Sulfated zirconia: An efficient catalyst for solvent-free synthesis of silyl ethers under mild conditions

Article abstract of DOI:10.1080/00397911.2010.497591

Oximes, allylic, aliphatic, aromatic, cyclic, acyclic, and hetero alcohols are silylated in short reaction times with good yields in the midst of a catalytic amount of sulfated zirconia solid acid catalyst and trimethylsilyl cyanide under nonbasic, solvent-free, and ambient temperature conditions. Selectivity toward O-silyl ether rather than N-silyl ether has been observed. This simple experimental procedure, combined with easy recovery and reusability of the catalyst, is expected to contribute to the development of a clean and environmentally friendly strategy for the synthesis of O-silyl ethers. Copyright

Full text of DOI:10.1080/00397911.2010.497591

Synthetic Communications1, 41: 2064–2072, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.497591
SULFATED ZIRCONIA: AN EFFICIENT CATALYST FOR
SOLVENT-FREE SYNTHESIS OF SILYL ETHERS UNDER
MILD CONDITIONS
Boningari Thirupathi, Avvari N. Prasad, Rapelli Srinivas, and
Benjaram M. Reddy
Inorganic and Physical Chemistry Division, Indian Institute of
Chemical Technology, Hyderabad, India
GRAPHICAL ABSTRACT
Abstract Oximes, allylic, aliphatic, aromatic, cyclic, acyclic, and hetero alcohols are
silylated in short reaction times with good yields in the midst of a catalytic amount of
sulfated zirconia solid acid catalyst and trimethylsilyl cyanide under nonbasic, solvent-free,
and ambient temperature conditions. Selectivity toward O-silyl ether rather than N-silyl
ether has been observed. This simple experimental procedure, combined with easy recovery
and reusability of the catalyst, is expected to contribute to the development of a clean and
environmentally friendly strategy for the synthesis of O-silyl ethers.
Keywords Heterogeneous catalyst; silylation of alcohols; solid acid; solvent-free; sulfated
zirconia
INTRODUCTION
Reliable, selective, and environmentally friendly chemical transformations are
crucial to the development of new therapeutics and the design of novel materials.
Heterogeneous catalysts that can be easily prepared and used to obtain organic
molecules selectivity are important to modern chemical synthesis. The development
of protecting groups that shield reactive functionalities has also proved indispensable
in the preparation of complex biologically active molecules.[1] Recently, several
Received December 14, 2009.
Address correspondence to Benjaram M. Reddy, Inorganic and Physical Chemistry Division,
Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500 607, India. E-mail: bmreddy@
iict.res.in; mreddyb@yahoo.com
2064

SOLVENT-FREE SYNTHESIS OF SILYL ETHERS
2065
Scheme 1. Efficient and solvent-free O-silylation of alcohols.
groups synthesized an anticancer drug NCX 4040 by selective silylation of 4-hydroxy
benzyl alcohol as a starting material.[2] Moreover, the protection of hydroxy groups
by silylation is imperative in peptide synthesis and as lipophilicity modifiers for the
peptides.[3] A variety of O-silylation methods for the preparation of silyl ether com-
pounds from alcohols have been developed. These methods utilized many silylating
agents, such as chlorotrimethylsilane,[4,5] trimethylsilyl azide,[6] triethylsilyl chlor-
ide,[7] allylsilane,[8] triethylsilyl hydride,[9] and hexamethyldisilazane[10] for the intro-
duction of a silyl group onto a variety of alcohols. Hexamethyldisilazane (HMDS) is
a cheap and commercially available compound that can be used for the preparation
of trimethylsilyl ethers from hydroxy compounds. All these methods frequently suf-
fer from drawbacks such as lack of reactivity and difficult removal of ammonium
salts. The poor silylating power of HMDS is also the main drawback to its appli-
cation. Therefore, a variety of catalysts have been employed for activating this
reagent, which include sulfuric acid,[11] ZrCl4,[12] nitrogen–ligand complexes of metal
chlorides,[13] montmorillonite,[10a] iodine,[14] H3PW12O40,[10c] LiClO4,[15] and CuSO4=
5H2O.[16] Although several of these procedures are useful, most of them suffer from
the use of homogeneous[17,18] and often corrosive catalysts, which are usually
nonrecoverable and thus potentially polluting,[13,10c] tedious workups, and=or long
reaction times. Consequently, new procedures that address these downsides are
desirable.
The concept of electrophile activation by Lewis acids has long been the core of
catalysis by transition metal complexes. Whereas Me3SiNu activation remains as an
important issue in the development of most effective and highly selective methods for
carrying out addition reactions, a new trend based on the simultaneous activation of
the Me3SiNu and the electrophile has been gaining importance. Electrophile
activation requires the application of a Lewis acid, and this role is usually played
by various transition metal salts and complexes.
Scheme 2. Plausible reaction mechanism for O-silylation of alcohols.

2066
B. THIRUPATHI ET AL.
On the other hand, sulfated zirconia ðSO24ꢀ=ZrO2Þ is a popular solid superacid
catalyst exhibiting the highest acid strength (H0 ꢁ ꢀ16.04).[19] Treatment of a variety
of alcohols and phenols with dihydropyrimidinones (DHP) in the presence of a cata-
lytic amount of sulfated zirconia provided the corresponding tetrahydropyrane
(THP) ethers in good yields. This procedure could also be efficiently applied to
highly acid-sensitive alcohols such as allyl and propargyl alcohols.[20] This solid cata-
lyst was also utilized for the tetrahydropyranylation of hydroquinone protected with
benzyl ether.[21] The present investigation was undertaken against this background.
RESULTS AND DISCUSSION
During the itinerary of our investigation on Strecker synthesis, 1,3-asymmetric
induction for optically pure a-aminonitrile synthesis,[22] and cyanosilylation of
aldehydes,[23] we found that trimethylsilyl cyanide (TMSCN) is activated by pro-
moted zirconia solid acid catalysts. Subsequently, we have initiated our exploration
by direct cyanation of secondary alcohols by using sulfated zirconia catalyst and
TMSCN. To our surprise, this expected reaction did not work well for the cyanation
of alcohols, but silylation of alcohols was accomplished within 2 min. Further, we
have reported previously the tetrahydropyranylation of alcohols and phenols under
solvent-free conditions by employing sulfated zirconia,[21] and other work on
sulfated zirconia catalyst.[24] We therefore commenced the reaction of TMSCN with
a wide variety of proton changeable compounds such as alcohols (primary, secondary,
tertiary, aliphatic, aromatic, cyclic, acyclic, and allylic), oximes, phenols, carboxylic
acids, amines, and thiols. Indeed, the reaction proceeds so fast that within a few
minutes, the products were readily isolated either by recrystallization or by column
chromatography.
Initially, we instigated our exploration with the screening of various solvents by
making use of sulfated zirconia catalyst for O-silylation. Preliminary experiments in
various solvents confirmed that the reaction progresses very well under solvent-free
conditions (Table 1) in comparison to different solvents. In addition, the reaction is
selectively applicable to sterically hindered alcohols, facilitating selective O-silylation
rather than N-silylation in the case of 2-amino-1,2-diphenylethanol (entry 8, Table 2)
and selective hydroxyl silylation more favorably than addition reactions (entry 2,
Table 2). Allylic alcohols proceeded somewhat slower, and O-silylation of citronellol
has taken long reaction time, resulting in less yield of product under catalyst-free
Table 1. Screening of solvents for O-silylation of alcohols
Entry
Amount of SO24ꢀ=ZrO2 catalyst (mg)
Solvent
Time (min)
Yield (%)a
1
2
3
4
5
6
7
50
50
50
50
50
50
—
Dichloro methane
Methanol
Toluene
15
20
60
80
15
02
10
82
75
54
72
65
96
90
Benzene
Acetonitrile
Neat
Neat
aIsolated yields of the pure product.

SOLVENT-FREE SYNTHESIS OF SILYL ETHERS
2067
Table 2. Catalyst-free and SO24ꢀ=ZrO2-catalyzed O-silylation of alcohols
Entry
1.
Substrate
Product
Time (min)
Yield (%)a
96b; 90c
02b; 10c
2.
15b; 180c
90b; 63c
3.
4.
03b; 20c
89b; 76c
03b; 08c
92b; 86c
5.
6.
02b; 15c
15b; 45c
92b; 88c
68b; 70c
7.
8.
02b; 10c
87b; 80c
05b; —c
95b; —c
9.
10.
11.
30b; 30c
02b; 08c
05b; 15c
Traces
92b; 90c
85b; 76c
aYields of isolated products after column chromatography otherwise mentioned.
bReaction with SO42ꢀ=ZrO2.
cReaction without catalyst.

2068
B. THIRUPATHI ET AL.
condition (entry 2, yield 63%, time 3 h, Table 2). However, this reaction proceeded
smoothly in the presence of sulfated zirconia catalyst (entry 2, yield 92%, time
15 min, Table 2). We also observed that TMSCN is inactive in case of tertiary alco-
hols (not included in the table) and amines toward N-silylation (entry 8, Table 2, also
for aniline). Therefore, this might provide access to selective O-silylation more easily
than N-silylation and addition reactions.
EXPERIMENTAL
Preparation of Catalyst
Zirconium hydroxide was prepared first from zirconium oxychloride by
hydrolysis with dilute aqueous ammonia solution. For this purpose, the requisite
quantity of ZrOCl2 ꢂ 8H2O (Loba Chime, GR grade) was dissolved in doubly dis-
tilled water, and aqueous NH3 was added dropwise with vigorous stirring to this
clear solution until the pH of the solution reached 8. The obtained precipitate was
washed thoroughly with hot distilled water several times until free from chloride ions
and dried at 393 K for 24 h. On the resulting hydrous zirconium hydroxide, sulfate
promoter was deposited by a wet impregnation method. To incorporate the pro-
moter, sulfuric acid (Aldrich, analytical reagent grade) was used as the precursor.
Detailed procedures for the preparation of this catalyst and its characterization
can be found elsewhere.[25,26] In particular, the X-ray powder diffraction and other
characterization results revealed the presence of ZrO2 tetragonal phase. The prefer-
ential formation and subsequent stabilization of tetragonal zirconia has been mainly
attributed to high acidity and activity of promoted zirconia solid acid catalysts.
Representative Experimental Procedure
A glass reactor was charged under nitrogen atmosphere with the substrate
(1.0 mmol), TMSCN (1.2 mmol) (2.2 mmol in case of 1,3 propane diol, 3.2 mmol
in case of glycerol reactions), and SO24ꢀ=ZrO2 catalyst (50 mg, 10 mol%) and stirred
at room temperature for an appropriate time. Caution: TMSCN must be used in a
well-ventilated hood because of its toxicity and moisture-sensitive nature. After
completion of the reaction, as indicated by thin-layer chromatography (TLC), the
reaction mixture was filtered off and washed with ethyl acetate (3 ꢃ 5 mL). The
organic layer was dried over anhydrous Na2SO4, and the solvent was evaporated
under vacuum to yield the corresponding product (column chromatography was
performed whenever required). The wet catalyst was used for recycling, and no
appreciable change in the activity was noted. NMR and mass spectrometric techni-
ques were utilized for the analysis of products. All products were identified by com-
paring their spectral data with literature. The spectral data for some selected
representative compounds are given.
Selected Data
Trimethyl(1-phenylethoxy)silane (Table 2, Entry 1). Colorless oil; 1H
NMR (300 MHz, CDCl3): d 0.14 (s, 9 H), 1.50 (d, J ¼ 6.446 Hz, 3H), 4.88–4.93

SOLVENT-FREE SYNTHESIS OF SILYL ETHERS
2069
(q, J ¼ 6.446 Hz, 1H), 7.24–7.29 (m, 1H), 7.31–7.37 (m, 4H); IR (neat) n 3448, 3030,
2966, 1605, 1450, 1367, 1253, 1096, 958, 944, 755, 698 cmꢀ1; EIMS: m=z 194 (Mþ),
193 [Mþ ꢀ H], 179 [Mþ ꢀ CH3], 149 [Mþ ꢀ (CH3)3], 105 [Mþ ꢀ OSi(CH3)3].
(E)-(3,7-Dimethylocta-2,6-dienyloxy)trimethylsilane (Table 2, Entry
1
2). Colorless oil; H NMR (300 MHz, CDCl3): d 0.19 (s, 9H), 1.69 (s, 3H), 1.76
(s, 6H), 2.02–2.26 (m, 4H), 4.20 (d, J ¼ 6.043 Hz, 2H), 5.15 (t, J ¼ 6.798 Hz, 1H),
5.37 (t, J ¼ 6.798 Hz, 1H); IR (neat): n 3453, 3028, 2956, 1947, 1600, 1493, 1450,
1252, 1103, 1058, 967, 889, 746, 695 cmꢀ1; EIMS: m=z 226.
Cyclohexanone O-trimethylsilyl oxime (Table 2, Entry 3). Colorless oil;
1H NMR (300 MHz, CDCl3): d 0.19 (s, 9H), 1.55–1.79 (m, 6H), 2.23 (t, J ¼ 6.798 Hz,
Hz, 2H), 2.50 (t, J ¼ 6.798 Hz, 2H); IR (neat) n 3320, 2933, 2858, 1635, 1447, 1250,
989, 913, 844, 752, 648 cmꢀ1; EIMS: m=z 186 (M þ 1) (Mþ), 169, 141, 115, 106, 92,
77, 51.
1
Benzhydryloxytrimethylsilane (Table 2, Entry 4). Colorless oil; H NMR
(300 MHz, CDCl3): d 0.19 (s, 9H), 5.84 (s, 1H), 7.27–7.32 (m, 2H), 7.35–7.45 (m,
8H); IR (neat): n 3029, 2957, 2863, 1949, 1597, 1450, 1253, 1186, 1065, 882, 745,
698, 605 cmꢀ1; EIMS: m=z 258 [M þ 1] (Mþ), 242 [Mþ ꢀ CH3].
1
Trimethyl(3-phenylpropoxy)silane (Table 2, Entry 5). Colorless oil; H
NMR (300 MHz, CDCl3): d 0.19 (s, 9H), 1.87–1.96 (m, 2H), 2.75 (t, J ¼ 7.365 Hz,
2H), 3.66 (t, J ¼ 6.23 Hz, 2H), 7.19–7.24 (m, 3H), 7.29–7.34 (m, 2H); IR (neat): n
3027, 2952, 2862, 1603, 1452, 1615, 1251, 1099, 963, 869, 840, 744, 697 cmꢀ1; EIMS:
m=z 209 [M þ 1] (Mþ), 193[Mþ ꢀ CH3], 134, 119, 105, 91, 56.
1
Trimethyl(naphthalen-2-yloxy)silane (Table 2, Entry 6). Colorless oil; H
NMR (300 MHz, CDCl3): d 0.19 (s, 9H), 6.87–6.91 (dd, J ¼ 6.798 Hz, J ¼ 2.266 Hz,
1H), 7.00 (s, 1H), 7.15 (t, J ¼ 6.798 Hz, 1H), 7.24 (t, J ¼ 6.798 Hz, 1H), 7.50–7.59
(m, 3H); IR (neat): n 3056, 2959, 1630, 1597, 1467, 1355, 1260, 1173, 970, 928,
855, 747, 471 cmꢀ1; EIMS: m=z 216 (Mþ), 181, 131.
1
Cyclohexyloxytrimethylsilane (Table 2, Entry 7). Colorless oil; H NMR
(300 MHz, CDCl3): d 0.19 (s, 9H), 1.25–1.46 (m, 5H), 1.54–1.66 (m, 1H), 1.75–
1.88 (m, 4H), 3.61–3.68 (m, 1H); IR (neat): n 3393, 2935, 2860, 1720, 1602, 1504,
1450, 1253, 1180, 1096, 845, 750, 693 cmꢀ1; EIMS: m=z 172 (Mþ), 158, 143.
1,2-Diphenyl-2-(trimethylsilyloxy)ethanamine (Table 2, Entry 8). Color-
less oil; 1H NMR (300 MHz, CDCl3): d 0.19 (s, 9H), 2.54 (s, 2H), 4.37 (d,
J ¼ 6.798 Hz, 1H), 4.95 (d, J ¼ 6.798 Hz, 1H) 7.57–7.64 (m, 10H); IR (neat): n 3380,
3331, 3276, 3062, 3030, 2956, 2885, 1949, 1598, 1493, 1453, 1251, 1092, 1066, 885,
842, 755, 699, 535 cmꢀ1; EIMS: m=z 286 [M þ 1], 196 [Mþ ꢀ OSi(CH3)3].
2,2,8,8-Tetramethyl-3,7-dioxa-2,8-disilanonane (Table 2, Entry 10). Col-
1
orless oil; H NMR (300 MHz, CDCl3): d 0.013 (s, 9H), 1.60 (q, J ¼ 6.043 Hz, 2H),
3.54 (t, J ¼ 12.086 Hz, 4H); IR (neat): n 3394, 2956, 1655, 1395, 1253, 1091, 975, 841,
753, 689 cmꢀ1; EIMS: m=z 220 (Mþ), 221 (M þ 1), 149, 131, 73.
2,2,8,8-Tetramethyl-5-(trimethylsilyloxy)-3,7-dioxa-2,8-disilanonane
1
(Table 2, Entry 11). Colorless oil; H NMR (300 MHz, CDCl3) d 0.19 (s, 18H),

2070
B. THIRUPATHI ET AL.
0.21 (s, 9H), 3.50–3.55 (dd, J ¼ 6.043, J ¼ 5.288 Hz, 2H), 3.61–3.66 (dd, J ¼ 6.043,
J ¼ 5.288 Hz, 2H), 3.68–3.80 (m, 1H); IR (neat) n 3404, 2958, 1638, 1404, 1253,
1147, 1100, 988, 841, 752 cmꢀ1. EIMS: m=z 308 (Mþ), 309 (M þ 1), 237, 219, 165,
147, 131, 117, 105, 103.
CONCLUSION
In conclusion, we have developed a novel protocol with short reaction times
under solvent-free conditions without the requirement of a base for silylation of
different types of alcohols and oximes with a simple reaction procedure by using
sulfated zirconia and trimethysilyl cyanide for the synthesis of trimethylsilyl ethers
in excellent yields.
ACKNOWLEDGMENTS
B. T. thanks the Council for Scientific and Industrial Research (CSIR), New
Delhi, for the senior research fellowship. A. N. P. and R. S. thank the CSIR,
New Delhi, for junior research fellowships.
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