First examples of gold nanoparticles catalyzed silane alcoholysis and silylative pinacol coupling of carbonyl compounds

Article abstract of DOI:10.1016/j.tetlet.2008.03.092

In this Letter, we illustrate the first catalytic application of supported Au nanoparticles in silane alcoholysis. The reaction proceeds with low amount of catalysts, under solvent free conditions for liquid substrates, without additional ligands and in a

Full text of DOI:10.1016/j.tetlet.2008.03.092

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3221–3224
First examples of gold nanoparticles catalyzed silane alcoholysis
and silylative pinacol coupling of carbonyl compounds
Patrizio Raffa a,b,*, Claudio Evangelisti b,c, Giovanni Vitulli c, Piero Salvadori b,c
a Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
b
`
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
c Advanced Catalyst s.r.l., Via Risorgimento 35, 56126 Pisa, Italy
Received 12 February 2008; revised 17 March 2008; accepted 19 March 2008
Available online 23 March 2008
Abstract
In this Letter, we illustrate the first catalytic application of supported Au nanoparticles in silane alcoholysis. The reaction proceeds
with low amount of catalysts, under solvent free conditions for liquid substrates, without additional ligands and in a very selective way
for primary alcohols in the examined cases. Additionally, a gold catalyzed silylative pinacol coupling of carbonyl compound is observed
for the first time and some hypotheses about the reaction mechanism are given.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Gold nanoparticles; Catalysis; Hydrosilanes; Alcoholysis; Pinacol coupling
ROH
+
R'3SiH
Since the discovery of the catalytic activity of gold nano-
particles in low temperature CO oxidations,1 there has been
an explosion in the interest shown in catalysis by gold. A
number of reviews have recently been reported regarding
the applications of supported Au nanoparticles,2–4 Au(I)
and Au(III) salts or complexes,4,5 as catalysts in several
reaction of interest such as oxidations, hydrogenations,
additions and so on, indicating the versatility of such sys-
tems. As relevant examples, Au(I) or Au(III) complexes,
and also a supported Au/CeO2 systems are reported to
be efficient catalysts for the hydrosilylation of carbonyl
compounds to the corresponding silyl ether.6–8
It has been recently shown that supported Au nanopar-
ticles obtained by metal vapourization are very active
catalysts for terminal alkynes hydrosilylation in heteroge-
neous phase.9 Following this finding, we investigated the
use of supported gold catalysts in reactions involving
hydrosilanes. In particular, we focused our attention on
silane alcoholysis (Scheme 1). Silyl ether (1, Scheme 1) for-
ROSiR'3
+ H2
1
Scheme 1.
mation is not only a fundamental process in the synthesis
of functional organosilicon compounds but also an impor-
tant technique for the protection of reactive hydroxy
groups during multistep organic syntheses.10 From the
standpoint of ‘green chemistry’,11 this transformation
should be conducted through catalytic dehydrogenative
silylation with a hydrosilane rather than through electro-
philic silylation with a silyl electrophile (R3Si–X) in combi-
nation with a stoichiometric base.12 The former produces
H2 as the sole byproduct instead of a base–HX salt in the
latter. Several metal complexes have successfully been used
as selective homogeneous catalysts, including Ir(I),13
Cu(I)14 and also Au(I)15 systems. In all these cases, com-
plex organic ligand and 0.5–1% loading of metal catalyst
are used to obtain good performances. Classical hetero-
geneous catalysts, such as Pd/C or Raney Ni are very active
in this reaction, but scarcely chemoselective.12 Recently, an
interesting heterogeneous version of the reaction was
*
Corresponding author.
E-mail address: raffa@sns.it (P. Raffa).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.092

3222
P. Raffa et al. / Tetrahedron Letters 49 (2008) 3221–3224
reported, in which a perfluorinated dirhodium(II) complex
adsorbed on silica resulted in a very efficient and reusable
catalyst under solvent free conditions.16 To our knowledge,
no example of heterogeneous gold catalyzed silane alcoho-
lysis has been reported.
For our tests we used Au/Al2O3 catalysts with 1% metal
loading prepared in two different ways: metal vapour syn-
thesis17 (AuMVS) and deposition–precipitation method18
(AuDP). Both catalysts contain small Au nanoparticles
(3–5 nm of mean diameter), but it has been reported that
tified products is obtained. After the same prolonged time,
the conversion of the electron-rich phenol (entry 7) in the
corresponding siloxane is only 5% in toluene. The use of
a reaction solvent in the case of solid substrate is needed
to have complete dissolution of the substrate. The reaction
rate proved to be greatly influenced by this parameter
(entries 7–9). For 4-benzoyloxyphenol (entries 7–9), reflux-
ing THF seems to be the better choice but the reaction is
still very slow. Salicylaldehyde is recovered unchanged
after 18 h (entry 10). The same reaction, carried out with
vanillin, provided a complex mixture of products with
74% conversion in 18 h (entry 11). To investigate in more
detail the influence of the presence of a CHO group on
the outcome of the reaction, we performed a series of trials
using benzaldehyde as the substrate.
Hydrosilylation of carbonyl compounds using Au(I) or
Au(III) complexes, and also a supported Au/CeO2 system
was reported to afford the corresponding silyl ether in good
yields.6–8 Surprisingly, we obtained the silylated pinacol
coupling product 3 (Scheme 2) together with the expected
silylalcohol 2 using our gold catalysts, as shown in
Table 2.22
in AuMVS the gold is essentially zerovalent, while the metal
19
surface of clusters is largely oxidized to Au(I) in AuDP
.
Our results of silane alcoholysis of various alcoholic sub-
strates are reported in Table 1.20,21
Entries 1–4 show that, for n-butanol, excess Et3SiH
increases reaction rate, but in any case PhMe2SiH is a
better choice than Et3SiH. Both AuMVS and AuDP catalysts
show practically the same activity. We generally used con-
ditions of entry 4 as optimized conditions for the other sub-
strates. With liquid substrates (entry 1–6), reactions were
carried out in the absence of solvent. Under the selected
conditions, the reaction proceeds smoothly only with pri-
mary alcohols of the analyzed series (entries 4–7), demon-
strating high chemoselectivity in these cases. tert-Butanol
is recovered practically unreacted after 3 h of reaction. If
the reaction time is prolonged to 18 h, we observe 55% of
tert-butanol conversion, but a complex mixture of uniden-
In most of the cases, the main product is 3 (meso/
dl = 50:50) instead of the expected 2. In particular, AuMVS
catalysts with PhMe2SiH afford 3 with a 80:20 selectivity
both for benzaldehyde and acetophenone (entries 2 and
4), while AuDP catalysts give predominantly the expected
Table 1
Gold catalyzed silane alcoholysis of various substrates
Entry
Alcohol
Silane
Cat
Solvent/T/time
Conversiona
1
2
3
4
Et3SiH
AuMVS 0.05%
—/100 °C/18 h
—/100 °C/6 h
—/100 °C/3 h
—/100 °C/3 h
82%
76%
95%
99% (92%)
Et3SiHb
OH
PhMe2SiH
PhMe2SiH
AuDP 0.05%
AuDP 0.05%
OH
5
6
PhMe2SiH
—/100 °C/3 h
99% (95%)
<1%c
OH
PhMe2SiH
PhMe2SiHb
AuDP 0.1%
—/100 °C/3 h
<1%d
49%
OH
7
8
9
AuDP 0.05%
Toluene/110 °C/3 h
1,4-Dioxane/100 °C/18 h
THF/65 °C/18 h
BnO
50% (44%)
OH
10
Et3SiH
AuMVS 0.1%
—/100 °C/18 h
<1%
CHO
OMe
OH
11
PhMe2SiH
AuMVS 0.1%
Toluene/110 °C/18 h
74% mixture of products
OHC
a
Determined by GC analysis; where not specified, 1 is detected as the sole product. In parenthesis, isolated yield of the product purified by column
chromatography (SiO2, hexane/AcOEt 4:1).
b
Silane/alcohol = 2.
55% conversion after 18 h, complex mixture of products.
5% conversion in silyl ether 1 after 18 h.
c
d

P. Raffa et al. / Tetrahedron Letters 49 (2008) 3221–3224
3223
Finally, we tested other functionalized substrates in the
reaction with PhMe2SiH under optimized conditions,
namely decylamine, dodecanethiol and methyl benzoate.
No reaction occurs in all these cases.
OSiR3
CHO
Aucat
OSiR3
+
R3SiH
OSiR3
3
2
In this Letter, we illustrated the first catalytic applica-
tion of supported Au nanoparticles in silane alcoholysis.
The reactions proceed with low amount of catalysts
(0.05% with respect to alcohol), under solvent-free condi-
tions for liquid substrates, without additional ligands and
in a very selective way for primary alcohols in the examined
cases. Since supported Au nanoparticles have proven to be
ineffective in hydrosilylation of some other functionalized
compounds, such as alkenes7, amines, thiols and esters, this
procedure seems to be suitable for the selective protection
of primary alcohols, or the synthesis of silyl ethers, in the
presence of various functionalities. Further investigation
is in progress about this opportunity. Moreover, the use
of low amounts of a supported catalysts (easier to recover
than homogeneous systems) and the absence of additional
ligand and solvents, make this reaction very promising. In
addition, we observed for the first time a gold catalyzed
silylative pinacol coupling of carbonyl compound and gave
some hypotheses about the reaction mechanism. Further
Scheme 2.
silylalcohol 2, with 60:40 selectivity, if PhMe2SiH is used
(entry 3). These findings suggests that the oxidation state
of gold could govern the product distribution: Au(I) spe-
cies may favour the classical hydrosilylation reaction, while
Au(0) species may favour the pinacol-type coupling.
Accordingly, if the AuDP catalyst is pre-treated with the
silane for 1 h at 100 °C before the addition of aldehyde
(reducing conditions), the selectivity is still switched to
the predominant formation of 3 (2:3 = 38:62). From a
mechanistic point of view, we can suppose that Au(I)
may favour an addition–elimination mechanism, while
Au(0) can act as a radical initiator which affords pinacol-
type coupling products with a meso/dl ratio of 50:50, by
analogy with reported mechanisms.8,23 A plausible mecha-
nism is depicted in Scheme 3.
Table 2
Au/Al2O3 catalyzed hydrosilylation of carbonyl compounds
Entry
Silane
Cat
Solvent/T/time
Conversiona
Selectivitya 2:3
1
2
3
4b
a
Et3SiH
AuMVS 0.1%
AuMVS 0.1%
AuDP 0.1%
—/100 °C/18 h
—/100 °C/18 h
—/100 °C/18 h
—/100 °C/24 h
95% (32%)
96% (50%)
77%
36:64
20:80
60:40
20:80
PhMe2SiH
PhMe2SiH
PhMe2SiHc
AuMVS 0.15%
99%
Determined by GC. In parenthesis, isolated yield (not optimized) of product 3 purified by column chromatography (SiO2, hexane/AcOEt 4:1).
Acetophenone used as substrate instead of benzaldehyde.
Silane/ketone = 2.
b
c
O
OSiR3
R'
O
R3Si
R'
Au
O
H
Ph R'
R'
R3Si
R'
O
R3Si
H
SiR3
R'
Au
Au
Au
H
O
OSiR3
H
R'
H2
Au(0)
H
Au(I)
R3Si
H
R3SiH
R3SiH
OSiR3
R'
R'
R3SiO
OSiR3
R'
Scheme 3. Plausible mechanisms for gold catalyzed hydrosilylation and silylative pinacol coupling.

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P. Raffa et al. / Tetrahedron Letters 49 (2008) 3221–3224
13. Field, L. D.; Messerle, B. A.; Rehr, M.; Soler, L. P.; Hambley, T. W.
Organometallics 2003, 22, 2387–2395.
14. Ito, H.; Watanabe, A.; Sawamura, M. Org. Lett. 2005, 7, 1869–1871.
15. Ito, H.; Watanabe, A.; Sawamura, M. Org. Lett. 2005, 7, 3001–3005.
16. Biffis, A.; Braga, M.; Basato, M. Adv. Synth. Catal. 2004, 346, 451–
458.
studies are required to define the scope of this new reaction
and to confirm the mechanistic suggestions. In conclusion,
a new reactivity for gold nanoparticles is shown, that we
hope could disclose new ways in gold catalyzed organic
reaction research.
17. Authentic sample from Caporusso. For catalyst characterization see
Ref. 9.
18. Prepared according to Haruta procedure: (a) Haruta, M. Cattech
2002, 6, 102–115; (b) Milone, C. et al. J. Catal. 2004, 222, 348–356.
19. Casaletto, M. P.; Longo, A.; Venezia, A. M.; Martorana, A.;
Prestianni, A. Appl. Catal., A 2006, 302, 309–316.
Acknowledgements
The author P.R. would like to acknowledge Dr. France-
sco Mazzini and Professor Igor Khovansky for the useful
suggestions and encouragement.
20. All reactants and reagents were purchased from Sigma–Aldrich and
used without further purification. GC analyses were performed with a
DB1 capillary column (30 m  0.52 mm, 5 lm) using He as the carrier
gas and a flame ionization detector (FID). 1H NMR spectra were
recorded with a Gemini 200 MHz instrument, in CDCl3 solution
using CHCl3 as internal standard. The metal content in the catalysts
was determined by atomic absorption spectrometry in an electro-
chemically heated graphite furnace with a Perkin–Elmer 4100ZL
instrument, after the dissolution of the solid in hot aqua regia.
21. General experimental procedure: Catalytic runs of solvent free
reactions have been carried out in Pyrex Carius tubes fitted with
Rotaflo taps. Reactions in solution were carried out in a two-necked
round flasks, equipped with refrigerator and magnetic stirrer. Under
inert atmosphere, to the indicated amount of Au/Al2O3 were added
via syringe 2 mmol of alcohol, 2 or 4 mmol of silane and, when
present, the reaction solvent. The suspension was stirred for a chosen
time at the reported temperature, filtered on Celite and the filtrate
analyzed by GC analysis. The formation of 1 was confirmed by
comparison with GC of authentic samples, obtained by standard
procedure (R3SiCl/Et3N/DMAP/CH2Cl2).
References and notes
1. Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987,
4, 405.
2. Haruta, M. Gold Bull. 2004, 37, 27–36.
3. Hutchigs, G. J. Catal. Today 2005, 100, 55–61.
4. Hutchings, G. J.; Hashmi, A. S. K. Angew. Chem., Int. Ed. 2006, 45,
7896–7936.
5. Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211.
6. Ito, H.; Yajima, T.; Tateiwa, J.; Hosomi, A. Chem. Commun. 2000,
981.
7. Corma, A.; Gonzalez-Arellano, C.; Iglesias, M.; Sanchez, F. Angew.
Chem., Int. Ed. 2007, 46, 7820–7822.
8. Lantos, D.; Contel, M.; Sanz, S.; Bodor, A.; Horvath, I. T. J.
Organomet. Chem. 2007, 692, 1799–1805.
9. Caporusso, A. M.; Aronica, L. A.; Schiavi, E.; Martra, G.; Vitulli, G.;
Salvadori, P. J. Organomet. Chem. 2005, 690, 1063–1066.
10. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 3rd ed.; Wiley & Sons: New York, 1999.
11. Anastas, P.; Warner, J. Green Chemistry: Theory and Practice; Oxford
University Press: New York, 1998.
22. General experimental procedure: Same as Ref. 21. Product 3 (meso/
dl = 50:50) for entry 1 was characterized by GC–MS and 1H NMR
analysis. GC–MS: m/z, (rel. int.) = 413 (M+À29, 1); 221 (99). H
NMR (200 MHz, CDCl3): 0.6–1.1 ppm, 30H, multiplet; 4.8 ppm, 1H,
singlet; 4.9 ppm, 1H, singlet, 7.1–7.5, 10H, multiplet.
12. Lukevics, E.; Dzintara, M. J. Organomet. Chem. 1985, 295, 265–315
(review article).
23. Hays, D. S.; Fu, G. C. J. Am. Chem. Soc. 1995, 117, 7283–7284.