Titanium silicates as efficient catalyst for alkylation and acylation of silyl enol ethers under liquid-phase conditions

Article abstract of DOI:10.1016/j.molcata.2011.06.013

The activity of titanium- and tin-silicate samples such as TS-1, TS-2, Ti-β and Sn-MFI has been investigated for acylation and alkylation of silyl enol ethers under mild liquid-phase conditions. Silyl enol ethers successfully react with acetyl chloride and tert-butyl chloride under dry conditions in the presence of above catalysts to produce the corresponding acylated and alkylated products, respectively. In the case of acetylation reaction, two different nucleophiles with carbon-center (C-atom) and oxygen-center (O-atom) in silyloxy group of silyl enol ether reacts with acetyl chloride to give 1,3-diketone and ketene-ester, respectively. The selectivity for alkylation is always ca. 100% and no side products are formed. Among the various solvents investigated, anhydrous THF was found to be the suitable solvent for alkylation; whereas dichloromethane exhibited high selectivity for diketones for acylation. The formation of nucleophiles from silyl enol ethers appears to be the key step for successful acetylation and tert-butylation by nucleophilic reaction mechanism. Sn-MFI showed less activity than that observed over the titanosilicates. The observed catalytic activity is explained on the basis of "oxophilic Lewis acidity" of titanium silicate molecular sieves in the absence of H ₂O under dry reaction conditions.

Full text of DOI:10.1016/j.molcata.2011.06.013

Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Titanium silicates as efficient catalyst for alkylation and acylation of silyl enol
ethers under liquid-phase conditions
Manickam Sasidharana, Asim Bhaumikb,∗
a Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan
b Department of Material Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The activity of titanium- and tin-silicate samples such as TS-1, TS-2, Ti-␤ and Sn-MFI has been investi-
gated for acylation and alkylation of silyl enol ethers under mild liquid-phase conditions. Silyl enol ethers
successfully react with acetyl chloride and tert-butyl chloride under dry conditions in the presence of
above catalysts to produce the corresponding acylated and alkylated products, respectively. In the case
of acetylation reaction, two different nucleophiles with carbon-center (C-atom) and oxygen-center (O-
atom) in silyloxy group of silyl enol ether reacts with acetyl chloride to give 1,3-diketone and ketene-ester,
respectively. The selectivity for alkylation is always ca. 100% and no side products are formed. Among
the various solvents investigated, anhydrous THF was found to be the suitable solvent for alkylation;
whereas dichloromethane exhibited high selectivity for diketones for acylation. The formation of nucle-
ophiles from silyl enol ethers appears to be the key step for successful acetylation and tert-butylation by
nucleophilic reaction mechanism. Sn-MFI showed less activity than that observed over the titanosilicates.
The observed catalytic activity is explained on the basis of “oxophilic Lewis acidity” of titanium silicate
molecular sieves in the absence of H2O under dry reaction conditions.
Received 18 April 2011
Received in revised form 27 June 2011
Accepted 30 June 2011
Available online 6 July 2011
Keywords:
Titanium silicates
Alkylation
Acylation
Silyl enol ethers
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
tion. For instance, Murakami and co-workers have proved that
alkali-exchanged zeolites such as Rb-, Cs-, and K–X have been effi-
Acylation and alkylation are important synthetic reactions for
synthesizing aromatic ketones, alkylated products, and a vari-
ety of key intermediates widely used in the fragrances, fine
chemicals, and pharmaceuticals industries. These reactions gen-
erally use homogeneous Lewis acids such as AlCl3, BF3, FeCl3,
etc. or Brönsted acids like HF and polyphosphoric acid as
catalysts in stoichiometric quantities [1] under homogeneous
reaction conditions. The corrosive nature coupled with problems
associated while disposing the spent Lewis acids and mineral
acids have led to environmental concerns that have stimulated
research aimed at developing safer alternate solid catalysts. Fur-
thermore, the acylation and alkylation reactions are generally
carried out through electrophilic pathway, which often leads more
side products due to their severe reaction conditions at high
ciently used for side-chain alkylation of toluene with methanol or
formaldehyde [2]. Solid bases such as alkaline earth oxides [3,4],
hydrotalcites [5,6], and aluminophosphate oxynitrides (ALPONs,
prepared by nitridation of AlPO4) [7] have been used for nucle-
ophilic reactions involving nucleophile or carbanion-type species
for formation of new carbon–carbon bonds through terminal alky-
lation like Knoevenagel and aldol condensations, and Michael
addition, among others. Unlike the electrophilic reactions at high
temperature, the nucleophilic reactions often lead to high con-
version of substrate as well as product selectivity under mild
liquid-phase conditions.
Microporous aluminosilicates, particularly MFI and Faujasite
type, have been extensively investigated to catalyze the nucle-
ophilic C–N (addition of ammonia and amines to olefins), C–O
(addition of H2O and alcohols to alkenes) and C–S (addition of H2S
to olefins) bond formation reactions [8–11] under vapor-phase con-
ditions at high temperature. However, nucleophilic carbon–carbon
bond formation reactions involving carbon-anion (carbanion) has
been scarcely studied both under liquid- as well as vapor-phase
conditions. The successful synthesis of titanium silicate molecular
sieves such as TS-1, TS-2, and Ti-␤ [12–15] and their poten-
tial application for a variety of oxidation reactions have been
extensively studied under heterogeneous conditions using dilute
H2O2 as oxidant [16,17]. However, the utility of these titanium
silicates in areas other than oxidation catalysis has not been stud-
temperature. Alternatively, nucleophilic reactions involving
a
nucleophile (electron rich atom or group of atoms) is an impor-
tant route for formation of carbon–carbon bonds with high product
selectivity under mild-conditions. In zeolite catalytic systems,
although it is difficult to obtain direct evidence for carbanions
participation, their presence can reasonably be inferred from
the reaction chemistry, e.g., in the aryl-ring side-chain alkyla-
∗
Corresponding author.
E-mail address: msab@iacs.res.in (A. Bhaumik).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.06.013

88
M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
ied in depth. We have previously demonstrated the efficiency
of the titanium silicates for carbon-carbon bond formation reac-
tions in Mukaiyama-type aldol condensation and Michael addition
reactions [18–20] in the absence of H2O. It was observed that
titanosilicates play an important role in activating the “leaving
group” and to produce the nucleophile which leads to smooth
nucleophilic reactions with high product selectivity. Normally, the
acylation reaction of silyl enol ethers with acid chloride [21–23]
and alkylation reaction using SN1 active tert-alkyl halide [24,25]
are generally catalyzed by homogenous Lewis-acids such as TiCl4,
SnCl4, ZnCl2, FeCl3, etc. in equimolar quantities at −40 ◦C to 70 ◦C.
Here we report, titanium silicate molecular sieves as efficient Lewis
acid catalysts for acylation and alkylation of silyl enol ethers with
acetyl chloride and tert-butyl chloride, respectively.
the calcined samples was estimated by inductively coupled plasma
emission spectrometry (ICP) on a Rigaku JY38S spectrometer and by
using a Shimadzu AA-6300 atomic absorption spectrometer (AAS)
fitted with a double beam monochromator. BET surface area and
micropore volumes were measured by N2 adsorption at 77 K on a
BELSORB 28SA analyzer after evacuation at 300 ◦C for 3 h. The reac-
tion products were analyzed on a Shimadzu 14B gas chromatograph
equipped with OV-1 capillary column fitted with an FID detector.
2.3. Catalytic reaction procedure
All the catalysts were calcined at 773 K for 6 h to remove the
organic templates and activated at 423 K for 2 h before the cat-
alytic reactions. In a typical reaction for tert-butylation, 10 mmol
of silyl enol ether was added to dry THF (15 mL) containing 20 wt%
of activated TS-1. To the above mixture, 10 mmol tert-butyl chlo-
ride was added and the reaction temperature was raised to 333 K.
The content was magnetically stirred for 16 h and the progress of
the reaction was monitored by gas chromatography. In the case
of acylation, dichloromethane was used as reaction medium. For
acylation, the mixture of silyl enol ether and acetylchloride in dry
dichloromethane was vigorously stirred at 313 K for 5 h. After com-
pletion of reaction, the catalyst was filtered and the excess acetyl
chloride was removed by washing with water. The products were
quantified by GC analysis and the unknown products were identi-
fied through authentic samples, and GC–MS splitting pattern.
2. Experimental
2.1. Synthesis of different titanium silicates
In a typical synthesis of TS-1 [13], 45 g of tetrapropylammonium
hydroxide (20% aqueous solution) was added to 27.1 g of tetraethyl
orthosilicate (TCI, 98%) under vigorous stirring for 3 h. Then 1.06 g
tetrabutyl orthotitanate dissolved in dry isopropanol was added
slowly to obtain a clear homogeneous solution and steadily stirred
for an extended period of time to evaporate the ethanol produced
from the hydrolysis to obtain the gel composition: 0.25 SiO2: 0.008
TiO2: 0.09 TPAOH: 6.3 H2O. The gel was crystallized under agitation
at 443 K for 24 h and the product was centrifuged to remove the very
fine particles, washed thoroughly and finally calcined at 773 K for
12 h in flowing air.
Ti-␤ was synthesized in a basic medium by modifying the
reported procedure [15]. Ti(OBu)4 and H2O2 were mixed with
water under vigorous stirring to get a titaniumperoxo complex. This
complex was added to tetraethylammonium hydroxide (Aldrich,
35% aqueous solution) followed by the addition of Aerosil-200 sil-
ica and the stirring was continued until a homogeneous thick gel
was obtained after the addition of 2% dealuminated-beta seed crys-
tals. The resulting gel (SiO2:0.025 TiO2:0.55 TEAOH:0.337 H2O2:6.3
H2O) was transferred into a Teflon-lined autoclave, and the crys-
tallization was carried out at 413 K for 5 days. The as-synthesized
catalysts were dried at 373 K and calcined in O2 flow for 12 h at
793 K.
3. Results and discussion
3.1. Activity of different titanium silicates for tert-butylation of
silylenol ether
The metallosilicates were thoroughly characterized by XRD,
UV–vis, and FTIR spectroscopies, scanning electron microcope
(SEM), elemental analysis by ICP and AAS, and sorption/surface
area measurements. Fig. 1 shows diffused reflectance spectra of
TS-1, TS-2, and Ti-␤ exhibiting a sharp absorption band around
218 nm, characteristic of titanium present in the tetrahedral envi-
ronment [13–15]. The six coordinated octahedral titanium species
normally appear at 270 nm was found to be negligible for all
The catalyst TS-2 was prepared using tetrbutylammonium
hydroxide (40%, Aldrich) [14] according to the gel composition:
SiO2:0.025 TiO2:0.35 TBAOH:33.3 H2O under static condition at
443 K for 48 h.
Sn-containing microporous silica with MFI framework topol-
ogy (Sn-MFI) was prepared according to the reported literature by
Ramaswamy and Mal [26]. TEOS, tetrapropylammonium hydrox-
ide, and SnCl4 were used under basic medium and the gel was
crystallized hydrothermally at 443 K for 40 h [26]. All the cata-
lysts were activated at 373 K under vacuum for 3 h prior to the
commencement of reaction.
2.2. Characterization of titanosilicates
Phase purity of titanosilicates was determined from powder
X-ray diffraction (XRD) patterns using a Rint 2000 diffractome-
ter with Cu K␣ radiation in the region of 2ꢀ = 5–50. UV–visible
diffuse reflectance spectra were recorded on a JASCO V–550 spec-
trometer using BaSO4 as a reference. Scanning electron microscopy
(SEM) observation was made on a JEOL JSM–T220 instrument after
suspending the sample in ethanol followed by gold sputtering.
FTIR spectra were recorded on a Shimadzu FTIR-8100 spectrom-
eter using a KBr pellet technique. The amount of Ti present in
Fig. 1. UV–vis diffuse reflectance spectra of: TS-2 (a), Ti-␤ (b) and TS-1 (c).

M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
89
O
H3C
Cl
C
H3C
H3C
THF
OSiMe3
CH3COCl
O
O
COCH3
O
CH3
Dichloromethane
+
C-acylation
O-acylation
Scheme 1. Acetylation and tert-butylation of silyl enol ethers over Ti-␤.
the titanium silicates investigated under the present study. Fur-
thermore, the absence of an absorption band at 300 nm suggests
that these samples are completely free from bulk anatase phase.
UV–visible spectrum of Sn-MFI exhibited exclusively tetrahedral
species from the absorption band at 212 nm [26]. The physical char-
acteristics of different titanosilicates and tin-silicates are listed in
Table 1. The SEM pictures and XRD patterns of the metallosilicates
suggest that they are highly crystalline (Figs. S1 and S2, support-
ing information). The FTIR spectra of these metallosilicates exhibit a
band around 950–970 cm−1 due to M–O stretching vibrations sug-
gesting the presence of metals in the silica network. ICP and AAS
chemical analyses data revealed Si/Ti mole ratios of TS-1, TS-2 and
Ti of 35.0, 47.0 and 41.0 respectively, whereas Si/Sn mole ratio in
Sn-MFI was 58.0 (Table 1).
The alkylation, more specifically, ␣-tert-butylation of silylenol
ethers of ketones with tert-butyl chloride is depicted in the
Scheme 1. It is worthy to point out that, primary and secondary alkyl
halides undergo reactions by SN2 pathway; however in the present
study, tert-butyl chloride was used as alkylating agent, which is
expected to react by SN1 reaction mechanism. Fig. 2 exhibits the
reaction progress with time for the tert-butylation of silyl enol ether
over various metallosilicates and separate experimental runs were
carried out at each point of graphs. Among the various titanosili-
cates investigated, large-pore Ti-␤ shows the best activity (59.1%)
than the other metallosilicates. Interestingly, the medium-pore TS-
1 also exhibited comparable activity to that of Ti-␤; however, the
activity of TS-2 (Si/Ti = 47) and Sn-MFI (Si/Sn = 58) show relatively
lower activities in accordance with lower metal contents. How-
ever, the intrinsic activity of all the metallosilicates increases rather
linearly as the reaction progress.
Fig. 2. Effect of reaction time over conversion for tert-butylation of cyclohexyl silyl
enol ether using different metallosilicates: Ti-␤ (a), TS-1 (b), TS-2 (c) and Sn-MFI (d).
ride leading to the formation of SN1 substitution products. The
interaction of Lewis acid oxo-philic sites of titanium-silicates with
silylenol ether leads to the formation of carbon-nucleophiles under
dry conditions. The nucleophilic species then attacks the car-
bocation (tert-butyl chloride) in SN1 fashion as observed under
homogeneous condition [24,25]. Entries 1–3 in Table 2 show the
suitability of different dry solvents for tert-butylation of silyl enol
ether with tert-butyl chloride and THF showed better activity than
dichloromethane or acetonitrile. The ␣-tert-butyl ketones were
the sole reaction products and no side products were observed.
The remaining material was the unreacted starting materials.
The reactivity of different silyl enol ethers for tert-butylation
is shown in entries 3–6. The cyclohexanone (entry 3) and 4-
methylcyclohexanone (entry 4) derivative of silyl enol ethers
exhibit higher conversion of 59.1 and 54.8%, respectively, than
the cyclopentanone derivative (entry 5, 47.3%). The open-chain
methyl trimethylsilyl dimethylketene acetal (entry 6) also pro-
3.2. Tert-butylation of various silyl enol ethers
Table 2 shows the tert-butylation of various silyl enol ethers.
The SN1 reaction involves the attack of carbon-nucleophile on
either side of stable planar electrophilic-carbon of tert-butyl chlo-
Table 1
Physiochemical characteristics of metallosilicates.
Catalyst
Si/M mole ratio (product)a
Particle size (␮m)b
BET surface areac (m2 g−1
)
Micropore volumec (mL g−1
)
TS-1
TS-2
Ti-␤
Sn-MFI
35.0
47.0
41.0
58.0
0.1–0.2
0.3–0.4
0.2–0.3
0.2–0.4
535
469
648
451
0.15
0.13
0.28
0.14
a
Si/M was calculated from elemental analysis by ICP and AAS.
Particle size was estimated from SEM analysis.
BET surface area and micropore volume were calculated from N2 sorption analysis.
b
c

90
M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
Table 2
Tert-butylation of silyl enol ethers over various metallosilicates.a
Entry
Catalyst
Metal content, wt.%
Reactant
Product
Yield, mole%
OSiMe3
O
1b
2c
3
Ti-␤
Ti-␤
Ti-␤
1.88
1.88
1.88
42.6
35.3
59.1
OSiMe3
OSiMe3
OSiMe3
O
O
O
4
Ti-␤
1.88
54.8
OSiMe3
O
5
6
Ti-␤
Ti-␤
1.88
1.88
47.3
46.7
O
OSiMe3
OMe
OSiMe3
O
O
O
O
O
7
TS-1
2.19
1.64
3.27
–
55.4
49.6
43.1
Nil
OSiMe3
OSiMe3
OSiMe3
OSiMe3
H
8
TS-2
9
Sn-MFI
10
11
No catalyst
SiO2 + H-ZSM-5
2.28
87.0
+
C3H7
Cl
C
12d
Ti-␤
1.88
No alkylation
–
a
Reaction conditions: silyl enol ether = 10 mmol, tert-butylchloride = 10 mmol, catalyst = 20 wt%, THF = 15 mL, temperature = 333 K, and reaction time = 16 h.
Acetonitrile as solvent.
Dichloromethane as solvent.
tert-butylation of 2-methyl-2-hexene under identical reaction conditions.
b
c
d
duced the corresponding alkylated product with moderate yield
reaction conditions but no alkylation was noticed. The above result
unambiguously proves nucleophilic alkylation over the titanium-
and tin-silicates. Furthermore, the similarity between medium-
pore TS-1 and large-pore Ti-␤ in their activity leads to whether
the reaction is intra porous or surface assisted. However, the basic
aspects regarding the nucleophilic reactions under homogeneous
conditions suggest that the formation of nucleophile is a key step
[27] and the charged nucleophiles can travel through the reaction
medium to attack the electron deficient or electrophilic carbon cen-
ter to give the desired product. Therefore, the facile formation of
nucleophile from silyl enol ethers by the catalytic action of titanosil-
icates could be the key step for successful tert-butylation of silyl
enol ethers.
(46.7%). Entries 7–9 exhibit the activities of TS-1, TS-2, and Sn-
ZSM-5. Similar to Ti-␤, the medium-pore titanium silicates (entries
7 and 8) also lead to facile reaction with good yield; however,
Sn-MFI realized lower yield than the medium-pore titanium sil-
icates. As expected, the reaction fails to undergo in the absence
of catalysts (entry 10). To obtain more insight over the role of
Brönsted acid-sites, the reaction was carried with a mixture of sil-
ica and H-ZSM-5 catalyst (entry 11, Al = 2.28 wt%) but no desired
product was obtained. Only cyclohexanone was formed due to
cleavage of silyl enol ethers. In order to prove the nucleophilic
reaction indirectly, tert-butylation was attempted with 2-methyl-
2-hexene (entry 12) instead of silyl enol ethers under identical

M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
91
Table 3
Acylation of silyl enol ethers over various metallosilicates.a
Entry
Catalyst
Reaction
Product distribution
Diketone (C-acylation)
Ketene-ester (O-acylation)
OSiMe3
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
CH3COCl
+
1b
2c
3d
4
Ti-␤
82.4
79.5
68.5
89.7
87.2
85.8
84.1
17.6
20.5
31.5
10.3
12.8
14.2
15.9
OMe
OSiMe3
+
OMe
Ti-␤
OSiMe3
+
OMe
Ti-␤
OSiMe3
+
OMe
Ti-␤
OSiMe3
+
OMe
5
TS-1
TS-2
Sn-MFI
OSiMe3
+
OMe
6
OSiMe3
+
OMe
OSiMe3
7
+
CH3COCl
8
Ti-␤
83.5
16.5
OSiMe3
+
CH3COCl
CH3COCl
9
Ti-␤
Ti-␤
Ti-␤
Ti-␤
Ti-␤
86.0
80.2
86.3
83.9
63.6
14.0
19.8
16.7
16.1
36.4
OSiMe3
+
10
11
12e
OSiMe3
CH3CH2COCl
+
OMe
OSiMe3
CH3COCl
CH3COCl
+
OMe
OSiMe3
+
13f
OMe
a
Reaction conditions: Silyl enol ether = 10 mmol, acetyl chloride = 10 mmol, catalyst = 20 wt%, dichloromethane = 15 mL, temperature = 313 K, and reaction time = 5 h.
b
c
d
e
f
THF.
Nitromethane.
Acetonitrile.
Reaction was performed with uncalcined Ti-␤ and 16.3% conversion was observed.
Small amount of water was added during the reaction and the conversion was 42.9%.
3.3. Acylation of silyl enol ethers
always quantitative although there is a variation in product dis-
tribution. Among the titanosiliates, the Ti-␤ and TS-1 showed high
tendency for carbon–carbon bond formation with minor portion
of ketene–ester formation. The acetylation of different silyl enol
ethers under standardized reaction conditions produced the corre-
sponding diketones from 80.2 to 86.0% (entries 8–10). The use of
other acylating agent such as propionyl chloride also produced a
mixture of diketone and ketene–esters with similar product selec-
tivity (entry 11). On contrary to homogeneous Lewis acids catalyst
[21,28], the Brönsted acidity originating from silanol groups cannot
be neglected over heterogeneous catalysts and produces apprecia-
ble amount of O-acylated products. The product distribution in the
acylation reaction suggest that the carbanion (nucleophile) formed
from silyl enol ether reacts with the electron deficient carbon of
–C O of acid chloride to give desired diketones. On contrary, Brön-
sted acid promoted reaction of chloride ions (of acetyl chloride)
with SiMe3 (leaving group) to form neutral SiMe3Cl prior to dou-
ble bond migration in the silyl enol ether leads to the formation
of carbon–oxygen bond to give ketene–esters. In order to obtain
Table 3 exhibits the acylation of silyl enol ethers with acetyl
and propionyl chloride over different metallosilicates under mild
liquid-phase conditions (Scheme 1). All the reactions were per-
formed under perfectly dry conditions as that of alkylation, and
all the catalysts invariably show quantitative conversion. Similar
to nucleophilic alkylation, the acylation over microporous met-
allosilicates produces diketones as the major product through
carbon–carbon bond formation. However, ketene-ester is also
formed as the minor side-product through carbon–oxygen bond
formation. Among the various dry solvents investigated for reac-
tion between methyl trimethylsilyl dimethylketene acetal and
acetyl chloride (entries 1–4), dichloromethane was found to be
an efficient solvent as it leads to significantly less amount of the
side-product ketene–ester and therefore, dichloromethane was
used as solvent for further investigation. The acylation reaction
also proceeds smoothly over different titanosilicates as well as
Sn-silicates (entries 5–7); the conversion of silyl enol ether is

92
M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
Ti
A
H3C
H3C
H3C
Me3Si
O
O
Ti-silicate
Dry THF
+
Cl
C
MeO
OMe
Ti
H3C
H3C
H3C
H3C
+
B
C
Cl
C
H3C
H3C
O
OMe
SiMe3
Scheme 2. Formation of nucleophile and subsequent carbon–carbon bond during alkylation and acylation over titanium silicates.
information on the role of external surface, the uncalcined Ti-␤ was
used in the acetylation (entry 12) and the observed conversion was
very low due to absence of reaction inside the pores and channels
(since they are filled with template). Hence, the small amount of
product formed over the uncalcined Ti-␤ is attributed to catalytic
activity of surface titanyl groups. Furthermore, decreased activity
was observed on addition of a small amount of water in the reac-
tion medium (entry 13). This result suggests poisoning of Lewis acid
sites, which are primary responsible for the nucleophilic reactions.
In addition, the presence of a little H2O could promote Brönsted acid
sites, which may possibly increase the O-acylated product. Further-
more, the stability of Ti-␤ and TS-1 were also studied by successive
reuse of these catalysts for four times after activating the catalyst
at 623 K for 3 h in air to remove the occluded solvent and organic
molecules present in the pores and channels. In the case of tert-
butylation after four successive reuses, the activities of Ti-␤ and
TS-1 were found to be 97.2 and 98.0%, respectively. In addition,
the analysis of reaction mixture after evaporation of solvents by
AAS indicates a negligible amount of leaching of metals and thus
confirms that the titanium active sites are quite stable under the
alkylation and acylation conditions.
titanium- and tin-silicates can activate C–Cl bond of (CH3)3C–Cl
(making Cl− a better leaving group) and generate carbocation
(CH3)3C+, which could be attacked by nucleophile silyl enol ether
to yield the ␣-alkylated ketone (Scheme 2, path B). Therefore, in
either case as soon as carbon-nucleophile is formed, it can travel
through the reaction medium to attack electron deficient or elec-
trophilic carbon either inside the channel or outside pores to give
final product. Furthermore, when the nucleophilic reaction occurs
before the migration of double bond, as in the case of acyla-
tion, the ultimate product is ketene–ester due to carbon–oxygen
bond formation. As expected, the addition of small amount of
water decreased the conversion (Table 3, entry 13) probably by
poisoning the Lewis acid sites. Thus it confirmed the role of
Lewis acid sites in these catalytic reactions. Ti4+ in the titanium
silicates with its d0 configuration behaves as oxophilic site to
activate the O–Si bond in silyl enol ethers even though some
Lewis acid sites are poisoned in the presence of water. Thus, the
Lewis acidity of titanium silicates in the complete absence of H2O
would promote carbon–carbon bond formation reactions in the
alkylation and acylation of silyl enol ethers under liquid-phase con-
ditions.
3.4. Probable reaction pathway over titanium silicates
4. Conclusions
In the absence of water TS-1 shows oxophilic Lewis acidic
property [29,30] while the possibility of Brönsted acid sites has
been suggested in the presence of H2O2 and H2O [31–35]. Since
both alkylation and acylation reactions were carried out under
absolutely dry conditions, Lewis acidity can play an important
role in the formation of nucleophiles according to our previous
reports [18–20]. The probable reaction mechanism may involve
the activation of enolic-oxygen of silyl enol ethers followed by
the migration of a double bond, thus facilitating the formation
of nucleophilic moiety at the ␤-carbon (Scheme 2, pathway A),
which subsequently lead to carbon–carbon bond formation [36,37].
In addition, the observed activity of Ti-␤ with large-pore size is
only marginally higher than that of TS-1 (Table 2, entries 3 and
7) indicating that a concerted mechanism involving delocalized
charges on the silyl enol ether and tertiary-carbon or carbonyl
carbon induced by Ti-sites would be less operative, although
such a process cannot be ruled out completely. On the other
hand it may be probable that Lewis acid sites present in the
The titanium silicates Ti-␤, TS-1, and TS-2 in the absence of
H2O successfully catalyze the carbon–carbon bond formation in
SN1 type tert-butylation and acetylation reactions using silylenol
ethers of ketones as nucleophilic reagent. Tert-butylation of silyl
enol ethers produced the corresponding ␣-tert-butyl ketone as
the sole product through carbon–carbon bond formation and no
side-products were observed. Among the various metallosilicates
investigated, Ti-␤ exhibited best activity in the alkylation reac-
tion. All the metallosilicates exhibited a facile reaction between
silyl enol ethers and acetyl chloride and led to quantitative conver-
sion. Diketone and ketene esters were formed due to the reaction
of C-nucleophile and O-nucleophile with –C O acetyl chloride,
respectively. The titanium silicates were reused four times with
negligible loss of activity and no leaching of titanium was observed
during the reactions. The titanium silicates with “oxophilic Lewis
acidity” promote nucleophilic tert-butylation and acetylation of
silylenol ethers under mild liquid-phase conditions.

M. Sasidharan, A. Bhaumik / Journal of Molecular Catalysis A: Chemical 346 (2011) 87–93
93
Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in
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