Thermodynamically leveraged tandem catalysis for ester RC(O)O-R′ bond hydrogenolysis. scope and mechanism

Article abstract of DOI:10.1021/acscatal.5b00950

Rapid and selective formal hydrogenolysis of aliphatic ester RC(O)O-R′ linkages is achieved by a tandem homogeneous metal triflate + supported palladium catalytic system. The triflate catalyzes the mildly exothermic, turnover-limiting O-R′ cleavage process, whereas the exothermic hydrogenation of the intermediate alkene further drives the overall reaction to completion.

Full text of DOI:10.1021/acscatal.5b00950

Letter
pubs.acs.org/acscatalysis
Thermodynamically Leveraged Tandem Catalysis for Ester RC(O)O−
R′ Bond Hydrogenolysis. Scope and Mechanism
Tracy L. Lohr,†,‡ Zhi Li,†,‡,∥ Rajeev S. Assary, 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
*
S Supporting Information
ABSTRACT: Rapid and selective formal hydrogenolysis of aliphatic
ester RC(O)O−R′ linkages is achieved by a tandem homogeneous metal
triflate + supported palladium catalytic system. The triflate catalyzes the
mildly exothermic, turnover-limiting O−R′ cleavage process, whereas the
exothermic hydrogenation of the intermediate alkene further drives the
overall reaction to completion.
KEYWORDS: C−O cleavage, biomass, tandem catalysis, thermodynamic leveraging, metal triflates, hydrogenolysis
ster functional groups are ubiquitous in nature as fat and
oil components, and they represent one of the three major
temperature (400−500 °C) thermal cracking over zeolites or
Al O to afford CO and the corresponding hydrocarbons
(Figure 1). Thus, there is currently a paucity of methodologies
E
2
3
2
6
biomolecule classes along with carbohydrates and proteins.
Esters are utilized extensively in products as diverse as motor
1
a
1a−c
fuels, polyesters, fine chemicals, and pharmaceuticals
where they frequently serve as carboxylic acid protecting
groups
1
c−e
1
(e.g., Taxel and Cetraxate drugs). For these
reasons, new atom-efficient/greener means to cleanly catalyze
ester modification processes are of great interest.
With respect to biomass, esters are most commonly found as
triglycerides or fats, where the component long chain free fatty
acids can be used for detergents, polymers, and surface
coatings, and are an unconventional source of diesel fuel
(
biodiesel), which has attracted growing attention as an
1g−i
alternative to traditional petroleum-based fuels.
Due to
the widespread presence of triglycerides in biomass, efficient
catalytic methods to break down esteric moieties for fuel or
other chemicals via C−O bond cleavage processes, while also
preserving the C3 functionality, are attractive. Although ester
hydrolysis and transesterification are common, cleavage of
esters to alkanes and carboxylic acids that is efficient, selective,
and proceeds under nonaqueous/aprotic conditions, would be
highly desirable.
Figure 1. Comparison of decarboxylative and nondecarboxy-lative
ester C−O hydrogenolysis processes.
for selective aliphatic ester Calkoxy−O bond cleavage under mild
conditions that avoids sacrificial decarboxylation processes.
Recently, our laboratory reported a thermodynamically
leveraged, mechanism-based tandem strategy for the selective
7
A common characteristic of metal-catalyzed ester Calkoxy−O
hydrogenolysis of diverse etheric C−O linkages. The reactions
are clean, selective, rapid, and mechanistically well-understood
from both experiment and theory. These results raise the
cleavages is the requirement that the alkoxy (R′) group be
2
2
either sp hybridized or π-conjugated (allylic or benzylic).
There are also reports of ester Calkoxy−O cleavage with
nonconjugated aliphatic substituents, but these typically involve
forcing decarboxylation conditions (250−500 °C) using noble
Received: May 6, 2015
Revised: May 14, 2015
3
4
5
metal/group 10 catalysts such as Pt, Pd, or Ni, or high
©
XXXX American Chemical Society
3675
DOI: 10.1021/acscatal.5b00950
ACS Catal. 2015, 5, 3675−3679

ACS Catalysis
Letter
question of whether this approach might be generalizable to
other biofeedstock oxygenates and what might constitute the
substrate 1a (entries 17−20, Table 1). To quantify product
8
yields, Hf(OTf) + Pd/C was run in neat 1a at 125 °C for 2 h
4
1
mechanistic constraints on those types of processes. Here we
report a new transformation utilizing a tandem metal triflate
Lewis acid + a heterogeneous Pd catalytic hydrogenation
system that mediates rapid, selective, and clean ester C−O
hydrogenolysis to the corresponding carboxylic acids and
hydrocarbons (Figure 1, Table 1).
in a sealed glass vessel, before cooling in an ice bath. H NMR
spectroscopic analysis versus internal standard gives yields of
95% and 97% for cyclohexane and AcOH, respectively (with
trace benzene; see Table S4 for AcOH yields).
The reaction mechanism was first probed via kinetic analysis,
which shows that the CH C(O)O−cyclohexyl cleavage reaction
3
is zero-order in substrate concentration, zero-order in H
2
pressure, and first-order in Lewis acid concentration (Figures
S1−S5), yielding the rate law shown in eq 1. The
Table 1. Screening of Tandem Ester C−O Hydrogenolysis
a
Catalysts and Conditions
0
0
1
υ = k[substrate] [H ] [Lewis acid]
(1)
2
zero-order in substrate kinetics indicates that, under the
conditions studied (0.5 mol % Lewis acid), the homogeneous
metal triflate catalyst is operating under saturation kinetics. The
RC(O)O−R′ cleavage event is reasonably turnover-limiting in
this tandem scenario, as supported by the first-order depend-
c
d
acid
hydrog. cat.
solvent
conv. (%)
ρ
e
e
e
e
1
2
3
4
-
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
5% Pd/SiO2
5% Pd/TiO2
5% Pd/Al O
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
THF
DMF
MeOH
N.R.
N.R.
N.R.
N.R.
5
-
ence on metal triflate and the zero-order dependence on H2
La(OTf)3
Yb(OTf)3
Sc(OTf)3
Ce(OTf)4
Fe(OTf)3
Al(OTf)3
Zr(OTf)4
Hf(OTf)4
HOTf
2.60
2.81
3.23
3.44
3.71
3.87
4.29
4.37
-
pressure. The overall reaction yields activation parameters of
‡
−1
‡
ΔH = 25(2) kcal mol and ΔS = −8(1) e.u., in an Eyring
‡
analysis (Table S1). The slightly negative ΔS indicates that the
5
transition state is to some degree organized and polar, as might
be anticipated from a Lewis acid-mediated charge separation
event stabilized by secondary substrate molecules.
6
7
8
9
33
56
56
The extent to which the RC(O)O−R′ bond cleavage
89
11
transition state involves some degree of charge separation
1
0
20
e
was further probed via acyl (R) substituent effects (Table 2).
1
1
HOAc
N.R.
85
-
1
2
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
Hf(OTf)4
4.37
4.37
4.37
4.37
4.37
4.37
4.37
4.37
4.37
1
1
1
1
3
4
5
6
74
Table 2. Influence of Carboxylate Substitution on RC(O)O−
a
31
R′ Hydrogenolysis Activity
2
3
5% Pd/BaSO4
15
5% Pt/C
10% Pd/C
10% Pd/C
10% Pd/C
10% Pd/C
52
e
1
7
8
9
0
N.R.
N.R.
N.R.
N.R.
e
e
e
1
1
2
b
R−
conv. (%)
Hf(OTf)4
H O
2
1
2
3
4
CH −
89
60
42
33
100
83
93
61
44
3
a
All reactions performed with 0.5 mol % acid catalyst, 0.2 mol % metal
hydrogenation catalyst, under 1 bar of H in neat substrate at 125 °C
for 1 h unless otherwise noted. Volatile cyclohexane observed (m/z
4) by gas-phase MS. DFT-computed free energy. NMR conversion
vs mesitylene internal standard. N.R. = no reaction. ρ = DFT-
computed effective metal ion charge density. N.R. entries run for 4 h.
CH CH −
3
2
2
(CH ) CH-
3 2
(CH ) C−
3
3
b
c
8
c
5
CF −
3
d
c,d
6
CF −
e
3
7
8
9
ClCH −
2
Cl CH −
2
2
As shown in Table 1, DFT calculations indicate that both C−
O bond cleavage and hydrogenation steps are thermodynami-
cally favorable for the representative ester cyclohexyl acetate,
Cl C−
3
a
All reactions performed with 0.5 mol % Hf(OTf) , 0.2 mol % Pd/C
4
(
1
10% metal loading), under 1 bar of H in neat substrate at 125 °C for
2
b
1
a. The elimination of acetic acid via C−O bond cleavage is
h unless otherwise noted. NMR conversion vs mesitylene internal
c
computed to proceed with ΔG° = −6.0 kcal/mol, while
cyclohexene hydrogenation proceeds with ΔG° = −18.4 kcal/
standard. 1,1,2,2-tetra-chloroethane used as internal standard to avoid
Friedel−Crafts reaction with mesitylene. Reaction time = 10 min.
d
9
mol. Next, the effects of Lewis acid, hydrogenation catalyst,
and solvent were investigated for the same model reaction,
which cleanly gives acetic acid and cyclohexane as the only
spectroscopically observable products (along with cyclohexene
Using cyclohexyl acetate as a benchmark, introducing
successive methyl groups at the acetate β-carbon atom
progressively depresses activity (entries 1−4, Table 2),
suggesting that carboxylate steric bulk hinders carbonyl group
activation by the metal triflate. Electronic effects were examined
by substituting the acyl hydrogens with fluorine or chlorine
moieties. Cyclohexyl trifluoroacetate is found to be far more
reactive (entries 5 and 6, Table 2), while the first chlorine
substitution slightly accelerates the cleavage rate (entry 7, Table
2) and successive chlorine additions slow the reaction rate
(entries 7−9, Table 2), most likely due to increasing steric
hindrance between the ester and triflate ligands on the Hf
7a,b,10
as an intermediate). Similar to ether hydrogenolysis,
Lewis
acid metal ions with high effective DFT-computed charge
density (ρ) are most active, among which Hf(OTf) exhibits
4
the highest catalytic activity (entries 2−9, Table 1). Brønsted
acids are not as effective (entries 10 and 11, Table 1). Pd/C
and Pd/SiO2 are similarly active as the hydrogenation
component, outperforming other supported Pd catalysts
(
entries 12−16, Table 1). Coordinating solvents such as
water and THF suppress the activity in comparison to the neat
3
676
DOI: 10.1021/acscatal.5b00950
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ACS Catalysis
Letter
center. Taken together, these results indicate that less basic, and
acid elimination to alkene, which subsequently undergoes
hydrogenation (entry 2, Table 3). Secondary esters are more
readily cleaved to the desired alkane + acetic acid (entry 3,
Table 3), while tertiary esters are the most reactive substrates
(entry 4, Table 3). Although 1-adamantyl acetate undergoes
RC(O)O−R′ hydrogenolysis to yield adamantane (entry 6,
Table 3), 2-adamantyl acetate only undergoes slight hydrolysis/
−
nonsterically hindered carboxylates (e.g., CF COO ) react
3
most rapidly in this transformation.
The present catalytic hydrogenolysis is far more sensitive to
alkoxy (R′) group identity (Table 3) than to that of the
Table 3. Alkoxy Group R′ Effects on Catalytic Ester
a
Hydrogenolysis Activity
condensation to yield trace amounts of (2-adamantyl) O (entry
2
5
, Table 3), presumably reflecting the inhibitory strain of olefin
formation. We do not believe carbonyl hydrogenation to the
alcohol is occurring, as entries 5 and 6 would then show similar
reactivity. Regardless of the substitution order, benzylic and
allylic esters are generally the most reactive. Devoid of β-H
atoms, benzyl acetate reacts rapidly to afford oligomers (entry
7
, Table 3). Esters with β-H atoms that facilitate styrene
formation yield the desired hydrogenolysis products with
higher efficiency, even at 25 °C (entries 8−9, Table 3). Finally,
allyl esters are extremely reactive, and rapid cleavage ensues at
2
5 °C (entries 10−11, Table 3). These results suggest that ester
RC(O)O−R′ hydrogenolysis activity tracks the stability of the
corresponding R′ carbocations: tertiary > secondary ≫
primary. The yields in Table 3 indicate that ester hydro-
genolysis, where feasible, is clean, with the majority of mass
balances >90%.
Selectivity was further probed using 1,2-bis-acetoxy-2-
phenylethane as a substrate (Table 3, entry 12), where the 2°
benzylic ester cleaves preferentially over 1° cleavage. No 2-
phenylethanol acetate is observed, indicating the 2° ester
position is more reactive, and that reactivity trends for esters on
the same molecule follow those of the individual esters. The
conversion is low at room temperature, which may reflect
chelation of Hf(OTf) by both ester functionalities.
4
Applying this catalytic protocol to the biomass relevant
triglyceride Tricaprylin (n-C H alkyl chains) at 200 °C/2 h
7
15
achieves up to 96% conversion to C -based hydrocarbons,
3
along with related 1,2, 1,3, and 1-oxygenates, as well as the n-
C H C(O)OH fatty acid in high yields (see Table S5). C
7
15
3
product selectivity is highly dependent on conversion and
conditions. Using less acidic M(OTf) (M = Al, Ce) affords
n
higher 1,2 and 1,3-oxygenate selectivity (up to 27 and 11%,
respectively, Table S7), while Hf(OTf) favors propane and the
4
1
-oxygenate (up to 72 and 22%, respectively, Table S7).
Addition of MeOH to the reaction mixture affords the
corresponding methyl ester (biodiesel) from n-C H C(O)OH
7
15
in quantitative yield.
To provide additional atomistic understanding of the
pathway for Hf(OTf) -mediated cyclohexyl acetate hydro-
4
a
genolysis, a detailed solution phase enthalpic profile was
computed by DFT techniques (Figure 2, see Supporting
Information). This profile includes the structures of all
intermediates (A to G) along the reaction coordinate. All
All reactions performed with 0.5 mol % Hf(OTf) , 0.2 mol % Pd/C
4
(
10% metal loading), and 1 bar of H2 in neat substrate at the
temperature and for the time indicated. N.D. = not determined. Yields
of products are shown in parentheses. Yields of AcOH are reported in
Table S4. AcOH (96% yield) and trace product detected in the liquid
phase. Gas-phase MS showed both 2-methylbutane and 2-methyl-
butene.
b
energies are with respect to that of isolated Hf(OTf) and the
4
substrate, denoted A (0.0 kcal/mol). From Figure 2 it can be
seen that the most favorable catalyst−cyclohexyl acetate
binding to occurs via the ester carbonyl group, and that the
A → B process is exothermic by 23.4 kcal/molmore so than
binding via the etheric oxygen, which (not shown) is
exothermic by ∼12 kcal/mol. RC(O)O−R′ bond cleavage
occurs via a transition state C (−8.1 kcal/mol) that requires an
intrinsic barrier of 16.3 kcal/mol from B. The cleavage of the
RC(O)O−R′ bond results in the formation of an intermediate
D (−14.3 kcal/mol) containing a carboxylate ion coordinated
in a bidentate fashion to the Hf(IV) ion, while the cyclohexyl
carboxylate group, likely reflecting the importance of stabilizing
a carbocationic intermediate and/or partially charge-separated
transition state. Under 1 bar of H and 125 °C, the primary
ester n-octyl acetate does not readily undergo RC(O)O−R′
hydrogenolysis. Instead, partial hydrolysis of the ester to acid
and alcohol, and subsequent dehydration of the alcohol to ether
2
(
making the process catalytic in water) is observed (entry 1,
Table 3). However, at 200 °C, n-octyl acetate undergoes acetic
3
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DOI: 10.1021/acscatal.5b00950
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ACS Catalysis
Letter
Figure 2. Calculated enthalpy profile (energies in kcal/mol) and rate-determining transition state for RC(O)O−R′ bond cleavage by Hf(OTf)
along with experimental and calculated KIE’s.
4
cation is weakly stabilized via interaction with one of the triflate
oxygens. Formation of cyclohexene and acetic acid (coordi-
nated to Hf) occurs via transition state E (1.2 kcal/mol) with
proton transfer from the cyclohexyl cationic moiety to the
carboxylate anion. This process (D → F) via E is computed to
be highest point in the enthalpy and free energy profiles (Table
S7) and is likely the rate-controlling step of the reaction
sequence. The calculated kinetic isotope effect (KIE) for this
step is 6.3, in good agreement with the experimental KIE of 6.5
AUTHOR INFORMATION
■
Corresponding Author
E-mail: t-marks@northwestern.edu.
Present Address
Z.L.) ShanghaiTech University, 100 Haike Road, Pudong
District, Shanghai 201210, China.
*
∥
(
Author Contributions
These authors contributed equally (T.L.L. and Z.L.).
‡
(
0.5). The computed apparent enthalpy of activation using an
Notes
energy span approach is 24.6 kcal/mol (ΔH(E-B)), in good
accord with the experimental value of 25(2) kcal/mol.
The authors declare no competing financial interest.
In summary, hydrogenolytic ester C−O cleavage catalyzed by
a tandem homogeneous metal triflate/supported Pd system
exhibits a broad scope, including secondary and tertiary esters
ACKNOWLEDGMENTS
This work was supported by the U.S. Department of Energy
under contract DE-AC0206CH11357. NSF grant CHE-
■
(
≤125 °C) and primary esters (200 °C) at 1 bar of H , with the
2
1
213235 on basic f-element chemistry supported T.L.L. and
resulting products being alkanes and carboxylic acids. This
result represents an atom economical, anhydrous, and most
importantly, selective (allylic ≥ benzylic >3° > 2° ≫ 1°)
approach to cleaving aliphatic ester RC(O)O−R′ functionalities
at lower temperatures and pressures than traditional decarbox-
ylative processes. The present tandem catalytic system also
produces biodiesel from triglycerides in high yield without
coproduction of undesired glycerol. Instead, the C3 backbone
is preserved and converted to hydrocarbons and/or oxygenates.
Further mechanistic and computational analysis of this catalytic
ester transformation will be reported in a future manuscript,
along with a full account of triglyceride transformations.
provided reactor equipment. 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 Sciences,
and Office of Basic Energy Sciences, which supported Z.L.,
R.S.A., and L.A.C. The Pd/TiO catalyst was provided by Mr.
2
M. S. Liu. We gratefully acknowledge the computing resources
provided on “Blues”, a 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,
and Office of Basic Energy Sciences, under Contract No. DE-
AC02-06CH11357. This research used resources of the
National Energy Research Scientific Computing Center
12
ASSOCIATED CONTENT
■
(
NERSC), which is supported by the Office of Science of the
U.S. Department of Energy under Contract No. DE-AC02-
5CH11231.
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b00950.
0
Experimental details, substrate synthesis, characteriza-
tion, kinetic plots for determining rate law, Eyring
analysis plots, KIE experiments, error analysis, and
computational details (PDF)
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