Silver-catalyzed hydrosilylation of aldehydes

Article abstract of DOI:10.1039/b609679d

Silver triflate, either alone or in the presence of an appropriate phosphine or NHC ligand, has been shown to catalyze the chemoselective hydrosilylation of aromatic and aliphatic aldehydes to yield silyl ethers, thus representing the first systematic app

Full text of DOI:10.1039/b609679d

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Silver-catalyzed hydrosilylation of aldehydes{
Bradley M. Wile and Mark Stradiotto*
Received (in Berkeley, CA, USA) 7th July 2006, Accepted 3rd August 2006
First published as an Advance Article on the web 23rd August 2006
DOI: 10.1039/b609679d
Silver triflate, either alone or in the presence of an appropriate
phosphine or NHC ligand, has been shown to catalyze the
chemoselective hydrosilylation of aromatic and aliphatic
aldehydes to yield silyl ethers, thus representing the first
systematic application of silver species as catalysts for the
hydrosilylation of unsaturated organic substrates.
The metal-catalyzed hydrosilylation of carbonyl compounds is an
atom-economical methodology that provides direct access to
1
synthetically useful O-protected silyl alcohols. In addition to the
diversity of main group and transition metal catalysts for carbonyl
hydrosilylation that have been identified over the past three
Scheme 1 Silver-catalyzed addition of silanes to aldehydes.
using 3 mol% AgOTf in THF. Although the initial and overall
reaction rates were lowered considerably upon the addition of
1,2
2b
2c
2d,e
decades, including those based on Ti, Ru, and Rh,
effective group 11 catalysts have also emerged. The groups of
20 mol% Et
3
P to 3 mol% AgOTf, the use of this catalyst mixture
3
Stryker, Lipshutz, Nolan, and others have demonstrated that
4
5
in THF enabled the clean formation of 2a in 98% yield after 24 h
10a,b
at 70 uC, in the absence of 3a or other by-products (entry 2).
While excellent yields of 2a were also obtained in toluene
Cu complexes featuring either phosphine or N-heterocyclic carbene
(NHC) ligands can function as active and selective catalysts for
carbonyl hydrosilylations, while a single report by Hosomi and co-
workers highlights the catalytic utility of Au species for such
or DMSO, 1,2-dichloroethane proved to be a less effective
solvent. By comparison, reactions employing 3 mol% Me
at 70 uC in THF afforded 3a quantitatively, and no reaction
was observed when a Et P–Me SiOTf catalyst system was used at
70 uC in THF.
The conversion of 1a to 2a mediated by Et
3
SiOTf
6
transformations. In contrast, reports focused on the application of
Ag species as hydrosilylation catalysts are absent from the
literature. Indeed, Ag salts are often viewed as being catalytically
inactive in hydrosilylation chemistry, and in this context are
commonly employed as innocent anion exchange agents for the
3
3
3
P–AgX mixtures in
THF was found to vary based on the counteranion employed,
with catalysts derived from AgCl (57%) proving inferior to those
7
in situ generation of cationic transition (and other) metal catalysts.
8
9
Inspired by recent reports from the groups of He and Li in
which Ag complexes have been shown to mediate the addition of
s-bonds to unsaturated organic molecules, we became interested
in evaluating the catalytic utility of Ag species in carbonyl
hydrosilylations, including examining the influence of supporting
ligands on such Ag-mediated transformations. Herein we report
the Ag-catalyzed chemoselective addition of silanes to aldehydes,
which to the best of our knowledge represents the first systematic
application of Ag species as catalysts for the hydrosilylation of
unsaturated organic compounds.
based on AgOTf (98%), AgBF
4
(93%), or AgSbF (95%).
6
However, the observation that a R P–AgCl mixture is capable
3
a
Table 1 Addition of Me PhSiH to aldehydes
2
b
b,c
% 3
b
d
Entry
Aldehyde
% Et
3
P
% 2
% other
TOF
1
2
3
4
5
6
7
8
9
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
1f
0
20
0
20
0
20
0
20
0
20
0
20
0
94
98
88
94
50
76
9
6
,1
,1
,1
,1
,1
58
,1
,1
2
1
40
1
20
12
1
133
29
121
49
121
,1
117
1
11
1
2
The hydrosilylation of benzaldehyde (1a) with Me PhSiH to
give the silyl ether 2a was selected as a preliminary test reaction for
Ag-mediated carbonyl hydrosilylations (Scheme 1). While reac-
tions conducted in THF at 24 uC employing 3 mol% AgSbF6
generated the benzaldehyde trimer (3a) in 97% yield after 15 min,
under analogous conditions employing 3 mol% AgOTf, 2a was
obtained in 94% yield, along with only 6% of 3a (Table 1, entry 1).
Similar results were obtained for reactions conducted at 70 uC
86
7
1
,1
,1
,1
,1
,1
,1
10
11
12
13
48
62
.99
48
99
6
24
,1
17
1
114
37
87
37
1f
1g
1g
14
a
20
Reactions in THF with 2 equiv. Me
versus 1. In the absence of Et P, the addition of silane resulted in
2
PhSiH and 3 mol% AgOTf
3
rapid change of the solution color from colorless to yellow, followed
by the immediate precipitation of Ag(s). Yields quoted with respect
Department of Chemistry, Dalhousie University, Halifax, Canada
B3H 4J3. E-mail: mark.stradiotto@dal.ca; Fax: +1 902 494 1310;
Tel: +1 902 494 7190
b
to 1 consumed (after 24 h at 70 uC for 20% Et
3
P, or 0.25 h at 24 uC
for 0% Et P) based on GC-MS and GC-FID data (average of two
3
{
Electronic supplementary information (ESI) available: Complete
experimental procedures and tabulated catalytic results. See DOI:
0.1039/b609679d
c
runs). Aldehyde trimer. Turnover frequency (TOF, h ) at 0.25 h,
d
21
based on the consumption of 1.
1
4
104 | Chem. Commun., 2006, 4104–4106
This journal is ß The Royal Society of Chemistry 2006

View Article Online
2
e
of mediating such transformations is noteworthy, given that AgCl
elimination in the presence of excess phosphine is often exploited
during the in situ preparation of cationic metal catalysts for
carbonyl hydrosilylations. For convenience, AgOTf was used in
THF for all subsequent catalytic investigations.
3 3
(Ph P) RhCl, has been shown to mediate the 1,4-conjugate
hydrosilylation of a,b-unsaturated aldehydes, in contrast to the
1,2-addition brought about by the Et P–AgOTf catalyst system
3
described herein in the hydrosilylation of 1g.
While we are currently unable to comment as to whether the
Ag-catalyzed hydrosilylation reactions detailed herein are homo-
geneous or heterogeneous in nature, the identification of
Intrigued by these preliminary catalytic results, we became
interested in assessing the impact of employing alternative
ligands in Ag-mediated hydrosilylations. Given the exceptional
selectivity exhibited by the catalyst system derived from a
Me PhSiOTf as a by-product in these reactions suggests the
2
possible intermediacy of AgH species. However, we do not view
combination of excess Et P relative to AgOTf (20 : 3), we
3
Me PhSiOTf as being responsible for the observed reduction
2
anticipated that the structurally related bidentate ligand 1,2-
bis(diethylphosphino)ethane (DEPE) might give rise to an even
more effective catalyst system. To our surprise, catalysts derived
from the combination of either 10 or 20 mol% DEPE with 3 mol%
AgOTf afforded significantly lower yields of 2a (45% and 68%)
than were obtained in analogous reactions employing 20 mol%
chemistry, given the distinct lack of hydrosilylation observed when
Me SiOTf is employed in place of AgOTf, in the presence or
3
absence of Et P (vide supra). Further experiments directed toward
3
identifying the reactive Ag species present in such catalytic systems
are currently underway in our laboratory.
In summary, this communication represents the first systematic
evaluation of Ag-catalyzed carbonyl hydrosilylation. We have
shown herein that AgOTf itself is able to catalyze aldehyde
reductions under mild conditions, and that a catalyst system
generated from AgOTf and an appropriate phosphine or
NHC ligand is capable of mediating the chemoselective
hydrosilylation of aromatic and aliphatic aldehydes. The
recognition of such Ag-mediated reactivity highlights the need
to consider the possible catalytic contributions of AgX species
that may be present during metal-mediated hydrosilylation
reactions.
Et P (98%). Although we are hesitant to draw detailed mechanistic
3
conclusions from such an observation, it is possible that the DEPE
ligands may serve to bridge rather than chelate in this system,
thereby generating what may be catalytically less-competent
1
1
oligomeric structures. In returning our focus to monophosphines,
n
the use of 20 mol% Bu
provided cleanly a yield of 2a (95%) comparable to that obtained
by use of 20 mol% Et P. In contrast, the combination of 3 mol%
AgOTf and 20 mol% of one of the more sterically hindered
3
P afforded an Ag-catalyst system that
3
i
t
branched trialkylphosphines Cy
triarylphosphine Ph P, afforded significantly lower yields of 2a
¡51%). To test the generality of ligation with a strong s-donor,
3 3 3
P, Pr P, or Bu P, or the
3
Acknowledgment is made to NSERC of Canada, the Canada
Foundation for Innovation, the Nova Scotia Research and
Innovation Trust Fund, and Dalhousie University for their
generous support of this work.
(
2
0 mol% of the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-
ylidene was used successfully in place of PR , giving 2a in 94%
3
yield.
Using favorable conditions identified from our reactivity survey
Notes and references
involving 1a (3 mol% AgOTf, 20 mol% Et P, THF, 70 uC), we
3
1
(a) J. F. Carpentier and V. Bette, Curr. Org. Chem., 2002, 6, 913; (b)
I. Ojima, Z. Li and J. Zhu, in Chemistry of Organic Silicon Compounds,
ed. Z. Rappoport and Y. Apeloig, Wiley, New York, 1998, vol. 2,
p. 1687.
sought to investigate the generality of the Ag-catalyzed hydro-
silylation of aldehydes. Whereas ortho-tolualdehyde (1b) was
reduced to 2b in 94% yield (entry 4), lower conversions were
obtained for the reduction of the more sterically hindered 2,6-
dimethylbenzaldehyde 1c (entry 6, 76%). Much faster reaction
rates under more mild conditions were observed when 3 mol%
AgOTf alone was employed as a catalyst for these transforma-
tions: however, significantly poorer selectivity for the desired
hydrosilylation product 2 was obtained (entries 3 and 5). The
aliphatic aldehydes 1d and 1e were also reduced more efficiently by
2 Selected examples: (a) D. J. Parks and W. E. Piers, J. Am. Chem. Soc.,
996, 118, 9440; (b) J. Yun and S. L. Buchwald, J. Am. Chem. Soc.,
999, 121, 5640; (c) C. Eaborn, K. Odell and A. Pidcock, J. Organomet.
1
1
Chem., 1973, 63, 93; (d) B. Tao and G. C. Fu, Angew. Chem., Int. Ed.,
2002, 41, 3892; (e) I. Ojima, M. Nihonyanagi, T. Kogure, M. Kumagai,
S. Horiuchi, K. Nakatsugawa and Y. Nagai, J. Organomet. Chem.,
1975, 94, 449.
3
W. S. Mahoney, D. M. Brestensky and J. M. Stryker, J. Am. Chem.
Soc., 1988, 110, 291.
use of Et P–AgOTf catalyst mixtures (entries 8 and 10) versus
3
4 (a) B. H. Lipshutz and B. A. Frieman, Angew. Chem., Int. Ed., 2005, 44,
345; (b) B. H. Lipshutz, C. C. Caires, P. Kuipers and W. Chrisman,
Org. Lett., 2003, 5, 3085; (c) B. H. Lipshutz, K. Noson, W. Chrisman
and A. Lower, J. Am. Chem. Soc., 2003, 125, 8779; (d) B. H. Lipshutz,
W. Chrisman and K. Noson, J. Organomet. Chem., 2001, 624, 367; (e)
B. H. Lipshutz, J. Keith, P. Papa and R. Vivian, Tetrahedron Lett.,
6
AgOTf alone (entries 7 and 9). Notably, when phosphine was
employed in the reduction of 1d, products derived from aldol
condensation processes represented less than 3% of the consumed
aldehyde.
1
998, 39, 4627.
(a) S. D ´ı ez-Gonz a´ lez, N. M. Scott and S. P. Nolan, Organometallics,
006, 25, 2355; (b) S. D ´ı ez-Gonz a´ lez, H. Kaur, F. K. Zinn, E. D. Stevens
and S. P. Nolan, J. Org. Chem., 2005, 70, 4784.
6 H. Ito, T. Yajima, J.-I. Tateiwa and A. Hosomi, Chem. Commun., 2000,
81.
In a report pertaining to carbonyl hydrosilylations catalyzed by
pybox)RhCl –AgX, the ability of unligated AgX salts to mediate such
transformations to a limited extent is mentioned briefly in a footnote;
pybox–AgX mixtures did not exhibit any catalytic activity:
H. Nishiyama, M. Kondo, T. Nakamura and K. Itoh,
Organometallics, 1991, 10, 500.
In an effort to assess the chemoselectivity of Ag-mediated
aldehyde hydrosilylation in the presence of either ketone or alkene
functionalities, the reduction of 4-acetylbenzaldehyde (1f) and
trans-cinnamaldehyde (1g) were examined. Whereas a mixture of
products was obtained by using AgOTf as a catalyst (entries 11
5
2
9
7
8
and 13), both substrates were reduced cleanly by use of a Et P–
3
(
3
AgOTf catalyst system, with 1,2 Si–H addition occurring
10c
exclusively at the aldehyde (entries 12 and 14).
Notably,
(
Ph PCuH) (Stryker’s reagent) has also proven effective for the
3
6
chemoselective hydrosilylation of aldehydes in the presence of
4e
(a) C.-G. Yang, N. W. Reich, Z. Shi and C. He, Org. Lett., 2005, 7,
4553; (b) Y. Cui and C. He, Angew. Chem., Int. Ed., 2004, 43, 4210.
4d
ketones and alkenes.
However, this Cu catalyst,
like
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 4104–4106 | 4105

View Article Online
9
(a) X. Yao and C.-J. Li, Org. Lett., 2005, 7, 4395; (b) Y. Luo, Z. Li and
C.-J. Li, Org. Lett., 2005, 7, 2675.
0 (a) Control experiments confirmed that 20 mol% Et P alone does not
catalyze these transformations. Moreover, while the experiments
reported herein were conducted using 99+% AgOTf (Aldrich),
no change in catalytic behavior was noted for reactions
employing 99.95+% AgOTf (Aldrich) that was found to contain
only the following trace elements by ICP analysis (ppm): Na (13.5);
Ca (1.6); B (0.8); Mg (0.3); Zn (0.1); Sb (0.1); (b) Et
3
SiH and
1
3
Ph SiH proved less effective for this transformation{; (c) The
2
2
1
identities of 2f and 2g were confirmed based on H and C NMR
13
data.
11 M.-C. Brandys and R. J. Puddephatt, J. Am. Chem. Soc., 2002, 124,
3946.
4
106 | Chem. Commun., 2006, 4104–4106
This journal is ß The Royal Society of Chemistry 2006