Switching diastereoselectivity of direct Mannich-type reaction of cyclic ketones by polymeric laponite nanoclay catalyst

Article abstract of DOI:10.1007/s13738-015-0772-z

A new polymeric laponite nanoclay heterogeneous catalytic system based on HPMC (hydroxypropyl methyl cellulose) was developed for direct Mannich-type reaction of ketones with substituted benzaldehydes and anilines to afford corresponding β-amino ketones in good to high yields. Interestingly, cyclic ketones exhibited different chemoselectivity. Cyclopentanone underwent aldol condensation to give crossed-aldol product, while cyclohexanone and cyclopentanone afforded corresponding Mannich adducts. In the case of cyclohexanone, stereoselectivity was changed depending on the nature of the substitution on benzaldehydes, in which, moderate electron-donating and electron-withdrawing groups afforded the anti isomer as major products, but strongly electron-donating substituted benzaldehydes led to syn isomer as the major Mannich adducts. Mannich reaction with cycloheptanone led to Mannich adducts with excellent syn selectivity.

Full text of DOI:10.1007/s13738-015-0772-z

J IRAN CHEM SOC
DOI 10.1007/s13738-015-0772-z
ORIGINAL PAPER
Switching diastereoselectivity of direct Mannich‑type reaction
of cyclic ketones by polymeric laponite nanoclay catalyst
Bagher Eftekhari‑Sis1 · Sahar Mohajer1 · Maryam Zirak2 ·
Gholam Reza Mahdavinia1 · Orhan Büyükgüngör3
Received: 11 March 2015 / Accepted: 2 November 2015
© Iranian Chemical Society 2015
Abstract A new polymeric laponite nanoclay heteroge-
neous catalytic system based on HPMC (hydroxypropyl
methyl cellulose) was developed for direct Mannich-type
reaction of ketones with substituted benzaldehydes and ani-
lines to afford corresponding β-amino ketones in good to
high yields. Interestingly, cyclic ketones exhibited different
chemoselectivity. Cyclopentanone underwent aldol conden-
sation to give crossed-aldol product, while cyclohexanone
and cyclopentanone afforded corresponding Mannich
adducts. In the case of cyclohexanone, stereoselectivity
was changed depending on the nature of the substitution
on benzaldehydes, in which, moderate electron-donating
and electron-withdrawing groups afforded the anti isomer
as major products, but strongly electron-donating substi-
tuted benzaldehydes led to syn isomer as the major Man-
nich adducts. Mannich reaction with cycloheptanone led to
Mannich adducts with excellent syn selectivity.
Introduction
Homogeneous catalysts, which are still being used in
industrial chemical processes, are strong mineral or
Lewis acids. These acids are highly toxic and danger-
ous to handle and transport, and corrosive to the storage
containers. Also, the final products of the catalyzed reac-
tion need to be separated from the catalyst generating a
great volume of waste because frequently those acids are
neutralized at the end of the reaction that causes environ-
mental problems. To overcome these troublesome disad-
vantages, a large number of studies have been developed
for application of heterogeneous catalysts. Therefore, the
use of heterogeneous catalysts such as clays and zeolites
has received great attention in organic synthesis. This has
been attributed to their inexpensive nature, greater selec-
tivity, environmental compatibility, recyclability, non-
corrosiveness, simplicity in operation and ease of separa-
tion [1–10]. Especially, clay catalysts make the reaction
process convenient, economic and act as Brønsted as well
as Lewis acids enabling them to function as efficient cata-
lysts for various organic transformations [11–16]. Poly-
mer-supported catalysts have also attracted much atten-
tion and rapidly increasing numbers of new polymeric
supports have recently been reported [17–30], since one
of the major benefits of polymer-supported catalysis, is
the recovery and reuse of immobilized species, which can
be extremely expensive.
Keywords Nanoclay · Polymeric catalyst · Mannich-type
reaction · β-Amino ketones
* Bagher Eftekhari-Sis
eftekharisis@maragheh.ac.ir; eftekhari.sis@gmail.com
β-Amino carbonyl compounds are attractive targets for
chemical synthesis because of their wide use as biologi-
cally active molecules [31]. Therefore, the development of
new synthetic methods leading to β-amino carbonyl com-
pounds or their derivatives has attracted much attention
in organic synthesis. The Mannich reaction is a classical
1
Department of Chemistry, University of Maragheh, P. O.
Box 55181-83111, Maragheh, Iran
2
Department of Chemistry, Payame Noor University, P. O.
Box 19395-3697, Tehran, Iran
3
Department of Physics, Ondokuz Mayis University,
TR-55139 Samsun, Turkey
1 3

J IRAN CHEM SOC
method for the preparation of β-amino ketones and alde-
hydes [31–33], and has been one of the most important
basic reactions in organic chemistry for its use in natural
product and pharmaceutical syntheses due to formation of
carbon–carbon and carbon–nitrogen bonds, simultaneously.
The classic Mannich reactions are usually carried out under
acidic or basic conditions, which offer a number of serious
disadvantages [32–35]. However, to avoid side reactions,
occasionally encountered in the presence of a strong acid
or a base, a number of alternative catalytic procedures for
Mannich reactions have been developed [36–46]. Unfor-
tunately, many of these procedures often require a large
excess of reagents, long reaction time and drastic reac-
tion conditions in organic solvents such as acetonitrile or
1,2-dichloroethane which are toxic. In some cases, a stoi-
chiometric amount of Lewis acid is required.
X‑ray crystal structure determination
Single crystals, suitable for X-ray crystallography, were
obtained via slow evaporation in EtOH. X-ray data were
collected on a STOE IPDS 2 diffractometer with a graph-
ite monochromator and Mo Kα radiation (λ = 0.71073 Å)
by the rotation method scans at 296 K. The structure was
solved using direct methods with SHELXS97 [49] and
refined on F2 by full-matrix least squares with anisotropic
displacements parameters for all non-hydrogen atoms with
SHELXL-97 [49]. All carbon-bonded hydrogen atoms
were geometrically fixed with C─H = 0.93, 0.97, and
0.98 Å for aromatic, methylene and methine H, respec-
tively, and constrained to ride on their parent atoms, with
Uiso(H) = 1.2Ueq(C). The nitrogen H atoms were located
from the difference Fourier map and allowed to refine
freely. The crystals data and pertinent details of the experi-
mental conditions are summarized in Table 1.
In continuing with our research on Mannich reaction
[39–41] and polymeric and solid support catalyst [47, 48],
herein we report a new polymeric laponite nanoclay as a
catalyst for the synthesis of β-amino carbonyl compounds
via direct Mannich-type reaction.
Results and discussion
Polymeric laponite nanoclay was synthesized via graft-
ing of acryl amide (AAm) onto HPMC using laponite RD
nanoclay as filler. The grafting was carried out using ceric
ammonium nitrate (CAN). It has been reported that the
anhydroglucose units are predominantly oxidized through
C2–C3 bond cleavage induced by Ce4+ ions and oxidiz-
ing corresponding bonds induce free radicals onto HPMC.
AAm monomer can graft onto HPMC backbones through
produced free radicals and cross-linking reaction can occur
in the presence of MBA cross-linker and finally a three-
dimensional network is produced. The dispersed laponite
sheets will be captured in the HPMC-g-PAAm networks
(Scheme 1).
The XRD patterns of pristine laponite RD and polymeric
laponite nanoclay were studied at 2θ = 2.5–15° and are
illustrated in Fig. 1. According to data, the XRD profile of
pristine laponite (a) shows a broad peak from 2θ = 2.5 to
2θ = 9.1 with a diffraction peak at about 2θ = 4.32 corre-
sponding to the distance of clay sheets with d spacing 20.4
Å. Stirring of laponite for 5 h subsequently in situ graft
copolymerization of AAm onto biopolymer in the presence
of laponite nanoclay leads to corresponding nano-compos-
ite that the XRD profile of the sample was shown in Fig. 1,
b. No diffraction peak was observed in polymeric laponite
nanoclay and it can be concluded that the clay layers are
completely exfoliated.
Experimental
Preparation of polymeric laponite nanoclay
An amount of 0.5 g of laponite RD was dispersed in 30 mL
of distilled water for 24 h. Then, 1 g of HPMC was added
to laponite RD solution and was allowed to completely dis-
solve. Afterward, acrylamide monomer (3 g) and methylen-
ebisacrylamide (MBA) (0.05 g) were added to the solution
and the temperature of the solution was adjusted at 30 °C.
The CAN initiator (0.1 g) was dissolved in 2 mL water and
added to the solution to initiate the polymerization. After
polymeric laponite nano composite formation, the product
was cut into small pieces and immersed into excess deion-
ized water to extract unreacted component. Then, the puri-
fied product was dried at 60 °C. The dried polymeric nano
composite was ground and kept from moisture and light.
General procedure for Mannich‑type reaction
To a mixture of 0.5 mmol of benzaldehyde, 0.5 mmol of
aniline and three equiv. of cyclohexanone, was added
0.04 g of polymeric laponite nanoclay and stirred at room
temperature for appropriate time. After completion of the
reaction, monitored by TLC, 5 mL EtOH was added and
catalyst was removed by filtration, and filtrate was concen-
trated under reduced pressure. The obtained crud product
was recrystallized from EtOH. All products are known and
The crosslinked HPMC-based polymeric laponite nano-
clay dispersed in water (0.1 g/100 mL water) and spread
onto copper grid. After air-drying, the TEM micrograph
was studied and the result was shown in Fig. 2. The micro-
graph of polymeric laponite nanoclay showed plates with
1
characterized by comparison of H NMR with authentic
samples.
1 3

J IRAN CHEM SOC
Table 1 Crystal data and
structure refinement for
CCDC deposit no.
I (CCDC 895762) [50]
II (CCDC 1043140) [50]
2-[(4-chlorophenylamino)
(p-tolyl)methyl]cyclohexanone
I, and 2-[phenyl(phenylamino)
methyl]cycloheptanone II
Empirical formula
Formula weight (g mol−1
Temperature (K)
Habitus, color
C20H22ClNO
327.84
C20H23NO
293.39
)
296
296
Prism, colorless
0.57 × 0.42 × 0.34
Monoclinic
P21/c
Prism, colorless
0.72 × 0.56 × 0.27
Monoclinic
P21/c
Crystal size [mm3]
Crystal system
Space group
Unit cell dimensions (Å, o)
a = 12.9387 (9)
b = 9.1452 (7)
c = 18.3327 (14)
β = 126.110 (4)
1752.5 (2)
4
a = 5.7534 (4)
b = 16.1336 (8)
c = 18.1980 (13)
β = 99.371 (6)
1666.65 (19)
4
Volume (Å3)
Z
Dc (gcm−1
)
1.243
1.169
F(000)
696
632
Diffractometer
STOE IPDS2
Rotation method scans
Mo Kα, 0.71073
0.22
STOE IPDS2
Rotation method scans
Mo Kα, 0.71073
0.07
Data collection method
Radiation type, wavelength (Å)
Absorption coefficient (mm−1
θ range for data collection (°)
Reflections collected
)
2.0–26.5
1.7–28.0
21,741
10,757
Independent reflections (Rint)
Observed reflections
3626 (0.036)
2462 [I > 2σ(I)]
3449 (0.041)
2170 [I > 2σ(I)]
Index ranges
−16 ≤ h ≥ 16
−11 ≤ k ≥ 11
−22 ≤ l ≥ 22
−7 ≤ h ≥ 7
−20 ≤ k ≥ 20
−22 ≤ l ≥ 18
Refinement
Full-matrix least squares on F2
3626, 0, 213
Full-matrix least squares on F2
3449, 0, 203
Data, restraints, parameters
R [F2 > 2σ(F2)], wR(F2), S
Δρmax, Δρmin (e Å−3
0.050, 0.137, 1.03
0.50, −0.17
0.048, 0.136, 1.01
0.26, −0.13
)
Scheme 1 Preparation of poly-
meric laponite nanoclay catalyst
1 3

J IRAN CHEM SOC
cyclohexanone was used, the reaction proceeded slowly and
afforded the desired Mannich adduct in low yield. So, the
optimum amount of cyclohexanone was determined to be
3 equiv. which furnished the corresponding Mannich prod-
uct in acceptable yield with excellent anti selectivity. Also,
we carried out the same reactions using different amount
of catalysts, and 0.04 g of polymeric laponite nanoclay per
0.5 mmol of 2-chlorobenzaldehyde was determined as an
optimum amount of catalyst. Also, different solvents were
examined and according to environmental view, we car-
ried out the reaction in water and solvent-free conditions,
and the Mannich adduct was obtained in good yield with
excellent anti selectivity under solvent-free conditions and
compared with conditions in which water was used as sol-
vent. Also, the catalytic activity of polymeric laponite nan-
oclay was investigated using HPMC as catalyst, in which
only corresponding imine was obtained, however, laponite
RD nanoclay led to Mannich adduct in good yield with
low diastereoselectivity. Moreover, the recyclability of the
polymeric laponite nanoclay catalyst was investigated. In
these experiments the product was isolated by filtration,
the solid residues were washed with dichloromethane, and
the remaining catalyst was dried under vacuum and then at
60 °C for 1 h, and reloaded with fresh reagents for further
runs. No considerable decrease in the yield and stereose-
lectivity was observed over four runs, demonstrating that
polymeric laponite nanoclay can be reused as a catalyst in
direct Mannich-type reactions.
Fig. 1 XRD patterns of a pristine laponite RD and b polymeric
nano-laponite
The scope of the reaction was investigated using differ-
ent anilines, benzaldehydes and ketones using polymeric
laponite nanoclay as catalyst under solvent-free conditions
at room temperature. The reaction was carried out by add-
ing of 0.04 g of polymeric laponite nanoclay to a mixture
of 0.5 mmol of benzaldehyde, 0.5 mmol of aniline and
three equiv. of ketones and stirring at room temperature to
appropriate time. The structures and yields of products are
summarized in Table 2.
As shown in Table 2, polymeric laponite nanoclay cat-
alyzed direct Mannich type reactions of cyclohexanone
exhibited interesting stereoselectivity depending on the
nature of the substituent on benzaldehydes. Benzaldehydes
with moderated electron-donating substituents (Me, entry
7) and electron-withdrawing substituents (Cl, NO2, entries
Fig. 2 TEM image of polymeric laponite nanoclay
undefined shape, which the size of plates were less than
30 nm. The data confirmed the presence of nanometric
laponite RD in polymeric matrix.
We first studied the catalytic activity of polymeric
laponite nanoclay in direct Mannich-type reaction of
cyclohexanone with aniline and 2-chlorobenzaldehyde
under different conditions (Scheme 2). When 1 equiv. of
Scheme 2 Polymeric laponite
nanoclay catalyzed Mannich-
type reaction of cyclohexanone
with aniline and 2-chlorobenza-
ldehyde
1 3

J IRAN CHEM SOC
Table 2 Direct Mannich-type reaction of cyclic ketones, anilines and benzaldehydes using polymeric laponite nanoclay as catalyst
Entry
X, Y, Z
Time (h)
Yield (%)a
anti/synb
1
2
3
CH2, H, H
24
24
2
64
>99/1
>99/1
CH2, H, 2-NO2
CH2, H, 2-Cl
62
75, 72, 72, 68c
98/2,
98/2,
97/3,
98/2
4
5
CH2, H, 4-NMe2
CH2, 4-Cl, H
5
80
60
32/68
>99/1
19
6
7
8
9
CH2, 4-Cl, 2-Cl
1.5
24
6
57
56
60
50
98/2
CH2, 4-Cl, 4-Me
CH2, 4-Cl, 4-OH-3-MeO
CH2, 4-Br, H
91/9
<1/99
86/14
2.5
d
10
11
12
CH2, 2-Cl, H
CH2, 2-Me, H
CH2CH2, H, H
24
24
24
–
–
d
–
–
60
6/94
13
14
CH2CH2, H, 4-Me
CH2CH2, 4-Cl, H
41
70
2/98
2
3
<1/99
15
S, H, 2-Cl
59
98/2
a
Yields refer to isolated products. bAnti/syn ratio was determined using 1H NMR spectra [39]. cCatalyst was used over four runs. dThe crossed
aldol product was obtained
2, 3 and 6) afforded the Mannich adducts with excellent
anti selectivity, while the strongly electron-donating sub-
stituted benzaldehydes such as 4-Me2N and 3-HO-4-MeO
(entries 4 and 8) furnished the Mannich adducts with high
to excellent syn selectivity. Also, aniline as well as para-
substituted anilines worked well in polymeric laponite nan-
oclay catalyzed direct Mannich type reaction and afforded
the corresponding β-amino ketones in good to high yields.
But, in the case of ortho-substituted anilines such as 2-Cl
and 2-Me (entries 10 and 11) the crossed aldol product
was obtained as sole product (Scheme 3). Further investi-
gation revealed that, initially obtained Mannich adducts
underwent removing of an aniline molecules to give aldol
product. This can be attributed to steric repulsion which
was introduced by Cl and Me groups on ortho position.
Another switching in selectivity was shown when cyclo-
heptanon was used as ketone component in Mannich type
reaction in the presence of polymeric laponite nanoclay
under solvent free conditions, which afforded the corre-
sponding β-amino ketones in good yields with excellent
syn selectivity (entries 12–14). The structures of all prod-
ucts were established by NMR in compared with reported
ones in the literature.
Scheme 3 Crossed aldol
condensation in the case of
ortho-substituted anilines
1 3

J IRAN CHEM SOC
Scheme 5 Crossed aldol condensation of cyclopentanone with ben-
zaldehyde
the relative configuration of the two stereocentres is R,S.
While, in the case of Mannich base II, the major isomer
has syn stereochemistry with 55.55 (19)° and 175.01 (14)°
torsion angles of C7─C1─C8─N1 and C2─C1─C8─N1,
respectively, and the relative configuration of the two ste-
reocentres is R,R.
Scheme 4 Proposed transition states in Mannich reaction of
cyclohexanone and cycloheptanone
The plausible reaction mechanism involves the nucleo-
philic addition of enol form of the ketones onto the in situ
generated imines. As shown in Scheme 4, reaction of
cyclohexanone with imines a, derived from moderately
electron-donating and electron-withdrawing substituted
benzaldehydes, proceeded via transition state A, in which
the minimal steric repulsions are exist and led to anti ste-
reoisomers [39]. While, in the case of strongly electron
donating substitutions (OH and Me2N) at para position
of benzaldehyde, enamine b is participated in the reaction
and the transition state B is overcoming, and syn isomer
is produced as major stereoisomer. In the case of cyclo-
heptanone, also two transition states are possible; C led
to anti and D afforded syn stereoisomer. Due to the steric
repulsion between aryl group of imine and methylene of
cycloheptanone ring in the transition state C, reactions pro-
ceeded via transition state D, and afforded syn stereoisomer
as major product.
In the case of cyclopentanone, reactions occurred rapidly,
and chemoselectively, corresponding crossed-aldol products
were obtained in high yields (Scheme 5). This can be attrib-
uted to the high acidity of the proton adjacent to the carbonyl
group of cyclopentanone that underwent elimination reaction.
An attempt to Mannich-type reaction of n-butanal, an
aliphatic aldehyde, and aniline with cyclohexanone in the
presence of catalytic amount of polymeric laponite nano-
clay at room temperature was failed. Similarly, Mannich
reactions of aldehydes and anilines with acyclic ketones
such as acetone and acetophenone derivatives were investi-
gated. The overall reaction is shown in Scheme 6.
Conclusion
In summary, a new heterogeneous catalytic system based
on HPMC-laponite nano composite was developed for
three-component direct Mannich type reactions of alde-
hydes, anilines, and ketones at room temperature under sol-
vent-free conditions. Reactions of cyclic ketones exhibited
interesting chemoselectivity and stereoselectivity depend-
ing on the ring sizes of ketones and nature of the substitu-
ent on the benzaldehydes. In the case of cyclopentanone,
chemoselectively, aldol condensation reaction took place
An ORTEP view of 2-[(4-chlorophenylamino)(p-tolyl)
methyl]cyclohexanone
I
and 2-[phenyl(phenylamino)
methyl]cycloheptanone II with atomic labelling is shown
in Fig. 3. The X-ray single crystal structures analysis
revealed that the major isomer of I possesses anti stereo-
chemistry, with 173.44 (18)° and −66.4 (2)° torsion angles
of C6─C1─C7─N1 and C2─C1─C7─N1, respectively, and
Fig. 3 ORTEP representations
(45 % probability level) of the
crystal structures of 2-[(4-chlo-
rophenylamino)(p-tolyl)
methyl]cyclohexanone I, and
2-[phenyl(phenylamino)methyl]
cycloheptanone II
1 3

J IRAN CHEM SOC
Scheme 6 Mannich-type reac-
tion of acyclic ketones
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tion with cyclohexanone and cycloheptanone led to the for-
mation of corresponding Mannich adducts. In the case of
Mannich reaction of cyclohexanone, stereoselectivity was
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