Rhodium-Catalyzed Synthesis of Benzosilolometallocenes via the Dehydrogenative Silylation of C(sp2)-H Bonds

Article abstract of DOI:10.1021/acs.orglett.5b01373

Use of a rhodium catalyst with electron-rich and bulky chiral diphosphine ligands having C₂-symmetry allowed efficient dehydrogenative silylation of the C(sp²)-H bond of ferrocenes leading to chiral benzosiloloferrocenes. The substra

Full text of DOI:10.1021/acs.orglett.5b01373

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed Synthesis of Benzosilolometallocenes via the
2
Dehydrogenative Silylation of C(sp )−H Bonds
,
†
†
†
,†,‡
Masahito Murai,* Koji Matsumoto, Yutaro Takeuchi, and Kazuhiko Takai*
†
‡
Division of Applied Chemistry, Graduate School of Natural Science and Technology and Research Center of New Functional
Materials for Energy Production, Storage and Transport, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530,
Japan
*
S Supporting Information
ABSTRACT: Use of a rhodium catalyst with electron-rich and bulky chiral
diphosphine ligands having C -symmetry allowed efficient dehydrogenative
2
2
silylation of the C(sp )−H bond of ferrocenes leading to chiral
benzosiloloferrocenes. The substrate scope was expanded to hydrogermane
and hydrosilanes having a ruthenocene backbone, which resulted in a new
approach to benzosilole- and benzogermole-fused metallocenes.
etallocenes are useful in material sciences as well as in
M
bioorganometallic chemistry due to their unique stimuli-
1
responsiveness based on redox activity. They are also
important and privileged units in catalysts and ligands for
2
synthetic organic chemistry. Therefore, development of
practical and efficient functionalization methods is important
for determining their inherent electronic characteristics.
Introduction of electron-donating or -withdrawing substituents
into the metallocene backbone is the most frequently used
strategy for functionalization. An alternative approach is
through extension of the π-system by annulation with aromatic
rings. Introduction of fused heteroaromatics is an especially
promising method because it can perturb the electronic
structure of the parent metallocene frameworks and provide
them with new properties such as charge-carrier mobility and
Figure 1. Comparison of the HOMO and LUMO energies of
heterocycle-fused ferrocenes estimated by DFT calculations
(LanL2DZ for Fe and B3LYP/6-31G(d) for other elements were
used). The values between each level are the energy gaps of the
frontier orbitals.
3
luminescence. On the basis of these past results, the synthesis
and that those of the LUMO were most effectively stabilized by
the silicon bridge due to the interaction between the low-lying
σ* orbital of the silicon and the π* orbital of the conjugated π-
3
a
3b
of several metallocenes, such as pyrrole-, thiophene-, and
phosphole-fused ferrocenes,
3c,d
have been reported. These
6
compounds were obtained mainly via complexation between
system of the backbone. This unique electronic structure
3a−c
the lithium salt of the heterocycle and iron chloride,
with
prompted development of a facile and efficient route to
benzosilolometallocenes. The present report describes the
rhodium-catalyzed dehydrogenative silylation and germylation
the sole exception of cyclization of 1-phosphanyl-2-(2-
lithiophenyl)ferrocene, which led to a phosphole-fused
3
d
2
ferrocene. Although these methods are useful, the use of
highly reactive lithium salt limits their application in the
construction of complexed metallocene frameworks.
of C(sp )−H bonds of ferrocenes and ruthenocenes as a new
7
,8
approach to benzosilolo- and benzogermolometallocenes.
Treatment of hydrosilane 1a, which can be readily prepared
from commercially available ferrocene in two steps, with a
A recent report described the catalytic synthesis of polycyclic
aromatic compounds fused by silicon-, germanium-, boron-,
and phosphorus-containing heterocycles based on the dehy-
catalytic amount of [RhCl(cod)] and rac-BINAP in dioxane at
2
70 °C afforded benzosiloloferrocene 2a in 44% yield (Table 1).
The yield increased to 72% when 3,3-dimethyl-1-butene was
added as a hydrogen acceptor. In this case, competitive
formation of the hydrosilylation product 3 with 3,3-dimethyl-1-
butene was obtained in 14% yield as a side product. In contrast
2
4,5
drogenative functionalization of C(sp )−H bonds.
The
success of these syntheses prompted an examination of
transition-metal-catalyzed dehydrogenative functionalization of
C−H bonds of metallocenes as a facile and efficient method to
tailor functionality. Preliminary data on the effect of π-
conjugation were estimated by DFT calculations (Figure 1).
The results suggested that the choice of the bridging atom had
a significant effect on the energy levels of the frontier orbital
4a
to a previous report on the synthesis of 9-silafluorenes,
Received: May 11, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01373
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Rh-Catalyzed Dehydrogenative Silylation and
2
Germylation of C(sp )−H Bonds of 1a and 4
entry
X
ligand
product
yield (%)
a
1
2
Si
Ge
rac-BINAP
2a
5
72 (44 )
b
a
PPh3
68 (45 )
a
b
Without 3,3-dimethyl-1-butene. 15 mol %.
silylative cyclization leading to 2a did not occur efficiently with
9
Wilkinson catalyst, RhCl(PPh ) . On the other hand,
3
3
benzogermoloferrocene 5 was obtained in 68% yield by using
PPh as a ligand in the presence of 3,3-dimethyl-1-butene via
3
2
4e
the dehydrogenative germylation of C(sp )−H bond of 4.
Despite advances in dehydrogenative C−H bond silylation,
few studies on the enantioselectivity of this transformation have
4c,7,10b,11
been reported.
Controlling both reactivity and stereo-
selectivity is challenging, and we have developed the rhodium-
catalyzed construction of axially chiral spirosilabifluorenes via 2-
4c
fold dehydrogenative silylation of bis(biaryl)dihydrosilanes.
Enantioselective desymmetrization via dehydrogenative silyla-
3
tion of C(sp )−H bonds using chiral diphosphine ligands with
10b
C -symmetry also has been reported from our group. On the
2
basis of these results, the asymmetric synthesis of benzosilo-
lometallocenes to induce planar chirality was investigated
1
2−14
(
Figure 2).
Fortunately, (R)-DTBM-SEGPHOS, which
was an effective ligand for the enantioselective silylation of the
3
10b
C(sp )−H bond,
gave 2a in 76% yield with 55% ee. In
contrast to the results shown in eq 1, reaction with (R)-DTBM-
SEGPHOS did not require a hydrogen acceptor. The absolute
15
configuration of 2a was assigned as S by a comparison of the
p
HPLC retention times for the major enantiomer obtained from
7
1
a with that for the literature-reported compound. Among the
Figure 2. Effect of phosphine ligands on enantioselective dehydrogen-
2
ative silylation of the C(sp )−H bond of 1a. (Isolated yields, ee, and
ligands examined, (R)-DTBM-Garphos, (R)-BINAP, and (R)-
S)-Josiphos were effective for enantioselective silylation to
provide 2a in 40% ee, 41% ee, and 47% ee, respectively. The
R)-(S)-BPPFA with a ferrocene backbone afforded 2a in high
absolute configuration for the major isomer are shown. Ee was
(
i
determined on a CHIRALPAK IB column with hexane/ PrOH = 99/1
as the eluent.)
(
yield (85%) but with low enantioselectivity (22% ee). Although
the combination of [Rh(OMe)(cod)]2 with (R)-DTBM-
SEGPHOS possessed similar catalytic activity (79% yield,
2
enantioselective C(sp )−H bond silylation of ferrocene using a
7a
combination of the rhodium complex and chiral diene ligand.
5
2% ee), the use of other rhodium and iridium precatalysts,
Although the maximum enantioselectivity was 86% ee, their
catalytic system requires 10 mol % of expensive rhodium
complex together with 10 equiv of a hydrogen acceptor to
prevent competitive hydrosilylation of the chiral diene ligand at
high temperature (135 °C). In contrast to the report of Shibata
et al., no side reactions, including cleavage of the C−Si bond,
occurred using the current rhodium−chiral diphosphine
system.
such as [Rh(cod)(OTf)] , [IrCl(cod)] , or [Ir(OMe)(cod)] ,
decreased both the yield and enantioselectivity. Other
transition-metal complexes, including Ru (CO) , Ir (CO) ,
2
2
2
1
6
3
12
4
12
Re (CO) , Pd(OAc) , Pd (dba) , and In(OTf) , were
2
10
2
2
3
3
completely ineffective, with the reaction resulting in recovery
of precursor 1a.
The effect of solvent and temperature on enantioselectivity
was also examined (Table 2). Acetonitrile and toluene were
ineffective at achieving enantioselectivity, while 1,2-dichloro-
ethane and cyclohexane provided 2a in moderate yield with
good enantioselectivity (entries 1−6). Further screening
revealed that using 1,2-dichloroethane as a solvent at a reaction
temperature of 50 °C increased the yield of 2a without loss of
enantioselectivity (entry 7). The reaction proceeded, even at 30
Under the reaction conditions shown in Table 2, hydrosilane
with an electron-withdrawing trifluoromethyl group afforded
the corresponding benzosiloloferrocene 2b in 93% yield with
1
7
80% ee (Figure 3). A chloride group, which allows further
derivatization through a transition-metal-catalyzed cross-cou-
pling reaction, was also tolerated to afford 2c in 80% yield and
77% ee. In contrast to our previous report, reductive
dechlorination leading to 2a did not occur when (R)-DTBM-
SEGPHOS was used as a ligand, which indicates the potential
utility of the current reaction in various functional material
°
C, with ee up to 83%, although prolonged reaction time (48
h) was required for the complete conversion of 1a (entry 8). In
the course of these investigations, Shibata et al. reported
B
DOI: 10.1021/acs.orglett.5b01373
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Effect of Solvent and Temperature on
Enantioselective Dehydrogenative Silylation of the C(sp )−
H Bond of 1a
Scheme 1. Parallel Competitive Silylation of 1a, 1b, and 7 in
the Separated Flask To Elucidate the Electronic Effects on
the Reactivity
2
a
entry
solvent
dioxane
yield (%)
ee (%)
1
2
3
4
5
6
76
87
24
67
86
83
93
88
55
61
3
THF
MeCN
CH ClCH Cl
77
74
37
79
83
2
2
cyclohexane
toluene
b
7
c
CH ClCH Cl
2
2
absorption of 2a with respect to phenylferrocene reflect (1)
effective electronic interaction between the ferrocene and the
benzosilole moieties and (2) a decreased HOMO−LUMO
energy gap as a consequence of the expanded π-conjugation.
These values are summarized in Table 3 along with the redox
8
CH ClCH Cl
2 2
a
Isolated yields, ee, and absolute configuration for the major isomer
are shown. ee was determined on a CHIRALPAK IB column with
hexane/ PrOH = 99/1 as the eluent. The absolute configuration of the
major enantiomer was S . 50 °C. 30 °C, 48 h.
i
b
c
p
Table 3. Photophysical and Electrochemical Properties
E1/2
λabs (nm) (ε × 10−3, M−1 cm−1
)
a
(mV vs Fc/Fc )
+
b
compd
phenylferrocene 288 (10.6), 440 (0.9)
306 (14.3), 348 (9.6), 456 (2.9)
−9
−42
n
2
a
a
−5
b
−3
In CH Cl (1 × 10 M). In CH Cl (1 × 10 M) with Bu NPF
2
2
2
2
4
6
(
0.1 M) as a supporting electrolyte.
potentials determined by cyclic voltammetry. CV charts of 2a in
Figure S3 contained reversible waves for the Fe(III)/Fe(II)
oxidation processes of the ferrocene moieties giving the
monocation radical. The oxidation potential of 2a shifted
cathodically by 33 mV when compared to that of phenyl-
ferrocene, again indicating the effective expansion of the π-
conjugated system.
Figure 3. Rh-catalyzed enantioselective synthesis of benzosilole-fused
metallocenes. (Ee was determined on a CHIRALPAK OD column
i
a
b
with hexane/ PrOH = 99/1 as the eluent. At 30 °C for 48 h. With
3
,3-dimethyl-1-butene (2 equiv).)
In conclusion, several benzosilole- and benzogermole-fused
ferrocenes and ruthenocene were prepared by rhodium-
catalyzed enantioselective intramolecular C−H silylation and
germylation. The process did not require the severe reaction
conditions or oxidants and may offer a practical and
environmentally friendly route to functionalized planar chiral
metallocenes. These results are of potential significance for
materials science as well as enantioselective synthesis. Further
4
a
syntheses. Cyclization of diethylsilane leading to 2d
proceeded with excellent enantiocontrol (93% ee). The
substrate scope could be expanded to hydrosilanes with a
18
ruthenocene backbone to furnish 6 in 94% yield with 82% ee.
To obtain insight into the reaction mechanism, the parallel
competitive cyclization of hydrosilanes 1a without any
substituents and 1b having an electron-withdrawing trifluor-
omethyl group was carried out in a separate flask. The result
demonstrated that formation of 2a proceeded faster than that
of 2b (Scheme 1, upper), reflecting the nucleophilic nature of
the silylrhodium species, which can be generated via oxidative
addition of the rhodium center to H−Si bond of 1. On the
other hand, 2-(dimethylsilyl)biphenyl 7 gave 9-silafluorene 8 in
3
2
investigation into enantioselective C(sp )−H and C(sp )−H
bond functionalization as well as applications of the resulting
chiral metallocenes are currently underway.
ASSOCIATED CONTENT
Supporting Information
■
*
S
2
4% yield under the same reaction conditions (Scheme 1,
4a
2
Experimental procedures, spectroscopic data for all new
lower). Although the dehydrogenative silylation of C(sp )−H
1
13
compounds, and copies of H and C NMR spectra. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.orglett.5b01373.
bonds has previously been reported to proceed via the C−H
19
bond activation rather than the electrophilic metalation, the
conversion of electron-rich 1a was faster than that of 7 under
the current reaction conditions using (R)-DTBM-SEGPHOS.
The UV−vis absorption study indicates that the benzosilo-
loferrocene 2a showed a maximum peak at 306 nm along with a
broad shoulder peak in the region of 400−550 nm (Figure S2,
Supporting Information). The bathochromic shifts of the
20
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: masahito.murai@okayama-u.ac.jp.
*E-mail: ktakai@cc.okayama-u.ac.jp.
C
DOI: 10.1021/acs.orglett.5b01373
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
P(4-MeOC
6
H
4
)
3
, 54%; PCy
3
, 24%; P(4-CF
3
C
6
H
4
)
3
, 30%; P(2-furyl) ,
3
2
%; P(OPh) , 23%; Xantphos, 32%.
3
The authors declare no competing financial interest.
(10) (a) Kuninobu, Y.; Nakahara, T.; Takeshima, H.; Takai, K. Org.
Lett. 2013, 15, 426. (b) Murai, M.; Takeshima, H.; Morita, H.;
Kuninobu, Y.; Takai, K. J. Org. Chem. 2015, 80, 5407. For our recent
work on the rhenium-catalyzed dehydrogenative borylation of
C(sp )−H bonds adjacent to nitrogen atom, see: (c) Murai, M.;
Omura, T.; Kuninobu, Y.; Takai, K. Chem. Commun. 2015, 51, 4583.
ACKNOWLEDGMENTS
■
2
This work was financially supported by a Grant-in-Aid (No.
5105739) from MEXT, Japan, and the MEXT program for
3
promoting the enhancement of research universities. We
gratefully thank Mr. Naoki Hosokawa (Okayama University)
for HRMS measurements and Prof. Koichi Mitsudo and Prof.
Seiji Suga (Okayama University) for CV measurements.
(11) During the review of the current manuscript, Hartwig et al.
reported enantioselective silylation of C(sp )−H bonds leading to
benzoxasiloles. See: Lee, T.; Wilson, T. W.; Berg, R.; Ryberg, P.;
2
Hartwig, J. F. J. Am. Chem. Soc. 2015, 137, 6742.
(12) Metallocenes with two different substituents in the one
cyclopentadienyl ring are chiral due to the loss of the plane of
symmetry. For representative pioneering works, see: (a) Thomson, J.
B. Tetrahedron Lett. 1959, 1, 26. (b) Cahn, R. S.; Ingold, C.; Prelog, V.
Angew. Chem., Int. Ed. Engl. 1966, 5, 385. For reviews, see:
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(
8
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(
(
15) When (R)-DTBM-SEGPHOS and 3,3-dimethyl-1-butene were
used as ligand and hydrogen acceptor, respectively, 2a was obtained in
3% yield with 50% ee.
16) Effect of metal complexes with (R)-DTBM-SEGPHOS at 70
C: [Rh(OMe)(cod)] , 79%, 52% ee (S major); [Rh(OTf)(cod)] ,
(
7
(
(
4
(
°
4
2
p
2
6%, 21% ee (Sp major); [IrCl(cod)] , 44%, 3% ee (Rp major);
2
F.; Reynolds, J. R. J. Am. Chem. Soc. 2011, 133, 10062. (d) Shimizu,
M.; Mochida, K.; Katoh, M.; Hiyama, T. J. Phys. Chem. C 2010, 114,
0004 and references cited therein.
[
Ir(OMe)(cod)] , 35%, 5% ee (R major).
2
p
(17) The reaction of 2-(hydrodimethylgermyl)phenylferrocene 4 in
1
(
the presence of [RhCl(cod)] and the chiral phosphines listed in
2
7) During our detailed investigation, we have learned that Prof.
Figure 2 afforded the inseparable mixture of products.
Takanori Shibata (Waseda University, Japan) also examined the
rhodium-catalyzed asymmetric synthesis of ferrocene-fused benzosi-
loles. We thank Prof. Shibata for providing this information prior to
their publication. For their recent report, see: (a) Shibata, T.; Shizuno,
T.; Sasaki, T. Chem. Commun. 2015, 51, 7802. Part of our current
work has already been reported: 95th Annual Meeting of the Chemical
Society of Japan; Chemical Society of Japan: Tokyo, March 2015; 2E5−
(
18) The absolute configuration of 2b, 2c, 2d, and 6 was deduced
from the configuration of 2a, which was assigned in ref 7a.
19) (a) Simmons, E. M.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132,
7092. (b) Kuznetsov, A.; Gevorgyan, V. Org. Lett. 2012, 14, 914.
20) The reaction with PPh (15 mol%) in place of (R)-DTBM-
(
1
(
3
SEGPHOS at 50 °C for 4 h gave 7% yield of 2a (from 1a) and 16%
yield of 8 (from 7). The conversion of 7 was slower than that of 1a
only when (R)-DTBM-SEGPHOS was used as a ligand (Scheme 1),
probably because the bulky rhodium center having a (R)-DTBM-
SEGPHOS ligand distorted the rhodacycle intermediate generated
from 7 and decreases its stability. Thus, we believe the present reaction
3
3. During the review of the current manuscript, He et al. reported
enantioselective silylation of ferrocene using the rhodium−chiral
phosphine catalyst system. See: (b) Zhang, Q.-W.; An, K.; Liu, L.-C.;
Yue, Y.; He, W. Angew. Chem., Int. Ed. 2015, 54, 6918.
(
8) The frontier energy levels for dibenzogermoloferrocene (X =
2
mechanism contains C(sp )−H bond activation, a mechanism similar
GeH in Figure 1) are −5.26 eV (HOMO) and −0.68 eV (LUMO),
2
to that reported in ref 19.
respectively.
(
9) Effect of other achiral ligands on the dehydrogenative silylation of
hydrosilane 1a with [RhCl(cod)] in dioxane at 70 °C: PPh , 48%;
2
3
D
DOI: 10.1021/acs.orglett.5b01373
Org. Lett. XXXX, XXX, XXX−XXX