Palladium-catalyzed Si-C bond-forming silylation of aryl iodides with hydrosilanes: An enhanced enantioselective synthesis of silicon-stereogenic silanes by desymmetrization

Article abstract of DOI:10.1039/c6ra12873d

An enantioselective Pd-catalyzed silicon-carbon bond-forming silylation reaction of aryl iodides with hydrosilanes for the synthesis of silicon-stereogenic silanes has been developed, in which a systematic optimization of a TADDOL-derived monodentate phos

Full text of DOI:10.1039/c6ra12873d

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Palladium-catalyzed Si–C bond-forming silylation
of aryl iodides with hydrosilanes: an enhanced
enantioselective synthesis of silicon-stereogenic
silanes by desymmetrization†
Cite this: RSC Adv., 2016, 6, 67113
Received 18th May 2016
Accepted 8th July 2016
a
a
a
a
a
*
Li Chen, Jiang-Bo Huang, Zheng Xu, Zhan-Jiang Zheng, Ke-Fang Yang,
DOI: 10.1039/c6ra12873d
a
a
ab
*
Yu-Ming Cui, Jian Cao and Li-Wen Xu
www.rsc.org/advances
An enantioselective Pd-catalyzed silicon–carbon bond-forming sily- silylation of aryl halides with hydrosilanes by transition-metal
lation reaction of aryl iodides with hydrosilanes for the synthesis of complexes, which can widen the applicability of silicon–
silicon-stereogenic silanes has been developed, in which a systematic carbon bond-forming silylation reactions to catalytic asym-
optimization of a TADDOL-derived monodentate phosphoramidite metric transformation with various aryl halides and functional
ligand set resulted in the identification of a new TADDOL-derived silanes. Meanwhile, optically active organosilicon compounds
phosphoramidite ligand that accesses chiral silanes with moderate bearing a silicon-stereogenic center have attracted much
to good yield and enantioselectivity under mild conditions.
attention in the elds of organic synthesis, functional material,
and bioorganic chemistry.4
Although the potential of palladium-catalyzed Si–C cross
coupling approach with hydrosilanes has been revealed since
1994,5 limited progress has been reported in catalytic silicon–
carbon-bond-forming silylation of aryl halides with hydro-
silanes.6 In this context, Masuda et al. described the rst
example of palladium(0)-catalyzed Si–C bond-forming silylation
of aryl halides with hydrosilane.7 And then in the subsequent
decade, other groups have also reported a series of catalytic
strategies for the palladium-promoted silylation of aryl halides
with hydrosilanes in the presence of various phosphine
ligands.8 However, the synthesis of silicon-stereogenic organo-
silicon compounds through palladium-catalyzed silicon–
carbon bond-forming silylation with aryl halides and hydro-
silanes is not an easy task because there is no successful
example in the past decade.4c Notably, as the only example in
this context, Yamanoi and Nishihara9 have ever reported an
enantioselective Pd2(dba)3-catalyzed silicon–carbon bond-
forming silylation of aryl halides with dihydrosilanes in the
presence of chiral TADDOL-derived phosphoramidite ligand
(Scheme 1), which afforded the optically active tertiary silanes
with low to moderate enantioselectivities (11–77% ee for 13
examples) as well as low to moderate yields. This pioneering
work also revealed the difficulty and challenge in the stereo-
selective construction of silicon-stereogenic silanes through
palladium-catalyzed silicon–carbon cross-coupling of aryl
halides with dihydrosilanes. Therefore, the development of an
efficient or improved asymmetric method for the enantiose-
lective silicon–carbon bond-forming silylation of aryl halides
with dihydrosilanes is eagerly awaited. Herein we report an
improved synthetic strategy with the development of highly
The selective construction of aryl carbon–silicon bonds is one of
the most important and challenging reactions in element-
organic chemistry and especially in the eld of organosilicon
chemistry,1 which enables the transformation of earth-
abundant silicon resources to synthetically useful organo-
silicon compounds. Accordingly, extensive investigation of
silicon–carbon bond-forming transformations has been carried
out for the synthesis and application of valuable organosilicon
compounds in versatile research elds.2 In this regard, the
classic method for the preparation of aryl silanes is the tradi-
tional alkali metals-involved Wurtz-type coupling of chlor-
osilanes with organic halides. An alternative and improved
method is the aryl magnesium reagent-based Grignard addition
to chlorosilanes.3 Despite the direct nucleophilic substitution of
chlorosilanes using an organometallic reagent is a useful
approach to the synthesis of aryl silanes, the nucleophilic
substitution reaction suffers from serious drawbacks such as
the requirement of strictly anhydrous conditions, possibly toxic,
difficult to handle, or less practical reaction temperatures. And
especially, it is impossible to be used for the construction of
chiral silanes by Wurtz- or Grignard-type addition. Gratifyingly,
recent efforts have focused on the development of catalytic
aKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education, Hangzhou Normal University, Hangzhou 311121, P. R. China. E-mail:
liwenxu@hznu.edu.cn; licpxulw@yahoo.com; Fax: +86 2886 5135; Tel: +86 2886 5135
bState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra12873d
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speculated that common TADDOL-derived phosphoramidite
ligands with alkyl substituents in the 3,5-position of phenyl
rings are not enough large with the aid of bulky group to control
the enantioselectivity of such reactions. For high levels of ster-
eocontrol, the TADDOL-derived phosphoramidite ligand would
be needed to extend substituents by introduction of bulky group
on ketal part and modication of nitrogen-center steric repul-
sion and electronic effect that could engage in a potential space
interaction between ligand and substrate. In this way, the
TADDOL-derived phosphoramidite could induce the molecular
reaction tube with specic responses to certain type of
substrates as
a single-molecular reactor. Based on this
hypothesis, we synthesized various TADDOL-derived phos-
phoramidite ligands bearing different substituents on ketal
(modication 1) or aryl ring (modication 2) or amine (modi-
cation 3) showed in Scheme 3.11 These phosphoramidite
ligands inherited the feature of TADDOL-based backbone and
contain the features which were important for this reaction in
the stereoselective induction of silicon–carbon bond-forming
silylation.
Despite the TADDOL-derived chiral phosphoramidites have
been widely used as effective P-ligands in various transition-
metal-catalyzed asymmetric transformations,11,12 the structural
modication of TADDOL-derived phosphoramidites with high
level of enantioselectivity for silicon–carbon bond-forming
silylation reaction is not an easy task. Then we began our
investigations using methylphenylsilane 1a and 2-iodoanisole
2a as model substrates in this reaction (Table 1 and Scheme 1).
With eight representative TADDOL-based phosphoramidites
bearing different substituents in hand (Scheme 3), we investi-
gated the enantioselective induction of these chiral phosphor-
amidites L1–L8 in silicon–carbon bond-forming silylation
Scheme 1 Previous and best result in the asymmetric palladium-
catalyzed Si–C bond-forming silylation of 2a with 1a that reported by
Nishihara and Yamanoi.9
efficient TADDOL-based chiral P-ligands – enhanced palladium-
catalyzed silicon–carbon bond-forming silylation of aryl iodides
with dihydrosilanes on the basis of previous ndings in chiral
phosphoramidite ligand chemistry.
Drawing inspiration from the powerful silicon–carbon cross-
coupling strategies, in which the hydrosilanes are merged with
aryl halides to generate aryl silanes, we envisioned that general
phosphine ligands could be used in this reaction. However, in
the preliminary investigation on the palladium-catalyzed
silicon–carbon bond-forming silylation of aryl iodides with
dihydrosilanes in the presence of commercially available P-
ligands or our ligands10 that could be applied in various cata-
lytic asymmetric transformations, almost no product was
detected in these cases (Scheme 2).
In light of previous progress in the palladium-catalyzed
silicon–carbon bond-forming silylation of aryl iodides,5–9 we
Scheme 2 The evaluation of commercially available BINAP, BINAPO,
and our ligands10 in the palladium-catalyzed silicon–carbon bond- Scheme 3 The optimization of TADDOL-based phosphoramidite by
forming silylation of aryl iodide 2a with dihydrosilane 1a.
exploiting steric repulsion and electronic effect.
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Table 1 The evaluation of modular TADDOL-based phosphoramidite
ligands in the palladium-catalyzed Si–C bond-forming silylation of 2a
with 1a
as chiral ligand, the solvent effect on the catalytic activity of
Pd2(dba)3 was really important and except THF, these solvents
were found to the inferior media to promote the enantiose-
lective silicon–carbon bond-forming silylation reaction with
varied yields and enantioselectivities (entries 1–8, Table S1 of
ESI†). For example, most of solvents led to the deceased enan-
tioselectivity (17–79% ee), whereas the protic solvent, such as
methanol, resulted in no reaction (entry 7, Table S1 of ESI†).
Notably, although Pd(PPh3)2Cl2, PdCl2, and Pd(OAc)2 were also
effective in this reaction (45–60% yields, 9–51% ee), Pd2(dba)3
turned out to be the best catalyst in term of enantioselectivity.
In addition, we continued to investigate various bases, such as
DBU, DIPEA, K2CO3, TMEDA, in the model silicon-bond form-
ing silylation reaction (entries 14–17, Table S1 of ESI†).
Surprisingly, TMEDA or K2CO3 was found to be the suitable
base in this reaction (76% ee or 80% ee respectively, entries 16
and 17), which further supported the choice of solvent, base and
ligand in Table 1. Notably, strong base, such as DBU, was
proved to be non-effective for this reaction (entry 14, Table S1 of
ESI†). Comparably, the use of DIPEA as base gave almost no
conversion under the present reaction conditions (entries 17
and 18, Table S1 of ESI†). Although the optimized reaction
conditions were not perfect in this reaction (up to 85% ee in this
model reaction), the importance of chiral TADDOL-based
phosphoramidite L8 as well as other factors was conrmed on
the basis of these results. To the best of our knowledge, the
enantioselectivity achieved in this work reached to the highest
level for such silicon–carbon bond-forming silylation reaction.
With the optimized conditions, the catalytic activity of
palladium/L8 complex generated in situ from TADDOL-based
phosphoramidite (L8) and Pd2(dba)3 was then investigated in
the silicon–carbon bond-forming silylation reaction of various
aryl iodides. As shown in Scheme 4, the Pd/L8 catalyst system
was also applicable to the silicon–carbon bond-forming silyla-
tion reaction of various aryl iodides that were converted into
corresponding silicon-stereogenic silanes in moderate
yields and promising enantioselectivities (up to 86% ee). Simi-
larly to previous report, the silicon–carbon bond-forming sily-
lation reaction of dihydrosilane with aryl iodides bearing
methyl or methoxyl groups at p-, m-, and o-position afforded
varied isolated yields (28–62% yields). Sterically hindered aryl
iodides, such as 2-iodoanisole (2a), 1-iodo-2-methylbenzene
(2d), 1-iodo-2,4-dimethylbenzene (2f), 1-iodonaphthalene (2g),
1-iodo-4,5-dimethoxy-2-methylbenzene (2j), and 1-iodo-2-
methoxynaphthalene (2k), were also resulted in good ee value
of the desired products (3d, 3f, 3j, and 3k), which supported the
crucial role of steric repulsion between substrate and catalyst in
this reaction. More importantly, the substituted methyl or
methoxyl group at different positions on the phenyl ring of aryl
iodides gave varied enantioselectivity for this silicon–carbon
bond-forming silylation reaction. For example, the Pd/L8 cata-
lyst exhibited different activity in enantioselective induction for
the silicon–carbon bond-forming silylation reaction of
methoxyl-substituted aryl iodides (2a–c) with methyl-
phenylsilane, in which the order of enantioselectivity for
methoxyl-substituted aryl iodides is 2-position (3a, 85% ee) > 3-
position (3b, 35% ee) > 4-position (3c, 14% ee). Therefore, these
Entry
P-Ligand
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L5
L6
L7
L8
90
59
42
38
51
56
25
50
40
75
28
10
42
79
36
85
a
The reaction was carried out with methylphenylsilane 1a (1.5 mmol).
2-Iodoanisole 2a (1.0 mmol), trimethylamine (3.0 equiv.), Pd2(dba)3
(0.025 mmol), ligand (0.075 mmol), in THF (2.0 mL), at ꢀ40 ꢁC, for 4
days. And the isolated yield is puried by ash column
b
chromatography. The enantioselectivity was determined by chiral
HPLC analysis employing a chiral stationary phase. And absolute
conguration was determined by comparing with the literature value.
reaction of methylphenylsilane 1a and 2-iodoanisole 2a in the
absence or presence of Pd2(dba)3. Interestingly, it was not
perfect in both catalytic activity and enantioselectivity for all the
TADDOL-derived phosphoramidites. As shown in Table 1, the
silicon–carbon bond-forming silylation reaction carried out
with TADDOL-derived phosphoramidites L1–L8 bearing
different substituents on three positions at ꢀ40 ꢁC proceeded to
give the desired product in varied yields (25–90% yields) and
low to good enantioselectivities (10–85% ee), which revealed the
great contribution of these substituents on chiral TADDOL-
based phosphoramidites to the enantioselective construction
of silicon-stereogenic center in this reaction. For example, the
TADDOL-derived phosphoramidite L1 (Ar ¼ Ph, X ¼ NMe2, Y1 ¼
Y2 ¼ Me) gave the desired product in good yield (90%) but with
only low enantioselectivity (40% ee). Gratifyingly, the use of
chiral TADDOL-derived phosphoramidites with N-methylani-
line as a substituent, L2, L6, and L8, led to the enantioselectivity
enhancement from 40% ee to 85% ee (Table 1, entries 2, 6, and
8). Especially, when the TADDOL-based phosphoramidite L8
(Scheme 3, Ar ¼ 3,5-(t-Bu)2, 4-OMePh, X ¼ NMePh, Y1 ¼ Y2 ¼
cyclohexyl) was used as a chiral ligand, the best enantiose-
lectivity was achieved in this case (85% ee).
Encouraged by the ligand-controlled enantioselective
enhancement outlined above, we turn attention to investigate
the effect of different solvents, palladium salts, and bases, on
the enantioselective silicon–carbon bond-forming silylation
reaction of 2a with 1a, since they had been demonstrated to be
important factors for catalytic asymmetric transformations. The
screening results from these investigations are presented in
Table S1 (see ESI†). With TADDOL-based phosphoramidite L8
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substrate-sensitive feature in the silicon–carbon bond-forming
silylation reaction, which is similarly to previous reports on
catalytic synthesis of silicon-stereogenic silanes by arylation of
hydrosilanes with aryl iodides. In any event, the enantiose-
lective silylation method described in this reaction is still an
effective and improved procedure for the catalytic synthesis of
various silicon-stereogenic silanes. Though only the good but
not excellent enantioselectivity of this silicon–carbon bond-
forming silylation reaction was observed, it is surprising that
the catalytic role of palladium/L8 complex was realized on the
silicon–carbon cross coupling reaction for the highest level of
enantioselectivity with palladium catalysis at present. The
probing exploration on the development of new TADDOL-
derived phosphoramidite ligands seems to be a good attempt
to improve the enantioselectivity in palladium-catalyzed
silicon–carbon bond-forming silylation reaction.
In summary, we have developed an arduous investigation for
the enantioselective synthesis of silicon-stereogenic silanes via
silicon–carbon bond-forming silylation reaction with aryl
iodides and dihydrosilane. The screening and optimization of
reaction conditions, especially with the development of new P-
ligand by modication of TADDOL-derived phosphoramidite
ligands, resulted in the determination of an efficient procedure,
which provided the corresponding silicon-stereogenic silanes in
moderate yields and low to good enantioselectivities (up to 86%
ee) under mild reaction conditions. In addition, to the best of
our knowledge, it is one of the best examples of asymmetric
palladium-catalyzed silicon–carbon cross coupling reaction for
a stereoselective synthesis of silicon-stereogenic silanes with
good enantioselectivity, in which the adaption of TADDOL-
based phosphoramidite L8 with the intermolecular interac-
tion between catalyst and substrate indicated that the control of
steric and electronic effect of chiral ligand is very important in
asymmetric catalysis. Further efforts will be devoted to develop
a chiral P-ligand to asymmetric palladium-catalyzed silicon–
carbon bond-forming silylation reaction with high level of
enantioselectivity.
Scheme 4 Catalytic asymmetric silicon–carbon bond-forming sily-
lation of aryl iodides with methylphenylsilane promoted by Pd/L8.
experimental results provided useful and comprehensive
information on the steric repulsion and electronic effect of aryl
iodides in this reaction. Supplementary ndings on the
importance of electronic effect were also achieved from the
experimental data of 3d–f (the order of ee value: 3f (86% ee) > 3d
(78% ee) > 3e (69% ee)). Thus in this reaction, both the elec-
tronic effect and steric repulsion of substituted aryl iodides
could not be ignored. Furthermore, 3j with 2-methyl substituent
on the aryl ring gave better enantioselectivity (80% ee) than that
without 2-methyl substituent on the aryl ring (3h, 21% ee),
which indicated the crucial role of steric effect in the stereo-
selective construction of silicon-stereogenic silane. However, it
should be noted that the crowded 2-iodo-1,3,5-
trimethoxylbenzene (2i) or tert-butyl(phenyl)silane gave the
corresponding silicon-stereogenic silane 3i or 3m respectively
in low yield and poor enantioselectivity (28% yield and 36% ee
for 3i, and 30% yield and 25% ee for 3m).
Acknowledgements
The authors gratefully thank the nancial support of the
National Natural Science Foundation of China (51303043,
21472031, and 21503060), Zhejiang Provincial Natural Science
Foundation of China (LR14B030001 and LY14B020013), Science
and Technology Department of Zhejiang Province
(2015C31138), and Hangzhou Science and Technology Bureau
of China (20140432B04).
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