Functionalization of amines by 'one pot-free solvent' reductive alkylation with a recyclable catalyst

Article abstract of DOI:10.1016/S0040-4039(00)01098-4

The ability to attach a preformed rhodium and iridium homogeneous catalyst to a support would have a distinct advantage over the 'one pot-free solvent' reductive alkylation of primary amines to form IPPD and DMPPD. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01098-4

Tetrahedron Letters 41 (2000) 6583±6588
Functionalization of amines by `one pot±free solvent'
reductive alkylation with a recyclable catalyst
Ramon Margalef-Catala, Carmen Claver, Pilar Salagre* and Elena Fernandez*
Á Â
Departament de QuõÂmica FõÂsica i Inorg aÁ nica, Universitat Rovira i Virgili, Pl. Imperial T aÁ rraco 1,
3005 Tarragona, Spain
4
Received 20 March 2000; accepted 11 May 2000
Abstract
The ability to attach a preformed rhodium and iridium homogeneous catalyst to a support would have a
distinct advantage over the `one pot±free solvent' reductive alkylation of primary amines to form IPPD
and DMPPD. # 2000 Elsevier Science Ltd. All rights reserved.
Environmentally benign processes which avoid the handling and generation of toxic materials
are prompting the chemical industry to develop new chemical reactions. Heterogenized
homogeneous catalysts obtained from organometallic complexes immobilized in solids such as
polymers or metal oxides,1,2 have become important in various areas of organic synthesis because
they are environmentally compatible, and they give good yields and selectivities. Recently,
layered compounds such as clay minerals and in particular smectites (e.g. montmorillonite whose
surface area, cation exchange capacity and swelling make it suitable to support catalysts) have
attracted considerable attention.3
�13
Much e€ort has been devoted towards the development of strategies for synthesizing N-phenyl-
0
0
N -isopropyl-p-phenylendiamine (IPPD) and N,N -di(1,4-dimethylpentyl)-p-phenylendiamine
(
14�22
DMPPD), which are used as antioxidant additives in rubber vulcanizers,
additives in biomimetic
sensors, anticorrosive protectors of metals and antiozonant agents.25,26 At present, a suitable
one pot±free solvent' synthetic procedure for producing the diamines IPPD and DMPPD with
23
24
`
27
heterogeneous catalysts based on Pt/C is being applied on an industrial scale. However, from a
synthetic standpoint one of the greatest challenges of this approach is to make the activity and
selectivity of the heterogeneous Pt/C catalyst reproducible.
Our interest in this area is to develop an alternative approach to that synthetic pathway in
which a primary amine is converted into a secondary amine by imine intermediate formation, but
being catalyzed by preformed rhodium or iridium complexes immobilized on montmorillonite.
*
Corresponding authors.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01098-4

6
584
This process is particularly attractive because the transformation takes place in a single step and
under milder and solvent free conditions.
The economic and technological factors involved in any large-scale chemical manufacture are
also taken into account in the proposed process because the immobilization of the transition
metal catalyst can allow an easy recovery and reusablility for further reactions.
The proposed `one-pot' reaction takes place as follows: the condensation reaction of ketones
with primary amines implies an addition in the ®rst step in which the basic nitrogen of amine
adds to the carbonyl carbon of ketone. The ensuing intermediate loses a water molecule to
generate the imine in the second step. Finally, in the third step, the imine intermediate undergoes
catalytic hydrogenation to yield the secondary amine.
Scheme 1.
Table 1
One-pot reductive alkylation of 4-aminediphenylamine 1a

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Each step in the process is well-documented in the literature: the synthesis of imines from
primary amines and aldehydes in the presence of montmorillonite K10, (referred to below as
2
8,29
MM-K10), gives high yields of the CˆN product and water elimination.
A more recent
approach reduces the considerable excess of support and shortens reaction time using microwave
irradiation.30 Heterogenized homogeneous catalytic hydrogenation of aldimines by removable
and reusable iridium complexes immobilized on montmorillonite clay has been reported by our
group as a highly ecient system to obtain secondary amines, conducting solution-like reactions
in the solid state and minimizing many of the barriers associated with the unremovable
homogeneous solution catalysts.
31
To test this chemistry, we examined the reaction between 4-aminediphenylamine 1 and acetone,
which have the double role of reagent and solvent of the reaction, therefore reducing the
employment of ecologically suspected solvents (Scheme 1). The preliminary experiments were
carried out in the presence of preformed cationic homogeneous catalysts so that we could determine
whether the rhodium and/or iridium systems were suitable for our objectives.
Overall reaction was obtained for the diamine IPPD
Rh(COD)(PPh ) ]BF homogeneous catalytic system was used, and the conversions and selec-
3
formation when the
[
tivities were even better than its iridium analogue complex [Ir(COD)(PPh ) ]BF and the hetero-
3
2
4
3
2
4
geneous catalytic system Pt/C, which was also checked under the same reaction conditions in our
laboratories (Table 1). It is noteworthy that no ketone reduction or imine hydrolysis were
observed during the reaction time.
The cationic rhodium complex was thus immobilized on the clay under N at room temperature
2
by stirring [Rh(COD)(PPh ) ]BF and commercial MM-K10 in dichloromethane for 24 h. Powder
3
2
4
X-ray di€raction of the resulting solid and conductimetric analysis of the solution suggested that
Scheme 2.

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586
Table 2
One-pot reductive alkylation of p-phenyldiamine 4a
31
the rhodium complex was mainly adsorbed on the external surface rather than intercalated in
the internal surface.9
To compare the activity and stability of the supported catalyst [Rh(COD)(PPh ) ]BF /MM-K10
,32
3
2
4
with its homogeneous counterpart, the `one pot' reductive alkylation of 4-aminediphenylamine 1
was carried out under the same standard conditions. The activity of the heterogenized catalyst
was found to be slightly higher than that of the homogeneous system, which could be attributed
to the fact that immobilization of the active catalyst on a rigid support prevents catalytically
inactive species from forming as a result of oligomerization reactions.33 The recovered support
catalyst [Rh(COD)(PPh ) ]BF /MM-K10 can be reused in consecutive reductive alkylations
3
2
4
showing a loss in activity and consequently in selectivity in 3 between runs 1 and 2, although the
total conversion and diamine percentage were constant in the following runs. Leaching of
rhodium complex is not responsible for the observed catalytic behaviour, as the ®ltrate from the
active catalyst suspension is completely inactive.
To con®rm the generality of this process, we faced the problem of the reductive alkylation of p-
phenyldiamine 4 with methyl-isoamyl-ketone, in the presence of iridium and rhodium catalytic
0
systems. Unlike the synthesis of IPPD, the production of N,N -di(1,4-dimethylpentyl)-p-phenyl-
enediamine (DMPPD) 9 may be accompanied by the formation of a considerable number of
intermediates (Scheme 2).

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587
Table 2 shows the results of the reductive alkylation of 4 with the homogeneous and heterogenized
rhodium and iridium catalysts. Interestingly, both the rhodium and iridium homogeneous
catalysts behave similarly although the selectivity on DMPPD 9 is higher in the case of
[
Rh(COD)(PPh ) ]BF (up to 88%). Signi®cant di€erences in the activities and selectivities were
3 2 4
observed when the reductive alkylation took place with the `one pot' heterogenized catalytic
systems [Rh(COD)(PPh ) ]BF /MM-K10 and [Ir(COD)(PPh ) ]BF /MM-K10, (the latter was
3
2
4
3 2
4
immobilized using the same procedure as for the rhodium analogue). Total conversion and
quantitative selectivities on 9 were achieved for these systems (96±98% on 9). On reuse the
catalytic activity of the rhodium-immobilized catalytic system maintained higher conversions and
selectivities than the iridium system which can be explained by the basis of the more active
metallic species being involved in the process for rhodium than iridium systems.
In summary, we reported herein the development of solid-supported transition metal
complexes realizing high catalytic activity in a free solvent media which would provide a safe
resource-saving and environmentally benign process.
Acknowledgements
This work was supported by DGES PB-97-0407-C05-01 and Generalitat de Catalunya DGR
grant 1998FI 00782).
(
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