Palladium-Catalyzed Enantioselective Carbene Insertion into Carbon-Silicon Bonds of Silacyclobutanes

Article abstract of DOI:10.1021/jacs.1c05879

We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si-C bonds with excellent enantioselectivity, which provides a rapid and distinct method to access silacyclopentanes with a three- or four-substituted stereocenter asymmetrically. Mechanistic studies using hybrid density functional theory suggest a catalytic cycle involving oxidative addition, carbene migratory insertion, and reductive elimination. In addition, roles of the chiral ligands in controlling the reaction enantioselectivity are also elucidated.

Full text of DOI:10.1021/jacs.1c05879

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Communication
Palladium-Catalyzed Enantioselective Carbene Insertion into
Carbon−Silicon Bonds of Silacyclobutanes
Jingfeng Huo, Kangbao Zhong, Yazhen Xue, MyeeMay Lyu, Yifan Ping, Zhenxing Liu, Yu Lan,*
and Jianbo Wang*
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ABSTRACT: We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si−C bonds with excellent
enantioselectivity, which provides a rapid and distinct method to access silacyclopentanes with a three- or four-substituted
stereocenter asymmetrically. Mechanistic studies using hybrid density functional theory suggest a catalytic cycle involving oxidative
addition, carbene migratory insertion, and reductive elimination. In addition, roles of the chiral ligands in controlling the reaction
enantioselectivity are also elucidated.
1a).14 In contrast, catalytic carbene insertion into Si−C bonds,
rganosilicons, compounds that contain Si−C bonds,
Ohave found increasing importance in material sciences which would serve as a straightforward approach for accessing
and pharmaceuticals.1−4 They also serve as valuable
organosilicons, remained elusive, and the corresponding
enantioselective version is expected to be attractive to access
chiral organosilanes.
intermediates or latent functional groups in chemical syn-
thesis.5,6 Catalytic carbene insertion into Si−H bonds
represents an efficient strategy to construct Si−C bonds,7
and a number of highly enantioselective insertion reactions
have been established with chiral transition-metal catalysts and
engineered enzymes.8−13 Mechanistically, these reactions are
initiated by the formation of metal carbene species, then
followed by concerted insertion into Si−H bonds (Scheme
We have recently proposed an alternative strategy for the
formal insertion of carbene into σ bonds, which is based on a
stepwise process (Scheme 1b).15−17 The catalytic cycle
involves oxidative addition, carbene formation, migratory
insertion, and reductive elimination. We hypothesized that
such a strategy could be applicable to a carbene-mediated Si−
C bond insertions. To examine this hypothesis, silacyclobu-
tanes (SCBs) were employed as the substrates. Silacyclobu-
tanes are a class of silicon-containing building blocks in organic
synthesis;18 in particular, alkynes, alkenes, and a variety of
other π or σ units have been applied to the ring expansion of
silacyclobutanes to construct sila-heterocycles.19−23 However,
the insertion of metal carbene into Si−C bonds of
silacyclobutanes under catalytic conditions remained unex-
plored.24 Our initial study with TMSCHN2 as the carbene
precursor demonstrated that the Pd-catalyzed carbene
insertion into the Si−C bond of SCB was indeed attainable.
We then conceived to challenge the enantiocontrol in this
carbene Si−C bond insertion reaction (Scheme 1c).
Scheme 1. Catalytic Carbene Insertion into σ Bonds
At the outset, (2,4,6-triisopropylphenyl)sulfonylhydrazone
(trisylhydrazone) was identified as the suitable carbene
precursor for this reaction, because the diazo intermediate
could be generated in situ from this type of sulfonylhydrazones
at low temperature.10,25 Thus, with trisylhydrazone 1a as the
carbene precursor, the Pd-catalyzed reaction with SCB 2a was
Received: June 7, 2021
Published: August 12, 2021
https://doi.org/10.1021/jacs.1c05879
J. Am. Chem. Soc. 2021, 143, 12968−12973
© 2021 American Chemical Society
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a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope of the Reaction with
Trisylhydrazones Derived from Aldehydes
a
a
Reaction conditions: trisylhydrazone 1b−w (0.1 mmol), silacyclo-
butane 2b (0.15 mmol), PdCp(η3-C3H5) (4 mol %), (R,R,R)-L9 (6
mol %), LiOtBu (0.3 mmol), dioxane/toluene = 1:1 (1.0 mL), 0 °C,
24 h. Isolated yields are reported.
a
Reaction conditions: 1a (0.10 mmol, 1.0 equiv), 2a (0.15 mmol, 1.5
equiv), [Pd] (4 mol %), ligand (6 mol %), LiOtBu (3.0 equiv),
dioxane/toluene = 1:1 (1.0 mL), 0 °C, 24 h. Isolated yields after
silica gel column chromatography. Enantiomeric excesses (ee) were
b
c
the Pd(II) catalyst, a series of chiral phosphoramidite ligands
were then screened. The modification of the N-substituents in
the ligand could significantly affect both the yield and ee of the
reaction, but for ligands L2−L8 the results were disappointing
(entries 7−13). The enantioselectivity was dramatically
improved when (R,R,R)-L9 was employed as the chiral ligand
(entry 14). For comparison, the corresponding (S,R,R)-L9 was
found to give a very low yield under the identical conditions
(entry 15). Thus, the matched chirality in the ligand is crucial
for the reaction. Further substitution on the BINOL moiety
d
determined by HPLC. 2b was the substrate under otherwise
identical conditions. The yield refers to 3b.
inspected (Table 1). The initial studies (entries 1−6) indicated
some Pd(II) catalysts combined with BINOL-phosphoramidite
ligand L1 could afford the desired insertion product 3a, in
which PdCp(η3-C3H5) gave the best results in terms of the
yield, but the ee was low (entry 6). With PdCp(η3-C3H5) as
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Scheme 3. Substrate Scope of the Reaction with
Trisylhydrazones Derived from Aromatic Ketones
Scheme 4. Scale-up Experiment and Applications
a
a
Reaction conditions: trisylhydrazones 5a−g (0.1 mmol), silacyclo-
butane 2b (0.15 mmol), PdCp(η3-C3H5) (4 mol %), (R,R,R)-L9 (6
mol %), LiOtBu (0.6 mmol), toluene (0.5 mL), rt, 36 h. Isolated
yields are reported.
(L10, L11) gave poor results (entries 16, 17). The chiral spiro
backbone-based phosphoramidite ligand L12 was examined,
and the yield could be improved but the ee was largely
diminished (entry 18). Finally, the SCBs bearing various
substituent groups at the aromatic moiety Ar were examined
(see Supporting Information (SI) for details). While the ee
remained constantly high over a series of SCBs, the SCB
bearing p-CF3 substituent 2b could afford the product 3b in
91% yield and 99% ee (entry 19).
Scheme 5. Proposed Reaction Mechanism
Subsequently, we started to explore the scope of this
reaction with a series of trisylhydrazones derived from
aldehydes (Scheme 2). For the reaction with trisylhydrazones
derived from aromatic aldehydes, the substrates bearing
substituents in para-, meta-, and ortho- positions with different
electronic properties all proceeded well to afford the
corresponding products 4a−v with high yields and excellent
enantioselectivities. Some functional groups, such as Bpin (4g)
and CO2Me (4f and 4l), were also tolerated, making the
products amenable for further modifications. Likewise, the
trisylhydrazones derived from naphthyl aldehydes (4n and 4o)
and heteroaromatic aldehydes (4q−t) gave the insertion
products in good yields without erosion of the enantiose-
lectivities. The reaction also worked with trisylhydrazones
derived from unsaturated aldehydes (4u and 4v). The absolute
configuration of product 4i was determined to be S by X-ray
crystallographic analysis (CCDC 2042635). We also carried
out the reaction with SCB in which the silicon is substituted by
an aryl and a methyl group. The chiral center on silicon should
be generated in the reaction; however, the diastereoselectivity
of the isolated product 4w was low. SCBs bearing alkyl
substituents were also examined; however, carbene insertion
products were not observed in these cases (see SI).
Under slightly modified conditions, this reaction also
occurred smoothly with the trisylhydrazones 5a−g derived
from ketones, generating silacyclopentanes bearing a quater-
nary chiral center (6a−g) (Scheme 3). The trisylhydrazones
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Figure 1. (a) Free energy profiles for path A and path B. The energies are in kcal/mol and represent the relative free energies, which were
calculated using the M06-2X method in toluene solvent. (b) Optimized structures and IGM analysis of the transition states TS-2 and TS-2′ for
forming (S)-3a and (R)-3a. For IGM analysis, blue, attraction; green, weak interaction; red, steric effect.
derived from cyclic ketones were competent substrates in this
reaction, affording the corresponding chiral spiro silanes (6c−
g). The absolute configurations of the products (6a and 6d)
were determined by X-ray crystallographic analysis (CCDC
2042632 and 2042637).
To show the synthetic practicality of this reaction, the
carbene insertion was conducted on a preparative scale. Thus,
the reaction of 2b with 1a on a 3.0 mmol scale was conducted
under the standard conditions to afford 3b in 94% yield with
excellent enantioselectivity (Scheme 4a). The product 3a could
be oxidized to give chiral diol 7 without erosion of ee (Scheme
4b). Furthermore, we have demonstrated that this palladium-
catalyzed method could be applied to the hydrazone substrates
derived from natural product (−)-perillaldehyde 8 as well as
drug molecules estrone 10 and iloperidone 12 (Scheme 4c).
On the basis of our previous studies,26,27 we propose the
putative pathways for the reaction, which involves carbene-
coordinated palladacycle species D as the key intermediate
(Scheme 5). For the generation of D, two pathways (Path A
and Path B) are considered, both starting with ligand and SCB-
coordinated Pd(0) species A (see SI for the details).28 In Path
A, the diazo substrate 1a′, generated in situ from the
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corresponding trisylhydrazone 1a, coordinates to A to form
palladium carbene species C by releasing N2. Then oxidative
addition with 2a generates the intermediate D. Alternatively, D
can be generated through Path B, in which oxidative addition
with 2a occurs first to form E, followed by interaction with 1a′
to generate intermediate D. From D, carbene migratory
insertion occurs with either the Pd−Si bond or Pd−C bond,
generating intermediate G or H respectively. Finally, reductive
elimination gives 3a and regenerates catalyst A.
To substantiate this hypothesis, DFT computation using
M06-2X method is performed (see SI for the details).29,30 The
ligand and SCB-coordinated Pd(0) complex A is chosen as the
starting point (Figure 1a).28 In Path A, the interaction of A
with the diazo substrate occurs first to generate palladium
carbene C with an energy barrier of 22.7 kcal/mol. Oxidative
addition of C with 2a then generates intermediate D with an
energy barrier of 11.3 kcal/mol. In Path B, oxidative addition
of A occurs with 2a first to generate five-membered ring
palladium intermediate E, which is endergonic by 4.7 kcal/mol.
Subsequent coordination with 1a′, followed by releasing of N2,
generates the common intermediate D with an energy barrier
of 22.7 kcal/mol. From intermediate D, migratory insertion of
the carbene moiety into the Pd−Si bond or Pd−C bond
releases 32.8 or 62.3 kcal/mol of free energy, with very low
energy barriers in both cases. Finally, reductive elimination
occurs from G or H to release final product 3a and palladium
catalyst, which is exergonic by 41.4 and 11.9 kcal/mol,
respectively. Hence, the rate-determining step in both Path A
and Path B is carbenation, and the total activation energy of
Path A is 22.7 kcal/mol and path B is 27.4 kcal/mol,
suggesting that Path A is more favorable.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c05879.
Experimental procedures and spectral data for all new
compounds and details of DFT calculation (PDF)
Accession Codes
CCDC 2042632, 2042635, and 2042637 contain the
supplementary crystallographic 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 Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Jianbo Wang − BNLMS, Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of
Education, College of Chemistry, Peking University, Beijing
100871, China; State Key Laboratory of Organometallic
Chemistry, Chinese Academy of Sciences, Shanghai 200032,
China; orcid.org/0000-0002-0092-0937;
Email: wangjb@pku.edu.cn
Yu Lan − School of Chemistry and Chemical Engineering,
Chongqing Key Laboratory of Theoretical and
Computational Chemistry, Chongqing University, Chongqing
400030, China; Green Catalysis Center, and College of
Chemistry, Zhengzhou University, Zhengzhou 450001,
China; orcid.org/0000-0002-2328-0020; Email: lanyu@
cqu.edu.cn
For Path A, the enantioselectivity-determining step will be
the oxidative addition of C with 2a, namely the formation of
intermediate D. To inspect the origin of the enantioselectivity,
independent gradient model (IGM) analysis of the oxidative
addition transition states for generating both enantiomers was
performed. As shown in Figure 1b, there is a clear steric
repulsion between the phenyl group of the carbene moiety and
the phenyl group of reacting 1,1-diphenylsiletane in the
transition state TS-2′ for generating (R)-3a. Furthermore,
there is a weak repulsion between the 1,1-diphenylsiletane
reactant and (R,R,R)-L9 ligand in this transition state. In
contrast, there is a weak attraction between the phenyl group
of the carbene moiety and the phenyl group of 1,1-
diphenylsiletane in the transition state TS-2 for generating
(S)-3a. The two transition states differ in free energy of
activation by 1.2 kcal/mol favoring the S product, which is in
good agreement with the experiments. Hence, the enantiose-
lectivity is mainly controlled by the steric effect of the oxidative
addition transition state, in which the chiral phosphoramidite
bearing bulky substituents is crucial for the enantioselectivity.31
In summary, we have developed an unprecedented
asymmetric carbene insertion reaction into the Si−C bonds
of silacyclobutanes catalyzed by palladium ligated with a chiral
phosphoramidite. The reaction provides a unique approach
toward the construction of silicon−carbon bonds in a
stereoselective manner. Mechanistic investigations through
computational methods reveal the details of the reaction
pathways and the factors governing the enantiocontrol of the
insertion reaction.
Authors
Jingfeng Huo − BNLMS, Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of
Education, College of Chemistry, Peking University, Beijing
100871, China
Kangbao Zhong − School of Chemistry and Chemical
Engineering, Chongqing Key Laboratory of Theoretical and
Computational Chemistry, Chongqing University, Chongqing
400030, China
Yazhen Xue − BNLMS, Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of
Education, College of Chemistry, Peking University, Beijing
100871, China
MyeeMay Lyu − BNLMS, Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of
Education, College of Chemistry, Peking University, Beijing
100871, China
Yifan Ping − BNLMS, Key Laboratory of Bioorganic
Chemistry and Molecular Engineering of Ministry of
Education, College of Chemistry, Peking University, Beijing
100871, China
Zhenxing Liu − Green Catalysis Center, and College of
Chemistry, Zhengzhou University, Zhengzhou 450001, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c05879
Notes
The authors declare no competing financial interest.
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of Silacycles Possessing a Tetraorganosilicon Stereocenter. J. Am.
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ACKNOWLEDGMENTS
■
The project is supported by NSFC (91956104 and 21822303),
BNLMS, and Laboratory for Synthetic Chemistry and
Chemical Biology of Health@InnoHK of ITC, HKSAR. We
thank the program for Science Technology Innovation Talents
in Universities of Henan Province (No. 20HASTIT004).
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catalyzed Intermolecular Exchange between C-C and C-Si σ Bonds. J.
Am. Chem. Soc. 2017, 139, 12414−12417.
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