Photodeoxygenation of phenanthro[4,5-bcd]thiophene S-oxide, triphenyleno[1,12-bcd]thiophene S-oxide and perylo[1,12-bcd]thiophene S-oxide

Article abstract of DOI:10.1080/17415993.2019.1615065

Sulfoxides, upon irradiation with ultraviolet (UV) light undergo α-cleavage, hydrogen abstraction, photodeoxygenation, bimolecular photoreduction, and stereo-mutation. The UV irradiation of dibenzothiophene S-oxide (DBTO) yields dibenzothiophene (DBT) as a major product along with ground-state atomic oxygen [O(³P)]. This is a common method for generating O(³P) in solution. The low quantum yield of photodeoxygenation and the requirement of UVA light are drawbacks of using this method. 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 have shown to deoxygenate up to three times faster than DBTO upon UVA irradiation; however, the photodeoxygenation of these sulfoxides does not appear to be limited to the production of O(³P). In this work, phenanthro[4,5-bcd]thiophene S-oxide, triphenyleno[1,12-bcd]thiophene-S-oxide, and perylo[1,12-bcd]thiophene-S-oxide were synthesized and their photodeoxygenation was studied. Phenanthro[4,5-bcd]thiophene-S-oxide, triphenyleno[1,12-bcd]thiophene-S-oxide, and perylo[1,12-bcd]thiophene-S-oxide deoxygenated upon UVA irradiation. However, the common intermediate experiments did not conclusively identify the photodeoxygenation mechanism of these sulfoxides.

Full text of DOI:10.1080/17415993.2019.1615065

Journal of Sulfur Chemistry
ISSN: 1741-5993 (Print) 1741-6000 (Online) Journal homepage: https://www.tandfonline.com/loi/gsrp20
Photodeoxygenation of
phenanthro[4,5-bcd]thiophene S-oxide,
triphenyleno[1,12-bcd]thiophene S-oxide and
perylo[1,12-bcd]thiophene S-oxide
Satyanarayana M. Chintala, John T. Petroff II, Andrew Barnes & Ryan D.
McCulla
To cite this article: Satyanarayana M. Chintala, John T. Petroff II, Andrew Barnes &
Ryan D. McCulla (2019): Photodeoxygenation of phenanthro[4,5-bcd]thiophene S-oxide,
triphenyleno[1,12-bcd]thiophene S-oxide and perylo[1,12-bcd]thiophene S-oxide, Journal of Sulfur
Chemistry, DOI: 10.1080/17415993.2019.1615065
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JOURNAL OF SULFUR CHEMISTRY
https://doi.org/10.1080/17415993.2019.1615065
Photodeoxygenation of phenanthro[4,5-bcd]thiophene
S-oxide, triphenyleno[1,12-bcd]thiophene S-oxide and
perylo[1,12-bcd]thiophene S-oxide
Satyanarayana M. Chintala, John T. Petroff II, Andrew Barnes and Ryan D. McCulla
Department of Chemistry, Saint Louis University, St. Louis, MO, USA
ABSTRACT
ARTICLE HISTORY
Received 6 February 2019
Accepted 25 April 2019
Sulfoxides, upon irradiation with ultraviolet (UV) light undergo α-
cleavage, hydrogen abstraction, photodeoxygenation, bimolecular
photoreduction, and stereo-mutation. The UV irradiation of diben-
zothiophene S-oxide (DBTO) yields dibenzothiophene (DBT) as a
major product along with ground-state atomic oxygen [O(3P)].
This is a common method for generating O(3P) in solution. The
low quantum yield of photodeoxygenation and the requirement
of UVA light are drawbacks of using this method. The sulfox-
ides 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 have shown to deoxygenate up to
three times faster than DBTO upon UVA irradiation; however, the
photodeoxygenation of these sulfoxides does not appear to be
limited to the production of O(3P). In this work, phenanthro[4,5-
bcd]thiophene S-oxide, triphenyleno[1,12-bcd]thiophene-S-oxide,
and perylo[1,12-bcd]thiophene-S-oxide were synthesized and their
photodeoxygenation was studied. Phenanthro[4,5-bcd]thiophene-
S-oxide, triphenyleno[1,12-bcd]thiophene-S-oxide, and perylo[1,12-
bcd]thiophene-S-oxide deoxygenated upon UVA irradiation. How-
ever, the common intermediate experiments did not conclusively
identify the photodeoxygenation mechanism of these sulfoxides.
KEYWORDS
Photodeoxygenation;
sulfoxides; reactive oxygen
species; triplet oxygen;
toluene oxidation; 1-octene
oxidation; polycyclic
sulfoxide
CONTACT Ryan D. McCulla
ryan.mcculla@slu.edu, rmccull2@slu.edu
Department of Chemistry, Saint Louis
University, 3501 Laclede Ave., St. Louis, MO 63103, USA
© 2019 Informa UK Limited, trading as Taylor & Francis Group

2
S. M. CHINTALA ET AL.
1. Introduction
Ground-state atomic oxygen [O(3P)] is a reactive oxidant which is selective towards cer-
tain functional groups such as thiols, alkenes, sulfides, and aromatics [1–10]. Photolysis
of dibenzothiophene S-oxide (DBTO) upon irradiation with UVA light produces diben-
zothiophene (DBT) and an oxidant purported to be ground-state atomic oxygen [4]. The
chemical yield of formation of DBT from DBTO for this reaction is 95% [11]. However,
the quantum yield of this reaction is very low at approximately 0.003 [4]. Photodeoxy-
genation of DBTO is a clean method of producing O(3P) in solution as it does not yield
any by-products. Therefore, various derivatives of DBTO have been employed as O(3P)
precursors to oxidize simple organic and biomolecules in condensed phase [3,5–7,12,13].
However, the requirement of high energy UVA light to induce the photodeoxygenation
reaction combined with low photodeoxygenation quantum yield limits O(3P) from being
utilized extensively as an oxidant in biological systems. Therefore, better precursors that
absorb in the visible range and have a higher quantum yield of photodeoxygenation are
desirable.
Photodeoxygenation of DBTO is believed to produce O(3P) in solution; however, there
is no spectroscopic evidence to support it (Figure 1). O(3P) cannot be spectroscopi-
cally detected in solution due to the absence of an assessable spectroscopic signature.
Therefore, O(3P) has been identified and characterized indirectly by its reactivity profile
[1,2,4,6,10,14,15]. Recently more indirect evidence has been provided to show that oxi-
dation reactions by photodeoxygenation of DBTO occur through freely diffusing O(3P)
[16]. This has been done by the irradiation of a DBTO derivative inside of nanocap-
sules in the presence of an O(3P) accepting molecule outside the nanocapsules. The
nanocapsules were not permeable to either the DBTO derivative or the O(3P) accepting
molecule. The results showed that a small oxidant produced by photodeoxygenation, likely
O(3P), diffused through the pores of the nanocapsules to oxidize the oxygen accepting
molecule.
The most common technique to indirectly detect the generation of O(3P) in solu-
tion is the common intermediate experiment [4,10,12,14,17]. Common intermediate
experiments are based on the principle that two different reactions may have a com-
mon intermediate if both reactions yield the same products in the same ratio. Hence,
common intermediate experiments were used to test if the sulfoxides 1–5, shown in
Figure 2, produce O(3P) by photodeoxygenation. These sulfoxides absorb near or into
the visible region and have higher quantum yields of photodeoxygenation compared
Figure 1. Photodeoxygenation of DBTO.

JOURNAL OF SULFUR CHEMISTRY
3
Figure 2. Sulfoxides previously tested for O(3P) precursors.
to DBTO. However, common intermediate experiments showed that these sulfoxides
might have different pathways of photodeoxygenation in addition to the unimolecular
S–O bond cleavage mechanism leading to O(3P) as the oxidized product yields obtained
were lower when compared to DBTO [14]. Similarly, common intermediate experi-
ments also indicated that sulfoxide 4 does not produce O(3P) upon irradiation with
UVA light [14]. A potential reason for the inability to release O(3P) is that the struc-
ture of the sulfoxide dinaphtho[2,1-b:1’,2’-d] thiophene S-oxide is not planar. The T2
triplet state has been shown to be dissociative for aryl sulfoxides with planar geom-
etry, and it is unclear if T2 triplet state is dissociative for sulfoxides with non-planar
geometry [10,18].
Although, the sulfoxides in Figure 2 have been suggested to have multiple pho-
todeoxygenation mechanisms, the approach of benzannulation of DBTO structure has
shown to improve the quantum yield of photodeoxygenation and extend the UV–Vis
absorption into the visible region. In this work, using the similar approach, sulfoxides
phenanthro[4,5-bcd]thiophene S-oxide (6O), triphenyleno[1,12-bcd] thiophene S-oxide
(7O), and perylo[1,12-bcd]thiophene S-oxide (8O), which are shown in Figure 3, were cho-
sen because of their rigid planar structures as shown by the computation analysis. These
sulfoxides were synthesized, and the common intermediate experiments were performed
to verify if they produce O(3P) upon UVA irradiation.

4
S. M. CHINTALA ET AL.
Figure 3. Sulfoxides and sulfides used in this study.
2. Experimental and computational methods
2.1. Materials
All commercially available materials were procured from Sigma Aldrich, Fisher Chem-
icals, or Oakwood Chemicals and are used directly without any purification expect as
mentioned. Anhydrous N-Methyl-2-pyrrolidone (NMP) obtained from Sigma Aldrich
was further dried using 3 Å molecular sieves. Hexanes and tetramethylethylenediamine
(TMEDA) were also dried using 3 Å molecular sieves. The concentration of n-butyllithium
(n-BuLi) used was 1.6 M. HPLC grade toluene and acetonitrile were used for common
intermediate experiments and determination of quantum yield of photodeoxygenation
experiments.
2.2. General methods
The UV–Vis absorbance spectra for sulfoxides 6O–8O were obtained using Shimadzu
UV-1800 UV spectrophotometer. Shimadzu GC – 2010 plus was used to obtain the con-
centrations of products formed from the common intermediate experiments. An Agilent
1200 Series HPLC with a quaternary pump, diode-array detector and Higgins Analytical
CLIPEUS C18 column (5 μm, 150 × 4.6 mm) was used for HPLC analysis. A Bruker DRX-
400 NMR was used to obtain NMR spectra. The solvents CDCl3 and deuterated dimethyl
sulfoxide (DMSO-d6) were used for obtaining NMR spectra. The chemical shift of CHCl3
peak was set to 7.27 ppm and used as the reference peak for 1H NMR performed in CDCl3.

JOURNAL OF SULFUR CHEMISTRY
5
The chemical shift of dimethyl sulfoxide (DMSO) was set to 2.50 ppm for 1H NMR spectra
obtained in DMSO-d6. Similarly, the chemical shift of CHCl3 was set to 77.23 ppm for 13
C
NMR obtained in CDCl3. The emission spectra of sulfoxide 8O were obtained by using
a Photon Technology International spectrofluorometer equipped with a 75 W Xenon arc
steady state lamp, excitation monochromator, sample holder, emission monochromator
and a PMT detector.
2.3. Common intermediate experiments
4 ml of saturated solutions of sulfoxides 6O–8O were prepared and transferred into 5 ml
quartz cuvettes (1 cm × 1 cm). The solutions were prepared in toluene for the oxidation
of toluene common intermediate experiment. For the oxidation of 1-octene common
intermediate experiment, the sulfoxide solution was prepared in acetonitrile (ACN). The
concentration of 1-octene in the prepared solution was 500 mM. The solutions were
degassed by purging them by Argon. The degassed solutions were irradiated with Luzchem
UVA bulbs (LZC – UVA), which are centered at 350 nm. For irradiation of sulfoxide 8O
with 420 nm light, Luzchem 420 nm bulbs (LZC-420) were used. Dodecane was used as
the internal standard. The change in the sulfide concentration was measured using HPLC
analysis. The concentrations of products formed were obtained using GC-FID analysis.
2.4. Determination of quantum yield of photodeoxygenation
Saturated solutions of sulfoxides 6O–8O were prepared in acetonitrile. The solutions were
transferred into quartz cuvettes (1 cm × 1 cm) and degassed using Argon sparging. The
degassed solutions were irradiated using 75 W Xenon lamp focus on a monochromator.
The reactions were carried out to conversions lower than 20%. The increase in the sulfide
concentration was measured by HPLC analysis. Photorearrangement of azoxybenzene to
2-hydroxyazobenzene was used as an actinometer [19].
2.5. Computational method
A comparative study has shown that HSHEH1PBE method with 6-311G(d,p) basis set is a
better method at predicting the vibrational frequencies of flufenpyr and amipizone more
accurately compared to DFT/B3LYP and B3PW91 methods [20]. Therefore, HSHEH1PBE
method with 6-311G(d,p) basis set was used in this work. Initial guess geometries were
generated by standard bond angles and lengths. The neutral species of DBTO, 6O, 7O and
8O were analysed separately for molecular structure using HSEH1PBE methods with the
6-311G(d,p) basis set [20,21]. The geometry optimizations were performed with Gaussian
09 [22].
3. Results and discussion
3.1. Computed structures of compounds 6O–8O
The computed structures for sulfoxides 6O–8O are shown in Figure 4. These struc-
tures were obtained by HSEH1PBE/6-311G(d,p) geometry optimizations with Gaussian09
[20,21]. The optimized structures of sulfoxides DBTO, 6O–8O were planar and the

6
S. M. CHINTALA ET AL.
Figure 4. Computed Structures of Sulfoxides 6O–8O.
Table 1. Dihedral Angles of Sulfoxides 6O–8O.
Sulfoxide
C1–C2–C3–C4 Dihedral Angle
C5–C6–S–O Dihedral angle
DBTO
0.006°
62.0°
63.8°
63.1°
63.0°
6O
7O
8O
0.005°
0.003°
0.001°
dihedral angles C1–C2–C3–C4 and C5–C6–S–O of sulfoxides DBTO, and 6O–8O are
shown in Table 1. The dihedral angle C5–C6–S–O was used to show the angle created by the
S–O bond and the plane of the aromatic system. No significant differences were observed.
The calculations of structural geometry optimization, zero-point correction, thermal cor-
rection to energy, thermal correction to enthalpy, and thermal correction to Gibbs free
energy are provided in the supporting information.
3.2. Synthesis of compounds 6O–8O
Compounds dichloro iodobenzene, 6O, 7O, 7 and 8 were synthesized according to the
reported procedures [23–28]. Compounds 6O and 7O have similar synthetic procedures
which are shown in Scheme 1 [23]. The first step of their syntheses involves refluxing of
the polycyclic aromatic hydrocarbon with n-BuLi in the presence of TMEDA followed
by addition of S2Cl2 at 0°C. The next step is the oxidation of sulfide to sulfoxide by
dichloro iodobenzene and water in acetonitrile. Compound 8 was prepared in two steps
[24]. The first step was nitration of perylene using nitric acid in a dioxane/water sol-
vent mixture which gave 1-nitroperylene (8A). The second step involved heating of 8A
in the presence of sulfur to yield 8. Sulfoxide 8O was obtained by oxidation of 8 using
meta-chloroperbenzoic acid (mCPBA) as shown in Scheme 1. The temperature of the reac-
tion was maintained under 5°C to reduce the formation of the sulfone by-product. The
detailed synthetic procedures of dichloro iodobenzene, 6–8 and 6O–8O are provided in
the supporting information.
3.3. UV-Vis spectra of sulfoxides 6O–8O
Photodeoxygenation of DBTO requires irradiation of UVA light as the UV–Vis absorp-
tion of DBTO does not extend past 350 nm [14]. This is one of the drawbacks of utilizing

JOURNAL OF SULFUR CHEMISTRY
7
Scheme 1. Synthesis of sulfoxides 6O–8O. N-Methyl-2-pyrrolidone (NMP), 3-chloroperbenzoic acid
(mCPBA), tetramethylethylenediamine (TMEDA).
photodeoxygenation of DBTO for generation of O(3P) in biological systems. The UV-Vis
spectra of sulfoxides 6O–8O were obtained to determine if the absorption extends past
into visible wavelength range. The UV-Vis spectra of 6O–8O are shown in Figure 5. The
UV–Vis cut-off wavelengths (ε < 100 M−1 cm−1) for 6O–8O are 364, 358, and 566 nm
respectively. Only sulfoxide 8O had UV-Vis absorption spectra which extended into the
visible wavelength range. In addition, sulfoxide 8O has two luminescence peaks at 437 and
414 nm when excited using 380 nm light. The emission spectrum of 8O is provided in the
supporting information.
3.4. Quantum yield of photodeoxygenation of 6O–8O
The quantum yield of photodeoxygenation of DBTO is wavelength dependent and is
approximately 0.003 at 320 nm in ACN [4]. The sulfoxides shown in Figure 2 have up to
three times higher quantum yield of photodeoxygenation compared to DBTO. The quan-
tum yields of photodeoxygenation of 6O–8O were measured based on the increase in
the corresponding sulfide concentration. The quantum yields of photodeoxygenation of
6O–8O are listed in Table 2. For photolysis of 6O–8O, the wavelength with the highest
extinction coefficient above 320 nm in their corresponding UV–Vis spectrum was used as
the irradiation wavelength.

8
S. M. CHINTALA ET AL.
Figure 5. UV–Vis spectra of 6O–8O.
Table 2. Quantum yields of photodeoxygenation of
compounds DBTO and 6O–8O.
Compound
DBTOa
6O
7O
8O
Quantum yieldb ꢀ
Wavelength (nm)
+sulfide
0.0026 0.0004
0.0007 0.0003
0.0028 0.0001
0.00010 0.00001
320 12
320
329
320
6
6
6
aQuantum yield of photodeoxygenation of DBTO from Ref. [4].
bQuantum yields reported are within 95% confidence intervals
and were calculated by increase in sulfide concentration.
Photodeoxygenation of 6O–8O yielded their corresponding sulfides. No other signifi-
cant products were obtained in HPLC analysis. The quantum yields of photodeoxygenation
of sulfoxides 6O–8O ranged from 0.0001–0.0028. The values of quantum yield of pho-
todeoxygenation of sulfoxides 6O and 8O are 0.0007 0.0003 and 0.00010 0.00001
respectively. These values are lower than the quantum yield of photodeoxygenation of
DBTO. The quantum yield of photodeoxygenation of 7O was 0.0028 0.0001, which was
in the range of the quantum yield of photodeoxygenation of DBTO.
3.5. Oxidation of toluene as a common intermediate experiment
The oxidation of toluene has been previously used to verify whether aryl sulfoxides pro-
duce O(3P) on photodeoxygenation [4,9,12,14]. As shown in Figure 6, oxidation of toluene
by photodeoxygenation of DBTO yields benzaldehyde, benzyl alcohol, o-cresol, m-cresol,
and p-cresol. These products are always obtained in a certain product ratio as shown in
Table 3. Therefore, if the oxidation of toluene by photodeoxygenation of a sulfoxide yields
benzaldehyde, benzyl alcohol, o-cresol, m-cresol and p-cresol in the same ratio as in the
case of DBTO then the sulfoxide and DBTO likely have a common intermediate of pho-
todeoxygenation. Toluene was oxidized by photodeoxygenation of sulfoxides 6O–8O by

JOURNAL OF SULFUR CHEMISTRY
9
Figure 6. Oxidation of toluene by photodeoxygenation of DBTO.
Table 3. Product distribution and yields of toluene oxidation by photolysis of DBTO and 6O–8O.
Percentage product yield (%)a
Compound
Benzaldehyde
Benzyl alcohol
o-Cresol
m/p-Cresolb
Total product yield (%)
CH3: Ringc
DBTO
6O
7O
5
2
2
5
3
1
1
1
6
3
3
4
3
1
1
1
17.6 0.9
13
7
3
1
1
1
42
26
15
28
6
4
4
2
1: 3
1: 4
1: 2
1: 2
14
7
1
1
8O
12.9 0.1
5.6 0.2
aPercentage product yields are calculated with respective to formation of sulfide and are within 95% CI.
bCalculated as one peak.
cRatio of sum of yields of PhCHO and benzyl alcohol to combined yields of cresols.
Figure 7. Oxidation of 1-octene by photodeoxygenation of DBTO.
irradiation with UVA light. The oxidized toluene product ratios and total product yields
were then obtained. The details of the common intermediate test are shown in Table 3.
Sulfoxides 6O–8O oxidized toluene to benzaldehyde, benzyl alcohol, o-cresol, m-cresol
and p-cresol. The ratios of CH3:ring oxidation obtained for DBTO and 6O–8O were
1:3, 1:4, 1:2 and 1:2, respectively. Although the ratios of CH3:ring oxidation obtained
for 6O–8O are similar to the CH3:ring oxidation ratio obtained for DBTO, the total
product yields of 6O–8O were lower to that of DBTO. The total product yields of
toluene oxidation obtained for DBTO and 6O–8O were 42 6%, 27 2%, 15 4%, and
28 2%, respectively. This is because the yields of cresols (ring oxidation) obtained for
6O–8O were lower compared to DBTO. The ratios of the oxidized products obtained for
6O–8O suggests O(3P) formation. However, the lower total product yields suggest other
photodeoxygenation pathways that do not generate O(3P).
3.6. Oxidation of 1-octene common intermediate experiment
As the oxidation of toluene common intermediate experiment indicated that 6O–8O may
produce O(3P) with lower product yields, and thus, the oxidation of 1-octene was per-
formed to determine if the total product yields of 6O–8O would be lower as well. O(3P)
reacts with 1-octene to yield 1-octanal and 1,2-epoxyoctane as shown in Figure 7 [2]. To
confirm that O(3P) was the intermediate formed during the photodeoxygenation process,
1-octene was oxidized by photodeoxygenation of sulfoxides 6O–8O in acetonitrile. The
details are shown in Table 4.

10
S. M. CHINTALA ET AL.
Table 4. Product distribution and yields of 1-octene oxidation by photolysis of DBTO and 6O–8O.
Product yieldsa
Compound
1-octanal (%)
24
8.6 0.6
4.4 1.1
1,2-epoxyoctane (%)
35
Total product yield (%)
59
Aldehyde: epoxideb
DBTO
6O
7O
2
3
6
1: 1.4
5.1: 1
2.2: 1
–
1.7 0.1
2.0 1.0
Not detected
10.3 0.7
6.4 2.1
–
8O
Not detected
aPercentage product yields are calculated with respective to formation of sulfide and are within 95% CI.
bRatio of yield of 1-octanal to yield of 1,2-epoxyoctane.
Sulfoxides 6O and 7O along with DBTO upon UVA irradiation oxidized 1-octene to
1-octanal and 1,2-epoxyoctane. However, the ratio of 1-octanal:1,2-epoxyoctane obtained
for 6O, 7O were very different than the ratio obtained for DBTO. The ratio of 1-octanal:1,2-
epoxyoctane obtained for DBTO was 1:1.4 with the total product yield of 59 6%. Sulfox-
ide 6O yielded 1-octanal and 1,2-epoxyoctane in the ratio of 5.1:1 with total product yield
of 10.3 0.7%. Sulfoxide 6O yielded more 1-octanal compared to 1,2-epoxyoctane, and the
total product yield obtained for 6O was lower than that of DBTO. Sulfoxide 7O showed a
1-octene oxidation pattern similar to 6O. Sulfoxide 7O on reaction with 1-octene yielded
more 1-octanal compared to 1,2-epoxyoctane. The ratio of 1-octanal: 1,2-epoxyoctane was
2.1:1. Like 6O, the total product yield obtained for 7O, 6.4 2.1%, was significantly lower
to DBTO. Sulfoxide 8O did not yield any observable 1-octene oxidation products. As the
absorption spectrum of sulfoxide 8O extends into the visible wavelength range, oxida-
tion of 1-octene common intermediate experiment was performed by irradiation of 8O
by 420 nm light. This resulted in photodeoxygenation of 8O to 8; however, no 1-octene
oxidation products were observed in the GC-FID analysis.
Photodeoxygenation of DBTO occurs through a unimolecular scission of S–O bond in
pure acetonitrile [4]. However, in an aqueous medium, the mechanism of photodeoxy-
genation of DBTO is dependent on the pH of the solution [15]. At neutral and acidic
pH, photodeoxygenation of DBTO has been suspected to have the unimolecular scission
mechanism. In basic pH, DBTO photodeoxygenates through a bimolecular photoinduced
electron transfer mechanism. As the common intermediate experiments of 6O–8O was
performed in acetonitrile, we considered all the unimolecular mechanisms of oxidation
that have been considered for DBTO photodeoxygenation to rationalize the results of the
common intermediate experiments of 6O–8O. In previous studies, three different path-
ways of oxidation of a reactant by the photodeoxygenation of DBTO has been suggested as
shown in Scheme 2 [2,4,16]. The first mechanism is the release of a freely diffusing O(3P)
into the bulk solution, which has been recently supported with evidence that the oxidant
can pass through a nanocapsule barrier that prevents the diffusion of the sulfoxide [16].
In addition to freely diffusing O(3P), the oxidation of the reactant can occur before the
oxidant escapes the solvent cage (Scheme 2(a)) [2]. Oxidation prior to escape from the
solvent cage has been suggested to occur by two different mechanisms. One possibility is
the formation of discrete O(3P) that reacts with a reactant molecule prior to escape from
the solvent cage (Scheme 2(b)). A second possibility is an oxygen-atom transfer from an
excited state DBTO directly to the reactant (Scheme 2(c)). While the previous experiments
were unable to distinguish between 2b and 2c for DBTO, the ratio of oxidation products

JOURNAL OF SULFUR CHEMISTRY
11
Scheme 2. Different pathways of oxidation by DBTO photodeoxygenation.
produced by either 2b or 2c would have to be the same as freely diffusing O(3P) (2a) to
conform with the experimental results.
Unfortunately, the data obtained from the common intermediate experiments do not
conclusively identify the photodeoxygenation mechanisms of 6O–8O. The oxidation of
toluene by photodeoxygenation of 6O–8O in neat toluene yielded similar product ratios
and slightly lower product yields compared to DBTO. In contrast, the oxidation of 1-octene
dissolved in acetonitrile by photodeoxygenation of 6O–8O resulted in different product
ratios and significantly lower product yields compared to DBTO. In toluene, the lower
yields obtained for 6O–8O compared to DBTO could be explained if 6O–8O deoxygenates
by an additional mechanism that does not produce O(3P). However, the very low yields
and the different products observed for 1-octene are inconsistent with 6O–8O generating
O(3P) in acetonitrile.
One possible reason for this difference is if 6O–8O undergo photodeoxygenation by a
different mechanism than DBTO. For example, if 6O–8O undergo photodeoxygenation by
an oxygen-atom transfer similar to Scheme 2(c), it is plausible that the observed oxidized
products would be different than O(3P) generated by DBTO. However, this would require

12
S. M. CHINTALA ET AL.
the oxygen-atom transfer mechanism of 6O–8O to give the same ratio of oxidized products
as O(3P) in toluene. Another potential rationale for this difference is the mechanism of
photodeoxygenation for 6O–8O is different in toluene than acetonitrile. For example, the
observed products could arise if 6O–8O generated O(3P) in toluene but not in acetonitrile.
However, it seems somewhat unlikely that a modest change in solvent would result in such
a significant change in the mechanism.
4. Conclusion
The photodeoxygenation process of sulfoxides 6O–8O was investigated and the nature of
the oxidant produced during this process was studied. The common intermediate experi-
ments did not conclusively identify the oxidant generated during the photodeoxygenation
of 6O–8O. This oxidant produced during photodeoxygenation of 6O–8O oxidized toluene
to cresols, benzyl alcohol, and benzaldehyde. Similarly, oxidation of 1-octene yielded 1-
octanal and 1,2-epoxyoctane. Additionally, the product yields of the common intermediate
experiments of 6O–8O were lower compared to DBTO. While the quantum yield of pho-
todeoxygenation of 7O was in the range of DBTO, sulfoxides 6O and 8O had lower
quantum yields of photodeoxygenation compared to DBTO.
Funding
This work was supported by grants CHE-1255270 from the National Science Foundation and donors
to the Herman Frasch Foundation for Chemical Research.
Disclosure statement
No potential conflict of interest was reported by the authors.
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