Iridium(I)-catalysed tandem hydrosilylation-protodesilylation of imines

Article abstract of DOI:10.1002/ejoc.200500168

In the presence of alkylsilanes, the cationic Ir^I complex [{Ir[bis(pyrazol-1-yl)methane](CO)₂}BPh₄] (1) catalyses the reduction of a range of imines, including N-alkyl and N-aryl imines, and both aldimines and ketimines. Excellent conversions directly to the product amines are achieved rapidly at room temperature in methanol solution. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejoc.200500168

SHORT COMMUNICATION
Iridium(
I
)-Catalysed Tandem Hydrosilylation-Protodesilylation of Imines
Leslie D. Field,[a] Barbara A. Messerle,*[b] and Sarah L. Rumble[b]
Keywords: Hydrosilylation / Imines / Iridium / Solvent effects / Homogeneous catalysis
In the presence of alkylsilanes, the cationic IrI complex
[{Ir[bis(pyrazol-1-yl)methane](CO)2}BPh4] (1) catalyses the
reduction of a range of imines, including N-alkyl and N-aryl
imines, and both aldimines and ketimines. Excellent conver-
sions directly to the product amines are achieved rapidly at
room temperature in methanol solution.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Introduction
The catalysed reduction of imines is a useful method for
the synthesis of primary and secondary amines. The amine
products of the reductions are often useful building blocks
for biologically and synthetically important compounds.[1]
Hydrosilylation of imines, in which the Si–H bond is added
across the C=N bond, is an attractive alternative approach Figure 1. [{Ir(bpm)(CO)2}BPh4] [1; bpm = bis(pyrazol-1-yl)meth-
ane]
to the hydrogenation of imines as it is experimentally sim-
ple, does not require high pressure or temperature, and
makes use of readily available silanes. Most of the effective
catalysts for imine hydrosilylation are transition metal com-
Results and Discussion
plexes, using metals including Rh,[2–4] Ru,[5,6] Ti,[7,8] Cu,[9]
and Ni[10] however a recent report, using high throughput
screening methods, also identified a number of new Sn and
Zn catalysts.[11] There have been only limited reports of im-
ine hydrosilylation employing complexes of Ir,[12] although
Ir complexes are known to promote the hydrogenation of
imines.[13–22] The development of catalysts which are ef-
ficient for both N-aryl and N-alkyl imine substrates, as well
as aldimines and ketimines, remains an important goal for
hydrosilylation research.
The hydrosilylation of the acyclic aldimine N-(benzyl-
idene)aniline (2a) was explored using [{Ir(bpm)(CO)2}-
BPh4] (1) as catalyst and the tertiary silane triethylsilane
(Et3SiH) as reducing agent in both [D8]THF and CD3OD
solvents (Scheme 1). In [D8]THF, a moderate conversion
was achieved (52% after 24 h at 60 °C), giving a 1:1 mixture
of the N-silylamine 3a and the desilylated amine 4a. On
addition of water to this product mixture, in the presence
of catalyst, almost quantitative conversion to the desilylated
product was obtained in 2 h.
Recently we reported the efficient hydrosilylation of a cy-
clic imine, 2-methyl-1-pyrroline, catalysed by the cationic IrI
dicarbonyl complex [{Ir(bpm)(CO)2}BPh4] [1; bpm = bis-
(pyrazol-1-yl)methane], as well as the use of this catalyst for
a tandem hydroamination/hydrosilylation procedure, which
indicated its potential as an imine hydrosilylation cata-
lyst.[23] We report the catalytic activity of this IrI complex
in the hydrosilylation of imines, including acyclic and cyclic
imines (Figure 1).
[a] School of Chemistry, University of Sydney,
NSW 2006, Australia
Fax: +61-2-9351-6650
E-mail: L.Field@chem.usyd.edu.au
[b] School of Chemistry, University of New South Wales,
NSW 2052, Australia
Fax: +61-2-9385-6141
E-mail: b.messerle@unsw.edu.au
Scheme 1.
Eur. J. Org. Chem. 2005, 2881–2883
DOI: 10.1002/ejoc.200500168
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2881

L. D. Field, B. A. Messerle, S. L. Rumble
SHORT COMMUNICATION
When the solvent was changed to methanol, a marked hour for all of the acyclic imine substrates examined, and
rate enhancement was observed, with quantitative conver- only 1.5 hours for the cyclic indolenine. The silylated amine
sion to the desilylated amine 4a achieved in less than 10 min intermediates 3 were not observed in any of the reactions
at room temperature (in air).
conducted in alcohol solution. In the absence of a metal
The efficiency of the hydrosilylation in methanol solvent catalyst there is only very minor conversion (Ͻ10%) of im-
for a representative set of imine substrates 2a–c and 5 is ine to amine after 30 min under identical reaction condi-
given in Table 1 with 2 equivalents triethylsilane in the pres- tions.
ence of 1 mol-% complex 1 in CD3OD at room temperature
(in air). The efficiency of conversion of the N-phenyl imines
was higher than for the other imines studied.
By reducing the catalyst loading to 0.5 mol-% and re-
cording the first NMR spectrum at 2 min a turnover fre-
quency can be estimated for the hydrosilylation of ketimine
2b. After 2 min the reaction had proceeded to 83% conver-
sion, giving an initial turnover frequency of approximately
5000 h–1. The results in Table 1 clearly indicate that in
methanol solvent, the complex 1 is an efficient catalyst for
the reduction of aldimines and ketimines, as well as N-aryl
and N-alkyl imines.
Table 1. Efficiency of imine hydrosilylation using Et3SiH, catalysed
by [Ir(bpm)(CO)2][BPh4] (1), in methanol solvent.
Mechanistically, since only the desilylated amine is ob-
served in methanol solvent, the reaction could proceed in a
tandem fashion by hydrosilylation of the imine to give an
intermediate N-silylamine, followed by a rapid methanolysis
of the nitrogen–silicon bond. Nitrogen–silicon bonds are
generally labile and silyl-protected amines are frequently de-
protected with methanol or water or dilute acid. This rea-
soning is consistent with the fact that the N-silylamines are
observed as the predominant reaction products when the
reaction is carried out in the absence of a protic solvent.
The reaction could also proceed by direct hydrogenation of
the imine by hydrogen gas, produced in situ from the reac-
tion between the silane and methanol since complex 1, and
related complexes, are known to catalyse the very rapid sil-
anolysis of hydroxy groups in alcohols with the formation
of H2 and silyl ethers.[24] The iridium complex 1 does act as
a catalyst for the direct hydrogenation of imines.[25] How-
ever, when the hydrosilylation reaction is conducted in
CD3OD as a solvent, there is no incorporation of deute-
[a] Quantitative conversion. This is defined as Ͼ99% conversion.
This is the time where no remaining substrate peaks are evident in
1
the 300 MHz H NMR spectrum of the reaction mixture. [b] The
first NMR spectrum was recorded at 10 min, at which point these
reactions had gone to completion. Therefore the actual time to
completion may be significantly less than 10 min.
2
rium at the imine carbon (by, H NMR) and this indicates
that the products are not formed by reduction of the imine
with HD which would be formed in the reaction between
The reduction reaction is efficient and clean with Ͼ98% CD3OD and Et3SiH. A third viable alternative is that meth-
conversion to the desilylated amine product in less than 0.5 anol reacts directly with a metal-bound species in the cata-
Figure 2. Proposed mechanism for hydrosilylation of an imine in methanol.
2882
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Eur. J. Org. Chem. 2005, 2881–2883

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-
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Experimental Section
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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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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
www.eurjoc.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2883