Ionic liquids-promoted addition of arylsulfinic acids to p-quinones: A green synthesis of diaryl sulfones

Article abstract of DOI:10.1055/s-2004-829145

Arylsulfinic acids undergo smooth conjugate addition to p-quinones in air- and moisture-stable second generation room temperature ionic liquid [bmim]BF₄ under mild conditions to produce the corresponding arylsulfonylhydroquinones in excellent yields with high selectivity. In this reaction, ionic liquid plays the dual role as the solvent and the catalyst. The quinones show enhanced reactivity in ionic liquid thereby reducing the reaction times and improving the yields significantly. The presence of ionic liquids helps to avoid the use of either acid or base catalysts for this conversion. The recovered ionic liquid was reused for four to five times with consistent activity.

Full text of DOI:10.1055/s-2004-829145

PAPER
1849
Ionic Liquids-Promoted Addition of Arylsulfinic Acids to p-Quinones:
A Green Synthesis of Diaryl Sulfones
A
G
ree
n
S
.
ynthesis of
D
S
iaryl Sulfone
.
s
Yadav,* B. V. S. Reddy, T. Swamy, N. Ramireddy
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Fax 91(40)27160512; E-mail: yadav@iict.ap.nic.in
Received 28 January 2004; revised 20 April 2004
Abstract : Arylsulfinic acids undergo smooth conjugate addition to
p-quinones in air- and moisture-stable second generation room tem-
perature ionic liquid [bmim]BF4 under mild conditions to produce
the corresponding arylsulfonylhydroquinones in excellent yields
with high selectivity. In this reaction, ionic liquid plays the dual role
as the solvent and the catalyst. The quinones show enhanced reac-
tivity in ionic liquid thereby reducing the reaction times and im-
proving the yields significantly. The presence of ionic liquids helps
to avoid the use of either acid or base catalysts for this conversion.
The recovered ionic liquid was reused for four to five times with
consistent activity.
Figure 1 Chemical structure of representative ionic liquids
Their high polarity and ability to solubilize both organic
and inorganic compounds can result in enhanced rates of
chemical processes and can provide higher selectivities
compared to conventional solvents. As a result of their
green credentials and potential to enhance rates and selec-
tivities, ionic liquids are finding increasing applications in
organic synthesis.8 Recent reports have shown that they
can also promote and catalyze many transformations of
commercial importance under mild conditions without the
need of any additional acid catalyst.9,10
Keywords: ionic liquids, p-quinones, sulfinic acids, diaryl sulfones
The conjugate addition of arylsulfinic acids to p-quinones
to form carbon–sulfur bonds constitutes a key reaction in
biosynthetic processes as well as in organic synthesis.1
The simple and direct method for the synthesis of 2,5-di-
hydroxydiaryl sulfones involves the nucleophilic addition
of arylsulfinic acids to quinones and is generally catalyzed
by an acid or a base catalyst.2–4 Recently, the reaction has
been studied using various solvents at different pH condi-
tions.5 However, many of these methods often involve the
use of an acid or a base catalyst which always demand
aqueous work-up for the catalyst separation, recycling and
disposal. Since organosulfur compounds have become in-
creasingly useful and important in drugs and pharmaceu-
ticals, the development of simple, convenient, and
environmentally benign approaches is desirable.
In this article, we describe a novel use of ionic liquids for
the conjugate addition of arylsulfinic acids to p-quinones
to produce diaryl sulfones in high yields under mild and
neutral conditions (Scheme 1).
Scheme 1
In recent years, second generation air and moisture stable
room temperature ionic liquids (Figure 1) such as
[bmim]BF4 and [bmim]PF6 are being used as green sol-
vents for the immobilization of transition metal based cat-
alysts, Lewis acids and enzymes.6 They are referred to as
‘designer solvents’ as their properties such as hydrophilic-
ity, hydrophobicity, Lewis acidity, viscosity and density
can be altered by the fine-tuning of parameters such as the
choice of organic cation, inorganic anion and the length of
alkyl chain attached to an organic cation. These structural
variations offer flexibility to the chemist to devise the
most idealized solvent, catering to the needs of any partic-
ular process.7
Accordingly, treatment of p-benzoquinone with phenyl-
sulfinic acid in [bmim]BF4 afforded 2,5-dihydroxydiphe-
nyl sulfone 3a in 92% yield. The reaction went to
completion in a short time (25 min). The product thus ob-
tained was isolated by simple extraction with diethyl
ether. The ionic liquid was further washed with diethyl
ether and reused several times without further purifica-
tion. Encouraged by the results obtained with p-benzo-
quinone, we turned our attention to various substituted
quinones and sulfinic acids. Interestingly, numerous
quinones such as 1,4-benzoquinone and its methoxy,
ethoxy, methyl, dimethyl analogues and 1,4-naphtho-
quinone afforded quite effectively Michael adducts in
high yields (Table 1, entries a–n). In case of mono-substi-
tuted quinones such as toluquinone (2-methyl-1,4-benzo-
quinone) and 2-methoxy-1,4-benzoquinone, the addition
occurs at the 5- and 6-positions with the former predomi-
SYNTHESIS 2004, No. 11, pp 1849–1853
x
x.
x
x
.
2
0
0
4
Advanced online publication: 13.07.2004
DOI: 10.1055/s-2004-829145; Art ID: Z01804SS
© Georg Thieme Verlag Stuttgart · New York

1850
J. S. Yadav et al.
PAPER
nating by 9:1 (Table 1, entries c–h). Like benzoquinone, The reaction was studied in both hydrophilic [bmim]BF4
1,4-naphthoquinone also afforded 2-phenylsulfonylnaph- and hydrophobic [bmim]PF6 ionic liquids and [bmim]BF4
thalene-1,4-diol under identical conditions (Table 1, en- was found to give the best results. Comparatively low
tries m,n). In all cases, the reactions proceeded efficiently conversions (20–50%) were obtained when [bmim]PF6
at room temperature in [bmim]BF4 ionic liquid without was used as a solvent. Furthermore, the use of organic sol-
the need of any additional acid catalyst. The probable vents such as acetonitrile and tetrahydrofuran also gave
mechanism seems to be addition of sulfinic acid to the p- poor yields (30–65%) even after long reaction times (ap-
quinone, which is activated by ionic liquid. The initially proximately 8.0–12 h). The quinones exhibit enhanced re-
formed addition product may undergo tautomerization to activity in the ionic liquid thereby reducing the reaction
give the desired hydroquinone as shown in Scheme 2.
times and improving the yields significantly. For instance,
Table 1 Addition of Arylsulfinic Acids to p-Quinones Using [bmim]BF4 Ionic Liquid
Entry
Quinone
1
Producta
3
R
Reaction Time (min)
25
Yield (%)b
92
a
Ph
b
c
4-MeC6H4
Ph
30
35
95
85 (10)c
d
e
4-MeC6H4
Ph
30
40
87 (6)c
83 (8)c
f
4-MeC6H4
Ph
50
35
86 (7)c
85 (9)c
g
h
i
4-MeC6H4
Ph
40
30
89 (5)c
92
j
4-MeC6H4
Ph
25
35
90
93
k
l
4-MeC6H4
Ph
30
25
91
88
m
n
4-MeC6H4
20
92
a All products were characterized by 1H NMR, IR and mass spectroscopy.
b Isolated and unoptimized yields.
c Yield indicated in parenthesis refers to the other regioisomeric product observed in the 1H NMR spectrum.
Synthesis 2004, No. 11, 1849–1853 © Thieme Stuttgart · New York

PAPER
A Green Synthesis of Diaryl Sulfones
1851
Scheme 2
ane to afford the pure diaryl sulfone. The products thus obtained
were characterized by comparison of their NMR, IR, Mass, TLC,
mixed TLC analysis and physical data with authentic samples. The
spectral data of all the products were identical with those of authen-
tic samples.5
treatment of 2-methylbenzoquinone with phenylsulfinic
acid in [bmim]BF4 afforded the product as a mixture of 2-
methyl-5-phenylsulfonylhydroquinone (3c) and 2-meth-
yl-6-phenylsulfonylhydroquinone (3c¢) in 85 and 10%
yields respectively whereas the same reaction in acetoni-
trile gave the products 3c¢ and 3c in 65% overall yield in
the ratio of 2:1. In addition, the ionic liquid was easily re-
covered after the reaction and reused several times with-
out loss of activity; even after the fourth cycle the product
3a was obtained with similar yield and purity to that ob-
tained in the first cycle. The yields were generally high to
quantitative and the reaction was complete in a few min-
utes. The scope and generality of this method is illustrated
with respect to various substituted quinones and sulfinic
acids and the results are presented in Table 1. The use of
ionic liquids as the reaction media for this transformation
helps to avoid the use of moisture-sensitive reagents or
corrosive acids as promoters, thereby minimizing the pro-
duction of acid waste during workup.
2-Phenylsulfonyl-1,4-hydroquinone (3a)
Solid;3a mp 193 °C.
IR (KBr): 3286, 2362, 1597, 1507, 1461, 1365, 1288, 1137, 1084,
818, 729 cm–1.
1H NMR (200 MHz, CDCl3): d = 6.70 (d, 1 H, J = 8.1 Hz), 6.85 (dd,
1 H, J = 1.9, 8.1 Hz), 7.19–7.20 (d, 1 H, J = 1.9 Hz), 7.40–7.55 (m,
3 H), 7.90 (d, 2 H, J = 8.1 Hz), 8.85 (br s, 1 H, OH), 9.0 (br s, 1 H,
OH).
MS (EI): m/z (%) = 250 (M+, 100), 185 (10), 160 (30), 151 (30), 108
(95), 77 (70), 51 (80).
2-(4-Methylphenylsulfonyl)-1,4-hydroquinone (3b)
Solid; mp 211–212 °C.
IR (KBr): 3286, 1596, 1507, 1465, 1367, 1283, 1137, 1087, 914,
878, 817, 712 cm–1.
In summary, we have described a simple, convenient and
efficient protocol for the conjugate addition of arylsulfinic
acids to p-quinones to produce diaryl sulfones using ionic
liquids as recyclable solvents. The ionic liquid plays a
dual role as reaction solvent and promoter. The p-quino-
nes exhibit enhanced reactivity in ionic liquids, thereby
reducing the reaction times and improving the yield sig-
nificantly. The simple experimental procedure, combined
with ease of recovery and reuse of this novel reaction me-
dia, is expected to contribute to the development of a
1H NMR (200 MHz, CDCl3): d = 2.45 (s, 3 H), 6.60 (d, 1 H, J = 1.7
Hz), 6.75 (d, J = 7.9 Hz, 1 H), 6.90 (dd, J = 1.7, 7.9 Hz, Hz, 1 H),
7.30 (d, J = 8.1 Hz, 2 H), 7.80 (d, J = 8.1 Hz, 2 H), 8.90–9.08 (br s,
2 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 20.6, 113.7, 118.3,
122.4, 126.3, 127.3, 127.3, 128.8, 128.8, 138.6, 143.1, 147.8, 149.3.
MS (EI): m/z (%) = 264 (M+, 40), 165 (10), 154 (100), 136 (90), 120
(20), 95 (30), 81 (40), 69 (70), 55 (80).
2-Methyl-5-phenylsulfonyl-1,4-hydroquinone (3c)
green strategy for the synthesis of 2,5-dihydroxydiaryl Solid;5 mp 140 °C.
sulfones.
IR (KBr): 3463, 3384, 1615, 1472, 1418, 1298, 1223, 1192, 1147,
1081, 1013, 874, 725 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.15 (s, 3 H), 6.70 (s, 1 H), 7.20
(s, 1 H), 7.45–7.60 (m, 3 H), 7.95 (d, 2 H, J = 8.1 Hz), 8.80 (br s, 2
H, OH).
Melting points were recorded on Buchi R-535 apparatus and are un-
corrected. IR spectra were recorded on a Perkin-Elmer FT-IR 240-
c spectrophotometer using KBr optics. H NMR spectra were re-
1
corded on Gemini-200 spectrometer in CDCl3 using Me4Si as inter-
nal standard. Mass spectra were recorded on a Finnigan MAT 1020
mass spectrometer operating at 70 eV. TLC was monitored on 0.25
mm precoated silica gel plates (60F-254). The commercially avail-
able ionic liquids were purchased from Fluka Chemical Company
and used as received without further purification. The purity of
[bmim]BF4 ionic liquid is 97% (NMR). All the solvents were dis-
tilled, dried, and stored under N2 prior to use.
MS (EI): m/z (%) = 264 (M+, 100), 122 (80), 111 (20), 94 (50), 77
(50), 66 (30), 51 (50).
2-Methyl-5-(4-methylphenylsulfonyl)-1,4-hydroquinone (3d)
Solid; mp 175–176 °C.
IR (KBr): 3424, 3366, 2927, 1595, 1411, 1275, 1136, 1081, 866,
811, 673 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.20 (s, 3 H), 2.40 (s, 3 H), 5.40
(br s, 1 H, OH), 6.75 (s, 1 H), 7.05 (s, 1 H), 7.29 (d, 2 H, J = 8.0 Hz),
7.80 (d, 2 H, J = 8.0 Hz), 8.60 (br s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 16.1, 20.9, 112.9,
119.2, 123.4, 127.6, 127.6, 129.1, 129.1, 132.9, 138.9, 143.2, 147.5,
147.8.
Sulfonation of Quinones; General Procedure
A mixture of the p-quinone (1 mmol) and arylsulfinic acid (1 mmol)
in 1-butyl-3-methylimidazolium tetrafluoroborate (3 mL) was
stirred at r.t. for the appropriate time (Table 1). After completion of
the reaction, as indicated by TLC, the reaction mixture was washed
with Et2O (3 × 10 mL). The combined Et2O extracts were concen-
trated in vacuo and the resulting product was directly charged on
small silica gel column and eluted with a mixture of EtOAc–n-hex-
MS (EI): m/z (%) = 278 (M+, 100), 122 (85), 92 (30), 66 (30).
Synthesis 2004, No. 11, 1849–1853 © Thieme Stuttgart · New York

1852
J. S. Yadav et al.
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2-Methoxy-5-phenylsulfonyl-1,4-hydroquinone (3e)
MS (EI): m/z (%) = 278 (M+, 50), 136 (100), 118 (90), 91 (90), 65
Solid; mp 199–200 °C.
(30), 43 (20).
IR (KBr): 3348, 1738, 1619, 1502, 1477, 143, 1387, 1286, 1194,
153, 1050, 978, 860, 799, 754 cm–1.
1H NMR (200 MHz, CDCl3): d = 3.80 (s, 3 H), 6.40 (s, 1 H), 7.23
(s, 1 H), 7.40–7.55 (m, 3 H), 7.80 (d, 2 H, J = 8.1 Hz), 8.05 (br s, 1
H, OH), 8.65 (br s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 55.5, 100.6, 113.6,
126.2, 127.0, 127.0, 128.8, 128.8, 133.2, 138.9, 139.2, 149.4, 153.1.
2,5-Dimethyl-3-(4-methylphenylsulfonyl)-1,4-hydroquinone
(3j)
Solid; mp 149 °C.
IR (KBr): 3439, 3284, 1728, 1598, 1423, 1324, 1259, 1225, 1181,
1120, 1068, 1009, 855, 668, 594 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.23 (s, 3 H), 2.30 (s, 3 H), 2.50
(s, 3 H), 4.35 (br s, 1 H, OH), 6.97 (s, 1 H), 7.40 (d, 2 H, J = 8.0 Hz),
7.85 (d, 2 H, J = 8.0 Hz), 10.35 (br s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 11.9, 15.7, 21.0,
119.9, 119.9, 123.9, 124.9, 125.9, 125.9, 129.3, 129.3, 138.6,
143.9, 147.5, 148.3.
MS (EI): m/z (%) = 280 (M+, 10), 135 (15), 106 (20), 77 (100), 69
(60), 51 (15).
2-Methoxy-5-(4-methylphenylsulfonyl)-1,4-hydroquinone (3f)
Solid; mp 151 °C.
MS (EI): m/z (%) = 292 (M+, 100), 220 (15), 136 (95), 107 (50), 91
(80), 79 (70), 65 (75), 39 (75).
IR (KBr): 3293, 2364, 1699, 1620, 1514, 1440, 1351, 1282, 1240,
1139, 1086, 1047, 824, 666 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.38 (s, 3 H), 3.78 (s, 3 H), 6.38
(s, 1 H), 7.15 (s, 1 H), 7.20 (d, 2 H, J = 8.0 Hz), 7.70 (d, 2 H, J = 8.0
Hz), 8.45 (br s, 1 H, OH), 9.20 (br s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 20.9, 55.4, 100.7,
113.5, 126.3, 126.9, 126.9, 128.7, 128.7, 138.9, 139.1, 142.8, 149.5,
153.2.
2,6-Dimethyl-3-phenylsulfonyl-1,4-hydroquinone (3k)
Solid;3a mp 146 °C.
IR (KBr): 3421, 3219, 2365, 1613, 1568, 1469, 1347, 1216, 1128,
1083, 854, 816, 726 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.08 (s, 3 H), 2.20 (s, 3 H), 4.75–
4.90 (br s, 1 H, OH), 6.60 (s, 1 H), 7.40–7.60 (m, 3 H), 7.78 (d, 2 H,
J = 8.0 Hz), 9.80 (br s, 1 H, OH).
MS (EI): m/z (%) = 294 (M+, 100), 187 (10), 156 (60), 138 (100),
123 (60), 111 (60), 91 (65), 65 (65), 53 (30).
MS (EI): m/z (%) = 278 (M+, 100), 212 (30), 197 (30), 183 (20), 151
(20), 136 (30), 77 (30), 67 (30), 51 (20).
2-Ethoxy-5-phenylsulfonyl-1,4-hydroquinone (3g)
Solid; mp 190 °C.
IR (KBr): 3341, 1713, 1599, 1461, 1282, 1138, 845 cm–1.
2,6-Dimethyl-3-(4-methylphenylsulfonyl)-1,4-hydroquinone
(3l)
1H NMR (200 MHz, CDCl3): d = 1.35 (t, 3 H, J = 7.0 Hz), 3.95 (q,
2 H, J = 7.0 Hz), 6.45 (s, 1 H), 6.85 (s, 1 H), 7.35–7.50 (m, 3 H),
7.81–7.91 (m, 2 H), 8.01–8.15 (br s, 1 H, OH), 8.82–8.93 (br s, 1 H,
OH).
13C NMR (proton decoupled, DMSO-d6): d = 14.2, 64.4, 104.0,
106.6, 126.2, 127.5, 127.5, 128.7, 128.7, 132.9, 138.1, 141.5, 148.5,
149.5.
Solid; mp 161–162 °C.
IR (KBr): 3422, 3217, 1597, 1568, 1469, 1348, 1211, 1126, 1086,
1011, 858, 819, 684 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.18 (s, 3 H), 2.23 (s, 3 H), 2.40
(s, 3 H), 4.30 (br s, 1 H, OH), 6.70 (s, 1 H), 7.25 (d, 2 H, J = 8.0 Hz),
7.78 (d, 2 H, J = 8.0 Hz), 9.95 (br s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 13.1, 17.2, 21.0,
119.9, 119.9, 125.2, 126.5, 126.5, 129.4, 129.4, 130.2, 139.7, 143.5,
146.1, 150.0.
MS (EI): m/z (%) = 294 (M+, 80), 266 (30), 124 (100), 96 (35), 69
(40), 43 (40).
MS (EI): m/z (%) = 292 (M+, 100), 136 (90), 107 (20), 91 (20), 67
(20), 43 (20).
2-Ethoxy-5-(4-methylphenylsulfonyl)-1,4-hydroquinone (3h)
Solid; mp 156 °C.
IR (KBr): 3343, 1597, 1522, 1472, 1434, 1282, 1201, 1132, 1087,
843 cm–1.
2-Phenylsulfonyl-1,4-dihydroxynaphthalene (3m)
Solid; mp 179 °C.
1H NMR (200 MHz, CDCl3): d = 1.45 (t, 3 H, J = 6.9 Hz), 2.41 (s,
3 H), 4.13 (q, 2 H, J = 6.9 Hz), 5.15 (br s, 1 H, OH), 6.39 (s, 1 H),
7.15 (s, 1 H), 7.30 (d, 2 H, J = 8.1 Hz), 7.75–7.81 (d, 2 H, J = 8.1
Hz), 9.03 (br s, 1 H, OH),
13C NMR (proton decoupled, DMSO-d6): d = 14.4, 20.9, 63.9,
101.8, 113.9, 116.5, 127.4, 127.4, 129.1, 129.1, 138.9, 139.2, 143.0,
149.5, 152.6.
IR (KBr): 3416, 3338, 2364, 1577, 1451, 1275, 1131, 1069, 859,
723 cm–1.
1H NMR (200 MHz, CDCl3): d = 6.90 (s, 1 H), 7.40–7.60 (m, 5 H),
7.90 (d, 2 H, J = 8.0 Hz), 8.05 (d, 1 H, J = 8.1 Hz), 8.23 (d, 1 H,
J = 8.1 Hz), 9.48–9.59 (br s, 1 H, OH), 9.82 (s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 103.6, 122.2, 123.0,
126.1, 126.5, 127.0, 127.0, 128.1, 128.3, 128.9, 128.9, 128.9, 133.0,
141.8, 145.5, 146.2.
MS (EI): m/z (%) = 308 (M+, 40), 280 (10), 150 (20), 124 (30), 98
(25), 69 (70), 57(100).
MS (EI): m/z (%) = 300 (M+, 100), 158 (25), 151 (20), 130 (25), 102
(30), 77 (25), 51 (20).
2,5-Dimethyl-3-phenylsulfonyl-1,4-hydroquinone (3i)
Solid;3a mp 155–156 °C.
2-(4-Methylphenylsulfonyl)-1,4-dihydroxynaphthalene (3n)
Solid; mp 169 °C.
IR (KBr): 3439, 3284, 1728, 1598, 1423, 1324, 1259, 1225, 1181,
1120, 1068, 1009, 855, 668, 594 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.15 (s, 3 H), 2.25 (s, 3 H), 4.35
(br s, 1 H, OH), 6.80 (s, 1 H), 7.45–7.65 (m, 3 H), 7.85 (d, 2 H,
J = 8.1 Hz), 10.2 (br s, 1 H, OH).
IR (KBr): 3429, 2366, 1528, 1459, 1317, 1273, 1128, 862, 763
cm–1.
1H NMR (200 MHz, CDCl3): d = 2.40 (s, 3 H), 6.89 (s, 1 H), 7.30
(d, 2 H, J = 8.1 Hz), 7.45–7.60 (m, 2 H), 7.79 (d, 2 H, J = 8.1 Hz),
Synthesis 2004, No. 11, 1849–1853 © Thieme Stuttgart · New York

PAPER
A Green Synthesis of Diaryl Sulfones
1853
8.10 (d, 1 H, J = 7.9 Hz), 8.25 (d, 1 H, J = 7.9 Hz), 9.45 (br s, 1 H,
OH), 9.65 (br s, 1 H, OH).
13C NMR (proton decoupled, DMSO-d6): d = 20.9, 103.6, 120.4,
122.3, 123.1, 126.2, 126.6, 127.2, 127.2, 128.1, 128.3, 129.5, 129.5,
139.0, 143.7, 145.4, 146.3.
MS (EI): m/z (%) = 314 (M+, 90), 158 (90), 151 (100), 130 (60), 102
(60), 91 (50), 77 (25), 65 (25), 51 (20).
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Acknowledgment
BVSR thank CSIR New Delhi for the award of fellowship.
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Synthesis 2004, No. 11, 1849–1853 © Thieme Stuttgart · New York