CpMn(CO)3-catalyzed photoconversion of thiols into disulfides and dihydrogen

Article abstract of DOI:10.1021/om1005947

The UV photolysis of CpMn(CO)₃ with thiols at room temperature effected the following catalytic transformation: 2 RSH → R ₂S₂ + H₂. This reaction is a cleaner and greener way toward making disulfides, as it produces dihydrogen as the only side-product. The manganese system exhibits high chemoselectivity as the transformation proceeds efficiently even in the presence of numerous functional groups. A manganese dicarbonyl complex, CpMn(CO)₂RSH, and cyclopentadiene have also been detected using FTIR and NMR spectroscopic techniques, respectively. Based on our experimental data, a mechanism has been proposed to account for the catalysis.

Full text of DOI:10.1021/om1005947

Organometallics 2010, 29, 4459–4463 4459
DOI: 10.1021/om1005947
CpMn(CO)3-Catalyzed Photoconversion of Thiols into
Disulfides and Dihydrogen
Kheng Yee Desmond Tan, Jun Wei Kee, and Wai Yip Fan*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
Received June 17, 2010
The UV photolysis of CpMn(CO)3 with thiols at room temperature effected the following catalytic
transformation: 2 RSH f R2S2 þ H2. This reaction is a cleaner and greener way toward making
disulfides, as it produces dihydrogen as the only side-product. The manganese system exhibits high
chemoselectivity as the transformation proceeds efficiently even in the presence of numerous
functional groups. A manganese dicarbonyl complex, CpMn(CO)2RSH, and cyclopentadiene
have also been detected using FTIR and NMR spectroscopic techniques, respectively. Based on our
experimental data, a mechanism has been proposed to account for the catalysis.
Scheme 1. Oxidative Coupling of Thiols into Disulfides and
Hydrogen
Introduction
Organosulfur compounds play vital roles in chemistry and
biology,1,2 and their transformations have always been of
interest, in particular the oxidation of thiols into disulfides.
Such reactions, however, usually require a basic catalyst3 or
stoichiometric amounts of oxidants (e.g., sulfoxides,4 metal
oxidants,5 peroxides,6 halogens,7 and air),8 which commonly
generate unwanted side-products. In addition, many of such
protocols require costly and toxic reagents and/or long
reaction times.
The synthesis of disulfides from thiols sometimes suffers
from overoxidation, producing sulfoxides, sulfones, thio-
sulfinates, and thiosulfonates.9-11 Tanaka et al. previously
reported that the selective oxidative coupling of thiols by
transition-metal complexes into disulfides is a viable alter-
native to the use of oxidants,12 but such reactions remain
largely unexplored. Gondi et al. also reported the usage of
nanophase-manganese(VII) oxide coated clay in the trans-
formation of thiols into disulfides.13
In this work, we investigate a greener catalytic system: the
photocatalyzed conversion of thiols into disulfides under an
inert atmosphere using an organometallic manganese com-
plex, CpMn(CO)3 (1), with dihydrogen as the only side-
product. Interestingly, the catalytic system exhibits tolerance
toward numerous functional groups. We also aim to better
understand the mechanism of transition-metal-catalyzed
dehydrogenation of thiols by using FTIR and NMR spec-
troscopy and mass spectrometry to detect key intermediates
and products.
Results and Discussion
*To whom correspondence should be addressed. Fax: (þ65) 6779-
1691. E-mail: chmfanwy@nus.edu.sg.
(1) Cremlyn, R. J. In An Introduction to Organosulfur Chemistry;
Wiley: New York, 1996.
(2) Whitham, G. H. In Organosulfur Chemistry; Oxford University
Press: Oxford, 1995.
(3) Joshi, A. V.; Bhusare, S.; Baidossi, M.; Qafisheh, N.; Sasson, Y.
UV laser irradiation of complex 1 with aliphatic and aro-
matic monofunctional thiols in the absence of oxygen afforded
the corresponding disulfides in excellent yields, with dihydro-
gen as the only side-product (Scheme 1). Although broadband
irradiation effects the same transformation, full conversion into
the products requires a longer time. 1H NMR and EI-MS have
been used to confirm the identities of the organic products. In
addition, isolated yields of the disulfides are in excellent agree-
ment with our method of using NMR for quantifying the
product within the reaction mixture (Table 1).
We have also detected dihydrogen in quantitative yields of
up to 80% (relative to the disulfides) using EI-MS (Figure 1),
which leads us to propose that the transformation is indeed
very clean. This is further supported by the absence of the
other commonly encountered overoxidation products, such
as sulfones and sulfoxides.
Tetrahedron Lett. 2005, 46, 3583.
(4) Wallace, T. J. J. Am. Chem. Soc. 1964, 86, 2108.
(5) Ramesha, A. R.; Chandrasekaran, S. J. Org. Chem. 1994, 59,
1354.
(6) Hajipour, A. R.; Mallakpour, S. E.; Adibi, H. J. Org. Chem. 2002,
67, 8666.
(7) Ali, M. H.; McDermott, M. Tetrahedron Lett. 2002, 43, 6271.
(8) Shah, S. T. A.; Khan, K. M.; Fecker, M.; Voelter, W. Tetrahedron
Lett. 2003, 44, 6789.
(9) Iranpoor, N.; Mohajer, D.; Rezaeifard, A. R. Tetrahedron Lett.
2004, 19, 3811.
(10) Freeman, F.; Angeletakis, C. N. J. Am. Chem. Soc. 1983, 105,
4039.
(11) Lazzaro, F.; Crucianelli, M.; Angelis, F. D.; Neri, V.; Saladino,
R. Tetrahedron Lett. 2004, 45, 9237.
(12) Tanaka, K.; Ajiki, K. Tetrahedron Lett. 2004, 45, 25.
(13) Gondi, S. R.; Son, D. Y.; Biehl, E. R.; Vempati, R. K. Phos-
phorus, Sulfur, Silicon 2010, 185, 34.
Of more interest are the aliphatic thiols (Table 1, entries 1-4),
as the disulfide formation does not proceed spontaneously upon
r
2010 American Chemical Society
Published on Web 09/07/2010
pubs.acs.org/Organometallics

4460 Organometallics, Vol. 29, No. 20, 2010
Table 1. Oxidative Coupling of Monofunctional Thiols into the Corresponding Disulfidesa
Tan et al.
a Irradiation (355 nm, ∼13 mJ/pulse) of complex 1 (9.19 μmol, ∼3 mol %), and RSH (3.06 mmol) in 0.5 cm3 cyclohexane for 2.5 h at 25 °C.
b Determined by 1H NMR using toluene (2a-d) and ethyl acetate (2e) as the internal standards. c Isolated by silica gel column chromatography.
d Obtained using the ratio of the number of moles of thiol converted against complex 1.
Figure 1. Photolysis of complex 1 with 1d in cyclohexane. (a) Gas-phase mass spectrum indicating the formation of dihydrogen.
(b) Mass spectrum of dihydrogen (m/e value of 2) scanned over time.
stirring in air. Although the stirring of 1e in the presence of
oxygen is known to spontaneously produce 2e (Table 1, entry 5),
the conversion is sluggish. In a control experiment without
complex 1, less than 1% conversion of 1e into 2e has been
detected. The catalytic transformation of the aromatic thiol is,
thus, still of value in this case.
Since the conversion of monofunctional thiols to the respec-
tive disulfides was shown to proceed efficiently, we went on to
investigate the one-pot irradiation of complex 1 with thiols in
the presence of numerous functional groups. This was achieved
via two methods: (i) the usage of bifunctional thiols (Table 2),
and (ii) the usage of 1d with additives (Table 3).
The catalytic system exhibited tolerance toward a wide
variety of functional groups, such as alcohols, alkenes,
alkynes, and esters, as evidenced by the unaffected disulfide
yields (Table 2, entries 1 and 3; Table 3, entries 7 and 8). The
results indicate the strong preference for the formation of
Mn-S interactions, although complexes such as CpMn-
(CO)2X (e.g., X is an alkene14 or an alkyne)15 can be formed
quite readily.
yields, although the process is still catalytic. The lowest yield
obtained is that with the phosphine (Table 3, entry 6), which
does not come as a surprise, as the complex CpMn(CO)2PPh3 is
very stable.16
The catalysis was attempted with two other metal complexes,
CpRe(CO)3 and C6H6Cr(CO)3, both of which possess a similar
electronic structure to complex 1. Both complexes, however,
were unable to effect the transformation under the same
reaction conditions; the thiol remained unchanged. In an
attempt to better understand the transformation, the effect of
dioxygen was studied. The amount of disulfide produced in the
presence of dioxygen is significantly lower, which may be
attributed to the competing decomposition of complex 1.
Using FTIR spectroscopy, two additional metal-carbonyl
peaks (νCO: 1882 and 1942 cm-1, cyclohexane solvent) have
been detected in the reaction mixture throughout the cata-
lysis of 1d (Figure 2a). The peak intensities showed a gradual
increase toward a maximum absorbance value before they
eventually diminished at the end of the reaction. A control
experiment without the thiol has also been performed, and the
same peaks were not observed. These two peaks have been
assigned to CpMn(CO)2RSH (I). Upon comparison, the νCO
The presence of other functional groups such as ethers,
ketones, nitriles, siloxanes, and phosphines reduces the disulfide
(14) Salzer, A.; Buchmann, B. J. Organomet. Chem. 1985, 295, 63.
(15) Antonova, A. B.; Kolobova, N. E.; Petrovsky, P. V.; Lokshin,
B. V.; Obezyuk, N. S. J. Organomet. Chem. 1997, 137, 55.
(16) Ginzburg, A. G.; Lokshin, B. V.; Setkina, V. N.; Kursanov,
D. N. J. Organomet. Chem. 1973, 55, 357.

Article
Organometallics, Vol. 29, No. 20, 2010 4461
Table 2. Oxidative Coupling of Bifunctional Thiols into the Corresponding Disulfidesa
a Irradiation (355 nm, ∼13 mJ/pulse) of complex 1 (9.19 μmol, ∼3 mol %) and RSH (3.06 mmol) in 0.5 cm3 cyclohexane (1f, 1g), benzene (1h-j), and
THF (1k) for 2.5 h at 25 °C. b Determined by 1H NMR using toluene as the internal standard. c Obtained using the ratio of the number of moles of thiol
converted against complex 1.
Table 3. Oxidative Coupling of 1d to 2d in the Presence of Additivesa
a Irradiation (355 nm, ∼13 mJ/pulse) of complex 1 (9.19 μmol, ∼3 mol %), RSH (3.06 mmol), additive (3.06 mmol, ∼100 mol %), and 0.5 cm3
cyclohexane for 2.5 h at 25 °C. b Determined by 1H NMR using toluene as the internal standard. c Obtained using the ratio of the number of moles of thiol
converted against complex 1. d Benzene was used as the solvent.
frequencies of this manganese complex are found to resemble
those of CpMn(CO)2MeOH (νCO: 1850 and 1921 cm-1, 50/50
ether/methanol solvent)17 and CpMn(CO)2PPh3 (νCO: 1885
and 1946 cm-1, heptane solvent).18 It is well known that νCO

4462 Organometallics, Vol. 29, No. 20, 2010
Tan et al.
Figure 2. Photolysis of complex 1 with 1d in cyclohexane. (a) IR bands at t = 0.5 h of intermediate I and unreacted complex 1.
(b) 1H NMR of cyclopentadiene in CDCl3 at t = 2.5 h.
Scheme 2. Proposed Catalytic Cycle for the Catalytic
Transformation of Thiols into Disulfides by Complex 1
cyclopentadiene in close to stoichiometric amounts with respect
to the original amount of complex 1 added (Figure 2b).
We propose a catalytic cycle for the transformation of
thiols into disulfides by complex 1 based on the experimental
evidence (Scheme 2). Upon photolysis of complex 1, Mn-
CO bond cleavage occurs to give the 16-electron intermedi-
ate, CpMn(CO)2. This reactive species subsequently reacts
with a molecule of thiol to generate intermediate I, which was
detected in our system. Thereafter, migration of the hydro-
gen atom in I to the cyclopentadienyl ring produces the
16-electron η4-cyclopentadiene intermediate II. This propo-
sition is supported by the previous findings of Top et al., in
which the photolysis of complex 1 in the presence of a proton
source leads to decomplexation and formation of the free
cyclopentadiene in high yields.16 The formation of cyclopen-
tadiene is regarded as a competitive channel to the catalysis.
Another molecule of thiol coordinates to II and produces
intermediate III, followed by the elimination of a molecule of
hydrogen, which restores the cyclopentadienyl ring and
produces the disulfide-bound intermediate IV. The dissocia-
tion of the disulfide from IV, whose vacant site is taken up by
another thiol molecule, effectively completes the catalytic
cycle.
Conclusion
The catalytic conversion of thiols into disulfides has been
accomplished using complex 1 as the precursor, with hydro-
gen gas as the only side-product. The reaction also exhibited
tolerance toward the presence of numerous functional
groups. A catalytic mechanism has been proposed for the
transformation based on our experimental data.
Experimental Section
frequencies taken in polar solvents generally exhibit a large red-
shift compared to hydrocarbon solvents. The manganese com-
plex also appears to closely resemble the phosphine derivative
since π back-bonding from manganese to both phosphorus and
sulfur is expected to take place. In addition to the disulfide
All reactions and manipulations were performed under
vacuum using standard vacuum-line and Schlenk techniques.
Materials. Cyclopentadienyl manganese tricarbonyl (CpMn-
(CO)3, 98%), cyclopentadienyl rhenium tricarbonyl (CpRe-
(CO)3, 99%), ethyl 2-mercaptoacetate (98%), furfuryl mercaptan
(98%), 1-butanethiol (98%), 1-octanethiol (98%), 1-dodecanethiol
(98%), 1-hexadecanethiol (98%), and 2-mercaptoethanol (98%)
were purchased from Alfa Aesar. Benzene (99.8%), benzene
chromium tricarbonyl (C6H6Cr(CO)3, 98%), benzonitrile (99%),
benzophenone (99%), chloroform-d (CDCl3, 99.8 atom % D,
stabilized with 0.5 wt % Ag, contains 0.03% TMS), cyclohexane
1
signals, H NMR spectroscopy revealed the production of
(17) Top, S.; Kaloun, E. B.; Toppi, S.; Herrbach, A.; McGlinchey,
M. J.; Jaouen, G. Organometallics 2001, 20, 4554.
(18) Hester, D. M.; Sun, J.; Harper, A. W.; Yang, G. K. J. Am. Chem.
Soc. 1992, 114, 5234.

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Organometallics, Vol. 29, No. 20, 2010 4463
(99.5%), diethyl ether (99.9%), ethyl acetate (99.8%), hexane
(97%), m-toluidine (99%), tetrahydrofuran (99%), toluene
(99.8%), triethylamine (99.5%), triphenylphosphine (99%),
1-hexene (97%), 1-hexyne (97%), and (3-mercaptopropyl)tri-
methoxysilane (97%) were purchased from Sigma-Aldrich. All
solvents and reagents were used without further purification.
Instrumentation. Photolysis experiments were performed using
a Quanta-Ray PRO-series pulsed Nd:YAG laser (355 nm) and a
Legrand broadband lamp (400-800 nm). Hydrogen gas was
detected using a Balzer Prisma QMS 200 residual mass analyzer.
IRspectra wererecorded witha Shimadzu IR Prestige-21 Fourier-
transform IR spectrometer (1000-4000 cm-1, 1 cm-1 resolution,
8 scans co-added for spectral averaging) using a 0.05 mm path
produced was quantified using 1H NMR spectroscopy by
calibration with toluene and ethyl acetate (2e and Table 3,
entries 4 and 5) as the internal standards. The products were
purified by silica gel chromatography for isolated yield deter-
mination. Hydrogen gas was detected by sampling the head-
space above the catalytic mixture throughout the reaction, and
the calibration was achieved via the introduction of a known
amount of pure hydrogen gas.
1H NMR and EI-MS Data for the Disulfides. Dibutyl disulfide
(2a). 1H NMR, δ (CDCl3): 2.69 (t, 4H, CH2S). EI-MS, m/z
1
(Mþ): 178. Dioctyl disulfide (2b). H NMR δ (CDCl3): 2.68
(t, 4H, CH2S). EI-MS, m/z (Mþ): 291. Didodecyl disulfide (2c).
1H NMR δ (CDCl3): 2.68 (t, 4H, CH2S). EI-MS, m/z (Mþ): 403.
1
1
length calcium fluoride cell. H NMR spectra were recorded in
Dihexadecyl disulfide (2d). H NMR δ (CDCl3): 2.68 (t, 4H,
CDCl3 on a Bruker AV 500 Fourier-transform spectrometer
operating at ca. 500 MHz. The chemical shifts were reported
relative to TMS. Mass spectra of the disulfide products were
recorded with a Finnigan Mat 95XL-T spectrometer.
CH2S). EI-MS, m/z (Mþ): 515. Di(p-tolyl) disulfide (2e). 1H
NMR δ (CDCl3): 2.32 (s, 6H, CH3). EI-MS, m/z (Mþ): 248.
Bis(2-ethoxy-2-oxoethyl) disulfide (2f). 1H NMR δ (CDCl3):
3.58 (t, 4H, CH2S). EI-MS, m/z (Mþ): 238. Difurfuryl disulfide
(2g). 1H NMR δ (CDCl3): 3.69 (s, 4H, CH2S). EI-MS, m/z (Mþ):
226. Di(2-hydroxyethyl) disulfide (2h). 1H NMR δ (CDCl3):
2.89 (t, 4H, CH2S). EI-MS, m/z (Mþ): 154. Bis(4-hydro-
xyphenyl) disulfide (2i). 1H NMR δ (CDCl3): 6.76 (d, 4H, ortho
to -OH). EI-MS, m/z (Mþ): 250. Bis[3-(trimethoxysilyl)propyl]
Procedure for the Catalytic Runs. The metal complex (9.19
μmol, ∼3 mol %) and the respective thiol (3.06 mmol) were
added into a round-bottom flask that contained 0.5 cm3 of the
solvent (cyclohexane for 1a-g and benzene for 1h-j) in the
absence of oxygen. For the functional group tolerance investi-
gations, the desired additive (3.06 mmol, ∼100 mol %) was
introduced accordingly into 0.5 cm3 of the cyclohexane and
benzene (Table 3, entry 4). The solutions were laser irradiated
(355 nm, ∼13 mJ/pulse) or broadband irradiated (400-800 nm,
11 W) and stirred at room temperature for 2.5 h. Precipitation of
the disulfide products was observed for the runs involving 1h-j,
and so the reaction mixtures were dried upon completion
and redissolved in 0.5 cm3 of THF. The amount of disulfide
1
disulfide (2j). H NMR (CDCl3): 2.70 (t, 4H, CH2S). EI-MS,
m/z (Mþ): 391.
Acknowledgment. K.Y.D.T. and J.W.K. thank NUS
for studentships. This project was supported by research
grants provided by MOE Tier 2 grant nos. T208B1111
and 143-000-430-112.