Efficient and convenient deprotection of thiocarbonyl to carbonyl compounds using 3-carboxypyridinium and 2,2′-bipyridinium chlorochromates in solution, dry media, and under microwave irradiation

Article abstract of DOI:10.1007/s00706-003-0095-0

A synthetic utility of 3-carboxypyridinium (CPCC) and 2,2′- bipyridinium (BPCC) chlorochromates in deprotection reactions is reported. Different types of thioamides, thioureas, thiono esters, and thioketones are deprotected to their corresponding carbonyl compounds with these reagents in good to excellent yields. The reactions were carried out in solution, under solvent-free conditions, and under microwave irradiation. The results show that with both reagents the rates of the reactions and the yields are usually highest under microwave irradiation. Springer-Verlag 2003.

Full text of DOI:10.1007/s00706-003-0095-0

Monatshefte f €u r Chemie 135, 411–418 (2004)
DOI 10.1007/s00706-003-0095-0
Efficient and Convenient Deprotection
of Thiocarbonyl to Carbonyl
Compounds Using 3-Carboxypyridinium
0
and 2,2 -Bipyridinium Chlorochromates
in Solution, Dry Media, and under
Microwave Irradiation
ꢀ
Iraj Mohammadpoor-Baltork , Hamid Reza Memarian,
and Kiumars Bahrami
Department of Chemistry, Isfahan University, Isfahan 81746-73441, Iran
Received May 19, 2003; accepted (revised) May 28, 2003
Published online December 23, 2003 # Springer-Verlag 2003
0
Summary. A synthetic utility of 3-carboxypyridinium (CPCC) and 2,2 -bipyridinium (BPCC) chloro-
chromates in deprotection reactions is reported. Different types of thioamides, thioureas, thiono esters,
and thioketones are deprotected to their corresponding carbonyl compounds with these reagents in
good to excellent yields. The reactions were carried out in solution, under solvent-free conditions, and
under microwave irradiation. The results show that with both reagents the rates of the reactions and the
yields are usually highest under microwave irradiation.
Keywords. Chlorochromates; Deprotection; Thiocarbonyl compounds; Carbonyl compounds.
Introduction
Protection and deprotection of functional groups is of importance in organic syn-
thesis. The conversion of thiocarbonyls to their corresponding carbonyl compounds
is an important chemical transformation. Several methods and reagents, such as
bromate and iodate solutions [1], dimethyl selenoxide [2], sodium peroxide [3],
diaryl selenoxide [4], t-butyl hypochlorite [5], benzeneseleninic anhydride [6],
NOBF [7], thiophosgene [8], tetrabutylammonium hydrogensulfate=NaOH [9],
4
þ
dimethyl sulfoxide=iodine [10], soft NO species [11], m-chloroperbenzoic acid
[
[
12], trifluoroacetic anhydride [13], manganese dioxide [14], diaryl telluroxide
15], clay supported ferric nitrate [16], N-nitrosamines [17], p-nitrobenzaldehyde=
ꢀ Corresponding author. E-mail: Imbaltork@sci.ui.ac.ir

4
12
I. Mohammadpoor-Baltork et al.
TMSOTf [18], 2-nitrobenzenesulfonyl chloride=potassium superoxide [19], clayfen
or clayan=MW [20], and Caro’s acid supported on silica-gel [21] have been
reported for this purpose. However, some of these methods are not suitable for
deprotection of primary thioamides, and some of the others also show limitations
such as long reaction times, low yields of the products, the use of expensive
reagents, and tedious work-up.
We have recently reported bismuth(III) nitrate pentahydrate and oxone as
convenient reagents for the deprotection of thioamides and thioureas [22]. How-
ever, bismuth(III) nitrate pentahydrate was not effective for the conversion of
thiono esters and thioketones to their corresponding esters and ketones. With
oxone thiono esters are converted to their esters while thioketones remained
intact. In continuation of our research in this area, we were interested to find
an efficient method for the deprotection of all the above mentioned thiocarbonyl
compounds. In this respect, we wish to report, that 3-carboxypyridinium
0
(CPCC) and 2,2 -bipyridinium (BPCC) chlorochromates are able to transform
thiocarbonyls to their carbonyl compounds efficiently under different reaction
conditions.
Results and Discussion
Deprotection Reactions with CPCC
3-Carboxypyridinium chlorochromate (CPCC) is an inexpensive and easily pre-
pared reagent. Several synthetically useful organic transformations such as
cleavage of the carbon-nitrogen double bonds [23], oxidative deprotection of tri-
methylsilyl and tetrahydropyranyl ethers [24], and oxidation of 1,4-dihydropyri-
dines [25] have been previously reported.
In this paper, at first the deprotection of thioamides, thioureas, thiono esters,
and thioketones with CPCC was investigated using refluxing acetonitrile. As
0
0
shown in Table 1, 1a–1z, 1a –1l were reacted with 2 molar equivalents of
0
0
the reagent to afford the corresponding carbonyl compounds 2a–2z, 2a –2l in
8–97% yields within 2–120 min. The deprotection of these thiocarbonyl com-
6
pounds was also performed under solvent-free conditions with 2.5 molar equiv-
alents of the reagent. Under these conditions, the carbonyl compounds were
obtained in 60–95% yields within 3–40 min. It is important to note that grind-
ing is necessary for the reactions to proceed in high yields. The reactions
without grinding resulted in low yields, 15–25%, even on heating. Finally,
the transformation of thiocarbonyls to their carbonyl compounds was investi-
gated under microwave irradiation. The deprotection was carried out in the
presence of 2 molar equivalents of CPCC with reaction periods ranging
between 1–20 min and the pure products were obtained in 80–98% yields.
Under microwave irradiation, acetonitrile was used for homogenization of the
reaction mixture. The polar character of this solvent also seems to increase
the reaction temperature, so the reaction is completed in a short time [26].
The results show that the rate of the reactions and the yields are usually higher
under microwave irradiation compared to solution and solvent-free reactions
(
Scheme 1).

Deprotection of Thiocarbonyl to Carbonyl Compounds
413
Table 1. Conversion of thiocarbonyl to carbonyl group with CPCC
a
Yield=% (t=min)
ꢁ1
Substrate
Product
ꢀꢀ =cm
Mp or bp [31]
Solution
Solid-phase
MW
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
2a
2b
2c
2d
2e
2f
94 (7)
93 (15)
92 (15)
93 (15)
92 (15)
91 (13)
91 (20)
92 (12)
90 (7)
95 (2)
95 (2)
93 (2)
98 (2)
96 (2)
94 (5)
94 (1)
96 (8)
97 (5)
94 (6)
97 (7)
95 (6)
96 (5)
97 (6)
93 (13)
96 (5)
92 (12)
94 (10)
94 (10)
95 (12)
96 (7)
96 (3)
96 (2)
93 (2)
95 (13)
96 (10)
97 (10)
92 (3)
80 (20)
96 (15)
97 (15)
92 (2)
92 (2)
96 (2)
97 (12)
93 (2)
96 (2)
94 (2)
3340, 3168, 1680
3440, 3350, 1682
3415, 3245, 1685
3420, 3315, 1658
3326, 1648
3385, 3190, 1665
3312, 1662
3330, 1650
3312, 1638
3424, 1650
3240,1648
3324, 1645
3310, 1647
3325, 1644
3330, 1657
3340, 1648
3320, 1650
3300, 1645
3285, 1646
3285, 1655
3376, 1658
3248, 1640
3312, 1667
3405, 1680
3280, 1651
3320, 1644
1650
82
93 (10)
91 (5)
131–132
95
94 (10)
91 (3)
145–146
238–239
>320
93 (30)
93 (2)
g
h
i
2g
2h
2i
154–156
163
93 (3)
96 (5)
92 (5)
105–106
60–61
j
2j
94 (10)
96 (10)
92 (10)
96 (15)
97 (20)
91 (30)
95 (5)
90 (15)
94 (20)
95 (10)
93 (5)
k
l
2k
2l
143
153
m
n
o
p
q
r
s
2m
2n
2o
2p
2q
2r
2s
157–158
202–203
198–199
145
93 (10)
92 (30)
92 (10)
90 (20)
92 (20)
90 (40)
90 (40)
93 (30)
91 (10)
91 (3)
90 (70)
94 (10)
92 (50)
95 (70)
96 (35)
96 (15)
96 (2)
211–212
146–148
153–155
159–160
130–131
171–173
168
t
2t
u
v
w
x
y
z
2u
2v
2w
2x
2y
2z
90 (2)
90 (3)
215–216
232
91 (40)
96 (30)
85 (75)
90 (20)
70 (120)
70 (120)
68 (120)
91 (10)
90 (10)
96 (15)
86 (40)
90 (20)
85 (25)
90 (10)
91 (25)
90 (25)
70 (30)
92 (20)
60 (35)
60 (30)
60 (35)
90 (10)
90 (10)
92 (20)
85 (35)
90 (15)
87 (25)
90 (15)
238–240
101–103
247
0
0
a
2a
0
0
b
2b
3210, 1750, 1714
1646
0
0
c
2c
106–107
127–130
89–90
0
0
d
2d
1637
0
0
e
2e
1638
0
0
f
2f
1720
210–212=760
196–197=760
75–77
0
0
g
2g
1725
0
0
h
2h
0
1653
0
i
2i
1650
136–137
49
0
0
j
2j
1650
0
0
k
2k
0
1660
41–43
0
l
2l
1715
82–84
a
Isolated yields
Deprotection Reactions with BPCC
0
2,2 -Bipyridinium chlorochromate (BPCC) is a commercially available oxidizing
agent and has been used for the oxidation of different organic compounds [27, 28].

4
14
I. Mohammadpoor-Baltork et al.
Scheme 1
As with CPCC, the deprotection reactions with this reagent were investigated
in solution, under solvent-free conditions, and under microwave irradiation. The
treatment of a variety of thioamides, thioureas, thiono esters and thioketones
0 0
1
a–1z, 1a –1l with 2.5 molar equivalents of BPCC in refluxing acetonitrile
0
0
afforded the corresponding carbonyl compounds 2a–2z, 2a –2l in 68–98% yields
within 2–150 min (Table 2). Under solvent-free conditions these reactions pro-
ceeded within 2–40 min in the presence of 3 molar equivalents of the reagent,
and the corresponding carbonyl compounds were obtained in 60–95% yields.
The deprotection was also carried out under microwave irradiation in the presence
of 2.5 molar equivalents of BPCC and the pure carbonyl compounds were obtained
in 80–99% yields after 1–20 min (Scheme 2). Under microwave irradiation, as
with CPCC the reaction times are shorter and the yields are higher.

Deprotection of Thiocarbonyl to Carbonyl Compounds
415
Table 2. Conversion of thiocarbonyl to carbonyl group with BPCC
a
Yield=% (t=min)
Substrate
Product
Solution
Solid-phase
MW
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
2a
2b
2c
2d
2e
2f
94 (10)
94 (10)
93 (5)
93 (15)
93 (15)
91 (13)
90 (15)
89 (13)
92 (20)
92 (2)
94 (2)
94 (2)
95 (3)
95 (2)
95 (2)
94 (5)
93 (1)
95 (9)
96 (5)
92 (6)
95 (7)
94 (7)
99 (7)
95 (7)
93 (12)
98 (5)
94 (12)
94 (10)
92 (10)
93 (14)
95 (7)
96 (3)
96 (3)
93 (3)
92 (10)
93 (10)
93 (10)
93 (2)
80 (20)
98 (15)
98 (15)
93 (3)
91 (3)
96 (3)
94 (12)
97 (3)
92 (3)
93 (3)
93 (10)
90 (5)
93 (30)
92 (2)
g
h
i
2g
2h
2i
90 (3)
91 (5)
96 (7)
93 (5)
j
2j
92 (15)
95 (15)
92 (15)
98 (20)
95 (20)
93 (35)
92 (5)
90 (12)
94 (25)
91 (10)
95 (10)
93 (5)
k
l
2k
2l
m
n
o
p
q
r
s
2m
2n
2o
2p
2q
2r
2s
90 (30)
92 (10)
93 (30)
93 (15)
90 (35)
91 (40)
92 (30)
95 (5)
91 (90)
92 (10)
94 (60)
93 (75)
95 (40)
90 (5)
t
2t
u
v
w
x
y
z
2u
2v
2w
2x
2y
2z
96 (2)
93 (5)
91 (2)
90 (3)
92 (50)
97 (30)
86 (150)
92 (25)
70 (120)
70 (120)
68 (120)
92 (10)
91 (10)
94 (20)
80 (40)
92 (20)
85 (30)
90 (15)
90 (15)
90 (25)
85 (30)
93 (15)
63 (30)
60 (30)
60 (35)
90 (10)
91 (10)
94 (20)
85 (35)
91 (15)
85 (20)
92 (15)
0
0
a
2a
0
0
b
2b
0
0
c
2c
0
0
d
2d
0
0
e
2e
0
0
f
2f
0
0
g
2g
0
0
h
2h
0
0
i
2i
0
0
j
2j
0
0
k
2k
0
0
l
2l
a
Isolated yields
0
In order to show the advantage of 2,2 -bipyridinium chlorochromate (BPCC)
over pyridinium chlorochromate (PCC), we compared the results obtained for
some of the reactions using these two reagents. It was found that in comparison
with pyridinium chlorochromate, the yields of the reactions are considerably higher
0
with 2,2 -bipyridinium chlorochromate (Table 3).

4
16
I. Mohammadpoor-Baltork et al.
Scheme 2
Table 3. Comparison of some of the results obtained with BPCC and PCC
a
BPCC:Yield=% (t=min)
a
PCC:Yield=% (t=min)
Substrate
Product
Solution
Solid-phase
MW
Solution
Solid-phase
MW
1
1
1
j
2j
92 (15)
92 (5)
90 (12)
92 (10)
90 (35)
92 (6)
98 (5)
92 (10)
55 (40)
62 (40)
63 (60)
30 (12)
32 (10)
35 (35)
75 (6)
81 (5)
60 (10)
p
s
2p
2s
94 (60)
a
Isolated yields
Varma et al. have also reported the deprotection of thioketones under micro-
0
wave irradiation [20]. Although, in one case (compound 1j ) the results of the
present work are comparable to that of Varma et al., this similarity may be
expected for other thioketones too. However, in this work we have investigated
the deprotection of other thiocarbonyl compounds including thioamides, thioureas,
and thiono esters.
In conclusion, in this study a new and efficient procedure for the deprotection
of thioamides, thioureas, thiono esters, and thioketonens is described. The method
offers several advantages including high yields of the products, short reaction
times, and experimental simplicity, which makes it a useful and attractive process
for this transformation.
Experimental
The thiocarbonyl compounds are either commercially available or were prepared as following: thio-
amides from the reaction of the corresponding amides with P4S10 [29]; thiono esters and thioketones
from the reaction of the corresponding carbonyl compounds with Lawesson’s reagent [30]. Yields refer
to isolated products. All of the products were characterized by comparison of their physical and
spectral data with those of authentic samples. CPCC and BPCC were prepared according to the
reported procedures [23, 27].
General Procedure for the Conversion of Thiocarbonyls to Carbonyl
Compounds in Acetonitrile
3
In a 50 cm round-bottomed flask a solution of 1 mmol of thiocarbonyl compound in 10cm of CH CN
3
3
was treated with 2 mmol of CPCC or 2.5mmol of BPCC and the reaction mixture was stirred under
reflux for the time indicated in Tables 1 and 2. The progress of the reaction was monitored by TLC
3
(
CCl :EtOAc¼ 4:1). The reaction mixture was filtered and the solid material was washed with 15cm
4
of CH CN. The filtrate was evaporated and the crude product was either recrystallized from
3
EtOH=H O or purified by chromatography on silica gel to afford the pure product (Tables 1 and 2).
2

Deprotection of Thiocarbonyl to Carbonyl Compounds
417
General Procedure for the Conversion of Thiocarbonyls to Carbonyl Compounds
under Solvent-free Conditions
A mixture of 1 mmol of substrate and 2.5 mmol of CPCC or 3 mmol of BPCC in a mortar was
prepared. The mixture was ground with a pestle for the length of time according to Tables 1 and 2.
The progress of the reaction was monitored by TLC. The mixture was extracted with CH Cl .
2
2
Evaporation of the solvent followed by recrystallization from EtOH=H O or chromatography on
2
silica-gel afforded the pure product (Tables 1 and 2).
General Procedure for the Conversion of Thiocarbonyls to Carbonyl Compounds
under Microwave Irradiation
Thiocarbonyl compound (1mmol) and 2 mmol of CPCC or 2.5 mmol of BPCC were mixed and then
3
.5 cm of CH CN were added. The mixture was subjected to microwave irradiation at 900 W for the
0
3
appropriate time according to Tables 1 and 2. After completion of the reaction (TLC), the mixture was
extracted with CH Cl . The solvent was evaporated and the crude product was either recrystallized
2
2
from EtOH=H O or subjected to chromatography on silica-gel to afford the pure product (Tables 1
2
and 2).
Acknowledgements
We are grateful to the Isfahan University Research Council for partial support of this work.
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