Efficient room-temperature O-silylation of alcohols using a SBA-15-supported cobalt(II) nanocatalyst

Article abstract of DOI:10.1002/cbdv.201200071

The O-silylation of OH groups of alcohols and phenols with hexamethyldisilazane (HMDS) was achieved in high-to-excellent yields using catalytic quantities of a SBA-15-supported cobalt(II) nanocatalyst (typically 0.5 mol-%) at room temperature and under solvent-free conditions. Furthermore, the heterogeneous catalyst showed an excellent durability and can be conveniently reused by filtration for at least twelve times without any noticeable loss of activity. Copyright

Full text of DOI:10.1002/cbdv.201200071

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
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Efficient Room-Temperature O-Silylation of Alcohols Using a SBA-15-
Supported Cobalt(II) Nanocatalyst
by Fatemeh Rajabi*a), Rafael Luqueb), and Juan Carlos Serrano-Ruizb)
a) Department of Science, Payame Noor University, P.O. Box 19395-4697, Tehran,
Iran (fax: þ982813344081; e-mail: f_rajabi@pnu.ac.ir)
b
´
´
) Departamento de Quꢀmica Organica, Universidad de Cordoba, Campus de Rabanales, Edificio Marie
Curie (C-3), Ctra Nnal Iva, KM 396, E-14014 Cordoba (e-mail: q62alsor@uco.es)
The O-silylation of OH groups of alcohols and phenols with hexamethyldisilazane (HMDS) was
achieved in high-to-excellent yields using catalytic quantities of a SBA-15-supported cobalt(II)
nanocatalyst (typically 0.5 mol-%) at room temperature and under solvent-free conditions. Furthermore,
the heterogeneous catalyst showed an excellent durability and can be conveniently reused by filtration
for at least twelve times without any noticeable loss of activity.
Introduction. – The use of protecting groups for functional groups is highly
significant in organic synthesis, with silylation being one of the most relevant protection
strategies. Silyl ethers are among the most frequently used reagents for OH-group
protection due to their stability in non-acidic reagents and high solubility in non-polar
solvents [1–3], as well as to their ability to increase volatility of the compounds to
facilitate their analysis by gas chromatography and mass spectroscopy [4][5]. Chiral
silyl ethers are also valuable building blocks to prepare biologically important
molecules including neocarzinostatins, prostaglandins, thromboxane, and nucleosides
[6][7]. Also, they serve as precursors to prepare SiꢀO bonded polymeric and
organosilicon compounds [8].
Three different approaches are commonly used for silylation of OH groups, and
these can be categorized as follows: 1) silylation of OH compounds by silyl halides or
silyl triflates in the presence of stoichiometric amounts of a base; 2) cleavage of SiꢀN or
SiꢀS bonds with alcohols; 3) dehydrogenative coupling reaction between SiꢀH and OH
groups. Although these procedures have been found to be reliable, they often involve a
challenging removal of bases and hazardous reagents added to the reaction, as well as a
low reactivity [1][2]. Current major drivers for a future more-sustainable society plead
for alternative chemical transformations of high efficiency, low cost, and easy
separation with a minimum waste production.
In this regard, hexamethyldisilazane (HMDS) has been reported as an inexpensive,
commercially available, and easy-to-handle chemical for the protecting of a range of
substrates [2][9]. However, HMDS has an intrinsic low silylation activity towards OH
groups. A variety of catalysts have been reported to activate HDMS for the
introduction of Si-protecting group into OH compounds [10]. These include sulfonic
acids [11] and sulfates [12], zirconium sulfophenyl phosphonate [13], lithium
ꢁ 2012 Verlag Helvetica Chimica Acta AG, Zꢂrich

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CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
perchlorate [4], H-bzeolite [14], triflates [15][16], and silica-supported perchloric acid
[10].
The reported procedures improve silylation of OH groups, in many instances,
however, long reaction times, harmful organic solvent media, drastic reaction
conditions, or tedious workup are among the disadvantages. Furthermore, most of
these reagents have significant activity losses due to the leaching of the active center in
solution and cannot be reused.
To maximize activities and selectivities for this important reaction, as well as
minimizing waste and the use of harsh reaction conditions and/or hazardous reagents,
the design of novel cost-effective, recyclable, non-corrosive, and stable high-efficiency
catalysts is a major priority in heterogeneous catalysis. In addition, the development of
solvent-free technologies to minimize the amount of effluents has become one of the
most important challenges for ꢃgreen chemistryꢄ. Recently, the combination of two
prominent green chemistry principles, namely solvent-free conditions and solid
supported catalysts, has become very popular and received substantial interest
[17][18].
In continuation of our recent findings on the use of heterogeneous CoII nanocatalyst
for the development of useful synthetic methodologies in organic synthesis [19][20],
herein, we report a simple and efficient method for the silylation of various OH groups
with a catalytic amount of CoII/SBA-15 under solvent-free conditions. The catalytic
activity of CoII/SBA-15 was first examined for the room-temperature silylation of
benzyl alcohol using HMDS. Once conditions are optimized for this substrate, the
methodology is extended to a wide range of alcohols under the same mild conditions
and without solvent.
Experimental. – Catalyst Preparation. Salicylaldehyde (2 mmol, 0.244 g) was added to an excess of
abs. MeOH, to which (3-aminopropyl)(trimethoxy)silane (2 mmol, 0.352g) was subsequently added. The
soln. instantly became yellow due to imine formation. After 3 h, Co(OAc)2 ·2 H2O (1 mmol, 0.248 g) was
added to the soln., and the mixture was further stirred for 3 h to allow the new ligands to complex Co. A
color change from pink to olive green was observed. Mesoporous silica (average pore diameter 60 ꢅ; 3 g)
was activated by refluxing in conc. HCl (6m) and then washed thoroughly with the deionized H2O and
dried before undergoing chemical-surface modification. This activation treatment readily hydrolyzed the
siloxane SiꢀOꢀSi bonds to SiꢀOH species, which will be key to anchor the Co complex. The complex and
the activated silica were then mixed, and the mixture was stirred overnight. The solvent was removed
using a rotary evaporator, and the resulting olive green solid dried at 808 overnight. The final product was
washed with MeOH and H2O (to remove all physisorbed metal species), until the washings were
colorless. Further drying of the solid product was carried out in an oven at 808 for 8 h.
Catalytic Experiments. A typical silylation of OH groups was performed as follows (Scheme): 1 mmol
of alcohol/phenol, was added to a mixture of CoII/SBA-15 (0.005 mmol, 0.017 g) and HMDS (0.6 mmol).
The mixture was then stirred at room temperature for the appropriate time. After completion of the
reaction, hexane (10 ml) was added, and the CoII catalyst was removed from the mixture by filtration,
rinsed with hexane (10 ml), and reused. The filtrate was washed with H2O, the solvent was evaporated,
Scheme

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
1825
and the final product was isolated in most cases in almost pure form. Further purification was carried out
by column chromatography on silica gel, eluting with AcOEt/petroleum ether. All compounds were
characterized by IR, NMR, and mass spectra.
Results and Discussion. – The aforementioned catalyst has been recently
established as a highly active catalyst for a wide range of reactions including oxidations,
acetalizations, etherifications, and others [19–21]. Optimization reactions performed
for the silylation of benzyl alcohol with HMDS over CoII/SBA-15 are compiled in
Table 1. The optimum benzyl alcohol/(CoII/SBA-15)/HMDS molar ratio was found to
be 1:0.005 :0.6 in order to achieve completion of the reaction within 15 min, producing
the corresponding silyl ether in 99% isolated yield (Table 1, Entry 3).
Table 1. Effect of Amount of Catalyst on the Yield of Silylation of Benzyl Alcohol Using HMDSa)
Entry
Catalyst [mol-%]
Time [min]
Yield [%]b)
1
2
3
4
none
0.2
0.5
1
60
60
15
5
6
57
99
99
a) Reaction conditions: molar ratio alcohol/HMDS 1 : 0.6 at room temperature. b) Isolated yields.
To ascertain the truly heterogeneous nature of the proposed protocol, the silylation
of benzyl alcohol was conducted under normal reaction conditions, and typically
stopped halfway through reaction completion (ca. 50% conversion). At this time, the
solid catalyst was filtered off, and the filtrate was allowed to react for the normal
reaction time. Analysis of the mixture clearly demonstrated that the reaction
completely stopped, then the solid was filtered off, and no further conversion was
observed after filtration. Based on atomic absorption spectroscopy (AAS) analysis, it
was found that no detectable Co species migrated to the solution under the investigated
conditions (<0.5 ppm). This clearly indicates the heterogeneous nature of the protocol
in which the supported Co catalyst promotes the reaction and not any Co species
leached into solution. In good agreement with previous leaching experiments, the solid
catalyst maintains the initial activity of the fresh catalyst upon consecutive reusing,
preserving over 95% of their initial activity after twelve reaction cycles (Fig.).
However, further uses of the catalyst showed that, from the 13th cycle, the catalyst
slightly deactivated and needed longer reaction times to achieve yields of 95%.
Furthermore, it was confirmed by AAS, a trace of Co (0.8 ppm) was detectable in the
solution.
Having satisfactorily established optimized reaction conditions for benzyl alcohol,
we then proceeded to explore the scope of the catalytic silylation reaction. As shown in
Table 2, a wide range of OH groups, including aliphatic alcohols, phenols, diols, and
triols, furnished the corresponding silyl ethers in excellent yields and at short reaction
times. The CoII/SBA-15 catalyst showed outstanding activity for the silylation of bulky
phenols (Table 2, Entries 7 and 8), as well as it proved very efficient for silylation of OH
groups without affecting other functional groups present in the molecule such as
lactones (Entries 9 and 10) or chloride (Entries 11 and 13). The reaction was also

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Table 2. Scope of Substrates in the Co-catalyzed O-Silylation of Substituted Alcohols with HMDSa)
Entry
1
Substrate
Product
Time [min]
15
Yield [%]b)
98
2
3
4
10
30
15
99
97
99
5
6
7
30
45
90
99
99
92
8
9
60
60
99
95
10
75
92
11
12
13
90
90
90
99
99
99
a) Reaction conditions: molar ratio alcohol/(CoII/SBA-15)/HMDS 1 : 0.005 : 0.6. b) Total yield.
amenable to different substituents as well as to interesting biomass platform molecules
(e.g., glycerol), which could be selectively and fully silylated (including the less reactive
OH group in position 2) in a short time of reaction at room temperature.
Conclusions. – A highly efficient silylation reaction of a wide range of alcohols and
phenols was carried out in the presence of catalytic amounts of CoII/SBA-15 material at
room temperature and without solvent. This material showed excellent catalytic

CHEMISTRY & BIODIVERSITY – Vol. 9 (2012)
1827
Figure. Re-use of the Co/SBA-15 catalyst in the silylation of benzyl alcohol under optimized conditions
activity, affording silylation products in excellent yields and in short reaction times. The
catalyst could be easily separated from the reaction mixture by simple filtration, and it
could be reused twelve times with retention of its high catalytic activity. The present
methodology provides a novel, very mild, solvent-free procedure for the preparation of
various silyl ethers in good-to-excellent yields. Further investigations to extend the
scope and synthetic applications of CoII/SBA-15 catalyst are currently under way in our
laboratories.
F. R. is grateful to Payame Noor University and Iran National Science Foundation (INSF) for
support of this work. R. L. gratefully acknowledges support from the Ministerio de Ciencia e Innovacion,
˜
Gobierno de Espana through a Ramon y Cajal Contract (ref. RYC-2009-04199) and funding from
Consejeria de Ciencia e Innovacion, Junta de Andalucia (project P10-FQM-6711) and Ministerio de
Economia y Competitividad (MEC) for funding under project CTQ2011-28954-C02-02.
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Received February 24, 2012