C O M M U N I C A T I O N S
Figure 3. Proposed mechanism for the catalysis.
the amine bound in the cavity to the hydroxylamine. Oxidation of
the hydroxylamine to the nitroso compound and further oxidations
occur as shown in Scheme 1.
The catalysts 2R and 2â are remarkably good artificial enzymes
for amine oxidation. These results are very encouraging both for
extending the reaction to other oxidations and using 2R or 2â in
selective synthesis.
Figure 1. Kinetic constants for the oxidation of various substrates in the
presence of 2R or 2â and 570 mM hydrogen peroxide (pH 7.0, 25 °C). All
amines were oxidized to the corresponding nitro derivatives, except 12,
which was oxidized to azo- and azoxy- compounds. Details are found in
Supporting Information, Table S1. * indicates reaction conditions: 36 mM
H2O2, pH 8.0 @ at 60 °C.
Acknowledgment. We thank the Lundbeck and Villum Kahn
Rasmussen foundations and the SNF(FNU) for support.
Supporting Information Available: Procedures for kinetic experi-
ments, Hanes plots, etc. This material is available free of charge via
References
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Figure 2. Plot of kcat vs pH for oxidation of 3 and 5 catalyzed by 2â. The
T ) 25 °C and [H2O2] ) 36 mM for 5 and 72 mM for 3.
hydroperoxide is somewhat less efficient. With oxone as oxidant,
no catalysis was observed. The pH dependency in the area where
2â is stable was also investigated (Figure 2). The rate of oxidation
of 3 increases somewhat at the higher pH, while the oxidation of
5 appears independent of pH. The rate of the catalyzed reaction
depends little on the hydrogen peroxide concentration (Figure S4,
Supporting Information) and works well in the entire [H2O2] range
from 5.7 to 570 mM.
Cyclodextrin 2â does not catalyze epoxidation of styrene in the
presence of H2O2, as it does in the presence of oxone.5c This is
taken as evidence for dioxiranes not being intermediates in these
amine oxidation reactions.
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We therefore propose the mechanism in Figure 3. Hydrogen
peroxide reacts with the ketone to form the hydroperoxide adduct.
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