Cerium triflate: An efficient and recyclable catalyst for chemoselective thioacetalization of carbonyl compounds under solvent-free conditions

Article abstract of DOI:10.1071/CH09296

A simple and efficient chemoselective thioacetalization of carbonyl compounds has been achieved using Ce(OTf)₃ (10 mol-%) as a catalyst under solvent-free conditions. Advantages of the methodology include very short reaction times, excellent yi

Full text of DOI:10.1071/CH09296

Communication
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2010, 63, 135–140
Cerium Triflate: An Efficient and Recyclable Catalyst
for Chemoselective Thioacetalization of Carbonyl
Compounds under Solvent-Free Conditions
Anil Kumar,A,B M. Sudershan Rao,A and V. Kameshwara RaoA
AChemistry Group, Birla Institute of Technology and Science, Pilani 333 031, India.
BCorresponding author. Email: anilkumar@bits-pilani.ac.in
A simple and efficient chemoselective thioacetalization of carbonyl compounds has been achieved using Ce(OTf)3
(10 mol-%) as a catalyst under solvent-free conditions. Advantages of the methodology include very short reaction times,
excellent yields, the catalytic use of a water tolerant Lewis acid, and simple recovery and reuse of the catalyst.
Manuscript received: 19 May 2009.
Manuscript accepted: 26 August 2009.
O
Ce(OTf)3 10 mol-%
Neat, RT
Introduction
RꢁS
SRꢁ
Rꢀ
2 RꢁSH
ꢂ
Thioacetals have been extensively investigated as carbonyl
protecting groups due to their inherent stabilities towards both
acidic and basic conditions as compared with acetals and
oxathioacetals.[1] They are used as intermediates for the con-
version of the carbonyl group to the parent hydrocarbon by
reductive desulfurization.[2] Thioacetals have also been widely
used in organic synthesis as masked acyl anions[3] or masked
methylene functions[4] in carbon-carbon bond forming reac-
tions; as well as in the synthesis of complex organic molecules
and natural products.[5] Thioacetals are commonly prepared
by the condensation of carbonyl compounds with thiols or
dithiols using protic acids, Lewis acids, metal complexes includ-
ing p-TSA,[6] BF3–OEt2,[7] ZnCl2,[8] SO2,[9] TMSCl–NaI,[10]
R
Rꢀ
R
1
2
3a–u
R ꢃ aryl, Rꢀ ꢃ H, Rꢁ ꢃ alkyl or aryl
R ꢃ alkyl, Rꢀ ꢃ H or alkyl, Rꢁ ꢃ alkyl or aryl
Scheme 1.
reaction methodologies,[25] we report herein the chemoselective
thioacetalization of aldehydes and aliphatic ketones using
Ce(OTf)3 as a catalyst under solvent-free conditions (Scheme 1).
Result and Discussion
TMSOTf,[11] AlCl3,[12] TiCl4,[13] silica supported reagents,[14]
Different variations of catalyst, solvent systems, and cat-
alyst loading were examined to find the optimal reaction
conditions for thioacetalization (Table 1). The reaction of 4-
chlorobenzaldehyde (1a) and thiophenol (2a) to give thioacetal
of 4-chlorobenzaldehyde (3a) was selected as an appropri-
ate model reaction. Among the catalysts screened, Ce(OTf)3,
Yb(OTf)3, and Y(OTf)3 were found to give excellent yield
of 3a (Table 1, entries 11, 16, 17), but Ce(OTf)3 gave
the highest yield of 3a under solvent-free conditions. Next
we optimized the catalyst loading using different amount
of catalyst and found that 10 mol-% of the catalyst under
solvent-free conditions was sufficient to give the desired
product in excellent yield (Table 1, entries 9–12). No sig-
nificant improvement in the yield of 3a was observed
on increasing the catalyst amount further from 10 mol-%
to 20 mol-% (Table 1, entries 11, 12). Notably, the reaction was
slower in a homogeneous medium when compared with solvent-
free conditions. When the model reaction was carried out in
various solvents such as DCM, DMSO, THF, acetonitrile, water,
and ionic liquid [bmim][BF4], using Ce(OTf)3 as catalyst, only
[bmim][BF4] and acetonitrile were found to be effective reac-
tion media to give 3a in 83 and 88% after 2 and 12 h respectively
NiCl2,[15] Bi(NO3)3,[16] metal triflates,[17] molecular I2
,
[18]
InCl3[19], ionic liquids,[20] and polymer supported reagents.[21]
Despite of the potential utilities of these methods giving high
yield of thioacetals, the majority of these are associated with
certain limitations such as strong oxidizing conditions, long
reaction times, the need for azeotropic distillation, harsh reac-
tion conditions, use of halogenated solvents and toxic agents,
unwanted side reaction, expensive catalysts, moisture sensitivity,
and tedious workup conditions. A mild, simple, and more effi-
cient method is still desirable for the chemoselective protection
of carbonyl groups as thioacetals.
Recently, there has been a growing interest in solvent-free
reactions in organic synthesis.[22] There are several advantages
associatedwithsolvent-freereactionsincludingsafety, economy,
ease of workup procedure, short reaction times, and greener
nature.[23] Lanthanide triflates are unique Lewis acids, and
they function well in aqueous media unlike common Lewis
acids that decompose readily in the presence of water.[24]
Ce(OTf)3 is a mild Lewis acid that has attracted little atten-
tion as a catalyst in organic synthesis.[24c] In continuation of
our efforts towards development of new, greener, and efficient
© CSIRO 2010
10.1071/CH09296
0004-9425/10/010135

136
A. Kumar, M. S. Rao, and V. K. Rao
(Table 1, entries 1–8). Importantly, the reaction did not proceed
in the absence of Ce(OTf)3, confirming the effectiveness of the
catalyst. The role of Ce(OTf)3 is not clear but it is expected that
under solvent-free conditions it activates the carbonyl group for
the initial addition of a thiol molecule. This is followed by the
dehydration of the intermediate hemithioacetal, and subsequent
attack by a second thiol molecule.
After standardizing the reaction conditions, different alde-
hydes and ketones were allowed to react with thiols (2a–d)
to give the corresponding thioacetals (3a–u). As shown in
Table 2, a wide range of aromatic aldehydes containing electron-
withdrawing groups, electron-donating groups and hindered
groups; aliphatic aldehydes; cyclic and acyclic aliphatic ketones
were transformed to the corresponding thioacetals in excellent
yields and short reaction time. The reaction was compatible with
wide range of functional groups that may compete for coordi-
nation with the catalyst. The regioselectivity of the method was
demonstrated by the thioacetalization of 1m to corresponding
thioacetal 3p without any competitive conjugate thia-Michael
addition.
The thioacetals were each characterized by 1H and 13C NMR,
and mass spectroscopic data (see Accessory Publication). For
example, 3a showed a singlet at δ 5.37 ppm for one proton of
α-CH group, a doublet and doublet of doublet at δ 7.25 and
Table 1. The comparison of different reaction conditions for synthesis
of 3a
1
7.37 ppm for 10 and four aromatic protons respectively in H
Sr. no. Catalysts
Solvent
Time
Yield [%]A
NMR spectra. The presence of a peak at δ 60.11 ppm for α-CH
carbon along with other aromatic carbons in 13C NMR spectra,
and a peak at m/z 342.92 in the mass spectrum also confirmed
the structure of 3a.
1
2
3
4
5
6
7
8
Ce(OTf)3 (10 mol-%)
[Bmim][BF4] 2 h
83
12
–
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (1 mol-%)
Ce(OTf)3 (5 mol-%)
Ce(OTf)3 (10 mol-%)
Ce(OTf)3 (20 mol-%)
Sm(OTf)3 (10 mol-%)
Ho(OTf)3 (10 mol-%)
Er(OTf)3 (10 mol-%)
Yb(OTf)3 (10 mol-%)
Y(OTf)3 (10 mol-%)
DCM
H2O
12 h
12 h
B
MeOH
THF
15 min
12 h
60
20
88
15
18
20
52
92
94
10
15
20
83
90
73
As noted in Table 2, the difference in reactivity of the cat-
alyst towards aldehydes and ketones provided the impetus to
examine the chemoselectivity of the catalyst. In order to study
the chemoselectivity, experiments with an equimolar mixture of
two different carbonyl compounds were performed (Table 3). For
instance, when an equimolar mixture of 1a and acetophenone
(1s) was allowed to react with 2 equivalents of 2a in presence
of 10 mol-% of Ce(OTf)3, only 3a was obtained while 1s was
recovered quantitatively. Similarly, when an equimolar mixture
of cyclohexanone (1r) and 1s was allowed to react with 2 equiv-
alents of 2a, the thioacetal of cyclohexanone (3u) was obtained
exclusively. These results showed that the catalyst is able to dif-
ferentiate aldehydes and ketones as well as aliphatic ketones
and aromatic ketones. Thus selective protection of aldehydes or
aliphatic ketones is possible using this method. The reason for
the observed chemoselectivity is not clearly known, but may be
CH3CN
DMSO
DMF
Neat
Neat
Neat
12 h
12 h
12 h
5 min
5 min
5 min
5 min
1 h
9
10
11
12
13
14
15
16
17
18
Neat
Neat
Neat
1 h
Neat
1 h
Neat
Neat
15 min
5 min
5 min
Triflic acid (20 mol-%) Neat
AIsolated yields.
BNo product formation was observed, aldehyde recovered quantitatively.
Table 2. Thioacetalization of different carbonyl compounds using Ce(OTf)3 under solvent-free conditionsA
Entry
R
Rꢀ
Rꢀꢀ
Ph
ProductB
3a
Time [min]
Yield [%]C
1
4-ClPh
H
5
92
90
89
93
88
90
89
87
78
56
90
85
90
87
89
90
86
87
89
88
91
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
4-ClPh
4-ClPh
4-ClPh
4-CH3Ph
4-CH3OPh
4-OHPh
2,6-Cl2Ph
2-NO2Ph
CH2CO2CH3
C4H4O
4-OH-3-CH3OPh
3-ClPh
3-CH3OPh
3-NO2Ph
PhCH=CH
CH3CH2CH2
CH3CH2
CH3CH2
(CH3)2CHCH2
-CH2(CH2)3CH2-
H
H
H
H
H
H
H
H
CH3
H
H
H
H
H
H
H
H
4-CH3OPh
3-CH3OPh
HOCH2CH2
3b
3c
3d
3e
3f
3g
3h
3i
5
5
5
5
5
5
8
10
60
5
8
5
5
8
5
5
5
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
CH3
CH3
15
15
15
3u
AReaction conditions: RT (25◦C), carbonyl compounds 0.711 mmol, thiol 1.56 mmol, and Ce(OTf)3 27 mg (10 mol-%).
BStructure of all the products were well characterized by 1H, 13C NMR, and mass spectroscopic data.
CIsolated yield after purification by column chromatography.

Cerium Triflate: An Efficient and Recyclable Catalyst for Chemoselective Thioacetalization
137
S
S
Table 3. Chemoselective thioacetalization using Ce(OTf)3 as catalyst
S
O
HO
OH
CHO
under solvent-free conditions
OH
Ce(OTf)3
RT, 5 min
ꢂ
ꢂ
Sr. no.
Mixture of carbonyl
compounds
Product
Time
[min]
Yield
[%]A
SH
Cl
Cl
89%
0%
Cl
1a
3d
2d
4
CHO
O
CH3
Scheme 2.
1
3a
5
92
Cl
Table 4. Reuse of the catalyst for thioacetalization of 1a with 2a to
give 3a
CHO
O
O
CH3
CH3
Run
1
92
2
92
3
88
4
87
5
85
2
3
4
3f
3i
5
10
15
90
76
90
Yield [%]A
OCH3
AYield of isolated product.
CHO
aqueous layer was evaporated under reduced pressure to give
the recovered catalyst, i.e. Ce(OTf)3, in an almost quantitative
yield. The recovered catalyst was subsequently reused for the
thioacetalization of 1a. As indicated in Table 4, it maintained
good catalytic activity, yielding 3a in 92, 92, 88, 87, and 85%
yield in five successive runs respectively.
NO2
O
CH3
O
3u
Conclusions
AYield of isolated product.
In conclusion, we have presented here a simple, economical
and efficient method for chemoselective thioacetalization of
carbonyl compounds using 10 mol-% of Ce(OTf)3 as catalyst
under solvent-free conditions at room temperature. The present
protocol is endowed with some merits, such as excellent func-
tional group compatibility, short reaction times, excellent yields,
solvent-free conditions, catalytic use of water tolerant Lewis
acids, wide applicability, and recyclability of the catalyst.
due to mild catalytic activity of Ce(OTf)3. Selectivity of thiol
towards aldehydes and aliphatic ketones, as compared with aro-
matic ketones, may be explained on the basis of difference in
relative electron density (global electrophilicity, ω)[26] at the car-
bonyl carbon in these compounds. Such difference in reactivity
of 1r and 1s towards thioacetalization has also been explained by
electron density calculations at the carbonyl carbon using semi-
empirical molecular orbital calculations.[20b] The competitive
formation of thioacetal (3d), versus cyclic 1,3-oxathiolane (4),
from the reaction of 1a with 2d (Scheme 2) could be explained
by the difference of local nucleophilicity i.e. the nucleophilic-
ity of OOH and SSH in 2d. The formation of thioacetal (3d) can
be seen as an orbital controlled soft-soft pathway.[27] Moreover,
selectiveformationof 1d over 4iscatalystandreactioncondition
specific, as when 2d was reacted with 1a in an ionic liquid using
Yb(OTf)3 as catalyst, formation of 4 was observed as major
product.[25c]
The reaction of 1a was explored with different thiols, includ-
ing 4-methoxythiophenol (2b), 3-methoxythiophenol (2c), and
2-mercaptoethanol (2d) (Table 2, entries 2–4), and all of them
gave excellent yields of the corresponding thioacetals (3b–d).
However, it is worth to note that when 4-nitrothiophenol was
used it did not result in thioacetal formation. Again, this may be
explained based on global nucleophilicity of the SSH (S-atom of
SH group) in different thiols, i.e. relative electron donating abil-
ity of the sulfur atom during nucleophilic addition at carbonyl
carbon.
Experimental
CeriumtriflatewaspurchasedfromSigma-Aldrich.Allcommer-
cial reagents and solvents were used without further purification
unless otherwise specified. NMR spectra were recorded on a
Bruker Heaven Avance 11400 spectrophotometer using TMS as
an internal standard and CDCl3 as a solvent and the chemi-
cal shifts were expressed in ppm. The IR spectra were recorded
using KBr pellets on Shimadzu Prestige-21 FTIR spectrophoto-
meter and νmax was expressed in cm−1. EI-MS spectra were
recorded on a Jeol SX 102/DA-6000 (6 kV, 10 mA) mass spectro-
meter.Thepurityoftheproductswasdeterminedonsilica-coated
aluminium plates (Merck).
General Procedure for the Thioacetalization
To a mixture of 4-chlorobenzaldehyde 1a (100 mg, 0.711 mmol)
and thiophenol 2a (172 mg, 1.56 mmol) was added Ce(OTf)3
(27 mg, 10 mol-%) and the whole reaction mass was stirred at
room temperature. The reaction progress was followed by TLC
(8:2 hexane/ethyl acetate). After stirring the reaction mixture
for 5 min diethyl ether (10 mL) and water (10 mL) was added,
and the organic layer was separated. The solvent was dried over
anhydrous sodium sulfate and evaporated under reduced pres-
sure. The residue was percolated through a band of silica gel
(60–120) using hexane/ethyl acetate (9:1 v/v) as an eluent to
give pure compound 3a in 92% yield.
In view of environmentally-friendly methodologies, recovery
and reuse of the catalyst is highly preferable. With this issue in
mind, we examined the possibility of recycling of the catalyst
using thioacetalization of 1a with 2a as model reaction. The
catalyst was recovered from the reaction mixture in the aque-
ous phase after extraction of the product with diethyl ether. The

138
A. Kumar, M. S. Rao, and V. K. Rao
General Procedure for Recovery and Reuse of Catalyst
3j: Methyl 3,3-bis(phenylthio)butanoate. Isolated yield:
56%. δH (CDCl3, 300 MHz) 7.66 (d, J 7.4 Hz, 4H), 7.42–7.31
(m, 6H), 4.16 (q, J 7.1 Hz, 2H), 2.75 (s, 2H), 1.62 (s, 2H), 1.28
(t, J 7.2 Hz, 3H). δC (CDCl3, 75 MHz) 169.67, 137.65, 131.67,
129.83, 129.09, 61.42, 59.97, 46.72, 28.28, 14.81. m/z (ESI)
332.34 [M+].
3k: 2-(Bis(phenylthio)methyl)furan. Isolated yield: 90%. δH
(CDCl3, 300 MHz) 7.37 (t, J 4.2 Hz, 4H), 7.30–7.24 (m, 6H),
6.23 (d, J 1.5 Hz, 1H), 6.15 (d, J 3.0 Hz, 1H), 5.47 (s, 1H),
5.27 (s, 1H). δC (CDCl3, 75 MHz) 151.66, 142.92, 134.08,
133.44, 129.27, 128.56, 110.90, 109.43, 53.56. m/z (ESI) 298
[M+].
3l: 4-(Bis(phenylthio)methyl)-2-methoxyphenol. Isolated
yield: 85%. δH (CDCl3, 300 MHz) 7.34 (d, J 3.6 Hz, 4H), 7.24 (d,
J 4.0 Hz, 6H), 6.89 (s, 1H), 6.80 (q, J 8.4 Hz, 2H), 5.65 (s, 1H),
5.39 (s, 1H), 3.80 (s, 3H). δC (CDCl3, 75 MHz) 146.73, 145.76,
134.54, 132.89, 131.62, 129.21, 128.14, 121.47, 114.40, 110.59,
60.54, 56.15. m/z (ESI) 354.21 [M+].
3m: ((3-Chlorophenyl)methylene)bis(phenylsulfane). Iso-
lated yield: 90%. δH (CDCl3, 300 MHz) 7.33 (d, J 3.0 Hz,
5H), 7.28–7.22 (m, 6H), 7.20 (d, J 6.1 Hz, 3H), 5.35 (s, 1H).
δC (CDCl3, 75 MHz) 142.13, 134.64, 134.33, 133.23, 130.03,
129.33, 128.54, 128.45, 126.46, 60.36. m/z (ESI) 343 [M+].
3n: ((3-Methoxyphenyl)methylene)bis(phenylsulfane). Iso-
lated yield: 87%. δH (CDCl3, 300 MHz) 7.35 (dd, J 6.3, 2.7 Hz,
4H), 7.27–7.21 (m, 6H), 7.20–7.13 (m, 1H), 7.02–6.85 (m,
2H), 6.77 (dd, J 8.2, 2.2 Hz, 1H), 5.39 (s, 1H), 3.73 (s, 3H).
δC (CDCl3, 75 MHz) 160.01, 141.58, 134.94, 132.93, 129.85,
129.24, 128.20, 120.65, 114.46, 113.43, 60.81, 55.63. m/z (ESI)
339.63 [M + H+].
After extracting the product using diethyl ether, the aqueous
layer containing cerium triflate was concentrated under reduced
pressure and dried under vacuum to recover the catalyst. The
flask containing recovered Ce(OTf)3 was again charged with 1a
(100 mg, 0.711 mmol) and 2a (172 mg, 1.56 mmol) and same
procedure was repeated as given in the general procedure for the
thioacetalization.
Spectroscopic Data for the Thioacetals (3a–u)
3a: ((4-Chlorophenyl)methylene)bis(phenylsulfane). Iso-
lated yield: 92%. δH (CDCl3, 300 MHz) 7.32 (dd, J 8.1, 5.1 Hz,
4H), 7.25 (d, J 7.2 Hz, 10H), 5.37 (s, 1H). δC (CDCl3, 75 MHz)
138.65, 134.41, 133.14, 129.61, 129.45, 129.32, 128.99, 128.45,
60.11. m/z (ESI) 342.92 [M+].
3b:
((4-Chlorophenyl)methylene)bis((4-methoxyphenyl)
sulfane). Isolated yield: 90%. δH (CDCl3, 300 MHz) 7.26 (d,
J 8.1 Hz, 4H), 7.17 (q, J 8.5 Hz, 4H), 6.77 (d, J 8.2 Hz, 4H),
5.10 (s, 1H), 3.77 (s, 6H). δC (CDCl3, 75 MHz) 160.46, 139.14,
136.43, 133.49, 129.63, 128.78, 124.67, 114.80, 62.42, 55.68.
m/z (ESI) 404 [M + H+].
3c:
((4-Chlorophenyl)methylene)bis((3-methoxyphenyl)
sulfane). Isolated yield: 89%. δH (CDCl3, 300 MHz) 7.31 (d,
J 8.0 Hz, 2H), 7.24 (d, J 8.9 Hz, 4H), 7.16 (t, J 7.9 Hz, 2H), 6.93
(d, J 7.6 Hz, 2H), 6.85 (s, 2H), 6.79 (d, J 8.3 Hz, 2H), 5.41 (s,
1H), 3.72 (s, 6H). δC (CDCl3, 75 MHz) 159.92, 159.40, 135.21,
130.04, 129.72, 129.03, 124.98, 124.80, 117.85, 114.40, 59.44,
55.70. m/z (ESI) 403.96 [M + H+].
3d:
2,2ꢀ-((4-Chlorophenyl)methylene)bis(sulfanediyl)
diethanol. Isolated yield: 93%. δH (CDCl3, 300 MHz) 7.48–7.37
(m, 2H), 7.29 (d, J 8.3 Hz, 2H), 5.15 (s, 1H), 4.09 (s, 2H), 3.70
(dd, J 10.8, 5.5 Hz, 4H), 2.80 (dt, J 12.8, 6.3 Hz, 2H), 2.70–2.51
(m, 2H). δC (CDCl3, 75 MHz) 139.60, 133.58, 129.51, 128.87,
61.64, 52.59, 35.25. m/z (ESI) 279 [M+].
3o: ((3-Nitrophenyl)methylene)bis(phenylsulfane). Isolated
yield: 89%. δH (CDCl3, 300 MHz) 8.12 (s, 1H), 8.07 (d, J
8.1 Hz, 1H), 7.65 (d, J 7.8 Hz, 1H), 7.41 (t, J 7.9 Hz, 1H),
7.34 (dd, J 6.5, 2.6 Hz, 4H), 7.29–7.20 (m, 6H), 5.47 (s, 1H).
δC (CDCl3, 75 MHz) 148.43, 142.36, 134.30, 133.63, 133.54,
129.76, 129.48, 128.93, 123.32, 60.11. m/z (ESI) 353 [M+].
3p: (3-Phenylprop-2-ene-1,1-diyl)bis(phenylsulfane). Iso-
lated yield: 90%. δH (CDCl3, 300 MHz) 7.52–7.40 (m, 3H),
7.38–7.15 (m, 12H), 7.04 (d, J 7.3 Hz, 1H), 6.32 (d, J
15.7 Hz, 1H), 6.24–6.10 (m, 1H). δC (CDCl3, 75 MHz) 133.87,
133.75, 132.89, 132.44, 129.36, 129.30, 129.18, 128.96, 128.45,
128.27, 128.11, 127.01, 59.05, 56.94. m/z (ESI) 335.39
[M + H+].
3e: (p-Tolylmethylene)bis(phenylsulfane). Isolated yield:
88%. δH (CDCl3, 300 MHz) 7.32–7.40 (m, 4H), 7.24–7.28 (m,
6H), 7.13 (t, J 8.2 Hz, 2H), 6.84 (d, J 7.8 Hz, 1H), 6.76 (t, J
7.5 Hz, 1H), 6.32 (s, 1H), 5.70 (s, 1H). δC (CDCl3, 75 MHz)
153.94, 134.02, 133.05, 130.09, 130.00, 129.30, 128.51, 124.82,
120.94, 117.54, 60.56, 56.77. m/z (ESI) 322.56 [M+].
3f: ((4-Methoxyphenyl)methylene)bis(phenylsulfane). Iso-
lated yield: 90%. δH (CDCl3, 300 MHz) 7.38–7.29 (m, 5H),
7.28 (s, 1H), 7.26–7.17 (m, 6H), 6.79 (d, J 8.7 Hz, 2H), 5.42 (s,
1H), 3.77 (s, 3H). δC (CDCl3, 75 MHz) 159.67, 135.14, 132.77,
132.04, 129.49, 129.21, 128.08, 114.22, 60.11, 55.67. m/z (ESI)
339.28 [M + H+].
3q: Butane-1,1-diylbis(phenylsulfane). Isolated yield: 86%.
δH (CDCl3, 300 MHz) 7.49–7.42 (m, 4H), 7.34–7.24 (m, 6H),
4.40 (t, J 6.6 Hz, 1H), 1.83 (dd, J 15.0, 6.9 Hz, 2H), 1.62 (dq, J
14.5, 7.3 Hz, 2H), 0.89 (t, J 7.3 Hz, 3H). δC (CDCl3, 75 MHz)
134.78, 133.08, 129.24, 127.99, 58.56, 38.33, 20.72, 13.13. m/z
(ESI) 297 [M + Na+].
3g: 4-(Bis(phenylthio)methyl)phenol. Isolated yield: 89%.
δH (CDCl3, 300 MHz) 7.33 (dd, J 6.2, 2.5 Hz, 4H), 7.27–7.13
(m, 8H), 6.70 (d, J 8.3 Hz, 2H), 5.40 (s, 1H), 4.98 (s, 1H).
δC (CDCl3, 75 MHz) 155.62, 135.00, 132.86, 132.29, 129.76,
129.24, 128.14, 115.72, 60.11. m/z (ESI) 325.97 [M + H+].
3r: Propane-1,1-diylbis(phenylsulfane). Isolated yield: 87%.
δH (CDCl3, 300 MHz) 7.50–7.38 (m, 4H), 7.34–7.24 (m, 6H),
4.35 (t, J 6.4 Hz, 1H), 1.95–1.81 (m, 2H), 1.13 (t, J 7.3 Hz, 3H).
δC (CDCl3, 75 MHz) 134.82, 133.05, 129.27, 127.99, 60.42,
29.38, 12.10. m/z (ESI) 261.22 [M + H+].
3h:
((2,6-Dichlorophenyl)methylene)bis(phenylsulfane).
Isolated yield: 87%. δH (CDCl3, 300 MHz) 7.42–7.34 (m,
4H), 7.27–7.14 (m, 8H), 7.10 (t, J 8.0 Hz, 1H), 6.20 (s, 1H).
δC (CDCl3, 75 MHz) 136.86, 135.42, 133.90, 133.85, 130.52,
130.15, 129.39, 128.67, 128.26, 60.33. m/z (ESI) 377.12 [M+].
3i: ((2-Nitrophenyl)methylene)bis(phenylsulfane). Isolated
yield: 78%. δH (CDCl3, 300 MHz) 7.92 (d, J 7.9 Hz, 1H), 7.79
(d, J 8.1 Hz, 1H), 7.53 (t, J 7.6 Hz, 1H), 7.39–7.29 (m, 5H), 7.27–
7.20 (m, 6H), 6.43 (s, 1H). δC (CDCl3, 75 MHz) 148.22, 135.00,
133.72, 133.59, 133.29, 131.12, 129.48, 129.04, 128.78, 124.88,
54.93. m/z (ESI) 354.55 [M + H+].
3s: Butane-2,2-diylbis(phenylsulfane). Isolated yield: 89%.
δH (CDCl3, 300 MHz) 7.64 (d, J 7.1 Hz, 4H), 7.33 (q, J 5.4 Hz,
6H), 1.75 (q, J 7.3 Hz, 2H), 1.37 (s, 3H), 1.10 (t, J 7.3 Hz, 3H). δC
(CDCl3, 75 MHz) 137.34, 132.53, 129.36, 128.93, 65.14, 34.68,
28.00, 9.94. m/z (ESI) 296 [M + Na+].
3t: (4-Methylpentane-2,2-diyl)bis(phenylsulfane). Isolated
yield: 88%. δH (CDCl3, 300 MHz) 7.65 (d, J 7.3 Hz, 4H), 7.34
(d, J 6.7 Hz, 6H), 2.19–2.03 (m, 1H), 1.73 (d, J 4.9 Hz, 2H), 1.41
(s, 3H), 0.94 (d, J 6.7 Hz, 6H). δC (CDCl3, 75 MHz) 137.22,

Cerium Triflate: An Efficient and Recyclable Catalyst for Chemoselective Thioacetalization
139
132.72, 129.30, 128.87, 65.23, 50.70, 28.80, 25.53, 25.26. m/z
(ESI) 303.73 [M + H+].
(e) B. Das, R. Ramu, M. R. Reddy, G. Machender, Synthesis 2005,
250. doi:10.1055/S-2004-834934
(f) S. Rudrawar, R. C. Besra,A. K. Chakraborti, Synthesis 2006, 2767.
(g) M. H. Ali, M. G. Gomes, Synthesis 2005, 1326. doi:10.1055/
S-2005-865303
3u: Cyclohexane-1,1-diylbis(phenylsulfane). Isolated yield:
91%. δH (CDCl3, 300 MHz) 7.69 (d, J 6.6 Hz, 4H), 7.35–7.33
(m, 6H), 1.74 (d, J 5.0 Hz, 4H), 1.65 (s, 4H), 1.32 (s, 2H). δC
(CDCl3, 75 MHz) 137.31, 132.04, 129.30, 128.90, 66.29, 37.54,
25.54, 23.10. m/z (ESI) 301 [M + H+].
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Accessory Publication
1
Copies of H and 13C NMR spectrum of compound 3a–u are
available on the Journal’s website.
(b) S. K. De, Tetrahedron Lett. 2004, 45, 2339. doi:10.1016/
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Acknowledgements
(d) S. Muthusamy, S. A. Babu, C. Gunanathan, Tetrahedron 2002, 58,
7897. doi:10.1016/S0040-4020(02)00897-9
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This work was supported by Department of Science and Technology,
New Delhi (SR-FTP-CS-34–2007) and Council of Scientific and Industrial
Research (CSIR), New Delhi (01(2214)/08/EMR-II). The author M.S.R. is
thankful to Birla Institute of Technology and Science, Pilani for institutional
research fellowship.
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