Photocatalytic Aerobic Thiol Oxidation with a Self-Sensitized Tellurorhodamine Chromophore

Article abstract of DOI:10.1021/acs.organomet.7b00166

Aerobic oxidation of thiols to disulfides was achieved photocatalytically using a tellurorhodamine chromophore (9-mesityl-3,6-bis(dimethylamino)telluroxanthylium hexafluorophosphate) as both the sensitizer and catalyst. The proposed mechanism, supported experimentally and computationally with DFT, involves the formation of a tellurorhodamine telluroxide from reaction with water and singlet oxygen generated by irradiation of the tellurorhodamine. The oxidation to the telluroxide is accompanied by the formation of hydrogen peroxide. The telluroxide oxidizes thiols to regenerate the tellurorhodamine and the disulfide plus water. Mechanistically, DFT suggests adding two thiols to the telluroxide with the loss of H₂O to give a trigonal-bipyramidal Te(IV), which undergoes concerted loss of disulfide to regenerate 1. Oxidation of thiophenol and 2-naphthalenethiol was complete after 2 h of irradiation with visible light under atmospheric conditions. Oxidation of the electron-poor 2,6-dichlorothiophenol, the sterically bulky tert-butylmercaptan, and aliphatic dodecanethiol was slower. The two aliphatic thiols displayed competing catalyst degradation. The corresponding selenorhodamine chromophore (9-mesityl-3,6-bis(dimethylamino)selenoxanthylium hexafluorophosphate) does not form the corresponding selenoxide under similar conditions and photooxidizes thiophenol and 2-naphthalenethiol much more slowly (≤6% conversion after 2-3 h).

Full text of DOI:10.1021/acs.organomet.7b00166

Article
pubs.acs.org/Organometallics
Photocatalytic Aerobic Thiol Oxidation with a Self-Sensitized
Tellurorhodamine Chromophore
Luke V. Lutkus,† Haley E. Irving,† Kellie S. Davies,‡ Jacqueline E. Hill,‡ James E. Lohman,†
Mathew W. Eskew,† Michael R. Detty,*,‡ and Theresa M. McCormick*,†
†Department of Chemistry, Portland State University, Portland, Oregon 97201, United States
‡Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States
S
* Supporting Information
ABSTRACT: Aerobic oxidation of thiols to disulfides was
achieved photocatalytically using a tellurorhodamine chromo-
phore (9-mesityl-3,6-bis(dimethylamino)telluroxanthylium
hexafluorophosphate) as both the sensitizer and catalyst. The
proposed mechanism, supported experimentally and computa-
tionally with DFT, involves the formation of a tellurorhod-
amine telluroxide from reaction with water and singlet oxygen
generated by irradiation of the tellurorhodamine. The
oxidation to the telluroxide is accompanied by the formation
of hydrogen peroxide. The telluroxide oxidizes thiols to
regenerate the tellurorhodamine and the disulfide plus water.
Mechanistically, DFT suggests adding two thiols to the
telluroxide with the loss of H2O to give a trigonal-bipyramidal Te(IV), which undergoes concerted loss of disulfide to
regenerate 1. Oxidation of thiophenol and 2-naphthalenethiol was complete after 2 h of irradiation with visible light under
atmospheric conditions. Oxidation of the electron-poor 2,6-dichlorothiophenol, the sterically bulky tert-butylmercaptan, and
aliphatic dodecanethiol was slower. The two aliphatic thiols displayed competing catalyst degradation. The corresponding
selenorhodamine chromophore (9-mesityl-3,6-bis(dimethylamino)selenoxanthylium hexafluorophosphate) does not form the
corresponding selenoxide under similar conditions and photooxidizes thiophenol and 2-naphthalenethiol much more slowly
(≤6% conversion after 2−3 h).
aerobic thiol oxidation.6 Organotellurium compounds have also
INTRODUCTION
■
been used as catalysts for the oxidation of thiols to disulfides
Homogeneous metal-catalyzed aerobic oxidation is a widely
with H2O2.13−16 The proposed active oxidant in these systems
used method for small-molecule transformations but often is
restricted by the stoichiometric use of heavy-metal cooxidants.1
is a telluroxidea Te(IV) compoundthat is reduced back to
The use of photoinduced redox catalysis for such trans-
the corresponding telluridea Te(II) compoundduring
formations has increased in recent years due to its potential for
the green production of small molecules at both the laboratory
and industrial scale.2−4 Photocatalysts such as TiO2 that utilize
atmospheric oxygen as an oxidant are only active in the UV
region, which drastically reduces their light absorption
efficiency.5 Organic dyes have been developed as photoredox
catalysts for a variety of transformations, including the
oxidation of thiols to disulfides,6 anti-Markovnikov additions
to alkenes,7,8 and oxidation of amines to iminium salts.9
Chalcogenorhodamine dyes such as those shown in Scheme
1 are similar in structure to the rhodamine, oxazine, thiazine,
and azine dyes that have been used as visible-light-driven
photocatalysts in reactions that use singlet oxygen (1O2) as an
oxidant.10 While both seleno- and tellurorhodamines generate
1O2 efficiently upon irradiation, tellurorhodamines undergo
photooxidation to telluroxides, while selenorhodamines do not
undergo oxidation to the corresponding selenoxide.11,12
thiol oxidation. Glutathione has been used as a stoichiometric
reducing agent for telluroxides formed either by 1O2
oxidation11,12 or by hypochlorite oxidation17 of tellurium-
containing chromophores. Telluroxides have also been shown
to oxidize a variety of substrates without the addition of
cooxidants.18
1
As an oxidant, O2 is advantageous, both economically and
environmentally. However, a short lifetime, limited scope, and
1
lack of selectivity hinder the use of O2 for many aerobic
oxidation reactions.19 The ease of 1O2 oxidation of tellurides to
telluroxides, which can then be used as a chemical oxidant, can
be extended to tellurium chromophores, which self-sensitize the
Special Issue: Tailoring the Optoelectronic Properties of Organo-
metallic Compounds with Main Group Elements
Organotellurium compounds have been used as redox
auxiliaries with a porphyrin photosensitizer for photocatalytic
Received: March 6, 2017
© XXXX American Chemical Society
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1
Scheme 1. Oxidation of Tellurorhodamine 1 to Tellurorhodamine Telluroxide 2 with O2 and Water and the Structure of
Selenorhodamine 3
1
production of O2 to produce catalytic systems requiring only
atmospheric O2, water and visible light irradiation.
the triplet state (τT) for Te-containing chromophores is shorter
than τT for comparable Se-containing dyes and relaxation of the
triplet by pathways other than quenching with oxygen becomes
competitive (i.e., phosphorescence) in air-saturated solutions.22
The fraction of triplets producing 1O2 can be reduced as triplet
lifetimes become shorter, and the smaller value of Φ(1O2) for 1
relative to 3 is certainly consistent with this analysis.
Herein, we report the use of tellurorhodamine 1 (Scheme 1)
as a visible-light-active photocatalyst for the selective aerobic
oxidation of thiols to disulfides under ambient temperature and
pressure in air according to eq 1. The role of telluroxide 2
(Scheme 1) as the active form of the catalyst and other Te(IV)
intermediates is described. The performance of 1 as a
photocatalyst is contrasted with the performance of selenorhod-
amine 3, which does not form the corresponding selenoxide
intermediate. We provide a detailed mechanism for the
photooxidation reaction based on experimental results and
DFT calculations.
Oxidation with H2O2. In earlier work, we examined a
number of tellurides (Chart 1) as catalysts for the activation of
Chart 1. Diorganotelluride Catalysts for the Activation of
H2O2
catalyst
2RSH + 1/2O2 ⎯⎯⎯⎯⎯⎯→ RSSR + H2O
light
(1)
RESULTS AND DISCUSSION
Synthesis of Chalcogenorhodamine Dyes 1 and 3. The
chalcogenorhodamines 1 and 3 were prepared by addition of
the Grignard reagent prepared from 2-bromomesitylene to the
corresponding chalcogenoxanthone 4 or 5 (Scheme 2).11,20
■
Scheme 2. Synthesis of Tellurorhodamine 1 and
Selenorhodamine 3
H2O2 for the oxidation of thiols to disulfides, with the
corresponding telluroxide being the active form of the
catalyst.15,16 Rate constants for oxidation of the tellurides to
the telluroxides with H2O2, kH O , are 2 M−1 s−1 for
2
2
telluropyrylium compound 622 and 3.6−28 M−1 s−1 for neutral
diorganotellurides 7−9.16 We had thought that tellurorhod-
amine 1 would be an excellent candidate as a catalyst for the
oxidation of thiols to disulfides with H2O2: 1 is oxidized to
telluroxide 2 with H2O2, 2 is reduced by glutathione to return
1, and the 9-mesityl group protects both the reduced and
oxidized forms.11 However, as described below, kH O for
Following addition of the Grignard reagent, the reaction
mixtures were stirred for 24 h and were then poured into 10%
aqueous HPF6 to precipitate the dyes, which were isolated in
80% yield for 1 and 62% yield for 3.
Photophysical Properties of Chalcogenorhodamine
Dyes 1 and 3. Tellurorhodamine 1 absorbs light with λmax of
600 nm in MeOH (ε = 8.6 × 104 M−1 cm−1), and
selenorhodamine 3 absorbs light with λmax of 568 nm in
MeOH (ε = 1.03 × 105 M−1 cm−1). The quantum yield for 1O2
generation (Φ(1O2)) for 1 is 0.75 0.03,11 and Φ(1O2) for
2
2
oxidation of 1 to 2 is orders of magnitude smaller than kH O for
2
2
the diorganotellurides of Chart 1.
The oxidation of 1 (1 × 10−5 M) to 2 with H2O2 in 10%
H2O/90% MeOH at 293 K was followed by UV−vis
spectroscopy, as shown in Figure 1. Reaction with 0.05 M
H2O2 was complete after 4800 s, and telluroxide 2 was
identified as the product of oxidation (Figure 1a).11 The rate of
appearance of 2 was followed at 680 nm as a function of time
for three concentrations of H2O2 (0.01, 0.03, and 0.05 M).
Typical first-order rate plots of the data for the three H2O2
concentrations are shown in Figure 1b. A plot of the pseudo-
first-order rate constants as a function of H2O2 concentration is
shown in Figure 1c. For the oxidation of 1 with H2O2, kH O in
selenorhodamine 3 is 0.85
0.03, as determined by time-
12
1
resolved spectroscopy of O2 phosphorescence.
Values of Φ(1O2) are related to quantum yields for triplet
generation (ΦT) and, strictly on the basis of spin−orbit effects,
the value of ΦT for 1 containing the heavier Te atom (filled 4d
shell) would be expected to be greater than that of 3 containing
the lighter Se atom (filled 3d shell).21 However, the lifetime of
2
2
90% aqueous MeOH at 293 K is (1.1 0.01) × 10−2 M−1 s−1,
B
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Figure 1. Oxidation of a 1 × 10−5 M solution of 1 with H2O2 in 10%
H2O/90% MeOH to give telluroxide 2 at 293 K: (a) spectral changes
for the conversion of 1 to 2 with 0.05 M H2O2; (b) typical first-order
rate plots for the appearance of 2 monitored at 680 nm at 0.01, 0.03,
and 0.05 M H2O2; (c) plot of mean pseudo-first-order rate constants
from triplicate runs as a function of H2O2 concentration. Error bars
represent 1 standard deviation.
which is 180-fold smaller than kH O for 6 and 2500-fold smaller
2
2
than kH O for 7 under similar conditions. For comparison,
selenorhodamine 3 does not react with 0.05 M H2O2 over a 2 h
time period.
2
2
Figure 2. Photooxidation of a 1 × 10−5 M solution of 1 to give
telluroxide 2 irradiated with 24 W white LEDs (a) in 10% H2O/90%
MeOH (spectrum taken every 2 min for 18 min) and (b) in air-
saturated CDCl3 containing 2.5% water (spectrum taken every 2 min
for 16 min).
Photochemical Reactivity of Chalcogenorhodamine
Dyes 1 and 3. Solutions of either 1 or 3 (1 × 10−5 M) in
degassed solutions of MeOH or CDCl3 under a N2 atmosphere
showed less than 5% bleaching upon irradiation with white
LEDs (24 W, LED spectrum in Figure S1 in the Supporting
Information) over a 2 h time period. Irradiation of a 1 × 10−5
M solution of 1 in air-saturated 10% H2O/90% MeOH with the
white LEDs gave photooxidation to telluroxide 2 (λmax 664 nm)
over an 18 min period with an isosbestic point at 616 nm
(Figure 2a). In contrast, no loss of the chromophore of 3 in air-
saturated 10% H2O/90% MeOH was observed with irradiation
with the white LEDs over a 2 h time period.
Table 1. Oxidation of Aromatic and Aliphatic Thiols (5 ×
10−2 M) to Disulfides with 1 mol % of 1 or 3 (5 × 10−4 M)
under Aerobic Conditions in 2.5% H2O/CDCl3
disulfide
irradiation
time (h)
conversion
a
entry photocatalyst
thiol
thiophenol
(%)
1
2
3
4
5
6
7
1
3
1
3
1
1
1
2
3
2
2
2
2
2
≥99
5
thiophenol
Irradiation of a 1 × 10−5 M solution of 1 in air-saturated dry
CDCl3 gave a different outcome. After 30 s of irradiation, the
appearance of 2 at 650 nm was observed, but 2 was not stable
to the conditions of the reaction. The ability of the telluroxide
O atom in tellurorhodamine telluroxides to act as an inter- or
intramolecular nucleophile has been described.11 The addition
of 2.5% water to the CDCl3 solution prior to irradiation
increased the stability of 2, and photooxidation of 1 to 2 was
observed with an isosbestic point at 600 nm (Figure 2b).
Photocatalytic Thiol Oxidation. Photocatalytic oxidations
of aliphatic and aromatic thiols to disulfides were monitored by
1H NMR spectroscopy using CDCl3 containing 2.5% H2O by
volume as solvent and 1 mol % of tellurorhodamine 1 or
selenorhodamine 3 (relative to thiol) as the photocatalyst. The
reaction mixtures were irradiated with white LEDs under
ambient conditions. The results are summarized in Table 1.
Thiophenol and 2-naphthalenethiol were completely oxidized
to disulfides (Table 1, entries 1 and 3, respectively) within 2 h,
as shown in Figure 3 for oxidation of 2-naphthalenethiol.
2-naphthalenethiol
2-naphthalenethiol
2,6-dichlorothiophenol
tert-butylmercaptan
≥99
6
52
38
25
b
b
1-dodecanethiol
a
b
Determined by 1H NMR spectroscopy. Loss of 1 was observed with
time.
Oxidation was selective, as no other products were apparent
upon completion of reaction.
The completion of thiol oxidation was indicated by a
colorimetric change in the appearance of the reaction mixture.
The persistence of 2i.e., 2 no longer being reduced to 1
was visually identifiable by the bathochromic absorbance shift
resulting in a change from a blue to a green solution (Figure S2
in the Supporting Information). After complete conversion of
thiol to disulfide, the addition of more thiol resulted in the
immediate reduction of 2 to 1 and further thiol oxidation was
observed with continued irradiation with visible light.
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(Table 1, entry 7). Oxidation of both tert-butyl mercaptan and
1-dodecanethiol gave significant catalyst degradation after 2 h
of irradiation, as observed spectrophotometrically by loss of the
chromophore of 1.
Thiol Reduction of Telluroxide 2. The reduction of 2 to 1
was examined by stopped-flow spectroscopy using thiophenol,
2-naphthalenethiol, 2,6-dichlorothiophenol, and 1-dodecane-
thiol as reductants. A solution of 2 (5 × 10−6 M) for the
stopped-flow studies was prepared by dissolving 1 in 10%
water/90% MeOH and photooxidizing the solution to produce
2.
The rates of reduction of 2 using 100 equiv of thiol (5 × 10−4
M) were followed by stopped-flow spectroscopy monitoring
the change in absorbance of 2 at 655 nm as shown in Figure 4.
2-Naphthalenethiol, thiophenol, and 2,6-dichlorothiophenol
gave pseudo-first-order rate constants of 1.46 0.08, 1.06
0.06, and 0.808 0.007 s−1, respectively.
Figure 3. 1H NMR spectra following the photooxidation of 2-
naphthalenethiol with 1 mol % of 1 under visible light irradiation in
CDCl3 containing 2.5% water at 0, 1, and 2 h. The thiol proton signal
at 3.5 ppm was used to monitor reaction progress.
If the pseudo-first-order rate constants are divided by the
thiol concentration (5 × 10−4 M), approximate second-order
rate constants for 2-naphthalenethiol, thiophenol, and 2,6-
dichlorothiophenol are calculated to be 2.9 × 103, 2.1 × 103,
and 1.6 × 103 M−1 s−1, respectively. Initial rates for the
oxidation of thiophenol to diphenyl disulfide with H2O2 in
aqueous MeOH have been reported.23 On the basis of an initial
As described above, irradiation of degassed solutions of 1
under a N2 atmosphere in either CDCl3 or MeOH did not give
2. Similarly, irradiation of degassed solutions of 1 and
thiophenol did not give diphenyl disulfide. These data are
consistent with O2 being an integral component of the
photocatalysis.
Using selenorhodamine 3 as the photocatalyst with
thiophenol (Table 1, entry 2) and 2-naphthalenethiol (Table
1, entry 4) resulted in conversions to disulfide of ≤6% after 2−
3 h of irradiation. The observed conversion is likely the result of
thiol oxidation by 1O2 produced during irradiation rather than 3
acting as a photocatalyst. No color change and no loss of
absorbance of 3 were observed by UV−vis absorption
spectroscopy during irradiation, indicating that 3 was not lost
during the reaction and does not produce observable amounts
of selenoxide. Similarly, irradiation of degassed solutions of 3
and thiophenol did not give diphenyl disulfide.
Oxidation of 2,6-dichlorothiophenol was slower than the
oxidation of either thiophenol or 2-naphthalenethiol, as nearly
half of the initial thiol concentration remained after 2 h (Table
1, entry 5). Catalyst degradation was not observed by
comparing the absorbance of 1 before and after irradiation
for 2 h. Oxidation of tert-butyl mercaptan was slower still with
38% conversion after 2 h (Table 1, entry 6). Oxidation of the
aliphatic 1-dodecanethiol gave only 25% oxidation after 2 h
rate of (1.5
0.4) × 10−7 M−1min−1 for the appearance of
diphenyl disulfide with 1.0 × 10−3 M thiophenol and 3.75 ×
10−3 M H2O2, a second-order rate constant of (6.7 1.7) ×
10−4 M−1 s−1 can be calculated. The rate constant for
thiophenol oxidation with H2O2 is ∼10−6 that of thiophenol
oxidation with 2, suggesting that H2O2 produced during the
reaction would not contribute significantly to thiol oxidation.
The slower rate of reduction of 2 with 2,6-dichlorothiophe-
nol is consistent with the 52% yield of disulfide after 2 h (Table
1, entry 4). The slower rate of reduction is likely a result of the
electron-withdrawing chlorine groups decreasing the nucleo-
philicity of the thiol S atom.
A solution of 2 was titrated with thiophenol, 2-
naphthalenethiol, or 2,6-dichlorothiophenol in order to
determine the number of molar equivalents of thiol required
for complete conversion back to 1. Two molar equivalents of
aromatic thiol proved sufficient for complete reduction of 2 to
Figure 4. Reduction of 2 to 1 as monitored by stopped-flow spectroscopy from the diminishing of the telluroxide peak at 655 nm under pseudo-first-
order conditions (a) by 2,6-dichlorothiophenol (green squares), thiophenol (red diamonds), and 2-naphthalenethiol (blue triangles); (b) by 1-
dodecanethiol.
D
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1, as determined by UV−vis spectroscopy (Figures S3−S5 in
the Supporting Information).
3). Selenanes D are viable intermediates and have been
observed by mass spectrometry and 77Se NMR spectroscopy.27
Loss of H2O2 from the hydroxy(perhydroxy)tellurane B forms
the telluroxide. The intermediates shown in Scheme 3 with
tellurorhodamine 1 have been studied by DFT, as discussed
below.
In contrast to reactions with the aromatic thiols, reduction of
2 using excess 1-dodecanethiol under pseudo-first-order
conditions did not result in a first-order loss of 2 (Figure
3b), which is indicative of a competing reaction. X-ray crystal
structures in earlier studies documented intramolecular and
bimolecular addition of nucleophiles to the 9-position of the
tellurorhodamine telluroxides, which disrupts the cyanine-like
chromophore.11 Similar chemistry with 1-dodecanethiol would
lead to loss of catalyst. Although the 9-mesityl group sterically
protected telluroxide 2 from nucleophilic addition to the 9-
position during reductions with 2 equiv of glutathione,11
addition of an excess of 1-dodecanethiol to telluroxide 2
appears to be competitive with reduction of 2 to 1.
Mechanism of Photooxidation of 1 to 2. Once formed,
telluroxide 2 is an excellent oxidizing agent for converting thiols
to disulfides. The mechanism of photooxidation of 1 to 2 was
examined both computationally and experimentally. The data
are consistent with the general mechanism proposed in Scheme
3 for oxidation of diorganotellurides to telluroxides. The initial
While H2O2 is produced upon photooxidation of 1 to 2 in
the presence of H2O, one can ask whether the presence of the
thiol influences the production of H2O2. A solution of 1 (5 ×
10−4 M) and thiophenol (5 × 10−2 M) in chloroform and water
(2.5% by volume) was irradiated with the white LEDs. Upon a
visual change in color, from a blue to a green solution, water
was added to create a biphasic solution and the concentration
of H2O2 in the aqueous layer was determined to be (1.8 0.1)
× 10−4 M using oxo[5,10,15,20-tetrakis(4-pyridyl)-
porphyrinato]titanium(IV) as described above,26 which corre-
sponds to roughly 0.4 equiv of H2O2 relative to 1. On the basis
of the rates of oxidation of 1 and thiophenol described above
with H2O2, the H2O2 produced during the photocatalysis does
not compete as an oxidant with 2 for the oxidation of
thiophenol to the disulfide or with 1O2 for the oxidation of 1 to
2.
Scheme 3. Proposed Mechanism of Telluride Oxidations
DFT Calculations. Structural optimizations were performed
with Gaussian 09 software28 at the B3LYP level29−31 using 6-
31+G(d) (C, O, S, N, H atoms)32−34 and LanL2DZ (Te
atoms)35−37 split basis sets.38 The HOMO and LUMO
molecular orbital maps of the reduced and oxidized mesityl-
substituted tellurorhodamine 1 are shown in Figure 5.
a
1
with O2 in the Photooxidation of 1 to 2
a
The inset shows the structure of the hydroxyl(perhydroxy)selenane
D.
formation of the pertelluroxide intermediate A of Scheme 3 has
1
been proposed as the starting point for addition of O2 to
diorganotellurides.22,24 The pertelluroxide A can oxidize a
second diorganotelluride (i.e., a second molecule of 1), leading
to reduction of A to the telluroxide (i.e., 2) and oxidation of the
diorganotelluride to the telluroxide (i.e., 2).22,24 The
stoichiometry of this chemistry was established in flash
photolysis studies of telluropyrylium dye 6 in MeOH (Chart
1).22 The reversible addition of H2O to the diorganotelluroxide
B forms the (dihydroxy)tellurane C.22,25
Figure 5. Frontier molecular orbitals (FMOs) from the DFT
optimized geometries of the reduced tellurorhodamine 1 and oxidized
2 of the (a) LUMO and (b) HOMO of 1 and the (c) LUMO and (d)
HOMO of 2.
Irradiation of 1 in the presence of water facilitated telluroxide
formation. Photooxidation of 1.0 × 10−5 M 1 to 2 in 10% H2O/
90% MeOH also produced (8.4
0.6) × 10−6 M H2O2.
The HOMOs of both the reduced species 1 and oxidized
species 2 are located primarily on the N and C atoms of the
3,6-diaminoxanthylium core with minimal contributions from
the Te atoms (Figure 5b,d). The LUMO of the reduced species
1 has a major contribution from the Te atom (Figure 5a),
which is absent in the LUMO of telluroxide 2. Furthermore, the
LUMO of the oxidized species 2 has a major contribution from
the C atom in the 9-position within the xanthylium core, which
supports the addition of a nucleophile at this position to give
tellurorhodamine degradation products postulated for reactions
with aliphatic thiols.
Concentrations of H2O2 were measured by an established
spectrophotometric technique using oxo[5,10,15,20-tetrakis(4-
pyridyl)porphyrinato]titanium(IV).26 The approximately 1
equiv of H2O2 produced upon photooxidation of 1 to 2 in
the presence of H2O suggests that oxidation of 1 to 2 via the
pertelluroxide A is less important in the presence of water.
The addition of water to pertelluroxide A would form the
hydroxy(perhydroxy)tellurane B. We were unable to observe
the hydroxy(perhydroxy)tellurane analogue of telluroxide 2 by
125Te NMR (δ 883 for the 125Te signal of 2).11 However, the
addition of H2O2 to selenoxides gives the hydroxy-
(perhydroxy)selenane D (the selenium analogue of B, Scheme
Oxidation of 1 to 2. Reaction energy diagrams of oxidation
and reduction of the telluride complex were determined using
E
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the calculated Gibb’s free energies (ΔG) according to the
Thiol Reduction of 2 to 1. To simplify the calculations, the
reduction of 2 with a thiol to give 1 was calculated using
methanethiol, which upon oxidation gives dimethyl disulfide.
The ΔG value of the reduction of 2 to 1 and subsequent
disulfide formation was calculated to be −46.6 kcal/mol. The
computationally examined structures in Figure 7 were based on
previously hypothesized substitution reactions, which involved
a (dihydroxy)tellurane intermediate (C in Scheme 3).15,16
There is a slight downhill transition from telluroxide 2 to the
(dihydroxy)tellurane C (ΔG = −0.45 kcal/mol) in Figure 7,
which is consistent with an equilibrium involving telluroxide 2
and H2O.22,25 Substitution of each of the hydroxy ligands with
methanethiol (generating water) and subsequent reductive
elimination of dimethyl disulfide re-forms 1, which supports the
2:1 thiol to telluroxide ratio for telluroxide reduction that was
observed experimentally.
equation
ΔG = [∑ Eprod] − [∑ Ereac
]
1
The energy diagram of the reaction of 1 with O2 to form 2 is
shown in Figure 6. The total ΔG of oxidation of 1 to 2 with 1O2
Intermediate E, in which the Te(IV) center has two
methanethiolate ligands, conceptually can undergo reductive
elimination via a stepwise process15,16 or via a concerted
reductive elimination, which is supported by the DFT
calculations.39 The transition state for the reductive elimination
(TSE) from E to 1 is 26 kcal/mol higher than intermediate E
and shows shortening of the S−S bond by a decrease in the S−
Te−S bond angle.
Figure 6. Reaction energy diagram of the formation of 2 from the
reaction of 1 with O2 and H2O.
1
1
was calculated to be 12.7 kcal/mol. Initial coordination of O2
The inset of Figure 7 shows the changes in geometry around
the Te center in going from intermediate E (inset left) to
transition state TSE (inset center) to 1 (inset right). The
Te(IV) center of E has trigonal-bipyramidal geometry with the
axial thiolate ligands distorted from linearity by the equatorial
lone pair of electrons. As E progresses toward the transition
state, the geometry around Te changes from trigonal
bipyramidal to something approaching square pyramidal: the
two Te−C bonds, one elongated Te−S bond (from 2.69 to3.28
Å), and the lone pair are nearly coplanar. The second (axial) S
is part of a 57.8° S−Te−S bond angle with the two S atoms
separated by 2.98 Å. Such a pseudorotation, but in the reverse
direction, has been suggested for the oxidative addition of
to the Te(II) of 1 to give pertelluroxide A (see also Scheme 3)
is slightly uphill in energy (1.17 kcal/mol). Coordination of
H2O followed by a proton shift forms the hydroxy-
(perhydroxy)tellurane B as a Te(IV) intermediate (see also
Scheme 3). Previous to this study, the role of H2O in the
oxidation mechanism was not well understood. The oxidation
of 1 occurred slowly in anhydrous solvent and H2O addition
allowed the reaction to proceed at a faster rate. We propose this
specific water-coordination mechanism as a plausible explan-
ation for the latter. The final step in the formation of 2 is
elimination of H2O2 (ΔG = −18.5 kcal/mol), which supports
the experimentally observed results.
Figure 7. Tellurorhodamine structures in the reaction pathway for reduction of telluroxide 2 to tellurorhodamine 1 with methanethiol. Energies are
relative to 1 in the balanced reaction and follow from Figure 6. The inset shows changes in geometry around the Te atom in the progression from E
(left) to TSE (center) to 1 (right).
F
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Scheme 4. Mechanism of Photooxidation of Thiols and O2 to Disulfides Using Tellurorhodamine 1 as Photocatalyst
(dimethylamino)-9H-telluroxanthen-9-one (4)20 as described in ref
11. Dye 1 was isolated in 80% yield, and its spectroscopic properties
were identical with those reported for 1 in ref 11: mp >260 °C; H
NMR (500 MHz, CD2Cl2) δ 7.52 (d, 1 H, J = 3.0 Hz), 7.51 (d, 1 H, J
= 10.0 Hz), 7.06 (s, 2 H), 6.76 (d × d, 2 H, J = 3.0, 10.0 Hz), 3.21 (s,
12 H), 2.42 (s, 3 H), 1.83 (s, 6 H); λmax in MeOH (ε, M−1 cm−1) 600
nm (8.6 × 104).
bromine to diorganoselenides and tellurides.40 As the reaction
progresses through the transition state, the disulfide is
produced (S−S bond length of 2.09 Å) via reductive
elimination regenerating 1 with a Te(II) center.
1
CONCLUSIONS
■
Catalytic amounts of tellurorhodamine 1 were used to oxidize
aromatic thiols to disulfides under aerobic conditions with
visible-light irradiation. Catalytic amounts of selenorhodamine
3 with visible-light irradiation gave slower and/or poorer
oxidation of thiols to disulfides. While both 1 and 3 produce
1O2 efficiently upon irradiation, the self-sensitized generation of
1O2 upon irradiation of 1 leads to oxidation of 1 to telluroxide
2, which is the true oxidant for the conversion of thiols to
disulfides. Selenorhodamine 3 does not form the corresponding
selenoxide intermediate under the conditions of the reaction,
and disulfide production is likely from the generation and
Preparation of 3,6-Bis(dimethylamino)-9-(2,4,6-trimethyl-
phenyl)-9H-selenoxanthen-9-ylium Hexafluorophosphate (3).
2-Bromomesitylene (0.86 g, 4.3 mmol) was added to a stirred
suspension of magnesium (0.12 g, 4.8 mmol) in anhydrous THF (2
mL). The mixture was stirred at ambient temperature for 3 h, and the
resulting Grignard solution was transferred via cannula to a stirred
suspension of 3,6-bis(dimethylamino)-9H-selenoxanthen-9-one (5;20
0.10 g, 0.29 mmol) in anhydrous THF (8 mL). The resulting mixture
was heated at reflux for 24 h, cooled to ambient temperature, and
quenched by the addition of glacial acetic acid (1 mL). The resulting
mixture was poured into 50 mL of 10% (by weight) aqueous HPF6.
After 12 h, the resulting precipitate was collected via filtration and
washed with water (10 mL) and diethyl ether (4 × 10 mL) to give
1
subsequent reaction of O2.
1
The overall photocatalytic pathway is summarized in Scheme
4. Two equivalents of the telluroxide 2 can be formed from
pertelluroxide A reacting with a second molecule of A. We
propose a second pathway that also produces telluroxide 2.
Formation of a hydroxy(perhydroxy)tellurane from the reaction
0.11 g (62%) of 3 as a green solid: mp 175−177 °C; H NMR (500
MHz, CDCl3) δ 7.35 (d, 2 H, J = 10.0 Hz), 7.34 (d, 2 H, J = 2.0 Hz),
7.04 (s, 2 H), 6.81 (d × d, 2 H, J = 2.0, 10.0 Hz), 3.28 (s, 12 H), 2.42
(s, 3 H), 1.83 (s, 6 H); 13C NMR (75.5 MHz, CD2Cl2) δ 163.2, 153.7,
146.2, 139.6, 137.4, 135.9, 133.5, 129.0, 119.7, 115.8, 109.1, 40.9, 21.3,
19.7; λmax in MeOH (ε, M−1 cm−1) 568 nm (1.03 × 105); λmax in
CH2Cl2 (ε, M−1 cm−1) 567 nm (7.76 × 104); HRMS (ESI) m/z
449.1518 (calcd for C26H29N280Se+ 449.1490). Anal. Calcd for
C26H29N2Se•PF6: C, 52.62; H, 4.93; N, 4.72. Found: C, 52.41; H,
5.05; N, 4.65.
1
of 1 with O2 and water (structure B in Scheme 4) is followed
by loss of H2O2 to give telluroxide 2. Telluroxide 2 oxidizes two
thiol molecules via sequential addition of thiols to generate first
intermediates D and then intermediate E plus a molecule of
water. Reductive elimination of disulfide from intermediate E
regenerates tellurorhodamine 1, which reenters the photo-
catalytic cycle. By analogy to telluride catalysts for the activation
of H2O2,15,16,39 tellurorhodamine 1 (the telluride state) is the
resting form of the catalyst and tellurorhodamine telluroxide 2
is the active form of the catalyst. This mechanistic analysis is
supported experimentally and computationally.
Photooxidation of 1. In Aqueous MeOH. A solution of 1 (1 ×
10−5 M) in 90% MeOH/10% water was irradiated with white LEDs
and monitored by UV−vis absorption every 2 min to give the spectra
shown in Figure 2a. From this photooxidized sample, the H2O2
concentration was determined spectrophotometrically.26 A stock
solution of oxo[5,10,15,20-tetrakis(4-pyridyl)porphyrinato]titanium-
(IV) (5 × 10−6 M) in perchloric acid (2 M) was prepared. Aliquots
(10 μL) of the photooxidized sample were added to 3 mL of the stock
solution, and the change in absorption at 432 nm was used to
determine the H2O2 concentration by comparing to a reference of an
aliquot (10 μL) of an aqueous urea peroxide solution (1.0 × 10−5 M).
The samples were referenced to a blank using water (10 μL). The
analysis was run in triplicate, and the concentration of H2O2 was
EXPERIMENTAL SECTION
■
Preparation of 3,6-Bis(dimethylamino)-9-(2,4,6-trimethyl-
phenyl)-9H-telluroxanthen-9-ylium Hexafluorophosphate
(1).11 Tellurorhodamine 1 was prepared by the addition of the
Grignard reagent prepared from 2-bromomesitylene to 3,6-bis-
G
DOI: 10.1021/acs.organomet.7b00166
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Article
determined to be (8.4 0.6) × 10−6 M (0.84 equiv in comparison to
1).
titration of 2 with various thiols, and calculated total
energies (PDF)
Cartesian coordinates of the calculated structures (XYZ)
In Aqueous Chloroform after Thiophenol Oxidation. A solution of
1 (5 × 10−4 M) and thiophenol (5 × 10−2 M) in chloroform (2 mL)
and water (50 μL) was irradiated with white LEDs. Upon a visual
change in color, from a blue to a green solution, water (2 mL) was
added to create a biphasic solution. From the aqueous layer an aliquot
(10 μL) was used to determine the H2O2 concentration using
oxo[5,10,15,20-tetrakis(4-pyridyl)porphyrinato]titanium(IV) as de-
scribed above. The concentration of H2O2 was determined from
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.R.D.: mdetty@buffalo.edu.
*E-mail for T.M.M.: t.m.mccormick@pdx.edu.
ORCID
Mathew W. Eskew: 0000-0002-5648-5233
Michael R. Detty: 0000-0002-8815-1481
Theresa M. McCormick: 0000-0003-1745-2911
■
triplicate runs and was (1.8
0.1) × 10−6 M (0.36 equiv in
comparison to 1).
Photocatalytic Thiol Oxidation. Stock solutions of thiophenol,
2-naphthalenethiol, 2,6-dichlorothiophenol, tert-butyl mercaptan, and
dodecanethiol (1 × 10−1 M) were prepared in 1 mL of CDCl3. A stock
solution of 1 (1 × 10−3 M) or 3 (1 × 10−3 M) was prepared in 2 mL of
CDCl3 with cyclohexane added as an internal standard. A 0.2 mL
portion of the stock solution of 1 or 3 and 0.2 mL of one of the thiol
stock solutions were mixed in 5 mm NMR tubes, resulting in a final
concentration of 5 × 10−2 M of thiol and 5 × 10−4 M of 1 or 3 for each
sample. A 10 μL aliquot of water was added to each NMR tube. The
samples were irradiated by 24 W white LEDs with magnetic stirring
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported in part by the National Science
Foundation (CHE-1566142). T.M.M. thanks Portland State
University for financial support and Prof. R. Simoyi for use of
the stopped-flow spectrometer.
1
(using a straightened paperclip sealed in a glass capillary), and the H
NMR spectrum was taken every 20 min.
Reduction of 2 to 1 by Aromatic Thiols. A solution of 2 (1 ×
10−5 M) was prepared by dissolving 1 in 95% MeOH and oxidizing the
solution with self-sensitized generation of 1O2. The reduction of 2 to 1
with sequential additions of 0.5 equiv of thiophenol, 2-naphthalene-
thiol, or 2,6-dichlorothiophenol was followed by UV−vis absorption
spectroscopy. The addition of 2 equiv of thiol gave complete reduction
of 2 to 1 (Supporting Information).
Stopped-Flow Experiments. Stock solutions of thiophenol, 2-
naphthalenethiol, 2,6-dichlorothiophenol, and dodecanethiol (5 ×
10−4 M) were prepared in 99% MeOH. A stock solution of 2 (5 ×
10−6 M) in 99% MeOH was used for each scan. The rate of the
consumption of 2 was monitored at 655 nm for each thiol. The
acquired scans from mixing equal volumes of the stock solutions of 2
and thiol are shown as an average of triplicate measurements in Figure
4. The sample-handling unit was fitted with two drive syringes that
were mounted inside a thermostated-bath compartment, which
allowed for variable-temperature experimentation. The optical
detection cell was set up in the 10 mm path length.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00166.
General methods, NMR spectral data for 1 and 3,
elemental analysis of 3, the spectrum of the LED light
source, photos of the observed color change of 1 and 2,
H
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