Mild hydrosilylation of amides by platinum N-heterocyclic carbene catalysts

Article abstract of DOI:10.1002/ejic.201402083

The platinum-catalyzed hydrosilylation of amides to afford amines selectively is reported. By using defined platinum/N-heterocyclic carbene complexes, the reduction of secondary and tertiary amines takes place under mild conditions. The selective catalytic hydrosilylation of amides is reported. By using defined platinum/N-heterocyclic carbene (NHC) complexes, the reduction of secondary and tertiary amines proceeds in good yields under mild conditions. Copyright

Full text of DOI:10.1002/ejic.201402083

SHORT COMMUNICATION
DOI:10.1002/ejic.201402083
Mild Hydrosilylation of Amides by Platinum
N-Heterocyclic Carbene Catalysts
Sabine Pisiewicz,[a] Kathrin Junge,[a] and Matthias Beller*[a]
Keywords: Homogeneous catalysis / Hydrosilylation / Reduction / Amides / Amines / Platinum
The platinum-catalyzed hydrosilylation of amides to afford
amines selectively is reported. By using defined platinum/N-
heterocyclic carbene complexes, the reduction of secondary
and tertiary amines takes place under mild conditions.
amines. Recently, we reported the use of zinc[2]- and cop-
per[3]-based systems for the selective hydrosilylation of sec-
Introduction
The selective reduction of amides to amines is of continu- ondary and tertiary amides. In addition, it was shown that
ing interest for organic chemistry. In general, most of the the combination of the iron carbonyl complex [Fe3(CO)12]
known processes require harsh reduction conditions and and polymethylhydroxysiloxane (PMHS) allowed the
are performed with stoichiometric amounts of metal hydrosilylation of tertiary amides to amines.[4] Furthermore,
hydrides based on boron or aluminum.[1] Alternatively, Nagashima and co-workers demonstrated that iron carb-
catalytic hydrosilylations offer mild and efficient access to onyl complexes catalyzed similar reductions with either
Figure 1. Synthesized Pt–NHC complexes (yields of isolated products are given in parentheses).
[a] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,
1,1,3,3-tetramethyldisiloxane (TMDS) or 1,2-bis (dimeth-
ylsilyl)benzene under thermal or photoassisted condi-
tions.[5,6] Moreover, several elegant protocols with other
transition-metals catalysts based on rhodium,[7] titanium,[8]
Albert-Einstein-Straße 29a, 18059 Rostock, Germany
E-mail: matthias.beller@catalysis.de
http://www.catalysis.de/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402083.
Eur. J. Inorg. Chem. 2014, 2345–2349
2345
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
SHORT COMMUNICATION
ruthenium,[9] iridium,[10] and platinum[11] partly using di- low conversion (Table 1, entries 2–4). Other solvents such as
verse ligands are well described. For example, Nagashima toluene, acetonitrile, Et2O, dioxane, and methylene chloride
and co-workers used commercially available platinum cata- were investigated, but the best results were obtained in
lysts such as Karstedt’s solution and hexachloroplatinic THF.
acid for the hydrosilylation of secondary amides to the cor-
responding amines.[12] Although several catalytic pro-
Table 1. Reduction of N,N-dimethylbenzamide with Ph2SiH2 in the
presence of different Pt catalysts.[a]
cedures for the hydrosilylation of amides have been de-
scribed in recent years, there is still space for improvement
with regard to catalyst efficiency and tolerance of func-
tional groups.
On the basis of the seminal work of Wanzlick[13] and
Ölefe[14] and later on that of Lappert[15] and Arduengo,[16]
Entry
Catalyst
Conv. [%][b]
Yield [%][b]
N-heterocyclic carbenes (NHC) are nowadays frequently
used as ligands in homogeneous catalysis. With respect to
hydrosilylation reactions, the reduction of aldehydes, esters,
imines, ketones, and amides was successfully achieved by
Darcel et al. by using well-defined piano-stool iron–NHC
complexes.[17] Furthermore, Adolfsson and co-workers used
an iron–NHC complex in combination with PMHS and
nBuLi to reduce tertiary amides.[18] Additionally, Kühn and
co-workers synthesized bis-N-heterocyclic carbene rhodium
complexes, which proved to have good activity in the hydro-
silylation of acetophenone.[19] Earlier on, the usefulness of
defined platinum carbene complexes was reported by the
group of Markó for the hydrosilylation of alkenes with high
efficiency and selectivity.[20] Given that the electronic prop-
erties of NCH ligands are similar to those known for elec-
tron-rich phosphines, similar catalytic behavior should also
arise. Because of their increased stability towards heat,
moisture, and oxygen, NHC ligands are a valuable alterna-
tive to phosphine ligands. In comparison to the Karstedt
catalyst, platinum–NHC complexes are more easy to handle
and do not require an argon atmosphere. Additionally, they
do not form platinum(0) nanoparticles during hydro-
silylation, which contaminate the reaction products and are
responsible for the formation of undesired side products.[21]
Owing to these advantages and as part of our current inter-
est in the hydrosilylation of amides, we evaluated known
platinum–NHC complexes in this reaction (Figure 1).
1
2
3
4
5
6
7
8
–
PtI2
0
0
20
10
99
99
10
–
30
25
20
99
10
10
75
99
10
–
25
25
20
99
PtCl2
Karstedt catalyst
1
2
3
4
5
6
7
9
10
11
[a] Reaction conditions: 8a (0.5 mmol), [Pt] (1 mol-%), Ph2SiH2
(2 equiv.), dry THF (3 mL), 40 °C, 1 h. [b] Conversion and yield
were determined by GC by using n-hexadecane as an internal stan-
dard.
However, upon using Pt–NHC complex 1 or 7, an excel-
lent yield (99%) of desired N,N-dimethylbenzamine (8b)
was obtained (Table 1, entries 5 and 11). The key step in
this catalytic reaction is the formation of active species 1a
(Scheme 1), which is more easily formed in complexes 1 and
7. Both complexes are more prone to liberate the 1,6-diene
ligand compared to other complexes.[23]
Scheme 1. Proposed initiation process.
Results and Discussion
Notably, all platinum NHC complexes resulted in 100%
At the start of our investigations, on the basis of the pre- yield of the product after a reaction time of 12–24 h. Be-
vious work of Markó and co-workers as well as that of cause of its high reactivity and better availability, complex
Strassner and co-workers seven different platinum–NHC 1 was used in further investigations.
complexes were synthesized (Figure 1).[22] In general, all
Next, we investigated the influence of different silanes on
complexes were easy available by addition of tBuOK to a the reaction (Table 2). Other silanes containing at least two
suspension of the corresponding imidazolium salt and Kar- geminal hydrides on the Si center were also reactive
stedt’s catalyst at 0 °C. Reactions were completed by stirring (Table 2, entries 2 and 3), but they delivered substantially
the mixture at room temperature overnight. The resulting lower yields of the product. Unfortunately, TMDS and
suspensions were filtered and washed with cooled isopropyl PMHS, which are commonly used in hydrosilylation reac-
alcohol. For the initial catalytic experiments, we chose the tions, gave only low yields of the product (Table 2, entries 5
reduction of N,N-dimethylbenzamide (8a) with Ph2SiH2 as and 6). This was probably due to gel formation, which was
the model system. Typically, the catalyst (1 mol-%) was observed in case of PMHS after a few minutes into the
used in dry THF as the solvent. As shown in Table 1 no reaction. Attempts to get a homogeneous reaction mixture
reaction occurred in the absence of any catalyst (Table 1, failed, and calibration by using an internal standard was
entry 1). The use of simple platinum salts also gave only impossible in this case. Furthermore, isolation of the
Eur. J. Inorg. Chem. 2014, 2345–2349
2346
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
SHORT COMMUNICATION
desired product by column chromatography proved to be ity at all (Table 2, entries 6–8) Advantageously, Pt–NHC
difficult and tedious, because during the workup process complexes are more stable than the Karstedt catalyst and
again gel formation was observed. This finding is in con- do not have to be stored under an atmosphere of argon.
trast to a recent report from the Nagashima group in which
silanes containing dual Si–H groups were essential for the
reaction.[12] Finally, monosilanes did not show any reactiv-
Table 2. Reduction of N,N-dimethylbenzamide (8a) with different
silanes.[a]
Entry
Silane
Conv. [%][b]
Yield [%][b]
1
2
3
4
5
6
7
8
Ph2SiH2
Et2SiH2
PhSiH3
TMDS
99
50
20
15
20
–
99
45
20
15
15
–
PMHS
(EtO)2MeSiH
Me2PhSiH
[(MeO)3SiO]2MeSiH
–
–
–
–
[a] Reaction conditions: 8a (0.5 mmol),
1
(1 mol-%), silane
(2 equiv.), dry THF (3 mL), 40 °C, 1 h. [b] Determined by GC by
using n-hexadecane as an internal standard.
Scheme 2. Proposed reaction mechanism. LnM represents active
species 1a.
Table 3. Reduction of tertiary amides.[a]
[a] Reaction conditions: amide (0.5 mmol), 1 (1 mol-%), Ph2SiH2 (2 equiv.), dry THF (3 mL), 40 °C, 1 h. [b] Determined by GC using n-
hexadecane as an internal standard.
Eur. J. Inorg. Chem. 2014, 2345–2349
2347
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
SHORT COMMUNICATION
Due to the fact, that no additional mechanistic studies
Experimental Section
were realized, we propose that the platinum–NHC complex
catalyzed hydrosilylation proceeds similar to the Ir- and Ru-
catalyzed hydrosilylation of amides (Scheme 2).[10b] At first,
ligand displacement results in a highly active monocarbene
platinum complex 1a. Reaction with the hydrosilane should
lead to the active hydridoplatinumsilyl species 1b, which is
capable of transferring R3Si+ to the carbonyl functionality
General Procedure for the Platinum–NHC-Catalyzed Hydro-
silylation of Amides: A 25 mL oven-dried Schlenk tube containing
a stirrer was charged with the respective tertiary amide (0.5 mmol)
and the platinum–NHC complex (1 mol-%). After purging the
Schlenk tube with argon, dry THF and Ph2SiH2 (2 equiv.) were
added. Finally, dry hexadecane was added as an internal standard.
Given that the reaction does not seem to be very sensitive to trace
to produce the oxocarbenium ion. Subsequent hydride re- amounts of water, wet hexadecane could also be used. The mixture
was stirred at 40 °C for 1 h. The yield was determined by GC.
duction yields the silyl-substituted aminal A. Then, the
iminium ion B is formed, which is followed by a second
hydride reduction giving the amine.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental and analytical details.
Once the optimized reaction conditions were identified,
the scope and limitations of the Pt-NHC complex catalyzed
reduction of amides by using Ph2SiH2 were explored. A
variety of tertiary amides including aromatic, heteroarom-
atic, and heterocyclic amides were reduced smoothly to the
corresponding amines (Table 3). Arenes containing elec-
tron-withdrawing or electron-donating groups in the para
position formed the desired amines in high yields (Table 3,
entries 2, 5, 8, 9). Amides containing halides or sterically
hindered substituents without electron-donating groups at
the aromatic ring led to lower yields (Table 3, entries 3, 4,
6, 7, 10).
Acknowledgments
The research was funded by the German Federate State of
Mecklenburg-Western Pomerania and the Bundesministerium für
Bildung und Forschung (BMBF). The authors thank Dr. W. Bau-
mann, Dr. C. Fischer, S. Buchholz, S. Schareina, A. Koch, and S.
Rossmeisl (all at LIKAT) for their excellent technical and analytical
support.
Finally, the reduction of secondary amides was explored.
In fact, these substrates required more drastic reaction con-
ditions (100 °C, Scheme 3). Notably, the product yield de-
creased with growing steric demand of the amide substitu-
ents. In the case of N-phenyloctanamide, only poor conver-
sion (Ͻ10%) was observed. Unfortunately, an increased
loading of the catalyst or silane did not improve the reac-
tion outcome.
[1] a) J. Seyden-Penne, Reductions by the Alumino- and Borohyd-
rides in Organic Synthesis, 2nd ed., Wiley-VCH, New York,
1997; b) N. I. Houpis, A. Molina, R. A. Reamer, J. E. Lynch,
R. P. Volante, P. J. Reider, Tetrahedron Lett. 1993, 34, 2593–
2596; c) G. W. Gribble, Chem. Soc. Rev. 1998, 27, 395–404.
[2] a) S. Das, D. Addis, S. Zhou, K. Junge, M. Beller, J. Am. Chem.
Soc. 2010, 132, 1770–1771; b) S. Das, D. Addis, K. Junge, M.
Beller, Chem. Eur. J. 2011, 17, 12186–12192.
[3] S. Das, B. Join, K. Junge, M. Beller, Chem. Commun. 2012, 48,
2683–2685.
[4] S. Zhou, K. Junge, D. Addis, S. Das, M. Beller, Angew. Chem.
Int. Ed. 2009, 48, 9507–9510; Angew. Chem. 2009, 121, 9671–
9674.
[5] Y. Sunada, H. Kawakami, T. Imaoka, Y. Motoyama, H. Naga-
shima, Angew. Chem. Int. Ed. 2009, 48, 9511–9514; Angew.
Chem. 2009, 121, 9675–9678.
[6] H. Tsusumi, Y. Sunada, H. Nagashima, Chem. Commun. 2011,
47, 6581–6583.
[7] a) R. Kuwano, M. Takahashi, Y. Ito, Tetrahedron Lett. 1998,
39, 1017–1020; b) B. Li, J.-B. Sortais, C. Darcel, Chem. Com-
mun. 2013, 49, 3691–3693.
[8] a) S. Bower, K. A. Kreutzer, S. L. Buchwald, Angew. Chem. Int.
Ed. Engl. 1996, 35, 1515–1516; Angew. Chem. 1996, 108, 1662–
1664; b) K. Selvakumar, J. F. Harrod, Angew. Chem. Int. Ed.
2001, 40, 2129–2131; Angew. Chem. 2001, 113, 2187–2189.
[9] a) Y. Motoyama, K. Mitsui, T. Ishida, H. Nagashima, J. Am.
Chem. Soc. 2005, 127, 13150–13151; b) D. V. Gutsulyak, G. I.
Nikonov, Angew. Chem. Int. Ed. 2010, 49, 7553–7556; Angew.
Chem. 2010, 122, 7715–7718.
Scheme 3. Pt–NHC complex catalyzed reduction of secondary
amides.
[10] a) C. Cheng, M. Brookhart, Angew. Chem. Int. Ed. 2012, 51,
9422–9424; Angew. Chem. 2012, 124, 9556–9558; b) S. Park,
M. Brookhart, J. Am. Chem. Soc. 2012, 134, 640–653; c) C.
Cheng, M. Brookhart, J. Am. Chem. Soc. 2012, 134, 11304–
11307.
Conclusions
[11] S. Hanada, Y. Motoyamam, H. Nagashima, Tetrahedron Lett.
2006, 47, 6173–6177.
[12] S. Hanada, E. Tsutsumi, Y. Motoyama, H. Nagashima, J. Am.
Chem. Soc. 2009, 131, 15032–15040.
[13] a) H.-W. Wanzlick, Angew. Chem. Int. Ed. Engl. 1962, 1, 75–
80; Angew. Chem. 1962, 74, 129–134; b) H.-W. Wanzlick, H. J.
In summary, we reported the first use of platinum–NHC
complexes in the reduction of amides. The catalytic system
showed tolerance to several functional groups, and different
tertiary amides containing substituted arenes and hetero-
cycles were reduced in good yields within short times.
Eur. J. Inorg. Chem. 2014, 2345–2349
2348
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
SHORT COMMUNICATION
Schönherr, Angew. Chem. Int. Ed. Engl. 1968, 7, 141–142; An-
gew. Chem. 1968, 80, 154.
K. Ölefe, J. Organomet. Chem. 1968, 12, 42–43.
P. J. Cardin, B. Cetinbaya, M. F. Lappert, Chem. Rev. 1972, 72,
545–574; M. F. Lappert, R. K. Maskell, J. Organomet. Chem.
1984, 265, 217–228.
[19] C. S. Straubinger, N. B. Jokic, M. P. Högerl, E. Herdtweck,
W. A. Herrmann, F. E. Kühn, J. Organomet. Chem. 2011, 696,
687–692.
[20] I. E. Markó, S. Stérin, O. Buisine, G. Mignani, P. Branlard, B.
Tinant, J.-P. Declercq, Science 2002, 298, 204–206.
[21] J. Stein, L. N. Lewis, Y. Gao, R. A. Scott, J. Am. Chem. Soc.
1999, 121, 3693–3703.
[22] a) G. Berthon-Gelloz, J.-M. Schumers, G. De Bo, I. E. Markó,
J. Org. Chem. 2008, 73, 4190–4197; b) M. A. Taiga, S. Ahrens,
T. Strassner, J. Org. Chem. 2011, 696, 2918–2927.
[23] G. Berton-Gelloz, O. Buisine, J.-F. Briere, G. Michaus, S.
Sterin, G. Mignani, B. Tinant, J.-P. Declercq, D. Chapon, I. E.
Marko, J. Org. Chem. 2005, 690, 6156–6168.
[14]
[15]
[16] A. J. Arduengo III, Acc. Chem. Res. 1999, 32, 913–921.
[17] a) D. Bézier, F. Jiang, T. Roisnel, J.-B. Sortais, C. Darcel, Eur.
J. Inorg. Chem. 2012, 1333–1337; b) H. Li, C. Luis, M. Castro,
J. Zheng, T. Roisnel, V. Dorcet, J.-B. Sortais, C. Darcel, Angew.
Chem. Int. Ed. 2013, 52, 8045–8049; Angew. Chem. 2013, 125,
8203–8207.
[18] A. Volkov, E. Buitrago, H. Adolfsson, Eur. J. Org. Chem. 2013,
Received: February 19, 2014
Published Online: April 4, 2014
2066–2070.
Eur. J. Inorg. Chem. 2014, 2345–2349
2349
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim