Nickel-mediated hydrogenolysis of C-O bonds of aryl ethers: What is the source of the hydrogen?

Article abstract of DOI:10.1021/ja300326t

Mechanistic studies of the hydrogenolysis of aryl ethers by nickel were undertaken with (diphosphine)aryl methyl ethers. A Ni(0) complex containing Ni-arene interactions adjacent to the aryl-O bond was isolated. Heating led to aryl-O bond activation and generation of a nickel aryl methoxide complex. Formal β-H elimination from this species produced a nickel aryl hydride which can undergo reductive elimination in the presence of formaldehyde to generate a carbon monoxide adduct of Ni(0). The reported complexes map out a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel. Investigations of a previously reported catalytic system using isotopically labeled substrates are consistent with the mechanism proposed in the stoichiometric system, involving β-H elimination from a nickel alkoxide rather than cleavage of the Ni-O bond by H₂.

Full text of DOI:10.1021/ja300326t

Communication
pubs.acs.org/JACS
Nickel-Mediated Hydrogenolysis of C−O Bonds of Aryl Ethers:
What Is the Source of the Hydrogen?
Paul Kelley, Sibo Lin, Guy Edouard, Michael W. Day, and Theodor Agapie*
Department of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard MC 127-72,
Pasadena, California 91125, United States
*
S Supporting Information
We recently reported that diphosphine arene pincer ligands based
ABSTRACT: Mechanistic studies of the hydrogenolysis of
aryl ethers by nickel were undertaken with (diphosphine)aryl
methyl ethers. A Ni(0) complex containing Ni−arene interac-
tions adjacent to the aryl−O bond was isolated. Heating led
to aryl−O bond activation and generation of a nickel aryl
methoxide complex. Formal β-H elimination from this
species produced a nickel aryl hydride which can undergo
reductive elimination in the presence of formaldehyde to
generate a carbon monoxide adduct of Ni(0). The reported
complexes map out a plausible mechanism of aryl ether hydro-
genolysis catalyzed by nickel. Investigations of a previously
reported catalytic system using isotopically labeled substrates
are consistent with the mechanism proposed in the stoichio-
metric system, involving β-H elimination from a nickel
alkoxide rather than cleavage of the Ni−O bond by H2.
on a p-terphenyl linker support mono- and dinuclear nickel com-
23,24
plexes that exhibit strong nickel−arene interactions.
The
m-terphenyl diphosphine motif was also found to predispose the
metal center toward interaction with the carbon at the 2′-position of
the central ring. Toward exploiting potential C−X activation chem-
istry, ligand variants with ether substitution at this position were
25
prepared. The present study utilizes diphosphines with tert-butyl
1a) or dimethylamino (1b) substitution para to the ether on the
central ring (Scheme 1); these two ligands lead to nickel complexes
(
Scheme 1. Stoichiometric Cleavage of an Aryl C−O Bond
Facilitated by Nickel
laboration of the aryl C−O bond to a variety of functional
E
groups has emerged as a versatile synthetic tool in organic
1
methodology, as phenol precursors are readily available and
synthetic modification of the aromatic ring is facile. The strong aryl
C−O bond, however, is typically difficult to activate. Nickel-based
catalysts have proven versatile in the conversion of substrates with
2
−13
14−16
aryl C−O
or C−S
bonds. Although cross-coupling of
phenolic substrates tends to require prior conversion to the more
reactive sulfonates, recent advances show that aryl phosphates,
2
aryl esters, aryl carbamates, aryl ethers, and even free phenols can
3−13
be used as electrophiles in cross-coupling reactions.
In a
complementary approach, the conversion of aryl C−O to aryl−H
bonds has been recognized as a valuable strategy for removing an
oxygen-based directing group from an aryl ring. Silanes have been
10,11
utilized as a hydride source for this transformation.
Addition-
ally, stoichiometric intramolecular aryl C−O activation has been
17,18
reported with rhodium and palladium pincer complexes.
with similar reactivity but complementary crystallinity that allows for
solid-state characterization of the resultant species.
In the context of biomass conversion to alternative fuels and
chemicals, the depolymerization of lignin, a significant component
of biomass containing aryl ether linkages, is a considerable chal-
Addition of 1 equiv of Ni(COD) to the diphosphines (1a,b)
2
at 20 °C led to generation of new species (2a,b) according to
19−21
lenge.
Recently, an appealing strategy involving the cleavage of
31
NMR spectroscopy (Scheme 1). The P NMR chemical shifts of
the resulting species (2a, 41.0 ppm; 2b, 40.7 ppm) are similar to
those reported for the p-terphenyl diphosphine-supported Ni(0)
lignin-like aryl C−O bonds via nickel-catalyzed hydrogenolysis was
22
reported by Hartwig et al. Given the general interest in the
conversion of aryl C−O bonds, detailed mechanistic insight includ-
ing the nature of the intermediates is instrumental in developing
practical catalysts. Herein, we report detailed studies of the nickel-
mediated reductive cleavage of an aryl ether with pendant phos-
phines and extend the mechanistic implications of these studies to a
catalytic system.
23
(
40.4 ppm). Similarly, the protons assigned to the central arene
resonate upfield compared with those of the free phosphine in the
Received: January 12, 2012
Published: March 6, 2012
©
2012 American Chemical Society
5480
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Journal of the American Chemical Society
Communication
Figure 1. Solid-state structures of 2b (left) 5b (center), and 6a (right). Solvent molecules, anions, and select hydrogen atoms not shown for clarity.
1
H NMR spectrum of 2a,b, whereas the ether OCH peak is only
importantly, the structure shows cleavage of the aryl C−O bond
and displacement of oxygen by nickel.
Additional experiments were performed to confirm the
nature of compound 3. The reaction of 4b with methanol at
3
slightly shifted. These data are consistent with the formation of a
Ni(0) species with interactions between the metal center and the
aromatic π-system, but not the ether oxygen. A single-crystal X-ray
diffraction (XRD) study of 2b confirmed the spectroscopic find-
ings. In the solid state the metal center is bound by two phos-
phines and interacts with two carbon centers of the central arene
2
0 °C led to the re-formation of species 3b, albeit not quan-
1
titatively ( H NMR spectroscopy). Treatment of 5b with
sodium methoxide at 20 °C led to the formation of 3b, 4b, and
other unidentified species. Although species 3 could not be
obtained by these routes without contamination from complex 4,
the independent methods of generation are consistent with 3
being a Ni(II) aryl methoxide. This is the product of oxidative
addition of the aryl C−O bond to Ni(0). Similar stoichiometric
reactions mediated by rhodium and palladium have been
reported.
(
Figure 1). The short Ni−C distances (1.96−2.09 Å) indicate
strong interactions between the nickel center and central arene.
Consequently, the C−C distances of the central ring are consistent
with partial localization of the double bonds. For 2b the C8−C9
and C10−C11 bonds (1.368(2) and 1.360(2) Å, respectively) are
shorter than the rest of the central arene C−C bonds by >0.06 Å.
The aryl C−O bond points away from the metal center, consistent
with partial sp hybridization of the 2′-position of the central ring
(
intermediate displaying η -interactions between Ni(0) and the
double bond adjacent to the oxygen was found computationally to
31
1
7,18
The origin of the hydride in 4 was studied by using a version
of the diphosphine ligand deuterated at the methoxy position
3
(
1b-d ). This precursor led to the formation of a species (4b-d )
3
1
2b, C18, Figure 1) due to the Ni−C interaction. Notably, an
1
with no signal in the hydride region of the H NMR spectrum, but
otherwise displaying the same peaks as 4. This is consistent with the
formation of a nickel deuteride, which confirms the methoxy group
as the source of hydrogen (deuterium). β-H elimination could occur
2
precede C−O bond activation in the cross-coupling of phenol
derivatives. Complex 2b is the only example of such an arres-
ted intermediate characterized by crystallography, according to a
Cambridge Structural Database search.
8,26
32−34
via a five-coordinate intermediate,
although in coordinatively
saturated systems it has been proposed to occur via alkoxide
27
35−37
38,39
dissociation
or in bimolecular fashion.
Given the strain of
Complexes 2a,b were found to convert to new species in
solution at 45 °C. After the first 12 h a single product (3a,b,
respectively) was observed by NMR spectroscopy, in mixture
the chelates in complex 4, phosphine dissociation may also be
32,33,40,41
possible,
tion.
opening up a cis coordination site for β-H elimina-
Further studies are necessary to determine the mech-
4
2−44
1
with starting material. Monitoring by H NMR spectroscopy
anism in the present system.
revealed that the peaks corresponding to the central arene ring
Although its generation does not require dihydrogen, hydride
4 is a potential intermediate in the hydrogenolysis of the aryl−O
bond. Complexes 2a,b were heated separately to 100 °C,
shifted downfield. Additionally, the OCH resonance shifted nearly
3
0.5 ppm downfield relative to precursors 1a,b. Upon further
1
heating, additional species, 4a,b, were observed. The central arene
proton signals of these species are also shifted relative to 1a,b,
generating new species, 6a,b, in >90% yield ( H NMR spectros-
copy) within 2 h. A Toepler pump experiment showed that
respectively, in the aromatic region. Intriguingly, the OCH signal
∼0.9 equiv of combustible gas was generated in this reaction.
3
1
is absent, and an upfield triplet was observed in the hydride region
The Ni−H peak is absent from the H NMR spectra of the
1
of the H NMR spectrum (−2.85 ppm, 4a; −2.87 ppm, 4b).
products, and a new triplet is present (6a, 6.82 ppm; 6b,
6.51 ppm), assigned to a new aryl ipso-C−H. Infrared (IR) spectros-
copy revealed an absorption indicative of a terminal Ni(0)−
These upfield peaks are consistent with the formation of Ni(II)
hydrides. Compound 4b was isolated in 55% yield by precipitation
from THF upon stirring 2b at 20 °C for 8 days; however, efforts
to obtain X-ray-quality single crystals of 4b have been unsuccessful
to date. Treatment with methyl iodide at 20 °C for 14 h generated
a new species, assigned as a Ni(II) iodide (5b); an XRD study of
this species confirmed the above assignment (Figure 1). The
nickel center is found in the same plane as the central arene. The
Ni−C distance (1.919(1) Å) is consistent with an aryl C−Ni
−1
−1
CO (6a, 1929 cm ; 6b, 1917 cm ). An XRD study of 6a
confirms the formation of a nickel carbonyl complex, with the
metal center bound to the two phosphines and interacting with
the π-system of the central aromatic ring. This binding mode is
consistent with reductive elimination forming an aryl−H bond.
The metal center is close to only one carbon atom from the
central ring (6a, C18, Figure 1), and the Ni−C distance is
longer (2.2763(7) Å) than in 2b. These structural parameters
suggest that the metal center is less π-basic in 6 compared to 2,
likely due to the electron-withdrawing properties of the car-
bonyl ligand.
bond. The diphosphine ligand framework, bound in pseudo-C
2
2
8
fashion, acts as a classical tridentate diphosphine aryl pincer,
2
9,30
but with six-member chelates involving aryl−aryl linkages.
The
solid-state structure is consistent with the NMR spectroscopic
data for 4 and 5, indicating the absence of the methoxy group
and the lack of interaction with the π-system of the arene. Most
The formation of compound 6 indicates that, in addition
to reductive elimination of a C−H bond, decarbonylation of
5
481
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Journal of the American Chemical Society
Communication
formaldehyde had occurred to generate H . To determine the
the incorporation of deuterium in the resultant naphthalene
2
50,51
origin of the hydrogen involved in the C−H reductive elimin-
(>90% Naph-D).
The above results support a mechanism in
ation, deuterium-labeled 2-d was investigated. Upon heating,
which the aryl C−H is derived primarily from the ether substrate
rather than dihydrogen. This finding is consistent with the mech-
anism observed for the stoichiometric reaction of the aryl ether
with pendant phosphines (1): aryl C−O bond activation by nickel
is followed by β-H elimination and reductive elimination (Scheme
3). Although the overall catalytic reaction is a hydrogenolysis of
3
1
the product does not display the aryl C−H peak in the H
NMR spectrum but otherwise displays the same spectro-
scopic features as 6, indicating that the deuterium originating
from the methoxy group was transferred to the aryl. Performing
the reaction in the presence of 1 atm H does not affect the
2
isotope incorporation, indicating that Ni−methoxide cleavage
by H does not compete with β-H elimination. No reaction was
Scheme 3. Mechanisms for the Conversion of Aryl Alkyl
Ethers to Arenes
2
observed upon heating of isolated 4b for 6 h at 100 °C. In
contrast, in the presence of paraformaldehyde, conversion of 4b to
6b occurred within 4 h at 60 °C. This behavior indicates that
formaldehyde facilitates the reductive elimination step. π-Acidic
ligands have been previously reported to facilitate reductive
45,46
elimination.
In the present case, isomerization to a cis-hydride
aryl species also must occur prior to reductive elimination. The
reaction of 4b with carbon monoxide generated 6b as well, albeit
more slowly and in a mixture with unidentified species.
Silanes have been used as hydride sources for the reductive
1
0,11
cleavage of aryl C−O bonds.
To test the effect of added
silane, 2b was heated to 80 °C in benzene in the presence of 2
equiv of Et SiD and generated only <10% 6b-d . A larger excess
3
1
of Et SiD (13 equiv) led to more deuterium incorporation
3
(
∼20% 6b-d ). Treatment of 2b-d with Et SiH (2 equiv)
1
3
3
47
generated 6b-d and 6b in a 1:1 ratio. These results indicate
1
that the mechanism involving β-H elimination (Scheme 1) is
favored vs interception of the nickel methoxide by silane at low
concentration of silane. The increased isotopic incorporation
from Et SiH vs Et SiD is consistent with normal isotope effects
3
3
for β-H elimination (conversion of 3b to 4b) and trans-
42−44,48,49
metalation between 3b and silane to generate 4b.
The generation of 6 and the intermediates characterized
above maps out a potential pathway for the cleavage of aryl
alkyl ethers. Notably, this process does not require dihydrogen.
To investigate the relevancy of this process for the catalytic
reaction, aryl alkyl ethers were subjected to hydrogenolysis
ethers, the reactive nickel hydride species may not result from the
cleavage of a Ni−alkoxide bond by H , but rather from β-H
2
elimination from an alkoxide ligand. Isotopic labeling
experiments with silanes as source of hydride (or deuteride)
indicated isotopic scrambling and proved inconclusive
22
under reported reaction conditions (Scheme 2). 2-Methoxy-
naphthalene and 2-hexyloxynaphthalene were selected as sub-
(
see SI).
Since the mechanism above does not require H for the
2
conversion of aryl ether to arene, the catalytic trials were also
Scheme 2. Isotopic Labeling Studies of Catalytic
Hydrogenolysis of Aryl Alkyl Ethers
performed in the absence of H ; <5% conversion was ob-
2
served. In a separate experiment, a mixture of 1,3-bis(2,6-
t
diisopropylphenyl)imidazolinium chloride (SIPr·HCl), NaO Bu,
and Ni(COD) in toluene was heated under H , concentrated
2
2
under vacuum to a dark red wax, reconstituted with m-xylene,
and treated with aryl ether substrate under N . Ether conver-
2
sion to arene was observed, albeit in lower yields (∼50%) com-
pared to standard conditions (∼75%). These experiments
support the hypothesis that H is necessary for the forma-
2
tion of the active catalyst but not for the actual catalytic
transformation.
In summary, the mechanism of nickel-mediated aryl ether
hydrogenolysis was investigated. A substrate with pendant
phosphines allowed the isolation and characterization of inter-
mediates along the reaction pathway. These support a mech-
anism involving Ni(0) coordination to arene, oxidative addition
of the aryl C−O bond followed by β-H elimination, and form-
aldehyde-assisted reductive elimination of the aryl−H bond.
Dihydrogen (1 atm) does not compete with the above pro-
cesses. Isotopic labeling investigations of a catalytic system
based on a nickel-NHC catalyst support a similar mechanism
strates. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium chlo-
t
ride (SIPr·HCl), NaO Bu, and Ni(COD) in m-xylene generate
2
the catalytic species in situ. Heating to 120 °C for 16 h under 1
atm of H leads to the formation of naphthalene and aliphatic
2
alcohol (hexyl silyl ether analyzed upon derivatization).
To elucidate the origin of the hydrogen delivered to the naphthyl
group, substrates labeled with deuterium at the oxygen-bound
carbon were prepared (NaphOCD and NaphOCD (CH ) -
3
2
2 4
CH , Naph = 2-naphthyl). Hydrogenolysis under H leads to
that does not require H for turnover. Overall, the present studies
3
2
2
5
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Journal of the American Chemical Society
Communication
provide mechanistic snapshots of a transformation of interest in
organic methodology and with potential for biomass conversion.
Future work will focus on detailed investigations of each step of this
mechanism toward developing practical catalysts.
(23) Velian, A.; Lin, S.; Miller, A. J. M.; Day, M. W.; Agapie, T. J. Am.
Chem. Soc. 2010, 132, 6296.
(
24) Lin, S.; Day, M. W.; Agapie, T. J. Am. Chem. Soc. 2011, 133,
828.
25) Chao, S. T.; Lara, N. C.; Lin, S.; Day, M. W.; Agapie, T. Angew.
3
(
Chem., Int. Ed. 2011, 50, 7529.
(26) Li, Z.; Zhang, S.-L.; Fu, Y.; Guo, Q.-X.; Liu, L. J. Am. Chem. Soc.
ASSOCIATED CONTENT
■
*
S
Supporting Information
2009, 131, 8815.
6
(27) An η -aryl ether nickel complex has been characterized:
Experimental procedures, characterization data, and crystallo-
graphic details (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Campora, J.; del Mar Conejo, M.; Reyes, M. L.; Mereiter, K.;
Passaglia, E. Chem. Commun. 2003, 78.
(
(
28) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759.
29) Steinke, T.; Shaw, B. K.; Jong, H.; Patrick, B. O.; Fryzuk, M. D.
AUTHOR INFORMATION
■
Organometallics 2009, 28, 2830.
(30) Kaufhold, O.; Stasch, A.; Pape, T.; Hepp, A.; Edwards, P. G.;
Newman, P. D.; Hahn, F. E. J. Am. Chem. Soc. 2008, 131, 306.
Corresponding Author
agapie@caltech.edu
(31) Reaction of 1 with Ni(II) reagents results in stoichiometric
Notes
cleavage of the ArO−CH bond. Investigations of this transformation
will be reported in a future manuscript.
3
The authors declare no competing financial interest.
(32) Ozawa, F.; Ito, T.; Yamamoto, A. J. Am. Chem. Soc. 1980, 102,
ACKNOWLEDGMENTS
6457.
■
(33) Komiya, S.; Morimoto, Y.; Yamamoto, A.; Yamamoto, T.
We thank Lawrence M. Henling for crystallographic assistance.
We are grateful to Caltech, BP, and NSF GRFP (S.L.) for
funding. The Bruker KAPPA APEXII X-ray diffractometer was
purchased via an NSF CRIF:MU award to Caltech, CHE-
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51) After the quenched reaction mixture of NaphOCD (CH ) CH
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