Combined Dynamic Kinetic Resolution and C?H Functionalization for Facile Synthesis of Non-Biaryl-Atropisomer-Type Axially Chiral Organosilanes

Article abstract of DOI:10.1002/chem.202100237

Although asymmetric C?H functionalization has been available for the synthesis of structurally diverse molecules, catalytic dynamic kinetic resolution (DKR) approaches to change racemic stereogenic axes remain synthetic challenges in this field. Here, a c

Full text of DOI:10.1002/chem.202100237

Communication
doi.org/10.1002/chem.202100237
Chemistry—A European Journal
&
Organic Chemistry
Combined Dynamic Kinetic Resolution and CÀH Functionalization
for Facile Synthesis of Non-Biaryl-Atropisomer-Type Axially Chiral
Organosilanes
Zhao Li+,[a] Xu Wang+,[a] Yu-Ming Cui,*[a] Jun-Han Ma,[a] Li-Lei Fang,[a] Lu-Lu Han,[a] Qin Yang,[a]
Zheng Xu,[a] and Li-Wen Xu*[a, b]
Dedicated to Prof. Albert S. C. Chan on the occasion of his 70th birthday
economy and step economy.[5,6] In contrast to the intensively
Abstract: Although asymmetric CÀH functionalization has
investigated directing group-assisted mono ortho CÀH bond
been available for the synthesis of structurally diverse
activation of substituted arenes (Figure 1a), the asymmetric
molecules, catalytic dynamic kinetic resolution (DKR) ap-
functionalization of two or more arene CÀH bonds to access to
proaches to change racemic stereogenic axes remain syn-
axially chiral products is much less explored. In this regard,
thetic challenges in this field. Here, a concise palladium-
Wang and co-workers reported the first chiral spiro-Cp/Rh-cata-
catalyzed DKR was combined with CÀH functionalization
lyzed enantioselective o- and m-CÀH dual activation to synthe-
involving olefination and alkynylation for the highly effi-
size CÀN axially chiral N-aryloxindoles.[7] Compared with gener-
cient synthesis of non-biaryl-atropisomer-type (NBA) axial-
al o- and m-CÀH dual activation, achieving the site- and enan-
ly chiral oragnosilanes. The chemistry proceeded through
tioselective activation of both ortho CÀH bonds is more chal-
two different and distinct DKR: first, an atroposelective CÀ
H olefination or alkynylation produced axially chiral vinylsi-
lanes or alkynylsilanes as a new family of non-biaryl atro-
pisomers (NBA), and second, the extension of this DKR
strategy to twofold o,o’-CÀH functionalization led to the
multifunctional axially chiral organosilicon compounds
with up to >99% ee.
lenging likely due to the racemic axially chirality[8]: 1) the ste-
reoselective functionalization reactions through o- and m-CÀH
activation are not suitable for the construction of axially chiral
biaryl atropisomers, especially through twofold CÀH functional-
ization on both ortho CÀH bonds of an aryl ring; 2) although a
Axially chiral compounds are widespread in natural products[1]
and biologically active molecules[2] and found in numerous ap-
plications, especially in asymmetric synthesis[3] serving as chiral
auxiliaries, ligands, and organocatalysts. Therefore, the catalytic
synthesis of such compounds has been a research topic of
great interest. Among the many elegant methods[4] for their
synthesis, the direct asymmetric functionalization of the inert
CÀH bond has attracted growing attention in terms of its atom
[a] Z. Li,+ X. Wang,+ Y.-M. Cui, J.-H. Ma, L.-L. Fang, L.-L. Han, Q. Yang, Dr. Z. Xu,
Prof. Dr. L.-W. Xu
Key Laboratory of Organosilicon Chemistry and
Material Technology of Ministry of Education, and
Key Laboratory of Organosilicon Material Technology of Zhejiang Province
Hangzhou Normal University
No. 2318, Yuhangtang Road, Hangzhou 311121 (P. R. China)
E-mail: ym_cui@hznu.edu.cn
liwenxu@hznu.edu.cn
[b] Prof. Dr. L.-W. Xu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Suzhou Research Institute and Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
Hangzhou 311121 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202100237.
Figure 1. Catalytic asymmetric synthesis of axially chiral molecules through
transition-metal-catalyzed CÀH functionalization: from classic biaryl atro-
pisomers to non-biaryl atropisomers (this work).
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chiral axis can be constructed by introducing two substituents
through o- and m-CÀH activation whether they are the same
or not, it is limited from the kinetic resolution process of race-
mic atropisomers through o- and o’-CÀH activation.
To date, although unsymmetrical dual CÀH activation has
been described for prochiral biaryls (Figure 1b),[9] no example
was reported on the asymmetric variant of arene o- and o’-CÀ
H functionalization to produce axially chiral and non-biaryl
atropisomeric compounds. Inspired by previous reports on the
synthesis of biaryl or non-biaryl atropisomers,[10] we anticipated
that an ideal enantioselective construction could be realized
from dynamic kinetic resolution (DKR) of racemic biaryls or
non-biaryls relative to CÀH functionalization. In the past years,
DKR of racemic starting materials has become a powerful and
practical alternative to prepare stereochemically pure final
products based on the dynamic stereocontrol.[11] Several
groups have discovered that the racemization of configuration-
ally stable biaryls could proceed smoothly to give axially chiral
biaryls based on DKR,[12] but there is no report on the enantio-
selective construction of non-biaryl atropisomers through DKR
combined with CÀH functionalization. In this regard, the devel-
opment of an effective strategy for the synthesis of non-biaryl
atropisomers that that are easily sensitive to temperature or
organic solvent because of axial rotation and possible racemi-
zation, in particular dynamic kinetic resolution combined with
CÀH functionalization, is highly desired. Herein, we describe a
straightforward approach to axially chiral styrene-type organo-
silanes through DKR of racemic starting materials through
mono and double ortho CÀH olefination/alkynylation (Fig-
ure 1d).
Scheme 1. Atroposelective synthesis of axially chiral vinylsilanes via mono
ortho-olefination.
activated alkenes. It was demonstrated that the substituent at
the nitrogen atom of the oxime directing group had major in-
fluence on the results of coupling reactions and OMe outper-
formed OBn and OH groups. As to the vinylsilane coupling
partners, triethylvinylsilane and trimethyl(1-phenylvinyl)silane
are also compatible affording the corresponding and stable
non-biaryl atropisomers (for the determination of enantiomeri-
zation barrier, see the Supporting Information, e.g., DG## =
148.1 kJmolÀ1 for 3g) with excellent enantioselectivities, albeit
in decreased yields (3e and 3 f). Changing the substituent
from Me to Ph on the upper cyclohexene or bottom aromatic
ring had negligible influence on reaction results (3a vs. 3d
and 3j vs. 3m). The substrates bearing electron-donating o-
MeO group and electron-withdrawing o-Cl and p-Br atoms pro-
ceeded well. Furthermore, the reactive benzothiophene was
also tolerated and delivered the desired 3n in 52% yield and
90% enantiomeric excess (ee). Notably, the successful exten-
sion to vinylsilanes as the coupling partners constitutes the
first example of asymmetric synthesis of axially chiral com-
pounds by atroposelective olefination of arene CÀH bonds
using unactivated alkenes.
Initially, to examine the feasibility of the dynamic kinetic res-
olution, we evaluated a palladium-catalyzed site-selective CÀH
olefination of 2-amino biaryls with readily available vinylsilanes
as a family of unactivated alkenes.[13] However, our attempt to
synthesize axially chiral vinylsilanes failed. Notably, while signif-
icant progress has been made on atroposelective CÀH olefina-
tion of rigid biaryls under metal catalysis, most of successful
examples are restricted to electronically activated or biased al-
kenes such as acrylates or styrenes.[14] The asymmetric CÀH ole-
fination of more flexible prochiral vinyl arenes with unactivated
alkenes to access axially chiral styrenes bearing a chiral axis be-
tween a substituted alkene and an unsymmetrical arene re-
mains a formidable task.[15,16] Previous observations showed
the significance of judicious choice of the proper chiral cata-
lysts and directing groups, which can cooperatively act to
modulate the reactivity and exert chiral induction. Then, a
series of reaction parameters have been evaluated in the
model reaction of 1a [(E)-3-methyl-2-(naphthalen-1-yl)cyclo-
hex-2-enone O-methyl oxime] with vinylsilane 2a, and the ex-
perimental results revealed the optimized reaction conditions
as following: Pd(OAc)2, Ac-l-Ala-OH as a chiral ligand, and
AgOAc as an additive, in MeOH and at 408C (Tables S1 and S2
in the Supporting Information).
Unlike the CÀH olefination reactions proceeding through
PdII/0 manifold, the alkynylation of arene CÀH bonds with al-
kynyl bromide as the coupling partner was thought to proceed
through PdII/IV catalytic cycle. The distinction in catalytic cycle
may require arenes with different steric and electronic proper-
ties which will create chance for introducing two kind of differ-
ent substituents. As demonstrated in Scheme 2, most of sub-
strates 1 smoothly underwent dynamic kinetic resolution com-
Next, the scope of DKR of styrene-type racemic starting ma-
terial 1 through CÀH olefination reaction was examined
(Scheme 1). Generally speaking, the vinylsilanes coupled with
vinyl arenes in inferior reactivities and enantioselectivities to
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Table 1. Catalytic synthesis of axially chiral organosilanes via double o-
and o’-CÀH functionalization based on DKR.[a]
Entry
1
Substrate
Partner
Product
Scheme 2. Atroposelective synthesis of axially chiral alkynylsilanes via mono
ortho-alkynylation.
2
3
bined with CÀH alkynylation reactions with silyl-protected al-
kynyl bromides under a slightly modified conditions, furnishing
the alkynylated products in moderate to good yields and up to
>99% ee. Specifically, triethylsilyl (TES) and tert-butyldimethyl-
silyl (TBDMS)substituted alkynyl bromides were also well toler-
ated, exerting high level of chiral induction. The only exception
is the product 5i containing an electron-withdrawing F atom
on the phenyl ring, which was obtained in comparable yield
but with lower enantioselectivity (80% ee). In the absence of
ortho-substituents, the alkynylated product was obtained in a
racemic form (5l), further proving the high steric demand for
axially chiral styrenes.
4
5
6
Having established the effective mono functionalization of
arene CÀH bonds as well as the dynamic kinetic resolution of
racemic starting materials 1, subsequently, we studied the sub-
strate scope toward the asymmetric double CÀH activation. As
shown in Table 1, the steric and electronic properties of sub-
stituents on the aromatic ring had a remarkable effect on reac-
tivity and enantioselectivity. When meta-substituted arenes
were subjected to di-o-functionalization, asymmetric di-alkyny-
lation was realized in one pot, whereas no di-olefination prod-
uct was formed, probably because the latter is more sensitive
to bulkiness around the olefination position. The di-alkynyla-
tion reaction of substrates with meta OMe and Me groups pro-
ceeded well in moderate yields and high enantioselectivities
(7a and 7b). Meanwhile, trace amounts of mono-alkynylated
products were detected during the double CÀH activation pro-
cess (e.g., 5l in Scheme 2). In contrast, 6c containing an elec-
tron-withdrawing F atom at meta position only gave much
lower enantioselectivity (34% ee) but still satisfactory reactivity
(51% yield). For the substrates with symmetrically substituted
arenes, a two-step method was adopted to obtain axially chiral
products. To demonstrate the versatility of the present
method, the different combinations of the coupling partners
were tested. For example, 8a was formed in 71% yield and
>99% ee when coupling 6d with the acrylate and triisopropyl-
7
8
[a] Reaction conditions: 6a–e (0.5 mmol), 2 or 4 (1.5 mmol), Pd(OAc)2
(10 mol%), Ac-l-Ala-OH (20 mol%), Ag2CO3 (3 equiv.), MeOH (5 mL), at
408C under air atmosphere. [b] The olefins and alkynes applied in se-
quentially for entries 4–8, and the feeding sequence of olefin or alkyne
that showed in the partner is from top to bottom.
silyl (TIPS) protected alkynyl bromide sequentially. If trimethyl-
vinylsilane and TIPS-protected alkynyl bromide were used as
olefinating and alkynylating partners in turn, 8b containing
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both vinylsilane and alkynylsilane moiety was afforded in 52%
yield and >99% ee. When activated and unactivated alkenes
were tested, the coupling sequence had major influence on
the coupling results (Table 1, entries 6 and 7). As a result, the
yield of 8c decreased from 41 to 20% while maintaining the
excellent enantioselectivity. Similarly, successive o-and o’-alky-
nylation gave the atropoisomerically pure dialkynylated pro-
duct 8d in 36% isolated yield. In addition, we verified that the
racemic starting materials obtained from the first step of CÀH
functionalization can also successfully complete the dynamic
kinetic resolution process under the optimal reaction condi-
tions.
type organosilanes with excellent enantioselectivities. Both the
oxime ether directing group and silyl substituent are readily
modifiable so that the resulting products exhibit diverse syn-
thetic applications in synthetic chemistry.
Acknowledgements
Financial support from National Natural Science Foundation of
China (No. 22072035 and 21773051), Zhejiang Provincial Natu-
ral Science Foundation of China (No. LY20B030006 and
LZ18B020001) is gratefully acknowledged.
To investigate the efficiency and practicality of this protocol,
gram-scale syntheses of products 3a and 5a were carried out
(Scheme 3). Satisfyingly, negligible deviation in the reactivity
and enantioselectivity was observed by the DKR strategy. The
utility of the developed method was further highlighted by se-
lective transformations of the functional groups in the prod-
ucts (Scheme 3). The oxime ether directing group and silyl sub-
stituent in 3a were readily removed in one step. Desilylation
of 5a was easily realized in the presence of tetrabutylammoni-
um fluoride TBAF. It is worth mentioning that the terminal al-
kynes 10 are versatile synthetic intermediates that underwent
smooth Sonogashira coupling, bromination, and click reaction,
delivering the corresponding products in good yields without
affecting the enantioselectivities. The configuration stability of
the olefinated and alkynylated products was proved by HPLC
analysis and the determined rotational barriers (see the Sup-
porting Information for details). The structure of 13 was con-
firmed and the absolute configuration was determined to be R
by XRD.[17]
Conflict of interest
The authors declare no conflict of interest.
Keywords: atropisomer · CÀH activation · dynamic kinetic
resolution · organosilicon · palladium
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Manuscript received: January 21, 2021
Accepted manuscript online: January 22, 2021
Version of record online: February 8, 2021
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Chem. Eur. J. 2021, 27, 4336 –4340
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