Organic Oxidations Using Geomimicry

Article abstract of DOI:10.1021/acs.joc.5b02109

Oxidations of phenylacetic acid to benzaldehyde, benzyl alcohol to benzaldehyde, and benzaldehyde to benzoic acid have been observed, in water as the solvent and using only copper(II) chloride as the oxidant. The reactions are performed at 250°C and 40 bar, conditions that mimic hydrothermal reactions that are geochemically relevant. Speciation calculations show that the oxidizing agent is not freely solvated copper(II) ions, but complexes of copper(II) with chloride and carboxylate anions. Measurements of the reaction stoichiometries and also of substituent effects on reactivity allow plausible mechanisms to be proposed. These oxidation reactions are relevant to green chemistry in that they proceed in high chemical yield in water as the solvent and avoid the use of toxic heavy metal oxidizing reagents.

Full text of DOI:10.1021/acs.joc.5b02109

Article
pubs.acs.org/joc
Organic Oxidations Using Geomimicry
Ziming Yang,† Hilairy E. Hartnett,*,†,‡ Everett L. Shock,*,†,‡ and Ian R. Gould*,†
†The School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
‡School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, United States
S
* Supporting Information
ABSTRACT: Oxidations of phenylacetic acid to benzaldehyde, benzyl alcohol to
benzaldehyde, and benzaldehyde to benzoic acid have been observed, in water as
the solvent and using only copper(II) chloride as the oxidant. The reactions are
performed at 250 °C and 40 bar, conditions that mimic hydrothermal reactions
that are geochemically relevant. Speciation calculations show that the oxidizing
agent is not freely solvated copper(II) ions, but complexes of copper(II) with
chloride and carboxylate anions. Measurements of the reaction stoichiometries
and also of substituent effects on reactivity allow plausible mechanisms to be
proposed. These oxidation reactions are relevant to green chemistry in that they
proceed in high chemical yield in water as the solvent and avoid the use of toxic
heavy metal oxidizing reagents.
solvated, and formation of complexes between simple cations
and anions is common under hydrothermal conditions.31
Speciation calculations relevant to geochemical conditions
show that the oxidizing species in these oxidation reactions is
not freely solvated copper(II) ions, but that the reactions
involve copper(II) complexes.
INTRODUCTION
■
Hydrothermal organic reactions under geochemically relevant
conditions have attracted attention for possible green chemistry
applications.1 The primary reasons are that water is the
solvent1−4 and that many functional group transformations can
be accomplished without the need for additional reagents.1−5
Most hydrothermal organic reactions in geochemically relevant
systems, however, take place in the presence of minerals and
other inorganic species.6−12 Organic/inorganic hydrothermal
reactions have been less well explored for green chemistry
applications. As part of our investigations of mineral effects on
hydrothermal organic reactivity,13 we have discovered some
simple organic oxidation reactions that use copper in oxidation
state (II). In the context of classical organic chemistry,
oxidations generally use heavy, often toxic, or expensive
metals,14−17 whereas enzymatic and also geologic redox
reactions use more Earth-abundant metals such as iron, nickel,
zinc, and copper.18−21 Copper(I) and copper(II) are mild
oxidizing agents for organic chemistry; nevertheless, copper
salts have previously found use in several organic oxidations.22
In particular, copper salts are useful catalysts for organic
oxidations where oxygen (O2) is the oxidant,16,22−24 including
some hydrothermal reactions.25 Cu(II) has been used as an
oxidant in the formation of copper(I) coordination polymers
under hydrothermal conditions,26 but there are very few reports
of Cu(II) ions as a simple oxidizing reagent for organic
reactions, especially in the absence of molecular oxygen.27,28
Here we report a series of simple organic oxidation reactions
that use copper(II) chloride as the oxidizing agent in water as
the solvent, in the absence of oxygen and at a temperature that
mimics geochemically relevant conditions. Compared to
ambient, water at temperatures above 100 °C has a lower
dielectric constant and a higher dissociation constant Kw.29,30
Consequently, organic compounds are much more soluble as
compared to ambient conditions, but ions tend to be less
RESULTS AND DISCUSSION
■
Oxidation Reactions. The oxidation reaction sequence is
summarized in Scheme 1. Conversion of phenylacetic acid (I)
and benzyl alcohol (II) to benzaldehyde (III), and also
benzaldehyde (III) to benzoic acid (IV), can be readily
accomplished under the present conditions with high chemical
yield. The reactions are performed in sealed glass tubes in pure
water at 250 °C in the absence of oxygen, and with cupric
chloride (CuCl2) as the only additive. The reaction pressure, ca.
40 bar, is determined by the water saturation vapor pressure at
250 °C.29 At the end of the reaction, the products are simply
extracted into an organic solvent with no additional workup.
The oxidizing reagent is indicated as Cu(II) in Scheme 1, but
this does not mean that freely solvated Cu(II) ions are involved
in the reactions.
Reactant Speciation Under Hydrothermal Conditions.
As mentioned above, water at the higher temperatures and
pressures that characterize geochemically relevant conditions
has a much lower dielectric constant than at ambient. The
experimental temperature and pressure are 250 °C and ca. 40
bar, respectively, and under these conditions the dielectric
constant is ca. 27.1,30 causing ionic solutes to speciate
compared to ambient.31 An accurate description of speciation
of inorganic and organic species in high temperature and high
Received: September 8, 2015
© XXXX American Chemical Society
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Scheme 1. Oxidation Reaction Sequence Linking Phenylacetic Acid (I) via Benzyl Alcohol (II) and Benzaldehyde (III) to
a
Benzoic Acid (IV)
a
Six equivalents of Cu(II) are consumed going from (I) to (IV); the six proposed oxidation steps are shown in red. The oxidizing species at each
step is not freely solvated Cu(II) ions; see text.
pressure water is critical to the understanding of many
geochemical processes, and geochemists have developed
thermodynamic tools that can predict the speciation of many
inorganic and organic solutes under hydrothermal conditions.32
The results of such thermodynamic calculations for water at
250 °C and 40 bar, with 0.2 molal CuCl2 and 0.1 molal acetic
acid as solutes, are shown in Figures 1 and 2. Acetic acid is used
Figure 2. Log molal concentrations of acetic acid, acetate and copper-
acetate complexes at equilibrium for an aqueous solution of 0.2 molal
CuCl2 and 0.1 molal acetic acid, at 250 °C and 40 bar at equilibrium,
as a function of solution oxidation state, defined in terms of oxygen
fugacity. The black curves correspond to acetic acid and acetate, the
abbreviation Ac stands for acetate, the blue curves correspond to
aqueous Cu(II)-acetate complexes and the red curves to aqueous
Cu(I)-acetate complexes.
Figure 1. Distribution of the major aqueous copper species (expressed
as a percent of the total copper in solution) as a function of solution
The dominant aqueous forms of Cu(II) and Cu(I) as
oxidation state, defined in terms of oxygen fugacity, for 0.2 molal
functions of solution oxidation state at equilibrium are
CuCl2 in the presence of 0.1 molal acetic acid in water at 250 °C and
summarized in Figure 1. Chloride complexes dominate the
copper speciation under all oxidation conditions; the
predominant form of Cu(II) in solution is the CuCl+ cation
40 bar at equilibrium. The blue curves correspond to aqueous forms of
Cu(II) and the red curves to aqueous forms of Cu(I). At relatively
oxidizing conditions the monocation Cu(II)Cl+ dominates, and at
relatively reducing conditions (more negative values of log f O2) the
−
while the predominant form of Cu(I) is the CuCl2 anion.
−
monoanion Cu(I)Cl2 dominates. Copper-acetate complexes also
Neutral complexes rarely account for more than 20% of the
copper in solution, and the uncomplexed ions are minor
components in the copper speciation. Both oxidation states of
copper are present at percent levels over the entire f O2 range
considered, and the transition from dominantly oxidized to
dominantly reduced occurs around log f O2 = −20. The strong
chloride complexation of copper suggests that uncomplexed
Cu(II) ions are unlikely to be the oxidizing species that take
part in the reactions. Together with the known temperature
dependence of Kw, these complexes partially account for the pH
of the solution being considerably lower than at ambient,
Figure 1. The pH decreases further as the solution becomes
more reducing, as shown in the inset of Figure 1.
form but at concentrations too low to be observed in this figure (see
Figure 2). The inset shows the calculated variation in solution pH with
oxidation state, showing lower pH in the more reduced solutions.
in place of phenylacetic acid because the relevant thermody-
namic data are not available for the aromatic acid, and acetic
acid should be a good model for the carboxylic acid and
carboxylate functional groups under these conditions. The
calculations were performed by combining mass action, mass
balance, and charge balance reactions for Cu(I) and Cu(II)
hydroxide, chloride, and acetate complexes, together with
activity coefficients calculated with an extended Debye−Huckel
The calculated speciation of acetic acid and copper−acetate
complexes is shown in Figure 2, which plots the log molality of
aqueous species vs log f O2. Note that copper−acetate
complexes form at much lower concentrations (generally
̈
equation, in a geochemical speciation code33 using equilibrium
constants evaluated with the revised Helgeson−Kirkham−
Flowers equations of state.31,34
B
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Scheme 2. Proposed Mechanism for Oxidative Decarboxylation of Phenylacetic Acid to Benzyl Alcohol in Water at 250°C and
a
in the Presence of CuCl2
a
The Cu(II) and Cu(I) species are almost certainly not present as the uncomplexed cations, see text.
<10−6 molal) relative to copper−chloride complexes which are
present at the percent level (>10−2 molal) (see Figure 1). The
associated form of acetic acid (CH3CO2−H) predominates under
all conditions, but acetate (CH3CO2 ) becomes somewhat
more abundant at high values of f O2 (see further below). The
Cu(Ac)+ complex is calculated to be as abundant or slightly
more abundant than the free acetate ion, which suggests the
aqueous species involved in the first step of the reaction
mechanism could be the metal−organic complex rather than
the anionic form of the acid. Hydroxide complexes of copper
are also formed according to the calculations, at concentrations
similar to those of the acetate complexes.
Phenylacetic Acid Oxidation Mechanism. At 250 °C in
water in the absence of CuCl2, phenylacetic acid (I) undergoes
very slow decarboxylation (<5% conversion in 6 h) to yield
toluene.35 In the presence of 0.20 molal CuCl2, however,
reaction is much faster so that conversion is essentially
complete after 2 h. The product is now benzaldehyde, formed
in >90% chemical yield. Experiments were performed in the
presence of 0.20 molal CuCl2 to determine the number of
moles of Cu(II) consumed. The decrease in Cu(II) ion
concentration was measured spectrophotometrically at 700 nm
and was compared to the number of moles of acid reacted,
determined gas chromatographically (see Experimental Sec-
tion). The measured consumed mole ratio (Cu(II)/acid)
ranged from 3.6 to 3.9 depending upon reaction time and
approached 4.0 at high conversion, suggesting that 4
equivalents of Cu(II) are required to convert phenylacetic
acid (I) into benzaldehyde (III). A white insoluble solid is
observed to form at high conversions, consistent with
formation of an inorganic copper product.
Increased conversion of phenylacetic acid was observed with
increasing Cu(II) concentration, suggesting that Cu(II)
reduction is involved in the rate-determining step. Benzyl
alcohol (II) also reacts with Cu(II) to give benzaldehyde (see
below), suggesting that II is on the pathway from I to III.
Reaction of sodium phenylacetate with 0.20 molal CuCl2
resulted in higher conversion (85% conversion after 0.17 h)
compared to the acid (68% after 0.17 h), which further suggests
participation of a deprotonated form of the acid in the reaction,
but not necessarily the freely solvated phenylacetate ion. The
reaction is reminiscent of the known decarboxylations of
benzoic and other carboxylic acids that are catalyzed by
copper(I)36 and copper(II) salts,37,38 and, in particular,
oxidative decarboxylations of carboxylic acids where copper(II)
is the oxidizing reagent.39−41 Based on the previously proposed
mechanisms of copper(II) decarboxylations and also the
speciation calculations described above, a plausible mechanism
for the phenylacetic acid reaction can be proposed, Scheme 2.
Consistent with both the mechanisms suggested for
copper(II) catalyzed decarboxylation of benzoic acids36,37 and
the results of the speciation studies described above, formation
of a complex between a copper(II) cation and the carboxylate is
proposed as the primary intermediate. Decarboxylation of this
phenylacetate/copper(II) complex forms a reduced copper ion,
which will presumably exist primarily as the CuCl2 anion
−
under the reaction conditions (Figure 1), and the benzyl
radical. Whether these products are formed in a concerted
process as indicated by the curved arrows in Scheme 2 cannot
be determined from the experimental results, but the curved
arrows clarify the origin of the copper(II) to copper(I)
reduction process and the formation of the benzyl radical.
The next step, single electron transfer oxidation of the benzyl
radical by copper(II), is already known to occur rapidly at
ambient in water and has been described in the literature.42 In
addition, independent photochemical generation of benzyl
radicals under hydrothermal reaction conditions in the presence
of Cu(II) has also previously been shown to result in oxidation
of benzyl radicals, with benzyl alcohol and benzaldehyde as the
products of this reaction.43 Thus, both uncomplexed copper(II)
ions at ambient conditions and copper(II) chloride complexes
formed under hydrothermal conditions are capable of oxidizing
benzyl radicals. The product of one-electron oxidation of the
benzyl radical is a benzyl cation, which is known to rapidly add
a water molecule44 and form benzyl alcohol (II) upon
deprotonation.
The mechanism is consistent with ring substituent effects.
Reaction for 0.17 h of 0.20 molal Cu(II) with 0.05 molal
phenylacetic acids that are substituted in the para-position with
−CH3, −t-Bu, and −F substituents gave conversions of 67%,
68%, and 66%, respectively. These can be compared to 68%
conversion for the unsubstituted acid under the same
conditions. Thus, there is no significant influence on the
reaction rate from an electron-donating or -withdrawing
substituent on the benzene ring. This is consistent with a
rate-determining step that involves formation and fragmenta-
tion of a phenylacetate/copper complex via a radical process,
Scheme 2, i.e. one that does not develop a charge on the ring.
The phenylacetic acid oxidation could not be stopped at benzyl
alcohol even with 2 equivalents of copper(II) because the
alcohol oxidation was sufficiently fast that benzaldehyde was the
major product at all reaction times.
Benzyl Alcohol Oxidation Mechanism. According to
Scheme 1, benzyl alcohol (II) is on the reaction pathway from
phenylacetic acid to benzaldehyde; therefore, II should also be
oxidized to benzaldehyde with 2 equivalents of Cu(II). This
was confirmed by experiment. Reaction of 0.05 molal benzyl
alcohol with 0.10 molal Cu(II) for 1 h resulted in 65%
conversion to benzaldehyde with a chemical yield of 92%.
Under these conditions, >90% of the product is benzaldehyde;
benzoic acid dominated the other products. Experiments were
performed to determine the stoichiometry of the alcohol
oxidation reaction. Reaction of 0.05 molal benzyl alcohol with
0.20 molal CuCl2 resulted in 43% conversion of the alcohol in
0.5 h. As before, the quantity of Cu(II) consumed was
measured spectrophotometrically and compared to the quantity
of alcohol consumed measured by gas chromatography. The
molar ratio of consumed Cu(II)/alcohol was 1.8−2.4 depend-
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Scheme 3. Proposed Mechanism for Oxidation of Benzyl Alcohol Acid to Benzaldehyde in Water at 250°C and in the Presence
a
of CuCl2
a
The Cu(II) and Cu(I) species are almost certainly not present as uncomplexed cations, see text.
Scheme 4. Proposed Mechanism for Oxidation of Benzaldehyde to Benzoic Acid in Water at 250°C and in the Presence of
a
CuCl2
a
The first step involves hydration followed by one-electron oxidation, path A, or one-electron oxidation followed by hydration, path B. The Cu(II)
and Cu(I) species are almost certainly not present as uncomplexed cations; see text.
ing upon reaction time. From this we conclude that 2
equivalents of Cu(II) are indeed required to oxidize II to III.
In contrast to the reactions of the substituted phenylacetic
acids, ring substitution of the benzyl alcohols resulted in large
changes in reaction rate. Reaction for 30 min of 0.10 molal
Cu(II) with benzyl alcohols substituted in the para-position
with −OCH3, −CH3, and −CF3 substituents resulted in
conversions of 99%, 78%, and 17%, respectively, compared to
29% conversion for the unsubstituted alcohol under the same
conditions. These observations strongly support formation of a
positive charge on the benzene ring in the rate-determining
step, Scheme 3. Benzyl alcohol does not have strongly Lewis
basic electrons equivalent to those formally associated with the
carboxylate anion, and so there is no reason to propose the
intermediacy of a copper/alcohol complex in this case. One-
electron oxidation of benzyl alcohol to form an aromatic radical
cation as the rate-determining step would be consistent with
the observed substituent effects. One-electron oxidation is
commonly observed in organic oxidations by metal cati-
ons,14−16,45 and even though the Cu(II)Cl+ cation would be
expected to be a weaker one-electron acceptor than the
uncomplexed Cu(II)2+ dication, endothermic electron transfer
will be facilitated by the thermal energy associated with the
reaction conditions. Although confirmation of this one-electron
transfer mechanism would require kinetic analysis of the driving
force for electron transfer,46 in the absence of accurate redox
potentials for the benzyl alcohols and the Cu(II) complexes
under the experimental conditions the mechanism can only be
tentatively assigned to single electron transfer on the basis of
the observed substituent effects. The aromatic radical cations
formed upon one-electron oxidation that have benzylic
hydrogens are known to be very strong Brønsted acids,47 and
rapid deprotonation will generate another benzylic radical,
which upon further one-electron oxidation by Cu(II), as
discussed above, forms protonated benzaldehyde that upon
deprotonation generates III, Scheme 3.
the reaction selectivity is high. The chemical yield of benzoic
acid based on aldehyde consumption is over 90%. In an
experiment in which 0.05 molal benzaldehyde was reacted in
the presence of 0.20 molal CuCl2 for 6 h, the measured molar
ratio of consumed Cu(II)/benzaldehyde was found to be 2.1−
2.4. From this we conclude that 2 equivalents of Cu(II) are also
required to oxidize benzaldehyde to benzoic acid. Oxidation of
benzaldehyde is considerably slower than oxidation of benzyl
alcohol and also requires an oxygen atom, which presumably
comes from the water. In conventional aqueous oxidations of
aldehydes to carboxylic acids, water first adds to the carbonyl to
form the hydrate, which is subsequently oxidized (Scheme 4,
pathway A).48 Under hydrothermal conditions, however,
dehydration reactions (loss of H2O) tend to dominate over
hydration (addition of H2O),49 which suggests that an alternate
mechanism should be considered, i.e., oxidation followed by
H2O addition (Scheme 4, pathway B). Reaction for 6 h of 0.20
molal Cu(II) with benzaldehydes substituted in the para-
position with −CH3 and −CF3 substituents resulted in
conversions of 45% and 31%, respectively, compared to 39%
for the unsubstituted aldehyde. Faster reaction with the
donating −CH3 substituent and slower reaction with the
−CF3 substituent suggest development of a positive charge on
the benzene ring in the rate-determining step. However, these
substituent effects are much smaller than those observed in the
alcohol oxidations. In addition, interpretation of the substituent
effects is complicated in this case because the equilibrium
constants for hydrate formation are also substituent dependent.
Electron-donating groups lower the equilibrium constant for
hydrate formation,50 which means that the substituent effects
on oxidation and hydrate formation are opposing.51 The steps
that contribute to determining the rate of reaction via pathway
A are likely to be (reversible) hydrate formation, and the one-
electron oxidation reaction. Hydrate formation is unlikely to
contribute to determining the reaction rate in pathway B, since
radical cations tend to be much stronger electrophiles than
their corresponding neutral structures,51 and H2O addition to
the radical cation is thus likely to be much faster than oxidation
of the neutral. This suggests that pathway A in Scheme 4 is
more likely than pathway B; however, we cannot distinguish
Benzaldehyde Oxidation Mechanism. Benzaldehyde
(III) can also be oxidized to benzoic acid (IV) under the
reaction conditions. Reaction of 0.05 molal benzaldehyde with
0.10 molal Cu(II) at 250 °C for 2 h results in ca. 14%
conversion, with benzoic acid (IV) as the major product. Again,
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benzylalcohol (98%), p-methylbenzylalcohol (98%), p-methoxybenzyl-
alcohol (98%), hydrochloric acid, sodium hydroxide (98%), cupric
chloride (97%), dichloromethane (99.9%), and n-decane (99%). Fresh
solutions of cupric chloride with concentrations ranging from 0.05 to
0.2 molal were prepared using high-purity water (18.2 MΩ·cm
resistivity) obtained from a commercial water purification system.
Sodium phenylacetate was prepared by neutralizing phenylacetic acid
with sodium hydroxide. The purities of all organic materials (except
phenyl acetate) were verified by gas chromatography.
these two pathways with certainty using the current
experimental data.
Hydrothermal Reaction Conditions. Previous work on
green organic oxidations has emphasized the importance of
eliminating halogenated solvents and toxic reagents such as
Cr(VI).52 Here, the reactions are performed in water as the
solvent using the relatively benign Cu(II). Interestingly,
conditions can be found such that the major product of the
benzyl alcohol oxidation reaction is benzaldehyde. This is
interesting because most oxidations of aldehydes in aqueous
media form the corresponding carboxylic acid (the aldehyde
cannot be isolated) due to oxidation of the hydrate that forms
from the aldehyde primary product.48 Even alcohols with lower
propensities for hydrate formation, such as benzyl alcohol, are
normally oxidized in nonaqueous solvents for this reason.53 At
high temperatures, the enthalpic drive to form the hydrate due
to covalent bond formation is offset by the entropic drive for
dehydration. This diminished propensity for hydrate formation
under hydrothermal conditions favors formation of the
aldehyde as the major product even though water is the
solvent. Of course, with additional copper(II) reagent and
longer reaction times the reaction can be pushed to the acid as
the major product, and it has been shown elsewhere that
copper salts can catalyze decarboxylation of benzoic acid with
extended reaction times.16 Nevertheless, the fact that benzyl
alcohol oxidation can be stopped at the aldehyde stage
highlights the fact that the hydrothermal reaction conditions
are important not just to provide thermal energy. As mentioned
above, at elevated temperatures the dielectric constant for water
is decreased making it an excellent solvent for organics and the
Kw is increased, which results in higher concentrations of
hydronium and hydroxide ions to catalyze the proton transfer
reactions of Scheme 1. The water properties at high
temperatures also significantly influence the speciation of the
ionic reactants, which may also be important in controlling the
selectivity of the reactions. Further characterization of the
Cu(II) complexes that are involved in the reactions studied
here will require real-time spectroscopic analysis under
hydrothermal conditions.
The vast majority of the organic material on Earth does not
participate in the more familiar, conventional surface carbon
cycle because it is located deep within the crust and therefore
undergoes chemical reactions under hydrothermal conditions,
such as those used in the oxidation reactions described here.54
In contrast to the majority of organic reactions close to
ambient, which tend to be controlled by enthalpic and kinetic
factors, reactions of this huge quantity of organic material under
hydrothermal geochemically relevant conditions tend to be
controlled by entropic and thermodynamic factors. Together
with the unique properties of water as a solvent under these
conditions mentioned above, this suggests that much new
useful organic chemistry may be uncovered via the develop-
ment of reactions that are inspired by reaction conditions
relevant to geology. To this end, the term “geomimicry” was
adopted,1−5 in analogy to the more established biomimicry.55
Methods. Hydrothermal experiments were performed in fused
silica glass tubes with a 6 mm inner diameter (ID) and a 12 mm outer
diameter (OD). The relatively large diameter tubes allow easy loading
of the solid starting materials and also guarantee an adequate quantity
of organic material for analysis after the experiments. The organic
starting material (0.10 mmol) and 2.0 mL of argon-purged cupric
chloride solution were loaded into each tube to obtain a molality of
0.05 molal for the organic species. The samples were frozen in liquid
nitrogen, degassed using three pump−freeze−thaw cycles on a small
vacuum line, and then sealed with a hydrogen flame under vacuum.
Each fused silica glass tube (∼15 cm long) was placed into a small steel
pipe (∼21 cm long, ID ≈ 2 cm) to contain the glass in the event of
tube failure. The pipe was closed with loose screw caps, and the screw
caps had a hole drilled through the end to prevent pressure from
building up inside the steep pipe. The glass-within-steel tube setup was
heated in a gas chromatography (GC) oven at 250 °C. At 250 °C the
pressure inside the silica glass tube is calculated to be ca. 40 bar based
on equilibrium thermodynamics calculations using SUPCRT92.32b
A
thermocouple inside the GC oven next to the sample tubes was used
to verify the reaction temperature. The estimated uncertainty in the
temperature of the sample was 2 °C over the duration of the
experiment. Approximately 10 min of preheating time were necessary
for the samples to reach the experimental temperature of 250 °C.
The experimental reaction times ranged from 10 min to 8 h at 250
°C and 40 bar, depending on the particular reaction. When the desired
reaction time was reached, the experiment was quenched by placing
the steel pipe into a room temperature water bath. The extraction
procedure was similar to that described in previous work.5a,43 After the
fused silica tube was opened using a tubing cutter, the sample was
transferred into a 20 mL glass vial, and the reaction tube was rinsed
twice with 10.0 mL of dichloromethane solution containing the
internal gas chromatography standard, n-decane (0.067% by volume).
The water and dichloromethane phases were combined in the 20 mL
vial. For the phenylacetic acid and the sodium phenylacetate samples,
two drops of 1 M aqueous HCl was added to the reaction tube after
opening to ensure protonation of any organic acid species and
extraction into the organic layer. The 20 mL vials were capped and
shaken vigorously to facilitate extraction of the organics into the
dichloromethane phase. The aqueous layer was separated into a small
centrifuge tube, and any suspended solid particles were separated by
centrifugation before the analysis of cupric ions in clear aqueous
solutions.
Quantitative analysis was performed for both the organic species in
the dichloromethane phase and the aqueous species that remained in
the water phase. The organic species were analyzed using gas
chromatography with a flame ionization detector. Reproducibility was
ensured by triplicate injections using an autosampler. The identities of
the organic products were verified by using authentic standards, and
their quantities were determined based on the calibration curves that
were referenced to the internal standard. Mass balance was calculated
by comparing the number of benzene rings in the products and the
reactants. The aqueous samples were analyzed quickly after
centrifugation, in order to minimize any air oxidation of Cu(I) to
Cu(II). The absorbance of Cu(II) ions in solution were determined at
700 nm (at room temperature) using a UV−visible spectropho-
tometer. The copper(II) concentrations were quantified using a
calibration curve built with authentic copper(II) standards ranging
from 0.01 to 0.20 molal. The 700 nm wavelength was selected to
ensure a measurable absorbance (i.e., greater than 0.1 and less than 1.0
absorbance units) over the range of copper(II) concentrations that
were studied. Measurement of copper(I) concentration in the aqueous
EXPERIMENTAL SECTION
■
Materials. The following materials were obtained from commercial
sources: phenylacetic acid (99%), p-fluorophenylacetic acid (98%), p-
methylphenylacetic acid (99%), p-tert-butylphenylacetic acid (98%), p-
trifluoromethyl-phenylacetic acid (97%), p-methoxyphenylacetic acid
(99%), benzaldehyde (99%), p-trifluoromethylbenzaldehyde (98%), p-
methylbenzaldehyde (97%), benzylalcohol (99.8%), p-trifluoromethyl-
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sample was not attempted since the solubility of copper(I) chloride is
low at room temperature.56 Triplicate measurements were taken to
ensure reproducibility of the copper(II) measurements using the
spectrophotometer. Based on repeated measurements, the uncertain-
ties in the reactant conversions are estimated to be 10% in all
experiments, the uncertainties in the chemical yields are estimated to
be 3%, and the estimated uncertainties in the molar ratios for
consumed reactants are 20%.
(13) Shipp, J. A.; Gould, I. R.; Shock, E. L.; Williams, L. B.; Hartnett,
H. E. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 11642−11645.
(14) Trahanovsky, W. S.; Wiberg, K. B. Oxidation in Organic
Chemistry, Vol. 5; Academic Press: New York, 1965.
(15) Ross, S. Oxidation Mechanisms. Application to Organic Chemistry;
Benjamin: New York, 1964.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: h.hartnett@asu.edu.
*E-mail: Everett.Shock@asu.edu.
*E-mail: igould@asu.edu.
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(23) Allen, S. E.; Walvoord, R. R.; Padilla-Salinas, R.; Kozlowski, M.
C. Chem. Rev. 2013, 113, 6234−6458.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The authors acknowledge helpful discussions with the members
of the Hydrothermal Organic Geochemistry (H.O.G.) group at
Arizona State University. We are thankful for National Science
Foundation Grant OCE-1357243 for the financial support.
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