O-benzenedisulfonimide as a reusable brnsted acid catalyst for hetero-michael reactions

Article abstract of DOI:10.1080/00397911.2011.614834

The hetero-Michael reactions among various oxygen, sulfur, and nitrogen nucleophiles and ,-unsaturated compounds were carried out in the presence of catalytic amounts of o-benzenedisulfonimide as Brnsted acid organocatalyst. The reaction conditions were very mild, and the yields of target products were good. The catalyst was easily recovered and purified, ready to be used in further reactions. This ability grants economic and ecological advantages. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2011.614834

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O-Benzenedisulfonimide as a Reusable
Brønsted Acid Catalyst for Hetero-
Michael Reactions
Margherita Barbero a , Silvano Cadamuro a & Stefano Dughera a
a Dipartimento di Chimica Generale e Chimica Organica, Università
di Torino, Torino, Italy
Accepted author version posted online: 27 Feb 2012.Version of
record first published: 13 Nov 2012.
To cite this article: Margherita Barbero , Silvano Cadamuro & Stefano Dughera (2013): O-
Benzenedisulfonimide as a Reusable Brønsted Acid Catalyst for Hetero-Michael Reactions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:5, 758-767
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Synthetic Communications1, 43: 758–767, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.614834
O-BENZENEDISULFONIMIDE AS A REUSABLE
BRØNSTED ACID CATALYST FOR HETERO-MICHAEL
REACTIONS
Margherita Barbero, Silvano Cadamuro, and Stefano Dughera
`
Dipartimento di Chimica Generale e Chimica Organica, Universita di Torino,
Torino, Italy
GRAPHICAL ABSTRACT
Abstract The hetero-Michael reactions among various oxygen, sulfur, and nitrogen nucleo-
philes and a,b-unsaturated compounds were carried out in the presence of catalytic amounts
of o-benzenedisulfonimide as Brønsted acid organocatalyst. The reaction conditions were
very mild, and the yields of target products were good. The catalyst was easily recovered
and purified, ready to be used in further reactions. This ability grants economic and ecologi-
cal advantages.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Brønsted acid; conjugate addition; homogeneous catalysis; Michael reaction;
recyclable catalyst
INTRODUCTION
The conjugate addition of oxygen, nitrogen and sulfur nucleophiles to unsatu-
rated carbonyl compounds by Michael reaction is one of the most advantageous and
easy methods for the forming of C-O, C-N, and C-S bonds in organic synthesis.[1]
In fact b-amino[2a,b] and b-oxy carbonyl groups[2a,c] are important functional-
ities present in numerous natural organic compounds such as alkaloids and polyke-
tides. b-Amino carbonyl compounds are also the key intermediates for the synthesis
of amino alcohol and b-amino acid derivatives, which possess various biological
activities.[3] Moreover, thio-Michael addition has emerged as one of the most power-
ful tools for C-S bond formation;[4] in fact thio-Michael addition provides a
Received August 1, 2011.
Address correspondence to Stefano Dughera, Dipartimento di Chimica Generale e Chimica
`
Organica, Universita di Torino, C. so Massimo d’Azeglio 48, 10125 Torino, Italy. E-mail:
stefano.dughera@unito.it
758

o-BENZENEDISULFONIMIDE AS A REUSABLE CATALYST
759
Figure 1. o-Benzenedisulfonimide.
widespread synthetic utility in many organic processes[5a–c] and in the synthesis of
medicinally important compounds.[5d]
Many different catalysts have been applied in hetero-Michael additions,[1]
including a number of Lewis acids.[6] The main disadvantages of many of these meth-
ods are the harsh reaction conditions, the relatively long reaction times, the poor sel-
ectivity, and often the use of toxic, corrosive, and expensive catalysts in high loading.
Moreover, their recovery and reuse is often impossible. To overcome this,
hetero-Michael additions have been successfully performed in acidic ionic liquids
[2-hydroxyethylammonium formate,[7a] 1-(4-sulfobutyl)piridinium sulfate,[7b] ethyl-
n-butylphosphonium
tosylate,[7c]
N-methyl-2-pyrrolidonium
dihydrogen
phosphate[7d]] and very recently it has been reported that 1,4-addition of anilines
to Michael acceptors proceeds easily in protic solvents[8a] or in water[8b] without
any catalyst. Moreover, it was also reported that the aza-Michael reaction between
various aliphatic amines and Michael acceptors furnished the Michael adducts in
good to excellent yield, without any solvent or catalysts.[8c]
The use of Brønsted acids as homogeneous catalysts is less widespread.
Spencer[9a] tested a number of Brønsted acids in aza, oxa and thia-Michael additions,
obtaining the best results using Tf2NH; Shen[9b] used TfOH as catalyst in an intra-
molecular oxa-Michael addition, and Goswani[9c] used 15% aqueous HCl in the
Michael addition between acrylates and fullerenol in the presence of an ammonium
salt as phase-transfer catalyst. Furthermore, the literature shows the use of some
Brønsted acids as heterogeneous catalysts (H2SO4 Á SiO2,[10a] HBF4 Á SiO2,[10b] acidic
clays,[10c] Nafion SAC-13,[10d] polystyrenesulfonic acid,[10e] and HClO4 Á SiO2[10f]) in
several hetero-Michael additions.
We have recently reported the use of o-benzenedisulfonimide[11a] (1, Fig. 1) in
catalytic amounts as a safe, non-volatile and noncorrosive, Brønsted acid in several
acid-catalyzed organic reactions.[11b,c] The catalyst, which has a high acidity (pka
À4.1 at 20 ꢀC),[11a] was easily recovered and purified, ready to be used in further
reactions, with economic and ecological advantages.
RESULTS AND DISCUSSION
In this paper we propose the use of 1 as a reusable Brønsted acid catalyst for
the hetero-Michael addition reactions between oxygen 2, sulfur 3, or nitrogen 4
nucleophiles and several a,b-unsaturated compounds 5–9 (Scheme 1).
Initially, to optimize the reaction conditions, the model reaction between
methyl vinyl ketone (5) and benzyl alcohol (2a) at rt and in the presence of 1 as cata-
lyst was studied under different conditions. As reported in Table 1 (entries 1–4),
polar solvents, slightly polar solvents, and H2O were tested, but the best results were
obtained in solvent-free reaction conditions (entries 5–7). The effect of the catalytic

760
M. BARBERO, S. CADAMURO, AND S. DUGHERA
Scheme 1. o-Benzenedisulfonimide 1 as a catalyst for hetero-Michael reactions.
amount of 1 was also studied. In the best conditions, an excellent yield (92%) of
Michael adduct 10a was achieved in a short reaction time (1 h) under solvent-free
conditions and in the presence of 5 mol % of 1 (entry 7).
Furthermore, 1 was recovered in excellent yields (89%) by simply evaporating
the aqueous washings under reduced pressure. The recovered 1 was reused as the
catalyst in another five consecutive runs between 2a and 5. The results are listed
in Table 2. The yields of 10a and the recovery of 1 were always good throughout
the course of the different runs.
The good yield, short reaction time, and simplicity of the procedure encour-
aged us to further exploit the generality and the scope of this reaction, catalyzed
by 1, using other alcohols 2 and other nucleophiles such as thiols 3. The results
are collected in Table 3. The Michael addition of various alcohols 2b–e to 5 produced
the corresponding adducts 10b–e in excellent yields (Table 3, entries 2–5) On the
other hand, no reactions occurred when less reactive phenols 2f and g were used
(Table 3, entries 6 and 7). Also, the reaction of aliphatic or aromatic thiols 3a–e with
Table 1. Trial reactions between 2a and 5
Entry
Solvent
1 (mol%)
Time (h)
Yield (%) of 10aa
1
2
3
4
5
6
7
MeCN
CH2Cl2
Toluene
H2O
5
5
5
5
1
2
5
24
6
42b
90c
24
24
5
35b
Tracesd
87c
89c
92c
Neat
Neat
3
Neat
1
aYields refer to the pure products.
bAfter 24 h, unreacted 2a and 5 were detected by GC and GC-MS. Nevertheless, the reaction was
stopped and the crude residue, purified in a silica-gel chromatography column (eluent: PE=Et2O
9.8:0.2), afforded pure 10a.
cOn the GC, GC-MS, and NMR analyses, the crude residue was the virtually pure 10a.
dAfter 24 h, unreacted 2a and 5 were detected by GC and GC-MS. Only weak traces of 10a were
detected.

o-BENZENEDISULFONIMIDE AS A REUSABLE CATALYST
Table 2. Consecutive runs with recovered 1
761
Entry
Time (h)
Yield (%) of 10aa
Recovery (%) of 1
1
2
3
4
5
6
1
92b
85
85
82
83
80
89, 49 mgc
86, 42 mgd
86, 42 mge
81, 34 mgf
79, 27 mgg
74, 20 mg
1
1
1.5
1.5
2.5
aYields refer to the pure products.
bThe reaction was performed with 5 mmol of 2a and 5 and 5 mol% of 1 (55 mg, 0.25 mmol).
cRecovered 1 was used as a catalyst in entry 2.
dRecovered 1 was used as catalyst in entry 3.
eRecovered 1 was used as a catalyst in entry 4.
fRecovered 1 was used as a catalyst in entry 5.
gRecovered 1 was used as catalyst in entry 6.
5 gave the corresponding adducts 11a–e in very good yields without any by-product
(Table 3, entries 8–12). It must be stressed that thiols can react without a catalyst.[4c]
However, the reaction time was longer (Table 3, entry 13).
To explore the synthetic usefulness of 1 in oxa-Michael and thia-Michael
reactions further, we also investigated the reactions of four other types of a,b
Table 3. Hetero-Michael reaction between 5 and alcohols 2 or thiols 3
Entry
R
Products and yields (%)a,b
Temp. (ꢀC)
Time (h)
1
2
Bn; 2a
nBu; 2b
sBu; 2c
tBu; 2d
Cy; 2e
10a; 92c
10b; 85c
10c; 88c
10d; 82c
10e; 84c
10f;À
Rt
Rt
Rt
Rt
Rt
50
50
Rt
Rt
Rt
Rt
Rt
Rt
1
1
3
1
4
1
5
1
6
Ph; 2f
4-MeOC6H4; 2g
Bn; 3a
24
24
0.5
0.5
0.5
0.5
0.5
3
7
10g;À
8
11a; 87c
11b; 91c
11c; 85c
11d; 82c
11e;92d
11e;90d,e
9
nBu; 3b
sBu; 3c
tBu; 3d
Ph; 3e
10
11
12
13
Ph; 3e
aAll the reactions were performed with 5 mol% of 1; the reactants are in equimolar amounts (5 mmol).
bYields refer to the pure and isolated products.
cOn the GC, GC-MS, and NMR analyses, the crude residues were the virtually pure hetero-Michael
adducts 10 and 11.
dThe crude residue was purified in a silica-gel chromatography column (eluent: PE=Et2O 9.8:0.2).
eThe reaction was performed without catalyst 1.

762
M. BARBERO, S. CADAMURO, AND S. DUGHERA
Table 4. Hetero-Michael reaction between 6–9 and alcohol 2a or thiols 3a, 3e
Entry
Reactants
Products and yields (%)a,b
Temp. (ꢀC)
Time (h)
1
2
r.t
2
—
50
24
3
4
—
—
50
50
24
24
5
6
7
r.t.
50
50
1
1
1
8
9
—
—
50
50
24
24
aAll the reactions were performed with 5 mol% of 1; the reactants are in equimolar amounts (5 mmol).
bYields refer to the pure and isolated products.
cOn the GC, GC-MS, and NMR analyses, the crude residues were the virtually pure hetero-Michael
adducts 10 and 11.

o-BENZENEDISULFONIMIDE AS A REUSABLE CATALYST
763
unsaturated compounds 6–9 with selected 2 or 3. The reaction between 2a and 6
furnished 10 h in very good yields (Table 4; entry 1). However, it must be stressed
that the reactions of the weak nucleophile 2a with 7–9 did not proceed at all, even
at 50 ꢀC (Table 4; entries 2–4). Thiols 3a and 3e reacted easily at rt or when heated
to 50 ꢀC with 6 and 7 (Table 4; entries 5–7), whereas no results were obtained with 8
and 9 (Table 4; entries 8–9). All these examples demonstrate the usefulness and the
simplicity of this method because it requires mild reaction conditions and short reac-
tion times; the target products are obtained in good yields and good selectivity, and
the absence of by-products is observed.
To further expand the scope of our work, we decided to study the aza-Michael
addition too. To start, we tested the reaction between 4a and 5 as reported in Table 5.
First of all, it was necessary to cool at 0 ꢀC (Table 5, entry 2) to minimize the forma-
tion of two by-products, namely imine 13a (that formed as a result of nucleophilic
attack of 4a on the carbonyl group of 5) and adducts 14a (that formed as a result
of the further nucleophilic attack of 12a on 5; Table 5, entry 1). In contrast, no side
products were detected using the less nucleophilic aniline 4e (Table 5, entry 6). Also
the reaction of other aliphatic amines 4b–d gave the corresponding Michael adducts
12 b–d in good yields and short reaction times (Table 5, entries 3–5).
Finally, amines 4a, b, and e, stronger nucleophiles than 2 or 3, reacted easily at
rt or at 50 ꢀC with 6–9 (Table 6; entries 1–3, 6, 8–10), producing the adduct 12f–l in
good yields. It must be stressed that the reactions between 4a, b and 9 furnished the
Table 5. Hetero-Michael reaction between 5 and amines 4
Entry
R
Products and yields (%)a,b
Temp. (ꢀC)
Time (h)
1
2
3
4
5
6
Bn; 4a
Bn; 4a
nBu; 4b
sBu; 4c
tBu; 4d
Ph; 4e
——c
r.t
0
0.5
0.5
0.5
0.5
0.5
1
12a; 85d
12b; 82d
12c; 82d
12d; 77d
12e; 89e
0
0
0
r.t.
aAll the reactions were performed with 5 mol% of 1; the reactants are in equimolar amounts (5 mmol).
bYields refer to the pure and isolated products.
cOn the GC and GC-MS analyses of the crude residue, three products were detected: 12a, 13a, and 14a.
However, it was impossible to separate them by chromatography column.
dOn the GC and GC-MS analyses of the crude residues, weak traces of compounds 13 and 14 were
detected. The crude residues, purified in a silica-gel chromatography column (eluent: PE=Et2O 9.8:0.2),
afforded pure compounds 12.
eThe crude residues were purified in a silica-gel chromatography column (eluent: PE=Et2O 9.8:0.2). For
more details see also the Supplementary Material.

764
M. BARBERO, S. CADAMURO, AND S. DUGHERA
Table 6. Hetero-Michael reaction between 6–9 and amines 4a, b, e
Entry
Reactants
Products and yields (%)a,b
Temp. (ꢀC)
Time (h)
1
2
3
0
1
r.t.
r.t.
4
1
f
4
5
4a 7e
4a 7g
r.t.
r.t.
24
1
12 h 82c
6
50
24
7
8
4e 7e
—
50
24
2
r.t.
9
50
50
1
10
24
2
11
12
50
50
2
(Continued )

o-BENZENEDISULFONIMIDE AS A REUSABLE CATALYST
Table 6. Continued
765
Entry
13
Reactants
Products and yields (%)a,b
Temp. (ꢀC)
Time (h)
24
f
—
50
aAll the reactions were performed with 5 mol% of 1; the reactants are in equimolar amounts (5 mmol).
bYields refer to the pure and isolated products.
cOn the GC, GC-MS, and NMR analyses, the crude residues were the virtually pure hetero-Michael
adducts 12.
dThe crude residues were purified in a silica-gel chromatography column; eluent: PE-Et2O (9.8:0.2).
eThe reaction was performed without catalyst 1.
fOn the GCand GC-MS analysesofthe crude residue, two productswere detected:12hand N,N-bis(2-ethox-
ycarbonylethyl)benzylamine. However, it was impossible to separate them by chromatography column.
gThe reaction was performed using the benzylammonium salt 16 as catalyst (5 mol%; 81.5 mg).
hOn the GC and GC-MS analyses of the crude residue two products were detected: N-benzylideneaniline
(15c) and N-(1-phenyl-2-nitroethyl)aniline. However, it was impossible to separate them by chromato-
graphy column. For more details see also the Supplementary Material.
immines 15a, b, derived from Michael adducts after the loss of nitromethane
(Table 6; entries 11 and 12).
In the literature it is reported that the aza-Michael reaction proceeds
without any catalyst, reacting various aliphatic amines with a number of Michael
acceptors.[8c] For example, the reaction between benzyl amine and methyl acrylate
furnishes after 18 h at rt a mixture of mono- and di-Michael adducts (in total 90%
yield). In contrast, aniline does not react in these conditions. In the light of these, per-
forming the reaction between 4a, e, and 7 without 1 we obtained about the same results
reported in the literature (Table 6; entries 4 and 7). From our experimental data it was
evident that the presence of catalyst 1 significantly accelerated the reaction between 4a
and 7 (Table 6, entry 3), allowed the reaction between 7 and weaker nucleophile 4e
(Table 6, entry 6), and prevented the formation of the di-adduct. It is reasonable to
assume to have a protonation of amine 4a by strong acid 1 with the formation of
ammonium salt 16. Despite this, 4a and 7 reacted very fast. So, we prepared the
ammonium salt 16 and used it as a catalyst in the reaction between 4a and 7. We
obtained the Michael adduct 12 h with a yield and time comparable to those obtained
for the reaction performed with 1 (Table 6; entry 5). In the light of these, it can be
assumed that 16 transfers a proton on the carbonyl group of the Michael acceptor 7.
The stable intermediate 17 reacts very easily with 4a to give 12 h (Scheme 2). However,
because of the high stability of the cation 17, the direct protonation of 7 by 1 can not be
excluded, even in the presence of 4a. Finally, the absence of the possible di-adduct
N,N-bis(2-ethoxycarbonylethyl)benzylamine could be explained by the greater rate,
in the presence of a catalyst, of the reaction between 4a and 7. Clearly, also in all the
other aza-Michael reactions conducted with aliphatic amines, it could be assumed that
ammonium salt of 1 acts as a catalyst, activating the Michael acceptors.
In conclusion, the synthetic usefulness of o-benzenedisulfonimide (1) as a cata-
lyst in hetero-Michael reactions has been demonstrated. The advantages of the
method are mild, solvent-free reaction conditions, stoichiometric reagent ratios,

766
M. BARBERO, S. CADAMURO, AND S. DUGHERA
Scheme 2. Aza-Michael reaction catalyzed by 16.
and use of a catalytic amount of the readily available and easy-handling Brønsted
acid catalyst. Furthermore, in comparison with other catalysts extensively used for
hetero-Michael reactions, 1 turned out to be safe, nonvolatile, noncorrosive and
benchstable. A further valuable aspect of the use of 1 is its easy recovery in good
yield from the reaction mixture, because of its complete solubility in water and its
reuse without loss of catalytic activity in other reactions.
EXPERIMENTAL
4-Benzyloxybutan-2-one (10a): Representative Procedure for the
Preparation of Michael Adducts 10, 11, and 12
o-Benzenedisulfonimide (1 5 mol%; 55 mg, 0.25 mmol) was added to a mixture of
benzyl alcohol (2a; 0.5,4 g, 5 mmol) and methyl vinyl ketone (5 0.36 g, 5 mmol) The
mixture was stirred at rt. The reaction was monitored by gas chromatography (GC)
and (GC–mass spectrometry MS) until the complete disappearance of 2a and 5 (1 h).
The reaction mixture was poured into Et2O-H2O (50 ml, 1:1). The aqueous layer was
separated and extracted with Et2O (2 Â 50 ml). The combined organic extracts were
washed with H2O (2 Â 50 ml) and dried over Na2SO4. After solvent removal under
1
reduced pressure, the crude residue was the virtually pure (GC, GC-MS, H NMR,
13C NMR) title compound 10a. The aqueous layer and aqueous washings were col-
lected and evaporated under reduced pressure. After removal of the water, virtually
pure (1H NMR) o-benzenedisulfonimide (1) was recovered (49 mg, 89% yield).
The recovered 1 was employed in other five catalytic cycles under the con-
ditions described, reacting with 2a and 5; Table 2 reported the yields of 10a and
the yields of recovered 1.
Supplementary Material
General experimental details, physical and spectral data of the products 10, 11,
and 12; and preparation and physical and spectral data of product 16 are included in
the Supplementary Material, available online.

o-BENZENEDISULFONIMIDE AS A REUSABLE CATALYST
767
ACKNOWLEDGMENT
This work was supported by the University of Torino.
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