Selective Activation of C=C Bond in Sustainable Phenolic Compounds from Lignin via Photooxidation: Experiment and Density Functional Theory Calculations

Article abstract of DOI:10.1111/php.12509

Lignocellulosic biomass can be converted to high-value phenolic compounds, such as food additives, antioxidants, fragrances and fine chemicals. We investigated photochemical and heterogeneous photocatalytic oxidation of two isomeric phenolic compounds from lignin, isoeugenol and eugenol, in several nonprotic solvents, for the first time by experiment and the density functional theory (DFT) calculations. Photooxidation was conducted under ambient conditions using air, near-UV light and commercial P25 TiO₂ photocatalyst, and the products were determined by TLC, UV-Vis absorption spectroscopy, HPLC-UV and HPLC-MS. Photochemical and photocatalytic oxidation of isoeugenol proceeds via the mild oxidative "dimerization" to produce the lignan dehydrodiisoeugenol (DHDIE), while photooxidation of eugenol does not proceed. The DFT calculations suggest a radical stepwise mechanism for the oxidative "dimerization" of isoeugenol to DHDIE as was calculated for the first time.

Full text of DOI:10.1111/php.12509

Photochemistry and Photobiology, 2015, 91: 1332–1339
Selective Activation of C=C Bond in Sustainable Phenolic Compounds
from Lignin via Photooxidation: Experiment and Density Functional
Theory Calculations
Morgan Zielinski (Goldberg), Luke A. Burke and Alexander Samokhvalov*
Chemistry Department, Rutgers University, Camden, NJ
Received 9 June 2015, accepted 4 August 2015, DOI: 10.1111/php.12509
ABSTRACT
Chemical oxidation relies upon expensive and/or toxic chemicals,
while catalytic oxidation uses elevated temperatures and expen-
sive catalysts, e.g. Pt group metals (15). Photochemistry and
photocatalysis can be regarded as methods of “Green Chemistry”
Lignocellulosic biomass can be converted to high-value phe-
nolic compounds, such as food additives, antioxidants, fra-
grances and fine chemicals. We investigated photochemical
and heterogeneous photocatalytic oxidation of two isomeric
phenolic compounds from lignin, isoeugenol and eugenol, in
several nonprotic solvents, for the first time by experiment
and the density functional theory (DFT) calculations. Pho-
tooxidation was conducted under ambient conditions using
(16), since they proceed under ambient temperature and pressure,
and use air and sunlight (17).
Isoeugenol (Fig. 1b) is one of the most abundant (up to 7%)
chemical components in bio-oil (7). Isoeugenol and eugenol
(Fig. 1c) are used as fragrances (13) and antioxidants (18). Oxi-
dation of eugenols produces lignans (19) including dehydrodi-
isoeugenol (DHDIE) (20). DHDIE is used as a natural medicine
compound with anti-inflammatory (21) properties similar to those
of the nonsteroidal anti-inflammatory drugs, and selective
enzyme inhibiting properties (22). Synthetic medicinal drugs in
river water (23) and drinking water (24) pose a significant health
concern worldwide. Unlike certain recalcitrant synthetic drugs,
especially estrogen mimics (25) and endocrine disrupting phtha-
lates (26), bioactive phenolic compounds from lignin can be
easily removed from wastewater via photocatalytic mineralization
under ambient conditions and sunlight (14).
2
air, near-UV light and commercial P25 TiO photocatalyst,
and the products were determined by TLC, UV–Vis absorp-
tion spectroscopy, HPLC-UV and HPLC-MS. Photochemical
and photocatalytic oxidation of isoeugenol proceeds via the
mild oxidative “dimerization” to produce the lignan dehy-
drodiisoeugenol (DHDIE), while photooxidation of eugenol
does not proceed. The DFT calculations suggest a radical
stepwise mechanism for the oxidative “dimerization” of isoeu-
genol to DHDIE as was calculated for the first time.
INTRODUCTION
Photooxidation of isoeugenol with photosensitizer proflavin
dihydrochloride in methanol leads to several products (27) of
oxidative “dimerization”: DHDIE and two b-aryl ethers. Photo-
sensitized oxidation of isoeugenol with methylene blue (MB)
gives seven products in methanol, seven in ethanol, six in ace-
tone and five in acetonitrile (18). Photosensitized oxidation of
Biomass is thought to partially replace fossils in production of
“green” chemicals (1–3)—the small organic molecules from bio-
mass, e.g. monomers for the synthesis of sustainable bioinspired
polymers (4–6). According to the U.S. Biobased Industry Tar-
gets, >90% of organic chemicals can be produced by 2090 from
biomass and “platform chemicals” obtained from biomass (7).
Lignin is the second major (after cellulose) component of wood,
and is a biopolymer of a very complex structure (8); the simpli-
fied formula of its tentative “monomer” is explained in Fig. 1a.
In Fig. 1a, the R and R indicate the short oxygen-containing
3
isoeugenol with MB dye in water/CH CN produces several
dimeric cyclic or acyclic lignans (28). Photosensitized oxidation
of eugenol by Rose Bengal dye or chlorophyll leads to the prod-
ucts of quinoid structure (13), thus photosensitized oxidation is
nonselective vs functional groups: phenyl ring, C=C bond and
OH group. In all reported studies of photooxidation of eugenols,
organic photosensitizer was used and several reaction products
were obtained. Organic photosensitizer is difficult to remove
from reaction mixture after the reaction is completed, and it can
undergo photodecomposition. Photooxidation of eugenols in
solution in the nonprotic solvents without organic photosensitizer
dye has not been investigated, to the best of our knowledge.
The density functional theory (DFT) is an ab initio technique
of quantum chemistry that is commonly used to study the mech-
anisms of chemical reactions of organic compounds by calcula-
tion of the energies and geometries of transition states (TSs) and
intermediates (INT) (29). The structure–activity relationships in
the antioxidant effects of several phenolic compounds from lig-
nin were investigated by the DFT (30). The antioxidant activity
1
2
chains with alkyl, olefin or substituted aryl groups. The “bio-oil”
produced via depolymerization of lignin (9) at the LignoCellu-
lose Feedstock Biorefinery (7) contains the high concentrations
of valuable organic compounds. Phenolic compounds from lignin
(
10) belong to the “lignin platform” and hold an especially high
promise for synthesis of sustainable chemicals. The following
oxidation reactions of phenolic compounds from lignin are
known: 1) chemical oxidation (11); 2) heterogeneous catalytic
oxidation (12); 3) photochemical oxidation with photosensitizer
dye (13) and 4) heterogeneous photocatalytic oxidation (14).
*
Corresponding author email: alexsam@camden.rutgers.edu (Alexander Samokhva-
lov)
2015 The American Society of Photobiology
©
1
332

Photochemistry and Photobiology, 2015, 91 1333
placed into the RV and allowed to react for 4 h in the flow of air at
ꢀ1
2
5 mL min ; in the experiments without solvent, 10 g pure eugenol or
isoeugenol was used.
Product analysis. TLC was used to identify products of chemical
reactions using silica gel plates from Sigma Aldrich and a 60/40 mixture
of hexane/ethyl acetate as an eluent, with UV visualization (38). UV–
visible spectroscopy (UV–Vis) was applied to assess the progress of
photooxidation reactions, using the Cary Bio-Rad 50 spectrometer in
absorbance mode. Usually, 3 lL of a clear supernatant were diluted with
acetonitrile up to 3 mL, and a quartz cuvette with l = 1 cm was used.
For the HPLC with the UV–Vis and mass spectrometric (MS) detection
using an electrospray ionization (HPLC-UV-MS-ESI), the 6410 Triple
Quad LC/MS 1200 Series instrument from Agilent was used. A C18
reverse phase column with stationary phase at 3.5 lm in diameter was
used. Samples of reaction products were volumetrically diluted by a
factor of 1000 prior to injection into the HPLC-UV-MS-ESI instrument.
The mobile phase in the HPLC was a mixture of 0.1 vol. % formic acid
Figure 1. Molecular structures of (a) tentative “monomer” of lignin; (b)
isoeugenol; (c) eugenol.
3
in water and 0.1 vol. % formic acid in CH CN, and a gradient elution
was used; for each specimen, at least three replicas were obtained.
Reference compound. The reference compound DHDIE was synthe-
sized and purified following the modified procedure (39). Briefly, a solu-
tion of 70 g FeCl3 in 200 mL water was added drop-wise to a solution
of 50 g isoeugenol in 450 mL ethyl alcohol and 200 mL water, until the
solution turned dark green and cloudy. The original synthesis (39)
required a few crystals of DHDIE to initiate crystallization; instead, the
solution was kept in the freezer for 1 week, and it produced a reddish
white crystalline precipitate in a dark yellow solution. The crystals were
isolated via vacuum filtration, washed with hexane and recrystallized
from hot hexane. The chemical identity of the DHDIE obtained was
established by melting point (39) using a Mel-Temp apparatus, retention
time on the HPLC-UV chromatogram, and the mass spectra in the
HPLC-UV-MS as reported elsewhere (40).
Quantum chemical calculations. All calculations were carried out with
the Gaussian09 suite of computer programs (41) using its standard
threshold and defaults. The DFT theoretical method was employed using
the unrestricted PBE0 functional (42,43), 6 ꢀ 1 + G(d) basis set, and
analytical gradients.
Photocatalytic oxidation. Photocatalytic oxidation was conducted under
the conditions similar to those of the photochemical oxidation (above),
has been assigned to the combination of the heat of formation of
the radical and spin delocalization (30), while TSs leading to
reactive INT were not considered. To the best of our knowledge,
studies of the mechanism of oxidation of isoeugenol have not
been reported by the DFT calculations.
Heterogeneous photocatalysis occurs after absorption of the
photon by the photocatalyst. Photocatalytic reactions proceed
under the “green” ambient conditions (31) and utilize the abun-
dant, free, environmentally clean and virtually unlimited sunlight
or its near-UV fraction (32). Titanium dioxide TiO2 Degussa
P25 recently rebranded as Evonik Aeroxide (31) is a benchmark
photocatalyst with a high stability and activity in the UV range.
Titanium dioxide is among the 50 most used chemicals in the
2
world (33), but the only report on using TiO in photocatalytic
reactions of eugenols is a complete oxidation of eugenol in waste
waters (34) to carbon dioxide and water. Heterogeneous photo-
catalytic oxidation of phenolic compounds from lignin in solu-
tion in the nonprotic solvents to organic compounds has not
been reported, to the best of our knowledge.
Recently, we reported the synthesis, structural and spectro-
scopic characterization, and testing of the novel titanium oxide
based photocatalysts (35) and sorbents (36) that operate in solu-
tion under ambient conditions. Herein, we report the study of the
selectivity of photochemical and photocatalytic oxidation of
major phenolic compounds from lignin, isoeugenol and eugenol,
in several nonprotic solvents at 30°C, kexc ≥ 300 nm, with ambi-
2
except that 50 mg of TiO was added. After reaction, the samples of sus-
pension containing TiO were centrifuged at 16 060 9 g (Fisher Scien-
2
tific AccuSpin Micro). The supernatant was analyzed by the TLC,
UV–Vis, HPLC-UV and HPLC-MS-ESI methods as described
above.
RESULTS
Photochemical oxidation of eugenol and isoeugenol
ent air at 1 atm, with and without P25 TiO . We also report, for
2
the first time, the DFT calculations of the mechanism of oxida-
tive “dimerization” of isoeugenol.
Photochemical oxidation experiments were conducted in a Pyrex
RV, with the spectrum of transmitted light as in Fig. 2. This is a
strong emission line at k = 313 nm (marked with an asterisk *)
that would cause the photoexcitation of the molecules of eugenol
and isoeugenol which have absorption bands at ca 310–325 nm,
as determined by the UV–Vis spectroscopy in solution (data not
shown).
MATERIALS AND METHODS
Chemicals. Eugenol, isoeugenol and vanillin were from Sigma Aldrich,
and were used as received. Acetonitrile CH
3
CN, dimethylformamide
(
DMF) and dimethylsulfoxide (DMSO) with an assay >99.7% were from
3
Acetonitrile CH CN has initially been chosen as solvent, since
Sigma, Fisher and Acros Organics and were used as received. Titanium
dioxide P25 Evonik Aeroxide was from Acros Organics.
it 1) has a high solubility of oxygen under standard conditions
(44); 2) is optically transparent for the excitation light used by
us and 3) is a “yellow” solvent by the scale of being environ-
mentally and toxicologically “green” (45). We have found by
TLC that isoeugenol, but not eugenol, formed products of pho-
tooxidation when dissolved in acetonitrile. Furthermore, in the
UV–Vis absorption spectrum of the solution of isoeugenol but
not eugenol, there was a change in the relative amplitude/area of
absorption peaks at 240–280 vs 310–325 nm before and after
photooxidation. Thus, isoeugenol undergoes photochemical oxi-
dation in solution, while eugenol does not. In the reference
Photoreactor. The custom-built photoreactor contained a light source
and reaction vessel (RV) placed inside the liquid circulation thermostat,
Neslab model EX221 filled with the mixture of water and ethylene gly-
col, and operated at 30°C. The light source was a commercial 450 W
medium pressure quartz mercury vapor lamp from Ace Glass (37). The
RV made of Pyrex transmits light with k ≥ 300 nm, while the infrared
radiation emitted by the mercury lamp is absorbed by the thermostat car-
rier fluid. The size of RV was 200 mm in length and 32 mm in diameter,
and it received humidified ambient air (ca 90% relative humidity) as
oxidant.
Photooxidation in the absence of photocatalyst. Ten milliliter of the
.5 M solution of the chosen phenolic compound in a chosen solvent was
0

1
334 Morgan Zielinski (Goldberg) et al.
MS analysis, and did not find any peaks due to the products of
oxidation or photolysis. Therefore, the solvents acetonitrile,
DMF and DMSO are stable under the conditions of photochemi-
cal oxidation.
Figure 3 shows the reaction network of photochemical oxida-
tion of isoeugenol.
The lignan DHDIE is more valuable than vanillin, since the
former is a product of synthesis, rather than destruction of the
reactant molecule, so we decided to investigate the dependence
of the molar concentration of DHDIE product on the choice of
the nonprotic solvent. We determined molar concentrations of
the DHDIE product by the HPLC-UV method (47). Figure 4
shows the HPLC-UV calibration plot obtained with the DHDIE
standard at kabs = 275 nm.
As an injected volume of the solution of DHDIE has been
increased, there was a corresponding linear increase in the absor-
bance at 275 nm. The HPLC-UV calibration plot in Fig. 4 was
used to determine the molar concentrations of DHDIE obtained
in photochemical oxidation of isoeugenol. Figure 5a shows the
dependence of the molar concentration of the DHDIE product (in
mM) on the dipole moment of each solvent used in the photo-
chemical oxidation.
Figure 2. The spectrum of the light used in the photooxidation of
phenolic compounds from lignin in solution. The * mark indicates emis-
sion line which corresponds to optical absorption band of isoeugenol and
eugenol in solution.
experiments in the dark, no products of oxidation of isoeugenol
or eugenol have been detected, as expected.
3
Although DMSO, DMF and CH CN have very close values
of the dipole moment, they produced the significantly different
We have used the HPLC-UV method to identify and quantify
the products of the photooxidation of isoeugenol at
amounts of DHDIE. Figure 5b shows the dependence of the
kabs = 275 nm which corresponds to the absorption maximum of
isoeugenol in solution. The HPLC-UV chromatogram of the
solution of pure isoeugenol shows one peak as expected, at the
retention time 10.0 min. Under photochemical oxidation, isoeu-
genol reacts in solution to form two products as found by the
HPLC-UV-MS. One product is vanillin with retention time
8
.8 min as identified by injection of the vanillin standard.
Another product results in the HPLC-UV peak at retention time
1.0 min. We have checked a hypothesis that this product is
1
DHDIE, by comparing its HPLC-UV retention time and the mass
spectrum with those of the DHDIE standard synthesized and
purified by us (see Materials and Methods). The mass spectrum
of DHDIE has three characteristic peaks at m/z = 327, 349, and
6
75 a.m.u. (40). Assignments of those peaks are as follows:
Figure 3. Reaction network of the photochemical and photocatalytic oxi-
dation of isoeugenol in solution.
+
+
[
M + H] +for 327 a.m.u., [M + Na] for 349 a.m.u. and
[
2M + Na] for 675 a.m.u., with M being the molecule of
DHDIE; the MS peaks are consistent with those reported (40)
for DHDIE.
Without solvent, isoeugenol produced only the small amounts
of DHDIE, and eugenol did not yield any products. We have
selected several nonprotic solvents other than CH CN for the
3
photooxidation in solution. Dimethylsulfoxide has a high dipole
moment of 4.3 D (46) and is a “yellow” solvent by the scale of
“being green” (45). Dimethylformamide was also tested by us,
since it has a high dipole moment of 3.86 D. We checked the
possibility of the photooxidation of each solvent. First, all sol-
vents have absorption peaks only at k < 260 nm and therefore
their molecules, unlike the molecules of isoeugenol or eugenol,
do not absorb the near-UV light. Second, we have conducted the
reference HPLC-UV analysis of reaction mixtures, with optical
detection at the multiple wavelengths: kabs = 220, 254 and
2
75 nm, and did not find any peaks due to chemical substances
other than eugenol, isoeugenol, DHDIE and vanillin. Third, we
have checked the three solvents used after a “blank” experiment
of photooxidation without dissolved substrate by the HPLC-UV-
Figure 4. The HPLC–UV calibration plot of dehydrodiisoeugenol
(DHDIE) in solution at k = 275 nm.

Photochemistry and Photobiology, 2015, 91 1335
knowledge, the mechanism of oxidation of isoeugenol has not
been studied by the methods of quantum chemistry. We have
applied the DFT method to study the mechanism of reaction of
isoeugenol to form DHDIE.
Reaction of isoeugenol by the DFT calculations
A DFT computational experiment was undertaken to elucidate
the possible mechanisms for the dimerization of isoeugenol, in
particular, to calculate the pathways to the neutral radical–neutral
molecule product, including TSs and INT in the formation of the
cyclic dimeric product. It is assumed that this intermediate
cycloaddition product then undergoes facile loss of hydrogen to
give the final, conjugated product (DHDIE). All calculations
were carried out with the Gaussian09 suite of computer programs
(41) using its standard threshold and defaults. The DFT theoreti-
cal method was employed using the unrestricted PBE0 functional
(
42,43), 6 ꢀ 31 + G(d) basis set, and analytical gradients. The
PBE0 functional (Gaussian09 keyword PBE1PBE) has been
shown to be effective in radical and excited state calculations
(
50). The 6 ꢀ 31 + G(d) basis set is considered an effective
basis set for both excited state and cycloaddition reactions since
it contains diffuse functions needed in TSs and radical reactions
(51). All critical points were examined for the proper number of
imaginary frequencies, and the vectors for the imaginary fre-
quency in each TS structure followed the motions expected for
bond breaking/formation or rotation. In our DFT model, we
assumed that the “dimerization” of isoeugenol starts with the
abstraction of a hydroxyl hydrogen atom from the molecule of
isoeugenol. Scheme 2 gives the sequence of the neutral radical
and a molecule of isoeugenol combining to form a benzylic radi-
cal intermediate (step 1), and then the benzylic radical and car-
bonyl combining to form a dimer-like cyclic product (step 2).
Two starting structures in the optimization procedure for step 1
were tested using the cycloaddition dimer as a framework: one
resembling the concerted mechanism whereby the ringC–CH(Me)
Figure 5. Molar concentration of the dehydrodiisoeugenol (DHDIE) pro-
duct in the photochemical oxidation in solution: (a) vs the dipole moment
of the solvent; (b) vs oxygen solubility in the solvent.
molar concentration of the DHDIE product on the solubility of
oxygen (44,48) in each solvent. The DMF (1.31 mM dissolved
O ) and CH CN (2.42 mM O ) have the higher oxygen solubility
2 3 2
than DMSO, yet they yield the lower concentrations of DHDIE
compared to DMSO. Therefore, the much higher yield of the
DHDIE obtained in DMSO vs other solvents is due to some
property of DMSO. Both the radical and radical cation of isoeu-
genol were generated in the solution of isoeugenol (49) by pho-
tolysis at 308 nm which is close to k = 313 nm, the excitation
wavelength in our reactions, Fig. 2. Radical cation of isoeugenol
undergoes a quick loss of proton to yield the more stable radical
isoeugenol (49) (Scheme 1).
ꢀ
and HC–O bonds were each elongated to 2.0 A and the other
involving a two-step mechanism obtained by slightly elongating
ꢀ
the new ringC–CH(Me) bond in the dimer to 1.6 A, but increasing
ꢀ
the former HC–O bond to ca 3 A. The TS optimization procedure
for the concerted starting structure continued to the identical TS
structure as the first step in a two-step mechanism. The activation
ꢀ1
a
energy, E (kcal mol ) of bond breaking is calculated as the dif-
The DMSO is an electron donor that strongly solvates cations
46); it is therefore speculated that DMSO solvates a radical
ference in energy of the first step TS and a van der Waals structure
found for the two reactants, thus avoiding basis set superposition
error and concentrating on relative energies of any TS structure or
INT. The intermediate and two TS expected for a two-step mecha-
nism were found. However, there are at least two approaches in
the first step which form rotameric isomers of the radical interme-
diate, wherein the C–CHAr bond is at a dihedral angle of either
gauche 60° or anti 180° to the C–H bond at the site of attachment
on the keto ring. (The gauche is pictured in Scheme 2.) Here, the
benzylic radical centers are planar sp2 and the two carbons form-
ing the new bond are each pyramidalized toward an sp3 center, as
can be seen in the DFT-optimized structures in the Supporting
Information, vide infra.
(
cation of isoeugenol (49) and accelerates the rate of formation of
DHDIE as the only “dimeric” product. On the other hand,
reported mild chemical oxidation of isoeugenol by stable radical
DPPH in methanol yields a complex mixture of DHDIE and its
methylated derivatives (40).
In our photooxidation tests, isoeugenol underwent oxidative
dimerization,” while eugenol did not react. Contrary to that,
“
both isoeugenol and eugenol underwent “mild” chemical oxida-
tion (40) yielding the mixture of several lignans including dehy-
drodieugenol (product of oxidation of eugenol) and its
methylated derivatives. Thus, photochemical oxidation in DMSO
as reported by us has the following advantages vs reported
When the gauche and anti benzylic radical approaches were
considered, seven structures were found for the neutral radical–
neutral molecule dimerization reaction. The first structure is a
van der Waals complex of the reactants in a position resembling
the anti TS (TS1) (second structure) for the formation of the
“
mild” chemical oxidation (40): 1) photochemical oxidation
selectively proceeds via internal C=C bond in isoeugenol, but
not terminal C=C bond in eugenol and 2) photochemical oxida-
tion leads to fewer number of products. To the best of our

1
336 Morgan Zielinski (Goldberg) et al.
Scheme 1. Formation of the radical cation and radical of isoeugenol from isoeugenol under the near-UV light.
bond is the rate limiting step and the formations of the radical
intermediate through the gauche and anti rotamers (Scheme 2)
are competitive to each other. Figure 6 shows the calculated
reaction profile.
The spin density on the reactants and product lies on the keto
ring fragment, while the spin density of the INT is localized in
the conjugated –CHAr fragment. This might account for the
absence of cyclic product with eugenol rather than isoeugenol.
The unpaired electron in the radical intermediate with the euge-
nol isomer, if it could be formed, would not have resonance sta-
bilization with the Ar group due to the intervening CH group in
2
the –CHCH Ar fragment. Table 1 provides the total energies in
2
ꢀ1
Hartrees and energies in kcal mol
relative to the van der
Waals complex of the reactants. Also given is the energy and
relative energy for the TS2 leading directly to Int2 from the van
der Waals complex via the gauche approach.
After finding the reaction mechanism involving a neutral radi-
cal, the search for a similar pathway was attempted for the
dimerization of two neutral molecules, i.e. without involving a
hydrogen abstraction, but no TS structure could be found for the
dimerization of two neutral molecules. After the DFT computa-
tional study of photooxidation of eugenol and isoeugenol, we
proceeded to experimental work on photocatalytic oxidation in
the presence of titanium dioxide in solution.
Photocatalytic oxidation of eugenol and isoeugenol with P25
TiO
2
To the best of our knowledge, photocatalytic oxidation of eugenols
in the nonprotic solvents has not been reported. Titanium dioxide
TiO Degussa P25 recently rebranded as Evonik Aeroxide (31) is
2
a benchmark photocatalyst which consists of ca 80% anatase,
rutile and some amorphous phase (52) and has the total surface
area of 30–50 m g . We have chosen to use 50 mg TiO photo-
2
Scheme 2. Stage-wise reaction of the radical of isoeugenol with a mole-
cule of isoeugenol through radical intermediates.
2
ꢀ1
catalyst per 10 mL liquid phase (see Materials and Methods),
since such a solid/liquid ratio is close to the high limit of the range
ꢀ
1
third structure which is the anti 180° intermediate (Int1). This
anti structure can then rotate through an eclipsed rotational TSr
0.5–3.0 g L recommended in the recent critical note (53) for the
2
ꢀ1
photocatalysts with the total surface area 50–200 m g . Further-
more, for the photocatalytic reactions in solution (31), one needs
to take into account the adsorption of the reactant on the surface of
the photocatalyst. Adsorption of certain phenolic compounds
obtained from lignocellulosic biomass has been reported, e.g. sali-
(
(
fourth structure) to form the gauche 60° intermediate (Int2)
fifth structure). The gauche intermediate then goes through a TS
structure (TS CO) (sixth structure, center structure of Scheme 2)
which then forms the ring product (seventh structure).
The DFT-optimized structures are provided in the Supporting
Information: van der Waals reactants (Figure S1), TSs Int1
2
cylic acid (33) in solution on TiO . With the rather high concentra-
tions of isoeugenol and isoeugenol in solution used by us (0.5 M)
and the high liquid/solid ratio as above, one should expect that the
amount of adsorbed phenolic compounds will be insignificant vs
the total amount present.
We have determined the molar concentration of the DHDIE
product in the photocatalytic oxidation of isoeugenol in the pres-
(
Figure S2), TSr (Figure S3), Int2 (Figure S4), TS CO (Fig-
ure S5) and the cycloproduct (Figure S6); TS TS1 looks very
similar to Int1 and is not shown. An energetically unfavorable
TS2 (gauche) state is provided in Figure S7. The Cartesian
coordinates and Gaussian09 input files of the DFT-optimized
reactants, TSs TS1, Int1, TSr, Int2, TS CO, the cycloproduct
and intermediate TS2 are provided in the Table S1. It was
found that the TS for the formation of the anti 180° intermedi-
2
ence of TiO using the HPLC-UV-MS method. In the reference
experiments conducted in the dark in the presence of TiO , no
2
reaction products have been detected. Figure 7a shows the molar
ꢀ
1
ate is ca 2 kcal mol lower than the TS for the formation of
the gauche 60° intermediate (TS2). The formation of the C–O
concentration of DHDIE obtained in the presence of TiO
dipole moment of the nonprotic solvents used.
2
vs

Photochemistry and Photobiology, 2015, 91 1337
Figure 6. Reaction profile of the stepwise reaction of the radical of iso-
eugenol with the molecule of isoeugenol determined by the density func-
tional theory (DFT) calculations.
Table 1. Total energies and energies relative to the van der Waals com-
plex of the reactants. Energetically unfavorable TS2 (compared to TSr) is
shown in italic font.
Total energy,
Hartree
Energy vs the van der Waals complex of
1
ꢀ
Structure
reactants, kcal mol
Figure 7. Molar concentration of the dehydrodiisoeugenol (DHDIE) pro-
duct in the photocatalytic oxidation in solution: (a) vs the dipole moment
of the solvent; (b) vs oxygen solubility in the solvent.
Reactants
TS1
Int1
TSr
Int2
TS CO
Products
TS2
ꢀ1075.582
ꢀ1075.550
ꢀ1075.567
ꢀ1075.551
ꢀ1075.562
ꢀ1075.543
ꢀ1075.574
ꢀ1075.547
0.0
19.6
9.1
19.2
12.2
24.4
4.4
It is known that with water as solvent, a major ROS in the
photocatalytic oxidation of organic compounds (55) is hydroxyl
radical HO˙. This hydroxyl radical is produced from water via its
oxidation by the hole h+ formed in the valence band upon the
21.8
photoexcitation of TiO
aqueous suspensions of TiO
2
. Hydroxyl radicals formed in illuminated
were detected by the electron para-
2
One can see that the dependence is significantly different from
that for the photochemical oxidation, compare Figs 5a and 7a.
This indicates that there is some other factor that affects the yield
of the DHDIE in the photocatalytic oxidation. Figure 7b shows
the molar concentration of DHDIE obtained in the presence of
magnetic resonance (EPR) spectroscopy with spin trapping
method (56). Specifically, illumination at k > 300 nm of aque-
ous suspension of TiO with dissolved spin trap 5,5-dimethyl-1-
2
pyrroline N-oxide (DMPO) resulted in the DMPO-OH˙ spin
adduct via the interaction of hydroxyl radicals with DMPO. On
the other hand, in the polar nonprotic solvents, a major ROS is
TiO
2
vs the solubility of oxygen in each solvent. The solvents
CN) show the
ꢀ
with the higher oxygen solubility (DMF and CH
3
the superoxide anion-radical O2 as we noted in the recent review
ꢀ
higher yield of DHDIE that indicates the involvement of reactive
oxygen species (ROS) in the photocatalytic oxidation of isoeu-
genol. Phenolic compounds from lignin containing olefinic C=C
bond were reported to react with singlet oxygen in nonprotic sol-
vents forming products from [2 + 4] cycloaddition reactions (en-
doperoxides) as well as from [2 + 2] cycloaddition reaction (54).
Some of resultant peroxides have decomposed to yield the final
anisaldehyde product, i.e. olefinic C=C bond in the reactant has
been oxidized to aldehyde group (54). Singlet oxygen is known
to be very strong nonselective oxidant; formation of singlet oxy-
gen in significant amounts under our experimental conditions
would have resulted in the products of oxidation of C=C bond in
eugenol which did not form any products with or without tita-
nium dioxide photocatalyst. Therefore, formation of singlet oxy-
gen as reactive intermediate is unlikely under our experimental
conditions; to definitely answer this question, a separate mecha-
nistic study would be needed.
(31). In the nonprotic solvents, the anion-radical O2 is formed
by reduction of dissolved oxygen with electrons (57) generated
in the conduction band of the photoexcited photocatalyst as fol-
lows, where D is the molecule of the substrate:
þ
ꢀ
TiO2 þ hv ! h ðTiO2Þ þ e ðTiO2Þ;
ꢀ
ꢀ
e ðTiO2Þ þ O2 ! O þ TiO2;
2
þ
þ
h ðTiO2Þ þ D ! D þ TiO2:
ꢀ
Anion-radicals O2 were detected in the suspension of TiO
in
2
acetonitrile under the illumination at 370 nm in the presence of
oxygen by the EPR spectroscopy with DMPO spin trap (58).
Interestingly, without solvent, isoeugenol produces only the small
amounts of DHDIE in the photocatalytic oxidation (Fig. 7)
which possibly indicates that the (polar) solvent stabilizes the

1
338 Morgan Zielinski (Goldberg) et al.
charged ROS. We propose that the selective photocatalytic
oxidation of isoeugenol to DHDIE in acetonitrile (and possibly
Figure S7. The DFT-optimized structure for reaction of isoeu-
genol radical with the molecule of isoeugenol: transition state
TS2. Green atoms: carbon; red atoms: oxygen; white atoms:
hydrogen.
ꢀ
in DMF) proceeds via anion-radicals O ; experimental and com-
2
putational work is in progress to test this hypothesis.
Table S1. The Cartesian Coordinates in Angstroms and Gaus-
sian input files for Reactants, TS1, Int1, TSr, Int2, TS CO, Pro-
ducts and intermediate TS2.
CONCLUSION
Selective activation of chemical bonds in sustainable phenolic
compounds from plant biomass using light is important for
producing “green” chemicals. For the first time, we report a
selective activation of the C=C bond in isoeugenol vs isomeric
eugenol by photocatalytic and photochemical oxidation to yield
the lignan DHDIE. Selective photocatalytic oxidation with
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Figure S3. The DFT-optimized structure for reaction of isoeu-
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Figure S5. The DFT-optimized structure for reaction of isoeu-
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