[MoO2Cl2] as catalyst for hydrosilylation of aldehydes and ketones

Article abstract of DOI:10.1039/b414145h

The high valent molybdenum-dioxo complex [MoO₂Cl₂] catalyzes the addition of dimethylphenylsilane to aldehydes and ketones to afford the corresponding dimethylphenylsilyl ethers in quantitative yield.

Full text of DOI:10.1039/b414145h

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[
MoO Cl ] as catalyst for hydrosilylation of aldehydes and ketones{
2
2
Ana C. Fernandes, Ricardo Fernandes, Carlos C. Rom a˜ o and Beatriz Royo*
Received (in Cambridge, UK) 16th September 2004, Accepted 26th October 2004
First published as an Advance Article on the web 2nd December 2004
DOI: 10.1039/b414145h
The high valent molybdenum-dioxo complex [MoO Cl
2
2
]
Using the above catalytic conditions, we then examined the
scope of MoO Cl -mediated hydrosilylation with a variety of
catalyzes the addition of dimethylphenylsilane to aldehydes
and ketones to afford the corresponding dimethylphenylsilyl
ethers in quantitative yield.
2
2
aldehydes and ketones (see Table 1). As shown in Table 1, the
catalytic reaction is suitable for a wide scope of aromatic aldehyde
The activation of a Si–H bond is one of the key steps in
1
hydrosilylation and other catalytic reactions. Numerous transi-
Table 1 Hydrosilylation of aldehydes and ketones with PhMe
catalyzed by [MoO Cl ]
2 2
2
SiH
a
tion metal complexes activate Si–H bonds via oxidative addition
2
to form hydrido silyl species. Recently, Toste and coworkers have
proposed an alternative mechanism for the catalytic hydrosilyla-
tion of carbonyl groups, in which a metal hydride species is
produced by a [2+2]-type addition of the Si–H bond to a
3
metal–oxygen p-bond (see Scheme 1). Similar s-bond activations
have been performed by MLS and MLN multiple bonds
4
M 5 Ti, Zr). In order to further explore the use of metal–oxo
(
complexes as catalysts for the reduction of carbonyl groups, we
studied the catalytic activity of the dioxomolybdenum com-
pound [MoO Cl ] (1) in the hydrosilylation of aldehydes and
Reaction
conditions
b
c
Substrate
Product
Yield
2
2
ketones.{ So far, the cis-MoO
2
fragment has been the subject of
a
25 uC/4 h
96%
extensive studies related to oxidation and oxygen transfer reactions
but its catalytic activity in a reducing reaction has never been
5
studied.
d
b
c
d
25 uC/4 h
25 uC/4 h
25 uC/16 h
61%
We found that 1 is a highly effective catalyst for the
hydrosilylation of aromatic aldehydes and also ketones (see
eqn. 1 and 2).
97%
32%
67%
ð1Þ
ð2Þ
e
f
25 uC/16 h
40 uC/16 h
The catalytic activity of 1 was tested using 4-nitrobenzaldehyde
and dimethylphenylsilane with a reaction ratio of 1 : 1.2 in the
presence of 5 mol% of catalyst. The reaction was carried out at
e
25 uC in dichloromethane over 4 h to yield the corresponding silyl
68%
ether, dimethyl(4-nitrobenzyloxy)phenylsilane, in 96% yield. The
formation of the silylated compound proceeds without by-
products detectable by NMR spectroscopy.
g
40 uC/16 h
76%
a
All reactions were carried out in dichloromethane with 1.0 equiv.
SiH, using 5 mol% of
Scheme 1 [2+2]-Addition of a Si–H bond to a metal–oxo multiple bond.
of aldehyde or ketone, 1.2 equiv. of PhMe
2
b
c
[
MoO
purified material. Accompanied by 38% desilylated alcohol. 75 :
5 trans : cis.
2 2
Cl ]. Fully characterized by IR and NMR data. Isolated,
d
e
{
Electronic supplementary information (ESI) available: NMR and IR
data. See http://www.rsc.org/suppdata/cc/b4/b414145h/
broyo@itqb.unl.pt
2
*
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 213–214 | 213

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Ana C. Fernandes, Ricardo Fernandes, Carlos C. Rom a˜ o and
Beatriz Royo*
Instituto de Tecnologia Qu ´ı mica e Biol o´ gica da Universidade Nova de
Lisboa, Quinta do Marqu eˆ s, EAN, Apt. 127, 2781-901 Oeiras, Portugal.
E-mail: broyo@itqb.unl.pt; Fax: +351-214411277; Tel: +351-214469731
substrates, affording the desired silyl ether in quantitative yields. In
the case of the 4-bromobenzaldehyde, the corresponding silyl ether
was obtained accompanied by a substantial quantity of desilylated
alcohol. Benzaldehyde derivatives containing functional groups
were well tolerated, even though the yields isolated for the ester
and cyano functionalities were lower, 67% and 32% respectively.
Ketones are also converted into the silylated compounds although
longer times and higher temperatures are required.
Notes and references
{
Typical experimental procedure for the hydrosilylation of aldehydes and
ketones with dimethylphenylsilane: All operations were carried out under
nitrogen. In a small flask, a mixture of aldehyde or ketone (1.01 mmol),
PhMe SiH (1.20 mmol), and a catalytic amount of 1 (0.050 mmol) was
The catalytic reaction was found to work with triethylsilane but
the yields of the corresponding ethyl ethers were significantly
reduced (24% yield for (4-nitrobenzyloxy)triethylsilane and 18%
yield for (4-bromobenzyloxy)triethylsilane after being stirred for
2
dissolved in dichloromethane (5 mL). The mixture was stirred (the reaction
time and the temperature are indicated in Table 1) and monitored
periodically by TLC. Upon completion, the reaction mixture was diluted
with hexane, loaded directly on to a silica gel column and chromato-
graphed with the appropriate mixture of hexane and diethyl ether to give
the silyl ether.
10 h at room temperature in dichloromethane). No catalysis was
observed with the more sterically encumbered triphenylsilane.
Replacement of dichloromethane by toluene as solvent afforded
similar results.
1
1 G. W. Parshall, S. D. Itell, Homogeneous Catalysts: The Applications
and Chemistry of Catalysis by Soluble Transition Metal Complexes,
Wiley, New York, 1992.
When the catalytic reaction was monitored by H NMR no
intermediates in the reaction were observed. The reaction of 1 with
a stoichiometric amount of dimethylphenylsilane gave an intract-
able blue residue, probably due to decomposition of an
intermediate hydride species formed by a [2+2]-addition of the
Si–H bond to the MoLO bond. The [2+2]-type addition of a
2 J. Y. Corey and J. Braddock-Wilking, Chem. Rev., 1999, 99, 175.
3
J. J. Kennedy-Smith, K. A. Nolin, H. P. Gunterman and F. D. Toste,
J. Am. Chem. Soc., 2003, 125, 4056.
H. M. Hoyt, F. E. Michael and R. G. Bergman, J. Am. Chem. Soc.,
4
2004, 126, 1018; J. L. Polse, R. A. Andersen and R. G. Bergman,
halosilane (R
3
SiX) to a molybdenum–oxygen p-bond has been
J. Am. Chem. Soc., 1998, 120, 13405; Z. K. Sweeney, J. L.
Polse, R. A. Andersen and R. G. Bergman, J. Am. Chem. Soc.,
6
reported in the literature.
1
998, 120, 7825; Z. K. Sweeney, J. L. Polse, R. A. Andersen,
R. G. Bergman and M. G. Kubinec, J. Am. Chem. Soc., 1997, 119,
543.
In conclusion, we have demonstrated that the high oxidation
2 2
state molybdenum complex [MoO Cl ] is a highly effective catalyst
4
for representative hydrosilylations of aldehydes and ketones with
dimethylphenylsilane. Studies related to the addition of other
s-bonds to a MoLO unit are being pursued.
5 F. E. K u¨ hn, M. Groarke, E` . Bencze, E. Herdtweck, A. Prazeres, A.
M. Santos, M. J. Calhorda, C. C. Rom a˜ o, I. S. Gon c¸ alves, A. D. Lopes
and M. Pillinger, Chem. Eur. J., 2002, 8, 2370; R. H. Holm, Chem. Rev.,
1987, 87, 1401.
H. Arzoumanian, H. Krentzien, C. Corao, R. Lopez and G. Agrifoglio,
Polyhedron, 1995, 14, 2887.
This research was financially supported by FCT through project
6
POCTI 37726/QUI/01.
2
14 | Chem. Commun., 2005, 213–214
This journal is ß The Royal Society of Chemistry 2005