Hydrothermal photochemistry as a mechanistic tool in organic geochemistry: The chemistry of dibenzyl ketone

Article abstract of DOI:10.1021/jo500899x

Hydrothermal organic transformations under geochemically relevant conditions can result in complex product mixtures that form via multiple reaction pathways. The hydrothermal decomposition reactions of the model ketone dibenzyl ketone form a mixture of reduction, dehydration, fragmentation, and coupling products that suggest simultaneous and competitive radical and ionic reaction pathways. Here we show how Norrish Type I photocleavage of dibenzyl ketone can be used to independently generate the benzyl radicals previously proposed as the primary intermediates for the pure hydrothermal reaction. Under hydrothermal conditions, the benzyl radicals undergo hydrogen atom abstraction from dibenzyl ketone and para-coupling reactions that are not observed under ambient conditions. The photochemical method allows the primary radical coupling products to be identified, and because these products are generated rapidly, the method also allows the kinetics of the subsequent dehydration and Paal-Knorr cyclization reactions to be measured. In this way, the radical and ionic thermal and hydrothermal reaction pathways can be studied separately.

Full text of DOI:10.1021/jo500899x

Article
pubs.acs.org/joc
Hydrothermal Photochemistry as a Mechanistic Tool in Organic
Geochemistry: The Chemistry of Dibenzyl Ketone
Ziming Yang,† Edward D. Lorance,‡ Christiana Bockisch,† Lynda B. Williams,*,§ Hilairy E. Hartnett,*,†,§
Everett L. Shock,*,†,§ and Ian R. Gould*,†
†Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, United States
‡Department of Chemistry, Vanguard University, Costa Mesa, California 92926, United States
§School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, United States
S
* Supporting Information
ABSTRACT: Hydrothermal organic transformations under geochemically
relevant conditions can result in complex product mixtures that form via
multiple reaction pathways. The hydrothermal decomposition reactions of
the model ketone dibenzyl ketone form a mixture of reduction,
dehydration, fragmentation, and coupling products that suggest simulta-
neous and competitive radical and ionic reaction pathways. Here we show
how Norrish Type I photocleavage of dibenzyl ketone can be used to
independently generate the benzyl radicals previously proposed as the
primary intermediates for the pure hydrothermal reaction. Under
hydrothermal conditions, the benzyl radicals undergo hydrogen atom
abstraction from dibenzyl ketone and para-coupling reactions that are not
observed under ambient conditions. The photochemical method allows the primary radical coupling products to be identified,
and because these products are generated rapidly, the method also allows the kinetics of the subsequent dehydration and Paal−
Knorr cyclization reactions to be measured. In this way, the radical and ionic thermal and hydrothermal reaction pathways can be
studied separately.
reasons, organic chemical reactions under conditions that
mimic geochemical conditions are being studied as potential
“green chemistry” systems.9
INTRODUCTION
■
Over 99.99% of the Earth’s organic carbon does not actively
participate in the biospheric carbon cycle but is located within
the crust, mainly in continental and marine sedimentary basins
in the form of kerogen.1 It is estimated that more than
15 000 000 Gt of organic matter is located beneath the surface
of the Earth.1 The reactions of this huge quantity of organic
matter contribute to a wide range of geochemically important
processes. Reactions of organic material in the Earth’s crust
represent an important component of the deep (or geo-
chemical) carbon cycle, which is an important factor in
controlling atmospheric carbon dioxide levels.2 Subsurface
organic reactions are critical to petroleum generation and
maturation processes,3 can provide energy sources for deep
microbial communities,4 and have been implicated in the
formation of ore deposits.5
From an organic chemistry perspective, it is interesting that
the solvent for most of these reactions is water, usually at
elevated temperatures and pressures.6 Water at high temper-
atures and pressures is an excellent solvent for many organic
reactions.7−18 A wide range of reactions have now been
identified in hot subcritical water (at associated confining
pressures), which include fragmentations into smaller struc-
tures, additions to make larger structures, and functional group
interconversions. These reactions take place in water without
the addition of any external reagents or catalysts.8 For these
Many hydrothermal reactions are surprising compared with
corresponding processes under ambient conditions. Examples
include the endothermic elimination of hydrogen from alcohols
and alkanes to form carbonyls and alkenes and the dehydration
of alcohols to form alkenes.8−12 Alcohol dehydration is
particularly surprising since water is the solvent. The
equilibrium between alcohol and alkene + water is strongly
temperature-dependent, and for butanol the equilibrium is on
the side of butene + water at temperatures above 200 °C at the
vapor−liquid saturation pressure of water.5 At higher temper-
atures the entropic contribution to the free energy increases, so
eventually water elimination, which increases the entropy,
becomes favorable despite being endothermic. In general,
hydrothermal organic reactions tend to be controlled by
thermodynamics and entropy, as opposed to kinetics and
enthalpy, which are the primary controlling factors closer to
ambient conditions. Both ionic and radical reaction mecha-
nisms can occur simultaneously under hydrothermal con-
ditions, although it is generally thought that ionic reactions are
Received: April 22, 2014
Published: July 15, 2014
© 2014 American Chemical Society
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Scheme 1. Schematic Representation of Functional Group Interconversions That Connect Simple Hydrocarbons and
Carboxylic Acids, Proposed by Seewald for Hydrothermal Organic Reactions7,18
more prevalent at lower temperatures whereas radical reactions
dominate at higher temperatures.13
and their distributions were very different from those expected
on the basis of the known DBK-derived radical chemistry under
ambient conditions.19 We thus sought independent evidence
for the intermediacy of radicals in the bond cleavage reactions
of DBK under hydrothermal conditions.
Although many hydrothermal organic reactions have now
been studied, the emphasis has been mainly on product
distributions; mechanistic studies are less frequent, and direct
evidence for proposed intermediates is often lacking. Partly this
is because hydrothermal reactions can give complex product
mixtures.7,8,10,14,15 Other than kinetic measurements16 and in
some cases isotope effects,17 the mechanistic toolbox for
hydrothermal organic reactions is somewhat limited. One of the
goals of this work was to develop a new experimental probe for
hydrothermal organic reaction mechanisms.
The most abundant organic functional groups found in
geochemical environments are alkanes and carboxylic acids.7
Seewald has proposed a reaction scheme that shows how acids
and alkanes might be connected by a series of reversible and
irreversible functional group interconversions (Scheme 1).7,18
The critical structure in this scheme is the ketone, since this is
where irreversible carbon−carbon bond cleavage must occur.
We recently studied the hydrothermal reactions of the model
compound dibenzyl ketone (DBK) in order to investigate how
bond cleavage might compete with functional group
interconversions.14 At 300 °C and 700 bar, all of the reactions
in Scheme 1 were observed, except that carbon−carbon bond
cleavage gave mainly coupling products rather than carboxylic
acids.14 The coupling products were explained as arising from
homolytic bond cleavage to give primary benzyl and phenacyl
radicals followed by rapid decarbonylation to give a pair of
benzyl radicals (eq 1), although the observed coupling products
The ambient photochemistry of DBK and its analogues has
been extensively studied in a variety of reaction media,19 and
the primary photophysical and chemical processes are well-
established. Specifically, photochemical excitation of DBK (hν
in eq 1) gives the same primary radicals as previously proposed
in the hydrothermal reactions of DBK (Δ in eq 1). Here we
compare the thermal and photochemical reactions of DBK and
show how independent generation of the primary radicals
rationalizes the product distributions in the hydrothermal
reactions of DBK. The photochemical method allows the
primary radical coupling products to be identified. Because they
are also formed on a much shorter time scale, their follow-up
thermal reactions can be monitored directly. In short,
hydrothermal photochemistry is found to be a very useful
mechanistic tool for geochemically relevant hydrothermal
reactions.
HYDROTHERMAL REACTIONS OF DBK
■
The hydrothermal decomposition reactions of DBK give a
complex mixture of products that has been described in detail
previously (Scheme 2).14 One reaction pathway involves
reduction, dehydration, and further reduction to give the
corresponding alcohol (2c), alkene (2d), and alkane (2e)
(although alcohol 2c does not accumulate significantly because
of rapid dehydration to form alkene 2d), which corresponds to
the functional group interconversion reactions shown in
Scheme 1. The majority of the reaction products, however,
a
Scheme 2. Summary of the Main Products of the Hydrothermal Reaction of DBK at 300 °C and 700 bar
a
From ref 14. Fragmentation and coupling reactions that give the products 1, 2a, 2b, 3a−d, 4a, 4b1, and 4b2 compete with reduction and
dehydration reactions that form alcohol 2c, alkene 2d, and alkane 2e. The (′) symbol means that the product is formed in more than one
stereoisomeric and/or structural isomeric form.
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Figure 1. Gas chromatograms showing the product distributions for (A, black) the thermal reaction of DBK in water at 300 °C and 86 bar (no light)
for a time period of 7 days, (B, red) hydrothermal photolysis in water at 300 °C and 86 bar for a time period of 33 min, and (C, blue) photolysis of
DBK in methanol at room temperature (RT) for a time period of 33 min. The various products separate chromatographically into those that contain
one, two, three, and four benzene rings.
are derived from carbon−carbon and carbon−hydrogen bond
cleavage reactions and subsequent coupling processes, presum-
ably via radical intermediates formed via eq 1.
2.1% in high-pressure gold, low-pressure gold, and fused silica,
respectively. The conversions were slightly higher in gold,
which suggests that the fused silica surface does not
significantly catalyze the reaction.
A gas chromatogram showing the product distribution for a
7-day thermolysis experiment in a fused-silica tube is shown in
Figure 1A. This longer reaction time period allows direct
comparison to the photochemical experiments, but the product
distribution is similar to that observed after 3 days.
Bibenzyl (2a) is an expected product of the coupling of two
benzyl radicals, but it is formed in relatively low yield compared
with other products. The various cleavage and coupling
products are most easily categorized in terms of the number
of benzene rings they contain: specifically, toluene (1) with one
ring; bibenzyl (2a) and stilbene (2b) with two rings; and
products that contain three benzene rings (3a−d) or four
benzene rings (4a and 4b) (Scheme 2).
Reactions designed to mimic geochemical conditions are
often performed for extended time periods, and gold vessels are
often used to minimize inadvertent catalysis by the reaction
container. Gold is considered to be one of the more inert
container materials.20 Our previous experiments were per-
formed in small, pressurized gold tubes, and because the
photochemical reactions required the use of transparent fused-
silica tubes, we first compared the reactions of DBK in fused-
silica and gold.
Hydrothermal reactions of DBK were performed for 3 days
in a fused silica tube, in a gold capsule that was sealed inside a
fused-silica tube, and in a gold capsule with an external pressure
of 700 bar. At 300 °C, the pressure in the fused-silica tube was
determined to be ca. 86 bar (see the Experimental Section). At
this pressure there was a headspace both in the fused-silica and
the gold in fused silica tube experiments. Because gold is
malleable, the external pressure of 700 bar was transmitted to
the sample, and under these conditions there was no headspace.
The observed products were identical in the three samples,
although their distributions were slightly different. The DBK
conversions were also somewhat different: 3.6%, 2.9%, and
HYDROTHERMAL PHOTOLYSIS OF DBK
■
1. Products of Hydrothermal Photolysis. The products
of the 7 day, 300 °C hydrothermal reaction were compared
with those for 33 min hydrothermal photolysis at 300 °C and
also room-temperature photolysis in methanol. The con-
versions were low (5−8%) to minimize secondary reactions.
In agreement with the earlier literature,19 the photochemical
reaction of DBK at room temperature yields bibenzyl (2a) as
the only detectable product (Figure 1C). Hydrothermal
photolysis at 300 °C, however, gives a very different product
distribution (Figure 1B). Although bibenzyl is still observed,
the major products are now toluene (1) and some of the three-
and four-benzene-ring coupling products shown in Scheme 2.
The product distribution from hydrothermal photolysis (Figure
1B) resembles a simpler version of the 7 day pure thermal
reaction (Figure 1A). The hydrothermal photochemical
conversion of DBK is linear with time, as is formation of
toluene, bibenzyl, and the coupling products. Figure 2 shows
the product yields as functions of irradiation time for the
hydrothermal photochemical reaction compared with the
corresponding reaction in methanol at room temperature,
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decarbonylation of the phenacyl radical to yield another benzyl
radical should occur in less than 2 ns at 300 °C. The geminate
benzyl radical pair is formed in an overall triplet state, and
efficient diffusive separation of this primary radical pair is
expected to occur in fluid solution since recombination is spin-
forbidden.22 The pure thermal reaction forms the same
geminate benzyl/phenacyl radical pair, in this case in an overall
singlet spin state. Separated benzyl radicals then form by
decarbonylation and diffusive separation.
The hydrothermal reaction of the benzyl radicals that does
not take place under ambient conditions is hydrogen atom
abstraction from DBK to yield toluene (1) and a DBK-derived
radical, DBK• (shown as the process “H-abs” in Scheme 3).
Coupling of DBK• with a benzyl radical generates the primary
three-ring structures 3a, which form as a mixture of three
structural isomers. Coupling of two DBK• generates the
primary four-ring structures 4a (two stereoisomers are formed),
which undergo thermal dehydration to form isomeric structures
4b. The primary products of the hydrothermal photochemical
radical reactions are thus 1 (toluene), 2a (bibenzyl), 3a, and 4a
(Scheme 3).
Figure 2. Comparison of product concentrations vs irradiation time
for the photolysis of DBK, showing the formation of bibenzyl 2a (blue
●
) as the only product in methanol at RT and the formation of
●
○
bibenzyl 2a (red ), toluene 1 (red ), and the three-ring coupling
product 3a1 (red
) in water at 300 °C and 86 bar. The
△
concentrations of the products at 300 °C are corrected for the
different conversions of DBK at the two reaction temperatures to
allow direct comparison of the relative chemical yields at the two
temperatures.
The hydrogen atom abstraction process (H-abs) occurs in
competition with the conventional benzyl radical coupling to
form bibenzyl (2a). Therefore, changing the relative rates of the
competing processes should influence the product distribution.
The formation of bibenzyl is kinetically second-order with
respect to benzyl radical concentration, whereas the competing
hydrogen atom abstraction with DBK is pseudo-first-order. The
relative rates of these reactions should thus depend upon the
radical concentration. Experiments were performed at higher
light intensities where the radical concentration is higher.
Under these conditions, the yield of bibenzyl increased
compared with those of the coupling products, consistent
corrected for the different DBK conversions under the two
conditions. This plot clearly shows the large difference in the
product distributions under the two sets of conditions.
A mechanism that accounts for the fragmentation and
coupling products for both the pure hydrothermal and
photochemical hydrothermal reactions is shown in Scheme 3.
The first steps are those in eq 1. Excitation of DBK to the first
excited singlet state is followed by intersystem crossing (ISC)
to the excited triplet state, which undergoes efficient homolytic
bond cleavage to form a geminate benzyl/phenacyl radical pair.
From the previously measured activation parameters,21
Scheme 3. Proposed Mechanism for the Formation of the Fragmentation and Coupling Products of the Hydrothermal Reaction
a
of DBK at 300 °C
a
The steps to the left of the dotted line are the radical processes, while those to the right of the dotted line are the follow-up thermal reactions that
are mainly ionic. The first step in the photochemical reaction combines excitation of DBK to the first singlet state (hν), ISC to the triplet state, and
finally homolytic cleavage (C−C) to form a benzyl radical and a phenacyl radical. This radical pair is also formed in the thermal (non-
photochemical) reaction (Δ), although much more slowly and in an overall singlet state. The second step (−CO/sep) includes decarbonylation
(−CO) and separation of the radicals in the geminate pair (sep). These primary steps are those given in eq 1. A separated benzyl radical reacts with
DBK via hydrogen atom abstraction (H-abs) to yield toluene and a DBK-derived radical, DBK•. The (′) symbol associated with a structure indicates
that more than one stereoisomer or structural isomer is formed.
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with competing radical coupling and hydrogen atom
abstraction.
radicals form coupling products.19 According to Scheme 3, the
formation of a three-ring coupling product and the
accompanying toluene consumes two DBK molecules, while
the formation of a four-ring product and the accompanying two
toluenes consumes three DBK molecules. Thus, compared with
the quantum yield for formation of bibenzyl, the quantum
yields for the formation of the three-ring and four-ring
structures should be 2 and 3 times larger, respectively. On
the basis of the relative chemical yields of bibenzyl and the
three- and four-ring structures, the quantum yield for DBK
consumption under hydrothermal conditions can thus be
calculated relative to conditions where bibenzyl is the only
product.
Bibenzyl formation does not depend upon the DBK
concentration, whereas the hydrogen atom abstraction reaction
is pseudo-first-order with respect to DBK. It was found that the
product distributions could also be varied controllably by
changing the DBK concentration. Hydrothermal photolysis of
DBK at a concentration of 0.05 molal gave bibenzyl as the
major product, whereas at 1 molal concentration toluene and
the coupling products dominated the distribution. The
quantum yields for formation of the coupling products can
thus be obtained by comparing experiments at different
concentrations of DBK.
Support for the mechanism shown in Scheme 3 was also
obtained from experiments performed in the presence of
Cu(II) ion, which is known to trap benzyl radicals via
oxidation.23 Hydrothermal photolysis of DBK was performed
in the presence of 0.5 molal Cu(II) chloride for 15 min. The
extent of DBK conversion was similar to that without Cu(II),
but the yields of all of the radical products (1, 2a, 3a, and 4a)
were dramatically reduced by 80−90%. New oxidized products
were detected in the presence of Cu(II), including
benzaldehyde, benzoic acid, and phenylacetic acid. This
provides strong evidence that benzyl and benzylic radicals are
the precursors of the various coupling products of the
hydrothermal photochemistry of DBK.
2. Relative Product Yields Support the Reaction
Scheme. According to Scheme 3, one molecule of toluene is
formed for each one of the three-ring coupling structures, and
two molecules of toluene are formed for each one of the four-
ring coupling structures. Therefore, the concentration of
toluene in the product mixture, [1], should be equal to the
sum of the concentrations of all of the three-ring products
(∑[3-ring]) plus twice the sum of the concentrations of all of
the four-ring structures (∑[4-ring]) (eq 2):
Hydrothermal photolyses of DBK at 0.05 molal and 1.0
molal were compared with the ambient photolysis of DBK,
where bibenzyl was the only product, and at concentrations
that had the same light absorption as the two hydrothermal
samples (details are given in the Supporting Information). It
was found that the DBK consumption for hydrothermal
photolysis is higher per absorbed photon at the higher DBK
concentration by a factor of ca. 1.3 compared with the lower
concentration. The calculated hydrothermal quantum yield at 1
molal concentration, based on the observed product distribu-
tion and assuming that each of the three- and four-ring
structures consumes two and three DBK molecules,
respectively, is 1.37, which agrees quite well with the
experimentally determined difference. The relative quantum
yields thus also support the mechanism shown in Scheme 3.
4. Hydrogen Atom Abstraction. The critical step in the
hydrothermal reaction mechanism is hydrogen atom abstrac-
tion from DBK by benzyl radicals. This process is responsible
for the new products and is also responsible for consumption of
more than one DBK molecule per photon. H atom abstraction
does not occur under ambient conditions, so we examined this
reaction computationally to see whether the reaction barrier
could account for the different behaviors observed at the two
temperatures. Methyl benzyl ketone (MBK) was studied
instead of DBK for computational economy, using B3LYP/
aug-cc-pVDZ as implemented in Gaussian 09.24 MBK was
optimized both in the gas phase and in solvent [modeled using
the polarizable continuum model (PCM)] at 300 °C and 700
bar. Starting with the optimal conformer, the benzyl radical was
fixed at 2.5 Å from the α-hydrogen of MBK (Figure 4), and the
distance-constrained complex was fully optimized. From this
geometry, the distance to the benzylic carbon of the radical,
Cbenzyl−Hα, was shortened to 1.5 Å in steps of 0.1 Å and then to
1.35 Å in steps of 0.05 Å (with optimization of the distance-
constrained complex at each step), at which point the Cα−Hα
bond separation distance in MBK became quite large (>3 Å),
consistent with breaking of this bond. A search for the
transition state located an extremely steep apex to the reaction
coordinate (Figure 4). The transition-state structure was
[1] = ∑[3‐ring] + 2 ∑[4‐ring]
The two sides of eq 2 are plotted as functions of time in Figure
3. The slopes of the plots are very similar, supporting eq 2 and
(2)
Figure 3. Time dependence of the concentration of toluene, [1],
compared to that of the sum of the concentrations of 3-ring products
plus twice the sum of the concentration of the 4-ring products. These
should be equal according to eq 2; the small difference may be a
consequence of assumed gas chromatography response factors for the
coupling products (see the Supporting Information) or other
experimental uncertainties.
the mechanism shown in Scheme 3. The small differences in
the slopes are probably consequences of the assumptions for
the gas chromatography response factors for the various
coupling products (i.e., 1.5 and 2.0 times larger than for
DBK for the three and four ring structures, respectively).
3. Relative Quantum Yields Support the Reaction
Scheme. The formation of toluene and the coupling products
requires consumption of more than one DBK molecule per
absorbed photon. The quantum yield for bibenzyl formation in
the ambient photochemistry of DBK is ca. 0.7, is essentially
solvent-independent, and is the same as that for formation of
separated benzyl radicals, since all of the separated benzyl
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ambient conditions in fluid solution, since coupling occurs only
at the benzylic position. This is readily understood, since
coupling at other positions initially forms a structure that has
lost aromaticity. On this basis, we anticipate that the major
product of the reaction of a benzyl radical and a DBK• radical
would arise from coupling at the two benzylic positions to yield
structure 3a1 (shown in Schemes 3 and 4). However, non-
Scheme 4. Structural Isomers Assigned as Products of
Benzyl/DBK Radical (DBK•) Coupling Observed in the Gas
a
Chromatogram of Figure 1B
Figure 4. Electronic energy vs the α-hydrogen−benzylic carbon
separation distance, rC−Hα, for hydrogen atom abstraction from methyl
benzyl ketone (MBK) by a benzyl radical. The inset shows the
transition-state structure, located at rC−Hα = 1.41 Å, as computed using
B3LYP/aug-cc-pVDZ and PCM to model the solvent. The energies
are referenced to MBK + benzyl radical at an infinite separation
distance. The reaction was computed to be exothermic by 29 kJ/mol
at 300 °C and 700 bar. The computed free energy barrier for the
reaction is 132 kJ/mol at 300 °C and 700 bar.
a
The major product is assigned as 3a1. The products 3a2 and 3a3 are
shown as para-coupling isomers, although ortho-coupling structures
could also form. The dashed lines indicate the bonds that cleave to
yield fragments with the indicated m/z values. For 3a3, the m/z 181
peak in the mass spectrum is much larger that the m/z 209 peak.
benzylic coupling has occasionally been observed in photo-
chemical reactions of DBK under ambient conditions in
nonhomogeneous environments.29 The equivalent non-ben-
zylic coupling processes, shown in eqs 3a and 3b, yield
characterized by an associated imaginary vibrational frequency
of 1528 cm−1, which corresponds to the desired hydrogen
transfer motion. The decrease in energy after the transition
state is reached is artificially steep as a consequence of
constraining only the Cbenzyl−Hα coordinate and none of the
other relevant bond distances. The reaction was found to be
exothermic, as expected for a reaction that generates a more
resonance-stabilized radical, by a value of 29 kJ/mol. This is in
excellent agreement with the difference of 30 kJ/mol in the
experimentally measured Cbenzyl−Hα and Cα−Hα bond
dissociation energies [i.e., 377 kJ/mol for the benzylic C−H
bond in toluene25 and 347 kJ/mol for the benzylic (α) C−H
bond in MBK;26 bond dissociation enthalpies are essentially
independent of temperature25]. The activation free energy was
computed to be 132.4 kJ/mol at 300 °C, which translates into a
bimolecular rate constant of 10.3 M−1 s−1 using transition state
theory.27 The corresponding activation energy and rate
constant under ambient conditions were calculated to be
119.0 kJ/mol and 8.9 × 10−9 M−1 s−1, respectively, indicating
that the reaction is ca. 109 times slower than at 300 °C. This is
consistent with bibenzyl being the only observable product at
room temperature and H atom abstraction being competitive
with radical recombination at 300 °C.
5. Characterization of the Coupling Products. Three
structural isomers assigned to the coupling products 3a are
observed in the gas chromatogram/mass spectrum of the
product mixture from hydrothermal photolysis. One of these
isomers, corresponding to the peak with the shortest retention
time in Figure 1B, is formed in significantly higher yield than
the other two. These products are not isolable (the conversions
are low, and the reaction mixtures are complex), and therefore,
we can only make tentative assignments. Coupling of a benzyl
radical and a DBK• radical could in principle occur at various
positions since the spins are delocalized. For benzyl radical, the
measured spin densities are 3.19, 1.0, and 1.22 for the benzylic,
ortho, and para positions, respectively.28 These spin densities
do not explain the coupling of two benzyl radicals under
structures that are tentatively assigned to the peaks 3a2 and 3a3
in Figure 1B (also see Scheme 4). The structures assigned to
3a2 and 3a3 in eqs 3 and Scheme 4 are shown as para-coupling
isomers, although ortho-coupling isomers could also be formed
and not be detected if they did not separate well under the
chromatography conditions. 3a2 and 3a3 are formed in lower
yields than 3a1, which is understandable since according to eqs
3 they must be formed via intermediates that have lost
aromaticity. Formation of these non-benzylic coupling products
under hydrothermal conditions points to higher reactivity and
lower selectivity in the reactions of benzylic radicals at higher
temperatures.
The assignments of 3a1, 3a2, and 3a3 to the first, second, and
third peaks, respectively, in the gas chromatogram of Figure 1B
is supported by their mass spectra and also the observed follow-
up thermal chemistry discussed below. The mass spectra for the
peaks assigned to 3a1 and 3a2 are very similar and contain
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fragments of similar size (m/z 181 and 209), consistent with
cleavage on both sides of the carbonyl group to give benzyl ions
(Scheme 4). In the mass spectrum for the peak assigned to 3a3,
however, the m/z 209 peak is small and the m/z 181 peak is the
base peak, since in the case of 3a3 the m/z 181 peak
corresponds to a more stable diarylmethyl ion (see the
Supporting Information).
Five products assigned to structures that contain four
benzene rings are formed at early reaction times in sufficient
quantities for mass spectral analysis (Figure 1B). Unlike the
three-ring products, the time dependences of the concen-
trations of these structures are nonlinear (Figure 5). Two of
the most abundant, and the primary four-ring products 4a are
not even observed (Figure 1A). The secondary products can be
understood as arising from follow-up dehydration reactions of
the 3a and 4a isomers. It should be noted that dehydration
occurs efficiently despite the fact that the solvent is water and
there are no added reagents or catalysts.
Dehydration of alcohols is perhaps the best-characterized
hydrothermal reaction from a mechanistic perspective.11,12 This
reaction proceeds by acid-catalyzed elimination, with the water
acting as the acid catalyst. It is well-known that under
hydrothermal conditions Kw for water increases30 and the
dielectric constant of water decreases,30 and with the increased
thermal energy, the reactivity of the hydronium and hydroxide
ions should increase. The mechanisms of the dehydration
reactions of the ketones 3a and 4a are not as obvious, however,
since these are not alcohols.
Further photolysis-followed-by-thermolysis experiments were
performed in order to obtain a quantitative description of the
dehydration reactions of 3a and 4a. Photolysis was performed
at 300 °C for 2 min, followed by thermolysis at the same
temperature for variable time periods. The rates of follow-up
thermal dehydration were found to be very different for the 3a
isomers and the 4a isomers. The reactions of both
diastereomers of 4a were complete within ca. 15 min after
the light was shut off (both reacted at the same rate), whereas
the reactions of the 3a isomers were much slower. The different
isomers of 3a decreased in concentration at different rates, but
none decreased by more than a factor of ca. 2 over a period of 1
day. The reactions of the 3a isomers were so slow that accurate
measurement of their reaction kinetics was not possible, since
they were also being formed by thermal (non-photochemical)
decomposition of DBK in the postphotolysis thermal reaction
period. However, the kinetics of the conversions of the
diastereomers of 4a to the 4b isomers were readily measured
by quenching of photolysis samples after different follow-up
thermolysis periods (Figure 6). Dehydration of the 4a isomers
to the 4b isomers was remarkably fast: the pseudo-first-order
Figure 5. Concentration vs time plot for the photolysis of DBK in
○
water at 300 °C at 86 bar for ( ) the primary coupling product 4a
with the shorter retention time of the two diastereomers in Figure 1B
1
●
and ( ) the dehydrated four-ring product 4b .
these structures are formed in equal abundance and have
identical mass spectra, suggesting that they are the meso/(d,l)
diastereomeric pair formed by coupling of two DBK• radicals
(indicated as peaks 4a in Figure 1B; the structure is shown in
Scheme 3). The other four-ring products 4b1 and 4b2 indicated
in Figure 1B are isomers with a molecular ion peak at m/z 400,
consistent with formal loss of water compared with the 4a
isomers (Scheme 3). The rate of formation of the 4b isomers
increases as a function of irradiation time (Figure 5), consistent
with the formation of the 4b isomers by secondary thermal
dehydration reactions of the 4a isomers.
That 4b1 and 4b2 are derived from 4a was readily
demonstrated by an experiment in which hydrothermal
photolysis was followed by thermolysis without light at 300
°C for a further 30 min. In this experiment it was found that the
two peaks formed in the initial photolysis and assigned to the
4a isomers had completely disappeared in a follow-up thermal
reaction and that the peaks assigned to the 4b isomers were
correspondingly larger. This explains the nonlinear behavior of
the 4a and 4b1 concentrations with time shown in Figure 5.
FOLLOW-UP IONIC REACTIONS
■
In addition to providing insight into the thermal radical
reactions, the photochemical technique allows the follow-up
ionic thermal reactions to be more readily studied. The
photochemical reaction clearly shows that the primary three-
ring and four-ring radical coupling products are the 3a and 4a
isomers. It was impossible to identify these primary products in
the thermal-only experiment because of the simultaneous
formation of secondary products.14 In the thermal-only
reactions, the primary three-ring products 3a are not among
Figure 6. Time dependence of the concentration of 4a (expressed as a
fraction of the sum of the 4a and 4b1 concentrations) in the
dehydration reaction of 4a to form furan 4b1 in water at 300 °C and
86 bar. The 4a stereoisomer is the one with the shorter gas
chromatographic retention time in Figure 1B. 4a was formed by
hydrothermal photolysis of DBK for 2 min, and this was followed by
thermal dehydration for the various times indicated in the graph. The
curve through the data corresponds to a pseudo-first order rate
constant of 0.25 min−1.
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Scheme 5. Proposed Mechanisms for Brønsted Acid-Catalyzed Dehydrations of 3a and 4a To Give 3b(′) and 4b1 or 4b2(′),
a
Respectively, via Enol [3e(′) and 4e(′)] and Conjugated Alkene [3f(′) and 4f(′)] Intermediates
a
The (′) symbol indicates stereoisomers and/or structural isomers. Acid can catalyze each step; only the first protonated intermediate is shown.
Cyclization of the primary protonated intermediate is possible for 4a (but not for 3a) to give furan 4b1, a structural isomer of dienones 4b2(′). The
numbers in red are the heats of formation in kJ/mol at 300 °C and 700 bar, calculated using the PM6 method with the solvent modeled using PMC
as detailed in the Supporting Information. The values for 3b(′), 4b2(′), and 4b1 include the heat of formation of the water that is liberated in the last
reaction, which is calculated to be −271 kJ/mol using the PM6/PCM method. Furan 4b1 was calculated to be more stable than the corresponding
dienones 4b2(′) by ca. 60 kJ/mol under these conditions using this method.
Figure 7. Structures of (left) the (3Z,5E) isomer of dienone 4b2 and (right) the furan 4b1 computed using the semiempirical PM6 method with
PCM to model water as the solvent. The structures show a lack of planarity and lack of conjugation between the carbon−carbon and carbon−oxygen
double bonds in the dienone and increased planarity in the furan.
rate constant is ca. 0.25 min−1. It is clear that the reactivities of
the 3a and 4a isomers are very different and that any proposed
dehydration mechanism needs to account for this difference in
reactivity.
Scheme 5 and as observed in the final dehydration products 3b
and 4b2 in the gas chromatograms in Figure 1.
Heats of formation for the relevant intermediate structures
are summarized in Scheme 5. Formation of the enol was found
to be slightly endothermic for both 3a and 4a, as expected, and
the isomerization 3e/4e to 3f/4f was almost thermoneutral.
Deprotonation of the initially protonated carbonyl may skip the
enol and form the alcohols 3f/4f directly, but the energy
changes along the two reaction pathways to the dienes 3b and
the dienones 4b2, including the final dehydration step, are very
similar and cannot account for the large observed difference in
the reaction rates for 3a and 4a. The isomeric dienones 4b2
shown in Scheme 5 are sterically congested, and extensive
conjugation is not observed in any of the calculated structures
(e.g., see Figure 7). This lack of conjugation due to steric
crowding is also observed in the enol and alkene isomers 3e/4e
and 3f/4f. The heats of formation of the four stereoisomers of
the dienones 4b2 were found to be similar, although the
We investigated possible acid-catalyzed dehydration reactions
computationally. For these large structures, the semiempirical
PM6 method was selected.24,31 Calculations were performed at
300 °C and 700 bar (the conditions of the original gold tube
experiments of ref 14) in simulated hot pressurized water using
the PCM (details are provided in the Supporting Information).
A plausible mechanism for the dehydration of 4a/3a involves
protonation of a carbonyl followed by formation of the
corresponding enol (4e/3e), rearrangement to form an alcohol
(4f/3f), and conventional acid-catalyzed elimination of water to
generate the dienone/diene (4b2/3b) (Scheme 5). Most of
these structures could be formed in multiple stereoisomeric or
structural isomeric forms, as indicated by the (′) symbols in
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(3Z,5E) isomer is the most stable computationally and is the
least crowded (Figure 7). The uncertainty in the heats of
formation of isomeric organic structures such as those in
Scheme 5 has been estimated to be ca. 16 kJ/mol.32 One of the
products of dehydration of 4a is formed in significantly higher
yield than the others (Figure 1B). The energy of the (3Z,5E)
isomer is lower than those of the others by ca. 30 kJ/mol (i.e.,
by more than the reported estimated uncertainty in the heats of
formation), but this energy difference is not enough to account
for the formation of this isomer in significantly higher
abundance than the others. However, another dehydration
pathway is available to 4a that can form a product that is even
more stable than the dienones.
Protonated 4a can undergo the known Paal−Knorr
cyclization to form a furan,33 whereas the corresponding
protonated 3a cannot (Scheme 5). This additional reaction
pathway readily accounts for the significantly increased rate of
dehydration of 4a compared with 3a. The furan that would be
formed, shown as structure 4b1 in Scheme 5 and Figure 7, is a
structural isomer of the dienones 4b2 in Scheme 5. Computa-
tionally, this furan is found to be significantly more stable than
the dienones (by ca. 60 kJ/mol at 300 °C and 700 bar), which
in turn suggests that it is the major dehydration isomer
observed with the shortest retention time in the gas
chromatograms of Figure 1. Although still sterically congested,
furan 4b1 is more conjugated than the least hindered dienone
(Figure 7), which presumably contributes to its increased
stability.
to proceed via protonation of the oxygen followed by
conventional E1 or E2 elimination.11,12 The rate of this
dehydration is intermediate between those observed for the
overall more complex dehydrations of 3a and 4a, suggesting
that the reactions in Scheme 5 are plausible on the observed
time scales.
SEPARATING THE IONIC AND RADICAL REACTION
PATHWAYS
■
The photochemical reaction effectively separates the radical
reactions from the mainly ionic secondary dehydration
reactions that occur simultaneously with the radical reactions
in the pure thermal reaction (Scheme 3). This separation
allowed us to construct the reaction scheme for the formation
of the fragmentation and coupling products (Scheme 3), which
was not possible from the previous thermal-only studies.14
Hydrothermal photolysis followed by thermolysis eventually
results in a product distribution resembling that of the pure
thermolysis reaction (Figure 1A). As mentioned above,
thermolysis after photolysis results in the complete removal
of the 4a isomers via dehydration. Thermolysis after photolysis
does not completely remove the 3a isomers, but their product
distribution changes compared with that after short-time
photolysis alone. In particular, the major product of hydro-
thermal photolysis, 3a1, becomes the least abundant of the 3a
isomers upon thermolysis, and additional three-ring structures
are observed, although in greater yields than could be explained
by reactions of 3a alone.
Isomers are possible for the dienones but not for furan 4b1.
Three dehydration isomers are clearly observed in the gas
chromatograms shown in Figure 1, and smaller peaks due to
other isomers can also be detected at longer reaction times.
Dehydration may form all of the isomers, or interconversion
among the dienone isomers could also occur via protonation
and deprotonation reactions. The development of a true
thermodynamic equilibrium among the various isomers is
unlikely, however, as a result of further follow-up reactions of
dienones 4b2 as discussed below.
Studies of model reactions also support Scheme 5.
Enolization of ketones 3a and 4a can be tested using DBK
itself as a model. As mentioned above, we expect the inherent
Brønsted acid and base catalytic activity in water to be high at
300 °C. The hydrothermal reaction of DBK in deuterated water
at 300 °C for 10 min resulted in complete incorporation of four
deuteriums in the unreacted DBK, as determined by mass
spectrometry, presumably as a consequence of exchange of the
four enolizable hydrogens in DBK.34 Thus, enolization readily
occurs on the time scale of the dehydration reaction. The
second isomerization step can also be readily understood as
proceeding via acid catalysis (protonation followed by
deprotonation), similar to enolization (Scheme 5). That this
process could also be fast on the time scale of the 4a
dehydration is supported by the calculated heats of formation,
which indicate that the process 4e → 4f is even less
endothermic than the enolization step, as well as observation
of rapid interconversions among isomeric alkenes under similar
conditions.15
This observation is explained by recognizing that 3a1 can
undergo the same dehydration reaction as 4a to give 3b isomers
(Scheme 3). The structures assigned to 3a2 and 3a3, however,
cannot undergo corresponding dehydration but could in
principle undergo reactions similar to those of DBK itself.
Under the experimental conditions and time scales, conversion
to secondary products would be expected to be minimal since
the reactions of DBK itself are relatively slow.14 Toluene (1)
would be expected to be a major product of these reactions, but
structural isomers of bibenzyl (2a) and other coupling products
would also be expected to form eventually. In support of this,
small peaks in the gas chromatograms, close to authentic
bibenzyl, are observed upon extended thermolysis and appear
to be bibenzyl isomers on the basis of mass spectral analysis
(Figure 1).
After extended thermolysis, the major three-ring structure is
aldehyde 3d, which is formed as multiple stereoisomers
(Scheme 3). We have not yet investigated this reaction in
detail; however, the aldehyde yield after extended reaction
times is sufficiently large that it cannot be formed from
reactions of the primary three-ring products 3a. A potential
alternate source of 3d is via bond homolysis in dienones 4b2
(Scheme 3). The other product of this reaction would be
toluene (1), although formation of 3d and 1 from 4b2 requires
two hydrogen atoms. With extended reaction times, stilbene
(2b, mainly the trans isomer) becomes a major product in the
two-benzene-ring region at the expense of bibenzyl (Figure
1A). The conversion to stilbene from bibenzyl formally
generates hydrogen atoms that could be used in the formation
of 3d from 4b2.
The final step in the mechanism of Scheme 5 is dehydration
of the alcohol, which we modeled by studying the same
reaction in 1-phenylethanol (eq 4). Thermolysis of 1-
phenylethanol in water for 1 h at 300 °C resulted in conversion
of 50% of the alcohol to styrene. The mechanism of
hydrothermal alcohol dehydration has previously been shown
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carried out in the same fused silica tubes and apparatus as the high-
temperature photolysis experiments. Radical trapping experiments
were performed using 0.5 molal CuCl2 at 300 °C and 86 bar.
The extraction and analytical procedures were as described
previously.14 Gas chromatography with flame ionization detection
(FID) was used for quantitative analysis of the reaction mixtures.
Because of the low conversions and the number of products, it was not
possible to isolate the coupling products containing three or four
benzene rings. As described previously,14 the gas chromatography FID
response factors for these coupling structures were estimated on the
basis of the number of benzene rings that they contained; specifically,
the values for the three-benzene-ring and four-benzene-ring structures
were estimated to be 1.5 and 2.0 times larger than that for DBK,
respectively.
CONCLUSIONS
■
Hydrothermal photolysis of dibenzyl ketone allows the
identification of the primary radical coupling products in the
corresponding thermal reactions and reveals unexpected
reactions of benzylic radicals in high-temperature water.
Hydrothermal photolysis also allows separation of the radical
and ionic reactions that occur simultaneously in the purely
thermal experiment without photolysis. Reactions of the benzyl
radicals at high temperature that do not occur under ambient
conditions include ortho and para coupling and hydrogen atom
abstraction from DBK itself. The increased thermal energy and
increased Kw facilitate rapid Brønsted acid- and base-catalyzed
reactions that are not observed under ambient conditions,
specifically dehydration of the ketones 3a and 4a. Although
alcohol dehydration has been observed many times in
hydrothermal organic reactions, this work points to dehy-
dration as a generally favorable process for different kinds of
structures.5 The follow-up reactions in Scheme 5 are mainly
Brønsted acid-catalyzed, and it is well-known that natural
geologic systems are characterized by widely varying pH and
distributions of organic materials and functional groups.35
Isolation of the ionic pH-dependent reactions is thus a valuable
tool to aid in the understanding of the complex schemes that
often characterize geologically relevant organic processes.
ASSOCIATED CONTENT
■
S
* Supporting Information
Computational details, a description of the quantum yield
experiments, mass spectra of the coupling products, and the full
citation for ref 24 (as SI ref 5). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: lynda.williams@asu.edu.
*E-mail: h.hartnett@asu.edu.
*E-mail: everett.shock@asu.edu.
*E-mail: igould@asu.edu.
EXPERIMENTAL SECTION
■
Most of the chemicals were available from previous work.14 Copper
chloride (98%) was purchased from Sigma-Aldrich and used without
purification. The hydrothermal photolysis experiments were carried
out in fused silica glass tubes (GM Associates, Inc.) that had a 6 mm
outside diameter and a 2 mm inside diameter. DBK was weighed into
the tubes, and 0.2 mL of argon-purged deionized water was added to
obtain solutions that, when heated, ranged in concentration from
0.035 to 1 molal. It was previously confirmed that DBK is soluble at all
experimental concentrations at 300 °C.14 The samples were frozen in
liquid nitrogen, evacuated using three pump−freeze−thaw cycles, and
sealed under vacuum with a hydrogen flame to a length of ∼12 cm.
Each sample occupied ∼8 cm of the tube length at room temperature,
and the remainder was evacuated headspace. The volume of the
headspace was diminished by water expansion at 300 °C since the
density of water decreases ca. 30% in going from room temperature to
300 °C. The pressure in the tube at 300 °C was determined to be ca.
86 bar using SUPCRT 92.36 The sealed fused silica tubes were placed
vertically in a large brass block that was heated using cartridge heaters.
A thermocouple inside the brass block next to the sample tube was
used to monitor the reaction temperature. The samples were
preheated in the brass block at 300 °C for 5 min before irradiation
for time periods of 2 to 33 min. The 5 min preheating time was
established in an experiment where the temperature was measured
directly using a thermocouple placed inside the same silica glass tube
containing silicone oil. The light source was a 200 W mercury arc lamp
(Osram HBO200W) equipped with an arc lamp power supply system
(model 8500, Oriel, Stamford, CT, USA). A filter solution of
potassium chromate and sodium carbonate was used to isolate the 313
nm emission line and control the light intensity.37 After irradiation, the
samples were immediately quenched in a cold-water bath.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by National Science Foundation
Grant OCE 0826588. We thank all of the members of the
Hydrothermal Organic Geochemistry (HOG) Group at ASU
for many stimulating discussions and helpful suggestions. We
are also grateful to Pierre Herckes and Jinwei Zhang (Arizona
State University) for their assistance with the GC−MS analysis.
DEDICATION
■
I.R.G. dedicates this manuscript to the memory of N. J. Turro, a
master teacher and scholar, who used the photochemistry of
dibenzyl ketone to teach the chemical community so much.
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