Enantioselective hydrosilylation of aromatic alkenes catalyzed by chiral bis(oxazolinyl)phenyl-rhodium acetate complexes

Article abstract of DOI:10.1055/s-0032-1317677

Highly efficient and enantioselective hydrosilylation of aromatic alkenes catalyzed by the chiral rhodium acetate complexes with the bis(oxazolinyl)phenyl ligands has been reported that afforded chiral silane derivatives with up to 99% ee. Georg Thieme Ve

Full text of DOI:10.1055/s-0032-1317677

LETTER
▌2957
letter
Enantioselective Hydrosilylation of Aromatic Alkenes Catalyzed by Chiral
Bis(oxazolinyl)phenyl–Rhodium Acetate Complexes
Enantioselective Hydrosilylation of Aromatic Alkenes
Tatsuo Naito, Takuma Yoneda, Jun-ichi Ito, Hisao Nishiyama*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
Fax +81(52)7893209; E-mail: hnishi@apchem.nagoya-u.ac.jp
Received: 29.09.2012; Accepted after revision: 30.10.2012
We recently reported that the phebox-Rh complex 1
Abstract: Highly efficient and enantioselective hydrosilylation of
[phebox = bis(oxazolinyl)phenyl] serves as a chiral cata-
aromatic alkenes catalyzed by the chiral rhodium acetate complexes
lyst for enantioselective hydrosilylation of styrene deriva-
with the bis(oxazolinyl)phenyl ligands has been reported that af-
tives and cyclic alkenes with high enantioselectivity
(Scheme 1).7 Although the catalytic reaction was slow,
addition of a silver salt effectively enhanced the catalytic
activity, indicating that a cationic Rh species might be
formed in situ as a precursor. At the same time, we dem-
onstrated that Rh acetate complexes 2 exhibit high perfor-
mance in the asymmetric conjugate reduction of α,β-
unsaturated carbonyl compounds.8 Herein we wish to re-
port that the phebox-Rh acetate complexes 2 exhibit re-
markable reactivity in the enantioselective hydrosilylation
of alkenes with enhancement of the enantioselectivity and
the regioselectivity.
forded chiral silane derivatives with up to 99% ee.
Key words: asymmetric hydrosilylation, alkene, bis(oxazoli-
nyl)phenyl, rhodium, hydrosilane
Since enantioselective hydrosilylation of alkenes is a
practical methodology to access chiral silane com-
pounds,1 chiral transition metal catalysts have been devel-
oped in the area of Pt, Pd and Rh metals.2–5 In particular,
the MOP-Pd catalyst [MOP = 2-(diphenylphosphino)-2′-
methoxy-1,l′-binaphthyl] was developed to be an efficient
and enantioselective catalyst with high branch selectivity
in aliphatic alkenes with trichlorosilane.3a Moreover, the
DIOP-Rh catalyst [DIOP = 4,5-bis(diphenylphosphino-
methyl)-2,2-dimethyl-1,3-dioxolane] also serves as a
highly enantioselective catalyst in the intramolecular hy-
drosilylation.4a On the other hand, Rh catalysts sometime
face the drawback of a side reaction, namely dehydroge-
native silylation, giving vinylsilanes.6
phebox-Rh (2) (1 mol%)
hydrosilane (3) (1.2 equiv)
toluene, 30 °C, 1 h
Ph
4a
Si
Si
+
Ph
Ph
phebox-Rh cat. (1)
5a
6a
(1 mol%)
Si
(EtO)2MeSiH (3a)
OH
(1.2 equiv)
H2O2
Si
+
OH
Ph
Ph
Ph
+
toluene, 50 °C
KHCO3, KF
without Ag+: 94%, 92% ee, dr = 50:50 (72 h).
Ph
Ph
with AgPF6 (2 mol%): 90%, 94% ee, dr = 54:46 (12 h).
7a
8a
Scheme 2
i-Pr
R
O
O
O
N
N
AcO
First, we examined the hydrosilylation of p-phenylstyrene
(4a) catalyzed by the phebox-Rh acetate complexes 2
(Scheme 2 and Table 1). When the complex 2a was em-
ployed in the hydrosilylation of 4a with (EtO)2MeSiH
(3a), the catalytic reaction was completed within one hour
at 30 ºC, giving a mixture of α- and β-silylated com-
pounds, 5a and 6a, in 97% yield with dr = 63:37. Succes-
sive stereospecific Tamao–Fleming oxidation9 of 5a with
H2O2, KF and KHCO3 afforded the corresponding S-alco-
hol 7a with 95% ee (Table 1, entry 1). In this reaction, the
acetate complex 2a significantly enhanced the reaction
rate as well as enantioselectivity and regioselectivity as
compared to those obtained using 1 (Scheme 1). The prod-
uct yield and enantioselectivity and regioselectivity were
influenced by the source of hydrosilanes. (EtO)3SiH (3b)
and (MeO)3SiH (3c) gave good results with high regiose-
Cl
Rh OH2
Rh OH2
OAc
Cl
N
N
O
i-Pr
R
1
2a: R = i-Pr
2b: R = s-Bu
2c: R = t-Bu
2d: R = Ph
Scheme 1
SYNLETT 2012, 23, 2957–2960
Advanced online publication: 23.11.2012
DOI: 10.1055/s-0032-1317677; Art ID: ST-2012-U0828-L
© Georg Thieme Verlag Stuttgart · New York
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3
6
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2
1
4
1
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3
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0
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6

2958
T. Naito et al.
LETTER
lectivity and enantioselectivity, whereas Me2PhSiH (3d)
was not effective (Table 1, entries 2–4). The substituents
on the oxazoline ring also affected both the enantioselec-
tivity and the regioselectivity. Among 2a–d, the complex
2b with sec-butyl group showed the highest ee value of
99% (Table 1, entry 5). In contrast, bulky substituents,
tert-butyl and phenyl groups, decreased the enantioselec-
tivity and the regioselectivity probably due to steric hin-
drance (Table 1, entries 6 and 7).
Table 2 Enantioselective Hydrosilylation of 1-Alkenes with 3b Cat-
alyzed by 2aa
Entry Alkene
Yield (%) Ratio of
ee (%)c
of 5 and 6 5 and 6b
1d
2
4-MeC6H4 (4b)
95
97
96
98
99
97
99
97
88
92
67:33
62:38
85:15
85:15
91:9
98 (S)
97 (S)
97 (S)
96 (S)
98 (S)
98 (S)
98
4-MeOC6H4 (4c)
4-ClC6H4 (4d)
4-BrC6H4 (4e)
3
Next, asymmetric hydrosilylation of various 1-alkenes 4
with 2a was examined (Table 2). Hydrosilylation of the
styrene derivatives with electron-donating and electron-
withdrawing groups at para-position (4b–g) led to the
S-products in high yields with excellent enantioselectivity
(Table 2, entries 1–6). In particular, electron-withdrawing
substituents led to a significant improvement of the regi-
oselectivity with dr values of up to 91:9 (Table 2 entries
3–6). The meta-substituted styrene derivatives 4h,i were
also converted into the silylated compounds in high yields
with high enantioselectivity (Table 2 entries 7 and 8).
However, hydrosilylation of ortho-substituted styrenes 4j
and 4k led to a decrease in the product yield and the regi-
oselectivity (Table 2, entries 9 and 10). It is noteworthy
that the formation of vinylsilanes in the catalytic hydro-
silylation was not detected.
4d
5
4-F3CC6H4 (4f)
4-MeO2CC6H4 (4g)
3-MeOC6H4 (4h)
3-ClC6H4 (4i)
6
90:10
71:29
89:11
30:70
45:55
7
8
98 (S)
49 (S)
91 (S)
9e
10f
2-MeOC6H4 (4j)
2-ClC6H4 (4k)
a Reaction condition: phebox-Rh complex 2a (1.0 mol%), (EtO)3SiH
(3b; 1.2 equiv), alkenes 4 (1.0 mmol), toluene (1 mL), 30 ºC, 1 h.
b Determined by 1H NMR.
c Determined by HPLC of alcohol 7.
d Reaction time: 3 h.
e Reaction time: 19 h.
f Reaction time: 18 h.
Table 1 Enantioselective Hydrosilylation of Styrene with Hydrosi-
lanes 3 Catalyzed by Phebox-Rh Acetate Complexes 2a
in C6D6 at 30 ºC, the amount of the cis-isomer 9b de-
creased to be less than 1% after 20 minutes and the trans-
isomer 9a was formed in 74% yield accompanied with si-
lylated products. As a result, the outcome of the absolute
configuration in 9a and 9b could be identical. This type of
isomerization of alkene was reported in the enantioselec-
tive hydroboration of cis- and trans-β-methylstyrene cat-
alyzed by the chiral Rh complex.10 Although the reactions
of dihydronaphthalene 11 and acenaphthalene 13 were
sluggish, they were silylated with 91% ee (R) and 83% ee
(S), respectively. In particular, 11 exhibited high regiose-
lectivity (dr = 100:0) to exclusively afford the β-substitut-
ed product. However, it is difficult to explain the
predominant formation of the β-substituted product rather
than the α-substituted one. It is notable that the values ob-
tained with 2a were much higher than those obtained with
the chloride complex 1.7
Entry Hydrosilane
Rh cat. Yield (%)
Ratio of
ee
of 5a and 6a 5a and 6ab (%)c
1
2
3
4
5
6
7
(EtO)2MeSiH (3a) 2a
97
99
76
0
63:37
78:22
78:22
–
95 (S)
98 (S)
98 (S)
–
(EtO)3SiH (3b)
(MeO)3SiH (3c)
Me2PhSiH (3d)
(EtO)3SiH (3b)
(EtO)3SiH (3b)
(EtO)3SiH (3b)
2a
2a
2a
2b
2c
2d
99
77
94
75:25
33:67
18:82
99 (S)
2 (S)
69 (S)
a Reaction condition: phebox-Rh complexes 2 (1.0 mol%), hydrosi-
lane 3 (1.2 equiv), alkene 4a (1 mmol), toluene (1 mL).
b Determined by 1H NMR.
c Determined by HPLC of alcohol 7a obtained by Tamao oxidation.
A proposed mechanism is shown in Scheme 4. Hydride si-
lyl intermediates A and B could be involved as active spe-
cies. Remarkable enhancement of the catalytic activity of
the phebox-Rh acetate complex 2 implies that initial reac-
tivity toward hydrosilane of 2 is higher than that of the
chloride complex 1. In accordance to the Chalk–Harrod
mechanism,11–13 silylated products might be formed by al-
kene insertion into the Rh–H bond followed by reductive
elimination. Judging from the construction of the S-abso-
lute configuration of the product, Re-face of α-carbon ap-
proaches the Rh atom in A. In contrast, Si-face of α-
carbon might be blocked by the oxazoline substituent. We
observed that the regioselectivity of the branched and lin-
Furthermore, the Rh catalyst 2a was successfully utilized
in hydrosilylation of trans-β-methylstyrene (9a) to give
predominantly the α-silylated compound in high enantio-
selectivity as well as high regioselectivity (Scheme 3). In-
terestingly, cis-β-metylstyrene (9b) also afforded the α-
silylated compound in 96% ee with the identical absolute
configuration that was observed for 9a. In this case, the
rapid isomerization to 9a from 9b took place prior to the
hydrosilylation. When we monitored the hydrosilylation
of 9b with 3b (1.2 equiv) in the presence of 2a (1 mol%)
Synlett 2012, 23, 2957–2960
© Georg Thieme Verlag Stuttgart · New York

LETTER
Enantioselective Hydrosilylation of Aromatic Alkenes
2959
Acknowledgment
phebox-Rh cat (2a)
(1 mol%)
(EtO)3SiH (3b)
(1.2 equiv)
Ph
This research was supported by a Grant-in-Aid for Scientific Re-
search from the Japan Society for the Promotion of Science (No.
22245014).
OH
9a
or
Ph
toluene, 30 °C, 3 h
then H2O2, KF, KHCO3
Ph
10
References
9a: 98%, 98% ee (S), dr = 95:5
9b: 98%, 96% ee (S), dr = 93:7
9b
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2a (5 mol%)
3b (1.2 equiv)
toluene, 30 °C, 24 h
then H2O2, KF, KHCO3
OH
11
12
99%, 91% ee (R), dr = 100:0
(2) Yamamoto, K.; Hayashi, T.; Kumada, M. J. Am. Chem. Soc.
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2a (2 mol%)
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Organometallics 1996, 15, 2098.
3b (1.2 equiv)
THF, 30 °C, 5 h
then H2O2, KF, KHCO3
OH
13
14
97%, 83% ee (S)
Scheme 3
ear products was strongly affected by the para-substitu-
ents of the styrene derivatives. Thus, the electronic
property of the C=C bond is considered to be a significant
factor for the selectivity between A and B. Although detail
of this electronic effect remains unclear, electron-rich sty-
renes might be tightly coordinated to the trivalent Rh(III)
center rather than to electron-deficient alkenes.14,15 Such
steric repulsion between the alkene and the phebox ligand
may affect the regioselectivity of the silylated products.
Ar
β
α
α
β
Ar
Re
N
N
R
R
Rh
Rh
H
H
O
O
N
N
Si
Si
R
R
A
B
(7) Tsuchiya, Y.; Uchimura, H.; Kobayashi, K.; Nishiyama, H.
Synlett 2004, 2099.
(8) Kanazawa, Y.; Tsuchiya, Y.; Kobayashi, K.; Shiomi, T.;
Itoh, J.-I.; Kikuchi, M.; Yamamoto, Y.; Nishiyama, H.
Chem.–Eur. J. 2006, 12, 63.
(9) Tamao, K.; Ishida, N.; Tanaka, T.; Kumada, M.
Organometallics 1983, 2, 1694.
Si
S
Si
Ar
Ar
Scheme 4
(10) Doucet, H.; Fernandez, E.; Layzell, T. P.; Brown, J. M.
Chem.–Eur. J. 1999, 5, 1320.
(11) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16.
(12) Bergens, S. H.; Noheda, P.; Whelan, J.; Bosnich, B. J. Am.
Chem. Soc. 1992, 114, 2128.
(13) For the modified Chalk–Harrod mechanism for a theoretical
study on rhodium catalysts, see: Sakaki, S.; Suminoto, M.;
Fukuhara, M.; Sugimoto, M.; Fujimoto, H.; Matsuzaki, S.
Organometllics 2002, 21, 3788.
(14) For examples of transition-metal–π-styrene complexes, see:
(a) Fueno, T.; Okuyama, T.; Deguchi, T.; Furukawa, J. J.
Am. Chem. Soc. 1965, 87, 170. (b) Kurosawa, H.; Asada, N.
J. Organomet. Chem. 1981, 217, 259. (c) Munakata, M.;
In summary, we have reported the highly efficient chiral
phebox-Rh acetate complexes for the enantioselective hy-
drosilylation of alkenes using (EtO)3SiH to generate the
corresponding optically active silylated compounds.16
The acetate complexes 2 significantly enhance the catalyt-
ic activity and the outcome of the enantioselectivity of the
products. Hydrosilylation of both trans- and cis-β-methyl-
styrene was found to produce the same enantiomer with
branch selectivity as well as enantioselectivity.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 2957–2960

2960
T. Naito et al.
LETTER
CHIRALCEL® OB-H; hexane–2-propanol (90:10); flow
rate: 1.0 mL/min; λ = 215 nm; tR = 5.2 min (major), tR = 5.7
min (minor); 97% ee.
(S)-1-(4′-Bromophenyl)ethanol (7e): Daicel
CHIRALCEL® OB-H; hexane–2-propanol (95:5); flow rate:
0.5 mL/min; λ = 254 nm; tR = 15.9 min (major), tR = 18.5 min
(minor); 96% ee.
(S)-(–)-1-(4′-Trifluoromethylphenyl)ethanol (7f): Daicel
CHIRALCEL® OJ-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 8.5 min (major), tR = 9.2 min
(minor); 98% ee.
Methyl 4-(1-Hydroxyethyl)benzoate (7g): Daicel
CHIRALCEL® OB-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 19.5 min (major), tR = 24.0 min
(minor); 98% ee.
(S)-1-(3′-Methoxyphenyl)ethanol (7h): Daicel
CHIRALCEL® OB-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 18.4 min (major), tR = 28.0 min
(minor); 98% ee.
(S)-1-(3′-Chlorophenyl)ethanol (7i): Daicel
CHIRALCEL® OB-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 9.9 min (major), tR = 13.5 min
(minor); 98% ee.
(S)-1-(2′-Methoxyphenyl)ethanol (6j): Daicel
CHIRALCEL® OB-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 10.6 min (major), tR = 18.6 min
(minor); 49% ee.
Kitagawa, S.; Kosome, S.; Asahara, A. Inorg. Chem. 1986,
25, 2622. (d) Ohkita, K.; Kurosawa, H.; Hirao, T.; Keda, I.
J. Organomet. Chem. 1994, 470, 189. (e) Brown, T. J.;
Dickens, M. G.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009,
131, 6350.
(15) For examples of electronic effect in asymmetric Rh
catalysts, see ref. 10 and: Kwong, F. Y.; Yang, G.; Mak, T.
C. W.; Chan, A. S. C.; Chan, K. S. J. Org. Chem. 2002, 67,
2769.
(16) Typical Procedure for Hydrosilylation of 4a with
(EtO)3SiH (Table 1, Entry 2): To a mixture of the phebox-
Rh complex 2a (5.4 mg, 0.010 mmol), 4-phenylstyrene (4a;
180 mg, 1.0 mmol) in toluene (1 mL) was added (EtO)3SiH
(3b; 220 μL, 1.2 mmol) at 30 °C and the mixture was stirred
for 1 h. After concentration of the reaction mixture, the crude
product was purified by silica gel column chromatography
with hexane–EtOAc (100:1–50:1) as an eluent to give a
mixture of 5a and 6a (342 mg, 0.99 mmol; 5a/6a = 78:22).
To the mixture of 5a and 6a (342 mg, 0.99 mmol), KHCO3
(300 mg, 3.0 mmol) and KF (174 mg, 3.0 mmol) in THF (1
mL) and MeOH (1 mL) was added H2O2 (30%, 1 mL) at 0
ºC. After being stirred for 12 h, the reaction was quenched
by addition of sat. aq Na2S2O3 (10 mL). The mixture was
additionally stirred for 1 h and was extracted with EtOAc (3
× 6 mL). The extract was washed with aq NaCl (10 mL) and
was dried over MgSO4. After concentration, the residue was
purified by column chromatography on silica gel with
hexane–EtOAc (15:1–5:1) as an eluent to give 7a (81.2 mg,
0.41 mmol, 41%).
(S)-1-(2′-Chlorophenyl)ethanol (6k): Daicel
CHIRALCEL® OJ-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 9.7 min (major), tR = 10.3 min
(minor); 91% ee.
HPLC Analysis
:(S)-1-(para-Biphenyl)ethanol (7a): Daicel CHIRALPAK®
AD-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ
= 254 nm; tR = 14.8 min (S), tR = 16.0 min (R); 98% ee.
(S)-1-(4′-Methylphenyl)ethanol (7b): Daicel
(S)-1-Phenylpropanol (10): Daicel CHIRALCEL® OB-H;
hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254
nm; tR = 7.9 min (major), tR = 9.7 min (minor); 98% ee.
(R)-2-Tetralol (12): Daicel CHIRALCEL® OJ-H; hexane–
2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; tR =
12.5 min (major), tR = 13.4 min (minor); 91% ee.
(S)-1,2-Dihydroacenaphthylen-1-ol (14): Daicel
CHIRALCEL® OB-H; hexane–2-propanol (90:10); flow
rate: 1.0 mL/min; λ = 215 nm; tR = 7.2 min (minor), tR = 10.0
min (major); 83% ee.
CHIRALCEL® OJ-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 12.4 min (major), tR = 14.4 min
(minor); 98% ee.
(S)-1-(4′-Methoxyphenyl)ethanol (7c): Daicel
CHIRALPAK® AS-H; hexane–2-propanol (95:5); flow rate:
0.8 mL/min; λ = 254 nm; tR = 22.1 min (minor), tR = 27.8 min
(major); 97% ee.
(S)-1-(4′-Chlorophenyl)ethanol (7d): Daicel
Synlett 2012, 23, 2957–2960
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