1-Naphthyl-TADDOL/Emim BF4: A new catalytic system for the asymmetric addition of Chan's diene to aromatic aldehydes

Article abstract of DOI:10.2478/s11532-010-0100-5

A new organocatalytic system was tested as a promoter for the asymmetric addition of Chan's diene to aldehydes under solvent-free conditions. This new organocatalyst generated in situ by mixing 1-naphthyl-TADDOL derivative and Emim BF₄ was able

Full text of DOI:10.2478/s11532-010-0100-5

Cent. Eur. J. Chem. • 8(6) • 2010 • 1172-1178
DOI: 10.2478/s11532-010-0100-5
Central European Journal of Chemistry
1-Naphthyl-TADDOL/Emim BF4: A new catalytic
system for the asymmetric addition of Chan’s
diene to aromatic aldehydes
Invited Paper
Rosaria Villanoa,*, Maria R. Acocellab, Vincenzo De Siob, Arrigo Scettrib,*
aInstitute of Biomolecular Chemistry – CNR, Reg. Baldinca,
07040 Li Punti (Sassari), Italy
bDepartment of Chemistry, University of Salerno,
84084 Fisciano (Salerno), Italy
Received 29 May 2010; Accepted 26 July 2010
A new organocatalytic system was tested as a promoter for the asymmetric addition of Chan’s diene to aldehydes under solvent-
free conditions. This new organocatalyst generated in situ by mixing 1-naphthyl-TADDOL derivative and Emim BF4 was able to give
enantioenriched vinylogous aldols and hetero-Diels-Alder cycloadducts. A mechanistic investigation through the detection of nonlinear
effects confirmed the involvement of the ionic liquid in the formation of a new catalytic supramolecular species.
Abstract:
Keywords: Organocatalysis • Hydrogen bonding • Chan’s diene • Aldol reaction • Cycloaddition
© Versita Sp. z o.o.
conditions, at low temperature and in the presence of
organic solvents (Scheme 1) [5].
1. Introduction
More recently, Chan’s diene 5, a synthetic equivalent
of acetoacetate dianion, demonstrated an unexpected
reactivity towards electron-poor aldehydes: in the
presence of TADDOL 1a, under solvent-free conditions
at room temperature, starting materials 3 suffered a
competing reaction mechanism, affording, in addition
In the last two decades, tetraaryl-1,3-dioxolane-4,5-
dimethanols (TADDOLs) (Fig. 1) and their derivatives
have proven to be very useful chiral auxiliaries in a wide
variety of fundamental asymmetric metal-catalyzed
processes [1,2].
Additional chemical properties of TADDOLs derive to the vinylogous aldols 6, the pyran-4-one derivatives
from their particular structural feature: in fact, the 7 (Scheme 2) [3f]. Therefore, this organocatalytic
proximity of the –OH groups allows the formation of approach promoted a different mechanistic pathway
an intramolecular hydrogen bond and the free –OH compared to organometallic catalysts [6].
group shows a strong tendency to associate with
Regarding the mechanistic aspects, both the
appropriate H-acceptors. This peculiarity has allowed occurrence of the vinylogous aldol condensation and
their use in the resolution of racemic mixtures (through the formation of the products 7 through a hetero-
the formation of inclusion compounds) [2] and, more Diels-Alder reaction have been attributed to the
recently, in organocatalyzed reactions [3]. In particular, organocatalytic properties of the used TADDOL. In fact,
TADDOLs of type 1 have been conveniently employed according to Rawal and Ding’s model, the formation of
in the vinylogous aldol condensation [4] of cyclic masked an intermolecular hydrogen bond between the free –OH
forms of acetoacetate esters 2 to give product 4, known group of TADDOL and the aldehydic oxygen resulted in
precursor of δ-hydroxy-β-ketoesters 6, in good yields the carbonyl activation through the lowering of its LUMO
and enantiomeric excesses (ee.s) under metal-free energy [3a,c].
* E-mail: rosaria.villano@icb.cnr.it
scettri@unisa.it
1172

R. Villano et al.
variety of important chemical processes [8]. In fact,
their solvation properties and their ability to interact
with substrates, resulting in high degree of organisation
in the transition state can dramatically influence the
outcome of chemical reactions [9]. More recently, the
possibility to use ILs as additives (in rather reduced
amounts) in order to promote the formation of active
species is emerging, above all in biocatalytic processes
[10]. Particularly, as regards imidazolium-based ionic
liquids, the reactivity of appropriate H-acceptors could
be increased by hydrogen bond formation thanks to the
acidic character of C(2)-H proton of the heterocyclic
cation [11], as pointed out in procedures for Diels-Alder
cycloaddition [12,13] and Baylis-Hillman reaction [14].
Ar
OH
Ar
O
OH
Ar
1a: Ar= 1-naphthyl
O
Ar
1
Figure 1. Structure of 1-naphthyl-TADDOL
1a (20 mol %)
O
O
O
O
O
OH
+ RCHO
Toluene, -60°C
OSiR'3
R
O
2
3
4
2. Experimental Procedure
OH
O
R
OMe
2.1 General
6
All reactions were performed in oven-dried (140°C)
vials. Thin-layer chromatography was performed on
Merck Kiesegel 60 F254 plates eluting with the solvents
indicated, visualized by a 254 nm UV lamp and aqueous
ceric sulfate solution. Column chromatographic
purification of products was carried out using silica gel
60 (70-230 mesh, Merck). All reagents (Aldrich, Fluka
and Strem) were used without further purification.
Infrared spectra were recorded on a Bruker 22 series
FT-IR spectrometer. NMR spectra were recorded on a
Bruker DRX 400 (400.135 MHz for 1H and 100.03 MHz
for 13C) spectrometer. HPLC analysis were performed
with Waters Associates equipment (Waters 2487 Dual
λ absorbance Detector) using a CHIRALPAK AD or a
CHIRALCEL OD column with mixtures and flow rates
as indicated. Mass spectrometry analysis was carried
out using an electrospray spectrometer Waters 4 micro
quadrupole. MW-assisted reactions were carried out
using a single mode CEM Discover appparatus in
closed vessels under magnetic stirring with temperature
measurement by IR detector and power control.
Scheme 1. Vinylogous aldol addition of diene 2 to aldehydes,
according to Rawal’s procedure
Me3SiO
OSiMe3
OMe
+
RCHO
5
3
O
OH
O
O
+
R
OMe
R
O
OMe
6
7
yield up to 52%
e.e. up to 56%
yield up to 64%
e.e. up to 60%
Scheme 2.
Asymmetric addition of diene 5 to electron-poor
aldehydes promoted by 1a under solvent-free
conditions
General procedure for the synthesis of 6
and 7 at room temperature (Procedure 1)
2.2
Furthermore, in these last years, the need to develop
cleanerandmoreefficientorganicprocessesisincreased
and so, the elaboration of solvent-free organocatalytic
protocols has attracted particular interest. Although
alternative and benign solvents (water, supercritical
CO2, etc.) have been used, the realization of solvent-
free methodologies is still the best option.
In this context an ever increasing interest was
stimulated by the intrinsic properties of ionic liquids (ILs)
[7] whose use as alternative solvents often resulted
in an enhancement of reactivity and/or selectivity in a
In a dry vial (S,S)-1a (0.025 mmol), IL2 (100 µL),
aldehyde 3 (0.25 mmol) and diene 5 (0.33 mmol) were
added. The resulting mixture was stirred for 4.5 hours
at room temperature, then dry THF (1 mL) was added.
This solution was cooled at -78°C and TFA (0.1 mL) was
added dropwise, then it was permitted to warm to room
temperature and after completion of the desilylation
reaction, it was neutralized by addition of saturated aq
NaHCO3. The reaction mixture was extracted with AcOEt
1173

1-Naphthyl-TADDOL/Emim BF4: A new catalytic
system for the asymmetric addition of
Chan’s diene to aromatic aldehydes
and the combined organic phase was dried (MgSO4) NaHCO3. The reaction mixture was extracted with AcOEt
and concentrated. The residue was purified by non- and the combined organic phase was dried (MgSO4)
flash chromatography (petroleum ether/ AcOEt from 8/2 and concentrated. The residue was purified by non-
to 1/1) to give the products 6 and 7.
flash chromatography (petroleum ether/ AcOEt from 8/2
The spectroscopic (IR, 1H NMR and 13C NMR) data to 1/1) to give the products 6 and 7.
of aldols 6 and HDA cycloadducts 7 matched the ones
reported in the literature [15].
3. Results and Discussion
2.3 General procedure for the synthesis
of 6 and 7 by MW-assisted protocol
(Procedure 2)
On the ground of these considerations, an investigation
was devoted to examine the influence of commercially
TADDOL 1a (0.025 mmol), IL2 (100 µL), aldehyde available ionic liquids (ILs) on the preparative (6/7 ratios)
(0.25 mmol) and Chan’s diene (0.33 mmol) were and stereochemical aspects (level of enantioselectivity)
introduced into a vessel for MW reactor. After cooling on the reaction of electron-poor aldehydes of type 3
at –78°C for 30 min, the reaction mixture was submitted with Chan’s diene in the presence of TADDOL 1a as
to MW irradiation at P = 10W for a period of time organocatalyst.
Preliminarily, o-nitro-benzaldehyde
indicated in Table. Then, dry THF (1 mL) was added was chosen as representative substrate and a set of
and the resulting solution was cooled at -78°C and TFA imidazolium-based ionic liquids (Fig. 2) was used as
(0.1 mL) was added dropwise; after warming to room solvent.
temperature and after completion of the desilylation
Unfortunately, Chan’s diene 5 demonstrated a
reaction, it was neutralized by addition of saturated aq dramatic instability in all ionic liquids IL1-IL4, because of
nBu
Me
CHO
IL1: X- = (SO4Et)-
-
IL4: X- = BF4
N
N
-
NO2
IL2: X- = BF4
Me Si
O
OSiMe3
OMe
3
IL3: X- = Cl-
+
N
X
X
N
Me
Et
5
3a
Figure 2. Ionic liquids evaluated in the reaction.
IL
rt
O
Table 1. ILs catalyzed vinylogous aldol reaction of 5 on o-nitro-
benzaldehyde 3a.
OH
O
O
Entry
IL
Reac.
Time / h
6a
Yield / %
7a
Yield / %
a
a
OMe
O
OMe
+
1
2
3
4
5
6
-
4.5
4.5
24
39
traces
22
21
-
IL1
IL1
IL2
IL3
IL4
NO2
NO2
-
4.5
4.5
4.5
34
24
-
-
6a
7a
27
31
aYields refer to isolated chromatographically pure compounds, whose
structures were confirmed by spectroscopic data (1H-NMR, 13C-NMR, MS,
Scheme 3. Addition of diene 5 to 3a in the presence of ionic
liquids
IR).
Table 2. Solvent-free reaction of Chan’s diene 5 with electron-poor aldehydes 3 promoted by organocatalytic systems (Procedure 1).
Entry
R-
Catalyst
Product
Yield%a
ee%b
Product
Yield%a
ee%b
1
o-NO2C6H4
IL2
6a
34
-
7a
24
-
2
3
o-NO C H
o-NO2C6H4
p-CF2C6H4
p-CF3C6H4
p-CF33C66H44
p-CNC H
p-CNC6H4
o-CNC66H44
p-NO2C6H4
p-MeC6H4
p-MeC6H4
1a
IL2 / 1a
IL2
1a
IL2 / 1a
6a
6a
6b
6b
6b
6c
6c
6d
6e
6f
37
47
26
45
27
35
24
17
32
39
18
19
24
-
7a
7a
7b
7b
7b
7c
7c
7d
7e
7f
32
46
17
23
56
26
32
45
34
-
57
62
-
48
51
49
52
63
53
-
4
5
29
34
29
50
ND
40
57
42
6
7c
8
1a
IL2 / 1a
IL2 / 1a
IL2 / 1a
1a
9
10
11c
12c
IL2 / 1a
6f
7f
-
-
aYields refer to isolated chromatographically pure compounds, whose structures were confirmed by spectroscopic data (1H-NMR, 13C-NMR, MS, IR).
bE.e.s were determined by HPLC analysis.
cThe reaction was conducted for 24 h.
1174

R. Villano et al.
25
20
15
10
5
7
0
60
50
40
30
20
10
0
Curve B
Curve D
Curve A
Curve C
0
0
20
40
60
80
100
0
20
40
60
80
100
ee organocatalyst (%)
ee organocatalyst (%)
Experimental ee data for aldol 6a produced by
vinylogous aldol reaction catalyzed by a) enantioenriched
organocatalyst 1a (■; Curve A) [15] and b) enantioenriched
1a / IL2 system (●; Curve C).
Figure 3.
Experimental ee data for cycloadduct 7a produced by
reaction catalyzed by a) enantioenriched organocatalyst
1a (■; Curve B) [15] and b) enantioenriched 1a / IL2
system (●; Curve D).
Figure 4.
a very fast decomposition leading to the parent methyl
acetoacetate in almost quantitative yields. Conversely,
no significant decomposition of 5 was observed when
the reaction on 3a was performed under solvent-free
conditions in the presence of ILs used as additives
(Scheme 3, Table 1). Furthermore the preparative
outcome was found to depend on the anionic moiety of
the ionic liquid.
Very interestingly, in entry 4, where the highest
values of conversion could be observed, the vinylogous
aldol 6a and the cycloadduct 7a were obtained in
comparable yields with respect to the ones afforded
by the background reaction (entry 1) performed in the
absence of any catalyst. These results suggested that
no particular influence was exerted by the presence of
IL2 as additive. Therefore, the experimental conditions
of entry 4 seemed to be appropriate to point out different
catalytic properties of 1-naphthyl-TADDOL 1a and
1-naphthyl-TADDOL 1a / IL2 catalytic systems, through
the detection of a different products distribution (6a/7a
ratios) and /or different ee.s. In brief, a first indication
of the possible formation of a supramolecular system
1a / IL2, with different catalytic activity, could be taken
simply by comparing the results of these two series of
experiments.
a new catalytic system. Similar results were obtained
in entries 9 and 10. Of course, the results obtained by
using 1a / IL2 (Scheme 4, path c) could not derive by the
simple outcome of two independent catalytic activities
(Scheme 4, path a + path b), that is by 1a and by IL2
catalysis: in fact, in this case, by using 1a / IL2 as
catalyst we would had to observe a product with reduced
optical purity, compared to that produced by using only
1a, because the reaction in the presence of only IL2
produces a racemic product and this is in disagreement
with the experimental results.
Conversely, in the case of a non-activated aldehyde
(entries 11-12), aldol 6f proved to be the only reaction
product and the presence of the additive caused a
notable drop both of the yield and the ee, suggesting
also in this case the formation of a new catalytic species
with a different activity. The control of the formation of
a new catalytic system by classical analytic methods
(for example via NMR) appeared particularly difficult
because this supramolecular aggregate was generated
insolvent-freeconditionsandtheintroductionofasolvent
could introduce a perturbation in the studied system. In
recent years the detection of nonlinear effects (NLE)
has proven to be a powerful tool in investigations in situ
on the mechanistic aspects of an asymmetric process
[16] and, in many reactions, these studies were used
to identify a higher order molecularity of the catalyst
(dimeric or oligomeric species) [16a,17].
For this reason, a number of experiments was
performed on electron-poor aldehydes (Table 2,
entries 1-10). The comparison of entries 2 and 3, 5
and 6, 7 and 8 pointed out significant modifications
on 6/7 ratios and, usually, an increase of the level of
enantioselectivity suggesting the involvement of IL2 in
In a previous paper [15], o-nitro-benzaldehyde,
chosen as representative substrate, was reacted
with Chan’s diene in the presence of enantioenriched
1175
4

1-Naphthyl-TADDOL/Emim BF4: A new catalytic
system for the asymmetric addition of
Chan’s diene to aromatic aldehydes
Table 3. MW-assisted vinylogous aldol reaction of Chan’s diene 5 on electron-poor aldehydes 3 realized via Scheme 5 (Procedure 2).
Entry
R-
Catalyst
Product
Yield%a
ee%b
Product
Yield%a
ee%b
1c
2d
3c
o-NO2C6H4
o-NO C H
o-NO2C6H4
o-NO2C6H4
o-NO2C6H4
p-NO2C6H4
p-NO22C6H4
p-CNC 6H 4
p-CF C6H4
p-CF33C66H44
-
-
6a
6a
6a
6a
6a
6e
6e
6c
6b
6b
26
43
29
40
29
13
21
29
23
10
-
-
7a
7a
7a
7a
7a
7e
7e
7c
7b
7b
23
27
49
54
23
13
27
34
24
11
-
-
1a
18
28
44
32
41
36
28
41
54
61
71
42
50
47
45
57
4d
5d
6d
7d
8c
1a
IL2 / 1a
1a
IL2 / 1a
1a
9d
1a
IL2 / 1a
10d
aYields refer to isolated chromatographically pure compounds, whose structures were confirmed by spectroscopic data (1H-NMR, 13C-NMR, MS, IR).
bE.e.s were determined by HPLC analysis.
cThe reaction was conducted by submitting the pre-cooled reaction mixture to MW irradiation (15 min, P = 10 W).
dThe reaction was conducted by submitting the pre-cooled reaction mixture to cycles of MW irradiation (6 cycles, 5 min each cycle, P = 10 W).
6a + 7a
rac-6a + rac-7a
path a
path b
y=37%
y=32%
y=34%
y=24%
ee=19% ee=57%
1a
IL2
3a + 5
path c
1a/IL2
6a + 7a
y=47%
y=46%
ee=24% ee=62%
Scheme 4. Summary scheme of the reaction of Chan’s diene 5 with 3a, promoted by different catalytic systems
TADDOL derivative 1a under solvent-free conditions.
The resulting multishaped curves and
by micro-wave (MW) irradiation [18]. In this area,
the possibility to apply this alternative energy source
A
B
(respectively Figs. 3 and 4) suggested the involvement to solvent-free organocatalyzed reactions in order
of an oligomeric catalytic species both in the vinylogous to realize procedure at low environmental impact is
aldol addition and the cycloaddition reaction. Probably, particularly intriguing and so, a variety of reactions
in the absence of any solvent, the involvement of under solvent-free conditions has been widely explored,
structures with more than one TADDOL molecule without any significant loss of efficiency and selectivity.
(“autoassociation”) in the catalytic cycle is favoured Generally it is very difficult to realize vinylogous aldol
differently from Rawal and Ding’s model (vide supra), additions under MW irradiation, due to the thermal
proposed for reactions taking place in solution, and instability of the employed silyloxydienes. Therefore, it
involving one only TADDOL molecule in the transition seemed interesting to evaluate the catalytic properties
state.
of TADDOL derivative 1a, under MW-irradiation [19], in
In a similar way, 3a was reacted with Chan’s diene 5 the reaction of Chan’s diene 5 with aldehydic substrates
in the presence of IL2 / enantioenriched 1a system. The (Scheme 5).
attainment of multishaped curves C and D (Figs. 3 and 4
Taking in mind the well-known thermal instability of
respectively) notably differing from A and B represented 5 [20], the experimental conditions had to be carefully
a further confirmation of the participation of the ionic chosen in order to avoid overheating of the reaction
liquid to the formation of new oligomeric supramolecular mixture. Therefore, MW-assisted reactions were carried
species,resulting,inmostreportedcases,inanincreased out using a single mode cavity dedicated reactor (CEM
enantioselectivity. To the best of our knowledge, this Discover). A preliminary experiment was performed
is the first study of NLEs on organocatalytic reactions on o-nitro-benzaldehyde, chosen as representative
based on the hydrogen-bonding activation of a carbonyl substrate, in the absence of organocatalyst (entry 1,
compound.
Table 3) and the reaction mixture, previously pre-cooled
In the recent years an important contribution for at –78°C [21], was submitted to MW irradiation for
the achievement or the improvement of expeditious 15 min.
and selective synthetic procedures has been afforded
Power-controlled conditions (P = 10W) ensured
1176
5

R. Villano et al.
O
Me3SiO
OSiMe3
OMe
OH O
O
catalyst, neat
+ RCHO
+
MW (P=10W)
R
OMe
R
O
OMe
5
3
6
7
Scheme 5. Asymmetric addition of diene 5 to aldehydes under MW irradiation
valuesoftemperaturelowerthan35°Candtheattainment
4. Conclusions
of vinylogous aldol 6a and cycloadduct 7a (respectively
26% and 23% yield) confirmed an acceptable
compatibility of MW treatment with the thermal lability
of Chan’s diene 5. Although no significant improvement
was observed after more prolonged reaction time,
the mode of irradiation had a deep influence on the
preparative outcome. In fact, when entry 1 was repeated
by submitting the pre-cooled reaction mixture to cycles
of MW irradiation (6 cycles, 5 min each cycle, P = 10W)
a different efficiency and products distribution was
observed as supported by the attainment of 6a and 7a
respectively in 43% and 27% yield (entry 2). In entries
3 and 4 both the modes of MW irradiation were used
to examine the influence of the organocatalyst 1a. We
were pleased to observe significant enhancement of
the efficiency of the process and, most interestingly, the
achievement of comparable levels of enantioselectivity
for products 6a and 7a with respect to the conventional
procedure (cfr entry 2 of Table 2 with entries 3 and 4 of
Table 3).
As predictable, when 1a / IL2 catalytic system was
used, overheating was rather difficult to avoid because
of the increased polarity of the reaction mixture, so that
the thermal decomposition of Chan’s diene showed
to be the predominant process. In fact in entry 5,
the temperature enhancement up to 55°C resulted
in a notable decrease of both the yields of 6a and 7a.
Conversely, most interestingly, the employment of the
new catalytic species 1a / IL2 allowed to attain products
6a and 7a in higher ee.s (respectively 44% and 71%
ee).
In conclusion, naphthyl TADDOL derivative 1a has
confirmed its synthetic versatility since, in combination
with suitable imidazolium-based ionic liquids has been
conveniently used in a solvent-free procedure for
the reaction of Chan’s diene with activated aromatic
aldehydes. A mechanistic investigation, based on
the detection of nonlinear effects, has pointed out
the participation of several TADDOL molecules in the
catalytic cycle and then the involvement of the used
ionic liquid in the formation of a new supramolecular
catalytic species; the extension of this activation
strategy to other hydrogen-bonding organocatalysts
reasonably discloses interesting perspectives in
asymmetric synthesis, especially by considering that in
solvent-free conditions the intermolecular interactions
are enhanced. Furthermore, through an accurate
control of temperature, the reaction of 5 with electron-
poor aldehydes was performed under MW irradiation
affording the corresponding aldols 6 and cycloadducts
7 in satisfactory yields and ee.s at very low power
values (only 10 W) and in rather reduced reaction times
compared to the conventional thermal treatment. Finally,
in some cases, the use of naphthyl TADDOL derivative/
ionic liquid catalytic system under MW irradiation
resulted in an enhancement of enantioselectivity.
Acknowledgments
We are grateful to the University of Salerno for financial
support.
The scope of the procedure was evaluated by
submitting to the usual treatments other electron-poor
aldehydes: satisfactory results were obtained in entries
8 and 9, while, rather surprisingly, p-nitro-benzaldehyde
pointed out a rather modest reactivity resulting in a
significant drop of efficiency (entry 6). Conversely,
improved yields and ee.s both for products 6 and 7 were
afforded by the new 1a / IL2 catalytic system.
1177
6

1-Naphthyl-TADDOL/Emim BF4: A new catalytic
system for the asymmetric addition of
Chan’s diene to aromatic aldehydes
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