Catalytic Enantioselective Dehydrogenative Si-O Coupling to Access Chiroptical Silicon-Stereogenic Siloxanes and Alkoxysilanes

Jiefeng Zhu, Shuyou Chen, and Chuan He *

Communication

Catalytic Enantioselective Dehydrogenative Si−O Coupling to

Access Chiroptical Silicon-Stereogenic Siloxanes and Alkoxysilanes

‡

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sı Supporting Information

ABSTRACT: A rhodium-catalyzed enantioselective construction of triorgano-substituted silicon-stereogenic siloxanes and

alkoxysilanes is developed. This process undergoes a direct intermolecular dehydrogenative Si−O coupling between dihydrosilanes

with silanols or alocohols, giving access to a variety of highly functionalized chiral siloxanes and alkoxysilanes in decent yields with

excellent stereocontrol, that signiﬁcantly expand the chemical space of the silicon-centered chiral molecules. Further utility of this

process was illustrated by the construction of CPL-active (circularly polarized luminescence) silicon-stereogenic alkoxysilane small

organic molecules. Optically pure bis-alkoxysilane containing two silicon-stereogenic centers and three pyrene groups displayed a

remarkable g_l_u_mvalue with a high ﬂuorescence quantum eﬃciency (g_l_u_m= 0.011, Φ = 0.55), which could have great potential

application prospects in chiral organic optoelectronic materials.

ilicon-containing molecules are of great academic and

industrial importance with widespread applications in

atoms at the silicon atom has also been demonstrated for the

generation of silicon-stereogenic alkoxysilanes via the control

19−25

many areas. In the past few decades, the development of new

methods for the preparation of novel types of organosilicon

compounds has been intensively studied, which has led to their

broad use in a diverse range of organic, organometallic, and

of stoichiometric chiral reagents or chiral auxiliaries.

Apart

from the stoichiometric chiral substrates-induced approach,

asymmetric catalysis for the construction of silicon-stereogenic

26−34

silanes is undoubtably more fascinating.

To the best of

−3

polymeric chemistry.

In particular, siloxanes and alkox-

our knowledge, the ﬁrst example for the catalytic enantiose-

lective alcoholysis of dihydrosilanes with achiral alcohols was

also reported by Corriu and co-workers in the 1970s using a

ysilanes (silyl ethers) are fundamentally important skeletons,

which are widely used as privileged monomers in silicon-based

materials, and valuable protecting groups, reagents, inter-

16,17

chiral Rh/(S,S)-DIOP catalyst (19% ee obtained).

Later,

−10

mediates in organic synthesis.

Despite vast methods for the

the Xu group made several eﬀorts to access silicon-stereogenic

−10

synthesis of siloxanes and alkoxysilanes,

the access of these

alkoxysilanes via enantioselective alcoholysis of dihydrosi-

35,36

organosilicon compounds bearing silicon-stereogenic centers

in enantioenriched forms has been signiﬁcantly less explored,

which severely hampers their applications in the design and

development of new chiral silicon-based materials.

lanes

(Scheme 1b). Besides the alcoholysis of dihydrosi-

lanes, a few examples of transition-metal-catalyzed enantiose-

37−39

lective hydrosilylation of ketones

and desymmetrization

of tetraorganosilanes via cleavage of a Si−C bond along with

40,41

Historically, the synthesis of silicon-stereogenic alkoxysilanes

relied on the optical resolution and kinetic resolution with

formation of a Si−O bond

could also produce the silicon-

stereogenic alkoxysilanes. It is noteworthy that the catalytic

asymmetric synthesis of enantioenriched silicon-stereogenic

siloxanes has remained unknown up to date. Given the

increasing demand for the synthesis of novel functional-

materials-oriented chiral organosilicon compounds, the ex-

ploration and development of new general strategies for the

facile access of silicon-stereogenic siloxanes and alkoxysilanes

with high eﬃciency and enantioselectivity are highly desirable.

Recently, we have reported an enantioselective intra-

11−15

chiral alcohol auxiliaries.

Despite the success of these

resolution methods, they are limited in substrate scope along

with low eﬃciency and poor atom-economy. Thus, the

development of novel synthetic methods toward silicon-

stereogenic alkoxysilanes based on asymmetric synthesis is

more attractive but was proved to be very challenging. In the

970s, the Corriu group developed a seminal work for the

asymmetric alcoholysis of dihydrosilanes in the presence of

6,17

rhodium catalyst.

When a chiral alcohol, such as

molecular C−H silylation strategy for the synthesis of

(

−)-menthol, was used in the Si−O coupling reaction, a

42−45

enantioenriched silicon-stereogenic silanes.

With the

moderate enantioselectivity of the silicon-stereogenic center

was achieved (48% ee, determined after stereospeciﬁc

nucleophilic displacement, Scheme 1a). Later, Leighton and

co-workers realized a chiral alcohol substrate- and chiral

copper catalyst-induced variant, which produced the desired

silicon-stereogenic alkoxysilanes with high diastereoselectivity

Received: January 29, 2021

Published: April 1, 2021

(

90:10 dr, Scheme 1a). In addition, a number of asymmetric

substitutions of one of two amino or alkoxy groups or chlorine

2021 American Chemical Society

J. Am. Chem. Soc. 2021, 143, 5301−5307

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Journal of the American Chemical Society

Communication

Scheme 1. Asymmetric Synthesis of Si-Stereogenic

Alkoxysilanes

great potential applications in chiral organic optoelectronic

materials.

At the outset of our research, we mainly focused on the

catalytic asymmetric synthesis of enantioenriched silicon-

stereogenic siloxanes. We commenced our studies by

examining the intermolecular dehydrogenative Si−O coupling

reaction of tert-butyl(4-methoxyphenyl)silane 1a with

dimethyl(phenyl)silanol 2a in the presence of Rh catalyst. At

room temperature in the solvent of toluene, several chiral

diphosphine ligands were tested. Treatment of 1a with 2a by

using [Rh(cod)Cl] (1 mol %) as the catalyst and Josiphos L1

(

2.2 mol %) as the chiral ligand successfully delivered the

desired Si−O coupling silicon-stereogenic siloxane product 3a

in 36% yield with good enantiocontrol (74% ee) (Table 1,

Table 1. Development of Reaction Conditions

Entry

Ligand

Solvent

Yield (%)

ee (%)

toluene

CH CN

DCE

dioxane

Conditions: 1a (0.24 mmol), 2a (0.2 mmol), [Rh(cod)Cl] (1 mol

), ligand (2.2 mol %), in 2.0 mL solvent, under argon atmosphere,

12 h. The yield was determined by H NMR using CH Br as internal

2 2

standard. The ee values were determined by chiral HPLC.

continued interest in the construction of novel silicon-

stereogenic organosilicon compounds, we questioned whether

we could expand the toolbox to achieve a general

intermolecular dehydrogenative Si−H/O−H coupling, ena-

bling the streamlined synthesis of silicon-stereogenic siloxanes

and alkoxysilanes. Herein, we report the development of a

rhodium-catalyzed enantioselective intermolecular dehydro-

genative Si−O coupling reaction between dihydrosilanes with

silanols or alocohols, which gives access to a wide range of

highly functionalized chiral siloxanes and alkoxysilanes in good

to excellent yields and enatioselectivities (Scheme 1c). Further

utility of this enantioselective dehydrogenative Si−O coupling

was illustrated by the construction of CPL-active (circularly

polarized luminescence) silicon-stereogenic alkoxysilane small

organic molecules. Optically pure bis-alkoxysilane containing

two silicon-stereogenic centers and three pyrene groups

displayed a remarkable g_l_u_mvalue with a high ﬂuorescence

entry 1). Other Josiphos type ligands with diﬀerent electronic

properties (L2 and L3) did not increase the yields and

enantioselectivities (Table 1, entries 2 and 3). Interestingly,

when exchanging the tBu-substituent and the Ph-substituent

on the two phosphine atoms of the Josiphos ligand (L4), both

the yield and enantioselectivity were sharply increased

aﬀording 68% yield with 97% ee (Table 1, entry 4). Further

examination of the chiral ligands found that BINAP L5 and

Segphos L6 were also eﬀective for this reaction, albeit in a

relatively lower yields and ee (Table 1, entries 5−6).

Investigation of other common solvents in the presence of

L4 disclosed that DCE (1,2-dichloroethane) and 1,4-dioxane

could also aﬀord moderate yields of the desired siloxane 3a

with good enantioselectivities, while MeCN failed to produce

the corresponding product (Table 1, entries 7−9).

Having identiﬁed the optimized conditions for the

enantioselective dehydrogenative Si−O coupling reaction, we

quantum eﬃciency (g_l_u_m= 0.011, Φ = 0.55), which could have

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Communication

Table 2. Substrate Scope of Si-Stereogenic Siloxanes

Conditions: 1 (0.24 mmol), 2 (0.2 mmol), [Rh(cod)Cl] (1 mol %), L4 (2.2 mol %), in 2.0 mL toluene, under argon atmosphere, 12 h. Isolated

yields. The ee values were determined by chiral HPLC.

next assessed the scope of this transformation to establish the

general methodology for the construction of enantioenriched

silicon-stereogenic siloxanes (Table 2). First, the aromatic

rings (yellow part) of dihydrosilane substrates bearing diﬀerent

functional groups, including an electron-donating methoxy

group (1a), amino group (1b), electron-withdrawing ﬂuoro

group (1c), and phenyl, naphthyl groups (1d, 1e), all reacted

smoothly with dimethyl(phenyl)silanol 2a to aﬀord the desired

silicon-stereogenic siloxane products 3a−3e in moderate to

good yields (54−98%) with excellent enantioselectivities (96−

derivatives with high enantioselectivity, we questioned whether

we could expand this process to the coupling of dihydrosilane

with alcohol, challenging the historic problem of the catalytic

enantioselective alcoholysis of dihydrosilane for the con-

struction of architecturally complex and functionally diverse

enantioenriched silicon-stereogenic alkoxysilanes with high

eﬃciency and enantioselectivity. On the basis of the

established reaction conditions (for a detailed account of the

optimization study, see Supporting Information Table S1), a

wide range of alcohols displaying a variety of substituents were

found to be suitable substrates in the Rh-catalyzed

enantioselective dehydrogenative Si−O coupling reaction

(Table 3). Benzylic alcohol substrates bearing various

electron-donating or withdrawing substituents, such as methyl,

methoxy, bromo, and triﬂuoromethyl, were all well accom-

modated in this transformation to deliver the corresponding

silicon-stereogenic alkoxysilanes in 64−88% yields with 91−

99% ee (5a−5e). This process was also eﬀective with

naphthalene (5f) and heterocycles such as furan (5g) and

thiophene (5h). Sterically hindered secondary and tertiary

alcohols 5i and 5j also proceeded smoothly in the reaction

without any diﬃculty aﬀording the desired products with

excellent enantioselectivities (98−99% ee). Besides benzylic

alcohol, aliphatic alcohols with a number of diﬀerent

substituents were also suitable substrates undergoing this

dehydrogenative Si−O coupling giving the corresponding

products 5k−5n in 66−83% yields with 96−97% ee. To further

illustrate the utility of this methodology, we then examined the

reaction employing the core structures of several bioactive

molecules, pharmaceuticals, or materials building blocks, such

as phenothiaine (4o), D-ribofuranoside (4p), β-estradiol (4q),

8% ee). Second, the scope of silanols (blue part) was

investigated under the standard conditions. Methoxy, chloro,

and ﬂuoro groups (2f−2h) on the aromatic ring of the

dimethylphenylsilanols were well tolerated in the reaction,

producing the corresponding siloxanes 3f−3h in good yields

with 89−97% ee. Moreover, methyldiphenylsilanol (1i), tert-

butyldiphenylsilanol (1j), and trimethylsilanol (1k) were also

found to be competent substrates in the reaction, giving the

desired product 3i−3k in 56−75% yields without the loss of

enantioselectivities (92−97% ee). It is noteworthy that the

enantiopure bis-silicon-stereogenic siloxane 3l (>99% ee) could

easily be constructed via this strategy by the double coupling of

silanediol with dihydrosilane, which could be highly attractive

to organic material chemists due to their wide applications as

46−48

privileged monomers in silicon-based materials.

replacing the bulky tBu group into a methyl (1m), isopropyl

1n), cyclohexyl (1o), or 1-naphthyl (1p) group on the

Third,

(

dihydrosilane gave the corresponding siloxane products 3m−

p in moderate to good yields with 63−88% ee (purple part).

Since this intermolecular dehydrogenative Si−O coupling

reaction enables the facile access to silicon-stereogenic siloxane

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J. Am. Chem. Soc. 2021, 143, 5301−5307

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Communication

Table 3. Substrate Scope of Si-Stereogenic Alkoxysilanes

Conditions: 1 (0.24 mmol), 4 (0.2 mmol), [Rh(cod)Cl] (1 mol %), L4 (2.2 mol %), in 2.0 mL toluene, under argon atmosphere, 12 h. Isolated

yields. The ee values were determined by chiral HPLC. X-ray crystallographic analysis of 5o allowed determination of the absolute conﬁguration;

and conﬁgurations of the products 5 were assigned by analogy.

and pitavastatin fragment (4r). We were delighted to ﬁnd that

the corresponding silicon-stereogenic alkoxysilane products

UV−vis spectra; the absorption maxima are at around 317,

332, and 349 nm, respectively (Figure 1c). The emission

maxima vary from 396 to 508 nm. Both the monomer (around

400 nm) and excimer (around 500 nm) emission peaks were

observed for 6a and 6b. However, only a red-shifted,

broadened excimer emission peak (around 500 nm) was

seen for 6c, which may originate from the multiple pyrene

5o−5r could be obtained in good yields with excellent

stereoselectivities, irrespective of existing diverse functional

groups and complex molecular structures. Finally, the

interesting bis-silicon-stereogenic alkoxysilane 5s was also

obtained by the double coupling between 1,4-phenylenedime-

thanol with dihydrosilane in 52% yield with excellent

enantioselectivity (>99% ee).

units within the molecule in proximity (Figure 1d). It is

worthy of mention that 6c has a larger Stokes shifts (159 nm)

and a higher absolute ﬂuorescence quantum yield (0.55)

compared with 6a and 6b (see Supporting Information Table

S2 for details), which is suitable for the potential applications

in chiroptical materials. Then, we further investigated the

chiroptical properties of these π-conjugated silicon-stereogenic

alkoxysilanes via circular dichroism (CD) and circular

polarized luminescence (CPL) spectra. The CD spectra are

mirror images of each other, in which 6a, 6b, and their

enantiomers display clear Cotton eﬀects at about 330 and 348

nm, while (R, R)-6c and (S, S)-6c exhibit stronger opposite

signals at around 330 and 355 nm, respectively (Figure 1e).

Finally, the CPL experiment which reﬂected the chirality of

emitting excited states was conducted. To our delight, 6a−6c

are all CPL-active, displaying clear mirror images of each pair

of the enantiomers (Figure 1f). It is noteworthy that (R, R)-6c

and (S, S)-6c containing two silicon-stereogenic centers and

In recent years, there has been growing interest in the

chiroptical molecules that display circularly polarized lumines-

cence (CPL), which has great potential applications in 3D

displays, communication of spin information, information

49−53

storage and processing, CPL lasers, and biological probes.

Among various chiral luminescent systems, chromophores with

silicon-stereogenic centers exhibited potential intense CPL

54−59

emission properties due to their σ*−π* conjugation.

demonstrate the utility of these interesting chiral organosilicon

compounds, we ﬁnally synthesized three enantioenriched

silicon-stereogenic alkoxysilanes 6a−6c containing pyrene

groups under the standard conditions (Figure 1a) and

examined their photophysical properties. Under UV light

irradiation (365 nm), the solution of 6a and 6b in CH Cl

exhibited blue luminescence, while 6c displays bright green

luminescence (Figure 1b). These compounds show similar

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Journal of the American Chemical Society

J. Am. Chem. Soc. 2021, 143, 5301−5307

https://pubs.acs.org/doi/10.1021/jacs.1c01106.

Communication

Figure 1. Photophysical properties investigations. (a) Enantioselective synthesis of eﬀective CPL-active silicon-stereogenic alkoxysilanes. (b)

Fluorescence images of π-conjugated silicon-stereogenic alkoxysilanes (λ = 365 nm). (c) Absorption spectra of π-conjugated silicon-stereogenic

−

−3

alkoxysilanes in CH Cl (10 M). (d) Emission spectra of π-conjugated silicon-stereogenic alkoxysilanes in CH Cl (10 M). (e) CD spectra of

−

a−6c and their enantiomers in CH Cl (2.0 × 10 M) at room temperature. (f) CPL spectra of 6a−6c and their enantiomers in CH Cl (1.0 ×

−3

M) at room temperature, excited at 349 nm. (g) g_l_u_mvalues−wavelength curve for 6a−6c and their enantiomers.

three pyrene groups show intense CPL signals that range from

70 to 675 nm. The luminescence dissymmetry factors (g

were measured as −1.1 × 10 for (R, R)-6c and +1.1 × 10

In summary, we have developed a rhodium-catalyzed

enantioselective intermolecular dehydrogenative Si−O cou-

pling reaction between dihydrosilanes with silanols or

alocohols, which gives access to a wide range of highly

functionalized chiral siloxanes and alkoxysilanes in good to

excellent yields and enatioselectivities. Further utility of this

process was illustrated by the construction of CPL-active

silicon-stereogenic alkoxysilane small organic molecules

displaying large g_l_u_mvalues with high emission eﬃciency,

which could have great potential applications in chiral organic

optoelectronic materials.

)

lum

−2

−

for (S, S)-6c at 573 nm (Figure 1g). To the best of our

knowledge, it is fairly rare that small organic compounds with a

stereogenic center exhibit such a high g_l_u_mvalue with a high

ﬂuorescence quantum eﬃciency (g_l_u_m= 0.011, Φ = 0.55).

To further demonstrate the importance of silicon-stereogenic

centers in these chiroptical alkoxysilanes, the chiral-at-carbon

linked pyrene analogue of 6a was synthesized and numbered as

a′. The photophysical properties showed that the absorption

and CD spectra of 6a′ were similar to 6a. However, the

excimer emission peak of 6a′ became structureless. In addition,

both the CPL intensity and Φ of 6a ′ (g = 0.0030, Φ =

ASSOCIATED CONTENT

■

sı Supporting Information

lum

The Supporting Information is available free of charge at

.07) were lower than 6a (see Supporting Information Table

S2 and Figure S1 for details). This comparison indicates that

chromophores linked with silicon-stereogenic centers could be

quite attractive. We believe that these easily accessed silicon-

stereogenic alkoxysilanes with intense CPL properties could

have great potential application in the development of

interesting chiroptical devices in the near future.

Materials and methods, experimental procedures,

optimization studies, characterization data, photophys-

ical properties studies, H, C, and F NMR spectra,

HPLC spectra and mass spectrometry data of new

compounds (PDF )

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Journal of the American Chemical Society

Groups for the Creation of Structurally Controlled Siloxane-Based

Communication

Accession Codes

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data for this paper. These data can be obtained free of charge

via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

data_request@ccdc.cam.ac.uk, or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Joergensen, K. A. The Prevalence of Diarylprolinol Silyl Ethers as

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Chuan He − Shenzhen Grubbs Institute and Department of

Chemistry, Guangdong Provincial Key Laboratory of

Catalysis, Southern University of Science and Technology,

Shenzhen, Guangdong 518055, China; orcid.org/0000-

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002-9983-7526 ; Email: hec@sustech.edu.cn

and Application of Silicon-Stereogenic Silanes: A Renewed and

Authors

Significant Challenge in Asymmetric Synthesis. Chem. Soc. Rev. 2011,

Jiefeng Zhu − School of Chemistry and Chemical Engineering,

Harbin Institute of Technology, Harbin 150080, China;

Shenzhen Grubbs Institute and Department of Chemistry,

Guangdong Provincial Key Laboratory of Catalysis, Southern

University of Science and Technology, Shenzhen, Guangdong

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Shuyou Chen − Shenzhen Grubbs Institute and Department of

Chemistry, Guangdong Provincial Key Laboratory of

Catalysis, Southern University of Science and Technology,

Shenzhen, Guangdong 518055, China

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Catalyzed by Rhodium Complexes. J. Organomet. Chem. 1976, 120,

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Author Contributions

J.Z. and S.C. contributed equally to this work.

Notes

‡

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Asymmetric Silane Alcoholysis: Practical Access to Chiral Silanes. J.

The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGMENTS

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We are grateful for ﬁnancial support from the National Natural

Science Foundation of China (21901104), the Thousand

Talents Program for Young Scholars, the start-up fund from

Southern University of Science and Technology, and

Guangdong Provincial Key Laboratory of Catalysis (No.

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Synthesis of Organosilicon Compounds using a C2 Chiral Auxiliary.

020B121201002). We also thank Wei Lu (SUSTech) and

Bull. Chem. Soc. Jpn. 1997, 70, 1393−1401.

Chao Zou (SUSTech) for their kind help with the photo-

physical properties investigations. We dedicate this paper to

the 10th anniversary of the Department of Chemistry,

SUSTech.

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