Surfactant-assisted specific-acid catalysis of Diels-Alder reactions in aqueous media

Article abstract of DOI:10.1002/poc.1240

Surfactant-assisted specific-acid catalysis (SASAC) for Diels-Alder reactions of dienophiles 1 and 4 with cyclopentadiene 2 in aqueous media at 32°C was studied. This study showed that acidified aniohic surfactants (pH 2) such as sodium dodecyl sulfate (S

Full text of DOI:10.1002/poc.1240

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 764–770
Published online 17 August 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1240
Surfactant-assisted specific-acid catalysis of
Diels–Alder reactions in aqueous media
1
2
*
Egid B. Mubofu and Jan B.F.N. Engberts
1Chemistry Department, University of Dar es Salaam, Dar es Salaam, Tanzania
2Physical Organic Chemistry Unit, Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Received 16 March 2007; revised 4 June 2007; accepted 9 June 2007
ABSTRACT: Surfactant-assisted specific-acid catalysis (SASAC) for Diels–Alder reactions of dienophiles 1 and 4
with cyclopentadiene 2 in aqueous media at 32 8C was studied. This study showed that acidified anionic surfactants
(pH 2) such as sodium dodecyl sulfate (SDS) and linear alkylbenzene sulfonic acid (LAS) accelerate Diels–
Alder reactions. Conversely, under similar reaction conditions (pH 2) these reactions are inhibited by (acidified)
cationic surfactants such as dodecyltrimethylammonium bromide (DTAB), dodecyldimethylammonium bromide
(DDAB), and dodecylmethylammonium bromide (DMAB). A modest rate acceleration resulting from the surfactant
hydrogen-bonding capacity is also recorded for the Diels–Alder reaction of naphthoquinones (6) with cyclopentadiene
(2) in aqueous media at 32 8C. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: specific-acid catalysis; Diels–Alder reaction; surfactant-type Brønsted acid; micellar catalysis; micellar
inhibition
INTRODUCTION
Combined Lewis-acid surfactant catalysts form stable
colloidal dispersions with organic substrates and hence
are effective Lewis-acid catalysts. Combined Brønsted-
acid surfactant catalysts in water have also been
employed in a number of organic reactions in water.14–16
However, these catalysts have not been studied on
Diels–Alder reactions in water. The aggregation of
long-chain hydrocarbontails (C10–C20) in water results
in the semi-ordered structures of micelles with ionic
groups protruding into the aqueous phase whereas the
aliphatic chains are brought to close proximity in the core
of the aggregate excluding water. This means that when
organic compounds are introduced in an aqueous micellar
solution, dipolar and hydrophobic interactions will cause
binding of these compounds to the micelles.17,18 The
properties of the binding site will strongly depend on the
nature and concentration of the solubilizate and the nature
of the surfactant used. It is therefore reasonable to assume
that surfactant-assisted specific-acid catalysis (SASAC)
for Diels–Alder reactions in water should occur depend-
ing on the type of the surfactant and the overall influence
of hydrophobic effects, electrostatic interactions, and the
accompanying medium effects. It is anticipated that for
anionic surfactant molecules, the headgroups will con-
centrate the protons near the surface of the aggregates. In
this solution, hydrophilic dienophiles are expected to
reside near the surface of the micellar aggregates and
hence will be in close proximity to the bound protons and
as a result an enhancement of the rate of the Diels–Alder
Diels–Alder reactions provide important and useful
methods for the synthesis of six-membered ring com-
pounds including versatile synthetic intermediates and
natural products.1 The reaction involves the stereospecific
[4þ2] cycloaddition of a dienophile and a conjugated
diene. The reaction is accelerated when performed in
water2 and is catalyzed by both Lewis-acid3–5 and specific-
acid catalysts.6–8 Generally, the use of micellar and
vesicle-forming surfactants as catalysts is common for
Diels–Alder and other types of cycloaddition reactions in
aqueous solution.9 More interestingly, the combination of
water-stable Lewis-acids such as scandium triflate and
anionic surfactant (e.g. sodium dodecyl sulfate (SDS)),
gives remarkably efficient Lewis-acid catalysts for some
organic reactions in water.10 The use of scandium tris-
(dodecyl sulphate), Sc(DS)3, for example, has been
successfully applied to the three-component Mannich-
type reaction of aldehydes, amines, and silyl enolates
in water.11 Furthermore, the use of copper salt of
5,5-di-n-dodecyl-2-hydroxyl-1,3,2-dioxaphosphorinan-2-
12
one, Cu(dDP)2 and copper bis(dodecyl sulphate),
(Cu(DS)2),13 have shown an excellent rate acceleration
for Diels–Alder reactions in water.
*Correspondence to: E. B. Mubofu, Chemistry Department, University
of Dar es Salaam, P.O. Box 35061, Dar es Salaam, Tanzania.
E-mail: mubofu@chem.udsm.ac.tz
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 764–770

SASAC OF DIELS–ALDER REACTIONS
765
O
S
reaction is expected. On the contrary however, it is
Na+
anticipated that the cationic surfactants will repel the
protons from the Stern region of the micelle and hence
inhibit the Diels–Alder reactions. Consequently, Diels–
Alder reactions would efficiently be accelerated in the
specific-acid/anionic surfactant combination and will be
retarded in the specific-acid/cationic surfactant combi-
nation. To our knowledge, SASAC of Diels–Alder reac-
tions in aqueous media has not been reported previously.
In this study, we present kinetic data that illustrate the
catalytic efficiency of SASAC for Diels–Alder reactions.
The hydrogen-bond donor capacity of surfactants has
also been applied to increase the reactivity of these
cycloaddition reactions in aqueous media.
O
O
(a)
O
Sodium dodecylsulfate (SDS)
R3
+
Br-
N
R1
(b)
R2
= CH3
DTAB R1 = R2 = R3
DDAB R1 = H, R2 = R3 = CH3
MDAB R1 = R2 = H, R3 = CH3
Scheme 3
RESULTS AND DISCUSSION
conditions (Table 1). It is reasonable to assume that the
rate acceleration in the presence of SDS in water is due to
concentration of diene and dienophile in the micellar
reaction volume.19,20 Upon acidification of the surfactant
solution with hydrochloric acid up to pH 2, the reaction
rates are further improved (Fig. 1). At the cmc of SDS
(8.38 mM)21 at 32 8C, the apparent second-order rate
constant of the reaction is increased from 0.26 to
0.57 sꢀ1 Mꢀ1. Above the surfactant cmc, a gradual decre-
ase in the reaction rate occurs due to the dilution effect
upon increasing the surfactant concentration.
We suggest that the increased concentration of protons
near the surfactant headgroups leads to the reaction rate
enhancements for the Diels–Alder reaction. The rate
increase upon acidification of the SDS solution suggests
that the combination of hydronium ions and anionic
surfactant leads to a more effective specific-acid catalyst
for this Diels–Alder reaction. This observation serves as
an indirect proof that this type of bimolecular Diels–Alder
reaction occurs near the surface of the micellar head-
groups. Even if the precise preferred location of the less
hydrophobic dienophile is uncertain, these observations
suggest that it is close to the micellar surface while that
of the hydrophobic reactant (i.e. cyclopentadiene) is
probably deeper in the micelle.
Diels–Alder reactions in the presence
of SDS and acidified SDS
SASAC has been examined for the Diels–Alder reactions
of substituted 3-phenyl-1-(2-pyridyl)-2-propen-1-one 1
and 3-phenyl-1-(4-pyridyl)-2-propen-1-one 4 with cyclo-
pentadiene 2 (Schemes 1 and 2). First, the reaction of
dienophile 1a with cyclopentadiene 2 was carried out in
SDS solutions in water. The presence of SDS (Scheme 3a)
afforded reaction rates higher than those for the same
reaction in water under similar reaction conditions. At the
critical micelle concentration (cmc) of SDS, the accel-
eration reaches a maximum of almost 40 times faster than
the uncatalyzed reaction rates under similar reaction
R
N
O
+
N
O
2
1a R = NO2
1b R = H
R
3a R = NO2
3b R = H
Scheme 1
Table 1. The apparent second-order rate constants for
the Diels–Alder reaction of 1a with cyclopentadiene 2 in
different reaction media at 32 8C
Relative to the
uncatalyzed
reaction in water
N
k
O
Medium
(Mꢀ1 sꢀ1
)
N
+
Uncatalyzed reaction in water
SDS (10 mM in water))
SDS (10 mM in water, pH 2)
Hydrochloric acid (0.01 M)
LAS (0.8 mM)
0.0067
0.259
0.57
0.141
1.16
1
37
81
21
O
2
4
5
173
Scheme 2
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 764–770
DOI: 10.1002/poc

766
E. B. MUBOFU AND J. B. F. N. ENGBERTS
1.2
1
0.8
0.6
0.4
0.2
0
0
5
10
[LAS], mM
15
Figure 2. The influence of linear alkylbenzene sulfonic acid
(LAS) on the apparent second-order rate constants for
the Diels–Alder reaction of 3-(4-nitrophenyl)-1-(2-pyridyl)-2-
propen-1-one (1a) with cyclopentadiene (2)
Figure 1. Catalysis of the Diels–Alder reaction of
3-(4-nitrophenyl)-1-(2-pyridyl)-2-propen-1-one (1a) with
cyclopentadiene (2) using SDS (^) and acidified SDS (~)
is twice faster than that in SDS solution at pH 2. The
maximum catalytic efficiency of LAS is found to be at
relatively low acid strength (pH 3) and at a concentration
of 0.8 mM. The activity then drops drastically even
though the acid strength increases (Table 2). This demon-
strates that the catalytic efficiency of LAS depends on
both the surfactant properties of LAS and its specific-
acid strength. Dilution of diene and dienophile on more
micelles is in competition with an increase in acid
concentration.
Diels–Alder reactions using linear
alkylbenzene sulfonic acid (LAS)
The catalysis of Diels–Alder reactions with linear LAS,
an examined micelle-forming surfactant (Scheme 4a),
was also examined. Figure 2 shows the apparent
second-order rate constants for these reactions. They
are larger than those obtained when the reaction was
performed in aqueous acidified SDS (Table 1).
Catalysis by LAS on the Diels–Alder
reaction of dienophiles (1b, 4) with
cyclopentadiene 2 in water
The Diels–Alder reaction of 4-nitrophenyl-1-(2-pyridyl)-
2-propen-1-one 1a with cyclopentadiene 2 in linear LAS
is about 170 times faster than the uncatalyzed reaction
under similar reaction conditions. The maximum appar-
ent second-order rate constant in linear LAS is almost
eightfold faster than that in aqueous 0.01 M HCl and it
LAS was also found to be the catalyst of choice for the
Diels–Alder reaction of 3-phenyl-1-(2-pyridyl)-2-propen
1-one 1b and 3-phenyl-1-(4-pyridyl)-2-propen-1-one 4
dienophiles with cyclopentadiene 2 in water at 32 8C
SO3H
Table 2. Comparison of LAS and HC1 for their catalytic
activity on the Diels–Alder reaction of 1a with 2 in water at
32 8C
(a)
pH of LAS
in water
pH of HCl
in water
Linear alkylbenzene sulfonic acid
kLAS (Mꢀ1 sꢀ1
)
kH (Mꢀ1 sꢀ1
)
SO3H
1.37
2.08
2.35
2.99
3.10
0.52
0.54
0.63
1.12
1.16
1.37
2.00
2.42
2.90
3.00
0.20
0.14
0.09
0.04
0.034
(b)
Benzenesulfonic acid
Scheme 4
Copyright # 2007 John Wiley & Sons, Ltd.
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SASAC OF DIELS–ALDER REACTIONS
767
1.2
Table 3. The apparent second-order rate constants for the
Diels–Alder reaction of dienophiles 1 and 4 with cyclopen-
tadiene 2 in 1 mM linear alkylbenzene sulfonic acid com-
pared to the uncatalyzed reaction in water at 32 8C
1
0.8
0.6
0.4
0.2
0
Reaction
kLAS (Mꢀ1 sꢀ1
)
kw (Mꢀ1 sꢀ1
)
kLAS/kw
1a þ 2
1b þ 2
4 þ 2
1.124
0.1223
0.0267
0.0067
0.0049
0.0041
168
25
6.5
(Table 3). The catalysis by LAS on these reactions shows
that the bidentate character of the dienophile 1b provides
an advantage over the monodentate counterpart 4 (Fig. 3).
This behavior has been previously observed for these
reactions when catalyzed by conventional aqueous hydro-
chloric acid under similar reaction conditions.22 This
reactivity difference is caused by both hydrogen-bonding
interactions and electronic effects. The proton at the
pyridyl nitrogen of 1b may undergo intramolecular
hydrogen-bonding with the oxygen atom of the carbonyl
group. Consequently, this interaction will stabilize the
lowest unoccupied molecular orbitals (LUMO) of 1b, and
thereby enhancing its reactivity. On the other hand, the
proton attached at the pyridyl nitrogen of the dienophile
4 is at a too remote distance from the carbonyl oxygen
for the intramolecular hydrogen-bonding. In addition,
the intramolecular electrostatic interaction between the
positive charge on the pyridyl nitrogen and the negative
charge on the oxygen of the carbonyl carbon in dieno-
phile 1b is another possible reason of this reactivity
difference between these dienophiles.
0
5
10
15
20
Concentration, mM
Figure 4. Apparent second-order rate constants for the
Diels–Alder reaction of 3-(4-nitrophenyl)-1-(2-pyridyl)-2-
propen-1-one dienophile (1a) with cyclopentadiene (2) in
the presence of benzenesulfonic acid (^) and linear alkyl-
benzene sulfonic acid (~) at 32 8C in aqueous solution
phile 1a with cyclopentadiene 2 was also studied so as to
establish the influence of the hydrophobic alkyl moiety of
LAS on the catalysis of this reaction. The kinetic results
clearly indicate that the use of benzenesulfonic acid,
which lacks the hydrophobic alkyl moiety, under similar
reaction conditions led to much less effective catalysis
(Fig. 4).
The catalytic activity of benzenesulfonic acid and that
of linear LAS (Scheme 4a) on the Diels–Alder reactions
of 3-(4-nitrophenyl)-1-(2-pyridyl)-2-propen-1-one dieno-
0.18
0.16
0.14
0.12
0.1
Inhibition of Diels–Alder reactions in the
presence of cationic surfactants
After the successful acceleration of the Diels–Alder
reactions with acidified anionic surfactants, the study was
extended to acidified cationic surfactants (Scheme 3). Our
hypothesis was that cationic surfactants will repel the
protons from the Stern region of the micelles and thereby
inhibit the Diels–Alder reactions. Indeed, the Diels–Alder
reaction of 3-(4-nitrophenyl)-1-(2-pyridyl)-2-propen-1-
one dienophile (1a) with cyclopentadiene (2) at 32 8C,
showed inhibition by dodecyltrimethylammonium bro-
mide (DTAB) and dodecylmethylammonium bromide
surfactants at pH 2 (Fig. 5). The inhibition begins
immediately after the cmc of these surfactants which
is consistent with the notion that binding of 1a to the
micelles hampers specific-acid catalysis.
0.08
0.06
0.04
0.02
0
0
5
10
15
20
[LAS], mM
Figure 3. Comparison of the apparent second-order rate
constants for the reaction of dienophile 1b (~) and dieno-
phile 4 (^) with cyclopentadiene 2 at 32 8C in aqueous linear
alkylbenzene sulfonic acid
The inhibition was also observed when the reaction
of 3-phenyl-1-(4-pyridyl)-2-propen-1-one dienophile (4)
Copyright # 2007 John Wiley & Sons, Ltd.
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768
E. B. MUBOFU AND J. B. F. N. ENGBERTS
The hydrogen-bond interaction effect of
cationic surfactants on the Diels–Alder
reaction of 6 with 2
0.16
0.14
0.12
0.1
The extent to which water can affect a reaction through
hydrogen-bond interactions depends on the number and
the hydrogen-bond-accepting capability of the substitu-
ents in the substrates. We therefore extended our study
to an examination of the hydrogen-bonding influence
of cationic surfactants on the Diels–Alder reactions of
naphthoquinones 6 with cyclopentadiene 2 (Scheme 5).
The use of DDAB, and DTAB surfactants gave different
reaction rates (Fig. 7). The apparent second-order rate
constants for the Diels–Alder reaction of 6a with 2 in the
presence of a surfactant with a higher hydrogen-bond
donating capacity are slightly higher than those for a
structurally similar surfactant with a lower hydrogen-
bond donating capacity. This indeed suggests that there is
a kinetic effect due to hydrogen-bond interaction between
the surfactant ammonium hydrogen atom and the acti-
vated complex.
0.08
0.06
0.04
0.02
0
0
10
20
30
40
[Surfactant], mM
O
O
Figure 5. Apparent second-order rate constants for the
Diels–Alder reaction of 3-(4-nitrophenyl)-1-(2-pyridyl)-2-
propen-1-one dienophile (1a) with cyclopentadiene (2) at
32 8C inhibited by dodecyltrimethylammonium bromide (^)
and dodecylmethylammonium bromide (~) surfactants at
pH 2
+
R
O
R
O
2
6a R = H
6b R = COMe
7a R = H
7b R = COMe
with cyclopentadiene (2) was performed in dodecyldi-
methylammonium bromide (DDAB) cationic surfactant
at pH 2 at 32 8C (Fig. 6).
Scheme 5
0.026
0.024
0.022
0.02
11
10
9
8
0.018
0.016
0.014
0.012
0.01
7
6
5
4
3
2
0
5
10 15 20 25 30 35
[DDAB], mM
0
20
40
60
[Surfactant], mM
Figure 6. Apparent second-order rate constants for the
Diels–Alder reaction of 3-phenyl-1-(4-pyridyl)-2-propen-1-
one dienophile (4) with cyclopentadiene (2) at 32 8C inhib-
ited by dodecyldimethylammonium bromide (DDAB) surfac-
tant at pH 2
Figure 7. The influence of the surfactant hydrogen-
bonding capacity on the second-order rate constants for
the Diels–Alder reaction of naphthoquinone (6a) with cyclo-
pentadiene (2) at 32 8C using surfactants DTAB (^) and
DDAB (~)
Copyright # 2007 John Wiley & Sons, Ltd.
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SASAC OF DIELS–ALDER REACTIONS
769
that a bidentate dienophile reacts faster than a structurally
related monodentate dienophile when catalyzed by LAS.
21
19
17
15
13
11
9
EXPERIMENTAL
Materials
Naphthoquinone (Aldrich), ethanol (Merck), 4-acetyl-
pyridine (Aldrich), 2-acetylpyridine (Aldrich), benzal-
dehyde (Aldrich), ether (Merck), acetonitrile (Aldrich),
N, N-dimethyldodecylamine (Aldrich), N- methyldode-
cylamine (Aldrich), and DTAB (Aldrich) were of the
highest purity available. Technical grade (96%) linear
alkylbenzene sulfonic acid, which comprised of a mixture
of linear LAS with molecular weight 304, was employed.
Cyclopentadiene was cracked from its dimer (Merck)
immediately before use. Dimineralized water was double
distilled in a quartz distillation unit. The solvents were
used as received without further purification. Dienophiles
1b and 4 were prepared by aldol condensation of 2-
and 4-acetylpyridine with the corresponding substituted
benzaldehyde using documented procedures.3,23 Dieno-
phile 1a was purified from the stock sample prepared
before in the group.23
7
0
20
40
60
[ Surfactant], mM
Figure 8. The influence of the surfactant hydrogen-bond
donating capacity on the second-order rate constants for the
Diels–Alder reaction of 5-acetyl-2,4-naphthoquinone (6b)
with cyclopentadiene (2) at 32 8C using surfactants DDAB
(~) and MDAB (^)
1b and 4: To 100 ml of water cooled to 5 8C, 16.5 mmol
of the appropriate benzaldehyde and 17 mmol of the
appropriate acetylpyridine were introduced. The mixture
was thoroughly shaken to obtain a finely dispersed
emulsion. This was followed by the addition of 10 ml of
10% NaOH. The mixture was once again shaken and left
undisturbed overnight at 4 8C. The product which was
oil-like solidified upon shaking and was filtered and
washed with water giving good yields: 1b, 93%, 4, 77%.
The products were crystallized from ethanol to afford
pure products with melting points for 1b, 74.8–75.5 8C
(lit. 74.5–75.3 8C,23 74 8C24 and 4, 89.5–90.0 8C (lit.
The reaction of 6b with cyclopentadiene 2 was
expected to show higher reaction rates in these media
because the presence of another carbonyl group could
further enhance the hydrogen-bond stabilizing effect. The
reaction of 6b with cyclopentadiene 2 at 32 8C in DDAB
and dodecylmethylammonium bromide (DMAB) surfac-
tants solution are compared in Fig. 8.
As expected, the second-order reaction rate constant is
increased by increasing the hydrogen-bond donating
capacity of the dienophile.
1
89.0–89.2 8C,23 87–88 8C25). The H NMR spectra of
these products were checked and were consistent with the
previously reported results.23
CONCLUSIONS
Kinetic measurements
SASAC using acidified anionic surfactant and linear LAS
of Diels–Alder reactions in water has been investigated by
kinetic measurements. The catalysis is enhanced by the
acidification of the anionic surfactant and the reaction
rate is enhanced by about 40 times for SDS while it is
enhanced by about 170 times for LAS. When the
reactions are performed using acidified cationic surfac-
tants, rate inhibition is found. The results serve as an
indirect indication that bimolecular Diels–Alder reactions
occur close to the micellar surface (Stern region).
Moreover, the reactions are faster in these surfactant
media than in aqueous hydrochloric acid under similar
reaction conditions. Furthermore, we have established
Rate constants were determined by UV–VIS spectroscopy
(Perkin Elmer l2, l5, or l12 spectrometer). The
disappearance of the absorption of the dienophile at an
appropriate wavelength was monitored for at least four
half-lives. The dienophile was introduced into a 1 cm path
length quartz cuvet containing 3.5 ml of the appropriate
surfactant solution. After equilibration, 10–25 ml of a
concentrated stock solution of cyclopentadiene in
acetonitrile was added. The pseudo-first-order rate
constants were obtained using a fitting program. The
apparent second-order rate constant was then calculated
by dividing the observed pseudo-first-order rate constant
by the initial concentration of cyclopentadiene. Typical
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 764–770
DOI: 10.1002/poc

770
E. B. MUBOFU AND J. B. F. N. ENGBERTS
concentrations employed were [diene] ¼ 1 ꢁ 10ꢀ3–2 ꢁ
10ꢀ3 M and [dienophile] ¼ 1 ꢁ 10ꢀ5 M. The rate con-
stants were measured at least three times and the reported
rate constants are an average of three runs. The rate
constants were reproducible to within 3%.
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Preparation of the cationic surfactants
The alkylammonium surfactants were prepared by
protonation of the respective amines. Due to solubility
problems of these amines in aqueous systems, direct
protonation using aqueous HBr/HCl was not feasible. The
protonation was thus achieved by dissolving the amines in
diethyl ether. This was followed by bubbling in hydrogen
bromide gas until the surfactant salts precipitated out. The
surfactant salts were then vacuum-dried at room
temperature overnight. The cmc of each surfactant was
determined by conductivity at 32 8C. The cmc were 13.01
and 11.58 mM for DDAB and DMAB.
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The National Research School Combination Catalysis
(NRSCC, Netherlands) is gratefully acknowledged for
financial support. The University of Dar es Salaam (Tan-
zania) is acknowledged for a generous study leave to Egid
B. Mubofu.
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Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 764–770
DOI: 10.1002/poc