Etheric C-O bond hydrogenolysis using a tandem lanthanide triflate/supported palladium nanoparticle catalyst system

Article abstract of DOI:10.1021/ja306309u

Selective hydrogenolysis of cyclic and linear ether C-O bonds is accomplished by a tandem catalytic system consisting of lanthanide triflates and sinter-resistant supported palladium nanoparticles in an ionic liquid. The lanthanide triflates catalyze endothermic dehydroalkoxylation, while the palladium nanoparticles hydrogenate the resulting intermediate alkenols to afford saturated alkanols with high overall selectivity. The catalytic C-O hydrogenolysis is shown to have significant scope, and the C-O bond cleavage is turnover-limiting.

Full text of DOI:10.1021/ja306309u

Communication
pubs.acs.org/JACS
Etheric C−O Bond Hydrogenolysis Using a Tandem Lanthanide
Triflate/Supported Palladium Nanoparticle Catalyst System
Abdurrahman C. Atesin,† Natalie A. Ray,† Peter C. Stair,†,‡ and Tobin J. Marks*,†
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
‡Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
S
* Supporting Information
Scheme 1. Proposed Tandem Pathway for Lanthanide
ABSTRACT: Selective hydrogenolysis of cyclic and linear
Triflate/Pd Nanoparticle-Mediated Etheric C−O Bond
a
ether C−O bonds is accomplished by a tandem catalytic
system consisting of lanthanide triflates and sinter-resistant
supported palladium nanoparticles in an ionic liquid. The
lanthanide triflates catalyze endothermic dehydroalkox-
ylation, while the palladium nanoparticles hydrogenate the
resulting intermediate alkenols to afford saturated alkanols
with high overall selectivity. The catalytic C−O hydro-
genolysis is shown to have significant scope, and the C−O
bond cleavage is turnover-limiting.
Hydrogenolysis
fficiently converting abundant lignocellulosic biomass and
Ecoal into liquid fuels and commodity chemicals presents a
grand research challenge.1 In principle, such processes could
increase sustainable liquid fuel supplies to satisfy near-term
energy demands and decrease reliance on petroleum. However,
progress is currently hindered by a lack of energy-efficient/cost-
effective catalytic routes,2 and by the product oxygen content
which lowers energy density and complicates processing.1d,3
Since aliphatic and aromatic ethers and furans constitute a
significant component of biomass materials and cross-link many
of the structures, we sought mechanistic understanding-based
selective C−O cleavage strategies that could ultimately facilitate
conversion of these feedstocks into liquid fuels.4 Moreover,
selective C−O bond cleavage processes also have significant
potential in organic synthesis;5 however, these bonds are
typically unreactive in the absence of allylic or benzylic
junctures. Here we report a new, thermodynamically based
tandem strategy for catalytic ether hydrogenolysis which builds
on a transformation typically employed to achieve the
microscopic reverse, C−O fusion.
a
Ln = lanthanide; R1, R2 = organic functional group.
close the catalytic cycle to produce saturated alkanols (Scheme
2, step ii).11
The use of room temperature ionic liquids (RTILs) as
catalytic reaction media has advanced dramatically.12 These
robust, recyclable, nonvolatile, polar, typically aprotic solvents
often provide unique reaction efficiencies and selectivities (as in
Scheme 2. Approximate Reaction Coordinate Showing: (i)
Conversion of a Cyclic Ether into an Intermediate Alkenol,
Followed by (ii) Hydrogenation of the Alkenol to Yield a
Saturated Alcohol
To catalyze C−O bond hydrogenolysis, we focused on
coupling endothermic C−O bond scission via the microscopic
reverse of well-documented alkene hydroalkoxylation processes
(Scheme 1, Cycle A; ΔH ≈ +14 kcal/mol) with exothermic
CC hydrogenation (ΔH ≈ −25 kcal/mol) to yield saturated
alcohols (Scheme 1, Cycle B). We previously demonstrated6
that solutions of easily recycled, electrophilic lanthanide
triflates7 in ionic liquids are efficient and selective catalysts
for C−O fusion via intramolecular alkene hydroalkoxylation8
(reverse of Scheme 1, Cycle A).9,10 From microscopic
reversibility, this catalytic system should, in principle, be ideally
competent for the reverse but to date undocumented and
endothermic C−O scission to yield alkenols (Scheme 2, step i).
Coupling to an effective hydrogenation catalyst would then
Received: June 28, 2012
Published: August 13, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
lanthanide-mediated hydroalkoxylations),6 as well as environ-
mental attractions and facile product separation.13 While metal
nanoparticles and various organometallic complexes are
effective olefin hydrogenation catalysts in RTILs near room
temperature, preliminary studies with such catalysts and
allylphenol as a model dehydroalkoxylation product in
[EMIM][OTf] (ethylmethyl-imidazolium triflate) revealed
significant catalyst deactivation via agglomeration and/or
thermal decomposition at temperatures affording useful
dehydroalkoxylation rates (≥100 °C).
Table 1. Catalytic C−O bond Hydrogenolysis Mediated by
Ln(OTf)3 + Pd@ALD in [EMIM][OTf]
As an example, commercially available Pd supported on
BaSO4 was found to be stable in [EMIM][OTf] and to readily
hydrogenate allylphenol at room temperature. However, in
tandem catalytic experiments with 2,3-dihydro-2-methylbenzo-
furan (1) in [EMIM][OTf] using Yb(OTf)3 and the
aforementioned Pd catalyst at 185 °C/600 psi H2, only slow
C−O cleavage and hydrogenation to n-propylphenol (2) is
observed, with a maximum yield of 45% in 22 h (eq 1). In
marked contrast, Pd nanoparticles deposited on Al2O3 via
atomic layer deposition (Pd@ALD), are exceptionally resistant
to thermal sintering.14 When this catalyst is used in conjunction
with Yb(OTf)3 in [EMIM][OTf] under the same reaction
conditions, n-propylphenol is formed cleanly at 185 °C/600 psi
H2 with any hydrogenated arene byproducts below the
detection limits.
The scope of this tandem catalytic process was next
investigated over the range 110−185 °C and 100−600 psi
H2, under anhydrous conditions, with final products isolated
either by simple quantitative ether extraction or vacuum
transfer, thus allowing efficient catalyst and ionic liquid
1
recycling. Results are summarized in Table 1, with H/13C
NMR product characterization detailed in the Supporting
Information. Importantly, these conversions proceed cleanly
(>95% selectivity) to the indicated products. In general, etheric
bond cleavage is favored at more substituted C−O junctures,
with the C−O bonds of acyclic ethers cleaved more rapidly
than those of cyclics (entries 8,9). The latter observation is
consistent with the entropic advantage of creating more
particles/degrees of freedom that accompanies C−O scission,
and with kinetic parameters for the corresponding cyclization
process.6 Negligible C−O cleavage is observed under the
present conditions for substrates such as di-n-octylether and
tetrahydropyran, which lack a H atom β to the etheric O atom.
That such a substitution pattern is beneficial for rapid
hydrogenolysis is in good agreement with the regiochemistry
of the reverse hydroalkoxylation process (Scheme 1, cycle
A),6,8a,9 while encumbered substrates such as α-t-butyl-THF are
unreactive. Furthermore, the present turnover frequency (Nt)
dependence on Ln3+ ionic radius/electrophilicity (Table 1)
closely parallels that of the microscopic reverse reaction,6 with
Yb3+ > Sm3+ > La3+, again arguing that the same reaction
coordinate is traversed.
a
Turnover frequencies determined from aliquots taken during the
initial stages of the reaction with 1H NMR spectroscopic integration vs
internal standard. Reaction conditions: 600 psi H2; [La(OTf)3] = 1
mM; [substrate] = 0.01 M. For further experimental details, see the
Supporting Information.
corresponding hydroalkoxylation processes,6 where free HOTf
could not be detected in, or vacuum transferred from,
Ln(OTf)3-based catalytic reaction mixtures. Note that these
observations are consistent with quantum chemical calculations
on the Al(OTf)3-catalyzed hydroalkoxylation reaction coor-
dinate,10c as well as B3LYP DFT calculations on the present
systems.15 Furthermore, it was found previously that Nt for the
Ln(OTf)3-catalyzed ring closure process is depressed on
addition of an arylsilane proton trap,6a,16 arguing that
abstractable H+ ions (e.g., from a coordinated alcohol)10c are
involved in the cyclization; see Scheme 1, cycle A where a C−H
proton is transferred to an evolving Ln-alkoxide group. In the
present hydrogenolysis experiments, the same effects are
observed when 1.0 equiv PhSiMe3 is added to the Yb(OTf)3
+ Pd@ALD mediated 1 → 2 ether cleavage, and the yield of
alkanol is depressed by ∼95%, with benzene and Me3SiOTf
Additional support for the proposed pathway of Scheme 1 is
provided by control experiments indicating negligible turnover
in the absence of either the Ln(OTf)3 or Pd nanoparticle
catalysts. Note from Table 1, entry 1 that turnover is far slower
when 1.0 equiv HOTf is substituted for Yb(OTf)3 in these
reactions. Similar observations were made in studies of the
observed in H and 19F NMR analysis of the reaction mixture.
1
As shown in Figure 1, kinetic analysis of the 1 → 2 process in
Table 1, entry 1 with Yb(OTf)3 reveals first-order dependence
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Communication
recyclability. Current efforts are focusing on scope and
mechanism in terms of substrate and catalyst.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and product characterization. This materi-
al is available free of charge via the Internet at http://pubs.acs.
org.
AUTHOR INFORMATION
Corresponding Author
t-marks@northwestern.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported as part of the
Institute for Atom-efficient Chemical Transformations (IACT),
an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under contract DE-AC02-06CH11357 (catalytic C−O
cleavage processes) and by the NSF under grant CHE-0809589
(basic lanthanide chemistry).
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