Iridium(i)-Catalysed Tandem Hydrosilylation-Protodesilylation of Imines
SHORT COMMUNICATION
perature. Yields given are determined directly from the 1H NMR
spectra.
lytic cycle to give the free amine and a methoxysilane. This
pathway would rationalise the marked rate enhancement
which is observed in methanol if the release of product is
the slow step in the reaction Scheme. A possible mechan-
istic Scheme is shown in Figure 2.
Acknowledgments
Experiments are underway to more clearly define the
mechanism of the reaction and to determine the scope of
metal complexes and silanes which are effective in the cata-
lytic reduction of imines. The catalysed hydrogenation of
imines is effective using this catalyst, giving quantitative
yields, and is also being explored as an alternative means
of reduction.[25]
We gratefully acknowledge financial support from the University of
New South Wales, and the Australian government for an Australian
Postgraduate Award (S.L.R.).
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Conclusions
In conclusion, an efficient process for the synthesis of
amines using [{Ir(bpm)(CO)2}BPh4] (1) as an effective cata-
lyst for the reduction of imines has been demonstrated. The
hydrosilylation–protodesilylation of a range of imines af-
forded excellent conversions to the amines in short time
periods on reaction in methanol. The synthesis of enantio-
merically pure amines is also an important goal and studies
of the enantioselective reduction of imines using related me-
tal complexes incorporating chiral N-donor ligands are cur-
rently underway.
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Experimental Section
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Typical Experimental Procedure for Hydrosilylation in THF: The Ir
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complex
1 (3.7 mg, 5.2 μmol) and the imine 3a (47.3 mg,
0.26 mmol) were weighed into a NMR tube fitted with a concentric
Teflon valve. The tube was then evacuated and [D8]THF (0.6 mL)
was vacuum-transferred into the tube. Et3SiH (84 mg, 0.72 mmol)
was injected and the tube was frozen in liquid nitrogen before being
taken to the NMR machine. After rapid thawing, the tube was
inserted into the NMR probe heated to 60 °C and the reaction
progress was monitored by 1H NMR spectroscopy. Yields given are
1
determined directly from the H NMR spectra.
Typical Experimental Procedure for Hydrosilylation in MeOH: The
Ir complex 1 (2.2 mg, 3.1 μmol) and the imine 3a (56 mg,
0.31 mmol) were weighed into a 10-mL round-bottomed flask,
which was then fitted with a rubber septum. CD3OD (0.2 mL) was
added to give a yellow suspension. A silane solution, Et3SiH
(74 mg, 0.64 mmol) in CD3OD (0.8 mL), was injected into the flask
containing the catalyst and imine. All of the solid dissolved and
bubbles of gas were observed. After stirring for 5 minutes, the pres-
sure was released through a needle, the yellow solution was trans-
ferred to the NMR tube and the progress of the reaction was moni-
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2004, 10, 4546–4555.
[22] C. Blanc, F. Agbossou-Niedercorn, G. Nowogrocki, Tetrahe-
dron: Asymmetry 2004, 15, 2159–2163.
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22, 4393–4395.
[24] L. D. Field, B. A. Messerle, M. Rehr, L. P. Soler, T. W. Ham-
bley, Organometallics 2003, 22, 2387–2395.
[25] The Ir complex 1 (5.6 mg, 7.8 μmol) and the imine 3a (71 mg,
0.39 mmol) were dissolved in THF (2 mL) and stirred at 60 °C
in a stainless steel bomb under a hydrogen pressure of 120 psi
for 20 min. The amine 4a was formed in quantitative yield (by
NMR spectroscopy).
1
tored by H NMR spectroscopy. The first spectrum was taken at
10 minutes after the start of the reaction, so that for the reactions in
which quantitative conversion is quoted, reactions may have been
complete before this time. All reactions were done at room tem-
Received: March 7, 2005
Published Online: May 24, 2005
Eur. J. Org. Chem. 2005, 2881–2883
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2883