Angewandte
Communications
Chemie
À
C O Bond Cleavage
German Edition:
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Chemo- and Regioselective Hydrogenolysis of Diaryl Ether C O
Bonds by a Robust Heterogeneous Ni/C Catalyst: Applications to the
Cleavage of Complex Lignin-Related Fragments
Fang Gao+, Jonathan D. Webb+, and John F. Hartwig*
Abstract: We report the chemo- and regioselective hydro-
a homogeneous NHC–Ni system, inspired by related cross-
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genolysis of the C O bonds in di-ortho-substituted diaryl
coupling chemistry involving the activation of aromatic C O
ethers under the catalysis of a supported nickel catalyst. The
catalyst comprises heterogeneous nickel particles supported on
activated carbon and furnishes arenes and phenols in high
yields without hydrogenation. The high thermal stability of the
bonds.[10] The added NaOtBu led to chemoselective hydro-
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genolysis of the C O bonds over the competitive hydro-
genation of aryl groups observed with prior systems.[11] Later,
we discovered that heterogeneous nickel particles generated
in situ from [Ni(cod)2] without an added ancillary ligand
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embedded metal particles allows C O bond cleavage to occur
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in highly substituted diaryl ether units akin to those in lignin.
Preliminary mechanistic experiments show that this catalyst
undergoes sintering less readily than previously reported
catalyst particles that form from a solution of [Ni(cod)2].
catalyze the cleavage of C O bonds of biaryl ethers, including
those of electron-rich biaryl ethers, with catalyst loadings as
low as 0.5 mol% (vs. 20 mol% of the NHC–Ni catalyst).[12]
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Although this catalyst cleaved the C O bonds of some biaryl
ethers, it did not react with biaryl ethers containing highly
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T
he rising concerns about global climate change and the
electron-rich C O bonds and multiple substituents that mimic
depletion of fossil fuels have stimulated the pursuit of energy
and commodity chemicals from renewable sources.[1] Lignin,
the most abundant reserve of aromatic compounds besides
fossil fuels,[2] has the potential to supplement or replace coal
or crude oil as the source of arenes.[3] Although some progress
has been made in the transformation of lignin into liquid bio-
oils,[4] mixtures of saturated hydrocarbons,[5] emulsifying
agents,[6] and well-defined aromatic compounds,[7] further
studies are needed to create practical processes. For example,
high temperatures (> 2208C) and a large excess of hydrogen
(in the MPa range) are often required for the degradation of
lignin to an acceptable extent, and catalysts for the hydro-
genolysis of lignin lead to reduction of the arenes.[4–7] This
reduction of the arenes consumes hydrogen and prevents the
use of lignin as a source of aromatic compounds. Further-
those in lignin. For example, the reaction of biaryl ether 1 led
to less than 2% conversion, even at temperatures up to 1808C
[Eq. (1); cod = 1,5-cyclooctadiene, TMS = trimethylsilyl,
tmeda = tetramethylethylenediamine].
Clearly, a more active and robust set of catalysts are
needed to cleave the C O bonds in biaryl ether units like
those in lignin. Herein, we report the cleavage of biaryl C O
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more, few methods have been devised for cleavage of the
strong C O bonds of diaryl ethers; the presence of these
[8]
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bonds by readily available nickel nanoparticles supported on
activated carbon. These catalysts are more thermally stable
than homogeneous or unsupported nanoparticulate nickel
and therefore catalyze the hydrogenolysis of hindered,
electron-rich diaryl ethers relevant to those in lignin.[13]
Our efforts to evaluate the reactivity and selectivity of
a supported, nanoparticulate nickel catalyst under various
conditions for the cleavage of biaryl ethers began with
reactions of diphenyl ether (2a) itself. These studies showed
that the hydrogenolysis of 2a catalyzed by unsupported Ni
precursors was incomplete when conducted at 1808C, rather
than 1208C (Table 1, entries 1–3), even with prolonged
reaction times (data not shown). Faster aggregation of the
Ni nanoparticles formed in situ would account for the lower
activity of the catalyst generated from [Ni(CH2TMS)2-
(tmeda)] than that of the supported particles for reactions
at 1808C rather than 1208C (Table 1, entries 1 and 2).
entities significantly limits the theoretical yields of mono-
meric arenes and phenols in the above processes.[7] Therefore,
systems are needed that selectively catalyze the hydrogenol-
ysis of diaryl ethers, especially highly substituted biaryl ethers
relevant to those found in lignin, under relatively mild
conditions.
We previously reported the first examples of the catalytic
hydrogenolysis of diaryl ethers under atmospheric hydrogen
pressure (15 psi) at low temperatures (80–1208C) to form
arenes and phenols without competing hydrogenation of the
aromatic rings;[9] the transformations were catalyzed by
[*] Dr. F. Gao,[+] Dr. J. D. Webb,[+] Prof. Dr. J. F. Hartwig
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
E-mail: jhartwig@berkeley.edu
[+] These authors contributed equally.
We hypothesized that the localization of the nanoparticles
would inhibit such aggregation at higher temperatures, thus
leading to a longer-lived catalyst. Thus, we deposited [Ni-
Supporting information for this article is available on the WWW
1474
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1474 –1478