Rapid ether and alcohol C-O bond hydrogenolysis catalyzed by tandem high-valent metal triflate + supported Pd catalysts

Article abstract of DOI:10.1021/ja411546r

The thermodynamically leveraged conversion of ethers and alcohols to saturated hydrocarbons is achieved efficiently with low loadings of homogeneous M(OTf)_n + heterogeneous Pd tandem catalysts (M = transition metal; OTf = triflate; n = 4). For example, Hf(OTf)₄ mediates rapid endothermic ether a? alcohol and alcohol a? alkene equilibria, while Pd/C catalyzes the subsequent, exothermic alkene hydrogenation. The relative C-O cleavage rates scale as 3 > 2 > 1. The reaction scope extends to efficient conversion of biomass-derived ethers, such as THF derivatives, to the corresponding alkanes.

Full text of DOI:10.1021/ja411546r

Communication
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Rapid Ether and Alcohol C−O Bond Hydrogenolysis Catalyzed by
Tandem High-Valent Metal Triflate + Supported Pd Catalysts
Zhi Li,† Rajeev S. Assary,‡ Abdurrahman C. Atesin,† Larry A. Curtiss,‡,¶ and Tobin J. Marks*,†
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
‡Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
¶Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
S
* Supporting Information
supported hydrogenation catalysts, capitalizing on accrued
ABSTRACT: The thermodynamically leveraged conver-
sion of ethers and alcohols to saturated hydrocarbons is
achieved efficiently with low loadings of homogeneous
M(OTf)n + heterogeneous Pd tandem catalysts (M =
transition metal; OTf = triflate; n = 4). For example,
Hf(OTf)4 mediates rapid endothermic ether ⇌ alcohol
and alcohol ⇌ alkene equilibria, while Pd/C catalyzes the
subsequent, exothermic alkene hydrogenation. The relative
C−O cleavage rates scale as 3° > 2° > 1°. The reaction
scope extends to efficient conversion of biomass-derived
ethers, such as THF derivatives, to the corresponding
alkanes.
mechanistic and thermodynamic understanding of the hydro-
alkoxylation pathway.12 Various ether linkages are effectively
cleaved when a Pd nanoparticle/alumina catalyst is used for
hydrogenation.13 Mechanistic studies and DFT-level computa-
tion reveal that lanthanides with smaller ionic radii/greater
electrophilicity, i.e., higher effective charge density, exhibit
greater catalytic activity.14
Here, this concept is further explored with other metal triflates
by both experiment and theory. We report that higher valent
metal triflates, Hf(OTf)4 in particular, exhibit far higher activity
than lanthanide triflates and mineral acids, hydrogenolyzing not
only etheric C−O bonds but also alcoholic C−O bonds in a
broad spectrum of biomass-related substrates. This approach is
advantageous over previous protocols from this laboratory12,14
and others in many ways: (1) extended substrate scope including
C−O bonds of alcohols and primary ethers; (2) high yields of
alkanes without skeletal rearrangement; (3) commercially
available catalysts with low metal loadings; (4) lower reaction
temperatures; (5) solvent-free reaction conditions.
major challenge in converting biomass into chemicals and
A
hydrocarbon fuels is to efficiently cleave the ubiquitous
etheric and alcoholic C−O linkages within the feedstock
molecules and materials to lower the oxygen content and degree
of polymerization.1 For example, cyclic ethers such as
tetrahydrofuran/-pyran derivatives are important targets since
they can be obtained from renewable cellulosic biomass
resources,1f and efficient, selective routes to convert cyclic ethers
to hydrocarbons present a major challenge in biomass research.2
Among the strategies for catalytic biomass hydrodeoxygenation,3
multifunctional catalyst systems consisting of a supported
hydrogenation catalyst and an acid,4 either homogeneous5 or
supported,6 have been studied. In addition to mineral acids such
as sulfuric and phosphoric acids, molecular Lewis acids have
received growing attention for these transformations.7 In organic
synthesis, metal trifluoromethanesulfonate (triflate, OTf)
complexes have proven to be highly effective Lewis acid catalysts,
offering acidity, moisture and air stability, and recyclability.8
Furthermore, they promote C−O bond heterolysis to cationic
species which subsequently form C−C or C−O bonds with
nucleophiles.9 These characteristics raise the intriguing question
of whether metal triflates might be useful catalysts in hydro-
genolytic C−O bond cleavage processes and with what
generality.
The present strategy is based on thermodynamic analysis of
plausible reaction pathways from oxygenates to hydrocarbons,12
with computational results outlined in Scheme 1, ignoring small
ring strain effects.15 Although C−O bonds have substantial bond
enthalpies, their hydrogenolysis is decidedly exothermic.
In initial experiments, various triflates were screened for C−O
cleavage in 1,8-cineole (1), which should be reactive toward acid-
catalyzed ring-opening, driven by ring strain and the stability of
any tertiary carbocationic intermediates (Table 1). At 100 °C in
the absence of Pd, 1 undergoes facile conversion to water +
terpinene isomers 2. Upon prolonged exposure to the catalyst,
the terpinene isomers are converted to a complex mixture of
species. Introduction of Pd/C under an argon atmosphere
cleanly converts 2 to a mixture of p-menthanes 3 (cis- + trans-
isomers) and p-cymene 4 in a 1:2 ratio via transfer hydro-
genation.16 Pre-reducing the Pd catalysts17 with H2 significantly
enhances catalytic activity. These reaction conditions were next
used to screen a variety of Lewis and Brønsted acids for catalytic
C−O cleavage.
Previous exploratory/mechanistic studies in this laboratory
showed that lanthanide triflates in ionic liquids are efficient
catalytic systems for C−C bond-forming Friedel−Crafts
acylation10 and C−O bond-forming alkene hydroalkoxylation/
cyclization.11 More recently, etheric C−O hydrogenolysis,
traversing the microscopic reverse of catalytic hydroalkoxyla-
tion,11 was demonstrated using tandem lanthanide triflate +
From Table 1, which summarizes reaction conversions with
different catalysts, note that of the Lewis acids screened,
lanthanide triflates are minimally active, while Hf(OTf)4 and
Received: November 12, 2013
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
15).18 Nevertheless, since the final outcome of C−O cleavage
is hydrocarbon products, using hydrocarbon solvents was not
pursued further.
A broader screening of substrates and reaction conditions is
summarized in Table 2, starting with primary C−O bonds in a
Scheme 1. Catalytic Cycles and Computed Noncatalyzed
Enthalpic Parameters for the Hypothetical Tandem
Conversion of 2-Methyltetrahydropyran to n-Hexane via 5-
Hexen-1-ol as Intermediate
Table 2. Catalytic Alcohol and Ether C−O Hydrogenolysis
a
Mediated by Hf(OTf)4 and Pd/C
a
Table 1. Acid and Medium Screening for C−O Cleavage
b
c
entry
acid
solvent
t (min)
conv (%)
ρ
1
−
neat
90
30
30
10
10
10
10
10
10
10
90
90
90
90
10
N.R.
0.2
−
2
La(OTf)3
Yb(OTf)3
Ce(OTf)4
Sc(OTf)3
Fe(OTf)3
Al(OTf)3
Zr(OTf)4
Hf(OTf)4
HOTf
neat
2.60
2.81
3.44
3.23
3.71
3.87
4.29
4.37
−
−
−
−
−
3
neat
0.2
4
neat
5.7
5
neat
12.4
13.6
23.7
33.5
46.0
10.4
N.R.
N.R.
N.R.
N.R.
49.7
6
neat
7
neat
8
neat
9
neat
10
11
12
13
14
15
neat
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
THF
DMF
MeOH
H2O
n-octane
−
a
Reaction conditions: 0.5 mol% acid catalyst, 0.2 mol% Pd as 10% Pd/
C under Ar at 100 °C; 6 mmol substrate neat or 1.0 M in solvent. N.R.
= no products observed. THF = tetrahydrofuran. DMF = N,N-
b
a
dimethylformamide. MeOH = methanol. Conversions determined by
Unless noted, all reactions performed with 0.5 mol% Hf(OTf)4, 0.2
c
1H NMR of reaction mixture aliquots. Effective charge density of the
mol% Pd/C, 40 bar H2 without solvent. Conversions and selectivities
determined by 1H NMR of reaction mixture aliquots. Isolated yields in
metal center computed at the B3LYP level. See Supporting
Information for details.
b
parentheses. N.R. = no products observed. 1.0 equiv of H2O added.
c
d
pH ≈ 4 bar; catalysts pre-reduced with H2. Volatile products not
2
e
Zr(OTf)4 are far more active (Table 1, entries 2, 3, 8, and 9). This
trend is consistent with the higher effective charge density (ρ) of
central triflate metal ions.15 Note that HOTf is less active (Table
1, entry 10), arguing that these transformations are predom-
inantly catalyzed by Lewis acids rather than by HOTf from metal
triflate hydrolysis. Solvent screening (Table 1, entries 11−15)
shows that coordinating solvents significantly depress turnover,
and the reaction in ionic liquid 1-ethyl-3-methylimidazolium
triflate gives a complex mixture, probably reflecting severe
deactivation of the Pd/C.12 In contrast, n-octane as the reaction
solvent accelerates turnover, possibly reflecting better mass and
heat transfer, greater H2 solubility, reduced competitive
adsorption of unreactive molecules to the heterogeneous
catalyst, and/or azeotropic water removal (Table 1, entry
isolated. Conversion and yield by GC-MS. See SI for details.
high-pressure reactor. Pre-reduction of the catalyst is accom-
plished along with temperature elevation. When the present
catalysts are applied to 1-octanol, a roughly 1:1 n-octane:di-n-
octyl ether mixture is formed, with 5% 1-octanol remaining after
3 h at 180 °C; n-octane is the sole product after 12 h (Table 2,
entry 1). Note that neat, dry di-n-octyl ether is unreactive (Table
2, entry 2); however, adding 1 equiv of H2O gives a similar
progression as starting from 2 equiv of 1-octanol, presumably
because the same equilibrium between ether and alcohol is
quickly established in both cases (Table 2, entry 3). The
reactivity of secondary C−O bonds is greater than that of
B
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Journal of the American Chemical Society
Communication
primary substrates, achieving higher conversions to alkanes at a
lower temperature in a shorter time (Table 2, entries 4−6).
Unlike di-n-octyl ether, dry dicyclohexyl ether does not require
added H2O to undergo hydrogenolysis, arguing that direct C−O
heterolysis is more facile (Table 2, entry 6). Furthermore, tertiary
C−O bonds undergo rapid cleavage at 100 °C (Table 2, entries 7
and 8). Hydrogenolysis of THF and THF derivatives generates
alkanes as the ultimate products, although many alcoholic and
etheric intermediates are detected (Table 2, entries 9−13). For
example, in the reaction of 2-n-butyltetrahydrofuran (Table 2,
entry 12), observed intermediates include 1-octanol and di-n-
octyl ether, consistent with preference for secondary rather than
primary C−O cleavage.19 Finally, when tetrahydrofurfuryl
alcohol (Table 2, entry 13) is treated under conditions for
secondary C−O hydrogenolysis, tetrahydropyran is obtained as
the major product, presumably derived from the cyclization of
intermediate 1,5-pentanediol.20
These results provide a preliminary mechanistic picture of the
reaction pathways. First, primary C−O bonds are the most
resistant to cleavage, and primary ethers do not undergo direct
C−O cleavage at significant rates. Instead, in the presence of
water, there is equilibration with the parent primary alcohol
which undergoes acid-catalyzed dehydration to alkene and rapid,
irreversible hydrogenation to alkane (Scheme 2A). This explains
Figure 1. Computed liquid-phase enthalpies for the Hf(OTf)4-catalyzed
hydrogenolysis of 2-methyltetrahydropyran at the B3LYP level. All
values reported in kcal/mol in blue. The enthalpy change for the
noncatalyzed ether → hexane hydrogenolysis is shown in the dotted
box.
first part of the catalytic cycle (5 → 8), the key reaction is the
Lewis-acid-catalyzed ring-opening of ether 6 via transition state
TS1 upon ether binding and activation of the etheric oxygen.
This pathway is similar to the Ln(OTf)3-catalyzed tandem
hydrogenolysis of ethers to alcohols.14 The computed intrinsic
enthalpy barrier for this Hf(OTf)4-catalyzed ring-opening is 16.8
kcal/mol. Hydrogenation of alkenol 8 to alcohol 9 is found to be,
as expected, exothermic and kinetically less challenging than ring-
opening.12 In the next phase of the catalytic cycle (9 → 12), the
key endothermic reaction is alcoholic C−O bond cleavage.
Bond-breaking occurs via transition state TS2, which has an
intrinsic barrier of 24.5 kcal/mol (9 → 10). Upon cleavage of the
C−O bond of alcohol 9, the alcohol β-hydrogen transfers to the
Hf−OH ligand to form Hf···OH2 and alkene. Hydrogenation of
10 to 11 is analogous to the 8 → 9 hydrogenation. Removal of
hexane from 11 yields complex Hf(OTf)4−OH2, with water
bound strongly to the Hf4+ center (by 18.8 kcal/mol). Water
dissociation then completes the catalytic cycle.
Scheme 2. Reactivity Trends and Tentative Pathways for the
Catalytic Hydrogenolysis of Primary, Secondary, and Tertiary
C−O Bonds
The computational results in many ways concur with the
experimental observations. First, the computed barrier for the
etheric C−O bond cleavage of 6 by Yb(OTf)3 (most active
trivalent lanthanide catalyst)12 is 32.4 kcal/mol,15 while that of
Hf(OTf)4 is only 16.8 kcal/mol (7 → TS1), in good agreement
with the present catalyst activity profiles.22 We expect this
decrease to scale with the effective charge density at the central
metal ion. Second, water is shown to strongly coordinate to the
catalytic metal center, in agreement with the solvent screening
experiments, showing that coordinating solvents generally
depress catalytic activity. Third, the calculations suggest that
the Lewis acid catalyst may facilitate very rapid proton
elimination after the C−O dissociation (Figure S7), which may
explain why carbocation rearrangement products were not
observed.
In summary, we report a highly efficient and green tandem
catalytic system comprised of a high-valent metal triflate Lewis
acid and a supported Pd catalyst for the hydrogenolysis of ethers
and alcohols. This solvent-free catalyst system is capable of
deoxygenating a broad spectrum of alcoholic and etheric
substrates derivable from renewable biomass resources. Satu-
rated hydrocarbons are obtained as the major products with
why water is necessary for primary ether hydrogenolysis but not
for that of primary alcohols. Second, secondary C−O bonds are
more reactive toward cleavage. Dehydration equilibria between
secondary ethers and alcohols are still observed, but unlike
primary ethers, secondary ethers undergo significant cleavage
without the initial water addition (Scheme 2B). Lastly, tertiary
C−O bonds are the most reactive, with C−O cleavage so facile
that often alkene intermediates are detected, unlike the primary
and secondary cases (Scheme 2C). The recyclability of the
present catalysts was also examined. While over five consecutive
runs, all reactions proceed to complete conversion, the Pd/C
activity declines with time, and activity is not restored by
standard triflate catalyst dehydration procedures.21
Computational studies were next performed using B3LYP
DFT methods to map the energy landscape of the Hf(OTf)4-
catalyzed 2-methyltetrahydropyran conversion to n-hexane via
intermediate 5-hexen-1-ol (Table 2, entry 11; Figure 1).15 In the
C
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Journal of the American Chemical Society
Communication
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ASSOCIATED CONTENT
* Supporting Information
Experimental details, product characterizations, detailed compu-
tational results, and discussions. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
t-marks@northwestern.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy
under contract DE-AC0206CH11357. This material is based
upon work supported as part of the Institute of Atom Efficient
Chemical Transformation (IACT), an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences. NSF grant CHE-
1213235 on basic f-element chemistry supported Z.L. and
provided reactor equipment. We gratefully acknowledge the
computing resources provided on “Fusion”, a 320-node
computing cluster operated by the Laboratory Computing
Resource Center at Argonne National Laboratory. Use of the
Center for Nanoscale Materials was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. DE-AC02-06CH11357. This
research used resources of the National Energy Research
Scientific Computing Center (NERSC), which is supported by
the Office of Science of the U.S. Department of Energy under
Contract No. DE-AC02-05CH11231.
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