Rhodium-catalyzed carbon-silicon bond activation for synthesis of benzosilole derivatives

Article abstract of DOI:10.1021/ja3096174

A rhodium-catalyzed coupling reaction of 2-trimethylsilylphenylboronic acid with internal alkynes is developed for the synthesis of 2,3-disubstituted benzosilole derivatives. A range of functional groups, encompassing ketones, esters, amines, aryl bromides, and heteroarenes, are compatible, which provides rapid access to diverse benzosiloles. Sequential 2-fold coupling enables modular synthesis of asymmetrically substituted 1,5-dihydro-1,5-disila-s-indacene, a π-extended molecule of interest in organic electronics. In terms of the mechanism, the reaction involves cleavage of a C(alkyl)-Si bond in a trialkylsilyl group, which normally requires extremely harsh conditions for activation. Mechanistic studies, including effects of substituents, reveal that C-Si bond cleavage does not proceed through a hypercoordinated silicon species, but rather through a rhodium-mediated activation process. The potential use of the reaction in catalytic asymmetric synthesis of Si-chiral benzosiloles is also demonstrated.

Full text of DOI:10.1021/ja3096174

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Article
Rhodium-Catalyzed Carbon-Silicon Bond
Activation for Synthesis of Benzosilole Derivatives
Masahiro Onoe, Katsuaki Baba, Yoonjoo Kim, Yusuke Kita, Mamoru Tobisu, and Naoto Chatani
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 Nov 2012
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Journal of the American Chemical Society
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Rhodium-Catalyzed Carbon-Silicon Bond Activation for Syn-
thesis of Benzosilole Derivatives
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Masahiro Onoe,† Katsuaki Baba,† Yoonjoo Kim,† Yusuke Kita,†,§ Mamoru Tobisu,*,‡, †
and Naoto Chatani*,†
† Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565ꢀ0871, Japan
‡ Center for Atomic and Molecular Technologies, Osaka University, Suita, Osaka 565ꢀ0871, Japan
ABSTRACT: A rhodiumꢀcatalyzed coupling reaction of 2ꢀtrimethylsilylphenylboronic acid with internal alkynes is developed for
the synthesis of 2,3ꢀdisubstituted benzosilole derivatives. A range of functional groups, encompassing ketones, esters, amines, aryl
bromides, and heteroarenes, are compatible, which provides rapid access to diverse benzosiloles. Sequential twoꢀfold coupling
enables modular synthesis of asymmetrically substituted 1,5ꢀdihydroꢀ1,5ꢀdisilaꢀsꢀindacene, a πꢀextended molecule of interest in
organic electronics. In terms of the mechanism, the reaction involves cleavage of a C(alkyl)ꢀSi bond in a trialkylsilyl group, which
normally requires extremely harsh conditions for activation. Mechanistic studies including effects of substituents, reveal that CꢀSi
bond cleavage does not proceed through a hypercoordinated silicon species, but rather through a rhodiumꢀmediated activation proꢀ
cess. The potential use of the reaction in catalytic asymmetric synthesis of Siꢀchiral benzosiloles is also demonstrated.
10a
formation
of
MeꢀSi
bonds
in
SiMe4
and
Me3Si(CH2)3SO3Na10b are also reported. A more general stratꢀ
egy for activating C(alkyl)ꢀSi bonds is conversion to pentaꢀ or
hexacoordinated silicon species by adding an external or interꢀ
nal nucleophile.11 For instance, an MeꢀSi bond in pentacoordiꢀ
nate Me3SiF2− can be cleaved under palladiumꢀcatalyzed conꢀ
ditions, thus serving as a methyl donor in the crossꢀcoupling
process.12 Nucleophiles, such as alkoxides13 and carbonyl oxꢀ
ygens,14 can be used to activate C(alkyl)ꢀSi bonds to facilitate
alkyl group transfer in crossꢀcoupling processes when the nuꢀ
cleophiles are attached to the silicon atom (internal coordinaꢀ
tion). This intramolecular activation strategy is useful in the
context of the synthesis of silacycles via C(alkyl)ꢀSi bond
cleavage. A tethered alcohol has been reported to promote
intramolecular substitution at the silicon via the cleavage of a
C(alkyl)ꢀSi bond.15 In these cases, stoichiometric amount of a
base is generally required to generate an alkoxide, which can
form hypercoordinated silicon species by intramolecular attack.
Shindo reported that carbonyl oxygen can also serve as a tethꢀ
ered nucleophile in the iodineꢀpromoted cyclization of (Z)ꢀβꢀ
silylacrylic acid derivatives.16 Cramer recently reported a skelꢀ
etal rearrangement including the cleavage of a C(alkyl)ꢀSi
bond by catalytically generated rhodium alkoxide.17 Carbon
nucleophiles, such as organolithium and magnesium, also
promote displacement at a silicon center.18 In this transforꢀ
mation, a new CꢀSi bond is formed by cleaving a inert
C(alkyl)ꢀSi bond. In 2002, van Klink and Bickelhaupt reported
that intramolecular nucleophilic substitution by a tethered
Grignard reagent occurs at a trimethylsilyl group to form
dibenzosilole (Scheme 2a).19 Xi extensively investigated this
reaction using organolithium reagents and established a genꢀ
eral protocol for the synthesis of a range of silole derivatives
(Scheme 2b).20,21 Recently, Shirakawa and Hayashi applied
Introduction
Organosilicon compounds occupy a prominent position in the
field of organic chemistry, and their use covers synthetic reaꢀ
gents and organic materials.1ꢀ4 The method development for
the synthesis of organosilicon compounds therefore continues
to attract interest. To construct new carbonꢀsilicon (CꢀSi)
bonds, halosilanes or hydrosilanes are typically used as the
silicon source,1ꢀ5 but the tetraorganosilanes formed are normalꢀ
ly unreactive and cannot be transformed further. If a CꢀSi
bond transformation is achieved, new synthetic strategies that
enable access to intricate siliconꢀcontaining architectures beꢀ
come possible (Scheme 1).
Scheme 1. CꢀSi Transformation Enables New Synthetic
Strategy
The reactivity of a CꢀSi bond depends greatly on the hyꢀ
bridization of the carbon atom attached to the silicon: C(sp2)ꢀ
Si, C(sp)ꢀSi and C(allyl)ꢀSi bonds can be cleaved relatively
easily, compared with C(alkyl)ꢀSi, because of the hyperconjuꢀ
gative interactions with neighboring πꢀbonds.6,7 C(alkyl)ꢀSi
bonds incorporated in a strained ring, i.e., silacyclopropanes8
and silacyclobutanes,9 are exceptionally reactive, affording
ringꢀopened products under mild conditions. Catalytic transꢀ
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this process to the synthesis of benzosilole derivatives by
Page 2 of 14
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combining it with ironꢀcatalyzed carbolithiation reaction of
alkynes.22
Scheme 4. RhodiumꢀCatalyzed Coupling of 2ꢀ
Trimethysilylphenylboronic Acid with Alkynes
Scheme 2. Intramolecular Nucleophilic Substitution at a
5
Silicon Center for the Synthesis of Silole Scaffold
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It is worth noting that siloles and their benzoꢀfused anaꢀ
logs are attractive synthetic targets. Siloles (silacyclopentadiꢀ
enes) have received much attention as useful building blocks
of πꢀconjugated organic materials because of their unique
electronic properties, resulting from their lowꢀlying LUMO
levels, derived from σ*ꢀπ* conjugation in the ring.27 Additionꢀ
ally, their benzoꢀfused analogs,28 dibenzosiloles (silafluoꢀ
renes)29 and benzosiloles (silaindenes),30 are also known to be
promising candidates for organic materials. Various applicaꢀ
tions of siloleꢀcontaining organic electronic devices such as
lightꢀemitting diodes,31 fieldꢀeffect transistors,32 solar cells,33
and fluorescent sensors34 have been intensively studied. Tradiꢀ
tionally, silole derivatives have been synthesized under harsh
reaction conditions, using stoichiometric amount of strong
reducing reagent,35 highly basic organometallic reagents36 or
other reactive reagents.37,38 A serious drawback of these proꢀ
cedures is narrow functional group toleration as a result of the
vigorous reaction conditions, which limits the diversity of the
accessible products. This limitation has been overcome, in part,
by the recent development of transition metalꢀcatalyzed methꢀ
ods, including nickel,39 palladium,25,40 iridium,41 ruthenium,42
gold,43 and rhodium26,44 catalysis. Nevertheless, further develꢀ
opment of versatile and convergent catalytic methods is still
necessary for the exploration of new functional materials.
In 1992, Ojima reported that the rhodiumꢀcatalyzed reacꢀ
tion of diyne 1 with HSitBuMe2 under an atmosphere of CO
affords silole 2, in which an MeꢀSi bond derived from hyꢀ
drosilane is cleaved (Scheme 3).23 This reaction is notable in
that the C(alkyl)ꢀSi bond cleavage occurs in the absence of a
strongly nucleophilic activator, suggesting that the intermediaꢀ
cy of hypercoordinated silicon species is unlikely, and that
catalytic activation of C(alkyl)ꢀSi is involved. In spite of its
mechanistic uniqueness and potential use as a catalytic method
for the synthesis of siloles, the report contains this one isolated
example, and further investigation have never been reported.
Scheme 3. Ojima's RhodiumꢀCatalyzed Silylative Cyꢀ
clization of Diyne via the Cleavage of a MeꢀSi Bond
Results and Discussion
In 2009, our group reported a novel rhodiumꢀcatalyzed
coupling reaction of 2ꢀtrimethylsilylphenylboronic acid and
internal alkynes, leading to the formation of benzosilole derivꢀ
atives (Scheme 4).24 The most salient feature of this reaction is
the involvement of the activation of the MeꢀSi bond under
catalytic and virtually neutral conditions. Following our preꢀ
liminary report, several catalytic reactions that proceed via the
activation of C(alkyl)ꢀSi bonds have appeared.25,26 Xi demonꢀ
strated that palladium is also a competent catalyst for a similar
process.25 Matsuda has developed an intermolecular variant of
Ojima's reaction (Scheme 3) using disilane, in place of hyꢀ
drosilane.26 In this article, we provide a full account of our
study of rhodiumꢀcatalyzed benzosilole synthesis via the
cleavage of a CꢀSi bond, including the scope of both coupling
partners, mechanistic investigation of the CꢀSi bond cleavage,
and applications in the construction of stereogenic silicon
centers through catalytic enantioselective CꢀSi bond activation.
Previously, we reported rhodiumꢀcatalyzed transformations of
nitriles through the cleavage of carbonꢀcyano bonds with the
aid of organosilicon reagents.45 During the course of the study,
we examined the rhodiumꢀcatalyzed reaction of nitrile 3 with
hexamethyldisilane, expecting that a putative key intermediate
4 might be intercepted by a pendant chloroarene moiety to
form 5. However, we observed no proof for the formation of 5.
Instead, dibenzosilole 6 was isolated in 27% yield (55% of 3
was recovered) (Scheme 5). What intrigued us most was that
one of the methyl groups in a trimethylsilyl group was elimiꢀ
nated during the formation of 6. At the time we initiated these
studies, C(alkyl)ꢀSi bond cleavage by transitionꢀmetal catalysis
was rare.14a,23 Silole derivatives are attractive synthetic targets
because of their characteristic photophysical properties, as
mentioned above.31ꢀ34 We therefore decided to pursue the rhoꢀ
diumꢀmediated dibenzosilole formation reaction in more detail.
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further improved by using organic bases such as triethylamine
1
2
3
4
5
6
7
8
9
or 1,4ꢀdiazabicyclo[2,2,2]octane (DABCO) (entries 2 and 3).
Scheme 5. Initial Discovery of CꢀSi Bond Cleavage
Table 1. RhodiumꢀCatayzed Cyclization of Boronic Acid 9
via Cleavage of MeꢀSi Bonda
Cl
[RhCl(cod)]2
10 mol%
Me3Si SiMe3
ethylcyclohexane
160 °C, 15 h
Si
Me
Me
6
27%
CN
3
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Cl
SiMe3
N
N
SiMe3
Rh
5
a
4
not observed
Reaction conditions: 9 (0.5 mmol), [RhCl(cod)]2 (0.025
mmol), base (1.0 mmol), dioxane (1 mL), H2O (10 ꢁL), at 100 °C
for 15 h. b Isolated yield.
A hypothetical pathway to 6 is outlined in Scheme 6. Acꢀ
cording to our previous findings,45 rhodiumꢀmediated decyaꢀ
native silylation initially occurs to form silylated compound 7.
Arylrhodium species 8 was subsequently generated through
the activation of an ArꢀCl bond. If arylrhodium 8 undergoes an
intramolecular cyclization by substituting a methyl group on
the silicon center, dibenzosilole 6 is produced. As described in
the Introduction, such a displacement at silicon is known to
proceed when strong nucleophiles, such as organolithium or ꢀ
magnesium, are used.19ꢀ21 However, it was uncertain whether
transition metal species such as arylrhodium could promote a
similar substitution reaction.
The formation of 6, shown in Table 1, clearly suggests that
an arylrhodium(I) intermediate can activate a suitably posiꢀ
tioned CꢀSi bond of a trimethylsilyl group and induce cyclizaꢀ
tion to form a silole skeleton. To further exploit this new reacꢀ
tivity of arylrhodium(I) species toward a silyl group in the
synthesis of more demanding targets, we designed an annulaꢀ
tive coupling of 2ꢀtrimethylsilylphenylboronic acid (10a) with
an alkyne to build benzosilole derivatives (Scheme 7). Accordꢀ
ing to the pioneering work of Hayashi and coworkers,47a arylꢀ
rhodium 11 generated from 10a should add across an alkyne
to form alkenylrhodium 12, which would then undergo cyꢀ
clization through displacement at a silicon center, as for arylꢀ
rhodium 8. Although this carborhodation/cyclization strategy
has often been employed using arylboronic acids bearing an
ortho electrophiles, such as carbonyl, cyano, electronꢀdeficient
alkenes, and halide groups,46,48 its application to orthoꢀsilylꢀ
substituted phenylboronic acids has never been reported.
Scheme 6. A Plausible Pathway for Dibenzosilole 6
3
[Rh] Me3SiSiMe3
Cl
[Rh]
[Rh]
Scheme 7. RhodiumꢀCatalyzed TwoꢀComponent Couꢀ
pling Reaction of oꢀTrimethylsilylphenylboronic Acid and
Internal Alkyne
Me
Me
"Me"
Si
Me
SiMe3
Si
Me
8
Me
7
6
(HO)2B
[Rh]
SiMe3
9
As shown in Table 2, the designed annulation was
To verify the feasibility of our hypothesis, especially the
competence of organorhodium species in the CꢀSi bond cleavꢀ
age, we turned our attention to the use of boronic acid 9 as a
more suitable precursor to the arylrhodium 8 on the basis of a
wellꢀestablished boron to rhodium transmetallation process.46
Boronic acid 9 was therefore subjected to conditions typical of
those used for the generation of arylrhodium species, i.e.,
[RhCl(cod)]2 (5 mol%) and base (2 equiv) in dioxane/H2O
(100/1) at 100 °C (Table 1). When Na2CO3 was added as a
base, the expected dibenzosilole 6 was obtained in 89% yield
through the cleavage of an MeꢀSi bond (entry 1). Yields were
achieved
by
conducting
the
reaction
of
2ꢀ
trimethylsilylphenylboronic acid (10a) and diphenylacetylene
(13a) under identical conditions to those used for intramolecuꢀ
lar cyclization (Table 1). The yield of benzosilole 14a was
significantly affected by the base used. When inorganic bases
such as Na2CO3 were used, 14a was obtained in low to modꢀ
erate yield due to the formation of undesired protodeboronated
product (entry 1ꢀ3). A significant increase in yield was obꢀ
served when amine bases were used (entries 4ꢀ6). Notably,
when DABCO was used, the annulative coupling proceeded
even at ambient temperature, producing 14a in 69% yield (enꢀ
try 7).
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Page 4 of 14
without any loss of yield (entry 19). Benzosiloles bearing an
ester group at the 2ꢀposition were also accessible using alkynyl
esters (entries 20 and 21).
1
2
3
4
5
6
7
8
Table 2. RhodiumꢀCatalyzed TwoꢀComponent Coupling
Synthetic methods for benzosiloles are dominated by intramoꢀ
lecular processes,30,35c,40n,42a,b,43 in which the product diversity
relies on the structures of the starting compounds. In contrast,
the reaction established here is an intermolecular twoꢀ
component annulation. A wide range of substituted benꢀ
zosiloles can therefore be prepared simply by changing the
structure of the alkyne component, as demonstrated in Table 3.
This modular nature of the present method is particularly valꢀ
uable when applied to the synthesis of compound libraries.
Having traditionally inaccessible benzosiloles50 in hand has a
significant impact on the exploration of new organic functionꢀ
al materials, given the unique optoelectronic properties of
arylꢀsubstituted siloles.31ꢀ34,51 Some of the 2,3ꢀdiarylꢀ
substituted benzosiloles synthesized in this study exhibited
solidꢀstate photoluminescence, and the wavelength of the
maximum absorption can be tuned by the functional groups
introduced (see SI for details).
of 10a with 13a: Effect of Base and Temperaturea
9
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a
Reaction conditions: 10a (0.5 mmol), 13a (1.0 mmol),
Table 3. Scope of Alkynesa
[RhCl(cod)]2 (0.025 mmol), base (1.0 mmol), dioxane (1 mL),
H2O (10 ꢁL) for 15 h. b Isolated yield. c GC yield.
The scope of the alkyne partner is shown in Table 3. Not
only diphenylacetylene (13a, entry 1), but other substituted
diarylacetylenes 13bꢀ13f produced the corresponding benꢀ
zosiloles (entries 2ꢀ6). The high functional group compatibility
of the reaction gives rapid access to 2,3ꢀdiarylꢀsusbstituted
benzosilole derivatives bearing a range of electronꢀdonating
(entries 2 and 3) and ꢀwithdrawing groups (entries 4ꢀ6). Of
note is the applicability of ketone groups, which are incompatꢀ
ible with the highly basic conditions typically used for the
synthesis of silole derivatives.35,36 Moreover, bromoaryls,
which cannot survive under conventional palladiumꢀbased
crossꢀcoupling conditions, are readily introduced by this rhoꢀ
diumꢀcatalyzed protocol (entry 4). The obtained 14d serves as
a versatile platform for more elaborated benzosilole derivaꢀ
tives. Fused aromatics such as naphthyl, as well as hetꢀ
eroarenes, including thiophene and pyridine, were also incorꢀ
porated (entries 7ꢀ9). Aliphatic alkynes also serve as excellent
components in this annulative coupling to produce dialkylꢀ
substituted benzosiloles (entries 10ꢀ12). The reaction tolerates
a propargylic ether functionality, which is potentially reactive
under rhodium catalysis49 (entry 13). Cyclic alkyne 13n unꢀ
derwent annulation to form a twelveꢀmembered cycloalkaneꢀ
fused benzosilole 14n (entry 14). Several electronically or
stericallyꢀbiased unsymmetrical alkynes were also incorpoꢀ
rated, delivering the corresponding benzosiloles regioselecꢀ
tively. For instance, the use of alkylphenylacetylenes regioseꢀ
lectively yielded benzosiloles bearing an aryl group at the 2ꢀ
position as the major product. (entries 15 and 16). Although
terminal alkynes failed to react in this coupling, silylꢀprotected
alkynes 13qꢀ13s successfully participated to give the correꢀ
sponding benzosiloles. When 1ꢀ(trimethysilyl)propyne 13q
was used, benzosilole bearing a trimethylsilyl group at the 2ꢀ
position was obtained with high yield and regioselectivity
(86%, 11:1) (entry 17). Trimethylsilylphenylacetylene (13r)
was also efficiently incorporated, favoring a 2ꢀsilyl isomer,
albeit with diminished selectivity (94%, 2:1; entry 18). The
regioselectivity in this specific case was readily improved
(10:1) by using a more bulky triethylsilyl protecting group
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struction of simple silole structure. Thus, the reaction of
a
Reaction conditions: 10a (0.5 mmol), 13 (1.0 mmol),
1
2
3
4
5
6
7
8
alkenylboronate ester 18, which is readily prepared by palladiꢀ
umꢀcatalyzed silylboration of ethynylbenzene,54 with 13a furꢀ
nished trisubstituted silole 19 under the standard reaction conꢀ
dition (eq. 1). In this case, a PhꢀSi bond of diphenylmethylsilyl
group is selectively cleaved (vide infra for selectivity issues
regarding the silicon substituent). Since the regioselectivity of
a silylboration across a terminal alkyne is controllable,55 regiꢀ
oselective synthesis of various trisubstituted siloles would be
possible by sequential silylboration/rhodiumꢀcatalyzed annulaꢀ
tion with unsymmetrical alkynes (see entries 15ꢀ21 in Table 3).
[RhCl(cod)]2 (0.025 mmol), DABCO (1.0 mmol), dioxane (1
mL), H2O (10 ꢁL) at 80 °C for 15 h. b Isolated yield. c 10a (0.2
mmol), 13b (0.4 mmol), [RhCl(cod)]2 (0.01 mmol), DABCO (0.4
mmol), dioxane (0.4 mL), H2O (4 ꢁL). d 13c (0.6 mmol), DMF (1
mL). e 13e (0.6 mmol), NEt3 (1.0 mmol) in place of DABCO.
f
[RhCl(cod)]2 (0.05 mmol) at 100 °C for 72 h. g 13m (2.0 mmol)
and [RhCl(cod)]2 (0.05 mmol). h The values in parenthesis refer to
the ratio of regioisomers determined by NMR. i 13 (2.0 mmol),
[RhCl(cod)]2 (0.05 mmol), Na2CO3 (1.0 mmol) in place of
DABCO. At 100 °C. k At 130 °C.
j
9
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In the course of the investigation to determine the scope of
the arylsilane component, we encountered difficulties in puriꢀ
fying some of the starting arylboronic acids. We therefore
pursued the possibility of directly using protected boronic acid
substrates, which can be purified by chromatography (Table 4).
Neopentyl glycolate 10b and catecholate 10c could be emꢀ
ployed as surrogates for the boronic acid 10a under the same
reaction condition (entries 1ꢀ3). However, when bulkier pinaꢀ
colate ester 10d was used, the reaction was sluggish, and 10d
remained after 15 h. Increasing the amount of water effectiveꢀ
ly promoted the conversion of 10d, and the yield of 14a was
raised to 86% (entry 4). The use of boronate esters offers sigꢀ
nificant synthetic advantages over the use of boronic acids,
because boronate esters are readily available by transition metꢀ
alꢀcatalyzed borylation, especially through CꢀX52 or CꢀH53
bond activation.
Table 5. Scope of oꢀTrimethylsilylarylboronic Estersa
Table 4. Effect of Substituents on the Boron Moietya
Ph
Ph
B(OR)2
[RhCl(cod)]2
DABCO
dioxane/H2O (100/1)
80 °C, 15 h
5 mol%
Ph
Me
+
a
Si
Me
14a
Reaction conditions: 10 (0.5 mmol), 13a (1.0 mmol),
SiMe3
10a-d
Ph
[RhCl(cod)]2 (0.025 mmol), DABCO (1.0 mmol), dioxane (1
mL), H2O (10 ꢁL) at 80 °C for 15 h. b Dioxane (1 mL), H2O (67
ꢁL) (dioxane/H2O = 15/1).
13a
entry
1
yield (%)b
B(OR)2
B(OH)2
A plausible catalytic cycle of the reaction of boronic acid
10a and alkyne 13 is depicted in Scheme 8. Rhodium hydroxꢀ
ide I is initially generated from rhodium(I) chloride with the
aid of water and base and serves as a catalytically active speꢀ
cies. Rhodium complex I is subsequently converted into arylꢀ
rhodium II by transmetallation with boronic acid 10a, folꢀ
lowed by insertion of alkyne 13, leading to alkenylrhodium III.
The cyclization of III then occurs with cleavage of an MeꢀSi
bond in a trimethylsilyl group to form benzosilole 14, along
with methylrhodium IV. The hydrolysis of methylrhodium IV
regenerates rhodium hydroxide I with evolution of methane.56
The methane generation during the catalytic cycle was conꢀ
firmed by 1H NMR spectroscopy. The resonance indicative of
methane (δ = 0.20 ppm) was observed when the reaction was
(10a)
(10b)
95
96
O
B
O
O
B
O
O
2
3
95
(10c)
(10d)
4
B
56 (86)c
O
a
Reaction conditions: 10 (0.5 mmol), 13a (1.0 mmol),
[RhCl(cod)]2 (0.025 mmol), DABCO (1.0 mmol), dioxane (1
b
mL), H2O (10 ꢁL) at 80 °C for 15 h. Isolated yield. c Dioxane (1
mL), H2O (67 ꢁL) (dioxane/H2O = 15/1).
1
run in a sealed tube and monitored by H NMR spectroscopy
(see SI for the detail). When unsymmetrical alkynes are used,
the regioselectivity is determined in the step of the insertion of
alkynes (II→III). The regioselectivities observed with unꢀ
symmetrical alkynes in this study (entries 15ꢀ21 in Table 3)
suggest that the formation of alkenylrhodium III, which bears
a bulkier or more electronꢀwithdrawing substituent at the αꢀ
position, i.e., R2 in Scheme 8, is favored. The trend in regioꢀ
chemistry is in accord with that observed in reported catalytic
Having established that boronate esters are good substrates,
we next synthesized several substituted boronate esters and
conducted rhodiumꢀcatalyzed coupling with 13a (Table 5).
The πꢀextended silole 15 was assembled using a naphthaleneꢀ
based substrate. Introduction of electronꢀdonating and elecꢀ
tronꢀwithdrawing substituents into the benzosilole framework
was also achieved, as in 16 and 17. Moreover, our synthetic
method for benzosiloles was successfully applied to the conꢀ
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reactions via carborhodation of arylrhodium II across alꢀ
kynes.46
1
2
3
4
5
6
Scheme 8. Plausible Catalytic Cycle
Table 6. Scope of Silicon Substituentsa
B(OH)2
7
8
9
[Rh] OH
CH4
SiMe3 10a
I
B(OH)3
[Rh]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H2O
[Rh] Me
IV
SiMe3
II
R1
Si
R1
R2
R1
R2
R2
Me
[Rh]
Me
Si
Me
14
13
Me
Me
III
It is important to note that alkenylrhodium intermediate III
can follow several alternative pathways on the basis of the
reported reactivity of organorhodium(I) species (Scheme 9). In
addition to the MeꢀSi bond cleavage process that leads to the
formation of benzosilole 14 (path 1), alkenylrhodium III
could undergo 1,4ꢀmigration of a rhodium center to form V,57
which should eventually be protonated to form arylsilane VI
(path 2). However, we did not observe this type of byproduct
in all cases investigated in this study. In addition, there are
several reports where other pathways predominate over 1,4ꢀ
rhodium migration via CꢀH activation.17,47b On the basis of
these observations and literature, we consider that the rate of
path 2 is relatively slow compared with that of path 1 under
our rhodiumꢀcatalyzed conditions. Another alternative pathꢀ
way that intermediate III could follow is the oxidative addiꢀ
tion of the ArꢀSi bond, which affords rhodium(III) intermediꢀ
ate VII (path 3). Subsequent C(alkenyl)ꢀSi bondꢀforming reꢀ
ductive elimination from VII would result in the formation of
arylrhodium VIII. This unique silicon/rhodium transposition
process was reported by Cramer.17 The resultant arylrhodium
VIII can follow two possible pathways. One is protonation,
which affords alkenylsilane IX (path 3ꢀ1). However, we can
exclude this path since no IX was observed experimentally.
Another pathway involves the MeꢀSi bond cleavage, which
should afford identical products as path 1 (path 3ꢀ2). Based on
the Cramer's work17 as well as our own observation that a silyl
group at the vinylic position can participate the silole forꢀ
mation (eq 1), this represents another possible pathway to
silole 14.
a
Reaction conditions: 10 (0.5 mmol), 13a (1.0 mmol),
[RhCl(cod)]2 (0.025 mmol), DABCO (1.0 mmol), dioxane (1
mL), H2O (67 ꢁL), at 80 °C for 15 h. b Isolated yield. c Ratio of
(Please refer page 12 for Scheme 9. This is a twoꢀcolumn artꢀ
work.)
e
the products (21/14a) determined by GC. d NMR yield. Ratio of
the products (25/14a) determined by NMR.
Our proposed catalytic cycle consists of relatively wellꢀ
established elementary processes involving rhodium(I), except
for the CꢀSi cleavage step (III→14). To better understand this
elusive but intriguing process, we next investigated the scope
of the silicon substituents in boronate ester 10. Boronate esters
possessing sterically and electronically different silyl groups
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Journal of the American Chemical Society
were prepared and exposed to rhodiumꢀcatalyzed coupling
with 13a under the optimized conditions (Table 6). A more
1
2
3
4
5
6
7
8
hindered triethylsilylꢀsubstituted substrate 10h successfully
afforded the corresponding benzosilole 20 via cleavage of an
EtꢀSi bond, although the yield was slightly decreased comꢀ
pared to that using a trimethylsilyl substrate 10b (entry 1). The
sensitivity of CꢀSi bond substitution to steric effects led us to
examine a series of dimethylalkylsilyl (SiMe2R) groups. As we
expected, selective cleavage of methyl over bulkier alkyl
groups was observed. For example, a substrate bearing an
To shed further light on the mechanism of CꢀSi bond
cleavage, we investigated the electronic effect of the Ar group
of SiMe2Ar on the selectivity for MeꢀSi and ArꢀSi cleavage.
Boronate esters containing a series of paraꢀsubstituted arenes
on the silicon atom 10oꢀ10u were prepared, and reacted with
13a under the standard conditions (Table 7). All the substituꢀ
ents examined were tolerated, and the corresponding benꢀ
zosiloles derived from MeꢀSi bond cleavage (A) and ArꢀSi
bond cleavage (B) were obtained in each case. The ratio of the
products (A/B in Table 7) varied depending on the electronic
nature of the arene moiety on silicon atom.61 Based on these
results, the selectivity for ArꢀSi bond cleavage [B/(A/2+B)],
which is an index for the preference of ArꢀSi cleavage relative
to MeꢀSi cleavage was calculated. The Hammett plot of the
selectivity [B/(A/2+B)] against σp value afforded two interꢀ
9
n
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SiMe2 Bu group, as in 10i, smoothly underwent annulative
coupling to form siloles 21 and 14a in a ratio of 93/7 (entry 2).
Exclusive cleavage of Me is observed when the bulkier iꢀPr, tꢀ
Bu and benzyl groups are employed (entries 3ꢀ5). A phenyl
group on the silicon atom was next examined in order to deꢀ
termine the relative reactivities between C(sp3)ꢀSi and C(sp2)ꢀ
Si bond. The reaction of a substrate possessing a dimeꢀ
thylphenylsilyl group (SiMe2Ph) resulted in a mixture of 25
and 14a in a ratio of 59/41, suggesting that cleavage of both
MeꢀSi and PhꢀSi bonds occurs competitively under the catalytꢀ
ic conditions (entry 6).58 Phenyl cleavage occurred exclusively
over methyl cleavage when a diphenylmethylsilyl (SiMePh2)
group was introduced into the substrate (entry 7).
The substitution aptitude observed with SiMe2Bn and
SiMe2Ph groups (entries 5 and 6 in Table 6) contrasts with that
observed with the CꢀSi substitution reactions using organolithꢀ
ium reagents. It was reported that the cleavage of a CꢀSi bond
by an organolithium species occurred via pentacoordinated
organosilicate 26 (eq 2).20ꢀ21 In this system, when a substrate
bearing SiMe2Bn or SiMe2Ph is used, a benzyl or phenyl
group is predominantly eliminated over a methyl group, preꢀ
sumably because the thermodynamic stabilities of concurrently
produced organolithium species (Li+R−) determine the selecꢀ
tivity of the collapse of organosilicate intermediate 26.
secting lines as shown in Figure 1. A positive
was obtained with electronꢀdonating groups, whereas electronꢀ
deficient aryl groups resulted in a negative
value (–0.28).62
ρ value (+0.12)
ρ
Table 7. Electronic Effect of Aryl Group on Silicon Subꢀ
stituentsa
In stark contrast, a kinetic factor primarily determines the
selectivity in CꢀSi cleavage in our rhodiumꢀcatalyzed annulaꢀ
tion. In the case of SiMe2Bn, a methyl group, which is small,
is exclusively cleaved, and the benzyl group remains intact
(entry 5 in Table 6). In the case of SiMe2Ph, kineticallyꢀ
favored Me cleavage is competitive with a thermodynamicalꢀ
lyꢀfavored Ph cleavage, and a mixture of the two benzosiloles
is produced (entry 6 in Table 6).59 These prominent differꢀ
ences among the reactivities of the silicon substituents exclude
a mechanistic scenario in which a discrete pentacoordinated
silicate species is involved as an intermediate in the rhodiumꢀ
catalyzed benzosilole formation. Although the exact mechaꢀ
nism for the rhodiumꢀcatalyzed CꢀSi bond cleavage remains
elusive at present, a concerted pathway such as σꢀbond meꢀ
tathesis through a fourꢀcentered transition state 27 may acꢀ
count for the observed selectivity for the CꢀSi cleavage in the
rhodiumꢀcatalyzed cyclocoupling, in which a less hindered Cꢀ
Si bond is expected to be cleaved preferentially (eq 3).60
a
Reaction conditions: 10 (0.5 mmol), 13a (1.0 mmol),
[RhCl(cod)]2 (0.025 mmol), DABCO (1.0 mmol), dioxane (1
c
mL), H2O (67 ꢁL), at 80 °C for 15 h. b NMR yield. The ratio of
the products was determined by NMR after chromatography.
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sioned that the synthesis of 1,5ꢀdihydroꢀ1,5ꢀdisilaꢀsꢀindacene
1
skeleton, in which two siloles are fused with a benzene core,
would be possible by a twoꢀfold annulation, hopefully incorꢀ
porating two different alkynes. The analogous ladderꢀtype
ringꢀfused siloles are among the most attractive targets as orꢀ
ganic electronic materials because of their widely πꢀ
conjugated coplanar structures.66 Establishing a general conꢀ
vergent protocol for the synthesis of these siloleꢀfused polyꢀ
aromatics would therefore lead to numerous application. We
chose disilylꢀsubstituted phenylboronate ester bearing a bromo
group, i.e., 39, as an appropriate platform for sequential silole
formation. The boronate 39 was readily prepared from comꢀ
mercially available 1,4ꢀdibromobenzene (38) in three steps
(See SI for details). The boronate ester 39 was then assembled
with 4ꢀoctyne (13k) under the optimal conditions for the rhoꢀ
diumꢀcatalyzed annulation, affording benzosilole 40 in excelꢀ
lent yield, without loss of the bromo functionality. The bromo
moiety served as a handle for the introduction of a boryl group
via the palladiumꢀcatalyzed Miyaura borylation,52 furnishing
the boronate 41. The second rhodiumꢀcatalyzed annulation
with diphenylacetylene (13a) delivered the expected 1,5ꢀ
dihydroꢀ1,5ꢀdisilaꢀsꢀindacene derivative 42 with excellent
efficiency. The mild nature of our protocol allowed for the
synthesis of densely functionalized intermediate 40, thereby
enabling the incorporation of two different alkynes into the
silole structure in a sequential manner.
2
3
4
5
6
7
8
9
Figure 1. Hammett Plot for Selectivity of ArꢀSi Bond Cleavꢀ
age
0.1
Me
OMe
OBu
0.05
y = -0.2782x + 0.0035
R2 = 0.9839
NMe2
0
-0.05
-0.1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H
F
y = 0.118x + 0.0892
R2 = 0.9819
Cl
-0.15
-0.2
CF3
-0.9
-0.6
-0.3
0
0.3
0.6
P-value
A convex line in the Hammett plot usually suggests a
change in the rateꢀdetermining step with electronic variation of
the substituents.63 On the basis of the Hammett studies, we
propose that the rhodiumꢀcatalyzed ArꢀSi bond cleavage is not
a concerted mechanism, but an asynchronous electrophilic
aromatic substitution mechanism (eq 4).64 Thus, the intramoꢀ
lecular electrophilic attack of the alkenylrhodium 35 on the
aryl group on the silicon center initially generates an areniumꢀ
like intermediate 3665 (step a). Migration of the alkenyl moiety
on the rhodium to a silicon atom and the simultaneous cleavꢀ
age of an areniumꢀsilicon bond produce benzosilole 14a and
arylrhodium 37 (step b). The Hammett behavior is rationalized
by assuming a change in the turnoverꢀlimiting step as a result
of varying the electronics of the leaving aryl group. In the negꢀ
ative slope region (red dashed line), electrophilic attack of the
rhodium (step a) is rateꢀdetermining, because a more electronꢀ
donating group accelerates the formation of an ArꢀSi cleaved
product. In contrast, the cyclization stage (step b) becomes
rateꢀdetermining in the positive slope region (blue dashed line),
because the presence of highly electronꢀrich arene renders step
a significantly facile and reversible. However, the electronꢀ
donating group decelerate step b in turn by stabilizing the parꢀ
Scheme 10. Synthesis of Asymmetrically Substituted 1,5ꢀ
Dihydroꢀ1,5ꢀdisilaꢀsꢀindacene Derivative by Sequential
Introduction of Two Different Alkynes
tially positive intermediate 36, thereby exhibiting a positive
ρ
value. In this mechanistic scenario, the rate of ArꢀSi cleavage
relative to MeꢀSi cleavage decreases when the absolute value
of σp of the substituent is high, regardless of its sign.
As demonstrated in Table 6, when a SiMe2R group is used
in our rhodiumꢀcatalyzed annulation, selective cleavage of an
MeꢀSi bond occurs to form benzosiloles bearing a silicon steꢀ
reogenic center. If enantiotopic methyl groups can be discrimꢀ
inated by adding a suitable chiral ligand, the catalytic asymꢀ
metric construction of benzosiloles is possible. The available
methods for constructing a siliconꢀstereocenter are consideraꢀ
bly limited compared with those for carbonꢀbased chiral cenꢀ
ters. Although several classical methods for preparing Siꢀ
chiral compounds using stoichiometric amounts of chiral
sources are known,67 the catalytic asymmetric methods for
such compounds remains underdeveloped.68 With regard to
catalytic asymmetric synthesis of chiral siloles, there was no
Having successfully examined preparative and mechanistic
aspects of the rhodiumꢀcatalyzed benzosilole synthesis, we
next directed our attention to the synthesis of more intricate
silole derivatives that are otherwise inaccessible. We enviꢀ
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can transform inert CꢀSi bonds are underway in our laboratoꢀ
example of the synthesis of siloles bearing a silicon stereocenꢀ
ry.73
ter, at the time when we initiated our study.69 Recently, Shinꢀ
tani and Hayashi reported a notable catalytic asymmetric synꢀ
thesis of siliconꢀstereogenic dibenzosiloles via palladiumꢀ
catalyzed enantioselective intramolecular CꢀH bond arylaꢀ
tion.70
1
2
3
4
5
6
7
8
ASSOCIATED CONTENT
Supporting Information. Detailed experimental procedures and
characterization of new compounds are available free of charge
via the Internet at http://pubs.acs.org.
Our approach focused on the investigation of chiral ligꢀ
ands that can induce enantioselective MeꢀSi bond cleavage in
the reaction of 10j with 13a. The highly sterically congested
nature of substrate 10j renders the application of typical chiral
ligands difficult. After much work on ligand screening and
optimization of the reaction conditions (See SI for details of
the optimization studies), we identified Imamoto's71 QuinoxP*
as an effective ligand, producing benzosilole 22* bearing a
chiral silicon center with almost perfect enantioselectivity
(98% ee) (eq 5). Nevertheless, the efficiency of the reaction
was significantly lowered as a result of the inherent steric
bulkiness imposed by this chiral phosphine ligand, thereby
limiting the application of other alkynes to this catalytic
asymmetric variant.72 Although this means there is still much
room for improvement, a benzosilole containing stereogenic
silicon center was synthesized for the first time by harnessing
our own rhodiumꢀcatalyzed carbonꢀsilicon bond activation
strategy.
9
AUTHOR INFORMATION
Corresponding Author
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60
*Eꢀmail: tobisu@chem.eng.osakaꢀu.ac.jp;
chatani@chem.eng.osakaꢀu.ac.jp
Present Addresses
§ Department of Chemistry, Graduate School of Engineering Sciꢀ
ence, Osaka University, Toyonaka, Osaka, 560ꢀ8531, Japan
ACKNOWLEDGMENT
This work was supported by a GrantꢀinꢀAid for Scientific Reꢀ
search on Innovative Areas “Molecular Activation Directed toꢀ
ward Straightforward Synthesis” from The Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan. MT
thanks the Kansai Research Foundation for Technology Promoꢀ
tion, the Izumi Science and Technology Foundation, and the
Shorai Foundation for Science and Technology for their financial
support. We also thank Professors Satoh and Miura (Osaka Uniꢀ
versity) for their assistance in obtaining solidꢀstate photoluminesꢀ
cence spectra. Professors Hayashi (Kyoto University) and Imamoꢀ
to (Chiba University) are acknowledged for their generous donaꢀ
tion of chiral ligands. MO expresses his special thanks for JSPS
Research Fellowship for Young Scientists. We also thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka
University, for assistance with MS and HRMS. We deeply thank
one of our reviewers for suggesting the mechanistic options.
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Conclusions
We have developed a rhodiumꢀcatalyzed coupling of 2ꢀ
silylphenylboronic acids with alkynes for the synthesis of benꢀ
zosilole derivatives. From the preparative viewpoint, our proꢀ
tocol has two notable advantages over classical methods. One
is high functional compatibility. The mildness of the catalytic
conditions employed keeps reactive functional groups, such as
ketones, esters and halides (Cl, and Br), intact during the silole
formation process. The other aspect is the convergent nature
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accessible simply by changing the structure of the alkyne couꢀ
pling partner. These advantages also enable the synthesis of
asymmetrically substituted 1,5ꢀdihydroꢀ1,5ꢀdisilaꢀsꢀindacene.
From the mechanistic viewpoint, the present reaction involves
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(65) The σp rather than σ+ showed better correlation in our Hammett
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1974, 15, 331. (c) Corriu, R. J. P.; Moreau, J. J. E. J. Organomet.
Chem. 1974, 64, C51. (d) Corriu, R. J. P.; Moreau, J. J. E. J.
Organomet. Chem. 1975, 85, 19. (e) Corriu, R. J. P.; Moreau, J. J. E.
J. Organomet. Chem. 1976, 120, 337. (f) Ohta, T.; Ito, M.; Tsuneto,
A.; Takaya, H. J. Chem. Soc., Chem. Commun. 1994, 2525. (g)
Schmidt, D. R.; O'Malle, S. J.; Leighton, J. L. J. Am. Chem. Soc.
2003, 125, 1190. (h) Yasutomi, Y.; Suematsu, H.; Katsuki, T. J. Am.
Chem. Soc. 2010, 132, 4510. (i) Shintani, R.; Moriya, K.; Hayashi, T.
J. Am. Chem. Soc. 2011, 133, 16440. (j) Shintani, R.; Moriya, K.;
Hayashi, T. Org. Lett. 2012, 14, 2902. (k) Kurihara, Y.; Nishizawa,
M.; Yamanoi, Y.; Nishihara, H. Chem. Commun. 2012 in press (doi:
10.1039/c2cc36238d)
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(69) A few axially chiral silole derivatives are known: (a) Yasuike,
S.; Iida, T.; Okajima, S.; Yamaguchi, K.; Seki, H.; Kurita, J.
Tetrahedron 2001, 57, 10047. (b) Yamauchi, K.; Tamura, N.; Seiki,
T.; Kuninobu, Y.; Takai, K. Chemical Society Japan Annual Meeting,
2012, 1C8ꢀ45. Related axially chiral silacycle: (c) Tamao, K.;
Nakamura, K.; Ishii, H.; Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc.
1996, 118, 12469. Helically chiral siloles (silahelicenes) have also
been reported, see ref 41b.
(70) Shintani, R.; Otomo, H.; Ota, K.; Hayashi, T. J. Am. Chem. Soc.
2012, 134, 7305.
(71) Imamoto, T.; Sugita, K.; Yoshida, K. J. Am. Chem. Soc. 2005,
127, 11934.
(72) 4ꢀOctyne and 1ꢀphenylꢀ1ꢀhexyne afforded a complicated mixture
when exposed to the conditions shown in eq 5.
(73) During the reviewing process of our manuscript, catalytic ArꢀSi
bond activation through intramolecular substitution by oxygenꢀbased
nucleophile has been reported: (a) Shintani, R.; Maciver, E. E.;
Tamakuni, F.; Hayashi, T. J. Am. Chem. Soc. 2012, 134, 16955
(asymmetric). (b) Fujihara, T.; Tani, Y.; Semba, K.; Terao, J.; Tsuji,
Y. Angew. Chem., Int. Ed. 2012, in press. (doi:
10.1002/anie.201207148)
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