bridged-ring-fused CpRh catalysts in the asymmetric C-H
activation was explored. Recently, we reported an asymmetric
C-H activation reaction of N-methoxybenzamides and quinones
to
hydrophenanthridinones.[20a,
were intensively optimized,
enantioselectivity for the
prepare
various
synthetically
important
Though the reaction conditions
however, the highest
model reaction of N-
tricyclic
22]
methoxybenzamide 7a and quinone 8a can only reach 85% ee
in the presence of the optimal catalyst (R)-Rh-5 (5 mol%). In
addition, the ee values for most of the examples in the substrate
scope examination are lower than 90%. To our delight, when the
newly developed chiral CpM catalyst Rh-1 was employed in this
reaction, the desired product 9a was obtained in 54% yield with
91% ee (Table 2, entry 1). Solvent screening showed acetone
was the best solvent, giving the product in 71% yield with 92%
ee (entry 2). When the reaction was conducted at a lower
Figure 3. Overlay of the crystal structures of the chiral CpRhI complexes Rh-1
(red), (R)-Rh-4 (blue) and (S)-Rh-5 (green).
o
temperature of 0 C for 36 h, the product was obtained in 77%
yield with 97% ee (entry 3). Further lowering the reaction
temperature to -20 oC led to no improvement of
enantioselectivity (entry 4). The catalyst loading could be
reduced to 2.5 mol% without affecting the reaction outcome
(entry 5). It should be noted that while the lactone 9a' was
observed as the major byproduct, the lactam 9a'' from
Additional interesting aspects were found by measuring
some representative dihedral angles and Rh-C bond lenths of
the above complexes (Table 1). On the one hand, the angle
between the C3-C4-C5 plane and the C3-C4-C7 plane in Rh-1 is
significantly larger than those in (R)-Rh-4 and (S)-Rh-5 (entry 1).
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10.1002/anie.202010489
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dehydration-aromatization of 9a was not detected. When the
chiral catalysts Rh-2 and Rh-3 were employed, inferior results
were given (entries 6 and 7). For comparison, the binaphthyl
catalyst (R)-Rh-4 and the spirobiindanyl catalyst (R)-Rh-5 were
also tested under the same reaction conditions. The products
were obtained in lower yields (54% and 62%), suggesting that
the horizontally less extended feature of Rh-1 might make the
metal center more accessible by the reactants (entries 8 and 9).
Moreover, lower enantioselectivities were observed (52% and
77% ee). The superior performance of the chiral bridged-ring-
based CpRh catalyst in this reaction strongly indicates that it can
well complement the existing chiral CpM catalysts. Finally, the
optimal reaction conditions were identified as that shown in entry
5. For more details of optimization of reaction conditions, please
see the Supporting Information.
products 9f-9j in good yields (47-67%) and high
enantioselectivities (93-98% ee). But when p-nitrobenzamide
was used, trace amount of product was obtained, which might
be due to the strong electron-withdrawing property of nitro group.
Moreover, some disubstituted amide substrates were also
investigated, delivering the desired products 9l-9n in 43-82%
yield with 92-96% ee. It should be stressed that the outcomes
achieved here are far superior to those in our previous studies.
To check the practicality of this reaction, the model reaction was
conducted on a large scale with 2.0 mmol of benzamide 7a,
affording the targeted product 9a in 80% yield and 97% ee.
Table 3. Substrate scope.[a]
Table 2. Optimization of reaction conditions.[a]
entry
[Rh] (mol%)
T (oC)
t (h)
yield (%)
ee (%)
1
2
3
4
5
6
7
Rh-1 (5)
Rh-1 (5)
Rh-1 (5)
Rh-1 (5)
Rh-1 (2.5)
Rh-2 (2.5)
Rh-3 (2.5)
30
30
0
12
12
36
48
36
36
36
54
71
77
77
77
66
81
91
92
97
97
97
76
62
-20
0
0
0
8
9
(R)-Rh-4 (2.5)
(R)-Rh-5 (2.5)
0
0
36
36
54
62
52
77
[a] Under N2 atmosphere, 7a (0.1 mmol, 1.0 equiv), 8a (2.0 equiv), [Rh],
CsOAc (50 mol%), HOAc (2.5 equiv) in dichloroethane (DCE, 0.5 mL, for entry
1) or acetone (0.5 mL, for entries 2-9).
With the optimized reaction conditions in hand, the
substrate scope was investigated (Table 3). When different N-
alkoxy benzamide substrates were tested, high yields and high
enantioselectivities were observed (9a-c). But the substrate N-
methylbenzamide proved unreactive. When p-toluquinone was
used to react with N-methoxy benzamide 7a, the product 9d was
obtained in 62% yield with 90% ee. Then, various substituted N-
methoxy benzamides were examined. The ortho-methyl
substituted benzamide could be converted to the product 9e in
55% yield with 93% ee. Then, several para-substituted
benzamides were evaluated, providing the corresponding
[a] Under N2 atmosphere, 7 (0.1 mmol, 1.0 equiv), 8 (2.0 equiv), Rh-1 (2.5
mol%), HOAc (2.5 equiv), CsOAc (50 mol%), acetone (0.5 mL), at 0 oC for 36
h. [b] Large scale reaction with 7a (2.0 mmol, 1.0 equiv).
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In summary, we have developed a new class of chiral CpRh
cataysts bearing C2-symmetric chiral bridged-ring-fused Cp
ligands for the asymmetric C-H activation. Structure analysis
reveals that the side wall of the optimal catalyst Rh-1 is vertically
more extended, horizontally less extended, and closer to the
metal center in comparison with the widely used binaphthyl
based catalyst Rh-4 and the spirobiindanyl based catalyst Rh-5.
Moreover, Rh-1 showed superior catalytic performance over Rh-
4
and Rh-5 in the asymmetric C-H activation of N-
methoxybenzamides and quinones, which may be ascribed to its
unique structural features. A series of chiral tricyclic products
were prepared in up to 82% yield with up to 99% ee. Further
applications of these chiral bridged-ring-fused Cp metal
complexes to other valuable asymmetric C−H activation are
currently under exploration in our lab.
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant 21971263).
[11] J. Zheng, W. J. Cui, C. Zheng, S. L. You, J. Am. Chem. Soc. 2016, 138,
5242.
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Yan, J. Zhang, W. Guo, J. Jiang, J. Wang, Angew. Chem. Int. Ed. 2019,
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Chem. Int. Ed. 2017, 56, 4540.
Keywords: asymmetric catalysis • C-H activation • chiral
bridged-ring-fused cyclopentadiene • rhodium • steric map
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This article is protected by copyright. All rights reserved.

10.1002/anie.202010489
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
G. Li, X. Yan, J. Jiang, H, Liang, C.
Zhou, and Prof. Dr. J. Wang*
Page No. – Page No.
Chiral Bicyclo[2.2.2]octane-Fused
CpRh Complexes: Synthesis and
Potential Use in Asymmetric C−H
Activation
A new class of chiral cyclopentadienyl rhodium(I) complexes bearing C2-symmetric
chiral bridged-ring-fused Cp ligands has been prepared and successfully applied to
the asymmetric C-H activation reaction of N-methoxybenzamides with quinones,
affording a series of chiral hydrophenanthridinones in up to 82% yield with up to
99% ee.
This article is protected by copyright. All rights reserved.