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Table 3 Recovery and reuse of the catalyst
Science (JSPS), Global COE Program, The University of Tokyo,
MEXT, Japan, and NEDO. We also thank Mr Noriaki Kuramitsu
(The University of Tokyo) for STEM and EDS analysis.
Notes and references
a
1
Determined by H-NMR with respect to tetrachloroethane as internal
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THF–TFE (4 : 1) to afford the desired imine 3ga as a pure form
despite the instability of the aliphatic imine (Table 2, entry 9).
With regard to the amine part, most of the desired N-alkyl
imines were prepared successfully in excellent yields (Table 2,
entries 12–14). When aniline derivatives were used as amine
coupling partners, the oxidation step became slower under
standard conditions. To improve the reactivity, we conducted
the reaction at 60 1C in THF–TFE (4 : 1), and N-aryl imines were
obtained in excellent yields (Table 2, entries 15–18).
As previously reported,17c the combination of Au and Pd
generates bimetallic alloy NPs. The formation of the alloy
structure from the two metals gives NPs with different proper-
ties from those of the individual metal NPs: Au-NPs and Pd-
NPs. We can expect that the Pd in the alloy NPs expresses more
electropositive properties and Au expresses more electronega-
tive properties because of their electronegativities. PICB-Au/Pd
obtained in this way has a strong Lewis acid character on the Pd
part, resulting in an acceleration of the dehydration step with
the higher acidic character of TFE in the solvent, which
explains why high selectivity in favor of imine products is
obtained with the PICB-Au/Pd catalyst. Furthermore, as we have
shown previously, the Au/Pd alloy NPs are more active in
aerobic oxidation of alcohols than single-metal Au-NP catalysts,
and this might operate in favor of imine formation.15c
Finally, we performed recovery and reuse studies with the
PICB-Au/Pd catalyst for the direct imine formation with
4-methylbenzyl alcohol 1a and benzylamine 2a and found that
the catalyst could be reused at least five times without notice-
able loss of catalytic activity (Table 3). Pretreatment of the PICB-
Au/Pd was required to reactivate the catalyst, similar to the case of
amidation reactions.19 This pretreatment consisted of washing with
pure water and THF, followed by heating at 170 1C for 4 h without
affecting the catalyst morphology.
In summary, we have achieved the selective formation of
imines from alcohols and amines using TOPs under mild condi-
tions. The PICB-Au/Pd heterogeneous catalyst showed high activ-
ity and selectivity for this transformation and this catalyst can
easily be recycled several times without loss of activity. The
method has been generalized and a wide variety of substrates
has been tested (benzylic, allylic, propargyl and aliphatic alcohols;
primary amines; aniline); these have shown excellent activity in
this coupling and the imine products can be obtained as an
analytically pure product after simple filtration to remove the
catalyst and an aqueous workup to remove the inorganic com-
pounds. We are now studying further applications of this catalytic
system and conducting mechanistic studies.
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 355--357 357