Nickel-catalyzed reductive cleavage of aryl alkyl ethers to arenes in absence of external reductant

Article abstract of DOI:10.1039/c5sc00305a

The reductive cleavage of the C-O bonds of aryl ethers has great potential in organic synthesis. Although several catalysts that can promote the reductive cleavage of aryl ethers have been reported, all such systems require the use of an external reductant, e.g., hydrosilane or hydrogen. Here, we report the development of a new nickel-based catalytic system that can cleave the C-O bonds of ethers in the absence of an external reductant. The hydrogen atom required in this new reductive cleavage reaction is provided by the alkoxy group of the substrate, which serves as an internal reductant. The absence of an external reductant enables the unique chemoselectivity, i.e., the selective reduction of an alkoxy group over alkenes and ketones. This journal is

Full text of DOI:10.1039/c5sc00305a

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DOI: 10.1039/C5SC00305A
Journal Name
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ARTICLE
Nickel-Catalyzed Reductive Cleavage of Aryl
Alkyl Ethers to Arenes in Absence of External
Reductant
Cite this: DOI: 10.1039/x0xx00000x
Mamoru Tobisu,*,a,b,c Toshifumi Morioka,a Akimichi Ohtsukib and Naoto
Chatani*,a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
The reductive cleavage of the C-O bonds of aryl ethers has numerous potential in organic
synthesis. Although several catalysts that can promote the reductive cleavage of aryl ethers have
been reported, all of such systems require the use of an external reductant, e.g., hydrosilane or
hydrogen. Here, we report the development of a new nickel-based catalytic system that can
cleave the C-O bonds of ethers in the absence of an external reductant. The hydrogen atom
required in this new reductive cleavage reaction is provided by the alkoxy group of the substrate,
which serves as an internal reductant. The absence of an external reductant enables the unique
chemoselectivity, i.e., the selective reduction of an alkoxyl group over alkenes and ketones.
www.rsc.org/
development of a nickelꢀcatalyzed reductive cleavage reaction
for the conversion of aryl alkyl ethers to the parent arenes in the
absence of an external reductant.
Introduction
The reductive deoxygenation of phenols and their derivatives is
a fundamental synthetic transformation that has been widely
used in various areas of organic synthesis; for example,
reductive deoxygenation enables the use of oxygenꢀbased
functionalities as removable activating and directing groups in
arene functionalization reactions.1 Reductive deoxygenation is
Results and discussion
One possible mechanism for the reductive cleavage of aryl
methyl ethers with an external hydride reagent involves
oxidative addition of the C(aryl)ꢀO bond to form nickel(II)ꢀ
also
a potentially useful strategy for the synthesis of
deoxygenated analogs of phenolꢀbased natural products.2 The
reductive deoxygenation of phenols has traditionally been
achieved by converting them to activated sulfonates such as
triflates, followed by catalytic reduction of the activated species
in the presence of a hydride donor.3 Recent progress in nickelꢀ
catalyzed4 CꢀO bond activation reactions5 has enabled the direct
use of unactivated phenol derivatives. One of the most
important achievements in this area has been the development
of a catalytic process for the reduction of inert aryl ethers;6,7
this not only expands the substrate scope of reductive
deoxygenation reactions, but also provides a platform for the
development of new catalytic methods for the conversion of
aryl etherꢀcontaining biomasses to alternative sources of fuel
and chemicals, including the depolymerization of lignin.8
However, it is worth noting that these nickelꢀcatalyzed
reductive cleavage reactions of aryl ethers only proceed in the
presence of a stoichiometric amount of an external reductant,
e.g., hydrosilane6a,6b,6d or hydrogen.6c Here, we report the
methoxide
A, followed by reduction of an arylꢀnickel bond
(Scheme 1a). Although this mechanism seems plausible in view
of the reports of nickelꢀcatalyzed crossꢀcouplings of aryl ethers
with other nucleophiles,5 the key oxidative addition of a
C(aryl)ꢀOMe bond has never been verified experimentally.9
GómezꢀBengoa and Martin proposed a new mechanism for the
nickelꢀcatalyzed reductive cleavage of aryl ethers by
hydrosilane (Scheme 1b).6d Spectroscopic and computational
studies indicate that silylnickel(I) species can be generated
under these conditions and mediate C(aryl)ꢀOMe bond cleavage
via migratory insertion of an arene to form benzylnickel species
B,
followed by concerted
elimination of MeOSiR3.
Polyaromatic compounds such as naphthyl ethers serve as good
substrates, but anisole derivatives do not react unless they
contain a suitable directing group; these results are consistent
with the proposed mechanism involving a dearomatized
intermediate
B. It was envisaged that if oxidative addition of a
C(aryl)ꢀO bond occurs to form intermediate
A, β
ꢀhydrogen
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Table 1 Ni-catalyzed reductive cleavage of 1a: effect of ligandsa
elimination would lead to the formation of the same reductive
cleavage product via (Scheme 1c). This mechanistic variant
therefore enables the reductive cleavage to proceed without an
external reductant. Although the relevance of ꢀhydrogen
C
β
elimination of nickelꢀalkoxide species in catalytic reductive
cleavage of aryl ethers has been suggested before,6d,9b catalytic
systems that can effect such a reaction in the absence of an
external reductant have not been reported.
a Reaction conditions: 1a (0.25 mmol), Ni(cod)2 (0.050 mmol), ligand (0.050
Scheme 1 Mechanistic diversity of reductive cleavage of aryl methyl ethers
mmol), NaOtBu (0.50 mmol) in toluene (0.50 mL) at 160 °C for 18 h.
To achieve the catalytic reaction shown in Scheme 1c, we
evaluated the effect of ligands in the nickelꢀcatalyzed reduction
of 2ꢀmethoxynaphthalene (1a) to give naphthalene (2). As
previously reported, the addition of PCy3 and SIPr, which are
the most effective ligands for the reductive deoxygenation of 1a
with hydrosilane6a,6b,6d and hydrogen,6c respectively, did not
lead to the formation of appreciable amounts of the reduced
The effect of the alkoxy group on the reductive cleavage
reaction was then examined using an I(2ꢀAd) ligand (Table 2).
Primary alkoxy groups, including EtO and the longer C8H17O
product
2 in the absence of an external reductant (Table 1,
n
entries 1 and 2). Systematic screening of several other NHCꢀ
based ligands revealed that those bearing 1ꢀadamantyl [I(1ꢀAd)]
and cyclohexyl (ICy)10 groups on their imidazole nitrogen
groups, were efficiently removed under these condition, as in
i
the reactions of 1b and 1c, but the bulkier PrO group was
completely unreactive. When a substrate bearing a PhCH2O
group (i.e., 1e) was used, C(aryl)ꢀO bond cleavage was
atoms promoted the desired reaction to provide
yields (entries 7 and 8). The highest yield of
2
in modest
2
was obtained
accompanied by cleavage of C(benzyl)ꢀO bonds. A PhO group
remained virtually intact under these conditions, which is
consistent with our proposed mechanism (Scheme 1c). Alkoxy
groups on polyaromatic scaffolds such as naphthalenes 4-11
with an NHC containing 2ꢀadamantyl groups [I(2ꢀAd)] (entry
9). Although it was possible to reduce the amount of NaOtBu
(80% yield with 50 mol% NaOtBu), the use of 2 equivalents
resulted in better yield of
catalyst loading was reduced to 10 mol%, the yield of
2
(See ESI for details). When the
was
and phenanthrene
reductive cleavage under these conditions to provide the parent
arenes. Functional groups such as silyl ( ), and amide ( and 9)
3 successfully underwent nickelꢀcatalyzed
2
decreased to 52% (The catalyst loading could be reduced with
other substrates. See ESI for details). The reductive cleavage
proceeded in the presence of excess mercury; therefore we
suggest that the soluble nickel species is responsible for the
catalysis.
5
8
were compatible. Notably, a pendant alkene moiety, which
could be reduced under the previously reported conditions
using hydrosilane6a,b,d or hydrogen,6c remained intact under the
current conditions, as shown by the reactions of 10 and 11
.
These newly developed conditions were also used with the
labeled substrate 2ꢀCD3Oꢀnaphthalene (1a-d3). The results of
this experiment showed that the hydrogen delivered to the
reduced product was primarily derived from the methoxy group
of the substrate (eq 1); this supports our proposed mechanism
shown in Scheme 1c.
Heteroarenes, namely the electronꢀdeficient quinoline 12 and
electronꢀrich carbazole 13, were good substrates. In addition,
this catalytic reaction could also be applied to benzylic CꢀO
bonds (14ꢀ16). Unfortunately, benzylic CꢀO bond activation is
limited to polyaromatic substrates, and simple benzyl alkyl
ethers were unreactive under the current conditions.
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Table 2 Ni/I(2-Ad)-catalyzed reductive cleavage of naphthyl and
naphthylmethyl ethersa
Scheme 2 Comparison with reported catalytic systems
Table 3 Ni/I(2-Ad)-catalyzed reductive deoxygenation of anisole derivativesa
a Reaction conditions: aryl or benzyl ether (0.25 mmol), Ni(cod)2 (0.050
mmol), I(2ꢀAd)·HCl (0.050 mmol), NaOtBu (0.50 mmol), in toluene (0.50
mL) at 160 °C for 18 h. The yields shown are isolated yields unless otherwise
noted. b Yield was determined by GC analysis because of the volatility of the
product. c NaOtBu (0.13 mmol) was used.
a
Reaction conditions: aryl or benzyl ether (0.25 mmol), Ni(cod)2 (0.050
mmol), I(2ꢀAd)·HCl (0.050 mmol), NaOtBu (0.50 mmol) in toluene (0.50
mL) at 160 °C for 18 h. Yields shown are the isolated yields, unless
Lower reactivities of substrates lacking a fused aromatic
ring is a common problem in nickelꢀcatalyzed transformations
b
otherwise noted. Yield was determined by GC analysis because of the
of inert phenol derivatives.11 Similarly, 4ꢀtertꢀbutylanisole (15
)
volatility of the product. c 2ꢀNaphthol was obtained in 16%.
was found to be much less reactive under the current conditions
(Table 3). However, the introduction of a phenyl group, as in
16, significantly enhanced the reactivity, and the reductive
cleavage product was obtained in 51% yield. Note that
compound 16 is completely unreactive under previously
reported nickelꢀcatalyzed reductive cleavage conditions using
hydrosilane.6a,b,d Replacement of the methoxy group with an
ethoxy group further improved the efficiency of this catalysis.
Ethoxybenzenes bearing a phenyl group at the para and meta
For comparison, the reductive cleavage of 14 was also
examined in the presence of an external reductant (Scheme 2).
The reported method using hydrosilane6b was far less effective
(4% yield). The protocol using dihydrogen6c was also less
effective in the presence or absence of AlMe3. These results
highlight the superior catalytic activity of the Ni/I(2ꢀAd) system
for reductive cleavage reactions.
positions, i.e.
, 17 and 18, respectively, therefore underwent the
reaction to give good yields. In the case of methoxy arene
substrates,
intermediate
β
A
ꢀhydrogen elimination of the postulated
generates formaldehyde complex (see
C
Scheme 1c), which readily decomposes to give an inactive
Ni(CO) species via decarbonylation.6d Although some Ni(CO)
species can generate an active Ni(0) species by the dissociation
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of a CO ligand under the current conditions, Ni(CO) formation In summary, we have developed a novel procedure for the
would be expected to decrease the efficiency of catalyst nickelꢀcatalyzed reductive deoxygenation of aryl ethers. This
turnover. The higher reactivities of ethoxyarenes compared new reaction proceeds in the absence of an external
with methoxyarenes can be attributed to the lower tendency of stoichiometric reductant via a mechanism involving oxidative
the acetaldehyde complex, which is generated from the nickelꢀ addition of the CꢀO bond followed by
β
ꢀhydrogen elimination,
ethoxide intermediate, to undergo the
undesired which has not previously been exploited in a catalytic setting.
decarbonylation.12 As well as a phenyl group, several electronꢀ The present catalytic system not only provides an atom
withdrawing groups can accelerate the present reductive economical process by avoiding the use of an external
cleavage reaction. Particularly important examples include reductant, but also provides a significantly wider substrate
anisoles bearing a ketone functionality, e.g., 19 and 20, because scope. These results also show that the elusive oxidative
previous methods using an external reductant reduce the addition of a C(aryl)ꢀOMe bond is a reliable elementary step in
pendant ketone moiety. The current nickel catalyst system also nickelꢀcatalyzed CꢀO bond transformations of aryl ethers,
enables the selective conversion of aryl ethers bearing amide particularly when an NHCꢀbased ligand is used. Further studies
21, alkene 22, and pyridine 23 groups to the corresponding of nickelꢀcatalyzed transformations of aryl ethers using a
arenes. Ortho substituted anisoles, such as 2ꢀethoxyꢀ1,1'ꢀ Ni/NHC system are currently underway in our laboratory.
biphenyl, failed to react under the current conditions.
The key to a successful reaction is the use of a new NHC
ligand I(2ꢀAd). The electronic and steric properties of I(2ꢀAd)
were investigated to gain insights that will help in the future
design of better ligands for catalytic CꢀO bond activation
processes (Table 4). In terms of the electronic properties, the
Tolman electronic parameter (TEP)13 for the new I(2ꢀAd)
ligand was determined to be 2049, by synthesizing [IrCl(I(2ꢀ
Ad))(CO)2]. This value is similar to the TEP values obtained
Acknowledgments
This work was supported by a GrantꢀinꢀAid for Scientific
Research on Innovative Areas "Molecular Activation Directed
toward Straightforward Synthesis" from MEXT, Japan. M.T.
was also supported by the “Elements Strategy Initiative to Form
Core Research Center” from MEXT. T.M. thanks the Program
for Leading Graduate Schools: “Interactive Materials Science
Cadet Program.” for their support. We also thank the
Instrumental Analysis Center, Faculty of Engineering, Osaka
University, for assistance with HRMS. Professor Makoto
Yasuda and Professor Yoshihiro Nishimoto are thanked for
obtaining IR spectra, and Dr. Nobuko Kanehisa and Professor
Masato Ohashi for Xꢀray analysis.
for ICy10 and I(1ꢀAd), and I(2ꢀAd) is a significantly stronger
σꢀ
donor than PCy3, the commonest ligand used in nickelꢀ
mediated CꢀO bond activation.5 The steric properties were
determined by calculating the buried volume (%Vbur),14 based
on the crystal structure of [IrCl(I(2ꢀAd))(cod)]. The %Vbur of
I(2ꢀAd) (33.5) was found to be close to that for IMes (33.8),
exerting a significantly larger steric demand than ICy,10 but not
as large as those of I(1ꢀAd) and IPr. The catalytic activity in
this study clearly cannot be explained solely by these two static
parameters, and other factors such as dynamic behavior of the
Notes and references
a
Department of Applied Chemistry, Faculty of Engineering, Osaka
University,
Suita,
Osaka
565-0871,
Japan.
E-mail:
substituents15 should be considered for
understanding of the structureꢀactivity relationships.
a
complete
tobisu@chem.eng.osaka-u.ac.jp; chatani@chem.eng.osaka-u.ac.jp
b
Center for Atomic and Molecular Technologies, Graduate School of
Engineering, Osaka University, Suita, Osaka 565-0871, Japan
c ESICB, Kyoto University, Katsura, Kyoto 615-8510, Japan
Table 4 TEPa and %Vburb values for selected NHCs used in this study
†
Electronic Supplementary Information (ESI) available: Experimental
procedures and characterization data for all new compounds. See
DOI: 10.1039/b000000x/
1
2
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a Tolman electronic parameter. Values are taken from ref 13 unless otherwise
noted. b Buried volume (ref 14). Values are calculated based on crystal
structures of the corresponding [IrCl(NHC)(cod)] unless otherwise noted. See
ESI for details. c Determined by synthesizing [IrCl(I(2ꢀAd))(CO)2]. See ESI
for details. d Calculated based on the crystal structure of
3
4
Selected examples: (a) S. Cacchi, P. G. Ciattini, E. Morera and G.
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,
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5
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7
8
9
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isolation of an acetaldehydeꢀbound nickel(0) complex, while the
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