Rhodium-catalyzed asymmetric synthesis of silicon-stereogenic dibenzooxasilines via enantioselective transmetalation

Article abstract of DOI:10.1021/ja3076555

A rhodium-catalyzed asymmetric synthesis of silicon-stereogenic dibenzooxasilines has been developed. High enantioselectivities have been achieved by employing (S,S)-Me-Duphos as the ligand through " enantioselective transmetalation".

Full text of DOI:10.1021/ja3076555

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Communication
Rhodium-Catalyzed Asymmetric Synthesis of Silicon-Stereogenic
Dibenzooxasilines via Enantioselective Transmetalation
Ryo Shintani, Eleanor E Maciver, Fumiko Tamakuni, and Tamio Hayashi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2012
Downloaded from http://pubs.acs.org on September 24, 2012
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Journal of the American Chemical Society
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Rhodium-Catalyzed Asymmetric Synthesis of Silicon-Stereogenic
Dibenzooxasilines via Enantioselective Transmetalation
Ryo Shintani,*,† Eleanor E. Maciver,† Fumiko Tamakuni,† and Tamio Hayashi*,†,‡
6
7
8
9
†Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan
‡Institute of Materials Research and Engineering, A*STAR, 3 Research Link, Singapore 117602
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shintani@kuchem.kyoto-u.ac.jp; tamioh@imre.a-star.edu.sg
partial formation of a Rh((S,S)-Me-Duphos)2 complex,12 which
displays much higher catalytic activity with no
enantioinduction. The reaction also proceeded well by using
ABSTRACT: A rhodium-catalyzed asymmetric synthesis
of silicon-stereogenic dibenzooxasilines has been
developed. High enantioselectivities have been achieved
[Rh(OH)(cod)]2 instead of [Rh(OH)(coe)2]2 in combination
by employing (S,S)-Me-Duphos as the ligand through
with (S,S)-Me-Duphos, but 2a was obtained only with 10% ee
(entry 4). This observation can be explained by the slower
ligand exchange between 1,5-cyclooctadiene (cod) and (S,S)-
Me-Duphos and by the fact that [Rh(OH)(cod)]2 alone can
promote the formation of 2a as a racemate (93% yield). In
contrast, the use of [RhCl(coe)2]2 completely shut down the
reaction (entry 5), which is presumably because a chloro-
rhodium complex cannot undergo ligand exchange with 1a to
form an aryloxorhodium species for successive transmetalation
to take place (vide infra). Other chiral bisphosphine ligands
such as (R)-Binap, (R)-H8-Binap, (R)-Segphos, and (S,S)-
Chiraphos were inferior to (S,S)-Me-Duphos, giving product
2a with lower enantioselectivity (5–62% ee; entries 6–9).
“enantioselective transmetalation”.
Transmetalation is one of the key elemental steps for a wide
variety of transition metal-catalyzed processes involving
organometallic reagents such as palladium-catalyzed cross-
coupling reactions1 and rhodium-catalyzed conjugate addition
reactions.2 Although this elemental step also often appears as
part of the catalytic cycle for various asymmetric catalyses,
“enantioselective transmetalation” that constructs
a
stereogenic center at the metal(loid) of the transmetalating
reagent has never been reported as far as we are aware.3,4 In this
context, here we demonstrate the first example of such
enantioselective
transmetalation
using
prochiral
Table 1. Rhodium-Catalyzed Asymmetric Synthesis of
Dibenzooxasiline 2a from 1a
organosilicon compounds under the catalysis of a chiral
rhodium(I) complex, leading to the formation of silicon-
stereogenic organosilanes with high enantioselectivity (eq
1).5,6
Rh-catalyst (8 mol % Rh)
ligand (6 mol %)
OH
O
OSi(t-Bu)Ph2
catalytic
Rh*–X
+
catalytic
Rh*–Ar
+
Si(t-Bu)Ph2
Si
ethyl acrylate (1.2 equiv)
THF, 65 °C, 24 h
Ph
t-Bu
enantioselective
transmetalation
Ar Ar
Ar
X
(1)
1a
2a
3a
Si
Si*
R1
R1
Si-stereogenic
R2
prochiral
R2
yield
(%)a
ee of
entry
Rh-catalyst
ligand
2a/3ab
2a (%)c
91 (R)
—
(R1 ≠ R2)
1
2
[Rh(OH)(coe)2]2 (S,S)-Me-Duphos 89
98/2
—
To realize this mode of asymmetric catalysis, we chose
hydroxy-tethered tetraoragnosilanes bearing two of the same
aryl groups as the transmetalating reagent in combination with
an electron-deficient olefin as the electrophile in the presence
of a chiral rhodium complex.7 As a starting point, we decided
to employ 2’-(tert-butyl)diphenylsilylbiphenyl-2-ol (1a) as a
model substrate and conducted several reactions to find a set of
conditions that could effectively promote the formation of
silicon-stereogenic dibenzooxasiline 2a8 with high ee
through enantioselective transmetalation (Table 1). Thus, high
yield of 2a with 91% ee was obtained along with a minute
amount of byproduct 3a by conducting the reaction of 1a with
ethyl acrylate (1.2 equiv) in the presence of a catalyst
[Rh(OH)(coe)2]2
—
0
3d [Rh(OH)(coe)2]2 (S,S)-Me-Duphos 95
>99/1
0
4
5
6
7
8
9
[Rh(OH)(cod)]2 (S,S)-Me-Duphos 90
>99/1 10 (R)
[RhCl(coe)2]2
(S,S)-Me-Duphos
0
20
88
51
94
—
—
[Rh(OH)(coe)2]2 (R)-Binap
95/5
99/1
99/1
5 (R)
17 (S)
56 (S)
[Rh(OH)(coe)2]2 (R)-H8-Binap
[Rh(OH)(coe)2]2 (R)-Segphos
[Rh(OH)(coe)2]2 (S,S)-Chiraphos
99/1 62 (R)
1
c
a Combined isolated yield of 2a and 3a. b Determined by H NMR.
Determined by chiral HPLC on a Chiralpak AD-H column with
hexane/2-propanol = 500/1. d 10 mol % of ligand was used.
9
generated from [Rh(OH)(coe)2]2 (8 mol % Rh) and (S,S)-Me-
O
Duphos10 (6 mol %) in THF at 65 °C (entry 1). It is worth
noting that the phenyl group cleaved from 1a ended up on
ethyl acrylate in the forms of ethyl 3-phenylpropionate (1,4-
adduct; 3% of Ph on 1a), ethyl cinnamate (Heck-type product;
11% Ph), and ethyl 3,3-diphenylpropionate (1,4-adduct of the
cinnamate; 72% Ph) under these conditions. The
rhodium/ligand ratio and the choice of rhodium complex were
found to be critically important in the present reaction. No
reaction took place when the reaction was conducted without
any added phosphine ligands (entry 2), and the use of excess
(S,S)-Me-Duphos (10 mol %; 1.25 equiv to Rh) gave product
2a in 95% yield but with complete loss of enantioselectivity
(entry 3).11 We tentatively attribute this loss of ee to the
PPh2
PPh2
PPh2
PPh2
PPh2
PPh2
O
O
O
(R)-Binap
(R)-H8-Binap
(R)-Segphos
Me
P
P
Me
Me
PPh2
PPh2
Me
Me
Me
(S,S)-Me-Duphos
(S,S)-Chiraphos
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Under the reaction conditions described in Table 1, entry 1,
various substituted dibenzooxasilines are effectively
enantiospecificity (eq 2). Similarly, the reaction of (R)-2a with
1
2
3
4
5
6
7
8
2
n-butyllithium gives (S)-5 in 86% yield with 97%
enantiospecificity. It is worth noting that these compounds
exist as a 1:1 mixture of inseparable (interconvertible) atrop
isomers in solution at room temperature, but only one atrop
isomeric form with S configuration around the axis is obtained
in a solid state upon recrystallization of (S)-4 from hexane as
shown in Figure 2 with the dihedral angle of 79–86° (cf.
dihedral angle for 2e in Figure 1 is 18°).14
prepared with high enantioselectivity as summarized in Table
2, entries 2–8 (87–92% ee), including an example of
benzonaphthooxasiline 2d (entry 4). In addition, substrates 1
having two of the same substituted phenyl groups on the
silicon atom are also converted to the corresponding
dibenzooxasilines 2 with high efficiency (88–92% ee; entries
9–12). Unfortunately, the use of substrates having a less bulky
alkyl group such as cyclohexyl group on the silicon shows
lower reactivity and enantioselectivity in the present catalysis
(entry 13).13 The absolute configuration of product 2e in entry
5 was determined to be R by X-ray crystallographic analysis
with Cu-Kα radiation after recrystallization from hexane at –
20 °C (Figure 1).14
9
OH
1. RLi, Et2O, 35 °C
O
R
(2)
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Si
2. sat. NH4Claq
Ph
Si
Ph
t-Bu
t-Bu
(R)-2a
91% ee
(S)-4 (R = Me): 82% yield, 88% ee
(S)-5 (R = n-Bu): 86% yield, 88% ee
Table 2. Rhodium-Catalyzed Asymmetric Synthesis of
Dibenzooxasilines 2: Scope
3
X
X
X
2
9
[Rh(OH)(coe)2]2 (8 mol % Rh)
(S,S)-Me-Duphos (6 mol %)
OSiRAr2
O
OH
ethyl acrylate (1.2 equiv)
THF, 65 °C, 24 h
Si
SiRAr2
Ar
R
8
Y
Y
Y
1
2
3
ee of
entry
2 (X, Y, R, Ar)
yield (%)a 2/3b
2 (%)c
89
80
96
96
77
97
70
92
87d
90
82d
85
59
98/2
96/4
97/3
98/2
96/4
>99/1
98/2
>99/1
98/2
98/2
98/2
98/2
93/7
91
90
88
91
87
92
90
90
92
91
88
90
71
1
2
3
4
5
6
7
8
9
2a (H, H, t-Bu, Ph)
2b (2-Me, H, t-Bu, Ph)
2c (2-F, H, t-Bu, Ph)
Figure 2. X-ray crystal structure of (S)-4 (Flack parameter = –
0.02(2); only one out of two independent molecules in the unit cell is
shown and hydrogen atoms are omitted for clarity).
2d (2,3-(CH=CH)2, H, t-Bu, Ph)
2e (H, 8-Me, t-Bu, Ph)
A proposed catalytic cycle for the reaction of 1a to give 2a
is illustrated in Scheme 1.2,7d Initial ligand exchange of
hydroxorhodium complex with substrate 1a gives
aryloxorhodium species A. This then undergoes enantio-
discriminating transmetalation intramolecularly to give
phenylrhodium species with the release of silicon-stereogenic
2a as the product. The resulting phenylrhodium species
2f (H, 8-F, t-Bu, Ph)
2g (H, 9-OMe, t-Bu, Ph)
2h (H, 9-CF3, t-Bu, Ph)
2i (H, H, t-Bu, 4-MeOC6H4)
10 2j (H, H, t-Bu, 4-MeC6H4)
11 2k (H, H, t-Bu, 4-PhC6H4)
12 2l (H, H, t-Bu, 3-MeC6H4)
13e 2m (H, H, Cy, Ph)
undergoes
the following
processes
to
regenerate
aryloxorhodium A on the basis of the fate of the phenyl group
described for Table 1, entry 1. Thus, at the early stage of
catalysis, insertion of ethyl acrylate to the phenylrhodium
takes place to give oxa-π-allylrhodium B, which mostly leads
to the formation of a rhodium hydride and ethyl cinnamate
through β-hydrogen elimination (Scheme 2a).16 The rhodium
hydride presumably reacts with another molecule of ethyl
acrylate, followed by protonolysis of resulting intermediate C
with 1a, provides aryloxorhodium A along with ethyl
propionate. On the other hand, at the late stage of catalysis
where most of ethyl acrylate has been converted to ethyl
cinnamate and ethyl propionate, insertion of ethyl cinnamate
to the phenylrhodium species occurs to give intermediate D,
and protonolysis of this intermediate with substrate 1a leads
a
b
Combined isolated yield of 2 and 3 unless otherwise noted.
Determined by 1H NMR.
hexane/2-propanol.
Determined by chiral HPLC with
c
d
e
Isolated yield of 2.
The reaction was
conducted in dioxane at 80 °C.
to the formation of aryloxorhodium
A and ethyl 3,3-
diphenylpropionate (Scheme 2b).
Scheme 1. Proposed catalytic cycle for the reaction of 1a to give
2a ([Rh] = Rh((S,S)-Me-Duphos))
2a
Figure 1. X-ray crystal structure of (R)-2e (Flack parameter =
0.01(4); hydrogen atoms are omitted for clarity).
transmetalation
[Rh] OH
– H2O
O
[Rh]
1a
[Rh] Ph
Si(t-Bu)Ph2
We have also begun to explore the derivatization of silicon-
stereogenic dibenzooxasilines 2 obtained in the present
catalysis. In our preliminary experiments, we found that the
reaction of (R)-2a (91% ee) with methyllithium proceeds with
retention of configuration at the silicon stereocenter15 to give
ring-opened tetraorganosilane (S)-4 in 82% yield with 97%
A
(see Scheme 2)
The formation of minor byproduct 3a can be rationalized by
the following pathway as illustrated in Scheme 3. Instead of
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Journal of the American Chemical Society
Chem. Soc. 1983, 105, 6129. (b) Hatanaka, Y.; Hiyama, T. J. Am.
undergoing transmetalation between aryloxorhodium and one
Chem. Soc. 1990, 112, 7793. (c) Falck, J. R.; Bhatt, R. K.; Ye, J.
J. Am. Chem. Soc. 1995, 117, 5973. (d) Boudier, A.; Knochel, P.
Tetrahedron Lett. 1999, 40, 687. (e) Hölzer, B.; Hoffmann, R. W.
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Soc. 2011, 133, 20738 and the references cited therein.
1
2
3
4
5
6
7
8
of the phenyl groups on the silicon from intermediate A,
transmetalation with the cleavage of the biphenyl–silicon
bond gives arylrhodium species E, protonolysis of which with
1a provides 3a along with regeneration of aryloxorhodium A.
Scheme 2. Proposed pathways for the conversion of [Rh]–Ph to
aryloxorhodium A: (a) early stage of the catalysis, (b) late stage
of the catalysis
(4) Desymmetrization of diaryl sulfoxides by stoichiometric
asymmetric sulfoxide–magnesium exchange has been reported:
Hampel, T.; Ruppenthal, S.; Sälinger, D.; Brückner, R. Chem.
Eur. J. 2012, 18, 3136.
(a)
CO2Et
CO2Et
O
O
!-H
elim
[Rh]
[Rh]
9
[Rh] Ph
[Rh]
H
OEt
OEt
(5) For reviews on asymmetric synthesis of silicon-stereogenic
organosilanes, see: (a) Xu, L.-W.; Li, L.; Lai, G.-Q.; Jiang, J.-X.
Chem. Soc. Rev. 2011, 40, 1777. (b) Oestreich, M. Synlett 2007,
1629. For selected examples of the utility of silicon-stereogenic
oragnosilanes, see: (c) Rendler, S.; Auer, G.; Oestreich, M.
Angew. Chem., Int. Ed. 2005, 44, 7620. (d) Rendler, S.; Auer, G.;
Keller, M.; Oestreich, M. Adv. Synth. Catal. 2006, 348, 1171. (e)
Rendler, S.; Oestreich, M.; Butts, C. P.; Lloyd-Jones, G. C. J. Am.
Chem. Soc. 2007, 129, 502. (f) Klare, H. F. T.; Oestreich, M.
Angew. Chem., Int. Ed. 2007, 46, 9335.
(6) For examples of catalytic asymmetric preparation of silicon-
stereogenic organosilanes, see: (a) Hayashi, T.; Yamamoto, K.;
Kumada, M. Tetrahedron Lett. 1974, 15, 331. (b) Corriu, R. J. P.;
Moreau, J. J. E. J. Organomet. Chem. 1975, 85, 19. (c) Ohta, T.;
Ito, M.; Tsuneto, A.; Takaya, H. J. Chem. Soc., Chem. Commun.
1994, 2525. (d) Corriu, R. J. P.; Moreau, J. J. E. Tetrahedron Lett.
1973, 14, 4469. (e) Corriu, R. J. P.; Moreau, J. J. E. J. Organomet.
Chem. 1976, 120, 337. Yasutomi, Y.; Suematsu, H.; Katsuki, T. J.
Am. Chem. Soc. 2010, 132, 4510. (f) Shintani, R.; Moriya, K.;
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Moriya, K.; Hayashi, T. Org. Lett. 2012, 14, 2902. (h) Shintani,
R.; Otomo, H.; Ota, K.; Hayashi, T. J. Am. Chem. Soc. 2012, 134,
7305. See also: (i) Tamao, K.; Nakamura, K.; Ishii, H.;
Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc. 1996, 118, 12469.
(j) Schmidt, D. R.; O’Malley, S. J.; Leighton, J. L. J. Am. Chem.
Soc. 2003, 125, 1190. (k) Igawa, K.; Takada, J.; Shimono, T.;
Tomooka, K. J. Am. Chem. Soc. 2008, 130, 16132.
(7) Hiyama and Nakao developed related, but structurally different,
hydroxy-tethered tetraorganosilanes as transmetalating reagents
for palladium-catalyzed cross-coupling reactions and rhodium-
catalyzed conjugate addition reactions. For leading references,
see: (a) Nakao, Y.; Imanaka, H.; Sahoo, A. K.; Yada, A.;
Hiyama, T. J. Am. Chem. Soc. 2005, 127, 6952. (b) Nakao, Y.;
Chen, J.; Tanaka, M.; Hiyama, T. J. Am. Chem. Soc. 2007, 129,
11694. (c) Nakao, Y.; Takeda, M.; Matsumoto, T.; Hiyama, T.
Angew. Chem., Int. Ed. 2010, 49, 4447. (d) Nakao, Y.; Chen, J.;
Imanaka, H.; Hiyama, T.; Ichikawa, Y.; Duan, W.-L.; Shintani,
R.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 9137. (e) Shintani,
R.; Ichikawa, Y.; Hayashi, T.; Chen, J.; Nakao, Y.; Hiyama, T.
Org. Lett. 2007, 9, 4643.
(8) For nonasymmetric preparation and utility of dibenzooxasilines,
see: (a) Huang, C.; Gevorgyan, V. J. Am. Chem. Soc. 2009, 131,
10844. See also: (b) Krasnova, T. L.; Chernyshev, E. A.;
Sergeev, A. P.; Abramova, E. S. Russ. Chem. Bull. 1997, 46,
1487. (c) Kirillova, N. I.; Cusev, A. I.; Sharapov, V. A.;
Afanasova, O. B.; Zubarev, Y. E.; Alekseev, N. V.; Chernyshev,
E. A.; Struchkov, Y. T. J. Organomet. Chem. 1986, 306, 55 and
the references cited therein.
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+
B
C
Ph
H
CO2Et
1a
1a
Ph
CO2Et
CO2Et
+
+
A
A
H
Ph
(b)
CO2Et
O
[Rh]
Ph
CO2Et
Ph
1a
Ph
+
[Rh] Ph
A
OEt
Ph
Ph
D
Scheme 3. Proposed reaction pathway for the reaction of 1a to
give 3a
transmetalation
OSi(t-Bu)Ph2
O
[Rh]
[Rh]
Si(t-Bu)Ph2
A
E
3a
1a
In summary, we have developed
a
rhodium-catalyzed
asymmetric synthesis of silicon-stereogenic dibenzooxasilines
through enantioselective transmetalation of prochiral
organosilanes. High enantioselectivities have been achieved
by employing (S,S)-Me-Duphos as the ligand, successfully
demonstrating the proof-of-principle for “enantioselective
transmetalation”. Future studies will explore further expansion
of the scope of the present catalysis as well as development of
related asymmetric reactions.
ACKNOWLEDGMENT. Support has been provided in part
by a Grant-in-Aid for Young Scientists (B), the MEXT, Japan,
and in part by the Asahi Glass Foundation. E.E.M. thanks
JSPS for postdoctoral fellowship.
SUPPORTING INFORMATION. Experimental procedures
and compound characterization data (PDF) and X-ray data
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
REFERENCES
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cross-coupling). (c) Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., Ed.; Wiley: Hoboken, NJ,
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Boronic Acids, 2nd ed; Hall, D. G., Ed.; Wiley-VCH: Weinheim,
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(11) The same trend was observed for (S,S)-Chiraphos when 10
mol % (1.25 equiv to Rh) was used, giving compound 2a in 91%
yield with 0% ee (see Table 1, entry 9 for comparison), but no
significant change of ee was observed for (R)-Binap, (R)-H8-
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Binap, and (R)-Segphos (see Table 1, entries 6–8 for
(15) (a) Sommer, L. H.; Rodewald, P. G.; Parker, G. A. Tetrahedron
comparison).
Lett. 1962, 821. (b) Corriu, R. J. P.; Lanneau, G. F.; Leard, M. J.
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1
2
3
4
5
6
7
8
(12) We currently have no good understanding for the structure,
reactivity, and stereoselectivity of this species. For preliminary
31P NMR studies on Rh/(S,S)-Me-Duphos complexes, see the
Supporting Information.
(13) The use of even smaller R groups (e.g., R = Me) results in
formation of complex product mixture.
(14) CCDC-893230 and CCDC-893231 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. See also the
Supporting Information for details.
(16) (a) Zou, G.; Wang, Z.; Zhu, J.; Tang, J. Chem. Commun. 2003,
2438. (b) Lautens, M.; Mancuso, J.; Grover, H. Synthesis 2004,
2006. See also: (c) Lautens, M.; Roy, A.; Fukuoka, K.; Fagnou,
K.; Martín-Matute, B. J. Am. Chem. Soc. 2001, 123, 5358. (d) de
la Herrán, G.; Murcia, C.; Csákÿ, A. G. Org. Lett. 2005, 7, 5629.
(e) Kurahashi, T.; Shinokubo, H.; Osuka, A. Angew. Chem., Int.
Ed. 2006, 45, 6336.
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X
X
[Rh(OH)(coe)2]2 (8 mol % Rh)
(S,S)-Me-Duphos (6 mol %)
OH
Ar
O
ethyl acrylate (1.2 equiv)
THF, 65 °C, 24 h
Si
Ar
Si
Ar
R
R
enantioselective
transmetalation
Y
Y
up to 92% ee
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