Montmorillonite K10 as a suitable co-catalyst for atom economy in chelation-assisted intermolecular hydroacylation

Article abstract of DOI:10.1016/S0040-4039(03)00026-1

The clay montmorillonite K10 is an efficient acidic solid that can act as a reusable co-catalyst in the condensation reaction of aldehydes and amines to provide the imine intermediate which is then transformed into the ketone through the hydroiminoacylation reaction, in the presence of rhodium complexes.

Full text of DOI:10.1016/S0040-4039(03)00026-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1631–1634
Montmorillonite K10 as a suitable co-catalyst for atom economy
in chelation-assisted intermolecular hydroacylation
Xiomara Yan˜ez,a Carmen Claver,a Sergio Castillonb,* and Elena Fernandeza,*
aDepartament de Qu´ımica F´ısica i Inorga`nica, Universitat Rovira i Virgili, Pc¸a. Imperial Ta`rraco 1, 43005 Tarragona, Spain
bDepartament de Qu´ımica Anal´ıtica i Orga`nica, Universitat Rovira i Virgili, Pc¸a. Imperial Ta`rraco 1, 43005 Tarragona, Spain
Received 25 November 2002; accepted 20 December 2002
Abstract—The clay montmorillonite K10 is an efficient acidic solid that can act as a reusable co-catalyst in the condensation
reaction of aldehydes and amines to provide the imine intermediate which is then transformed into the ketone through the
hydroiminoacylation reaction, in the presence of rhodium complexes. © 2003 Elsevier Science Ltd. All rights reserved.
Carbon–hydrogen bond activation through catalysed
hydroacylation reaction is a useful tool to convert
aldehydes into ketones. A key step in this reaction is
the CꢀH bond activation in the aldehyde by a metal
complex to generate an acyl–hydride intermediate.1
However, these intermediates can take two different
pathways: hydrometalation of an olefin to give acyl–
metal–alkyl complexes which eventually provide the
expected ketone through reductive elimination2
(Scheme 1, path a), or decarbonylation to give an
alkane and a metal–carbonyl3 (Scheme 1, path b).
substrate has efficiently suppressed decarbonylation.5
The chelation-assisted intermolecular hydroacylation is
based on the conceptual transformation of chelated
carboxaldimines (usually pyridyl–carboxaldimine) into
carboxketimines through catalytic hydroiminoacyl-
ation.6 In this context a general synthetic method has
been recently developed to obtain, in one step, ketones
from aldehydes in the presence of chelating amines as
additives.7 However, the in situ condensation of alde-
hydes and amines seems to be one of the rate-determin-
ing steps of the global process. The alternatives to
enhance the reactivity in this step involve the formation
of a previous imine compound 1 from aniline and
benzaldehyde (Scheme 2, path b), followed by transimi-
nation with 2-amino-3-picoline towards the car-
boxaldimine 2 (Scheme 2, path b).7 A carboxylic acid
must also be added to catalyse both condensation and
transimination steps. The success of this reaction, there-
fore, depends on a large number of additives and
co-catalysts.
Decarbonylation is not competitive in the intramolecu-
lar hydroacylation,4 but becomes an important limita-
tion in the intermolecular version. In order to stabilise
the acyl–metal–hydride intermediate, the addition of an
excess of ethylene, or carbon monoxide, or vinylsilanes
with Co(I) catalysts, have been used as suitable strate-
gies.2 However, chelation by cyclometalation of the
All this prompted us to search for a way of facilitating
the formation of 2, avoiding additives. As part of our
research8 concerning the use of acidic solids as
catalysts9 to reduce the hazardous materials involved in
chemical processes, we have investigated the hydro-
acylation through hydroiminoacylation of aldimines in
presence of montmorillonite K10 (MK10) clay as acidic
co-catalyst (Scheme 2, path a). We started by examin-
ing the catalytic hydroacylation of benzaldehyde with
1-hexene to give heptanophenone, in the presence of 2
mol% of [Rh(PPh3)3Cl] and 2-amino-3-picoline as the
chelating amine reagent. It must be pointed out, as it
has been reported previously,7 that working at the
Scheme 1.
Keywords: aldehydes; ketones; clays; intermolecular hydroacylation.
* Corresponding author. Tel.: 34-977-558046; fax: 34-977-559563;
e-mail: elenaf@quimica.urv.es
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(03)00026-1

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X. Yan˜ez et al. / Tetrahedron Letters 44 (2003) 1631–1634
Table 1. Influence of the amines in the hydroacylation of
benzaldehyde with 1-hexene catalysed by [RhCl(PPh3)3]
and MK10a
Scheme 2.
reaction temperature of 130°C for 1 h, only 9% of yield
on heptanophenone was observed (Table 1, entry 1).
The use of organic acids as co-catalysts in this reaction
increased the percentage of heptanophenone, although
it seems to be dependent of the strength and amount of
the Brønsted acid. The stronger the acidity of the
co-catalyst, the lower conversion and selectivity on
heptanophenone were observed (Table 1, entry 2 and
3). It seems that p-toluen-sulphonic acid could react
with 2-amino-3-picoline diminishing the amount of the
base required in the hydroacylation reaction. Even the
addition of a larger quantity of benzoic acid does not
contribute to a major conversion of benzaldehyde into
heptanophenone (Table 1, entry 4). On the other hand,
the addition of MK10 provides values of conversion
and percentage of heptanophenone comparable to the
reaction with benzoic acid, (Table 1, entry 5). Montmo-
rillonite K10 is characteristic of a high surface area
(BET area=221 m2/g) and Brønsted acidity centres
(1.4×10−4 meq H+/m2). MK10 efficiently catalyses the
condensation of benzaldehyde and 2-aminopicoline due
to its acidic properties and to the shift of the equi-
librium towards the formation of 2 by removing water
from the reaction media.10 The subsequent rhodium
catalysed hydroiminoacylation converted aldimine 2
into ketimine 3, which was partially hydrolysed in situ
towards the corresponding hydroacylated product, hep-
tanophenone (42%) (Table 1, entry 5). Because of its
acidic properties, MK10 can catalyse aldimine forma-
tion but also ketimine hydrolysis. This methodology
has avoided using aniline and benzoic acid, as well as
forming intermediates 1 and 4.7 Taking into account
how the number of Brønsted acid centres affects to the
catalytic process, we prepared a more acidic clay than
montmorillonite K-10, by acidification of MK10 with
+
HNO3 to provide H-MK10 with 7.7×10−4 meq H+/m2.
However, the conversion significantly decreases, proba-
+
bly due to the interaction of H-MK10 with the reac-
tant picoline (Table 1, entry 6).
Different types of 2-aminopyridine derivatives were
examined to determine how the amine influenced both
aldimine formation through the NH2 functional group
and the chelating ability with the rhodium complex
through the nitrogen atom in the pyridinyl group. Of
all the 2-aminopyridine derivatives studied, 2-amino-3-
picoline contributed to the highest catalytic activity,
followed by 2-amino-5-picoline (Table 1, entries 5 and
8). However those amine derivatives having a methyl
group in meta position with respect to the NH2 func-
tional group provided the lowest conversion (Table 1,
entries 7 and 9). These results contrast with those
observed in a previously reported work,11 where 2-
amino-4-picoline showed the highest catalytic activity.
In addition, it should be pointed out that 2-amino-6-
picoline did not contribute to the hydroiminoacylation,
probably because of the steric hindrance provided by
the 6-methyl group, which disfavoured the formation of
the six-membered ring metallacyclic complex.11
Another interesting advantage of the use of MK10 as
acidic co-catalyst is that it can be easily separated from
the products by a simple filtration. The addition of two

X. Yan˜ez et al. / Tetrahedron Letters 44 (2003) 1631–1634
1633
Table 2. Hydroacylation of benzaldehyde with 1-hexene catalysed by [RhCl(PPh3)3] and MK10a
Entry
MK10 (mg)
2-Amino-3-picoline (mmol)
T (°C)
t (h)
Conv. (%)b
Aldimine 2 (%)b
Heptano-phenone (%)b,c
1
2
3
4
5
6
7
83
83
83
166
333
83
0.75
0.75
0.75
0.75
0.75
0.25
1.25
110
130
130
110
110
110
110
2
2
4
2
2
2
2
80
72
76
67
41
33
81
2
12
12
21
22
0
98
88
88
79
78
100
93
83
7
a Standard conditions: benzaldehyde (2.5 mmol), 1-hexene (12.5 mmol), [RhCl(PPh3)3] (0.05 mmol), MK10: MK10 (83 mg). Solvent: toluene (0.5
mL). T: 110°C. Time: 2 h.
b Percentages determined by GC.
c With the addition of two drops of HCl (1N) to the filtrates.
Finally, one other advantage of this new methodology
is the recycling capacity of MK10 after it is separated
from the products and washed with toluene and
CH2Cl2. As is shown in Figure 2, MK10 can be reused,
drops of HCl (1N) to the filtrates favoured the total
hydrolysis of ketimine 3 towards heptanophenone
(Table 2, entry 1). Similar results are obtained when the
filtrates are purified by column chromatography (SiO2,
n-hexane/ethyl acetate 4/1), to yield pure heptanophe-
none. The literature reports the use of the last proce-
dure (purification in acidic silica), to guarantee the
ketimine hydrolysis towards the ketone.7
The search of best reaction conditions is summarised in
Table 2. The suitable amounts of MK10 and 2-amino-
3-picoline are 83 mg (Table 2, entry 1, 4 and 5) and
0.75 mmol (Table 2, entries 1, 6 and 7), respectively.
Temperature has also been optimised at 110°C, (Table
2, entries 1 and 2), and the reaction time at 2 h, because
longer reaction times do not improve significantly the
conversion of the reaction (Table 2, entries 2 and 3).
Under these conditions the conversion achieved is
about 80% and the selectivity for heptanophenone is
98%. When the reaction was scaled up to 7.5 mmol of
benzaldehyde and 37.5 mmol of 1-hexene, (in the pres-
ence of 250 mg of MK10 and 0.15 mmol of
[Rh(PPh3)3Cl]), similar values were obtained for con-
version and selectivity, 80% and 97% respectively.
Figure 1.
Table 3. Scope of the hydroacylation reaction catalysed
by [RhCl(PPh3)3] and MK-10a
When the concentration of 1-hexene was changed from
12.5 mmol (5 equiv.) to 2.5 mmol (1 equiv.), based
upon benzaldehyde, the conversion and percentage on
heptanophenone dramatically decreased (Fig. 1). Thus
an excess of 1-hexene seems to be required.
We extended the study to explore several other olefins
and aldehydes as substrates in order to determine the
scope of the intermolecular hydroacylation co-catalysed
by MK10 and [Rh(PPh3)3Cl]. All terminal olefins were
hydroacylated in fairly good yields within 2 h (Table 3,
entries 1 and 2). A lower conversion and lower selectiv-
ity towards the corresponding ketone have been found
for the internal olefin cyclohexene (Table 3, entry 3),
which can be related to its higher steric hindrance. It is
worth saying that cyclohexene failed to undergo
hydroacylation in the literature reported.7a Also satis-
factory results in terms of selectivity were obtained by
using as well electron-releasing or electron-withdrawing
aryl substituents on the aldehyde substrate (Table 3,
entries 4 and 5).

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X. Yan˜ez et al. / Tetrahedron Letters 44 (2003) 1631–1634
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ten consecutive runs without a significant loss in activ-
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The results show that intermolecular hydroacylation is
efficiently carried out using MK10 as acidic co-catalyst.
This reduces the number of reactants required and the
intermediates formed, improving the atom economy of
the process. In addition, the co-catalyst can be easily
separated from the products and reused for a significant
number of consecutive runs, without affecting the activ-
ity. Further work on the extension of this methodology
towards other aldehydes and olefins (including not aro-
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Acknowledgements
This work was supported by Ministerio de Ciencia y
Tecnologia (BQU2001-0656). We thank the University
of Pamplona (Colombia) for leave (to X.Y.).
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Lett. 1997, 38, 2039; (b) Dewan, S. K.; Varma, U.; Malik,
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