R. A. da Silva, L. W. Bieber / Tetrahedron Letters 51 (2010) 689–691
691
dride species to continue the reaction. This cyclic catalytic process
is analogous to the related reductive amination of aldehydes cata-
lyzed by Ir-complexes in aqueous acid15 and organic medium16
where triethyl silane was used as the hydride donor. Also, a re-
cently described reductive amination by Ir-catalyzed hydrogena-
tion in organic solvents or ionic liquids at high temperature may
proceed by a similar mechanism.17
In comparison with the last three procedures, the reductive
amination described in the present work is characterized by an ex-
tremely simple and safe experimental procedure with no side
products. The method employs aqueous solvent, open atmosphere,
room temperature, inexpensive reagents, and simple Ir-salts as cat-
alysts in a very low concentration (5 ꢀ 10ꢁ5 mmol). All these fea-
tures fulfill the basic principles of green chemistry.
Acknowledgments
This work was supported by fellowships and grants from CAPES
and CNPq (Brasília). We thank Mr. R. M. Srivastava for revision of
the manuscript.
References and notes
Scheme 1. Possible mechanism of reductive alkylation catalized by Ir.
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tected. Consequently,
a low-valent iridium hydride species
produced by Zn-reduction in acidic medium can be envisaged
(Scheme 1). After the transfer of the hydride to iminium ion, Ir2+
is recycled to Ir0 by Zn-reduction and generates a new iridium hy-