Cooperative catalytic activation of Si-H bonds by a polar Ru-S bond: Regioselective low-temperature c-h silylation of indoles under neutral conditions by a Friedel-crafts mechanism

Article abstract of DOI:10.1021/ja111483r

Merging cooperative Si-H bond activation and electrophilic aromatic substitution paves the way for C-3-selective indole C-H functionalization under electronic and not conventional steric control. The Si-H bond is heterolytically split by the Ru-S bond of a coordinatively unsaturated cationic ruthenium(II) complex, forming a sulfur-stabilized silicon electrophile. The Wheland intermediate of the subsequent Friedel-Crafts-type process is assumed to be deprotonated by the sulfur atom, no added base required. The overall catalysis proceeds without solvent at low temperature, only liberating dihydrogen.

Full text of DOI:10.1021/ja111483r

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Cooperative Catalytic Activation of Si-H Bonds by a Polar Ru-S Bond:
Regioselective Low-Temperature C-H Silylation of Indoles under
Neutral Conditions by a Friedel-Crafts Mechanism
Hendrik F. T. Klare,†,‡ Martin Oestreich,*,† Jun-ichi Ito,§ Hisao Nishiyama,§ Yasuhiro Ohki,*,‡ and
Kazuyuki Tatsumi*,‡
†Organisch-Chemisches Institut, Westf€alische Wilhelms-Universit€at M€unster, Corrensstrasse 40, 48149 M€unster, Germany
‡Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
§Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8603, Japan
S Supporting Information
b
Scheme 1. Cooperative Heterolysis of H-H and Si-H
ABSTRACT: Merging cooperative Si-H bond activation
and electrophilic aromatic substitution paves the way for
C-3-selective indole C-H functionalization under electro-
nic and not conventional steric control. The Si-H bond is
heterolytically split by the Ru-S bond of a coordinatively
unsaturated cationic ruthenium(II) complex, forming a
sulfur-stabilized silicon electrophile. The Wheland inter-
mediate of the subsequent Friedel-Crafts-type process is
assumed to be deprotonated by the sulfur atom, no added
base required. The overall catalysis proceeds without solvent
at low temperature, only liberating dihydrogen.
Bonds by Polar M-S Bonds
for the observed C-3 regioselectivity,17,20 whereas C-H bond
activation usually occurs at the C-2 carbon atom.21 Falck
et al.7 as well as Ishiyama and Miyaura et al.9b reported iridium-
(I)-catalyzed C(sp2)-H silylations of heterocycles at elevated
reaction temperatures of 80 and 120 °C, respectively. The
inherent C-2 selectivity is steered toward the C-3 position by a
large group at the indole nitrogen atom. Methods selectively
addressing the C-3 position without the need for a directing
group are still elusive, and we describe herein a remarkably mild
electrophilic indole C-H functionalization through a coopera-
tive Si-H bond activation mechanism.
elective activation of C(sp2)-H bonds is a challenge in
S
transition metal catalysis,1 and the direct transformation of
C(sp2)-H bonds into C(sp2)-Si bonds also receives its share of
attention.2 Several innovative transition metal-catalyzed proto-
cols were disclosed using either Si-H3-7 or Si-Si8-10 coupling
partners (Si = R3Si or RF2Si), the former often requiring a
sacrificial alkene as a dihydrogen acceptor.4,5,7 The diverse
reaction mechanisms of these catalyses are not fully elucidated
in all cases but likely proceed through a step in which the transi-
tion metal is involved in the C(sp2)-Si bond-forming event.
Our laboratories considered an alternative approach to the
transition metal-catalyzed dehydrogenative coupling of a
C(sp2)-H bond and a Si-H bond that would follow a funda-
mentally different mechanism,11 inspired by the unique reactivity
of polar M-S bonds (M = late transition metal) in the cooperative
heterolysis of dihydrogen12,13 at low temperature (Scheme 1,
upper).14 We reasoned that the same system could also serve as
a reactive site for heterolytic silane activation15 (Scheme 1, lower),
generating a highly electrophilic silicon intermediate.
We selected the cationic complexes 112 and 222 (Chart 1)
introduced by us for the planned Si-H bond heterolysis. The
coordinatively unsaturated metal center in 1 and 2 is stabilized by
a bulky 2,6-dimesitylphenyl thiolate (SDmp) ligand,23 which also
retards formation of binuclear sulfur-bridged complexes. The
polar M-S bond of these (formally) 16-electron complexes
combines Lewis acidity at the metal center and Lewis basicity at
the adjacent sulfur atom. That electronic situation lends both 1
and 2 the reactivity to heterolytically split dihydrogen.12,22
The lability of the thiolate ligand in rhodium(III) complex 1a
and iridium(III) complex 1b during the Si-H bond heterolysis
results in decomposition.12c For that reason, we decided to test
ruthenium(II) complex 2 with a tethered thiolate ligand, thereby
imparting stability to the Ru-S bond, which was now expected
to be retained throughout the catalytic cycle. We were delighted
to find that 2 catalyzed the dehydrogenative C(sp2)-Si coupling
at the indole C-3 carbon atom independent of the nitrogen
protective group (3-5f7a-9a, Table 1, entries 1-3).24 C-3
It was our hope that the thus-formed silicon-substituted
sulfonium ion would be sufficiently electrophilic to be susceptible
to nucleophilic attack at the silicon atom by an electron-rich
heterocycle. This assumption is corroborated by synthetic work
from the Simchen laboratory,16,17 showing that pyrroles and indoles
undergo silylation at the C-3 carbon atom with Me3SiOTf in the
presence of excess base.18 Pure electronic control in a Friedel-
Crafts-type electrophilic aromatic substitution (SEAr)19 accounts
Received: December 21, 2010
Published: February 22, 2011
r
2011 American Chemical Society
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|

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Table 2. Indole C-H Bond Functionalization: Silanesa,b,c
Chart 1. Coordinatively Unsaturated Transition Metal
Thiolate Complexes with a Polar M-S Bond [ArF = 3,5-
Bis(trifluoromethyl)phenyl]
silane
Si-H
silylated indole
entry cmpd
T [°C] t [h] conv. [%]a cmpd yield [%]c
1
2
3
4
5
6a Me2PhSi-H
6b MePh2Si-H
6c Et3Si-H
rt
rt
1/3
1/8
4
>95
>95
>95
0
9a
9b
9c
9d
9e
96
93
89
-
Table 1. Indole C-H Bond Functionalization: Protective
Groups
90
90
90
6d Ph3Si-H
24
6e t-BuMe2Si-H
24
0
-
a,b,c For footnotes a-c, see Table 1. d Too high viscosity toward full silane
consumption is avoided by using excess indole. Reaction rates increase:
cf. Table 1, entry 3 vs Table 2, entry 1.
Scheme 2. C-3 Selectivity Put to the Test
indole
entry cmpd PG
silylated indole
T [°C] t [h] cmpd C-3:C-2b yield [%]c
1
2
3
3
4
5
i-Pr3Si
Bn
50
50d
rt
6
2
2
7a
8a
9a
>99:1
>99:1
>99:1
82
89
92
Me
a Conversion was monitored by GLC analysis. b Ratio of regioisomers
1
was determined by GLC analysis and H NMR spectroscopy prior to
electron-donating and withdrawing groups were tolerated, and the
reaction rates were in the same order of magnitude irrespective of
the electronic nature of the substituent. The reaction temperature
was determined by the melting point of the indole rather than lack
of reactivity. The C-3:C-2 ratio was flawless in all cases.
purification. c Yield refers to analytically pure product isolated after
purification by flash chromatography on silica gel. d The reaction
temperature corresponds to the melting point of the indole.
over C-2 selectivity was of course obtained for i-Pr3Si (in 3) and
Bn (in 4) protection, but electronic control also secured the same
superb site-selectivity for the Me group (in 5). The catalyses
neither required solvent25 nor base18 nor a hydrogen acceptor4,5,7
to maintain turnover, and catalyst loadings as low as 0.1 mol %
still promoted the above coupling of 5, yet at reduced reaction
rate. Complex 2 retained its reactivity after consumption of the
reactants, and g10 reuses were possible at similar reaction rates,
with unchanged regioselectivity.
Having established the new catalytic system, we next focused
on the silane scope in the C-H silylation of structurally unbiased
5 (Table 2). A comparison of representative triorganosilanes
6a-6e revealed that steric factors override electronic effects, that
is the Lewis acidity of the silicon atom. Nonhindered silanes 6a-6c
reacted with expected reaction rates in the order MePh2SiH >
Me2PhSiH > Et3SiH (Table 2, entries 1-3). Larger silanes 6d
(with three Ph groups) and 6e (with one t-Bu group) cannot
accommodate the congested environment around the Ru-S
bond in 2 (Table 2, entries 4 and 5).26
The pronounced electronic preference for C-H bond function-
alization in the indole C-3 position was further probed in the
reaction of indole 10 with C-3 blocked by a Me group, which
completely thwarted the dehydrogenative coupling (10f11a,
Scheme 2), clearly supporting a Friedel-Crafts mechanism.
We next examined the scope of the indole motif (Table 3).
Our survey included examples of 2-substituted indoles (12 and
13, Table 3, entries 1 and 2) and several indoles substituted
on the benzene ring (14-18, Table 3, entries 3-7). Both
On the basis of current spectroscopic and experimental
observations, we suggest a catalytic cycle (Scheme 3) that con-
sists of (i) a cooperative silane activation step (cf. Scheme 1)12
and (ii) an SEAr reaction. (i) Initial coordination of the Si-H
bond to the vacant ruthenium(II) site merges into the reversible
heterolytic Si-H bond cleavage by the polar Ru-S bond
(2f26). A 1H/2H scrambling experiment at room temperature
employing [2H]-6a and [1H]-6b in the absence of a nucleophile
demonstrated their reversible interaction with complex 2. Mass
spectrometric analysis of the isotopic distribution showed com-
plete scrambling (1H/2H ≈ 50:50) for [1H/2H]-6a and
[1H/2H]-6b. One- and two-dimensional NMR measurements
of 26 (using excess silane in the absence of indole) are consistent
with our hypothesis. At room temperature, dynamic processes
including reversibility result in line-broadening, while in turn, a
1
clearly resolved doublet with no JSi-H satellites at δ = -8.25
(2JP-H = 48.7 Hz) ppm [1H NMR (500 MHz) in 1,2-Cl2C6D4]
at -10 °C provided unambiguous evidence for a phosphine-
coordinated Ru-H. The corresponding doublet is seen in the
31P NMR (202 MHz) spectrum at δ = 39.8 (2JP-H = 48.4 Hz)
ppm while a singlet appears at δ = 23.2 ppm for 2. (ii) The
assumed sulfur-stabilized trivalent silicon cation (silicon-substi-
tuted sulfonium ion27) in 26 is then transferred onto nucleophilic
indole 5, yielding Ru-H complex 27 (26f27) and σ-complex
28 (5f28). The fate of Wheland intermediate 28 could either
be reduction to the indoline followed by oxidation to 9 mediated
by 2 (not shown) or proton abstraction from 28 facilitated by
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Table 3. Indole C-H Bond Functionalization: Indolesa
Scheme 3. Proposed Catalytic Cycle: Cooperative Ru-S-
Catalyzed Si-H Bond Activation Followed by Regioselective
Friedel-Crafts-Type Indole C-H Bond Functionalization
(BArF - Omitted for Clarity)
4
Scheme 4. Control Experiment with Deuterated Silane
a All reactions were conducted according to the general procedure (for
details, see Supporting Information): indicated indole (1.0 equiv) and
silane 6a (1.0 equiv) in the presence of 2 (1 mol %) at the indicated
reaction temperature. b C-3:C-2 > 99:1. c Yield refers to analytically
pure product isolated after purification by flash chromatography on
silica gel. d The reaction temperature corresponds to the melting point
of the indole.
Scheme 5. Control Experiment with Silicon-Stereogenic
Silane
the weakly basic sulfur atom in 27 (28f9).28 Complex 2 is
indeed capable of indoline-to-indole dehydrogenation, though at
elevated temperature. An independent experiment using a deut-
erated silane ruled out that pathway; no deuterium incorpora-
tion into the indole was detected ([2H]-6af9a, Scheme 4). The
deprotonation of 28 by 27 affords 29, which is known to
immediately release dihydrogen (27f29f2);22 a related iri-
dium complex, generated from 1b and dihydrogen, was crystal-
lographically characterized.12a The proposed dual role of
the sulfur atom of 2 is remarkable as it is vital in both steps (i)
and (ii).
To gain additional insight into these steps, we probed the
reversible Si-H bond activation (2f26) as well as the inter-
molecular transfer of the trivalent silicon group (26f27) with a
silicon-stereogenic silane (experimental data are detailed in the
Supporting Information). By this, we established that the former
(in the absence of indole) proceeds with partial racemization at
extended reaction times (not shown), whereas the latter (in the
presence of indole) proceeds with complete racemization [(SiR)-
6ffrac-9f, Scheme 5]. We interpret these findings as additional
evidence in support of an SEAr mechanism, involving cationic
silicon intermediates. Aside from the configurational lability of
the silicon in a sulfur-stabilized silicon cation, several events
might account for the observed racemization, e.g., shuffling of the
silicon moiety between the reactants (silane and indole), parti-
cularly at those high concentrations without solvent.
bond activation by cationic complex 2 is unlikely.29 Examples
of that in carbonyl reduction are known though.29b,c As
opposed to our proposal, no ligand at the ruthenium atom is
directly participating in that process.29c Its mechanism is likely
related to the B(C6F5)3-catalyzed silane activation,30 and
clean inversion of the configuration at the silicon atom
is observed.31 Indoles are, in fact, reduced to indolines under
these conditions.32
To recap, we disclosed a perfectly regioselective and mild
indole C-H functionalization. The catalysis proceeds neat at
room temperature, not requiring a base or a hydrogen acceptor.
The regioselectivity is now C-3 and not C-2 at the indole core.
We think that these unique features are made possible by
combining cooperative Si-H bond activation and Friedel-
Crafts-type SEAr. The ruthenium-coordinated sulfur atom in
catalyst 2 is believed to be decisive in both steps, stabilizing a
trivalent silicon cation and later facilitating proton abstraction.
Our laboratories are currently extending this strategy to related
heterocycles along with an in-depth mechanistic investigation.
The above mechanistic picture will still require further
verification but a conventional Lewis acid-mediated Si-H
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S
Supporting Information. Experimental details and char-
b
acterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
martin.oestreich@uni-muenster.de;
ohki@mbox.chem.nagoya-u.ac.jp; i45100a@nucc.cc.nagoya-u.ac.jp
’ ACKNOWLEDGMENT
This research was supported by the Deutsche Forschungsge-
meinschaft (International Research Training Group M€unster-
Nagoya, GRK 1143 with a predoctoral fellowship to H.F.T.K.,
2007-2010), the G-COE program in chemistry (Nagoya), and a
Grant-in-Aid for Scientific Research (No. 18GS0207) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan. We acknowledge Dr. Christian Kesenheimer (Nagoya) for
a preliminary experiment, Dr. Klaus Bergander (M€unster) for
expert advice with the NMR spectroscopic measurements, and
Barbara Hildmann (M€unster) for her skillful technical assistance.
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