Cationic nitridoruthenium(VI) catalyzed hydrosilylation of ketones and aldehydes

Article abstract of DOI:10.1016/j.tetlet.2011.01.139

The first example of a ruthenium nitrido compound as hydrosilylation catalyst is described, using phenylsilane as reductant. A variety of ketones and aldehydes are reduced to alcohols with good to high isolated yields. Some mechanistic insight on this new system is provided on the basis of the available experimental findings.

Full text of DOI:10.1016/j.tetlet.2011.01.139

Tetrahedron Letters 52 (2011) 1670–1672
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Cationic nitridoruthenium(VI) catalyzed hydrosilylation of ketones and
aldehydes
⇑
Thanh V. Truong, Erica A. Kastl, Guodong Du
Department of Chemistry, University of North Dakota, Grand Forks, ND 58202, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The first example of a ruthenium nitrido compound as hydrosilylation catalyst is described, using phen-
ylsilane as reductant. A variety of ketones and aldehydes are reduced to alcohols with good to high iso-
lated yields. Some mechanistic insight on this new system is provided on the basis of the available
experimental findings.
Received 10 January 2011
Revised 26 January 2011
Accepted 27 January 2011
Available online 1 February 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Hydrosilylation
Ruthenium nitrido
Catalysis
Aldehydes and ketones
High oxidation state transition metal complexes are known for
their activity in catalytic oxidation and oxygen atom transfer reac-
tions.1 Recently, a number of high valent rhenium and molybde-
num complexes bearing terminal oxo- or imido-groups have
been demonstrated to be efficient catalysts in a variety of reduc-
tion reactions,2,3 such as hydrosilylation of ketones and aldehydes,4
reduction of imines5 and amides,6 and hydrogenation of alkynes.7
Besides the synthetic utility of these reactions, they also pose some
+
-
ClO4
N
N
N
Ru
O
O
O
H3C
H
Figure 1. Cationic nitrido-ruthenium(VI) salen.
very interesting mechanistic questions as related to
r-bond
activation.8,9
So far this new type of catalysts for reductions are derived
exclusively from high valent rhenium and molybdenum with a d0
or d2 electronic configuration, specifically those of Re(V), Re(VII),
Mo(VI) and Mo(IV) compounds. In an effort to expand the scope
of catalysts and further our understanding on the novel mecha-
nisms, we have become interested in investigating the use of other
high valent metals in catalytic reductions. Herein we report the
hydrosilylation of carbonyl compounds catalyzed by a cationic
nitrido Ru(VI) complex, 1 (see Fig. 1).10 To the best of our knowl-
edge, this is the first example of hydrosilylation catalyst based on
high valent Ru(VI) with a terminal nitrido group.
The catalytic activity of RuN-salen compound 1 was examined
using acetophenone as a representative substrate under different
reaction conditions. Variations in solvents, silanes, and catalyst
loading are summarized in Table 1. Among the solvents examined,
non-polar toluene and benzene seem to be suitable for the reac-
tion, although 1 is barely soluble in them. Since toluene has a high-
Table 1
Catalytic hydrosilylation of acetophenonea
Entry
Catalyst loading
Silane
Solvent
Time (h)
Conversionb (%)
1
2
3
4
5
6
7
8
5 mol %
5 mol %
5 mol %
5 mol %
5 mol %
5 mol %
5 mol %
1 mol %
PhSiH3 Benzene
PhSiH3 THF
PhSiH3 CH2Cl2
PhSiH3 CH3CN
48
48
48
48
48
48
4
90
67
68
0
50
0
Et3SiH
Et3SiH
Toluene
Benzene
PhSiH3 Toluene
PhSiH3 Toluene
99
99
18
a
Reaction conditions: 0.6 mmol acetophenone, 1.5 equiv of silanes, and RuN
catalyst in refluxing solvents.
b
Determined by NMR.
er boiling point than benzene (110 °C vs 80 °C), we suspect that the
reaction temperature plays a key role for the difference observed in
toluene and benzene. In fact, it is found that benzene can be
equally effective as solvent when the reaction is carried out in a
sealed NMR tube where the reaction temperature can be higher
⇑
Corresponding author. Tel.: +1 701 777 2241; fax: +1 701 777 2331.
E-mail address: gdu@chem.und.edu (G. Du).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.139

T. V. Truong et al. / Tetrahedron Letters 52 (2011) 1670–1672
1671
than the normal boiling point of benzene. Tertiary silanes such as
Et3SiH are active for the hydrosilylation with a much slower rate
when compared with primary silane PhSiH3. The reaction also
provides an excellent conversion when the loading of catalyst 1
is reduced to 1 mol %, albeit with a longer reaction time. No hydro-
silylation reaction was observed at room temperature or without
using a catalyst.
100
80
60
40
20
0
With the reaction conditions established, we then examined the
scope of substrates of the hydrosilylation in this catalytic system. A
variety of carbonyl compounds was investigated with PhSiH3
(1.5 equiv) and 1 mol % of catalyst in refluxing toluene or benzene.
The results are summarized in Table 2. It is found that all the sub-
strates were efficiently converted to the corresponding silylethers,
which were further converted to alcohols upon aqueous acidic
workup. The reaction can be applied effectively to aliphatic ke-
tones and alicyclic ketones with high isolated yields (entries 2
and 3). The conversion of benzaldehydes proceeds considerably
faster, typically complete in less than two hours under same con-
ditions (entries 4–6). Substituted benzaldehydes bearing elec-
tron-donating and electron-withdrawing groups also afforded the
corresponding alcohols upon workup. Thus the present catalytic
system represents a new method for the reduction of ketones
and aldehydes, which is an extremely useful synthetic
transformation.11
It should be noted that although the conversion is generally
complete as judged from the disappearance of the starting car-
bonyl compounds, the isolated yields are sometimes modest due
to the presence of side reactions. For example, in the crude reaction
mixture of acetophenone reduction, ethylbenzene is detected as a
minor product via further deoxygenative reduction.12 This over-
reduction could be significant for aryl ketones or aryl aldehydes,
particularly if the reaction is allowed to run too long. It is also
noted that small amount of acetophenone reappears after acidic
workup, presumably through the intermediacy of silyl enol ether.13
It is worth mentioning that the minor products profile is different
from the ReVO-catalyzed hydrosilylation, where ethylbenzene and
di(phenylethyl)ethers are observed.4c
0
5
10
Time/h
15
20
Figure 2. Time profile of the hydrosilylation of acetophenone with PhSiH3
(1.5 equiv) and 1 mol % catalyst in refluxing toluene. The conversion (circles) is
determined by NMR and the line is a smoothing function and not a kinetic fit.
to afford RuIII,10 or they could be reduced by hydrosilanes to give
low valent Ru species.15 Therefore it is reasonable to postulate that
the active species in the reaction is actually RuIII. To probe this pos-
þ
ꢁ
sibility, firstly a RuIII compound, ½RuðsaldachÞðH2OÞ2 ꢀ½PF6 ꢀ was
prepared independently and subjected to same catalytic condi-
tions. It was found that no hydrosilylation of acetophenone was
observed within 48 h in refluxing toluene. Secondly, we investi-
gated the reaction of compound 1 with PhSiH3 in CD3CN because
of solubility consideration. It appeared that 1 can be reduced by
PhSiH3, rather slowly at room temperature, to give a dark greenish,
paramagnetic species. The exact nature of this species is unknown;
however, it is clear that it is different from the species in the cata-
lytic reaction when carbonyl substrates are present, which is typi-
cally of a brown-reddish color. We also monitored the time profile
of acetophenone hydrosilylation (Fig. 2), which shows no apparent
induction period that may exist due to the reduction of RuN by
hydrosilane before catalysis began. Based on these preliminary re-
sults, we postulate that the catalysis proceeds through a hetero-
lytic cleavage of the SiH bond activated by coordination to the
metal center, similar to that of monooxo ReV based systems.2 How-
ever, the different minor-product profiles in these two systems
may indicate some differences in the reaction pathways. Further
detailed studies are necessary to shed light on the reaction
mechanism.
16
The catalytic mechanism is unclear at the moment. Low valent
RuII/III are well known as hydrosilylation/hydrogenation cata-
lysts.14 RuVIN complexes easily undergo N–N coupling reactions
Table 2
Hydrosilylation of various carbonyl compounds catalyzed by RuNa
Entry
1
Substrate
Product
Time
Yieldb (%)
67
O
OH
In conclusion, we have demonstrated that the air-stable, cat-
18 h
ionic RuVIN-salen is an efficient catalyst for the reduction of car-
Ph
Ph
Ph
Ph
O
OH
bonyl compounds in the presence of
a tertiary silane. The
2
20 h
86
reaction proceeds smoothly with good to high yields. Research is
ongoing to expand the scope of the reduction and to elucidate
the reaction mechanism.
3
4
20 h
2 h
89
66
O
OH
OH
Acknowledgements
CHO
This work is supported financially by the ND EPSCoR through
NSF Grant #EPS-0814442 and the University of North Dakota.
OH
MeO
CHO
5
1.5 h
73
MeO
OH
References and notes
CHO
6
2 h
87
1. (a) Romao, C. C.; Kuhn, F. E.; Herrmann, W. A. Chem. Rev. 1997, 97, 3197–3246;
(b) Espenson, J. H. Adv. Inorg. Chem. 2003, 54, 157–202; (c) Holm, R. H. Chem.
Rev. 1987, 87, 1401–1449.
2. Du, G.; Abu-Omar, M. M. Curr. Org. Chem. 2008, 12, 1185–1198.
3. (a) Maddani, M. R.; Moorthy, S. K.; Prabhu, K. R. Tetrahedron 2010, 66, 329–333;
(b) Nolin, K. A.; Ahn, R. W.; Kobayashi, Y.; Kennedy-Smith, J. J.; Toste, F. D.
Chem. Eur. J. 2010, 16, 9555–9562; (c) Ziegler, J. E.; Du, G.; Fanwick, P. E.;
Br
Br
a
Reaction conditions: 0.6 mmol ketone or aldehyde, 1.5 equivalent of PhSiH3 and
catalyst (1 mol %) in heated toluene or benzene.
Isolated yields; complete consumption of starting carbonyl compounds was
observed in all cases.
b

1672
T. V. Truong et al. / Tetrahedron Letters 52 (2011) 1670–1672
Abu-Omar, M. M. Inorg. Chem. 2009, 48, 11290–11296; (d) Pontes da Costa, A.;
Reis, P. M.; Gamelas, C.; Romao, C. C.; Royo, B. Inorg. Chim. Acta 2008, 361,
1915–1921.
10. Man, W.-L.; Tang, T.-M.; Wong, T.-W.; Lau, T.-C.; Peng, S.-M.; Wong, W.-T. J. Am.
Chem. Soc. 2004, 126, 478–479.
11. (a) Carpentier, J. F.; Bette, V. Curr. Org. Chem. 2002, 6, 913; (b) Riant, O.;
Mostefai, N.; Courmarcel, J. Synthesis 2004, 18, 2943.
12. (a) Shu, R.; Harrod, J. F.; Lebuis, A.-M. Can. J. Chem. 2002, 80, 489–495; (b)
Zaccheria, F.; Ravasio, N.; Ercoli, M.; Allegrini, P. Tetrahedron Lett. 2005, 46,
7743–7745.
13. (a) Reyes, C.; Prock, A.; Giering, W. P. Organometallics 2002, 21, 546–554; (b)
Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090–3098.
14. Kitamura, M.; Noyori, R. In Ruthenium in Organic Synthesis; Murahashi, S. I., Ed.;
Wiley-VCH: Weinheim, Germany, 2004; pp 3–52.
15. (a) Valliant-Saunders, K.; Gunn, E.; Shelton, G. R.; Hrovat, D. A.; Borden, W. T.;
Mayer, J. M. Inorg. Chem. 2007, 46, 5212–5219; (b) Collman, J. P.; Slaughter, L.
M.; Eberspacher, T. S.; Strassner, T.; Brauman, J. I. Inorg. Chem. 2001, 40, 6272–
6280.
16. Man, W.; Kwong, H.; Lam, W. W. Y.; Xiang, J.; Wong, T.; Lam, W.; Wong, W.;
Peng, S.; Lau, T. Inorg. Chem. 2008, 47, 5936–5944.
4. (a) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste, F. D. J. Am. Chem.
Soc. 2003, 125, 4056–4057; (b) Du, G.; Fanwick, P. E.; Abu-Omar, M. M. J. Am.
Chem. Soc. 2007, 129, 5180–5187; (c) Du, G.; Abu-Omar, M. M. Organometallics
2006, 25, 4920–4923; (d) Fernandes, A. C.; Fernandes, R.; Romao, C. C.; Royo, B.
Chem. Commun. 2005, 213–214.
5. (a) Fernandes, A. C.; Romao, C. C. Tetrahedron Lett. 2005, 46, 8881–8883; (b)
Nolin, K. A.; Ahn, R. W.; Toste, F. D. J. Am. Chem. Soc. 2005, 127, 12462–12463.
6. Fernandes, A. C.; Romao, C. C. J. Mol. Cat. A: Chem 2007, 272, 60–63.
7. Fernandes, A. C.; Fernandes, J. A.; Paz, F. A. A.; Romao, C. C. Dalton Trans. 2008,
6686–6688.
8. Calhorda, M. J.; Costa, P. J. Dalton Trans. 2009, 8155–8161.
9. (a) Shirobokov, O. G.; Gorelsky, S. I.; Simionescu, R.; Kuzmina, L. G.; Nikonov, G.
I. Chem. Commun. 2010, 46, 7831–7833; (b) Peterson, E.; Khalimon, A. Y.;
Simionescu, R.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov, G. I. J. Am. Chem. Soc.
2009, 131, 908–909; (c) Fernandes, A. C.; Romao, C. C. Tetrahedron Lett. 2007,
48, 9176–9179.