Asymmetric hydrosilylation of cyclohexa-1,3-diene with trichlorosilane by palladium catalysts coordinated with chiral phosphoramidite ligands

Article abstract of DOI:10.1016/j.tetasy.2013.02.002

Chiral phosphoramidite ligands prepared from (S)-binaphthol and various secondary amines were examined for the palladium-catalyzed asymmetric hydrosilylation of cyclohexa-1,3-diene with trichlorosilane. With a sterically demanding phosphoramidite bearing bis(diphenylmethyl)amine, the high enantiomeric excess of 87% was achieved, which is the highest enantioselectivity in the hydrosilylation of cyclohexadiene reported to date. The catalytic activity in the present hydrosilylation was high with all of the ligands employed, while the enantioselectivities varied dramatically depending on the dialkylamine moiety of the phosphoramidite ligand.

Full text of DOI:10.1016/j.tetasy.2013.02.002

Tetrahedron: Asymmetry 24 (2013) 418–420
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Tetrahedron: Asymmetry
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Asymmetric hydrosilylation of cyclohexa-1,3-diene with trichlorosilane
by palladium catalysts coordinated with chiral phosphoramidite ligands
Hoon Seo Park a, Jin Wook Han a, , Ryo Shintani b, Tamio Hayashi b,c
⇑
a Department of Chemistry, College of Natural Sciences, Institute of Nanoscience and Technology, Hanyang University, Seoul 133-791, Republic of Korea
b Department of Chemistry, Faculty of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
c Chemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral phosphoramidite ligands prepared from (S)-binaphthol and various secondary amines were exam-
ined for the palladium-catalyzed asymmetric hydrosilylation of cyclohexa-1,3-diene with trichlorosilane.
With a sterically demanding phosphoramidite bearing bis(diphenylmethyl)amine, the high enantiomeric
excess of 87% was achieved, which is the highest enantioselectivity in the hydrosilylation of cyclohexa-
diene reported to date. The catalytic activity in the present hydrosilylation was high with all of the
ligands employed, while the enantioselectivities varied dramatically depending on the dialkylamine moi-
ety of the phosphoramidite ligand.
Received 29 January 2013
Accepted 5 February 2013
Available online 27 February 2013
Ó 2013 Elsevier Ltd. All rights reserved.
R
Me
1. Introduction
MeO
MeO
H
Palladium-catalyzed asymmetric hydrosilylations of carbon–
carbon double bonds with trichlorosilane have been intensively
investigated in order to achieve excellent regioselectivities and
enantioselectivities with the aid of chiral monodentate ligands,
represented by MOPs having an axially chiral binaphthyl moiety,
from early 1990s to early 2000s.1 During this period, the substrate
scope of the hydrosilylation reaction has also been expanded to
include styrenes, alkyl-substituted alkenes, and conjugated
1,3-dienes. With regard to the enantioselectivity, the highest val-
ues for the reaction of each olefinic substrate type have been ob-
tained by choosing an appropriate chiral monophosphine ligand.1
For example, chiral MOP ligands, which are readily changeable
responding to the substrate types by substituting a suitable func-
tional group at the 20-position of the binaphthyl unit, have been
developed to give the highest enantioselectivities for the catalytic
hydrosilylation. For the hydrosilylation of styrenes and alkyl-
substituted alkenes, the highest enantioselectivity has been re-
ported with H-MOP ligands 1 and 22 and MeO-MOP ligand 3,3
respectively. The MOP ligand has also been modified and opti-
mized for the asymmetric hydrosilylation of 1,3-dienes, and
Ar-MOP ligands 4 and 5 showed the highest enantioselectivity
for cyclic 1,3-dienes.4 Ferrocenylmonophosphines such as 6 have
shown excellent enantioselectivities and catalytic activities in the
reaction of styrene.5
PPh2
PPh2
PAr2
Me
R
1
3
4
R = H: (R )-Ar-MOP
Ar = Ph: (S )-H-MOP
(R)-MeO-MOP
2
5
Ar = 3,5-(CF3)2C6H3: (S)-
R = n-C8H17: (R)-
Me
Ph
Me
N
Me
Me
P
O
OMe
O
Ph
P
N
O
O
Fe
PPh2
7
6
(SA,RC,RC)-
8
(RA,RC,RC)-
(S)-
On the other hand, chiral phosphoramidites, which are readily
available from chiral diols and amines, have been reported to be
effective as ligands for various catalytic asymmetric reactions.6
Chiral phosphoramidites, such as 7 and 8, have already been uti-
lized in the palladium-catalyzed asymmetric hydrosilylation of
styrenes and demonstrated excellent enantioselectivities and cata-
lytic activities.7 However, no reports concerning the application of
chiral phosphoramidite ligands to the hydrosilylation of conjugated
dienes have appeared in the literature, despite its synthetic utility.
Palladium-catalyzed asymmetric hydrosilylations of 1,3-dienes
with trichlorosilane, which proceed in a 1,4-addition fashion,
produce enantiomerically enriched allylsilanes.4a The allylsilanes
obtained are converted further without a loss of their enantiomeric
purities into useful intermediates in stereospecific transformations,
⇑
Corresponding author.
E-mail address: jwhan@hanyang.ac.kr (J.W. Han).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.02.002

H. S. Park et al. / Tetrahedron: Asymmetry 24 (2013) 418–420
419
such as chiral allyl alcohols by Tamao oxidations8 and chiral homo-
allyl alcohols via reaction with aldehydes.9 Herein we report that a
chiral phosphoramidite ligand derived from (S)-binaphthol and a
secondary amine showed a higher enantioselectivity than 5 which
has been until now the most enantioselective ligand in the palla-
dium-catalyzed asymmetric hydrosilylation of cyclohexadiene.
Table 1
a
Palladium-catalyzed asymmetric hydrosilylation of cyclohexa-1,3-diene with HSiCl3
Entry
L⁄
Temp (°C)
Time (h)
Yieldb (%)
% eec (abs config)
1
2
3
4
5
6
7
8
9
9a
9a
9b
9c
7
9d
9e
9f
20
ꢀ10
20
20
20
20
20
20
20
20
84
20
22
22
22
22
22
20
82
97
91
93
93
87
79
95
80
87 (R)
87 (R)
19 (R)
34 (R)
57 (R)
70 (R)
6 (S)
2. Results and discussion
53 (R)
47 (R)
Chiral phosphoramidite ligands prepared from (S)-binaphthol
and various secondary amines were examined for their catalytic
activity and enantioselectivity in the palladium-catalyzed asym-
metric hydrosilylation of cyclohexa-1,3-diene with trichlorosilane
(Scheme 1). The reaction was carried out without solvent in the
presence of 1.0 mol % of palladium catalysts formed in situ by mix-
9g
a
The hydrosilylation was carried out without solvent. The catalyst was generated
in situ by mixing [PdCl(p
-C3H5)]2 and a chiral ligand L⁄. The initial ratio of cyclo-
hexa-1,3-diene/HSiCl3/Pd/L⁄ was 1.0/1.2/0.010/0.020.
b
Isolated yields by bulb-to-bulb distillation.
Determined by HPLC analysis of alcohol 11 with a chiral stationary column
c
ing [PdCl(p-C3H5)]2 with 2.0 equiv (to palladium) of ligand 7 or 9.
(Daicel Chiralpak OB-H).
The resulting 3-(trichlorosilyl)cyclohexene 10 was subsequently
reacted with benzaldehyde in DMF to give the corresponding
homoallyl alcohol 11, which was subjected to HPLC analysis with
a chiral stationary phase column for determination of the enanti-
oselectivities. The results summarized in Table 1 show that the cat-
alytic activities of the palladium complex with phosphoramidite
ligands are high enough to complete the reaction within 22 h at
20 °C regardless of the ligand used, but the enantioselectivities var-
ied dramatically according to the ligand.
Thus, the hydrosilylation proceeded at 20 °C for 20 h in the pres-
ence of the palladium/9a as a chiral catalyst to give (R)-3-trichloro-
silylcyclohexene 10 with 87% ee, which is higher than the
previously reported value of 83% ee obtained by the palladium/5
catalyst at ꢀ10 °C for 168 h (entry 1).4b,c However, the enantiose-
lectivity obtained by the palladium/9a catalyst was not improved
upon by lowering the reaction temperature (entry 2). The steric
bulkiness of the amine substituents plays a crucial role with regard
to the enantioselectivity of this hydrosilylation. Thus, the enanti-
oselectivities with the phosphoramidites from diethylamine 9b11
Among the chiral phosphoramidite ligands studied, the ligand of
choice was the sterically demanding 9a10 which consisted of an
axial chiral (S)-binaphthol and a bulky bis(diphenylmethyl)amine.
OH
Cl3Si
PhCHO
DMF
Ph
Pd/L* (1/2)
+
HSiCl3
1 mol % Pd
10
11
L*=
Ph
Ph
N
Ph
Ph
O
O
O
O
P
NEt2
P
N(Pr-i)2
P
O
O
(S)-9a
(S)-9b
(S)-9c
Ph
N
Ph
Ph
Me
Me
Me
O
O
O
O
P
N
P
P
N
O
O
Me
Ph
Ph
Ph
(SA,SC,SC)-9e
7
9d
(SA,SC,SC)-
(SA,RC,RC)-
OMe
Me
Me
O
O
O
P
N
P
N
O
Me
Me
OMe
(SA,SC,SC)-9f
(SA,SC,SC)-9g
Scheme 1. Pd-catalyzed asymmetric hydrosilylation of cyclohexa-1,3-diene with HSiCl3.

420
H. S. Park et al. / Tetrahedron: Asymmetry 24 (2013) 418–420
and diisopropylamine 9c12 were only 19% and 34% ee, respectively
(entry 3 and 4). A series of ligands with a stereogenic center on the
amine substituents were also examined. Phosphoramidite 7, which
was one of the most enantioselective ligands in the asymmetric
hydrosilylation of styrene,7a gave product 10 with a moderate
enantiomeric excess of 57%, whereas its diastereomer 9d6 gave 10
with a higher ee than 7 but inferior to 9a (entry 5 and 6). It should
be noted that the product obtained from the hydrosilylation with
both diastereomers 7 and 9d had the same (R)-absolute configura-
tion. Changing the phenyl group in 9d to a 2-methoxyphenyl 9f13 or
1-naphthyl group 9g,14 caused the enantioselectivity to decrease to
53% and 47% ee, respectively. Ligand 9e15 bearing (S,S)-2,5-diphe-
nylpyrrolidine, which is thought to be a cyclic version of (S,S)-
bis(1-phenylethyl)amine, showed almost no enantioselectivity in
the present reaction (entry 7). The results from electronic and steric
tuning based on the chirality of diastereomer 9d imply that the
enantioselectivity of the hydrosilylation of cyclohexadiene is
strongly dependent on the substituents on the nitrogen atom in
the phosphoramidite ligands.
(R,S)-isomer. It follows that allylsilane 10 is an (R)-isomer with
87% ee.
Acknowledgments
This work was supported by the National Research Foundation
of Korea Grant funded by the Korean Government (2011-0010550).
References
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3. Conclusion
In conclusion, we have shown that chiral phosphoramidite
ligand 9a prepared from (S)-binaphthol and an achiral amine,
bis(diphenylmethyl)amine is the most effective for the palladium-
catalyzed asymmetric hydrosilylation of cyclohexa-1,3-diene with
trichlorosilane, achieving a high enantioselectivity of 87% ee, which
is the highest reported value so far in the hydrosilylation of
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4. Experimental
4.1. Palladium-catalyzed asymmetric hydrosilylation of
cyclohexa-1,3-diene with HSiCl3 and the subsequent reaction
with benzaldehyde
The reaction conditions and results are summarized in Table 1.
A typical procedure is given for the hydrosilylation reaction by the
Pd/9a catalyst (entry 1). To a mixture of [PdCl(p-C3H5)]2 (3.7 mg,
0.010 mmol), the phosphoramidite ligand 9a (27 mg, 0.040 mmol),
and cyclohexa-1,3-diene (160 mg, 2.0 mmol) was added dropwise
HSiCl3 (0.24 mL, 2.4 mmol) at 0 °C. The reaction mixture was
stirred in a sealed tube at 20 °C for 20 h, and then distilled
(bulb-to-bulb) under reduced pressure to give 354 mg (82% yield)
of 3-(trichlorosilyl)cyclohexene 10.4b
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A
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0.51 mmol) obtained above and benzaldehyde (21 mL, 0.2 mmol)
in DMF (1 mL) was stirred at 0 °C for 2 h. Saturated aqueous so-
dium hydrogen carbonate was added to quench the reaction, and
the aqueous layer was extracted with diethyl ether. The extract
was dried over anhydrous MgSO4, and the solvent was evaporated.
The crude product was purified by preparative TLC on silica gel
(hexane/ethyl acetate = 4:1) to give (R)-((S)-cyclohex-2-en-1-
yl)(phenyl)methanol (R,S)-11.4a,9a The enantiomeric purity and
absolute configuration of 11 were determined by HPLC analysis
(Chiralpak OB-H, hexane/2-propanol = 9:1, flow 0.5 mL/min,
254 nm, t1 = 10.7 min (S), t2 = 16.3 min (R)) to be 87% ee of the