Enantioselective synthesis of planar-chiral benzosiloloferrocenes by Rh-catalyzed intramolecular C-H silylation

Article abstract of DOI:10.1039/c5cc00723b

The first synthesis of planar-chiral benzosiloloferrocenes was achieved by the intramolecular reaction of 2-(dimethylhydrosilyl)arylferrocenes. The enantioselective cross dehydrogenative coupling of an sp² C-H bond of ferrocene with a Si-H bond

Full text of DOI:10.1039/c5cc00723b

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DOI: 10.1039/C5CC00723B
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Enantioselective Synthesis of Planar-Chiral
Benzosiloloferrocenes by Rh-Catalyzed Intramolecular
C-H Silylation
1Cite this: DOI: 10.1039/x0xx00000x
Takanori Shibata,a,b,* Tsubasa Shizuno,a and Tomoya Sasakia
Received 00th January 2015,
Accepted 00th January 2015
DOI: 10.1039/x0xx00000x
www.rsc.org/
have been prepared (Scheme 1ꢀa).10 In 2014, three groups
The
intramolecular
reaction
achieved
of
the
2-
first
independently reported CꢀH arylation with haloarenes by using
chiral Pd catalysts in the presence of more than a stoichiometric
amount of base. Planarꢀchiral indenoneꢀ and quinolinoneꢀfused
ferrocenes were obtained in excellent ee (Scheme 1ꢀb).11 In this
communication, we report the cross dehydrogenative coupling
(CDC) of ferrocene and hydrosilane or hydrogermane by using a Rhꢀ
chiral diene catalyst. This is the first example of the enantioselective
synthesis of planarꢀchiral heteroleꢀfused ferrocenes (Scheme 1ꢀc).12
(dimethylhydrosilyl)arylferrocenes
synthesis of planar-chiral benzosiloloferrocenes. The
enantioselective cross dehydrogenative coupling of an sp2 C-H
bond of ferrocene with a Si-H bond proceeded efficiently with
the use of a Rh-chiral diene catalyst.
Dibenzosilole (9ꢀsilafluorene) is an important skeleton of many
functional molecules that are useful as conjugated organosilicon
materials in organic electronics and photonics.1 Therefore, a facile
method for the synthesis of multiꢀsubstituted dibenzosilole is
strongly desired. Among various catalytic protocols, Pdꢀcatalyzed
intramolecular CꢀH arylation of diarylsilane2 and consecutive
coupling of 2,2'ꢀdiiodoꢀ1,1'ꢀbiphenyl with dihydrosilane3 are typical
examples. However, these reactions require halogenated substrates
and suffer from the formation of byꢀproducts derived from halogens,
based on atomꢀeconomical concerns. The Irꢀcatalyzed [2+2+2]
cycloaddition of silanylbenzeneꢀtethered 1,6ꢀdiyne with alkyne
affords a perfectly atomꢀeconomical transformation for the synthesis
of multiꢀsubstituted dibenzosiloles.4 As another approach, the Rhꢀ
catalyzed intramolecular dehydrogenative coupling of sp2 CꢀH and
SiꢀH bonds has been reported5 and asymmetric variants followed, in
which a chiral spirocyclic silicon center was created by a Rhꢀchiral
diphosphine catalyst.6
a) C-H insertion of carbene (ref. 10)
Z
Z
O
chiral Cu cat.
O
H
Fe
Fe
Z = (CH2)2, CMe2
N2
b) C-H arylation by haloarene (ref. 11)
X
chiral Pd cat.
H
base
Z
Z
Fe
Fe
Z = C(=O)
C(=O)NR
c) This work: Si-H/C-H coupling
In contrast, the catalytic creation of planar chirality in
disubstituted ferrocenes initiated by enantioselective CꢀH bond
cleavage has attracted much attention over the past few years.7
Aminomethyl groupꢀdirected asymmetric CꢀH arylation, Heckꢀtype
reaction, and annulation have been realized through the use of Pdꢀ
chiral amino acid catalysts.8 We also reported isoquinolyl directedꢀ
CꢀH alkylation with alkenes by using an Irꢀchiral diene catalyst.9 As
for the intramolecular reaction, carbene CꢀH insertion of
diazoketoneꢀtethered ferrocenes by a chiral Cu catalyst is a
pioneering work, and planarꢀchiral cyclic ketoneꢀfused ferrocenes
chiral Rh cat.
Z
H
Fe
Fe
ZH
Z = SiR2, GeR2
Scheme 1 Enantioselective and intramolecular reaction for the
synthesis of planar-chiral ring-fused ferrocenes
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DOI: 10.1039/C5CC00723B
We chose 2ꢀ(dimethylhydrosilyl)phenylferrocene (1aa) as a
model substrate, which was readily prepared in two steps (Scheme Table 2 Screening of chiral diene ligands
2). Diazotization of 2ꢀbromoaniline followed the reaction with
[RhCl(coe)2]2 (10 mol%)
ferrocene,13 and aryllithium prepared from the obtained bromide
chiral diene (24 mol%)
1aa
2aa
+
3aa
underwent silylation by dimethylhydrosilane.
toluene, 135 °C, 1.5 h
i-Pr
Ar = C6H5: L1
4-MeC6H4: L2
O
1) NaNO2,
H2N
HCl aq.
2) ferrocene,
AcONa
1) n-BuLi
Fe
Fe
4-t-BuC6H4: L3
3-MeC6H4: L4
2-MeC6H4: L5
4-FC6H4: L6
O-2-Nap
Me
Me
OMe
Br
44%
SiHMe2
1aa 80%
2) Me2SiHCl
Br
Me
Ph
L9
Scheme
2
Preparation of 2-(dimethylhydrosilyl)phenyl-
4-CF3C6H4: L7
4-MeOC6H4: L8
Ar
ferrocene 1aa
L10
According to the literature,6 we intended to perform the
intramolecular reaction of ferrocene 1aa in the presence of a chiral
Rh catalyst prepared from [RhCl(cod)]2 and (S)ꢀBINAP (Table 1,
Entry 1). Desired CDC product 2aa was obtained in good yield and
enantioinduction was ascertained, albeit in low selectivity. The
major byꢀproduct was disilane 4aa, which resulted from the selfꢀ
coupling of hydrosilanes. The iridium counterpart was less effective
with respect to both yield and ee, and a significant amount of
desilylated product 3aa was formed (Entry 2).14 Cationic Rh catalyst
gave a higher yield along with the formation of 3aa (Entry 3). We
also investigated several chiral diphosphine ligands, but without
success.15
Ph
Chiral dienes
Entry
Chiral diene
Yield (%)a
2aa:3aab
1:0.17
1:0.29
1:0.23
1:0.24
1:0.46
1:1.31
1:0.39
1:0.24
1:0.29
1:0.12
Ee (%)c
1
2
78
80
82
89
73
61
75
61
64
82
58
63
65
59
35
62
52
60
ꢀ33
<2
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
3
4
5
6
7
8
Table 1 Rh- or Ir-BINAP-catalyzed C-H silylation of 1aa
9
10
catalyst (10 mol%)
a Total yield of 2aa and 3aa. b The ratio was determined by NMR
(S)-BINAP (24 mol%)
analysis. c Ee of 2aa.
1aa
toluene, 135 °C, 1 h
To improve the yield and ee, we screened several alkenes as
hydrogen acceptors (Table 3).19 Norbornene surely increased the
yield of 2aa and decreased the yield of 3aa with comparable ee
(Entry 2). Unstrained cyclic alkene gave negative results (Entry 3).
In the presence of 4ꢀvinylcyclohexene, the reaction proceeded
quantitatively with the almost perfect suppression of disilane
formation, but the ee was significantly decreased (Entry 4). In
Ph
Si
Me2
Fe
Fe
Fc
Fc
Me2Si SiMe2
2aa
3aa
4aa (Fc = ferrocenyl)
contrast, 3,3ꢀdimethylbutꢀ1ꢀene realized
a drastic increase in
Entry
Catalyst
Yield (%)a 2aa:3aab Ee (%)c Yield (%)d
enantiomeric excess and greater amounts of the alkene slightly
suppressed the formation of 3aa (Entries 5 and 6).20 The hydrogen
acceptor probably prevented the hydrogenation of the chiral diene
ligand and the enantioselectivity was improved.
1
2
3
[RhCl(cod)]2
[IrCl(cod)]2
[Rh(cod)2]BF4
78
65
97
1:0
1:0.42
1:0.20
ꢀ27
7
ꢀ21
31
6
ND
e
a Total yield of 2aa and 3aa. b The ratio was determined by NMR
analysis. c Ee of 2aa. d NMR yield of 4aa. e (S)ꢀBINAP (12 mol%)
was used.
Table 3 Effect of alkenes as a hydrogen acceptor
[RhCl(coe)2]2 (10 mol%)
L1 (24 mol%)
We next examined chiral dienes as ligands (Table 2). We were
pleased to find that Carreira’s dienes16 drastically improved the
enantioselectivity: chiral Rh catalysts prepared from [RhCl(coe)2]2
and dienes generally realized moderate enantioselectivity (ca. 60%),
but the ratio of 2aa and 3aa differed depending on the aryl group
(Entries 1ꢀ8). In each entry, disilane 4aa was formed as a byꢀproduct
to some extent. We determined that L1 was the best ligand, and most
strongly suppressed the formation of 3aa. Hayashi’s dienes L917 and
L1018 were ineffective in the present reaction.
alkene (5 equiv)
1aa
2aa
+
3aa
toluene, 135 °C, 1.5 h
Entry
Alkene
none
Yield (%)a 2aa:3aab Ee (%)c
1
2
78
89
1:0.17
1:0.03
1:0.34
1:0.01
1:0.11
1:0.07
58
57
59
40
82
85
norbornene
cyclooctene
3
50
4
4ꢀvinylcyclohexene
3,3ꢀdimethylbutꢀ1ꢀene
3,3ꢀdimethylbutꢀ1ꢀene
>99
72
5
6d
67
2 | J. Name., 2015, 00, 1-4
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DOI: 1C0.O10M39M/CU5CNCI0C0A72T3IOB N
a
b
Total yield of 2aa and 3aa. The ratio was determined by NMR
[RhCl(coe)2]2 (10 mol%)
L1 (24 mol%)
analysis. c Ee of 2aa. d Alkene (10 equiv) was added.
(10 equiv)
t-Bu
The present reaction was sensitive to the bulkiness around the
silicon atom (Scheme 3). Both the yield and ee decreased as the
Ge
Me2
H
Fe
Fe
GeHMe2
toluene, 135 °C, 6 h
substituents
(R)
increase
in
size.
2ꢀ
4
5: 40%, 33% ee
(Diphenylhydrosilyl)phenylferrocene (1ad) was transformed into the
corresponding benzosiloloferrocene 2ad in good yield, but the ee
was low.
Scheme 4 Reaction of 2-(dimethylhydrogermyl)phenyl-
ferrocene (4)
[RhCl(coe)2]2 (10 mol%)
L1 (24 mol%)
In conclusion, we first prepared ferroceneꢀfused benzosiloles
and benzogermole by enantioselective and intramolecular cross
dehydrogenative coupling of an sp2 CꢀH bond of ferrocene with
a SiꢀH or GeꢀH bond. We will evaluate the potential of a new
class of planarꢀchiral molecules as electronic organic materials.
(10 equiv)
t-Bu
Si
H
Fe
Fe
R2
SiHR2
toluene, 135 °C, 24 h
1ab (R = n-Bu)
1ac (R = i-Pr)
1ad (R = Ph)
2ab: 51%, 50% ee
2ac: 39%, 18% ee
2ad: 74%, 26% ee
Acknowledgement
This work was supported by GrantꢀinꢀAid for Scientific Research on
Innovative Areas, “Molecular Activation Directed toward
Straightforward Synthesis,” MEXT and JST, ACTꢀC, Japan.
Scheme 3 Effect of substituents on the silicon atom
We checked the substrate scope by introducing several
Notes and references
a
substituents
on
the
benzene
ring
of
2ꢀ
Department of Chemistry and Biochemistry, School of Advanced
(dimethylhydrosilyl)phenylferrocene (Table 4). Both of electronꢀ
Science and Engineering, Waseda University, Shinjuku, Tokyo 169ꢀ8555,
donating and –withdrawing groups could be installed into the paraꢀ Japan, Fax: (+81) 3ꢀ5286ꢀ8098; eꢀmail: tshibata@waseda.jp
b JST, ACTꢀC, 4ꢀ1ꢀ8 Honcho, Kawaguchi, Saitama 332ꢀ0012, Japan.
and metaꢀpositions with respect to the silyl group: moderate yield
† Electronic Supplementary Information (ESI) available: [details of any
and good ee were achieved (Entries 1ꢀ5). A methyl group at the 3ꢀ
supplementary information available should be included here]. See
position slightly affected the enantioselectivity (Entry 6).
Introduction adjacent to the silyl group gave a poor ee (Entry 7)
DOI: 10.1039/c000000x/
1
2
For recent reviews, see: (
a
) M. Shimizu and T. Hiyama, Synlett,
Table 4 Substrate scope of 2-(dimethylhydrosilyl)aryl-
2012, 23, 973; (
b
) J. Y. Corey, Adv. Organomet. Chem., 2011, 59,
ferrocenes
181.
R
(
a
) M. Shimizu, K. Mochida and T. Hiyama, Angew. Chem., Int. Ed.,
2008, 47, 9760; asymmetric variant; ( ) R. Shintani, H. Otomo, K.
Ota and T. Hayashi, J. Am. Chem. Soc., 2012, 134, 7305.
Y. Yabusaki, N. Ohshima, H. Kondo, T. Kusamoto, Y. Yamanoi and
H. Nishihara, Chem. Eur. J., 2010, 16, 5581.
) T. Matsuda, S. Kadowaki, T. Goya and M. Murakami, Org. Lett.,
2007, , 133; asymmetric variant: ( ) R. Shintani, C. Takagi, T. Ito,
M. Naito and K. Nozaki, Angew. Chem., Int. Ed., 2015, 54, 1616.
4
[RhCl(coe)2]2 (10 mol%)
3
R
b
5
L1 (24 mol%)
3
4
(10 equiv)
6
t-Bu
Si
Me2
H
Fe
Fe
–
SiHMe2
toluene, 135 °C, time
(a
9
b
1ba-1ha
2ba-2ha
5
6
7
T. Ureshino, T. Yoshida, Y. Kuninobu and K. Takai, J. Am. Chem.
Soc., 2010, 132, 14324.
Y. Kuninobu, K. Yamauchi, N. Tamura, T. Seiki and K. Takai,
Angew. Chem., Int. Ed., 2013, 52, 1520.
Entry
R
Time (h) Yield (%)a
2:3b
Ee (%)c
74
1
2
3
4
5
6
7
4ꢀMe (1ba)
4ꢀCF3 (1ca)
5ꢀMe (1da)
5ꢀCF3 (1ea)
5ꢀOCF3 (1fa)
3,5ꢀMe2 (1ga)
6ꢀMe (1ha)
24
4
56
55
44
53
55
75
67
1:0.11
1:0.14
1:0.06
1:0.17
1:0.16
1:0.13
1:0.09
Reviews of planarꢀchiral ferrocenes, see: (
H. Lang, Organometallics, 2013, 32, 5668; (b) S. Arae and M.
Ogasawara, Tetrahedron Lett., 2015, 56 in press
(doi:10.1016/j.tetlet.2015.01.130).
) D.ꢀW. Gao, Y.ꢀC. Shi, Q. Gu, Z.ꢀL. Zhao and S.ꢀL. You, J. Am.
Chem. Soc., 2013, 135, 86; ( ) C. Pi, Y. Li, X. Cui, H. Zhang, Y.
Han and Y. Wu, Chem. Sci., 2013, , 2675; (c) Y.ꢀC. Shi, R.ꢀF.
Yang, D.ꢀW. Gao and S.ꢀL. You, Beil. J. Org. Chem., 2013, , 1891.
T. Shibata and T. Shizuno, Angew. Chem., Int. Ed., 2014, 53, 5410.
a) D. Schaarschmidt and
78
4
86
,
9
80
8
9
(a
2
76
b
4
5
55
9
24
5
a Total yield of 2 and 3. b The ratio was determined by NMR
10 S. Siegel and H. Schmalz, Angew. Chem., Int. Ed., 1997, 36, 2456.
11 ( ) R. Deng, Y. Huang, X. Ma, G. Li, R. Zhu, B. Wang, Y.ꢀB. Kang
and Z. Gu, J. Am. Chem. Soc., 2014, 136, 4472; ( ) D.ꢀW. Gao, Q.
Yin, Q. Gu, and S.ꢀL. You, J. Am. Chem. Soc., 2014, 136, 4841; (
analysis. c Ee of 2.
a
b
c
)
The reaction of (dimethylhydrogermyl)phenylferrocene (4) also
proceeded to afford benzogermoloferrocene 5 (Scheme 4).21 The low
yield and ee were probably due to the bulkiness of a germanium
atom compared with a silicon atom.
L. Liu, A.ꢀA. Zhang, R.ꢀJ. Zhao, F. Li, T.ꢀJ. Meng, N. Ishida, M.
Murakami and W.ꢀX. Zhao, Org. Lett., 2014, 16, 5336.
12 Only an example of the asymmetric synthesis of heteroleꢀfused
ferrocenes by diastereoselective reaction: S. Yasuike, J. Hagiwara,
H. Danjo, M. Kawahata, N. Kakusawa, K. Yamaguchi and J. Kurita,
Heterocycles, 2009, 78, 3001.
13 (a) K.G. Rutherford and W. Redmond, Org. Synth., 1963, 43, 12; (b)
W. F. Little, C. N. Reilley, J. D. Johnson, K. N. Lynn and A. P.
Sanders, J. Am. Chem. Soc., 1964, 86, 1376.
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C5CC00723B
14 Compounds 2aa and 3aa have similar polarity and could not be
separated by preparative TLC, but could be separated by GPC (gel
permeation chromatography), and 2aa was fully characterized.
15 (a) The representative ee of 2aa by using chiral diphosphines:
tolBINAP (9% ee); xylylBINAP (15% ee); MeꢀDUPHOS (<2% ee).
(b) We have recently learned that Prof. Kazuhiko Takai (Okayama
University) also examined a rhodiumꢀcihral diphosphineꢀcatalyzed
asymmetric synthesis of ferroceneꢀfused benzosiloles. We thank
Prof. Takai for exchanging information prior to the publication.
16 (
a
) C. Fischer, C. Defieber, T. Suzuki and E. M. Carreira, J. Am.
Chem. Soc., 2004, 126, 1628; ( ) C. Defieber, H. Grützmacher and
E. M. Carreira, Angew. Chem., Int. Ed., 2008, 47, 4482.
b
17 K. Okamoto, T. Hayashi and V. H. Rawal, Chem. Commun., 2009,
4815.
18 (
a
) N. Tokunaga, Y. Otomaru, K. Okamoto, K. Ueyama, R. Shintani
and T. Hayashi, J. Am. Chem. Soc., 2004, 126, 13584; ( ) R.
Shintani and T. Hayashi, Aldrichimica Acta, 2009, 42, 38.
19 C. Cheng and J. F. Hartwig, Science, 2014, 343, 853.
b
20 Recrystallization of 2aa gave a single crystal with improved ee (99%
ee), and its absolute structure was determined by Xꢀray analysis; The
crystal data of compound 2aa: C18H18FeSi, M = 318.27, orthorhombic,
space group P212121 (no. 19), a = 9.6741(4) Å, b = 10.9465(6) Å, c =
29.0292(11) Å, V = 3074.1(2) Å3, Z = 8, µ(MoꢀKα) = 10.447 cmꢀ1;
number of reflections measured: total 29429 and unique 6966, R1 =
0.0371, wR2 = 0.1036, Flack parameter (Friedel pairs = 2036) 0.003(8).
CCDC 1045375.
21 M. Murai, K. Matsumoto, R. Okada and K. Takai, Org. Lett., 2014, 16,
6492.
4 | J. Name., 2015, 00, 1-4
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