Electron-deficient vanadium(IV) tetraphenylporphyrin: A new, highly efficient and reusable catalyst for chemoselective trimethylsilylation of alcohols and phenols with hexamethyldisilazane

Article abstract of DOI:10.1002/aoc.1828

In the present work, the application of electron-deficient tetraphenylporphyrinatovanadium(IV) trifluoromethanesulfonate, [V ^IV(TPP)(OTf)₂], in the trimethylsilylation of alcohols and phenols with hexamethydisilazane (HMDS) is reported. This new V(IV) catalyst was used as an efficient catalyst for silylation of not only primary alcohols but also sterically hindered secondary and tertiary alcohols with HMDS. Trimethylsilylation of phenols with HMDS was also performed to afford the desired Trimethylsilyl ethers (TMS) ethers. The chemoselectivity of this method was also investigated. This catalyst can be reused several times without loss of its activity. Copyright

Full text of DOI:10.1002/aoc.1828

Full Paper
Received: 13 March 2011
Revised: 27 June 2011
Accepted: 27 June 2011
Published online in Wiley Online Library: 10 August 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1828
Electron-deficient vanadium(IV)
tetraphenylporphyrin: A new, highly efficient
and reusable catalyst for chemoselective
trimethylsilylation of alcohols and phenols
with hexamethyldisilazane
Majid Moghadam∗, Iraj Mohammadpoor-Baltork∗,
Shahram Tangestaninejad, Valiollah Mirkhani, Ahmad Reza Khosropour
and S. Abdolmanaf Taghavi
In the present work, the application of electron-deficient tetraphenylporphyrinatovanadium(IV) trifluoromethanesulfonate,
[VIV(TPP)(OTf)2], in the trimethylsilylation of alcohols and phenols with hexamethydisilazane (HMDS) is reported. This new V(IV)
catalyst was used as an efficient catalyst for silylation of not only primary alcohols but also sterically hindered secondary and
tertiary alcohols with HMDS. Trimethylsilylation of phenols with HMDS was also performed to afford the desired Trimethylsilyl
ethers (TMS) ethers. The chemoselectivity of this method was also investigated. This catalyst can be reused several times
c
without loss of its activity. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: vanadium porphyrin; electron-deficient; alcohols; phenols; trimethylsilyl ether; HMDS
Introduction
LaCl3,[23] poly(N-bromobenzene-1,3-disulfonamide)andN,N,Nꢁ,Nꢁ-
tetrabromobenzene-1,3-disulfonamide,[24] Fe(TFA)3,[25] Fe3O4,[26]
(n-Bu4N)Br,[27] trichloroisocyanuricacid(TCCA),[28] HClO4 –SiO2,[29]
Pd[30] and ZrO(OTf)2,[31] have been reported for trimethylsilylation
of alcohols and phenols. Although these procedures provide an
improvement, many of the catalysts or activators need long re-
action times, drastic reaction conditions or tedious workups, or
are moisture-sensitive or expensive. Hence, new procedures to
circumvent these problems are still in demand.
Metalloporphyrins as synthetic models for cytochrome P-450
have been widely used as catalysts in the oxidation of organic
compounds. However, there are few reports on the use of these
compounds as Lewis acids. In the last decade, high-valence high-
valent metalloporphyrins have been used as mild Lewis acid
catalysts. Suda’s group has reported the use of chromium and
iron porphyrins in organic synthesis. They used Cr(tpp)Cl for
regioselective [3,3] rearrangement of aliphatic allyl vinyl ethers
and for Claisen rearrangement of simple aliphatic allyl vinyl ethers,
Fe(tpp)OTf for rearrangement of α,β-epoxy ketones into 1,2-
diketones and rearrangement of monoalkyl-substituted epoxides
intoaldehydes,andCr(tpp)OTfforhighlyregio-andstereoselective
rearrangement of epoxides to aldehydes.[1–7]
Vanadium is required for normal health, and could act in vivo
either as a metal cation or as a phosphate analog, depending
on the oxidation state, V(lV) or V(V), respectively. Vanadium in
a sea squirt (tunicate), a primitive vertebrate, is concentrated in
blood cells, apparently as the major cellular transition metal, but
whether it participates in the transport of dioxygen (as iron and
copper do) is not known. In proteins, vanadium is a cofactor in an
algal bromoperoxidase and in certain prokaryotic nitrogenases.
However, its oxidation state within organisms seems to be highly
variable. The biochemistry of vanadium potentially involves four
oxidation states that are relatively stable in aqueous solution.
The protection of hydroxy compounds is of great impor-
tance during various transformations in a synthetic sequence,
especially in the synthesis of fine chemicals and natural
products.[8,9] Several methods, such as acetylation, tetrahydropy-
ranylation, methoxymethylation and trimethylsilylation, have
been reported for masking of hydroxyl groups. The most promis-
ing method for the silylation of hydroxy functional groups
is the use of hexamethyldisilazane (HMDS). One disadvan-
tage of HMDS is its poor silylating power in the absence
of a suitable catalyst, needing forceful conditions and long
reaction times in many cases.[10] To solve this problem, a
+
These are V2+, V3+, VO2+ and VO2 (the oxidation states 2, 3, 4,
and 5, respectively).[32]
∗
Correspondence to: Majid Moghadam and Iraj Mohammadpoor-Baltork, De-
partment of Chemistry, Catalysis Division, University of Isfahan, Isfahan
81746-73441, Iran. E-mails: moghadamm@sci.ui.ac.ir; imbaltork@sci.ui.ac.ir
variety of catalysts, including (CH3)3SiCl,[11] ZrCl4,[12] ZnCl2,[13]
K-10 montmorillonite,[14] LiClO4,[15] H3PW12O40
,
iodine,[17]
[16]
InBr3,[18] zirconium sulfophenyl phosphonate,[19] CuSO4.5H2O,[20]
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan
81746-73441, Iran
sulfonic acid-functionalized nanoporous silica,[21] MgBr2·OEt2,[22]
c
Appl. Organometal. Chem. 2011, 25, 687–694
Copyright ꢀ 2011 John Wiley & Sons, Ltd.

M. Moghadam et al.
on a Philips PU9716 or a Shimadzu IR-435 spectrophotometer. All
analyses were performed on a Shimadzu GC-16A instrument with
a flame ionization detector using silicon DC-200 or Carbowax 20M
columns. Tetraphenylporphyrin, [VO(TPP)] and [V(TPP)Cl2] were
prepared according to the literature.[44–46]
[VIV(TPP)(OTf)2]
HMDS, CH3CN
OH
or
OH
R
R
OSiMe3
or
Ar OSiMe3
Ar
Scheme 1. Trimethylsilylation of alcohols and phenols with HMDS cat-
alyzed by [VIV(TPP)(OTf)2].
Preparation of Tetraphenyporphyrinatovanadium(IV) Trifluo-
romethanesulfonate, [VIV(TPP)(OTf)2]
Recently, we reported the use of tin(IV) porphyrins as efficient
and mild Lewis acids in organic transformations.[33–43] In continu-
ation of our search for mild Lewis acids, we report a highly efficient
method for trimethylsilylation of alcohols and phenols with HMDS
catalyzed by new, efficient and reusable tetraphenylporphyrinato-
vanadium(IV) trifluoromethanesulfonate, [VIV(TPP)(OTf)2] at room
temperature (Scheme 1).
To a solution of V(TPP)Cl2 (0.97 g, 1 mmol) in THF (50 ml), at 55 ◦C,
AgCF3SO3 (0.54 g, 2 mmol) was added. The solution was stirred
at 55 ◦C for 30 min. The AgCl precipitate was filtered through
a 0.45 µM filter. The resulting solution was evaporated at room
temperature. Then, the [VIV(TPP)(OTf)2] was extracted with CH2Cl2
and the crystals were obtained after evaporation of solvent at
room temperature. CHNS analyses: calcd, C, 57.44; H, 2.93; N, 5.83;
S, 6.67; found, C, 57.35; H, 2.88; N, 5.91, S, 6.65.
Experimental
General Procedure for Trimethylsilylation of Alcohol
and Phenols with HMDS Catalyzed by [VIV(TPP)(OTf)2]
Chemicals were purchased from Fluka and Merck chemical
companies. 1H NMR spectra were recorded in CDCl3 solvent on a
Bruker-Avance 400 MHz spectrometer. Infrared spectra were run
To a solution of alcohol or phenol (1 mmol) and HMDS (0.6 mmol
per OH group) in CH3CN (0.5 ml) was added [VIV(TPP)(OTf)2]
Scheme 2. Preparation route for catalyst.
c
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Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 687–694

Electron-deficient vanadium(IV) tetraphenylporphyrin
Table 1. Trimethylsilylation of alcohols with HMDS catalyzed by [VIV(TPP)(OTf)2] at room temperaturea
Entry
1
Substrate
Product
Time (min)
1
Yield (%)b
100
CH2OH
CH2OSiMe3
2
3
4
CH2CH2OSiMe3
CH2CH2CH2OSiMe3
CH2OSiMe3
1
1
1
100
100
100
CH2CH2OH
CH2CH2CH2OH
CH2OH
Cl
Cl
5
6
Cl
Cl
CH2OH
1
1
100
100
Cl
Cl
CH2OSiMe3
CH2OH
Cl
CH2OSiMe3
Cl
CH2OSiMe3
CH2OH
OMe
7
8
1
1
100
100
OMe
MeO
t-Bu
O2N
CH2OSiMe3
MeO
t-Bu
O2N
CH2OH
CH2OH
CH2OH
CH2OSiMe3
CH2OSiMe3
9
1
2
100
100
10
11
12
13
CH2OH
NO2
2
2
1
100
100
100
CH2OSiMe3
NO2
CH2OSiMe3
CH2OH
O2N
O2N
CH2OH
CH2OSiMe3
Br
Br
CH2OH
CH2OSiMe3
14
2.5
100
OH
OSiMe3
15
16
1.5
1
100
100
OH
OSiMe3
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M. Moghadam et al.
Table 1. (Continued)
Entry
17
Substrate
Product
Time (min)
1
Yield (%)b
100
OH
OSiMe3
OSiMe3
18
2
100
OH
19
CHCH3
OH
OH
CHCH3
2
100
OSiMe3
OSiMe3
20
1
100
OH
OH
21
22
OSiMe3
OSiMe3
1.5
1
100
100
CH3
CH3
23
24
1
100
100
H3C
C
CH3
H3C
CH3
C
OH
OSiMe3
OSiMe3
OH
2.5
25
2
95
C
C
OH
OSiMe3
a Reaction conditions: alcohol (1 mmol), hexamethydisilazane (0.6 mmol), catalyst (1 mol%), CH3CN (1 ml).
b GC yield.
(1 mol%) and stirred at room temperature. The progress of the
reaction was monitored by GC. After completion of the reaction,
the solvent was evaporated, Et2O (10 ml) was added and the
catalyst was filtered. The filtrates were washed with brine, dried
over Na2SO4 and concentrated under reduced pressure to afford
the crude product.
[VIV(TPP)(OTf)2] with AgOTf. Figure 1 shows the UV–vis spectrum
of this catalyst in which three bands at 456 (Soret band), 585 and
628 nm (Q bands) were observed. The diffuse reflectance of the
[VIV(TPP)(OTf)2] resembles the solution counterpart spectrum with
a slight shift. The presence of Soret and Q bands of vanadium
porphyrin clearly indicates the synthesis of the metalloporphyrin
(Fig. 2). TheFT-IRspectrumof thecatalyst, [VIV(TPP)(OTf)2], exhibits
the characteristic bands of the porphyrin units at 768 (C–HPyrrole),
1153 (C–HPh), 1225 (C N) and 2856 cm−1 (C-HPh) (Fig. 3). An
indication for conversion of [VIV(TPP)Cl2] to [VIV(TPP)(OTf)2] is the
bands at 1298 cm−1 and 1174 cm−1 that are related to stretching
vibrations of C–F and S O bonds, respectively.
Catalyst Reusability
The reusability of catalyst was checked in the silylation of
4-chlorobenzyl alcohol. At the end of each reaction, the solvent
was evaporated, Et2O (10 ml) was added, and the catalyst was
filtered and reused.
Trimethylsilylation of Alcohol and Phenols with HMDS
Catalyzed by [VIV(TPP)(OTf)2]
Results and Discussion
The Lewis acidity of this high-valent V(IV) species was investigated
in trimethylsilylation of alcohols and phenols with HMDS. First,
to show the effect of OTf− groups on the catalytic activity
of [VIV(TPP)(OTf)2], different vanadium(IV) porphyrins such as
[VIVO(TPP)], [VIV(TPP)Cl2] and [VIV(TPP)(OTf)2] were used as catalyst
Preparation and Characterization of Catalyst
Scheme 2 shows the preparation route for [VIV(TPP)(OTf)2].
Commercially available [VIVO(TPP)] was converted to [VIV(TPP)Cl2]
with SOCl2. Then, [VIV(TPP)Cl2] was converted to electron-deficient
c
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Appl. Organometal. Chem. 2011, 25, 687–694

Electron-deficient vanadium(IV) tetraphenylporphyrin
Table 2. Trimethylsilylation of alcohols with HMDS catalyzed by [VIV(TPP)(OTf)2] at room temperaturea
Entry
1
Substrate
Product
Time (min)
1.5
Yield (%)b
100
OH
OSiMe3
2
2.5
100
OH
OH
Cl
OSiMe3
Cl
O2N
OSiMe3
3
4
3
3
100
100
O2N
OH
OSiMe3
NO2
NO2
Me3SiO
OSiMe3
5
6
2.5
2.5
100
100
HO
OH
OSiMe3
OSiMe3
OH
OH
7
8
2.5
2.5
100
100
OH
OSiMe3
HO
Me3SiO
Me3SiO
OSiMe3
OSiMe3
OSiMe3
OH
HO
OH
OH
9
2.5
2.5
100
100
OSiMe3
OH
10
a Reaction conditions: phenol (1 mmol), hexamethydisilazane (0.6 mmol), catalyst (1 mol%), CH3CN (1 ml).
b GC yield.
in the trimethylsilylation of 4-chlorobenzyl alcohol with HMDS.
The results showed that the catalytic activity of these catalysts is as
follows: [VIV(TPP)(OTf)2] (100%) > [VIV(TPP)Cl2] (48%) > [VIVO(TPP)]
(21%). These observations show that introducing triflate groups
on the prophyrin moiety increases the electron-deficiency of
V(IV) species, which in turn increases the catalytic activity of
[VIV(TPP)(OTf)2] in the trimethylsilylation reactions.
The trimethylsilylation of alcohols was carried out with HMDS
in the presence of [VIV(TPP)(OTf)2]. Different amounts of catalyst
were used in the silylation of benzyl alcohol in CH3CN and the
best result was obtained in the presence of 1 mol% of catalyst.
Different ratios of HMDS were also used in the same reaction
and the highest yield of TMS ether was produced with 0.6 mmol
of HMDS. Under these optimized conditions, different alcohols
including primary (including benzylic and linear ones), secondary
and tertiary alcohols were subjected to trimethylsilylation with
HMDS in the presence of [VIV(TPP)(OTf)2] at room temperature
and the corresponding TMS ethers were produced in high yields
(Table 1).Inthecaseofbenzylicalcohols,thenatureofsubstituents
has no significant effect on the product yields and reaction times.
The silylation of phenols was also investigated in the presence
of [VIV(TPP)(OTf)2]. As can be seen in Table 2, different phenols
were efficiently converted to their corresponding TMS ethers in
short reaction times (Table 2).
Owing to the high reactivity of this catalyst in the trimethylsi-
lylation of alcohols and phenols with HMDS, the competitive
silylation of alcohols and phenols was carried out. In this man-
ner, an equimolar mixture of a primary alcohol with a secondary
or a tertiary alcohol or phenol was subjected to trimethylsilyla-
tion with HMDS. The results, which are summarized in Table 3,
show that primay alcohols were selectively silylated with HMDS
(0.6 mmol) in the presence of secondary and tertiary alcohols and
c
Appl. Organometal. Chem. 2011, 25, 687–694
Copyright ꢀ 2011 John Wiley & Sons, Ltd.
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M. Moghadam et al.
Table 3. Selective silylation of alcohols and phenols with HMDS catalyzed by [VIV(TPP)(OTf)2]a
Row
ROH
Silyl ether
Time (min)
1
Yield (%)b
98
CH2OH
OH
CH2OSiMe3
1
OSiMe3
0
CH2OH
OH
1
99
CH2OSiMe3
OSiMe3
2
0
1
1
86
14
CH2OH
CH2OSiMe3
3
OH
OSiMe3
98
0
CH2OSiMe3
OSiMe3
CH2OH
OH
4
1
100
100
CH2OH
OH
CH2OSiMe3
OSiMe3
5c
a Reaction conditions for a binary mixture: 1 mmol of each alcohol or phenol, hexamethydisilazane (0.6 mmol), catalyst (1 mol%), CH3CN (1 ml).
b GC yield.
c 1.1 mmol of hexamethydisilazane was used.
note that, owing to the lesser nucleophilicity of sterically hindered
alcohols and phenols in comparison with primary benzylic alco-
hols, only primary benzylic alcohols were silylated with 0.6 mmol
of HMDS (Table 4, entries 1, 2 and 4). However, in the presence of
1.1 mmol of HMDS, both phenol and benzyl alcohol were silylated
completely.
To show the effectivness of the presented method in the
silylation reactions, we compared the obtained results in the
trimethylsilylation of benzyl alcohol with HMDS catalyzed by
[VIV(TPP)(OTf)2] with some of those reported in the literature
(Table 4). It is clear that the presented method is superior in terms
of reaction time, catalyst amount or product yield.
Figure 1. UV–vis spectrum of [VIV(TPP)(OTf)2].
Catalyst Reusability
Thereusabilityofthecatalystwasalsoinvestigated. Inthismanner,
the silylation of benzyl alcohol with HMDS was chosen as model
reaction. At the end of each reaction, the solvent was evaporated
and Et2O was added, and the catalyst was filtered and reused in
the next run with fresh benzyl alcohol and HMDS. The results show
phenols. When benzyl alcohol was silylated in the presence of
a linear alcohol such as 1-octanol, 14% of 1-octyltrimethylsilyl
ether was also detected in the reaction mixture. It is important to
c
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Appl. Organometal. Chem. 2011, 25, 687–694

Electron-deficient vanadium(IV) tetraphenylporphyrin
Table 4. Comparison of the results obtained for the trimethylsilylation of benzyl alcohol catalyzed by [VIV(TPP)(OTf)2] with those obtained by the
recently reported catalysts
OSiMe3
OH
Catalyst
Temperature
Entry Catalyst
HMDS (mmol) Catalyst (mol%)
(◦C)
Time (min) Yield (%) Reference
1
[VIV(TPP)(OTf)2]
0.6
0.8
0.8
0.8
0.7
0.6
0.6
0.7
1.4
1.4
0.7
1
1
RT
RT
1
1
100
95
90
98
98
99
98
91
90
90
92
98
90
98
92
100
100
This work
[12]
[16]
[17]
[20]
[21]
[22]
[23]
[24]
[24]
[25]
[26]
[28]
[29]
[31]
[38]
[42]
2
ZrCl4
2
3
H3PW12O40
1
1
55–60 ◦C
RT
23
2
4
iodine
5
CuSO4. 5H2O
1
RT
12
55
5
6
Sulfonic acid@ nanoporous silica
3
RT
7
MgBr2·OEt2
5
RT
8
LaCl3
10
0.02 g
4
RT
3h
90
30
5
9
Poly(N-bromobenzene-1,3-disulfonamide)
N,N,Nꢁ,Nꢁ-tetrabromobenzene-1,3-disulfonamide
RT
10
11
12
13
14
15
16
17
RT
Fe(TFA)3
2.5
10
6
RT
Fe3O4
RT
Trichloroisocyanuric acid (TCCA)
HClO4 –SiO2
0.8
0.8
2
RT
4 h
2
2.5
0.5
1
RT
ZrO(OTf)2
RT
1
[SnIV(TPP)(OTf)2]
[SnIV(TPP)(BF4)2]
0.5
0.5
RT
1
1
RT
1
Figure 3. FT IR spectrum of [VIV(TPP)(OTf)2].
Figure 2. Diffuse reflectance UV–vis spectrum of [VIV(TPP)(OTf)2].
Acknowledgment
We are thankful to the Center of Excellence of Chemistry of
University of Isfahan for financial support of this work.
that the catalyst can be reused several times (five consecutive
times were checked) without loss of its catalytic activity.
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