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here operates at room temperature (B100 1C lower than homo-
geneous catalysts previously reported) directly from nitro-
derivatives. This strategy constitutes, therefore, a cost-effective
and environmentally-friendly methodology to prepare secondary
amines. In addition, this process exemplifies the concept of
modern catalysis based on the identification and optimization
of the active sites on reusable solids for atom-economical
domino reactions.
Financial support by Consolider-Ingenio MULTICAT 2010,
subprograma de Apoyo a Centros y Universidades de Excelencia
Severo Ochoa (SEV 2012 0267), and MAT2009-00889 projects
from MICINN is acknowledged. P. R.-M. thanks the Ministry of
Education for a concession of a FPU contract. A. L. P thanks ITQ
a
Fig. 3 Dependence on the catalytic activity of Pd/C on the particle size. The
Pd/C (c) catalyst presents a bi-modal particle size distribution with a minor number
of small (o3 nm) Pd particles that account for the catalytic activity observed. HR-TEM
images of each palladium/carbon: (A) Pd/C; (B) Pd/C (b) (C) Pd/C (c).
´
for a contract. We thank Dr J. C. Hernandez-Garrido for the
microscopic images.
results suggest that the reaction proceeds on the surface of very
small nanoparticles with a significant number of exposed
palladium atoms.
The process was scaled-up in the laboratory and very good
yields and selectivity were still found at the gram scale (Table S2,
ESI†). The possible leaching of metals during the reaction was
studied by the filtration test and the results show that no active
catalyst species in solution were released during the process
(Fig. S4, ESI†), and recycling of the catalyst was also successful
at the gram scale (Table S3, ESI†).
Notes and references
‡ A typical reaction procedure for the synthesis of cyclohexylaniline 5
from nitrobenzene 2: Pd/C (5 wt%, 21.2 mg, 0.01 mmol of Pd) and
hexane (0.5 mL) were placed into the reactor (2 mL capacity) equipped
with a magnetic stirrer. Nitrobenzene 2 (21 mL, 0.2 mmol) and methane-
sulfonic acid (10 mL, 0.15 mmol) were added, and after the micro-
reactor was sealed, air was purged by flushing out four times with
hydrogen and then pressurized with 6 equivalents of H2 (B10 bar). The
resulting mixture was magnetically stirred overnight under static pressure
at room temperature. During the experiment, the hydrogen pressure
was decreased as the reaction proceeds. The product composition was
determined by means of GC, once the catalyst particles were removed
from the solution by filtration. The products were identified using
GC-MS and also by 1H NMR and 13C NMR.
Proton Nuclear Magnetic Resonance (1H-NMR) analyses of
the liquid phase after the reaction showed the presence of
ammonia as ammonium salt (Fig. S5, ESI†), which supports the
hydrogen-borrowing mechanism shown in Fig. 2, but opens the
question if the role of the acid is activating the imine or
quenching ammonia, or both. To differentiate those possibilities,
a solid substitute of methanesulfonic acid such as Amberlyst A15,
used in separated particles to those of Pd/C, was introduced as
a solid acid. Since catalysis operates on the solid surface, the
use of a separated solid acid should hamper the reaction yield
by the restricted contact between solids. Indeed, it was found
that the yield of 5 (Table S4, ESI†) strongly decreased in the
presence of Amberlyst A15, supporting the hypothesis that
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c
8162 Chem. Commun., 2013, 49, 8160--8162
This journal is The Royal Society of Chemistry 2013