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Communication
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the inability of 1 to transfer hydride to the imine and
demonstrates that initial iminium generation is essential.
Moreover, this observation implies that the mechanism of H2
activation does not involve hydride migration from B to the
CNHC of 1 to generate a neutral boron−nitrogen FLP, but it is
consistent with a mechanism that necessitates the generation of
a Lewis acidic borenium ion. The activation of H2 proceeds
slowly, while the reaction of 1 and the iminium salt [PhCH
NHtBu][B(C6F5)4] results in immediate hydride transfer.
These preliminary data imply that H2 activation is rate-
determining, although a detailed mechanistic study is ongoing.
In summary, we have demonstrated that a borenium cation
can be employed as a Lewis acid in an FLP to activate H2. This
borenium cation, which is readily obtained from an air-stable
precursor, is a new, highly active metal-free catalyst for the
hydrogenation of imines and enamines at room temperature.
Moreover, the borenium cation hydrogenation catalyst exhibits
improved selectivity and functional group tolerance relative to
existing FLP catalysts. The required Lewis acidity for hydrogen
activation is derived from the cationic charge at boron rather
than the incorporation of fluorinated substituents. This finding
provides a new family of readily accessible Lewis acids for FLP
chemistry and catalysis. Accordingly, systematic modifications
of the borenium cation catalysts for chemo-, regio-, and
stereoselectivity are currently under intense study.
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̈
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental and crystallographic data. This material is
(7) (a) De Vries, T. S.; Prokofjevs, A.; Harvey, J. N.; Vedejs, E. J. Am.
Chem. Soc. 2009, 131, 14679. (b) De Vries, T. S.; Vedejs, E.
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(i) Bonnier, C.; Piers, W. E.; Parvez, M.; Sorensen, T. S. Chem.
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AUTHOR INFORMATION
■
Corresponding Author
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.W.S. gratefully acknowledges the financial support of NSERC
of Canada and the award of a Canada Research Chair. J.M.F. is
grateful for the support of an OGS Scholarship.
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