Journal of the American Chemical Society
Communication
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substrates for our catalytic system, while biological systems
generally cannot oxidize or are inhibited strongly by secondary
amines.14b,16 We believe the mechanistic pathway of our catalytic
system and that of amine oxidases diverge after the formation of
the key hemiaminal intermediate D (Figures 1b and 2).11 In
enzymatic systems, dehydration of amines occurs through
proton transfer (Figure 2a). Two mechanisms for this proton-
transfer process have been postulated, transamination (Figure
2a-(i)) and addition−elimination (Figure 2a-(ii)), and both were
confirmed to be operational depending on the type of amine
oxidase.14b,15b,16a,17 However, both mechanisms can be ruled out
on the basis of our control experiments.
To explain our experimental results, we propose an alternative
mechanism in which the quinone hemiaminal intermediate D is
complexed to the metal NC surface and undergoes hydride
transfer to afford the desired imine (Figure 2b-(i)). The hydride-
transfer mechanism is precedented in metal NC literature and
forms the basis of our proposed mechanism.18 It should be noted
that the oxidation of amines in our reaction could occur through
a single-electron-transfer mechanism (Figure 2b-(ii)), although
the use of radical inhibitors 2,2,6,6-tetramethylpiperidinoxyl and
2,6-di-tert-butyl-p-cresol only slightly diminished the yield of the
desired imine (SI Table 13).11
Finally, we attempted to recover and reuse PI/CB-Pt/Ir and
found that the heterogeneous catalyst could be recovered easily
by filtration and reused up to five times without loss of catalytic
activity (81−87% yield for first through fifth uses; Scheme 2).11
In summary, we have discovered that the combination of
heterogeneous bimetallic Pt/Ir alloyed nanoclusters and 4-tert-
butylcatechol acts as an effective catalytic system for the aerobic
oxidation of amines to imines under mild conditions.
Mechanistic studies suggest that the Pt/Ir NCs and the catechol
derivative work cooperatively to facilitate oxidation of the amines
to imines. We believe this is the first example of a metalloenzyme-
like cooperative catalytic system of metal NCs and a simple
redox-active organic cofactor for an aerobic oxidation reaction.
Further mechanistic studies and application of this catalytic
system to other redox processes are ongoing and will be reported
in due course.
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(11) Please see Supporting Information.
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ASSOCIATED CONTENT
* Supporting Information
Procedures, kinetic studies, and spectra. This material is available
■
S
AUTHOR INFORMATION
Corresponding Author
■
(16) (a) Lee, Y.; Huang, H.; Sayre, L. M. J. Am. Chem. Soc. 1996, 118,
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Notes
The authors declare no competing financial interest.
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(b) Mure, M.; Mills, S. A.; Klinman, J. P. Biochemistry 2002, 41, 9269.
(18) Conte, M.; Miyamura, H.; Kobayashi, S.; Chechik, V. J. Am. Chem.
Soc. 2009, 131, 7189.
(19) Alternatively, the rate-determining step could be derived from the
oxidation of D in which the intermediate is oxidized without the
involvement of the Pt/Ir NCs. We thank a reviewer for making this
suggestion.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of 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.
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