Photodeoxygenation of dinaphthothiophene, benzophenanthrothiophene, and benzonaphthothiophene: S-oxides

Article abstract of DOI:10.1039/c5pp00466g

Photoinduced deoxygenation of dibenzothiophene S-oxide (DBTO) has been suggested to release atomic oxygen [O(³P)]. To expand the conditions and applications where O(³P) could be used, generation of O(³P) at longer wavelengths was desirable. The sulfoxides benzo[b]naphtho-[1,2,d]thiophene S-oxide, benzo[b]naphtho[2,1,d]thiophene S-oxide, benzo[b]phenanthro[9,10-d]thiophene S-oxide, dinaphtho[2,1-b:1′,2′-d]thiophene S-oxide, and dinaphtho[1,2-b:2′,1′-d]thiophene S-oxide all absorb light at longer wavelengths than DBTO. To determine if these sulfoxides could be used to generate O(³P), quantum yield studies, product studies, and computational analysis were performed. Quantum yields for the deoxygenation were up to 3 times larger for these sulfoxides compared to DBTO. However, oxidation of the solvent by these sulfoxides resulted in different ratios of oxidized products compared to DBTO, which suggested a change in deoxygenation mechanism. Density functional calculations revealed a much larger singlet-triplet gap for the larger sulfoxides compared to DBTO. This led to the conclusion that the examined sulfoxides could undergo deoxygenation by two different mechanisms.

Full text of DOI:10.1039/c5pp00466g

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ARTICLE
Photodeoxygenation of dinaphthothiophene,
benzophenanthrothiophene, and benzonaphthothiophene S–
oxides
Received 00th January 20xx,
Accepted 00th January 20xx
X. Zheng, S. M. Baumann, S. M. Chintala, K. D. Galloway, J. B. Slaughter, and R. D. McCullaa
DOI: 10.1039/x0xx00000x
Photoinduced deoxygenation of dibenzothiophene S-oxide (DBTO) has been suggested to release atomic oxygen [O(3P)].
To expand the conditions and applications where O(3P) could be used, generation of O(3P) at longer wavelengths was
desirable. The sulfoxides benzo[b]naphtho-[1,2,d]thiophene S-oxide, benzo[b]naphtho[2,1,d]thiophene S-oxide,
benzo[b]phenanthro[9,10-d]thiophene S-oxide, dinaphtho[2,1-b:1’,2’-d]thiophene S-oxide, and dinaphtho[1,2-b:2’,1’-
d]thiophene S-oxide all absorb light at longer wavelengths than DBTO. To determine if these sulfoxides could be used to
generate O(3P), quantum yield studies, product studies, and computational analysis were performed. Quantum yields for
the deoxygenation were up to 3 times larger for these sulfoxides compared to DBTO. However, oxidation of the solvent by
these sulfoxides resulted in different ratios of oxidized products compared to DBTO, which suggested a change in
deoxygenation mechanism. Density functional calculations revealed a much larger singlet-triplet gap for the larger
sulfoxides compared to DBTO. This led to the conclusion that the examined sulfoxides could undergo deoxygenation by
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two
different
mechanisms.
Introduction
The photodeoxygenation of aromatic heterocyclic oxides, such
as dibenzothiophene S-oxide (DBTO), is believed to generate
ground-state atomic oxygen [O(3P)], and this oxidant displays a
reactivity considerably different than other reactive oxygen
species (ROS).1-7 Specifically, O(3P) reacts selectively towards a
series of small organic molecules, and the photodeoxygenation
of a DBTO derivative results in the near quantitative oxidation
of a specific cysteine residue of the plant protein adenosine-5’-
phosphosulfate kinase.8,9 The unique selectivity and ability to
photorelease O(3P) raises the possibility of targeting a specific
biomolecule for oxidation in a complex mixture; however, the
limited number of methods and conditions suitable for the
generation of O(3P) constrains potential applications.
While the photodeoxygenation of DBTO is one of the few
methods capable of cleanly producing O(3P) in solution, one
limitation of DBTO is the need for UV-A irradiation. Absorption
of wavelengths above 350 nm by DBTO is miniscule. Thus, any
sample containing molecules capable of absorbing UV-A
irradiation increase the risk of complicating photochemistry.
One approach to circumvent this obstacle is to extend the
chromophore of DBTO in order to red-shift the absorption
spectrum. To these ends, the ability of benzo[b]naphtho-
[1,2,d]thiophene
S-oxide
(
1),
benzo[b]naphtho[2,1,d]-
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was prepared by a new synthetic route. Full details of the sulfoxides
1-5 preparation are given in the electronic supporting information.
General procedure for oxidation of DBT and the corresponding
thiophene of 1-5. First, 120.0 mg (0.42 mmol) dinaphtho[1,2-
b:2',1'-d]thiophene was dissolved in 100 mL CH2Cl2 at -30 °C. Then
127 mg of 77 wt% mCPBA (0.57 mmol) was dissolved in another 100
mL of dichloromethane and added to the reaction mixture
dropwise. After the reaction mixture was warmed to room
temperature, the reaction mixture was washed with saturated
sodium bicarbonate solution. The organic layer was collected, and
the aqueous layer was extracted with CH2Cl2 three times. The
combined organic extracts were dried over Na2SO4 and then
concentrated under reduced pressure. The crude product was
subjected to column chromatography (CH2Cl2 as eluent) to afford
dinaphtho[2,1-b:1',2'-d]thiophene S-oxide (31.2 mg, 0.110mmol,
26.1% yield).
thiophene S-oxide (
2
), benzo[b]phenanthro[9,10-d]thiophene
), and
) to generate an
oxidant similar to the parent DBTO is examined in this work.
The irradiation of in 2-methylbutane yields a similar product
S-oxide (
3), dinaphtho[2,1-b:1’,2’-d]thiophene S-oxide (4
dinaphtho[1,2-b:2’,1’-d]thiophene S-oxide (
5
2
mixture as DBTO, which provides evidence for the generation
of a similar oxidant.1-3 Additionally, the photodeoxygenation of
2
can be carried out at wavelengths as high as 385 nm.
Production of O(3P) during the photodeoxygenation of
is
further supported by the similar yields of styrene oxidation
products arising from photodeoxygenation of
and O(3P)
2
2
generated by microwave discharge methods (MDM).4 Thus, we
posit the increased benzannulation of DBTO is a promising
strategy to achieve the production of O(3P) at wavelengths
stretching into the visible spectrum.
Photodeoxygenation of 1-5 is expected to produce an oxidant
similar to the parent DBTO. While the available evidence
suggests the oxidant released by DBTO is O(3P), direct
detection of O(3P) in solution is challenging due to its reactivity
and limited spectral window. Indirect evidence for identity of
this oxidant as discrete O(3P) arises from the comparison of
products from multiple sources of O(3P), such as the MDM and
General Methods
Absorption spectra were recorded using a NanoDrop 2000c UV-Vis
spectrophotometer using samples contained in 10 × 10 mm quartz
cell. NMR spectra were recorded using a Bruker DRX-400 NMR.
High-resolution mass spectra were obtained using a JEOL JMS-700
mass spectrometer. An Agilent 1200 Series HPLC fitted with a
quaternary pump and diode array detector was used for HPLC
chromatographs run on an Agilent Eclipse XDB-C18 column (5μm,
150 x 4.6mm). For gas chromatograph analysis, a Hewlett-Packard
5900 Series GC fitted with a flame ionization detector and a Sciencix
CTS-5 column (0.25mm x 30m x 0.25um) was used.
photodeoxygenation of 2 cited above. Additionally, the oxidant
reacts with O2 to generate O3 and displays diffusion-limited
rate constants of a magnitude only possible for a very small
oxidant.4,8,10 Regardless of the exact nature of the oxidant,
O(3P) is used in this article to refer to the oxidant generated
during the photodeoxygenation of DBTO
.
As discussed earlier, a distinguishing feature of O(3P) is a
unique selectivity. For example, hydroxyl radical (•OH) reacts
with a large array of organic compounds at nearly diffusion-
limited rates; whereas, rate constants for the reaction
between O(3P) and the same array of organic compounds
spanned over six orders of magnitude.8 More recently, this
selectivity also extends to biological samples.9-13 In addition to
the selective oxidation of cysteine mentioned earlier,
photodeoxygenation of a DBTO derivative generated fatty
aldehydes associated with plasmalogens over other possible
Determination of Quantum Yields
For quantum yields determined at 330 nm, a 75 W Xe lamp focused
directly on a monochromator (Photon Technologies International)
was used in most experiments. Slit widths were set to allow 6 nm
linear dispersion. Samples of 4.0 mL in 1 cm square quartz cells
were placed in a permanently mounted cell holder. The holder was
placed such that all exiting light entered the sample cell without
further focusing. All samples were sparged with argon (>20
minutes) prior to irradiation. All quantum yield experiments were
carried out at a concentration high enough to reach an optical
unsaturated fatty acids in low-density lipoprotein (LDL).11 The density >2 at the given wavelength and carried out to low
production of O(3P) is also known to induce single-strand
cleavage in DNA.12,13 These results suggest O(3P) could be used
points was performed and carried out with HPLC or GC analysis.
conversion (<25%). Analysis of the reaction mixtures at various time
Photolysis of azoxybenzene to yield the rearranged product,
to create specific oxidation products in more complicated
o-hydroxyazobenzene, was used as a chemical actinometer.20
biological samples. For these samples, the efficient production
of O(3P) at longer wavelengths, where these samples are more
Product studies from steady state irradiations
transparent, is desirable.
For DBTO and 1-5, solutions for irradiation with concentrations
ranging from 1-6 mM were prepared. No significant variation in the
Experimental and Computational Methods
observed yields was observed over this range of concentrations.
Prior to irradiation, samples were degassed by argon sparging (≥20
Materials
minutes). The solutions were irradiated with 14 UVA bulbs (LZC-
Commercial materials were obtained from Aldrich, Fisher or
UVA, 8 watts, broad spectrum with λmax= 350 nm) in a LZC-4X
AKscientific and used without modification, except as noted. HPLC
photoreactor from Luzchem. The conversion of DBTO and 1-5 to the
grade toluene and benzene were used for the solvent oxidation
corresponding thiophene was not allowed to exceed 30%. For
experiments. The corresponding thiophene of compounds 1-4 were
analysis, dodecane was used as internal standard. Small samples
prepared according to literature procedure with slight
were removed by syringe and subjected to HPLC or GC analysis.
modifications.14-19 Dinaphtho[1,2-b:2’,1’-d]thiophene S-oxide (5)
2 | J. Name., 2012, 00, 1-3
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Scheme 1
ARTICLE
Computational Methods
electrocyclization and aromatization. Oxidation of 8 by mCPBA
yielded 5.
Initial guess geometries were generated HF/6-31G(d) level of
theory. To determine the redox potentials of the sulfoxides, the
approach described by Davis and Fry was used.21 In this approach,
Photodexoygenation of 1-5
Photodeoxygenation of 1-5 was expected to generate O(3P) and the
the neutral species, radical cation, and radical anion of each corresponding thiophene. To confirm this hypothesis, the oxidation
compound were optimized separately using B3LYP/6-31G+(d) with
SMD/IEF-PCM solvation in acetonitrile. Vibrational analysis was
used to confirm the absence of imaginary frequencies and to
determine the thermal free energies at 298.15 K. To determine
excited-state energies, optimized geometries for the S0, S1, and T1
were determined at the B3LYP/6-31G(d,p) or TD-B3LYP/6-31G(d,p)
level of theory. Again, vibrational analysis was used to confirm the
absence of imaginary frequencies and provide thermal corrections.
To examine if a larger basis set or different levels of theory would
improve performance, the energies at these geometries were
further refined using M06-2X, CIS, or B3LYP level of theory with the
aug-cc-pv(T+d)Z basis set.22 All geometry optimizations were
performed with the Gaussian 09 or ORCA suite of programs.23,24
products that arose from the irradiation of DBTO or 1-5 were
compared. The enlarged chromophores of 1-5, compared to the
parent DBTO, were expected to red-shift their absorption spectra—
potentially into the visible region, and further, to allow the
photogeneration of O(3P) in samples containing compounds that
absorb in the UV-A region of the spectrum without the
complicating photochemistry.
UV-Vis Absorbance of 1-5. One of the main drawbacks of DBTO as a
O(3P) precursor for applications in biological samples is the need for
wavelengths below 350 nm to release O(3P). To examine the
potential of 1-5 to generate O(3P) at longer wavelengths, the
ground-state absorption spectra were obtained for 1-5 and are
shown in Figure 1. In Figure 1(A), the absorption spectra of 1, 2, and
DBTO are shown. Three absorption bands were observed for DBTO
with the λmax of the third band being 319 nm. For 1 and 2, the λmax
of the longest wavelength band was 353 and 341 nm, respectively.
Additionally, the extinction coefficient (ε) for 1 and 2 fell below 100
at 386 and 395 nm, respectively. As expected, the spectra of 1 and 2
extended past 350 nm; however, significant absorption for 1 and 2
did not extend into the visible.
Results and Discussion
Synthesis of Dinaphtho[2,1-b:1',2'-d]thiophene S-oxide (5).
Unlike the sulfoxides 1-4, whose corresponding thiophenes were
prepared by literature procedures,14-19 a new synthetic route was
utilized to prepare 5. As shown in Scheme 1, 5 was prepared in four
steps from 3,4-dibromothiophene (6). The initial Suzuki-Miyaura
coupling of
6 and trans-2-phenylvinylboronic acid was most
The additional benzo moiety was expected to red-shift the
efficient when using the ligand, 2-(2‘,6‘-dimethoxybiphenyl)-
dicyclohexyl-phosphine (SPhos). With these conditions, 3,4-bis-[(E)-
2-phenylethenyl]-thiophene (7) was isolated with a moderate yield
of 71.0%. The conversion of 7 to dinaphtho[2,1-b:1’,2’-d]thiophene
(8) involved a trans to cis conversion followed by a photoinduced
electrocyclization and oxidative aromatization. Side products
observed during the photocyclization were reduced by first
isomerizing 7 to the cis-conformations. Irradiation of 7 in hexanes
using broadly emitting UVA bulbs (centered at λmax= 350 nm)
resulted in isomerization without cyclization. After isomerization, I2
and propylene oxide were added to the solution, and than the
solution was irradiated at 254 nm for one additional hour to induce
longer wavelength band of 3-5 compared to
absorption spectra of 3-5 are shown in Figure 1(B). The
sulfoxide had the most red-shifted absorption spectrum. The
max of the longest wavelength band for 4 was 387 nm compared to
1 and 2. The
4
λ
361 and 363 nm for 3 and 5, respectively. The absorption of 4
extends the furthest into the visible region with its extinction
coefficient (ε) falling below 100 at 437 nm compared to 419 and
423 nm for 3 and 5, respectively.
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Table 1. Quantum Yield for the Deoxygenation of 1-5.a
Sulfoxide
λ (nm)
320
Φꢀsulfoxide
Φ+sulfide
DBTO
0.0026 ± 0.0004b
0.0042 ± 0.0018
0.0047 ± 0.0015
0.0078 ± 0.0012
0.0015 ± 0.0012
0.0035 ± 0.0009
1
2
3
4
5
330
0.0040 ± 0.0001
0.006 ± 0.003
330
330
0.0095 ± 0.0005
0.0027 ± 0.0018
0.008 ± 0.004
330
330
a Quantum yields for the photodeoxygenation of thiophene S-oxides
b
1-5 all measured in acetonitrile.
DBTO photodeoxygenation
quantum yield value from reference 2.
The quantum yields for deoxygenation for 1-5 are slightly higher
than those for DBTO. The largest quantum yields values of 0.0095
for Φ-sulfoxide and 0.0078 for Φ+sulfide were observed for 3.
Solvent Oxidation Products. Since O(3P) is difficult to detect, the
yields of oxidized products have been used previously to indicate
that the same oxidant was being produced.5,10,25 Yields of the
oxidized products resulting from the irradiation of the parent DBTO
in toluene were compared to previously reported studies.5,25 In our
experiments, products arising from oxidation of the methyl group
(PhCHO and BnOH) accounted for 8% of the oxidant produced in
the photodeoxygenation of DBTO, and oxidation of the ring (o-
cresol, m-cresol, and p-cresol) accounted for 38% as shown in Table
2. Previous studies have reported total yields of oxidized products
as 73% and 78%, and thus, our total yield of 46% was considerably
less. Additionally, the ratio of methyl to ring oxidation of 1:4.8 was
less than the previously reported values of 1:2 and 1:1.6.
Figure 1. UV Spectra of the DBTO and thiophene S-oxides 1-5. (A)
UV Spectra of DBTO (solid line), 1 (solid line with closed square),
and 2 (solid with closed circle) in acetonitrile. (B) UV Spectra of 3
(dotted line), 4 (solid line with open square), and 5 (solid line with
open circle) in acetonitrile. Inserts depict expanded view of longer
wavelengths.
Comparison of our yields to those previously reported for DBTO
indicate the yields of oxidized products are sensitive to the
experimental conditions. Sensitivity to conditions is a common
observation for the photodeoxygenation of DBTO. For example, the
presence of molecular oxygen (O2) can change the yields and ratio
of oxidized products that accompany the photodeoxygenation of
DBTO and its derivatives.5,11 Given the known sensitivity to
experimental conditions, the yields obtained from the
Quantum Yields of Deoxygenation. The quantum yield of
photodeoxygenation of DBTO has typically been found to range
from 0.002 to 0.010 in different organic solvents.2 Table 1 list the
quantum yields for the deoxygenation of 1-5 irradiated at 330 nm in
acetonitrile. To examine the photodeoxygenation, the quantum
yield for the loss of 1-5 (Φ-sulfoxide) and the appearance of the
corresponding thiophene (Φ+sulfide) were determined. The values
ranged from 0.0095 to 0.0027 for Φ-sulfoxide and 0.0078 to 0.0015 for
photodeoxygenation of DBTO in our experiments provided
suitable standard to compare yields obtained for compounds 1-5.
a
Φ
+sulfide. The quantum yields for the loss of sulfoxide (Φ-sulfoxide) and
the formation of the corresponding thiophene (Φ+sulfide) were within
the experimental error of each other. No other significant products
were detected by HPLC or GC analysis. As expected, the
photodeoxygenation of 1-5 directly resulted in the formation of
corresponding thiophene as the only identifiable major product.
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Table 2. Comparison of toluene oxidation product yields by DBTO and thiophene S-oxides 1-5.a
Sulfoxide
PhCHO
BnOH
o-cresol
m- & p-cresolb
Total %yield
Oxidation
oxidized toluene
CH3:Ringc
1 : 2.0
1 : 1.6
1 : 4.8
1 : 2.0
1 : 1.6
1 : 1.0
4.9 : 1
1 : 1.5
DBTOd
14 ± 3
17 ± 3
10 ± 1
13 ± 4
5±1
25 ± 3
26 ± 5
24 ± 6
22 ± 5
73 ± 7
DBTOe
78 ± 9
DBTOf
3±1
20±1
18±1
46 ± 2
1f
2f
3f
4f
5f
2.4±0.5
1.7±0.4
2.1±0.5
8.3±1.1
1.6±0.2
5.1±0.8
3.4±0.6
4±1
9.8±0.8
5.5±0.3
3.8±0.9
2.8±0.4
3.3±0.0
5.6±0.3
2.7±0.2
2.4±0.6
1.3±0.2
2.4±0.1
22.9 ± 1.3
13.3 ± 0.8
12.3 ± 1.6
24.2 ± 2.2
9.5 ± 0.5
11.8±1.8
2.2±0.5
a Yields of toluene oxidation products relative to the formation of the corresponding sulfide of DBTO and thiophene S-oxides 1-5. The
oxidation products are benzaldehyde (PhCHO), benzyl alcohol (BnOH), and all three cresol isomers. b Measured as a single peak by GC-FID. c
Ratio of PhCHO plus BnOH yields to combined cresol yields. d Values from reference 5. e Values from reference 25. f This work.
As shown in Table 2, a significant decrease in the total yield of must have produced an oxidant or species that preferentially
oxidized toluene products for 1-5 compared to DBTO was observed. oxidized toluene to benzaldehyde or benzyl alcohol over cresols.
For 1-5, the yield of all oxidized solvent products only accounted for
The yields of the oxidized solvent from the irradiation of 1-5 in
24.2% of the deoxygenation of 4 at the high end and ranged down
toluene suggested the mechanism was different than that of DBTO.
to 9.5%, which was observed for 5. This change represented a
To confirm this result, DBTO and 1-5 were irradiated in benzene,
nearly 2-fold (at least) decrease compared to the 46% overall yield
and two oxidation products were observed: phenol and biphenyl.
of oxidized solvent observed for DBTO. This decrease indicated a
significant fraction, possibly all, of the photodeoxygenation of 1-5
occurs through a different mechanism than the parent DBTO.
The ratio of the oxidized solvent products was used to determine if
a common oxidant was formed. With the notable exception of
compound 4, photodeoxygenation resulted in higher yields of ring
oxidation products (cresols) than the combined methyl oxidation
products (benzaldehyde and benzyl alcohol). The ratio of methyl
Table 3. Comparison of benzene oxidation product yields by DBTO
and thiophene S-oxides 1-5.a
oxidation products to cresols was 1:4.8 for DBTO in this work. The
ratio of methyl to ring oxidation for the photodeoxygenation 1-5
Sulfoxide
Phenol
34 ± 3
Biphenyl
0.6 ± 0.1
0.6 ± 0.1
0.6 ± 0.1
0.8 ± 0.1
1.1 ± 0.2
0.7 ± 0.1
Total
PhOH:PhPh
57:1
ranged from 1:2 to 1:1 with notable exception of 4 whose ratio was
4.9:1. The change in the ratio of oxidized products suggested a
fraction, possibly all, of the photodeoxygenation of 1-5 generating
an oxidant that was different than the parent DBTO. However, the
ratio of methyl to ring oxidation for 1-3 and 5 was similar to results
previously reported for DBTO of 1:1.6 and 1:2.5,24 The errors
associated with the ratio of oxidized solvent for DBTO and 1-5 were
all insignificantly small, and thus, these errors were listed in Table
S1 in the ESI.
DBTO
35 ± 3
1
2
3
4
5
12.8 ± 0.6
14.3 ± 1.3
11.1 ± 0.8
7.2 ± 0.9
8.4 ± 0.4
13.4 ± 0.6
14.9 ± 1.3
11.9 ± 0.8
8.3 ± 0.9
9.1 ± 0.4
21:1
24:1
14:1
7:1
12:1
a Yields of benzene oxidation products relative to the formation of
the corresponding sulfide of DBTO and the thiophene S-oxides 1-5.
The oxidation products were phenol and biphenyl.
As noted above, the ratio of benzaldehyde and benzyl alcohol to
cresols was 4.9:1 for 4, which was the reverse of all other examined
thiophene S-oxides that yielded more cresols. Thus, the
photodeoxygenation of 4 must proceed by a different mechanism, For the oxidation of benzene, a similar trend as observed in the
at least in part, than that of DBTO. Additionally, this mechanism oxidation of toluene emerged. For DBTO, the formation of phenol
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was favored over biphenyl in a ratio of 57:1 as shown in Table 3. For Triplet sensitized photodeoxygenation of DBTO. Deoxygenation of
1, 2, 3 and 5, a more than 2-fold decrease compared to DBTO in the DBTO has also been induced by selective irradiation of common
yield of phenol relative to the amount of deoxygenation was triplet sensitizers, namely anthraquinone and benzophenone, in
observed. Irradiation of 4 in benzene resulted in the lowest yield of toluene.25 Yields of the DBT ranged from 83 to 94%, and the
phenol at 7.2% and highest yield of biphenyl at 1.1%. The ratio of toluene oxidation products of PhCHO, BnOH, and all three cresol
7:1 phenol to biphenyl for 4 was significantly different than all the isomers were observed. The ratio of the products arising from
other thiophene S-oxides examined. This trend was also consistent methyl oxidation (PhCHO and BnOH) and ring oxidation (cresols)
with a change in mechanism for the photodeoxygenation of 1-5.
was compared to those obtained by the direct irradiation of DBTO.
Direct irradiation of DBTO resulted in a ratio of 1:1.6 for CH3 versus
ring oxidation; however, the ratio was reversed when a triplet
sensitizer was used. The ratio of CH3:ring oxidation was 3.5:1 for
anthraquinone and 8.1:1 for benzophenone, which was similar to
the 4.9:1 ratio observed for 4. Additionally, the overall yields of
toluene oxidation products dropped substantially in the sensitized
conditions compared to direct irradiation. This led to the conclusion
that the mechanism of DBTO deoxygenation in direct and sensitized
As will be discussed below, we posited this change was the
emergence of two competitive deoxygenation mechanisms. The
first mechanism being the release of O(3P) leading preferentially to
oxygen insertion into arene C–H bonds and cresol formation. The
second mechanism was postulated as being similar to triplet
sensitized deoxygenation of DBTO.25
Photodeoxygenation mechanisms
The results of the product studies indicated there was a change or conditions were different. The sensitized mechanism was believed
emergence of a competing mechanism in the photodeoxygenation to occur through the T1 state as shown in Figure 1.
of 1-5 compared to DBTO. For DBTO, three distinct mechanisms of
Bimolecular photoreduction photodeoxygenation of DBTO. In the
deoxygenation have been previously reported.2,25,26 Direct
presence of methoxide, photoinduced one-electron transfer from
irradiation of DBTO has been posited to undergo a unimolecular S–
methoxide to DBTO or any aryl sulfoxide was believed to initiate
O bond scission leading to DBT and O(3P). In addition to this
deoxygenation. The resulting 9-S-3 hydroxysulfuranyl radical anion,
unimolecular mechanism, bimolecular triplet sensitization and one-
of the sulfoxide was believed to protonate and undergo
a
electron reduction of DBTO have also been shown to result in
deoxygenation. Given the structural similarity of 1-5 to DBTO, it was
hypothesized the change in solvent oxidation products for 1-5 was
the result of a mechanism similar to triplet sensitized or one-
electron reduction mechanisms becoming competitive or dominant
over the release of O(3P).
heterolytic S–O bond cleavage. Further evidence for this
mechanism was obtained by selectively irradiating N-
methylcarbazole, a known electron donor, in the presence of DBTO
to generate the 9-S-3 hydorxysulfuranyl radical anion.
The working hypothesis for the change in solvent oxidation for 1-5
was that a mechanism similar to the triplet sensitized or one-
electron reduction mechanism was competitive with or dominant
over the release of O(3P). To gain insight into this possibility, redox
and excited state properties of 1-5 and DBTO were examined with
computational methods.
Direct irradiation of DBTO. In organic solvents, the deoxygenation
of DBTO by direct irradiation has been postulated to occur via
dissociative T2 state. This mechanism was based on CASSSCF and
MRMP2 calculations.27 These calculation demonstrated the T2 state
of thiophene S-oxides dissociates to the corresponding thiophene
and O(3P). At the S1 optimized geometry, the energy of T2 was
Computed Properties of DBTO and thiophenes 1-5
below that of S1 which indicated intersystem crossing (ISC) from S1 One potential cause for the difference in the direct irradiation of
to T2 was energetically feasible. Additionally, a considerable barrier DBTO and 1-5 would be a change in the excited state energies.
to S–O cleavage along the T1 surface was found. Thus, the Differences in the excited energies of 1-5 could change the
mechanism for O(3P) generation was proposed to result from the partitioning into T1 and T2 and account for the observed difference
direct excitation into the S1 state of DBTO followed by ISC into the in product yields. For example, an increase in the state energy of T2
dissociative T2 state as shown in Scheme 2.
could make it inaccessible from the S1 state. To examine if the
excited state energies of 1-5 were different than DBTO TD-DFT
methods were used.
Scheme 2. Potential deoxygenation mechanisms for DBTO.
Computed Excited State Energies. The S1 and T1 energies of DBTO
were previously determined from its fluorescence and
phosphorescence spectra.28 The S1 energy of DBTO was determined
from the red-edge of the fluorescent spectra as 85 kcal mol-1. The T1
energy of DBTO was determined from the phosphorescence spectra
of DBTO in ether/isopentane/ethanol (EPA) at 77 K as 61 kcal mol-1.
Given the size of thiophene S-oxides 3-5 and available
computational resources, the ability of TD-DFT and CIS methods to
obtain suitable excited state energies within a reasonable time was
evaluated.
The ability of several computational methods to predict the excited
state energies of DBTO was examined by comparing the computed
and experimental values. To these ends, TD-B3LYP and B3LYP with
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the 6-31G(d,p) basis set were used to find the optimized geometries DBTO. While the predicted excited state energies were unlikely
for the S0, S1, and T1 states of DBTO. For accurate energies of sulfur quantitatively accurate, the substantial S1-T1 energy gap for 1-5
containing molecules, a large basis set is often needed.29-32 Thus, compared to DBTO was a consistent feature.
energies at the B3LYP/6-31G(d,p) structures were further refined
Table 5. TD-B3LYP and B3LYP/6-31G(d,p) excited state energies
using the M06-2X, B3LYP, and CIS levels of theory with the aug-cc-
relative to S0.
pV(T+d)Z basis set.22,33 The predicted S1 and T1 energies are shown
a
a
b
Sulfoxide
S1
T1
∆EST
7.0
in Table 4.
DBTO
70.0
65.0
65.8
64.8
62.3
62.7
63.0
49.4
52.3
48.0
42.2
49.0
1
2
3
4
5
15.6
13.5
16.8
20.1
13.7
Table 4. Comparison of computed S1 and T1 energies of DBTO to the
experimentally determined values.a
Method
S1
70
72
83
T1
63
62
70
B3LYP/6-31G(d,p)
B3LYP/aug-cc-pV(T+d)Z//B3LYP/6-31G(d,p)
M06-2X/aug-cc-pV(T+d)Z//B3LYP/6-31G(d,p)
CIS/aug-cc-pV(T+d)Z//B3LYP/6-31G(d,p)
Experimental b
a
S1 and T1 energies in kcal mol-1 determined from the difference
112 80
85 61
between the excited and ground (S0) state energies (H at 0 K) at
B3LYP/6-31G(d,p) level of theory. b S1-T1 energy gap in kcal mol-1.
a S1 and T1 energies in kcal mol-1 for DBTO determined from the
difference between the excited and ground (S0) state energies (H at
0 K) at a given level of theory. b Experimental S1 and T1 energies
from reference. 34
The larger S1-T1 energy gap for 1-5 compared to DBTO could affect
the partitioning into the T1 state versus the release of O(3P) from
the T2 state. The more exothermic intersystem crossing (ISC) from
S1 to T1 or relaxation from T2 could promote partitioning into the T1
The predicted S1 and T1 energies at the B3LYP/6-31G(d,p) level of state. Notably, the largest S1-T1 energy gap was found for 4 whose
theory were 70 and 63 kcal mol-1, respectively. It was anticipated a deoxygenation resulted in preferential benzyl oxidation, which was
larger basis set with large exponential d-functions like identical to the oxidation pattern observed for triplet sensitized
aug-cc-pV(T+d)Z would better replicate the experimentally DBTO deoxygenation. Thus, the larger S1-T1 energy gap appeared to
determined excited state energies. However, only a marginal favor deoxygenation through an alternate mechanism at the
improvement was observed with the S1 and T1 energies being 72 expense of O(3P) generation for 1-5.
and 62 kcal/mol. Switching to the M06-2X functional with the aug-
As stated previously, attempts to find a stable T2 geometry failed
cc-pV(T+d)Z basis set yielded energies of 83 and 70 kcal mol-1, which
since T2 was a dissociative state of DBTO. Since partitioning into T2
was the proposed mechanism of O(3P) generation, a change in the
was not considered a significant improvement. Finally, the CIS/aug-
cc-pVT+dZ level of theory had poor performance predicting values
relative energy gap between T2 and S1 or T1 could affect the release
of 112 and 80 kcal mol-1 for the S1 and T1 state energies,
of O(3P). For example, an increase in the T2 energy relative to S1 or
potentially T1 for 1-5 would be expected to disfavour O(3P) release
respectively.
None of the examined computational methods were likely to give compared to DBTO. To examine this possibility, T2 energies at the
quantitatively valuable predictions of the excited energies of 1-5. optimized geometries of S0, S1 and T1 were determined. These
More expensive computational methods such as MRMP2, have energies are listed in Table S2 in the ESI. For 1-3, T2 was higher in
provided reliable estimates of the T1 state energy for DBTO; energy than S1 at all three geometries, and the relative energy of T2
however, the predicted S1 energy was off by 7 kcal mol-1 or more.27 compared to S1 and T1 at all three geometries was similar to DBTO.
Thus, using more computationally expensive methods such as
Sulfoxides 4 and 5 were exceptions. For sulfoxides 4 and 5, the T2
was 2-5 kcal mol-1 lower than the S1 at the T1 geometry, and the T2
MRMP2 or CASSCF were not likely to improve the predicted values.
Given the additional computational cost of the aug-cc-pV(T+d)Z
energies were consistently lower for 4 and 5 when compared to 1-3
basis set, the B3LYP/6-31G(d,p) level of theory was used to examine
and DBTO at all geometries. However, it was unclear if this was a
the excited state energies of 1-5 for any qualitatively useful trends.
significant result. While the methyl to ring oxidation ratio listed in
The excited state energies of DBTO and 1-5 are given in Table 5. Table 2 is different for 4 compared to 1-3 and DBTO, the 1:1.5 ratio
While a stable minima for the S0, S1 ,and T1 states were found, all for 5 is similar the other sulfoxides. If the lower T2 energies for 4
attempts to find a stable T2 geometry resulted in the S–O bond and 5 were significant, it would be expected that 4 and 5 would
scission. This was expected due to the dissociative T2 state of DBTO. have displayed similar oxidation product ratios. Thus, it was unclear
The predicted S1 energies of 1-5 were relatively similar, ranging if any mechanistic insight could be gained from this data.
from 65.8 to 62.3 kcal mol-1, which was slightly lower than the 70.0
Excited state energies and solvent oxidation products of 4 were
kcal mol-1 for DBTO. The T1 energies of 1-5 were lower than those
substantially different than those compared to 1-3, 5 and DBTO.
found for DBTO. The most striking difference was the T1 energies of
Unlike the 1-3, 5 and DBTO that have relatively flat structures, the
1-5 were all more than 10 kcal mol-1 lower than for DBTO. This
B3LYP/6-31G(d,p) ground-state optimized structure of
4 was
resulted in much larger S1-T1 gap for 1-5, which ranged 13.5 to 20.1
kcal mol-1, which was much larger than the 7.0 kcal mol-1 gap for
twisted as shown in Figure 2. The dihedral angle between the
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naphthyl moieties (Cx-C-C-Cy) was 24.2˚ for 4. The corresponding
dihedral for DBTO, 1 and 5 was near zero, and the dihedral for 1
and 3 was 6.2˚ and 13.2˚, respectively (data given in ESI).
Figure 2. B3LYP/6-31G(d,p) optimized structure of 4 showing Cx-C-C-
Cy dihedral angle of 24.2˚. Carbon=Black, White=Hydrogen,
Yellow=Sulfur, Red=Oxygen.
Computed Redox potentials. An increased susceptibility to
photoinduced electron transfer was an alternative possibility to
O(3P) release, which could explain the difference in solvent
oxidation patterns for 1-5 compared to DBTO. Davis and Fry have
shown that B3LYP can be used to accurately predict hydrocarbon
arene redox potentials.21 Following the method described by Davis
and Fry, the ionization potential and electron affinity of benzene,
naphthalene, chrysene, pyridine, pyrrole, pyrimidine, quinolone,
and thiophene were determined to examine if this method could be
applied to heteroaromatic compounds. While additional sulfur
containing aromatic heterocylces would have been desirable for
this test set, the selected molecules were chosen since their
oxidation potentials had been determined under similar
experimental conditions.35,36 Figure 3 illustrates the correlation
between the computationally predicted and experimentally
determined redox values for the test set.
Figure 3. Comparison of PCM-B3LYP/6-31+G(d) estimated potential
to those determined by polargraphy or cyclic voltammetry. Closed
circles: (hydrocarbon aromatics, benzene, naphthalene and
chrysene); Open circle (thiophene); Open square: (nitrogen-
containing aromatics, pyridine, pyrrole, quinoline, pyrimidine). (A)
Ionization potentials compared to oxidation potential determined in
reference 35. Regression analysis: solid line (only hydrocarbon
aromatics); dotted line (all compounds). (B) Comparison of PCM-
B3LYP/6-31+G(d) estimated electron affinities to reduction
potential determined by polarography in reference 36.
As shown in Figure 3(A), correlation between the predicted and
experimental oxidation potential for the hydrocarbons (benzene,
naphthalene, chrysene) was excellent (R=0.9991). This was
consistent with the findings of Davis and Fry.21 Comparison of the
hydrocarbons plus thiophene, pyridine, and pyrrole was worse with
Table 6. The computed redox potential for DBTO and 1-5.
Sulfoxide
IP (eV)
6.27
5.66
5.77
5.64
5.45
5.62
EA (eV)
2.33
2.63
2.60
2.72
2.78
2.28
DBTO
1
2
3
4
5
a
correlation coefficient of 0.91121. Correlation between
experimentally determined reduction potential and
computationally determined electron affinities was decent with a
correlation coefficient of 0.98844. This was considered sufficient to
determine if the redox potentials of 1-5 were drastically different
than DBTO.
a
absolute reduction and ionization potentials computed by density
functional theory (B3LYP/6-31+G(d)) with SMD/IEF-PCM solvation.
Table 6 lists the redox potentials of 1-5 and DBTO determined by
following the method described by Davis and Fry.21 The ionization
potentials of 1-5 ranged from 5.77 to 5.45, which was lower than
the 6.27 eV value for DBTO. The electron affinities (EA) of 1-5
ranged from 2.72 to 2.28. If 1-5 were more susceptible to reduction
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than DBTO, the electron affinity of 1-5 would be significantly larger DBTO. This indicated the photodeoxygenation of 1-5 and
than DBTO. The EA of DBTO was 2.33, which fell in the middle of DBTO occurred by two different mechanisms. For DBTO
,
the range of EA for 1-5. Thus, it was concluded that bimolecular photodeoxygenation occurred primarily through a dissociative
photoreduction was not responsible for the different oxidation T2 state to release O(3P). In the oxidation of toluene, this
pattern observed for 1-5.
resulted in cresols being the major products. For 1-5, a
competitive deoxygenation mechanism through the T1 state,
which favored oxidation of the benzylic position of toluene,
was presumed. Larger computed S1-T1 energy gaps favored
Photodeoxygenation mechanism of 1-5
The product studies and computed properties of 1-5 indicated
deoxygenation occurred by two different mechanisms.
Deoxygenation of DBTO has been proposed to occur by both the T2
and T1 states. The dissociative T2 state leads to O(3P), which
selectively oxidized toluene to cresols. Under bimolecular triplet
sensitized conditions which presumably bypass the T2 state,
deoxygenation from the T1 state was shown to result in a decrease
in the overall oxidation of toluene and preferential oxidation of the
benzylic position.25
deoxygenation through the T1 mechanism, and
4 was believed
to largely undergo deoxygenation by this mechanism.
Acknowledgements
Portion of this work was supported by the National Science
Foundation under CHE-1255270. The authors thank the
National Science Foundation for support under grant CHE-
0963363 for renovations to the research laboratories in
Monsanto Hall. This work was additionally supported by
donors to the Herman Frasch Foundation.
The direct irradiation of 1-5 also resulted in a decrease in the yields
of toluene oxidation products. However, unlike the triplet
sensitization of DBTO that leads preferentially to methyl oxidation
products, cresols were the major oxidation products from the
deoxygenation of 1-3 and 5. This suggested that 1-5 were
undergoing deoxygenation by two competing mechanisms. One
mechanism generates O(3P), presumably through a T2 state, and the
second mechanism appeared similar to triplet sensitization of DBTO
which does not generate O(3P) and proceed through a T1 state.
Thus, the variations in the oxidation yields between 1-5 were the
result of differing partitioning into these two competing
mechanisms. For example, the benzyl:ring oxidation of 1 is 1:2,
which is the most similar to that of 1:4.8 for DBTO, which
presumably produces the most O(3P). However, the total yield of
toluene oxidation relative to the formation of the corresponding
sulfide of 1 dropped to 23% compared to 48% for DBTO. This drop
indicates 1 also undergoes deoxygenation through the T1 state or a
mechanism similar to one arising from triplet sensitization. This T1
mechanism must also lead to an oxidant or species that
preferentially generates PhCHO and BnOH in low yields. This
concomitantly resulted in a lower yield of O(3P) which preferentially
oxidized toluene to the cresol products. Thus, the drop in total
oxidation and slightly smaller ratio of benzyl:ring oxidation is
consistent with 1 undergoing both mechanisms.
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