Photocatalytic transformation of organic and water-soluble thiols into disulfides and hydrogen under aerobic conditions using Mn(CO)5Br

Article abstract of DOI:10.1021/om200461j

The photolysis of Mn(CO)₅Br with thiols under aerobic conditions at room temperature produces the corresponding disulfides in high yields, accompanied by the evolution of hydrogen as the only other product. This transformation is a greener route toward the synthesis of disulfides and exhibits 100% atom economy. The catalytic system possesses high chemoselectivity, as evidenced by high disulfide yields even in the presence of numerous functional groups. A mechanism has been proposed to involve free radical species and is based on fac-Mn(CO)₃(RSH)₂Br being an important catalytic intermediate. Mn(CO)₅Br is also able to catalyze the conversion of naturally occurring water-soluble thiols such as cysteine and glutathione. Coupled with suitable enzymes that regenerate thiols from disulfides using proton sources, it is possible to envisage a combined catalytic cycle that is able to reduce protons to hydrogen efficiently.

Full text of DOI:10.1021/om200461j

ARTICLE
pubs.acs.org/Organometallics
Photocatalytic Transformation of Organic and Water-Soluble Thiols
into Disulfides and Hydrogen under Aerobic Conditions Using
Mn(CO)5Br
Kheng Yee Desmond Tan, Guan Foo Teng, and Wai Yip Fan*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
ABSTRACT: The photolysis of Mn(CO)5Br with thiols under aerobic conditions at
room temperature produces the corresponding disulfides in high yields, accompanied by
the evolution of hydrogen as the only other product. This transformation is a greener
route toward the synthesis of disulfides and exhibits 100% atom economy. The catalytic
system possesses high chemoselectivity, as evidenced by high disulfide yields even in the presence of numerous functional groups. A
mechanism has been proposed to involve free radical species and is based on fac-Mn(CO)3(RSH)2Br being an important catalytic
intermediate. Mn(CO)5Br is also able to catalyze the conversion of naturally occurring water-soluble thiols such as cysteine and
glutathione. Coupled with suitable enzymes that regenerate thiols from disulfides using proton sources, it is possible to envisage a
combined catalytic cycle that is able to reduce protons to hydrogen efficiently.
Scheme 1. Transformation of Thiols into Disulfides and
’ INTRODUCTION
Hydrogen Using Complex 1
Organic compounds containing the element sulfur such as
disulfides are extensively utilized in chemistry and biology.1À3 In
biological systems, disulfides are known to prevent oxidative
damage.4 Disulfides are also useful reagents from the chemical
viewpoint, especially for organic syntheses; that is, they are used
to sulfenylate various anions.5 In addition, they are often used as a
protecting group for thiols.6 In industry, disulfides are used to
vulcanize rubber and elastomers.3
Since thiols are readily available, the oxidative coupling of
thiols is one of the most common methods for disulfide synthesis.
Many reagents have been used to carry out the coupling
including permanganates,7 halogens,8 iron(III) chloride,9 per-
oxides,10 nitrogen monoxide,11 sodium perborate,12 sulfuryl
chloride,13 sulfoxides,14 copper(II) nitrate trihydrate,15 manga-
nese compounds,4,16,17 cesium fluorideÀCelite solid base,18 and
rosolic acid.19 Despite the large number of routes available, some
appear to be less desirable, which is mainly attributed to a few
reasons: (i) the need for expensive and toxic reagents in
stoichiometric amounts, (ii) the generation of unwanted side-
products, (iii) long reaction times, and (iv) the overoxidation of
disulfides.
thiols into disulfides and hydrogen using complex 1 yields
comparatively higher TON (turnover number) values and can
proceed in the presence of oxygen or air. During the course of our
investigation, we have found that water-soluble thiols such as
glutathione (GSH) and cysteine could be transformed into their
respective disulfides with hydrogen evolution as well. This has
initiated keen interest especially since commercially available
enzymes such as glutathione reductase and cysteine reductase
can easily reduce the respective disulfides into thiols using proton
sources.23,24 Hence the combination of the Mn(CO)5Br- and
enzyme-catalyzed cycles into one will effectively convert protons
into hydrogen without sacrificing any thiols in the process. In this
work, we propose a mechanism to account for the experimental
observations.
Tanaka et al. has previously investigated the usage of transition
metal complexes to produce disulfides from thiols,20 and better
selectivity was achieved, cf. the usage of oxidants. Karami et al.4
and Montazerozohori et al.21 have also reported on the oxidative
coupling of thiols using metals. We have recently reported on the
suitability of CpMn(CO)3 as a precursor for a greener catalytic
system: the conversion of thiols into disulfides with hydrogen as
the only other product.22 Although conditions for the transfor-
mation can be considered mild, the catalysis proceeds only in the
absence of oxygen.
’ RESULTS AND DISCUSSION
Broadband irradiation of complex 1 with mono-functional
aliphatic thiols in the presence of oxygen affords the corre-
sponding disulfides in high yield, with hydrogen as the only
other product (Scheme 1). Although visible light can also
effect the same transformation, the time required for full
conversion is twice as long compared to UV photolysis. For
consistency in the yield studies, we have used a 355 nm UV
laser as the source of irradiation. Interestingly, the
In this paper, we explore the catalytic properties of another
group VII metal complex: pentacarbonylmanganese(I) bromide,
Mn(CO)5Br (complex 1). For this system, the conversion of
Received: May 30, 2011
Published: July 05, 2011
r
2011 American Chemical Society
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Table 1. Oxidative Coupling of Mono-functional Thiols by Complex 1a
a Irradiation (355 nm, ∼20 mJ/pulse) of complex 1 (39.6 μmol, 5.0 mol %) and RSH (0.793 mmol) in 0.5 cm3 cyclohexane for 2 h at room temperature.
b Irradiation (355 nm, ∼20 mJ/pulse) of complex 1 (19.8 μmol, 0.25 mol %) and RSH (7.93 mmol) for 4 h at room temperature. c Determined via 1H
NMR using toluene as internal standard. d Purified and isolated using silica gel column chromatography. e Calculated using the ratio of the number of
moles of thiol transformed over complex 1.
Table 2a. Oxidative Coupling of 1c by Complex 1 in the Presence of Additivesa
a Irradiation (355 nm, ∼20 mJ/pulse) of complex 1 (39.6 μmol, 5.0 mol %), RSH (0.793 mmol), and additive (0.793 mmol) in 0.5 cm3 cyclohexane for
2 h at room temperature. b Determined via 1H NMR using toluene as internal standard. c Purified and isolated using silica gel column chromatography.
d Calculated using the ratio of the number of moles of thiol transformed over complex 1.
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Table 2b. Oxidative Coupling of Bis-functional Thiols by Complex 1a
a Irradiation (355 nm, ∼20 mJ/pulse) of complex 1 (39.6 μmol, 5.0 mol %) and RSH (0.793 mmol) in 0.5 cm3 solvent for 2 h at room temperature.
b Determined via 1H NMR using toluene as internal standard. c Purified and isolated using silica gel column chromatography. d Calculated using the ratio
of the number of moles of thiol transformed over complex 1. e In benzene. f In cyclohexane. g In D2O.
Figure 1. IR spectra taken over certain time intervals of the reaction mixture containing complex 1 and 1c in cyclohexane initially. The νCO bands of
complexes 1, 2, and 3 are shown.
photocatalysis proceeds most efficiently in the presence of
oxygen or air; otherwise the disulfide yield is almost negligible
under vacuum conditions. In addition, photolysis of aliphatic
thiols in air without complex 1 does not give any disulfides.
Identities of the organic products have been confirmed using
1H NMR spectroscopy and EI-MS. The isolated yields of the
purified disulfides are also in good agreement with the values
obtained using 1H NMR spectroscopy (Table 1).
Using EI-MS, hydrogen has been detected in 75À80% yields
relative to the disulfides. The commonly encountered over-
oxidation products such as sulfoxides and sulfones were not
detected, which suggests that the transformation is indeed clean.
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We focus on the effect of photolysis on complex 1 with
aliphatic thiols in cyclohexane solvent, since the aromatic thiols
already spontaneously convert into their corresponding disul-
fides upon stirring in air even in the absence of manganese. Such
transformations of thiols into disulfides and hydrogen are also
slightly endergonic (ΔGo values ranging from 20 to 30 kJ molÀ1).25
Besides exhibiting 100% atom economy, the transformation can
be performed in the absence of a solvent (Table 1, entry 6). This
lends further support for the reaction to qualify as a greener
approach toward the synthesis of disulfides. Interestingly, com-
paratively higher TON values were obtained under neat condi-
tions, which suggests the greater degree of stabilization of key
catalytic intermediates by coordinating to thiol molecules.
As the transformation of mono-functional aliphatic thiols into
their respective disulfides produces high yields, we proceeded to
explore the photolysis of complex 1 with thiols in the presence of
other functional groups. This was achieved via two methods: (i)
the usage of thiol 1c in the presence of additives (Table 2a) and
(ii) the usage of bis-functionalized thiols (Table 2b).
In the presence of numerous functional groups, the system is
still catalytic, albeit with reduced disulfide and hydrogen yields.
Complex 1 can be considered to be tolerant toward the presence
of the aldehyde, ether, phosphine, and silane functional groups,
as shown by the high disulfide yields (Table 2a, entries 4, 5, 6, and
9), and comparatively less tolerant toward various other func-
tional groups (Table 2a, entries 2, 3, 7, 8, and 10; Table 2b,
entries 1, 2, and 5). Nonetheless, the results suggest the strong
preference for complex 1 to coordinate to thiols over other
MnÀX interactions (e.g., X is an alkene or a phosphine). The
only exception is the presence of the carboxyl functional group
ÀCOOH, which completely quenches the catalytic property of
complex 1 (Table 2b, entry 6). However when Na2CO3 was
added to deprotonate the substrate, high disulfide yields can be
recovered (Table 2b, entry 7).
We have used thiol 1c as a test case for understanding the
reaction pathways toward thiol coupling by complex 1. In order
to determine the role of the bromide ligand in the catalytic
pathway, tetrabutylammonium bromide has been added to the
catalytic mixture. However, no difference in the disulfide yields is
detected. Furthermore the reactions involving complex 1 and 1c
do not show a precipitate forming upon the addition of AgNO3
to the mixture after catalysis. In addition, Sweany and Watzke
have previously reported that the halide does not dissociate upon
photolysis of complex 1 with a low-pressure Hg lamp.26 Thus, we
propose that the MnÀBr dissociation is unlikely to be one of the
important steps in the catalytic cycle.
IR spectra of the photolytic mixture have been obtained over
regular time intervals in an attempt to identify the metal-contain-
ing intermediates present during catalysis. The νCO carbonyl
stretching frequencies have been assigned to the respective
species (Figure 1) based on literature values. Complex 1 (νCO
:
2001, 2020, 2051, and 2134 cmÀ1) appears to undergo sequential
substitution of its CO ligands by thiol molecules via a dissociative
pathway to form cis-Mn(CO)4(RSH)Br (2)27,28 and subse-
quently fac-Mn(CO)3(RSH)2Br (3). The structures of com-
plexes 2 (νCO: 1968, 2013, 2027, and 2086 cmÀ1) and 3 (νCO
:
1962, 1994, and 2069 cmÀ1) are shown in Scheme 2.
Hieber and Gscheidmeier have reported the formation of the
fac-C2H4(SH)2Mn(CO)3Br species,29 which possesses νCO
stretching frequencies similar to those of complex 3. In addition,
it was noted that only the cis- and fac-isomers were obtained for
the monosubstituted and disubstituted intermediates, respec-
tively. The stronger π-back-bonding of CO compared to Br gives
the trans-CO a comparatively stronger metalÀcarbonyl bond.
Hence the CO cis to Br is more labile, and that favors the
formation of the fac-isomer.27,28
Although Reimann and Singleton previously reported the
formation of a trisubstituted complex, Mn(CO)2L3Br, we
could not detect such a species in our system.30 However
the bromo-bridged manganese carbonyl dimer (4) is observed
to form at about t = 20 min into the photolysis,31 but disulfide
formation is only seen to be produced much later, at t = 60 min
(refer to Figure 2). In addition, a control that involved the
irradiation of complex 4, generated independently from heat-
ing complex 1 alone in cyclohexane, with thiols shows no
disulfide formation.
Scheme 2. Structures of Complexes 2, 3, and 4
Figure 2. Plot of νCO intensities for complexes 1, 2, and 3 against time for the oxidative coupling of 1c by complex 1. The relative disulfide concentration
as measured using NMR spectroscopy is also shown.
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Scheme 3. One of the Proposed Reaction Pathways for the Oxidative Coupling of Thiols by Complex 1
1
of complex 3, which was subsequently discontinued. H NMR
spectroscopy was used to monitor the amount of organic
products over time, and the results showed that continuous
UV irradiation is required to produce significantly higher disul-
fide yields. Thus, this could suggest that one of the steps in the
catalytic cycle is triggered by light.
Scheme 4. Net Conversion of Protons into Hydrogen
The requirement for oxygen suggests a catalytic pathway
involving free radical species. According to a previous study,
the SÀH bond in thiols can be cleaved homolytically under UV
irradiation to give H atom and RS radicals.32 We postulate that
oxygen could also have extracted the H atom from the thiol to
form a peroxy radical. Since Perkin et al. have previously reported
the usage of pyrogallol to scavenge the peroxyl radical,33 we have
also adopted the same method to ascertain the possible genera-
tion of such a radical in our system. Indeed, addition of pyrogallol
(0.1À0.5 equivalent of Mn(CO)5Br) decreases the disulfide
yield by approximately 8 times. In addition, results of control
experiments show that pyrogallol does not undergo any side-
reaction with both complex 1 and the thiol.
To further substantiate the presence of radicals in the catalysis,
we have used TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl,
to establish the presence of S-centered radicals. Busfield et al.
have previously used the structurally similar aminoxyl radical
(1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl) to sca-
venge for carbon- and sulfur-centered radicals, but not oxygen-
centered radicals.34 In our experiments, although there is no IR
evidence to indicate reactions between TEMPO with either
complex 1 or the thiol, the addition of TEMPO (0.1À0.5
equivalents of Mn(CO)5Br) to the catalytic mixture still manages
to decrease the disulfide yields by approximately 50%. On the
basis of our experiments with the radical scavengers, we conclude
that free radicals, possibly both S-centered and O-centered
radicals play an important role in the catalytic process.
A catalytic cycle for the oxidative coupling of thiols into
disulfides by complex 1 is proposed on the basis of the experi-
mental data obtained thus far. The photolysis of complex 1
results in the stepwise dissociation of CO followed by thiol
substitution to afford the bis-substituted intermediate complex 3.
At this point, the presence of oxygen becomes essential, where it
may interact with the metal center of complex 3 or with its thiol
ligands instead. In the first case, complex 3 will most likely be
oxidized to form a Mn(II) or Mn(III) complex. It is possible that
such a complex would then act as the active catalyst. In the
second case, a hydrogen atom from one of the coordinated thiol
molecules on 3 is abstracted by oxygen to form intermediate I
and the peroxy radical as illustrated in Scheme 3. The radical
quickly recombines with I to form intermediate II followed by
the elimination of hydrogen upon coupling of the H atom of the
weak OÀH bond of the peroxy moiety with the H atom on the
adjacent RSH ligand. The resultant intermediate III contains a
RSOO fragment, which has been reported to undergo facile
dissociation to O2 and the RS radical.32,35 Thus, it is possible for
To further elucidate the reaction mechanism, a plot of
relative intensities against time (the band used for each
complex is in parentheses) is obtained for the manga-
nese complexes: 1 (2051 cmÀ1), 2 (1968 cmÀ1), and 3
(2069 cmÀ1). The amount of disulfide formed at each time
interval is also included in the same graph. As expected, typical
kinetic profiles indicating intermediary behavior of complexes
2 and 3 are observed. These profiles also show that complex 3
is formed from 2, which in turn originated from complex 1.
Interestingly, the graph indicates that disulfide formation is
detected shortly after the maximum concentration of complex
3 is reached (Figure 2). The rate of disulfide formation also
appears to be highest at this point before reaching a plateau at
longer time intervals. In contrast, the connection between the
disulfide formation and complexes 1 and 2 is not as straight-
forward.
Since complex 3 appears to be closely linked to the
formation of disulfides from thiols, we have prepared it
separately by reacting Mn(CO)5Br with thiol under vacuum
conditions. It was also mentioned earlier that oxygen or air is
essential in this catalytic system. To establish the roles of
oxygen and light further, complexes 3 and 1c have been
reacted under three conditions: (i) UV irradiation in air, (ii)
absence of irradiation in air, and (iii) UV irradiation without
air. The results show that disulfide formation is observed only
for condition (i), hence lending support to the requirement
for oxygen, light, and complex 3 to effect the catalysis.
In an attempt to better understand the effect of light on the
transformation, the catalysis was initiated by the UV irradiation
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Table 3. Oxidative Coupling of Water-Soluble Thiols by Complex 1 under Different Conditionsa
a Irradiation (355 nm, ∼20 mJ/pulse) of complex 1 (0.08 mmol, 10 mol %) and RSH (0.793 mmol) in 1.0 cm3 solvent for 5 h at room temperature.
b 1.0 cm3 D2O. c 0.3 cm3 D2O, 0.7 cm3 tert-butanol.
the oxygen to be eliminated from III, leading to the formation of
a SÀS bond between the two adjacent RS ligands in IV. Finally
the dissociation of the disulfide from IV frees up vacant slots for
the coordination of the thiol substrates and effectively completes
the catalytic cycle.
’ CONCLUSION
The photocatalytic conversion of organic thiols into their
respective disulfides has been achieved with Mn(CO)5Br in the
presence of oxygen. The transformation is tolerant toward many
functional groups, proceeds under mild conditions, and produces
hydrogen as the only other product. A catalytic mechanism has
been proposed on the basis of experimental evidence and may
involve fac-Mn(CO)3(RSH)2Br as a key intermediate. Interest-
ingly, water-soluble thiols are also able to undergo the same
transformation. By combining complex 1 with suitable enzymes
that can regenerate thiols from disulfides using proton sources, it
is possible to construct a catalytic cycle that is able to reduce
protons to hydrogen efficiently.
During the course of this work, we went on to explore the
conversion of water-soluble thiols into the respective disulfides
and hydrogen. This interest was prompted by the well-known
fact that a class of enzymes, the disulfide reductases, is able to
convert disulfides back into thiols. In an aqueous medium, the
hydrogen source is the proton, which usually appears in NADPH
form.36 Enzymes such as glutathione and cysteine reductases are
also commercially available and relatively easy to use.37,38 Hence
it is possible to complete a catalytic cycle by combining the
oxidative coupling of thiols into disulfides catalyzed by complex 1
with the enzymes used to reduce the disulfides back into the
thiols. The overall reaction would be the conversion of protons
into hydrogen (Scheme 4). In this way, the thiols and disulfides
act only as mediators or hydrogen atom carriers and will not be
consumed in the catalytic cycle.
’ EXPERIMENTAL SECTION
General Considerations. All reactions and manipulations were
carried out in the presence of air at room temperature unless otherwise
stated.
Materials. Ethyl 2-mercaptoacetate (98%), furfuryl mercaptan
(98%), pentacarbonyl manganese bromide (98%), 1-butanethiol
(98%), 1-dodecanethiol (98%), 1-octanethiol (98%), and 2-mercapto-
ethanol (98%) were purchased from Alfa Aesar. (2,2,6,6-Tetramethyl-
piperidin-1-yl)oxy (99%) was purchased from Avocado Research
Chemicals. Deuterium oxide (99.9%) was purchased from Cambridge
Isotope Laboratories. Pyrogallol (98%) was purchased from Fluka.
Acetonitrile (99.8%), ethyl acetate (99.8%), benzaldehyde (99.5%),
benzene (99.8%), benzyl mercaptan (99%), chloroform (99%), chloro-
form-d (CDCl3, 99.8 atom % D, stabilized with 0.5 wt % Ag, contains
0.03% TMS), cyclohexane (99.5%), cyclohexanethiol (97%), diethyl
ether (99.9%), L-cysteine ethyl ester hydrochloride (98%), L-glutathione
reduced (98%), silver nitrate (99%), sodium carbonate (99%), tert-
butanol (99.5%), tetrabutylammonium bromide (99%), toluene
(99.8%), triethylsilane (99%), triphenylphosphine (99%), 1-hexene
(99%), 1-hexyne (97%), 2-heptanone (99%), 3-mercaptopropionic acid
(99%), 4-mercaptophenol (97%), and (3-mercaptopropyl)trimethoxy-
silane (97%) were purchased from Sigma-Aldrich. All solvents and
reagents were used without further purification.
We have used cysteine and glutathione as representatives of
water-soluble thiols. Since the carboxyl group inhibits the catalytic
property of complex 1, this functional group in cysteine and
glutathione has been protected and deprotonated respectively.
Two experimental conditions have been used (Table 3), where in
the first case the catalysis is allowed to be carried out heteroge-
neously by dispersing the surface water-insoluble complex 1 in an
aqueous solution of the thiol. In the second case, a solvent pair of
70% tert-butanol and 30% D2O is employed to ensure complete
miscibility of complex 1 with the thiols. Not surprisingly the
disulfide yield for either substrate conducted in the mixed solvent
is higher than those in pure water (Table 3). In both cases, the yields
for the bulkier glutathione are lower than those for cysteine, hence
showing that steric effects play a role in the process. In addition, the
mixed-solvent catalysis is less efficient compared to those involving
alkanethiols in cyclohexane. It is possible that interactions between
complex 1 and the mixed solvent could have slowed the reaction.
Nevertheless, the results show that Scheme 4 could be explored
further in order to exploit thiols and disulfides as possible hydrogen
atom carriers toward the generation of hydrogen gas from proton
sources. Current work in our laboratory is now focused on finding
the optimum way of coupling the two processes using complex 1
and the enzymes together in an efficient manner.
Instrumentation. Photolysis experiments were conducted using a
Continuum Surelite III-10 Nd:YAG laser (355 nm), a Legrand broad-
band lamp (300À800 nm), and
a Philips broadband lamp
(400À800 nm). All IR spectra were obtained using a Shimadazu IR
Prestige-21 FTIR spectrophotometer (1000À4000 cmÀ1, 1 cmÀ1 re-
solution, 4 scans co-added for spectral averaging) using a 0.05 nm path
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1
length calcium fluoride cell. H NMR spectra were recorded at room
temperature in CDCl3 and D2O on a Bruker AV 500 Fourier-transform
spectrometer operating at ca. 500 MHz. The chemical shifts were
reported relative to TMS for spectra taken in CDCl3 and to the residual
solvent peak for spectra taken in D2O. Hydrogen gas was detected using
a Balzer Prisma QMS 200 residual mass analyzer. Mass spectra for the
organic and water-soluble disulfides were recorded with a Finnigan Mat
95XL-T spectrometer and a Finnigan MAT 731 LCQ spectrometer,
respectively.
EI-MS, m/z (M+): 403. Bis(2-ethoxy-2-oxoethyl) disulfide, 1H NMR δ
(CDCl3): 3.58 (t, 4H, CH2S). EI-MS, m/z (M+): 238. Difurfuryl
disulfide, 1H NMR δ (CDCl3): 3.69 (s, 4H, CH2S). EI-MS, m/z
1
(M+): 226. Di(2-hydroxyethyl) disulfide, H NMR δ (CDCl3): 2.89
(t, 4H, CH2S). EI-MS, m/z (M+): 154. Bis(4-hydroxyphenyl) disulfide,
1H NMR δ (CDCl3): 6.76 (d, 4H, ortho to ÀOH). EI-MS, m/z (M+):
250. Bis[3-(trimethoxysilyl)propyl] disulfide, 1H NMR (CDCl3): 2.70
(t, 4H, CH2S). EI-MS, m/z (M+): 391. Dicyclohexyl disulfide, 1H NMR
δ (CDCl3): 2.68 (m, 2H, CH2S). EI-MS, m/z (M+): 230. Dibenzyl
disulfide, 1H NMR δ (CDCl3): 3.60 (s, 4H, CH2S). EI-MS, m/z (M+):
246. Di(3-propionate) disulfide, 1H NMR δ (D2O): 2.54 (t, 4H, CH2S).
ESI-MS (À mode) m/z = 208. Cystine ethyl ester, 1H NMR δ (D2O):
3.27À3.36 (m, 4H, CH2S). ESI-MS (+ mode) m/z = 297.0. Glutathio-
nate disulfide, 1H NMR δ (D2O): 3.24À3.32 (m, 4H, CH2S). ESI-MS
(À mode) m/z = 633.1.
Catalytic Runs Involving Organic Thiols. The metal complex
(39.6 μmol, ∼5 mol %) and the respective thiol (0.793 mmol) were
added into a round-bottom flask that contained 0.5 cm3 of the solvent.
For the functional group tolerance investigations, the respective additive
(0.793 mmol, ∼100 mol %) was introduced accordingly into 0.5 cm3 of
the solvent. The solutions were laser irradiated (355 nm, ∼20 mJ/
pulse), or broadband irradiated (400À800 nm, 11W), and stirred for 2 h.
The amount of disulfide produced was quantified using 1H NMR
spectroscopy by calibration with toluene as the internal standard. The
products were purified by silica gel chromatography for isolated yield
determination. Hydrogen gas was detected by sampling the headspace
above the catalytic mixture throughout the reaction, and the calibration
was achieved via the introduction of a known amount of pure hydro-
gen gas.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: (+65) 6779-1691. E-mail: chmfanwy@nus.edu.sg.
’ ACKNOWLEDGMENT
Catalytic Runs Involving Water-Soluble Thiols. One equiva-
lent of glutathione was reacted with 2 equivalents of Na2CO3 to form
sodium glutathionate. The metal complex (0.08 mmol) and the respec-
tive water-soluble thiol (0.793 mmol) were added into a round-bottom
flask that separately contained (i) 1.0 cm3 of D2O and (ii) 0.3 cm3
of D2O, 0.7 cm3 of tert-butanol, and were laser irradiated (355 nm,
∼20 mJ/pulse) for 5 h.
K.Y.D.T. thanked NUS for a studentship. This project was
supported by research grants provided by MOE Tier 2 grant nos.
T208B1111 and 143-000-430-112.
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