Tetrabutylammonium phthalimide-N-oxyl: An efficient organocatalyst for trimethylsilylation of alcohols and phenols with hexamethyldisilazane

Article abstract of DOI:10.1007/BF03249088

Tetrabutylammonium phthalimide-N-oxyl (TBAPINO) was found to be an effective organocatalyst for rapid, simple and chemoselective protection of the hydroxyl group of alcohols and phenols using hexamethyldisilazane (HMDS) under mild conditions. The low catalyst loading, high to quantitative yields and simple removal of the catalyst from the reaction mixture illustrate the other attractive features of this protocol.

Full text of DOI:10.1007/BF03249088

J. Iran. Chem. Soc., Vol. 8, No. 2, June 2011, pp. 537-544.
JOURNAL OF THE
Iranian
Chemical Society
ꢀꢀꢀꢀꢀꢀꢀꢀ
Tetrabutylammonium Phthalimide-N-oxyl: An Efficient Organocatalyst for
Trimethylsilylation of Alcohols and Phenols with Hexamethyldisilazane
M.G. Dekamin*, N. Yazdaninia, J. Mokhtari and M.R. Naimi-Jamal
Pharmaceutical and Biologically-Active Compounds Research Laboratory, Department of Chemistry, Iran University
of Science and Technology, Tehran 16846-13114, Iran
(Received 10 July 2010, Accepted 29 October 2010)
Tetrabutylammonium phthalimide-N-oxyl (TBAPINO) was found to be an effective organocatalyst for rapid, simple and
chemoselective protection of the hydroxyl group of alcohols and phenols using hexamethyldisilazane (HMDS) under mild
conditions. The low catalyst loading, high to quantitative yields and simple removal of the catalyst from the reaction mixture
illustrate the other attractive features of this protocol.
Keywords: Trimethylsilylation, Alcohols, Phenols, Hexamethyldisilazane, Tetrabutylammonium phthalimide-N-oxyl,
Organocatalysis
INTRODUCTION
Thus, organocatalysts are making a valuable contribution to
green chemistry [1,2].
At the dawn of the 21st century, organic chemistry
witnessed a revolution in catalysis using small organic
compounds-Organocatalysis. Nowadays, organocatalytic
protocols are extensively pursued to replace the traditional
catalytic approaches based on the use of toxic or precious
transition-metals for the synthesis of organic compounds.
Organocatalysts have several significant advantages over
traditional metal catalysis. They are usually robust,
inexpensive, readily available and non-toxic. Many
organocatalysts are inert towards moisture and oxygen.
Because of these unique features, no reaction conditions such
as inert atmosphere, absolute solvents, etc. are to be observed
in many instances. Furthermore, due to the absence of
transition metals, organocatalytic methods seem to be
especially attractive for the preparation of compounds that do
not tolerate metal contamination, especially pharmaceuticals.
ꢀ
Protection of hydroxyl groups is a common practice in
synthetic organic and analytical chemistry. Various protecting
methods such as tetrahydropyranylation, esterification,
esterification and trialkylsilylation have been used for this
purpose [3,4]. Among these methods, trimethylsilylation of
hydroxyl group has received a lot of attention due to thermal
stability, solubility in non-polar solvents, and enhanced
stability of the resulting trimethylsilyl (TMS) ether derivatives
under a variety of conditions. Furthermore, the removal of the
protecting group can be easily accomplished by acid- or base-
induced hydrolysis [3,5]. Trimethylsilylation is also
extensively used for the derivatization of hydroxyl compounds
to increase their volatility for gas chromatography and mass
spectrometry [6]. Hence, different silylating agents such as
chlorotrimethylsilane [7-9], trimethylsilyl triflate [10],
hexamethyldisilane [11] and hexamethyldisiloxane have been
used in the presence of a broad range of catalysts and reagents
for the introduction of TMS group into a variety of alcohols or
*Corresponding author. E-mail: mdekamin@iust.ac.ir

Dekamin et al.
phenols [12].
Although these methods demonstrate improvements in
many cases, they suffer from the use of expensive, uncommon
or highly moisture sensitive silylating agents and catalysts, the
lack of reactivity and difficulty in the removal of by-products
produced during the reaction.
over 4 Å molecular sieve supplied by Carl Roth GmbH + Co,
76185 Karlsruhe, Germany. The catalyst was powdered and
dried at 70 °C for 1 h under reduced pressure. All reactions
were protected from air moisture using a CaCl2 guard tube.
Analytical TLC was carried out using Merck 0.2 mm silica gel
60 F-254 Al-plates.
Among different silylating agents, hexamethyldisilazane
(HMDS) is a stable, inexpensive, commercially available and
easy-to-handle compound which can be used for the
preparation of TMS ethers from alcohols and phenols. The
work-up of the reaction is not time consuming since the only
by-product of the reaction is ammonia which can be easily
removed from the reaction media. However, this reagent
requires to be activated by a catalytic system for its reaction.
Therefore, varieties of catalytic systems including inorganic
catalysts have been developed for activating this reagent [12-
23].
Recently, we have introduced tetrabutylammonium
phthalimide-N-oxyl (TBAPINO) or potassium phthalimide-N-
oxyl (PPINO) as efficient organocatalysts for cyanosilylation
of carbonyl compounds and cyclotrimerization of isocyanates
under mild conditions [24-27]. In continuation of our efforts to
develop the catalytic scope of the phthalimide-N-oxyl (PINO)
anion, we herein disclose the application of TBAPINO 1 as a
new and efficient metal-free catalyst for a chemoselective and
very simple protection of alcohols and phenols using HMDS
to afford corresponding trimethylsilyl ethers under mild
conditions (Scheme 1).
FT IR spectra were recorded as KBr pellets on a Shimadzu
FT IR-8400S spectrometer. 1H NMR (500 MHz) and 13C
NMR (125 MHz) spectra were obtained using a Bruker DRX-
500 AVANCE spectrometer. All NMR spectra were
determined in CDCl3 or DMSO at ambient temperature. GC
chromatograms were recorded on
instrument.
a
Shimadzu 2010
General Procedure for the Preparation of
Trimethylsilyl Ethers (3a-u or 5a-j)
To a mixture of alcohol or phenol (1 mmol), TBAPINO
(8.1 mg, 0.02 mmol) and EtOAc (1 ml) was added HMDS
(0.55 mmol) and the mixture was refluxed for an appropriate
length of time indicated in Tables 2 and 3. Upon completion of
the reaction as judged by TLC or GC, water (2.5 ml) was
added and the organic materials were extracted with EtOAc.
The organic phase was washed with brine followed by water
(2.5 ml) and dried over MgSO4. The solvent was evaporated
under reduced pressure to afford the product, in almost pure
form, which was further purified by column chromatography
on silica gel (Hexane/Ethyl acetate 8:2). All compounds were
well characterized by GC analysis, IR and NMR spectral data
as compared with those obtained from authentic samples or
reported in the literature [13,14,20,22].
EXPERIMENTAL
Chemicals and Apparatus
All chemicals were purchased from Merck or Aldrich and
used as received. EtOAc wasꢀpurchased from Merck and kept
RESULTS AND DISCUSSIONꢀ
Review of the literature shows that only few
O
Bu4N+
O
N
H
(1, 2 mol%)
O
N
+
R
OSiMe3
R
OH
Me3Si
SiMe3
EtOAc, reflux
R=Alkyl, Aryl
ꢀ
Scheme 1. Trimethylsilylation of alcohol and phenols catalyzed by TBAPINO 1
ꢀ
538

Tetrabutylammonium Phthalimide-N-oxyl: An Efficient Organocatalyst for Trimethylsilylation ofꢀ
organocatalytic protocols using Trichloroisocyanuric acid
achieved by using 2 mol% of the catalyst 1 in EtOAc under
reflux conditions (entry 3, Table 1). Indeed, when a mixture of
2a and 2 mol% of 1 in EtOAc was treated with 0.55 equivalent
of HMDS, the trimethylsilylation of benzyl alcohol occurred
smoothly under reflux conditions after 20 min to afford the
desired trimethylsilyl ether (3a) in 98% isolated yield. In the
absence of catalyst, the reaction was much less efficient for
the conversion of benzyl alcohol to the corresponding TMS
ether and only a trace amount of the product was formed after
24 h (Table 1, entry 9).
Then, the optimal conditions were applied to the
trimethylsilylation of other alcohols. The protection of a wide
range of primary and secondary alcohols was successfully
achieved in high to quantitative yields. The obtained results
have been summarized in Table 2. In the presence of this
catalyst, benzylic alcohols and primary alcohols bearing acid-
sensitive heteroaryl substituents were readily transformed into
their corresponding TMS ethers (3a-k) in excellent yields
(Table 2, entries 1-11). However, benzylic alcohols containing
electron-donating substituents (entries 2-5) demonstrated
shorter reaction times compared to alcohols with electron-
(TCCA) [28], N-bromosuccinimide (NBS) [29], 1,3-dibromo-
5,5-diethylbarbutric acid [30], N,N,N′,N′-tetrabromobenzene-
1,3-disulfonamide [31,32] and tetrabutylammonium bromide
(TBAB) [33] have been developed for the protection of
hydroxyl functional group in recent years. It is noteworthy that
all of the above-mentioned catalytic systems fall into Lewis
acid-activation of HMDS category, except TBAB. Therefore,
the development of new and alternative Lewis basic catalysts
which operate under milder conditions would be desirable. On
the basis of our experiences with PINO-catalyzed cyanide
addition reactions with trimethylsilyl cyanide (TMSCN) [24-
25], we postulated that the PINO nucleophile should activate
HMDS in an analogous fashion.
First, the catalytic applicability of TBAPINO was
examined in the activation of HMDS for the protection of
benzyl alcohol 2a as a model reaction to optimize the required
amount of HMDS, reaction media, substrate to catalyst mole
ratio and temperature (Table 1). The progress of the reaction
was easily monitored by TLC. The best result in terms of
turnover number (TON) and turnover frequency (TOF) was
Table 1. Optimization of the Trimethylsilylation of Benzyl Alcohol Catalyzed by TBAPINOaꢀ
ꢀ
HMDS (0.55 equiv.), TBAPINO (1)
PhCH2OTMS
PhCH2OH
ꢀ
ꢀ
Entry
Mol
Solvent
Time
(min)
Temp.
(°C)
Yield
(%)b
TONc
TOF
(h-1)d
(%)
2.0
2.0
2.0
2.0
1.5
1.0
3.0
2.0
-
1
2
3
4
5
6
7
8
9
CH3CN
DMSO
EtOAc
CH2Cl2
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
25
25
20
30
60
120
25
60
reflux
70 °C
reflux
reflux
reflux
reflux
reflux
rt
91
93
98
98
94
95
96
94
15
46
46
49
49
63
95
32
47
-
110
110
147
98
63
48
77
47
-
24 h
reflux
aHMDS (0.55 mmol) was added to a mixture of benzyl alcohol (1.0 mmol) and the required amount of
TBAPINO in 1 ml of the solvent. bIsolated yields (average of at least 2 runs) after complete conversion
of the substrate. cTurnover number: moles of product per mole of catalyst. dTurnover frequency: moles
of product per mole of catalyst per hour.
ꢀ
539

Dekamin et al.
Table 2. Protection of Alcohols Catalyzed by TBAPINO under Optimized Reaction Conditionsa
R"
R"
HMDS (0.55 equiv.), TBAPINO (1)
R
COH
R
COTMS
R'
EtOAc, Reflux
R'
(2)
(3)
Entry
R
R'
R"
Product
Time
(min)
Yield
(%)b,c
1
2
3
4
5
Ph
4-MeC6H4
4-MeOC6H4
3,4-(MeO)2C6H3
H
H
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
20
15
15
30
45
94
96
98
91
85
2-MeOC6H4
6
7
8
9
10
11
12
13
4-ClC6H4
2-ClC6H4
4-NO2C6H4
2-NO2C6H4
2-Furyl
2-Thienyl
Me
1-Hexyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
3f
3g
25
30
60
75
20
25
60
150
95
97
94
93
90
87
96
90
3h
3i
3j
3k
3l
3m
14
H
H
3n
120
95
CH3(CH2)16-
15
16
17
Vinyl
Ph
Ph
H
Me
Ph
H
H
H
3o
3p
3q
60
45
45
96
94
95
18
PhCO
Ph
H
3r
45
94
19
Cylohexyl
H
3s
150
92
20
21
3t
120
180
92
0
H
H
H
HO
Ph
Ph
Ph
3u
aHMDS (0.55 mmol) was added to a mixture of alcohol or phenol (1.0 mmol) and 8.1 mg of TBAPINO in EtOAc
(1 ml). bIsolated yields (average of at least 2 runs) after complete conversion of the substrates. All compounds
c
were characterized on the basis of spectroscopic data (IR, 1H NMR, 13C NMR) by comparison with those reported
in the literature [13,14,20,22].
ꢀ
540

Tetrabutylammonium Phthalimide-N-oxyl: An Efficient Organocatalyst for Trimethylsilylation ofꢀ
withdrawing substituents (entries 6-9). Aliphatic primary
other hand, in a mixture of benzyl alcohol 2a and phenol 4a,ꢀ
the primary alcohol was preferentially converted to the
corresponding TMS ether (78% 3a vs. 22% 5a). The sameꢀ
preference was also observed when the primary benzyl alcohol
2a was subjected to reaction with HMDS in the presence of
secondary or tertiary hydroxyl groups such as benzhydrol
(72% 3a vs. 28% 3q) and triphenyl carbinol (100% 3a vs. 0%
3q), respectively.
alcohols were also subjected to the same conditions. As a
result, the corresponding TMS ethers of these substrates (3l-o)
were obtained in high yields within longer time periods (Table
2, entries 12-15). It is noteworthy that acid sensitive, allyl and
saturated alcohols react under the optimized reaction
conditions without the formation of any side products such as
polymerisation, rearrangement or dehydration (entries 10-15)
[12]. Secondary alcohols also underwent trimethyl-silylation
under the optimized reaction conditions (Table 2, entries 16-
20). However, secondary alcohols required longer reaction
times compared to similar primary alcohols. Interestingly,
cholesterol 2t was involved smoothly in the optimized reaction
conditions to afford the desired protected product (3t) in 92%
yield (entry 20) [12]. On the other hand, tertiary alcohols such
as triphenyl carbinol remained intact even after 3 h under the
optimized reaction conditions (Table 2, entry 21) [33].
The hypercoordinate silicon species are well known due to
their role in developing alternative pathways in organic
synthesis [34-37]. Accordingly, significant progress has been
made during the recent years for the Lewis base-catalyzed
reactions using silylated reagents. Among the Lewis base
catalysts, oxygen-containing nucleophiles are more efficient
due to the high bond strength as well as the kinetic lability of
the Si-O bond. Therefore, oxygen-containing nucleophiles are
suitable activators in silylated reagents chemistry such as
HMDS [38-39]. This activity could be interpreted in terms of
appropriate hardness of the oxygen-containing nucleophiles,
as hard bases, to interact efficiently with the trimethylsilyl
moiety as a hard acid [40-42]. According to the literature and
reactivity of different alcohols or phenols in this study, the
mechanism purposed in Scheme 3 can be considered for the
conversion of alcohols or phenols to their corresponding
trimethylsilyl ether in the presence of TBAPINO 1 [25,34-44].
Accordingly, the initial attack of the more nucleophilic PINO
anion on trimethylsilyl groups of 6a or 6b produces the
pentacoordinated intermediate I. The intermediate I can form
the hexacoordinated intermediate II by the subsequent attack
of less nucleophilic alcohol (2) or phenol (4) to expand the
valency of silicon, which has a low-lying empty d- orbital.
Then, an intramolecular acid-base reaction produces the
hexacoordinated intermediate III which can then decompose
to give the product (3 or 5) and regenerate the catalyst 1
ꢀ
ꢁ%`ꢂ.V`IQ`V5ꢀꢂ`1IVꢂ.7ꢃ1C7C:ꢂ1QJꢀ Q`ꢀ R1``V`VJꢂꢀ ].VJQCꢃꢀ :JRꢀ
naphthols with HMDS under the optimized conditions was
also performed smoothly and cleanly in high yields. The
obtained results have been summarized in Table 3. In general,
phenols and naphthols underwent silylation at longer reaction
times compared to benzylic alcohols while the same trend of
substituent effects was observed. Also, no reactivity was
observed for the thiophenolic substrate in the optimized
reaction conditions even in the presence of activating amino
group at the ortho- position (Table 3, entry 10).
To investigate the chemoselectivity of the protocol, some
competitive reactions were designed. The related data clearly
demonstrate exclusive protection of phenolic functional group
in the presence of its aromatic amine competitor. Indeed, only
the product of protection of the phenolic functionality (5b)
was produced by using both 0.55 and 1.1 equivalent of HMDS
in the reaction conditions (Scheme 2, Table 3, entry 2). On the
OH
OSiMe3
NH2
OH
NH2
NHSiMe3
HMDS (1.1 equiv.), TBAPINO
EtOAc, reflux
+
100%
0%
ꢀ
Scheme 2. Chemoselective protection of phenolic functional group in the presence of its aromatic amine
competitor
ꢀ
541

Dekamin et al.
Table 3. Protection of Phenols Catalyzed by TBAPINO under Optimized Reaction Conditionsa
HMDS (0.55 equiv.), TBAPINO (1)
ArOH
ArOTMS
EtOAc, Reflux
(4)
(5)
ꢀ
Entry
Ar
Ph
Product
Time
(min)
Yield
(%)b,c
5a
5b
5c
5d
5e
5f
1
2d
3
60
45
45
25
45
40
91
94
90
95
94
92
2-NH2C6H4
3,4-(MeO)2C6H3
4
5
6
2-Benzyl-4-ClC6H4
2-EtC6H4
4-EtC6H4
75
95
7
2-NO2C6H4
1-Naphthyl
5g
8
9
5h
5i
60
60
86
82
2-Naphthyl
SSiMe3
SH
10
180
N.R
NH2
NH2
5j
aHMDS (0.55 mmol) was added to a mixture of alcohol or phenol (1.0 mmol) and 8.1 mg of TBAPINO in EtOAc
b
c
(1 ml). Isolated yields (average of at least 2 runs) after complete conversion of the substrates. All compounds
were characterized on the basis of spectroscopic data (IR, H NMR, 13C NMR) by comparison with those reported
1
in the literature [13,14,20,22]. dThe reaction was performed using both 0.55 and 1.1 equivalent of HMDS.
_
R'
R
OH
H
(2 or 4)
N
Me3Si
Me3Si
(6a)
Me
Me
+
Si Me Bu4N
O
N
H
_
R
H
H
O
N
O
O
Me
Me
N
+
Si
O
Bu4N
R'
Me
O
O
N
O
+
O Bu4N
(I)
N
R'= SiMe3
O
or
H
(II)
(1, TBAPINO)
_
_
R
R
R
OSiMe3
H
N
O
H
O
(3 or 5)
Me
+
Me
Me
R'
NH2
+
R'
Si
O
Bu4N
+
+
Si Me
O
Bu4N
Me
Me
NH3
(6c)
R'= SiMe3 (6b)
N
O
O
N
O
O
or
H (6c)
(III)
(IV)
ꢀ
Scheme 3. Plausible mechanism for the trimethylsilylation of alcohol and phenols catalyzed by TBAPINO
ꢀ
542

Tetrabutylammonium Phthalimide-N-oxyl: An Efficient Organocatalyst for Trimethylsilylation ofꢀ
Table 4. Comparison of some of the Results Obtained by Trimethylsilylation of Alcohols and Phenols with HMDS in
the Presence of TBAPINO (1), with some of those Reported by Lanthanum Trichloride (LaCl3, 2), Lithium
Perchlorate (LiClO4, 3), Trichloroisocyanuric Acid (TCCA, 4), 1,3-Dibromo-5,5-diethylbarbituric Acid (5),
Tetrabutylammonium Bromide (TBAB, 6)
Entry
Substrate
Method [Equivalent of HMDS/TOF (h-1)]
1a
2[13]b
3[19]c
4[28]b
5[30]b
6[33]d
-
1
2
C6H5CH2OH
0.55/147
0.55/47e
0.55/196
0.7/3.0
0.7/4.8
0.8/3.8ꢀ
0.7/19
4-MeC6H4CH2OH
-ꢀ
-ꢀ
0.8/6.2ꢀ
-
-ꢀ
3
4
4-ClC6H4CH2OH
C6H5OH
0.55/114
0.55/46
-ꢀ
-ꢀ
-ꢀ
-ꢀ
-
-
0.7/37ꢀ
-ꢀ
0.7/5.8ꢀ
0.8/6.4ꢀ
5
6
7
1-Naphthol
(C6H5)2CHOH
Cholesterol
0.55/43
0.55/63
0.55/23
0.7/3.2ꢀ
0.7/2.7ꢀ
-ꢀ
0.7/4.8ꢀ
-ꢀ
1.0/3.2ꢀ
-ꢀ
-
0.7/3.0
-
-ꢀ
0.7/54ꢀ
-ꢀ
-ꢀ
-ꢀ
aEtOAc as solvent under reflux conditions otherwise it has been noted. bCH2Cl2 as solvent. cSolvent-free conditions.
dCH3CN as solvent. eAt room temperature.
through the pentacoordinated intermediate IV.
ACKNOWLEDGEMENTS
To illustrate the efficiency of the proposed method, Table
4 summarizes the comparison of the results obtained by our
We are grateful for the financial support from The
protocol with some of those reported in the literature
Research Council of Iran University of Science and
[13,19,28,30,33].
Technology (IUST), (Grant No. 160/5632).
CONCLUSIONS
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