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hydrolysis to a much lesser extent, after correction with the blank
experiment. These results strongly suggest that gold catalyses the
hydrolysis of N-cyclohexylideneaniline 8 formed from 4. The
possible hydrolysis of cyclohexanimine 7 must not be excluded.
The activation of imine 8 requires an acid resistant to hydro-
genation. However, many Lewis metal cations, able to catalyse the
hydrolysis of 8, will be rapidly reduced under the strong hydro-
genating conditions of the reaction. Br o¨ nsted acids are of course
Fig. 6 (A) Fluorescence measurements of the reaction mixture for the
formation of cyclohexanone oxime 2 under Au/C and Pd/C-catalysed
reluctant to hydrogenation, but independent experiments with conditions. (B) Corresponding UV-Vis spectrum at 2 h.
acetic, sulfuric and p-toluenesulfonic acid, besides HCl, showed
low amounts of cyclohexanone 9 (Table S3, ESI†). In contrast, it
appears that gold nanoparticles possess certain Lewis acidity even multi-step cascade reaction that involves: (1) hydrogenation of 1 to
under hydrogenating conditions and, for instance, it has been aniline 3; (2) further hydrogenation of 3 to cyclohexanamine 6; (3)
reported that gold-supported nanoparticles are able to activate coupling of 3 and 6 by the hydrogen-borrowing mechanism to form
24
imines towards hydroalkynylation. However, the formation of amine 4; (4) formation and hydrolysis of the imine 8 to generate
the imine on supported-Pd sites makes occurrence of the Lewis cyclohexanone 9 and aniline 3 that is recycled for the coupling; and
activation of the imine in Au-supported sites difficult since both (5) formation of oxime 2. The cascade reaction can potentially be
sites are separated. Thus, study on whether one of the catalysts for extended since the production of hydroxylamine hydrochloride is
the oxime formation, palladium or gold, may have free movement based on the Pd/C-catalysed hydrogenation of ammonium nitrate
in the reaction or, in other words, be soluble, was performed. For and the latter is produced from nitric acid and ammonia (Fig. S6,
this, we carried out the formation of oxime from 8 with catalytic ESI†), all these chemicals being used in the process reported here.
amounts of palladium(II) chloride, gold(I) chloride, soluble palla-
dium nanoparticles, and gold nanoparticles (see Fig. 5C) but none
25–27
Notes and references
of them improved the formation of oxime from 8.
reaction occurred to a good extent when we used a sample of
However, the
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28
mely high catalytic activities for hydration reactions and since
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28
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nanometric gold clusters leached into solution are responsible
for the last catalysed step of the transformation of nitrobenzene
2
1
to cyclohexanone oxime 2, namely the hydrolysis of imine 8.
In summary, we have described here the synthesis of cyclo-
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