Iridium-catalyzed intermolecular dehydrogenative silylation of polycyclic aromatic compounds without directing groups

Article abstract of DOI:10.1002/chem.201406508

This study describes the iridium-catalyzed intermolecular dehydrogenative silylation of C(sp²)-H bonds of polycyclic aromatic compounds without directing groups. The reaction produced various arylsilanes through both Si-H and C-H bond activation, with hydrogen as the sole byproduct. Reactivity was affected by the electronic nature of the aromatic compounds, and silylation of elec-tron-deficient and polycyclic aromatic compounds proceeded efficiently. Site-selectivity was controlled predominantly by steric factors. Therefore, the current functionalization proceeded with opposite chemo- and site-selectivity compared to that observed for general electrophilic functionalization of aromatic compounds.

Full text of DOI:10.1002/chem.201406508

DOI: 10.1002/chem.201406508
Communication
&
CÀH Bond Activation
Iridium-Catalyzed Intermolecular Dehydrogenative Silylation of
Polycyclic Aromatic Compounds without Directing Groups
Masahito Murai,* Keishi Takami, and Kazuhiko Takai*[a]
strates as solvents (usually more than 50 equiv), and high tem-
Abstract: This study describes the iridium-catalyzed inter-
peratures. In addition, intermolecular dehydrogenative silyla-
molecular dehydrogenative silylation of C(sp2)ÀH bonds of
tion of polycyclic aromatic compounds has not been reported,
polycyclic aromatic compounds without directing groups.
because the reaction of fewer substrates in the solvent would
The reaction produced various arylsilanes through both
be desirable for these solid aromatic compounds.
SiÀH and CÀH bond activation, with hydrogen as the sole
byproduct. Reactivity was affected by the electronic
nature of the aromatic compounds, and silylation of elec-
tron-deficient and polycyclic aromatic compounds pro-
ceeded efficiently. Site-selectivity was controlled predomi-
nantly by steric factors. Therefore, the current functionali-
zation proceeded with opposite chemo- and site-selectivi-
ty compared to that observed for general electrophilic
functionalization of aromatic compounds.
In our effort to develop dehydrogenative silylation of sp2
and sp3 CÀH bonds,[8] the unexpected dehydrogenative silyla-
tion of naphthalene with silanes occurred in the presence of
a catalytic amount of an iridium complex.[9] This observation
prompted a more thorough examination of the reaction be-
cause of the following: 1) the reaction proceeded efficiently
with naphthalene using a slight excess of silanes (~3 equiv);
2) site- and chemoselectivity could be well-controlled (sterically
less demanding and electron-deficient aromatic compounds
react preferentially), affording mono- and disilylated naphtha-
lenes in good yields; and 3) the major product, b-silylnaphtha-
lene, can be converted easily to b-halonaphthalenes,[10] which
are difficult to access by typical electrophilic functionalization
with bromine or iodine (Figure 1). Hartwig et al. reported rho-
dium-catalyzed dehydrogenative silylation of functionalized
benzene derivatives during the preparation of this work.[7i,j]
Synthesis of functionalized polycyclic aromatic compounds is
interesting due to their potential as tools for the construction
of complicated aromatic p-systems.[1] Among them, arylsilanes
continue to be important because they are key components
for new organic functional materials. These compounds can be
prepared by the reaction of aryl Grignard or aryl lithium re-
agents with halosilanes,[2] and transition-metal-catalyzed cou-
pling reaction of aryl halides with hydrosilanes[3] or disilanes.[4]
However, dehydrogenative silylation of simple arenes with hy-
drosilanes through both CÀH and SiÀH bond activation would
be a more direct and attractive because of atom-efficiency and
environmental benignity.[5] Pioneering work was done by Cha-
tani and Murai who reported the ruthenium-catalyzed intermo-
lecular dehydrogenative silylation of aromatic compounds
using directing groups, such as oxazolyl and pyridyl groups.[6]
The intermolecular dehydrogenative CÀH silylation of simple
aromatic compounds without directing groups still remains
challenging. The seminal work by Curtis et al. using Vaska’s
complex[7a] resulted in several ruthenium, rhodium, and plati-
num catalysts that are effective for this transformation.[7] How-
ever, the reaction had several disadvantages, including the
substrate scope, need for a large excess of neat aromatic sub-
Table 1. Optimization of reaction conditions.
Entry
Catalyst/ligand
Solvent
Yield [%][a]
1
2
3
4
[RhCl(PPh3)3]
[RhCl(cod)]2/tmphen
[IrCl(cod)]2/tmphen
[Ir(OMe)(cod)]2/tmphen
[Ir(OMe)(cod)]2/tmphen
[Ir(OMe)(cod)]2/tmphen
[Ir(OMe)(cod)]2/dtbpy
[Ir(OMe)(cod)]2/1,10-phen
[Ir(OMe)(cod)]2/tmphen
[Ir(OMe)(cod)]2/tmphen
cyclohexane
cyclohexane
cyclohexane
cyclohexane
dioxane
1
1
53
71 (70)
70
8
48
42
30
62 (59)[d]
5
6[c]
7
(CH2Cl)2
cyclohexane
cyclohexane
cyclohexane
cyclohexane
8
9[b]
10[c]
[a] Determined by 1H NMR spectroscopy. The values in parentheses are
isolated yields. [b] Cyclohexene was used in place of 3,3-dimethyl-1-
butene. [c] Naphthalene/HSiEt3 =1:3. Yield is based on silane. [d] Mixture
of disilylated naphthalenes 2a and 2a’ were isolated in 13 and 12%
yields.
[a] Dr. M. Murai, K. Takami, Prof. Dr. K. Takai
Division of Chemistry and Biotechnology
Graduate School of Natural Science and Technology
3-1-1, Tsushimanaka, Kita-ku, Okayama 700-8530 (Japan)
Fax: (+81)86-251-8094
E-mail: masahito.murai@cc.okayama-u.ac.jp
ktakai@cc.okayama-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406508.
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lene was recovered intact.[7i,j] Note that the reaction proceeded
efficiently only when 3,3-dimethyl-1-butene was used as a hy-
drogen acceptor. Other olefins, such as cyclohexene, cyclooc-
tene, and norbornene, which were identified as optimized hy-
drogen acceptors in the previous work, were not applicable to
the current iridium-catalyzed system due to competitive hydro-
silylation (entry 9). When the ratio of naphthalene and hydrosi-
lane employed was changed to 1:3 under the reaction condi-
tions in entry 4, the yield of 1a was decreased to 59%, and
2,6- and 2,7-disilylnaphthalenes 2a and 2a’ were obtained in
13 and 12% yields, respectively (entry 10).
Figure 1. Challenges in the functionalization of polycyclic aromatic com-
pounds. NBS=N-bromosuccinimide.
Using the optimized reaction conditions, the scope of poly-
cyclic aromatic compounds for dehydrogenative silylation was
The present study describes the iridium-catalyzed inter-
molecular dehydrogenative silylation enabling the direct
use of polycyclic aromatic compounds without any direct-
ing groups as substrates, and the production of hydrogen
as the sole byproduct.
Table 2. Ir-catalyzed dehydrogenative silylation of polycyclic aromatic com-
pounds leading to mono- and disilylarenes 1 and 2.
Recently, the Wilkinson complex, [RhCl(PPh3)3], was
found to be effective for the synthesis of 9-silabifluorenes
by the intramolecular dehydrogenative silylation of
C(sp2)ÀH bonds.[8a] This successful result encouraged an
examination of the intermolecular version of this C(sp2)ÀH
bond silylation. Treatment of naphthalene (2 equiv) with
triethylsilane, 3,3-dimethyl-1-butene (2 equiv), and a cata-
lytic amount of [RhCl(PPh3)3] in cyclohexane at 1008C pro-
vided 2-silylnaphthalene 1a albeit in very low yield
(Table 1, entry 1). The combination of other rhodium com-
plexes, including [RhCl(cod)]2 (cod=1,5-cyclooctadiene)
and [Rh(OMe)(cod)]2, with a variety of phosphines and
phenanthroline-based ligands did not improve the yield
of 1a, and most of the naphthalene was recovered even
heated at high temperature for a long reaction time
(entry 2). In contrast, dehydrogenative silylation occurred
smoothly when iridium complexes were used as catalysts
together with phenanthroline-based ligands. For example,
2-silylnaphthalene 1a was obtained selectively in 53%
yield in the presence of a catalytic amount of [IrCl(cod)]2
and 3,4,7,8-tetramethyl-1,10-phenanthroline (referred to
as “tmphen”) (Table 1, entry 3). Note that the silylation oc-
curred selectively at the b-position to afford 1a and no
formation of its isomer, 1-silyl-naphthalene, was observed
under the reaction conditions. Among the iridium and
rhodium complexes examined, [Ir(OMe)(cod)]2 was most
effective for furnishing 1a in 70% yield (entry 4). Examina-
tion of the solvent indicated that dioxane was as effective
as cyclohexane (entries 5 and 6).[11] Next, the effect of li-
gands was investigated by using [Ir(OMe)(cod)]2 and cy-
clohexane as the metal complex and solvent, respectively.
Other nitrogen ligands, such as 4,4’-di-tert-butyl-2,2’-bipyr-
idine (dtbpy), 1,10-phenanthroline (1,10-phen), and 4,7-di-
methyl-1,10-phenanthroline also displayed good activity,
although none were superior to tmphen (entries 7 and 8).
In contrast, phosphine ligands, including PPh3, PCy3, dppf,
2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP), and
DTBM-SEGPHOS, are completely ineffective, and naphtha-
Entry Method[a] Product [%]
1b (R=Cl)
74
2
2b (R=Cl)
8
94
2c (R=4-MeOC6H4)
0
67
2d (R=4-CF3C6H4)
7
91
1
2
A[c]
B
1c (R=4-MeOC6H4)
72
31
3
4
A[b]
B[b]
1d (R=4-CF3C6H4)
72
7
5
6
A[b,c]
B[b]
7
B
36
8
9
A[b]
B[b]
67
14
7
45
2
23
10
11
A[b]
B[b]
64
35
0
37[d]
12
A[b]
31
31
13
14
A[b]
B[b]
54
63
3
20
[a] Method A: arene/HSiEt3 =2:1, [Ir(OMe)(cod)]2 (2.5 mol%), tmphen (5 mol%)
3,3-dimethyl-1-butene (2 equiv). Yields are based on silane. Method B: arene/
HSiEt3 =1:3, [Ir(OMe)(cod)]2 (5.0 mol%), tmphen (10 mol%), 3,3-dimethyl-1-
butene (3 equiv). Yields are based on arenes. [b] Dioxane was used as a solvent.
[c] For 4 h. [d] Mixture of 2,7- and 2,8-disilylanthracenes (1:1).
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investigated (Table 2). The reaction was examined under two
different sets of conditions (method A: arene/silane=2:1, and
B: arene/silane=1:3) for the selective synthesis of mono- and
disilylarenes. The reaction of 1,5-dichloronaphthalene gave the
monosilylated naphthalene 1b in 74% yield with method A,
and disilylated naphthalene 2b in 94% yield with method B,
respectively (Table 2, entries 1 and 2). In contrast with the pre-
vious rhodium-catalyzed system for the silylation of benzene
derivatives,[7i] no reductive dechlorination was observed under
the present reaction conditions (entry 3). Naphthalene deriva-
tives containing an electron-donating anisyl and an electron-
withdrawing trifluoromethylphenyl group were good sub-
strates. The use of dioxane as a solvent to dissolve these naph-
thalenes was effective, and selective silylation at the b-position
of the naphthalene rings furnished 1c, 1d, 2c, and 2d (en-
tries 3–6). However, dehydrogenative silylation of 2,3-dime-
thoxynaphthalene was slow and produced 1e in low yield
(entry 7). The current method also can be applied to silylation
of the CÀH bonds of expanded p-conjugated systems. Monosi-
lylation of benz[de]isoquinoline-1,3-(2H)-dione using method A
provided the expected coupling product 1 f in 67% yield
(Table 2, entry 8). Changing the ratio of the two substrates pro-
duced a mixture of the mono- and disilylated imides 1 f, 2 f,
and 2 f’, which could be separated easily using silica gel
column chromatography, in total 82% yield (entry 9). Anthra-
cene, phenanthrene, and pyrene also underwent dehydrogena-
tive silylation to afford the mono- and disilylated products 1g–
i and 2g–i depending on the reaction conditions (entries 10–
14). All of the reactions occurred selectively at the least steri-
cally hindered position of these polycyclic p-conjugated sys-
tems.
Table 3. Ir-catalyzed dehydrogenative silylation leading to functionalized
benzene derivatives 1 and 2.
[a] 3 equiv of HSiEt3 and 3,3-dimethyl-1-butene were used.
Scheme 1. Ir-catalyzed dehydrogenative silylation with benzyldimethylsilane
(Si=SiMe2Bn).
dimethylsilyl group can be converted easily to aryl groups by
Hiyama cross-coupling, this result confirmed the potential utili-
ty of the present reaction toward the synthesis of complexed
p-conjugated systems.[12]
The current catalyst system is also effective for the intermo-
lecular dehydrogenative silylation of functionalized benzenes
(Table 3).[7i,j] Electron-deficient 1,3-bis(trifluoromethyl)benzene
afforded the corresponding silylarene 1j in 98% yield. The sily-
lation of 1,3-dichlorobenzene involved high chemoselectivity
to provide 1k in 94% yield without the loss of a chlorine
group. In contrast, silylation of an electron-rich aromatic com-
pound, 1,3-dimethoxybenzene, was sluggish. This result is con-
sistent with the reactivity trend demonstrated by the dehydro-
genative silylation of 2,3-dimethoxynaphthalene (Table 2,
entry 7). Formation of 1,3,5-trisubstituted benzene derivatives
were predominant, and their regioisomers were not detected
despite the electronic nature of the starting arenes. Trifluoro-
methylbenzene reacted efficiently to furnish the mono- and
disilylated benzene 1m and 2m in 32 and 52% yields, respec-
tively. These results demonstrate that electron-deficient arenes
1j and 1k were effective substrates for the current dehydro-
genative silylation.
Intermolecular competition experiments (see Table S3 in the
Supporting Information for details) using benzene, naphtha-
lene, 1,5-dichloronaphthalene, 1,3-dichloro- and 1,3-dimethoxy-
benzene revealed that the relative reactivity of these sub-
strates was 1,3-dimethoxybenzene<benzene !naphthalene<
1,3-dichlorobenzene<1,5-dichloronaphthalene (Figure 2). In
Figure 2. Reactivity of arenes for the current Ir-catalyzed dehydrogenative si-
lylation.
Other hydrosilanes can be used as silyl group sources for
the current catalytic transformation. When dehydrogenative si-
lylation of 1,5-dichloronaphthalene with benzyldimethylsilane
was performed, the expected dehydrogenative coupling prod-
ucts were obtained as a mixture of mono- and disilylated ad-
ducts 1n and 2n in 85% total yield (Scheme 1). Chlorine
groups, which have been used in various cross-coupling reac-
tions, were also well-tolerated. Moreover, because the benzyl-
addition, intramolecular competition experiments showed that
dehydrogenative silylation of naphthalene occurred preferen-
tially over benzene under the present conditions (Scheme 2).
Considering that the number of potentially cleavable CÀH
bonds of naphthalene is less than that of benzene (6 for ben-
zene and 4 for naphthalene), the chemoselectivity of naphtha-
lene over benzene was very high. These results suggest that
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Scheme 2. Selective dehydrogenative silylation of naphthalene in the pres-
ence of benzene.
the reactivity order of the aromatic substrates is controlled by
their electronic density and efficiency for interaction with an
iridium center. Resonance stabilization energy of each benzene
ring of polycyclic aromatic hydrocarbons weakens upon fusion
of another benzene ring. Therefore, the double bonds tend to
behave like a simple alkene, promoting the approach of the
catalytically active Ir–Si species through p-coordination (see
Scheme 3). Based on these observations, Scheme 3 presents
Scheme 4. One-pot, site-selective silylborylation of pyrene and its synthetic
application to donor–acceptor substituted pyrene 4 (See the Supporting In-
formation for detailed reaction conditions).
could be converted to the novel donor–acceptor substituted
pyrene 4 by an additional two-step transformation.[18,19] Exten-
sion of p-conjugation along the long axis of pyrene is useful
for expanding the p-conjugation without generating molecular
twist. The absorption in the visible region and strong fluores-
cence proved the usefulness of 4 as a new component of
light-emitting materials and solar cells.
In conclusion, the present report describes the iridium-cata-
lyzed intermolecular dehydrogenative silylation of polycyclic
aromatic compounds by activation of both SiÀH and CÀH
bonds. The reaction occurred preferentially for electron-defi-
cient and polycyclic aromatic compounds. Site-selectivity was
controlled by steric factors to provide selectively functionalized
arenes, which are difficult to access by conventional electro-
philic functionalization.
Scheme 3. Plausible reaction mechanism (tmphen ligand is omitted for clari-
ty. Si indicates SiR3).
a plausible mechanism for the current intermolecular dehydro-
genative silylation. First, IrÀH species was generated by oxida-
tive addition of IrÀOMe to hydrosilane followed by the reduc-
tive elimination of SiÀOMe.[13] This IrÀH species was subse-
quently added to hydrosilane, inserted into 3,3-dimethyl-1-
butene, and reductively eliminated to convert the IrÀSi species.
Next, oxidative addition of the resulting IrÀSi species to the
aryl C(sp2)ÀH bond generated an IrÀSi intermediate, which
then underwent reductive elimination to afford the corre-
sponding arylsilanes along with the regeneration of the IrÀH
species.[14,15] Although CÀH bond activation without using a di-
recting group is generally difficult,[16] the interaction of the
benzene ring in polycyclic aromatic compounds with the IrÀSi
intermediate might fix the Ir center near the aryl CÀH bond
and promote this energetically unfavored step.[17] Steric factors
dominated the site-selectivity of this CÀH bond activation,
which resulted in exclusive formation of the sterically less hin-
dered arylsilanes.
Experimental Section
General procedure for the iridium-catalyzed intermolecular
dehydrogenative silylation of polycyclic aromatic com-
pounds (method A)
A mixture of [Ir(OMe)(cod)]2 (3.3 mg, 5.0 mmol), 3,4,7,8-tetramethyl-
1,10-phenanthroline (2.4 mg, 10 mmol), and cyclohexane or dioxane
(0.25 mL) was stirred at 258C for 30 min. The mixture was then
added to aromatic compound (0.40 mmol), 3,3-dimethyl-1-butene
(33.7 mg, 0.40 mmol), and triethylsilane (23.2 mg, 0.20 mmol), and
further stirred at 1008C for 4 or 9 h. The solvent was removed in
vacuo, and the residue was subjected to flash column chromatog-
raphy on silica gel with hexane/EtOAc as the eluent to afford the
corresponding arylsilanes.
Acknowledgements
This work was financially supported by a Grant-in-Aid (No.
26248030) from MEXT, Japan, and a Grant-in-Aid for Scientific
Research on Priority Areas (No. 25105739), and MEXT program
for promoting the enhancement of research universities.
The present reaction can be extended to subsequent CÀH si-
lylation and CÀH borylation of polycyclic aromatic hydrocar-
bons. For example, treatment of triethylsilane under the opti-
mized reaction conditions followed by addition of bis(pinacola-
to)diboron resulted in selective introduction of both silyl and
boryl functionalities at the 2- and 7-positions of pyrene, re-
spectively (Scheme 4). The resulting 2-boryl-7-silylpyrene 3
Keywords: CÀH bond activation · dehydrogenation · iridium ·
polycyclic aromatic compounds · silylation
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2011, 1, 170–174.
[18] For donor–acceptor-substituted pyrenes, see: a) L. Zçphel, V. Enkel-
mann, K. Mꢃllen, Org. Lett. 2013, 15, 804–807; b) S. N. Keller, N. L. Veltri,
T. C. Sutherland, Org. Lett. 2013, 15, 4798–4801.
[19] K. Funaki, T. Sato, S. Oi, Org. Lett. 2012, 14, 6186–6189. See also
ref. [10].
Received: December 16, 2014
[9] Hartwig et al. has reported iridium-catalyzed borylation of CÀH bonds.
For reviews, see: a) I. A. I. Mkhalid, J. H. Barnard, T. B. Marder, J. M.
Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
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CÀH Bond Activation
M. Murai,* K. Takami, K. Takai*
&& – &&
Iridium-Catalyzed Intermolecular
Dehydrogenative Silylation of
Polycyclic Aromatic Compounds
without Directing Groups
Direct silylation: Treatment of aromatic
compounds with hydrosilanes in the
presence of iridium catalyst afforded si-
lylarenes without any directing groups.
The reactivity was affected by the elec-
tronic nature of arenes, and silylation of
electron-deficient and polycyclic aro-
matic compounds proceeded more effi-
ciently (see scheme).
&
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Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!