Catalysis Science & Technology
Paper
retained under set hydrogenation conditions with a 98.6%
maximum formation of 4-bromo-N-phenylhydroxylamine.
4-Nitrobenzonitrile was chosen as it contains another
electron-withdrawing, but reducible, substituent (Table 4,
entry 4). Full conversion was reached in only 13 minutes with
a maximum N-AHA yield of 98%.
4-Nitroanisole is more challenging due to the electron-
donating methoxy substituent, making the nitro functional
group less electrophilic. A lowered electrophilicity means
lower activity when hydrogenated with the strongly nucleo-
philic Pt catalyst. For this reason, 96 minutes were required
for full conversion of 4-nitroanisole with a corresponding
maximum yield of 91.4% 4-methoxy-N-phenylhydroxylamine
(Table 4, entry 5).
Entry 6 in Table 4 shows the high chemoselectivity for
the nitro group in the hydrogenation of 3-nitrostyrene. An
89.3% yield of the corresponding N-AHA was obtained
without much hydrogenation of the CC bond (7.5% of
3-ethylaniline is formed).
This selected group of nitroaromatics was chosen, but
we believe that a much broader group of substituted
N-phenylhydroxylamines could be synthesized with high yields.
Finally, a “hot filtration” experiment was carried out to
exclude possible leaching of large amounts of platinum into
solution. At high pressure and in the presence of TMEDA,
the reactivity in terms of nitrobenzene hydrogenation
stopped completely after catalyst filtration from the reaction
solution and no sign of any additional reactivity was observed
(see the ESI†).
from the surface by amines and a high concentration of
molecular hydrogen.
With this knowledge, the conditions were optimized in
order to convert a variety of nitroarene substrates to N-AHAs.
Experiments performed at high H2 pressure (50 bar) with
TMEDA as amine additive resulted in excellent (more than
90%) N-AHA yields.
Acknowledgements
We would like to thank BASF Nederland B.V. for their
financial support and scientific contribution to this work.
Notes and references
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