Direct sulfonylation of Baylis-Hillman alcohols and diarylmethanols with TosMIC in ionic liquid-[Hmim]HSO4: An unexpected reaction

Article abstract of DOI:10.1016/j.tetlet.2011.06.096

A Bronsted acidic ionic liquid-[Hmim]HSO₄ promoted unexpected reaction of Baylis-Hillman alcohols and diarylmethanols with p-toluenesulfonylmethyl isocyanide (TosMIC) affording the corresponding sulfone derivatives instead of N-tosylmethyl amid

Full text of DOI:10.1016/j.tetlet.2011.06.096

Tetrahedron Letters 52 (2011) 4622–4626
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Direct sulfonylation of Baylis–Hillman alcohols and diarylmethanols with
TosMIC in ionic liquid-[Hmim]HSO4: an unexpected reaction
⇑
Garima, Vishnu P. Srivastava, Lal Dhar S. Yadav
Green synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A Brønsted acidic ionic liquid-[Hmim]HSO4 promoted unexpected reaction of Baylis–Hillman alcohols
and diarylmethanols with p-toluenesulfonylmethyl isocyanide (TosMIC) affording the corresponding sul-
fone derivatives instead of N-tosylmethyl amides is reported. After isolation of the product, the ionic
liquid [Hmim]HSO4 was easily recycled for further use.
Received 27 February 2011
Revised 21 June 2011
Accepted 26 June 2011
Available online 2 July 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquids
Sulfonylation
Baylis–Hillman alcohols
p-Toluenesulfonylmethyl isocyanide
(TosMIC)
Allylic sulfones
OAc(OH)
The tight restrictions on the release of waste and toxic emis-
sions for the reduction of environmental pollution have induced
a paradigmatic shift in the development of new synthetic strate-
gies. Thus, in addition to the required mildness and selectivity,
the issue of environmentally friendly reaction conditions has
become increasingly important in designing alternate synthetic
routes for fine chemicals. In this context, ionic liquids (ILs) are
emerging as effective promoters and alternative solvents for green
chemical reactions because of their many fascinating properties.
ILs are simple, easy to recycle, inexpensive to prepare, and their
properties can be fine tuned by changing the anion or the alkyl
group attached to the cation.1 Recently, Brønsted acidic ionic
liquids have been deemed promising alternatives for acid catalyzed
reactions and play a dual solvent–catalyst role in a variety of
organic reactions.2
COOEt
R1
COOEt
O
2
O
O
S
BF3.OEt2, CH3CN
(Ref. 7)
R1
S
O
O
HN
S
4
NC
OH
1
O
R2 R3
R3
R2
TosMIC
O
3
O
HN
5
InCl3, CH3CN
(Ref. 6a)
Scheme 1. Synthesis of N-tosylmethyl amides 4 and 5 using TosMIC.
p-Toluenesulfonylmethyl isocyanide (TosMIC)3 is a versatile
and widely exploited reagent in a diverse range of organic
reactions manifesting into valuable scaffolds, building blocks, and
heterocycles. The broad synthetic utility stems from its varied
reacts accordingly. Apart from the TosMIC mediated synthesis of
C-nucleosides4 and diastereoselective Passerini reactions,5 recent
endeavors show its application in transition metal catalyzed car-
bon–carbon bond formation by the reaction with aryl alcohols6a
and 1,3-dicarbonyl compounds6b affording N-tosylmethyl amides
and b-keto-(E)-enamino esters, respectively. Quite recently, Yadav
et al.7 have applied the Baylis–Hillman (BH) chemistry8 to access
functionalized allyl amides from the reaction between TosMIC 1
and BH acetates in the presence of Lewis acid (Scheme 1). In this
report they have mentioned that TosMIC could also react with
BH alcohols instead of BH acetates to afford allyl amides, but
conversion rate was poor (20–45%) even after long reaction time.7
functional groups: the isocyano group undergoes typical
tion reactions, the acidic -carbon atom and the sulfonyl group
in the -position serve two functions by acting both as a sulfonyl
leaving group and contributing to the enhanced acidity of the
-carbon. TosMIC can be viewed as a specialized type of N,S-acetal
a-addi-
a
a
a
due to the presence of the geminal isocyano and tosyl groups and
⇑
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.096

Garima et al. / Tetrahedron Letters 52 (2011) 4622–4626
4623
OH
100
EWG
78
80
EWG
O
76
75
R1
77
76
80
60
40
20
0
S
O
6
2
R1 = aryl, alkyl
1
O
S
R
EWG = CN (E isomer)
[Hmim]HSO4
80 oC, 2-14 h
71-80%
COOMe (Z isomer)
O
NC
OH
O
S
1
R2
R2 R3
TosMIC
3
3 O
[Hmim]=
N
N
R2, R3 = aryl
R
H
1
2
3
4
5
6
7
Reaction cycles
Scheme 2. Ionic-liquid promoted synthesis of sulfone derivatives 6 and 7 using
TosMIC and alcohols.
Figure 1. Reusability of ionic liquid [Hmim]HSO4.
Table 1
Reaction of BH alcohol 2a with TosMIC under different conditionsa
order to replace metal derived Lewis acids and to minimize the
use of common organic solvents. Surprisingly, instead N-tosylm-
ethyl amides 4 and 5 (Scheme 1), unexpected sulfone derivatives
6 and 7 were obtained under the reaction conditions (Scheme 2).
The sulfones 6 and 7 are well known to serve as useful building
blocks for a number of important carbon–carbon bond forming
O
S
Tol
OH
O
CN
Solvent
TosMIC+
CN
1
2a
6a
reactions due to the unique reactivity pattern of a-sulfonyl carba-
Entry
Solventb
Time (h)
Temp (°C)
Yieldc,d (%)
nions under various reaction conditions.11,12 Additionally, they are
the main constituents of some biologically important compounds
that have potential for the treatment of Alzheimer’s disease,13 can-
cer, and abnormal cell proliferation diseases.14 Although several
new methods have recently been reported to access benzylic and
allylic sulfones,15,16 the application of TosMIC for the synthesis of
sulfone derivatives from alcohols has not been yet documented
and represents a unique and unprecedented protocol adding a
new manifold in TosMIC chemistry.
1
2
3
4
5
6
7
8
9
10
[Hmim]HSO4
10
10
15
20
10
10
10
10
10
10
80
80
50
25
80
80
80
80
80
80
48
78
42
—
52
—
—
—
71
39
[Hmim]HSO4–H2O
[Hmim]HSO4–H2O
[Hmim]HSO4–H2O
[Hmim]H2PO4–H2O
[Bmim]Cl–H2O
[Bmim]BF4–H2O
[Bmim]PF6–H2O
[Hmim]HSO4–H2O
aq H2SO4 (20%)
e
In an exploratory experiment, 2-(hydroxyphenylmethyl) acry-
lonitrile (BH alcohol) 2a (1 mmol) and TosMIC (1.1 mmol) were
taken in a Brønsted acidic ionic liquid–water system [Hmim]
HSO4–H2O (1 mL, 10:1) and the mixture was stirred at 80 °C. After
10 h, the reaction mixture was cooled to rt, and extracted with
dichloromethane. The organic layer on usual processing and chro-
matography afforded a colorless crystalline solid (mp 125 °C) in
78% yield that to our surprise was found to be the trisubstituted
allyl sulfone 6a (Table 1, entry 1). The structure of 6a was assigned
on the basis of 1H, 13C NMR and mass spectroscopy as well as by
comparison with the literature data.16c The E/Z ratio was found
to be 94:6 as determined by 1H NMR analysis of the crude product.
To get an insight into the mechanism, we also performed the
reaction of TosMIC with a model substrate 2-(hydroxyphenylmeth-
yl) acrylonitrile (BH alcohol) 2a in various conditions and results
are compiled in Table 1. As it is evident from Table 1, [Hmim]
HSO4–H2O (pKa = 1.88) was found to be the best solvent system
a
All reactions were performed using 1 mmol of BH alcohol 2a and 1.1 mmol
TosMIC.
b
1 mL of solvent was taken. In case of ionic liquid–H2O system the ratio was
10:1.
c
Isolated yields after column chromatography.
In all cases E/Z ratio was found to be >90:<10 in crude products as determined
d
by 1H NMR analysis.
e
Instead of TosMIC, freshly prepared p-toluenesulfinic acid (1.2 mmol) was used.
Intrigued by the above reports6a,7 and in continuation of our re-
search program based on ionic liquid promoted organic synthesis9
along with BH chemistry,10 initially we attempted to develop a
green protocol for TosMIC mediated synthesis of various amide
derivatives (Scheme 1). Thus, BH alcohols and other aryl alcohols
were reacted with TosMIC implementing reusable Brønsted acidic
ionic liquid-[Hmim]HSO4 to act both as a solvent and catalyst in
O
OH2
OH
O S
[Hmim]HSO4
R2
R1
O
R1
R2
Alcohol
R1
H2O
R2
Sulfone
S
O
H
O
O
O
S
NC
S
N
H
p-toluenesulfinic acid
O
O
[Hmim]HSO4
H2O
[Hmim]HSO4
H2O
H
O
N
O
H2N
+
O
H
H
TosMIC
Scheme 3. Plausible mechanism for ionic-liquid promoted solfonylation of alcohols with TosMIC.

4624
Garima et al. / Tetrahedron Letters 52 (2011) 4622–4626
Table 2
a
Reaction of alcohols with TosMIC in [Hmim]HSO4 (Scheme 2)
Entry
Alcohol
Productb
Time (h)
10
Yieldc (%)
E/Z ratiod
OH
O
S
Tol
CN
O
1
78
94:6
CN
6a
OH
O
S
Tol
O
CN
2
3
14
10
74
70
92:8
Cl
CN
6b
6c
Cl
OH
O
S
Tol
O
CN
n-Octyl
90:10
n-Octyl
CN
OH
COOMe
COOMe
Tol
4
5
12
14
72
71
4:96
5:95
S
O
O
6d
COOMe
OH
COOMe
Tol
S
Cl
O
O
6e
Cl
OH
COOMe
COOMe
Tol
S
6
7
8
10
14
10
76
74
79
7:93
3:97
—
MeO
O
O
MeO
6f
OH
COOMe
COOMe
Tol
S
Br
O
O
6g
Br
OH
O
S
O
Tol
7a
OH
O
S
O
Tol
9
3
4
76
74
—
—
Cl
Cl
7b
OH
O
S
O
Tol
10
F
7c
7d
7e
F
O
S
OH
O
Tol
11
12
2
3
80
74
—
—
MeO
MeO
O
S
OH
O
Tol
OH
O
S
O
Tol
13
14
3
3
76
72
—
—
7f

Garima et al. / Tetrahedron Letters 52 (2011) 4622–4626
4625
Table 2 (continued)
Entry
Alcohol
Productb
Time (h)
Yieldc (%)
E/Z ratiod
OH
O
S
O
Tol
7g
a
See Ref. 17 for general procedure.
b
c
All the products are known compounds15c,e,16c and were characterized by comparison of their mp and spectral data with those of reported in the literature.
Yields of pure isolated products after column chromatography.
d
The selectivity was determined by 1H NMR analysis.
The reaction of TosMIC with methyl 2-(hydroxyphenylmethyl)
acrylate in [Hmim]HSO4 proceeded to give predominantly
trisubstituted allylic sulfone-(Z)-methyl 3-phenyl-2-(tosylmeth-
yl)acrylate (Table 2, entry 4). Other acrylate derived BH alcohols
such as p-chloro, p-methoxy, o-bromo derivatives reacted cleanly
with TosMIC under similar protocol to afford the corresponding
trisubstituted allylic sulfones (Table 2, entries 5–7) in good yields
(71–78%) with high diastereoselectivity (E/Z ratio from 3:97 to
7:93). Significantly, the reaction is highly stereoselective for both
acrylonitrile/acrylate ester-derived BH alcohols, but with reversed
stereochemical directive effect, that is, acrylonitrile-derived BH
alcohols (2a–c, EWG = CN) predominantly afforded E-allyl sulfones
(Table 1, entries 1–3) while acrylate ester-derived BH alcohols
(2d–g, EWG = COOMe) selectively afforded Z-allyl sulfones
(Table 2, entries 4–7) under the same reaction conditions. The
configuration of trisubstituted allyl sulfones 6a–g was assigned
by comparing the 1H NMR and 13C NMR data with those of the pub-
lished ones,15c,16c and was further confirmed by NOE experiments.
The products 6a–g were studied by NOE experiments. Products
6a–c showed NOE between methylene protons and vinyl protons
confirming the E configuration, and products 6d–g showed NOE
between methylene protons and aromatic protons and showed
no NOE between methylene and vinyl protons confirming the Z
configuration. Although no mechanistic studies have been carried
out, related stereochemical reversals are attributed to differences
in relative stabilities of intermediates as explained earlier by con-
sidering models (I and II) depicted in Figure 2.19 When R1 is a large
group (R1 = COOMe), model I is favored and thus predominantly
forms the Z isomer. If R1 is a small group (R1 = CN), then model II
is favored and, therefore, predominantly forms the E isomer.
The present protocol was also found suitable for the direct
sulfonylation diarylmethanols containing both electron withdraw-
ing and electron donating substituents (Table 2, entries 8–14) and
afforded the corresponding benzylic sulfones in good yields
(72–80%). Unfortunately, the reaction of 1-phenylethanol with
TosMIC did not afford the corresponding benzylic sulfone under
the present reaction conditions. Based on an observation presented
in Table 1, a plausible mechanism for this reaction is depicted in
Scheme 3.
H
R1
R1
H
EWG = CN
To l
O
To l
O
S
S
EWG
EWG
EWG = COOMe
OH2
OH2
O
O
I
II
Figure 2.
to access the corresponding sulfone 6a. Its superiority over
[Hmim]HSO4 (Table 1, entries 1 and 2) indicates the role of water
in hydrolysis of TosMIC (Scheme 3). The temperature appears
crucial because lowering the reaction temperature from 80 to
50 °C lowered the product yield, and the reaction did not take place
appreciably at 25 °C even after stirring for 20 h (Table 1, entries 3
and 4). The reaction was unsuccessful in ionic liquid–water
systems such as [Bmim]Cl–H2O, [Bmim]BF4-H2O, and [Bmim]PF6-
H2O, which indicates that a Brønsted acid is necessary to catalyze
the present reaction. However, the reaction proceeded in the
[Hmim]H2PO4-H2O system but relatively low yield (52 %) of
sulfone 6a was obtained (Table 1, entry 5). This is probably due
to the lower Brønsted acidity associated with [H2PO4].
We also attempted the direct sulfonylation of alcohols using
TosMIC in 20% aq. H2SO4 (pKa = 2.0) under the same reaction
conditions, but a reduced product yield was obtained (Table 1,
entry 10). Eventually, the recycling performance of ionic liquid
[Hmim]HSO4 in the same model reaction was also investigated.
After isolation of product 6a, the ionic liquid [Hmim]HSO4 was eas-
ily recovered, and reused17 at least in five runs without appreciable
decrease in catalytic activity as shown in Figure 1. Thus, [Hmim]
HSO4 plays a dual role, that is, as an acid catalyst and as well as
a good reusable solvent for the present unprecedented protocol
for the direct sulfonylation of alcohols with TosMIC. Owing to
the presence of the organic moiety in [Hmim]HSO4, it is a better
solvent than aq 20% H2SO4 for organic reactants 1–3. Here, TosMIC
is decomposed to expel p-toluenesulfinic acid which acts as the
actual nucleophile. A reaction of freshly prepared p-toluenesulfinic
acid with BH alcohol 2a proceeds smoothly supporting the role of
p-toluenesulfinic acid as the nucleophile (Table 1, entry 9). The
stabilized p-toluenesulfinate anion is commonly used in the prep-
aration of sulfones,16a,c,18 but the use of TosMIC as a source of the
p-toluenesulfinate anion for the direct sulfonylation of alcohols
under acidic conditions has not been explored so far. Although
our conceptualization of a novel synthesis of N-tosylmethyl amides
by avoiding the commonly used Lewis acid in volatile organic
solvents did not materialize (Scheme 1), the preparative value of
this transformation proceeding in reusable solvent cum catalyst-
[Hmim]HSO4–H2O system and its mechanistic importance
prompted us to pursue the reaction in some detail. Thus, we turned
our attention to apply this protocol on other alcohols and the
results are summarized in Table 2.
In conclusion, we have uncovered a Brønsted acidic ionic liquid-
[Hmim]HSO4 promoted unprecedented reaction of p-toluensulfo-
nylmethyl isocyanide (TosMIC) with alcohols affording the corre-
sponding sulfone derivatives in good yields. The present work has
explored TosMIC formally as a source of tosyl nucleophile in substi-
tution reaction with alcohols, which opens up a new aspect of syn-
thetic utility of both TosMIC and Brønsted acidic ionic liquids.
Acknowledgments
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing spectra. V.P.S. and Garima are grateful to CSIR, New
Delhi, for the award of a Senior Research Fellowship (SRF).

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Garima et al. / Tetrahedron Letters 52 (2011) 4622–4626
12. (a) Back, T. G. Tetrahedron 2001, 57, 5263; (b) Paquette, L. A. Synlett 2001, 1; (c)
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17. General procedure for the synthesis of [E]- and [Z]-allyl sulfones (6) and benzyl
sulfones (7): A stirred solution of p-toluenesulfonylmethyl isocyanide (TosMIC)
(1.1 mmol) and alcohols 2 or 3 (1 mmol) in 1 mL of [Hmim]HSO4–H2O (10:1)
was heated at 80 °C for 2–14 h (Table 2). The reaction progress was monitored
by TLC. Upon completion, the reaction mixture was cooled to rt and extracted
with dichloromethane (3 Â 10 mL). The combined organic phase was dried
over MgSO4, filtered and evaporated under reduced pressure. The resulting
crude product was purified by silica gel column chromatography using a
gradient mixture of hexane/ethyl acetate (8:2) as eluent to give the
corresponding pure sulfone derivatives 6 and 7. All the products are known
compounds15c,e,16c and were characterized by comparison of their mp and
spectral data with those of reported in the literature. After isolation of the
product, the remaining mother liquid containing the ionic liquid was washed
with dichloromethane (2 Â 5 mL) to remove any organic impurity, dried under
vacuum at 90 °C to afford [Hmim]HSO4 in an excellent yield (96%), which was
used in subsequent runs without further purification (Fig. 1). The recovery
yield of [Hmim]HSO4 in subsequent runs was >93%.
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Basavaiah, D.; Hyma, R. S.; Kumaragurubaran, N. Tetrahedron 2000, 56, 5905.