Highly efficient activation of organosilanes with η2-aldehyde nickel complexes: Key for catalytic syntheses of aryl-, vinyl-, and alkynyl-benzoxasiloles

Article abstract of DOI:10.1021/ja510089c

An η²-aldehyde nickel complex was utilized as an effective activator for an organosilane in order to generate a hypervalent silicate reactant for the first time. This method was successfully applied to the highly efficient syntheses of 3-aryl-, vinyl-, and alkynyl-2,1-benzoxasiloles from benzaldehydes with aryl-, vinyl-, and alkynylsilyl groups at the ortho position. Initial mechanistic studies revealed that an intermolecular aryl transfer process was involved in the reaction mechanism. The formation of an η²-aldehyde complex was directly confirmed by NMR.

Full text of DOI:10.1021/ja510089c

Communication
pubs.acs.org/JACS
Highly Efficient Activation of Organosilanes with η2‑Aldehyde Nickel
Complexes: Key for Catalytic Syntheses of Aryl‑, Vinyl‑, and Alkynyl-
Benzoxasiloles
Yoichi Hoshimoto,†,‡ Hayato Yabuki,† Ravindra Kumar,† Haruka Suzuki,† Masato Ohashi,†
and Sensuke Ogoshi*,†,§
‡
†Department of Applied Chemistry, Faculty of Engineering and Frontier Research Base for Global Young Researchers, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
§Advanced Catalytic Transformation program for Carbon utilization (ACT-C), JST, Suita, Osaka 565-0871, Japan
S
* Supporting Information
We have reported the electrophilic addition of Me3SiOTf to
ABSTRACT: An η2-aldehyde nickel complex was utilized
as an effective activator for an organosilane in order to
generate a hypervalent silicate reactant for the first time.
This method was successfully applied to the highly
efficient syntheses of 3-aryl-, vinyl-, and alkynyl-2,1-
benzoxasiloles from benzaldehydes with aryl-, vinyl-, and
alkynylsilyl groups at the ortho position. Initial mechanistic
studies revealed that an intermolecular aryl transfer process
was involved in the reaction mechanism. The formation of
an η2-aldehyde complex was directly confirmed by NMR.
(η2-PhCHO)Ni(PCy3)2 giving (η1:η1-Me3SiOCH(Ph))-Ni-
(PCy3)OTf.3s,t This result would show that η2-aldehyde nickel
complexes are sufficiently reactive toward organosilicon
compounds. Then, we envisioned the generation of a hyper-
valent silicate that could be triggered by the activation of an
aldehyde via back donation from an electron-rich nickel(0)
complex and its application toward organic synthesis (Scheme
2).6 Herein, we report the syntheses of 3-aryl-, vinyl-, and
Scheme 2. Generation of Hypervalent Silicate Triggered by
Activation of Aldehyde via Back Donation from Nickel(0)
ldehydes are versatile carbonyl compounds and are
A
frequently employed as a key component in a wide range
of chemistry fields. When they are used in organic synthesis, an
activation of the carbonyl group is often required.1a A number of
methods have been developed with typical and transition metals,
in which aldehydes are activated through a coordination to the
metals. An η1 coordination of the carbonyl oxygen to Lewis acidic
metals is well-known (Scheme 1a).1 An enhancement of the
alkynyl-2,1-benzoxasiloles from benzaldehydes with aryl-, vinyl-,
and alkynylsilyl moieties at the ortho position, which would
proceed via a hypervalent silicate intermediate generated by the
electrophilic addition of the organosilyl moiety to the η2-
aldehyde nickel complex. These reactions proceeded with 100%
atom efficiency under very mild reaction conditions (∼60 °C)
without the addition of external bases to activate the organo-
silanes, which were essential in most of the reported transition-
metal-catalyzed addition reactions of organosilanes.7
Scheme 1. Activation of Aldehyde via (a) η1 or (b) η2
Coordination
The reaction of o-dimethylphenylsilylbenzaldehyde (1a) with
10 mol % Ni(cod)2 and PCy3 was examined in toluene at 60 °C,
giving 3-phenyl-2,1-benzoxasilole (2a) in 47% yield after 18 h
through the Ph group migration from the silicon to the carbonyl
carbon (Table S1).8 This result might support the generation of a
hypervalent silicate intermediate, as we envisioned (vide supra).
Even at room temperature, the reaction took place more
efficiently in the presence of 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene (IPr). With 1 mol % Ni(cod)2 and IPr, the
reaction was completed within 0.5 h to furnish 2a in 99% yield
(Table 1). These reaction conditions are very similar to that of
the Ni0-catalyzed Tishchenko reaction;5f,g however, no for-
mation of an ester was observed. When neither, nor both,
electrophilicity at the carbonyl carbon is expected via this η1
coordination, which allows the aldehyde to react with a variety of
nucleophiles. On the other hand, application of the η2
coordination of aldehydes has been rather limited in transition-
metal-catalyzed organic syntheses,2,3t even though many
examples of the η2-aldehyde complexes have been reported in
the coordination chemistry field.3,4 The reactivity of the η2-
aldehyde complex is very intriguing since both the carbonyl
oxygen and the carbon show nucleophilic reactivity with the
contribution of an oxametallacyclopropane resonance form
(Scheme 1b).3g,s,t Nevertheless, this type of distinctive reactivity
of the η2-aldehyde complexes has rarely been utilized in the
reported catalytic transformation of aldehydes.5
Received: September 30, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja510089c | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
toluene even at −78 °C.11 With 4 mol % TNI, the target
benzoxasilole 2f was obtained in 99% yield at −30 °C. Substrates
with a 4-fluorobenzaldehyde structure (1g) gave 2g in 94% yield.
A variety of aryl groups on the silicon, such as tolyl (1h and 1i), 4-
methoxyphenyl (1j), pentafluorophenyl (1k), and 1-naphthyl
(1l), were successfully applied to the present system.
Prolongation of the reaction time was required for 1j and 1l,
showing that both electronic and steric environment around the
silicon influenced the reaction rate.
Table 1. Highly Efficient Synthesis of Arylbenzoxasiloles with
a
Ni0 Complex
Next, we examined the synthesis of 3-vinyl-2,1-benzoxasilole
(2m) from o-dimethylvinylsilylbenzaldehyde (1m) via vinyl
group migration.13 However, under the same reaction conditions
given in Table 1, the formation of 2m was completely hampered
by the formation of (η2:η2-CH2CHSi(Me)2C6H4CHO)Ni-
(IPr).8 As a result of further optimizations (Table S2),8 2m was
obtained in 90% yield by employing 2 mol % Ni(cod)2 and 4 mol
% IPr in THF at 60 °C (Table 2). Under these reaction
Table 2. Highly Efficient Synthesis of Vinyl- and
a
Alkynylbenzoxasiloles with Ni0 Complex
a
General conditions: 1 and Ni(cod)2/IPr (0.02 mmol) were reacted in
toluene (2 mL) at room temperature. Yields of isolated products are
b
c
given. TNI (4 mol %) was used at −30 °C. Ni(cod)2/IPr (4 mol %)
was used.
a
General conditions: 1, Ni(cod)2 (0.02 mmol), and IPr (0.04 mmol)
were reacted in THF (1−2 mL) at 60 °C. Yields of isolated products
b
c
Ni(cod)2 and/or IPr were used, 2a was not obtained even at
more than 80 °C and more than 12 h of reaction time.8 The
reaction of 1a with CsF, nBu4NF, or [(Me2N)3S][F2SiMe3] as an
activator of the silicon was examined at 80 °C, and 2a was not
obtained (Table S1).8 These results showed that the (η2-
aldehyde)Ni(IPr) complex would be a highly efficient reagent in
the generation of a hypervalent silicate under this reaction
condition.
are given. Ni(cod)2 (1 mol %) and IPr (2 mol %) were used. 1n
(20.0 mmol) was reacted with Ni(cod)2 (1 mol %) and IPr (2 mol %)
in THF (20 mL) for 24 h.
conditions, the substituted 3-vinyl-2,1-benzoxasiloles 2n and 2o
were furnished in 89 and 91% yields, respectively. The procedure
was expanded to the gram-scale synthesis of 2n (17 mmol, 3.71
g). In addition, 3-alkynyl-2,1-benzoxasilole 2p was also given in
95% yield under the same reaction conditions. In these reactions,
the Ni0-catalyzed intramolecular hydroacylation of alkenes5e or
alkynes5i,14 was not observed.
Since there are chemically labile O−Si and C−Si bonds,
benzoxasiloles are very attractive compounds. Therefore,
extensive studies have recently been conducted for their
application in organic synthesis, and these reports have clearly
shown a significant degree of utility of benzoxasiloles.9,12,15
However, preparation methods for 3-aryl-2,1-benzoxasiloles are
limited. Thus, the present method would have utility as well as
the advantages of simple experimental procedures, high atom
efficiency, and environmentally beneficial conditions (no waste
and mild temperature). Moreover, to the best of our knowledge,
this is the first report on the synthesis of 3-vinyl- and alkynyl-2,1-
benzoxasiloles.15u,v The following transformations of 2p
undoubtedly show its potential utility in organic synthesis
(Scheme 3). The terminal alkyne-substituted benzoxasilole 2q
was synthesized through the desilylation of the TMS group in 2p
by K2CO3, which might be employed for further functionaliza-
tion of the alkyne terminal. Treatment of 2p with KHCO3 and
The present methodology can be applied to the synthesis of
various 3-aryl-2,1-benzoxasiloles 2a−l (Table 1). Alkyl sub-
stituents on the silicon did not affect the efficiency of the reaction
(2a and 2b), but it was reported that this difference would
influence the stability of benzoxasiloles toward silica gel
chromatography.8,9 o-Diphenylmethylsilylbenzaldehyde (1c)
gave 2c in 99% yield as a mixture of diastereomers (53:47
dr).10 Introducing an electron-donating methoxy group into the
para position with respect to the dimethylphenylsilyl group
retarded the transformations; however, the yield of 2d was also
excellent (3.5 h, 98%). By contrast, the reaction of 1e, which
equips an electron-withdrawing CF3 group, was completed
within 0.25 h to give 2e in 96% yield. In the presence of 10 mol %
Ni(cod)2 and IPr, the reaction of chlorine-substituted 1f did not
take place at room temperature or 100 °C due to the deactivation
of the catalyst by the oxidative addition of the Ar−Cl bond to the
Ni0 center. Although lowering the reaction temperature might
have been effective in preventing this deactivation pathway, this
would have caused a significant decrease in the solubility of
Ni(cod)2 toward toluene. Thus, we employed (η6-toluene)Ni-
(IPr) (TNI) as a Ni0/IPr source since TNI dissolves easily into
B
dx.doi.org/10.1021/ja510089c | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
a
Scheme 3. Synthetic Applications of 2p
Scheme 5. Stoichiometric Reaction of 1d with TNI
a
Reaction was monitored by NMR analyses, and yield of 6 was
determined with TMS as an internal standard.
δC −1.2). In the 29Si NMR spectrum, the resonance of 6 was
observed at −8.8 ppm, which was slightly shifted from the −7.6
ppm of 1d. These results clearly support the η2 coordination of
1d toward Ni0 by CO;3s,t,5g,20 however, the interaction
between Si and O remains unclear at this time.6a,12 After
warming to room temperature, the quantitative conversion of 1d
to 2d was confirmed by 1H NMR.
a
Isolated yields are given.
The mechanism of the present reaction might include the
following steps. First, a coodination of 1 to Ni0 gives rise to the
formation of an η2-aldehyde nickel intermediate such as 6.
Through this first step, the aldehyde is activated as a nucleophile
that continuously activates the silicon via an intramolecular
addition reaction, which affords a hypervalent silicate inter-
mediate. Then, an intermolecular aryl transfer would take place
to give the product. In order to clarify the details of the
mechanisms, further mechanistic studies are ongoing by our
group.
In summary, a synthesis of 3-aryl-, vinyl-, and alkynyl-2,1-
benzoxasiloles has been developed, which proceeds via an
electrophilic addition of the organosilyl group to the η2-aldehyde
nickel complex as a key step. It is noteworthy that this work
represents the first use of the η2-aldehyde nickel complex as a
highly efficient activator of organosilicon compounds. Thus, the
present reaction required no additives to activate the silicon and
harsh reaction conditions.
H2O2 aq. in THF/MeOH resulted in the formation of phenyl
propargyl alcohol 3 in 69% yield, in which the TMS group
remained intact. On the other hand, by employing the
combination of TBAF and H2O2 aq. in DMF, both desilylation
and Tamao−Fleming oxidation took place to afford 2-(1-
hydroxyprop-2-yn-1-yl)phenol (4) in 81% yield. Interestingly, 2-
methylenedihydrobenzofuran-3-ol (5) was directly synthesized
in 78% yield as the sole product from 2p with KHCO3, KF, H2O2
aq. at 70 °C, which is a promising precursor for benzofuranes16
and spiroketals.17 It is noteworthy that 5 was previously prepared
via Pd-,16a,b Cu-,16c Ag-,18 or Au19-catalyzed cycloisomerization
from 2-(1-hydroxyprop-2-ynyl)phenols.
As part of the initial mechanistic studies, a crossover
experiment using 1d and 1j was examined (Scheme 4). With 2
a
Scheme 4. Crossover Experiment with 1d and 1j
ASSOCIATED CONTENT
* Supporting Information
Experimental details and spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
a
Reaction was monitored by GC analysis, and yields of product were
AUTHOR INFORMATION
Corresponding Author
ogoshi@chem.eng.osaka-u.ac.jp
■
determined with n-pentadecane as an internal standard.
mol % Ni(cod)2 and IPr, the reaction gave four benzoxasiloles
including crossover products 2r and 2a in almost the same yields
after 3.5 h (Figure S3).8 The exchange of the aryl groups between
1d and 1j was not observed at all during the reaction. In addition,
no aryl exchange reaction between 2d and 2j was observed when
the isolated 2d and 2j were subjected to the reaction conditions.
These results indicate that the present reaction involves an
intermolecular aryl transfer process.10
In order to gain insight into the reaction intermediates, a
stoichiometric reaction of 1d with TNI was conducted in
toluene-d8 at −50 °C (Scheme 5). The formation of 6 was
confirmed by 1H, 13C, and 29Si NMR analyses (Figures S6−8).8
In the 1H and 13C NMR spectra, the resonances of the carbonyl
hydrogen and carbon were observed at δH 5.90 and δC 86.1,
respectively, both of which were shifted upfield compared with
those of 1d (δH 9.60 and δC 192.4). In addition, the methyl
groups on Si in 6 were observed as two diastereotopic peaks due
to the formation of a chiral carbon center ligated to the nickel (6:
δH 0.52 (s, 3H), 0.46 (s, 3H); δC −0.1, −0.5. 1d: δH 0.59 (s, 6H);
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Scientific Research on Innovative
Area “Molecular Activation Directed toward Straightforward
Synthesis” (23105546) and Grants-in-Aid for Young Scientists
(A) (25708018) from MEXT and by ACT-C from JST. Y.H.
acknowledges support from the Frontier Research Base for
Global Young Researchers, Osaka University, on the Program of
MEXT.
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