(PhCH2PPh3)+Br3-: A versatile reagent for the preparation, deprotection, and oxidation of trimethylsilyl ethers

Article abstract of DOI:10.1080/10426500902915440

Benzyltriphenylphosphonium tribromide (BTPTB), as a stable solid reagent, is easily prepared by the reaction of benzyltriphenylphosphonium bromide with Br₂. This reagent can be used as an efficient catalyst for the conversion of alcohols to their corresponding trimethylsilyl ethers (TMS ethers) with hexamethyldisilazane (HMDS). Desilylation of TMS ethers is also catalyzed by BTPTB in MeOH at room temperature in high yields. BTPTB is also able to oxidize the TMS ethers to their corresponding carbonyl compounds in a mixture of MeOH/H₂O in good to high yields. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

Full text of DOI:10.1080/10426500902915440

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(PhCH2PPh3)+Br3 : A Versatile Reagent
for the Preparation, Deprotection, and
Oxidation of Trimethylsilyl Ethers
F. Shirini a , G. H. Imanzadeh b , A. R. Mousazadeh a b , I.
Mohammadpoor-Baltork c , A. R. Aliakbar a & M. Abedini a
a Department of Chemistry, College of Science , University of
Guilan , Rasht, Iran
b Department of Chemistry, College of Science , University of
Mohaghegh-Ardabili , Ardabil, Iran
c Department of Chemistry , Isfahan University , Isfahan, Iran
Published online: 24 Feb 2010.
To cite this article: F. Shirini , G. H. Imanzadeh , A. R. Mousazadeh , I. Mohammadpoor-Baltork , A. R.
-
Aliakbar & M. Abedini (2010) (PhCH2PPh3)+Br3 : A Versatile Reagent for the Preparation, Deprotection,
and Oxidation of Trimethylsilyl Ethers, Phosphorus, Sulfur, and Silicon and the Related Elements,
185:3, 641-646, DOI: 10.1080/10426500902915440
To link to this article: http://dx.doi.org/10.1080/10426500902915440
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Phosphorus, Sulfur, and Silicon, 185:641–646, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902915440
(PhCH2PPh3)+Br3−: A VERSATILE REAGENT FOR THE
PREPARATION, DEPROTECTION, AND OXIDATION
OF TRIMETHYLSILYL ETHERS
F. Shirini,1 G. H. Imanzadeh,2 A. R. Mousazadeh,1,2
I. Mohammadpoor-Baltork,3 A. R. Aliakbar,1 and M. Abedini1
1Department of Chemistry, College of Science, University of Guilan,
Rasht, Iran
2Department of Chemistry, College of Science, University of Mohaghegh-Ardabili,
Ardabil, Iran
3Department of Chemistry, Isfahan University, Isfahan, Iran
Benzyltriphenylphosphonium tribromide (BTPTB), as a stable solid reagent, is easily pre-
pared by the reaction of benzyltriphenylphosphonium bromide with Br2. This reagent can be
used as an efficient catalyst for the conversion of alcohols to their corresponding trimethylsi-
lyl ethers (TMS ethers) with hexamethyldisilazane (HMDS). Desilylation of TMS ethers
is also catalyzed by BTPTB in MeOH at room temperature in high yields. BTPTB is also
able to oxidize the TMS ethers to their corresponding carbonyl compounds in a mixture of
MeOH/H2O in good to high yields.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental
file.
Keywords Alcohols; benzyltriphenylphosphonium tribromide; deprotection; oxidation;
trimethylsilyl ethers
INTRODUCTION
During a multistep synthesis, different methods are used for the protection of al-
coholic hydroxyl groups, in which trimethylsilylation is one of the most widely used
methods for this purpose.1 Because of its availability, cheapness, and stability, hexamethyl-
disilazane (HMDS) is considered as one of the most useful reagents for this protection.
Even though the handling of HMDS is easy, and silylation using this reagent does not
need special precautions, the low silylating power is the main drawback to its applica-
tion. To improve the silylating power of this reagent, a variety of catalysts have been re-
ported; of them, sulfonic acid,2 K-10 montmorillonite,3 silica chloride,4 KBr,5 Al(HSO4)3,6
Fe(HSO4)3,7 ZrCl4,8 tungstophosphoric acid,9 lithium perchlorate,10 cupric sulfate pen-
tahydrate,11 Al(OTf)3,12 magnesium triflate,13 copper triflate,14 trichloroisocyanuric acid,15
Received 16 December 2008; accepted 13 March 2009.
We are thankful to the University of Guilan and Mohaghegh-Ardabili University Research Councils for the
partial support of this work.
Address correspondence to Dr. Farhad Shirini, Department of Chemistry, College of Science, The University
of Guilan, Rasht 41335, Iran. E-mail: shirini@guilan.ac.ir
641

642
F. SHIRINI ET AL.
sulfonic acid-functionalized nanoporous silica,16 iodine,17 H-β zeolite,18 lanthanum trichlo-
ride,19 mono- and dibromo-5,5-diethylbarbituric acids,20 1,3-dibromo-5,5-diethylbarbituric
acid,21 HClO4·SiO2,22 NH4SCN,23 and tribromomelamine24 are examples. Although these
procedures are an improvement, most of them suffer from disadvantages such as long reac-
tion times, forceful conditions, low selectivity, tedious workup, and use of toxic, expensive,
or moisture-sensitive reagents. Hence introduction of new procedures to circumvent these
problems is still in demand.
Very recently, benzyltriphenylphosphonium tribromide (BTPTB) was used in the
tetrahydropyranylation of alcohols25 as well as the thioacetalization of carbonyl com-
pounds.26 In continuation of our ongoing research program on the development of new
methods for the protection of hydroxyl groups,4–8, 27, 28 herein, in addition to the intro-
duction of a new and high yielding method for the preparation of BTPTB, we report the
applicability of this reagent in the promotion of the trimethylsilylation of alcohols with
HMDS (Scheme 1).
HMDS / BTPTB
ROH
ROTMS
CHCl3, reflux
Scheme 1
Initially, 4-chlorobenzyl alcohol was chosen as a model substrate for the silylation
reaction. It was treated with 2 eq. of HMDS in the presence of 0.01 eq. of BTPTB in CHCl3
(2 mL) under reflux conditions, and the reaction was completed in 20 min (95% isolated
yield). After optimization of the reaction conditions, different types of alcohols, including
benzylic and primary and secondary aliphatic alcohols, were subjected to trimethylsilyla-
tion under the determined conditions. As shown in Table I, the corresponding TMS ethers
were obtained in short reaction times in good to high yields. No elimination and rearrange-
ment byproducts were observed at all. Tertiary alcohols are silylated slower than the other
alcohols using this method, and the reaction is incomplete in most cases (Table I, entries
13–15). Therefore this methodology can be used for the selective silylation of benzylic and
primary and secondary aliphatic alcohols in the presence of tertiary alcohols. This is exem-
plified by the competitive reaction between 2-methylbenzyl alcohol and triphenylmethanol
(Table I, entry 16).
In 1987, Kajigaeshi et al. reported that organic ammonium tribromides, such as
benzyltrimethylammonium tribromide, generate HBr and MeOBr in methanol.29 On the
basis of this and obtained results, the mechanism shown in Scheme 2 is selected as the most
plausible one for trimethylsilylation of alcohols catalyzed by BTPTB.
Taking cues from the above-mentioned idea based on the probable generation of HBr
from the reaction of MeOH and BTPTB, we decided to investigate the deprotection of
TMS ethers in methanol using this reagent. Our investigation clarified that the conversion
of TMS ethers to their corresponding alcohols is easily performed in the presence of
catalytic amounts of BTPTB in MeOH. All reactions were performed at room temperature
in short reaction times in high yields (Scheme 3, Table II). The probable mechanism of the
reaction is shown in Scheme S1 (available online in the Supplementary Materials).
We have also found that by increasing the amounts of BTPTB, TMS ethers were
efficiently converted to their corresponding aldehydes and ketones. All reactions were

BTPTB AS A VERSATILE REAGENT
643
(PhCH2PPh3) Br3
+ ROH
HBr + ROBr + (PhCH2PPh3) Br
ROH
2ROTMS
HBr
(Me3Si)2NH
(Me3Si)2NH2 Br
H4N Br
NH3
HN(SiMe3)2
Scheme 2
BTPTB
ROTMS
ROH
MeOH, r.t.
Scheme 3
Table I Trimethylsilylation of alcoholsa
Entry
Substrate
Product
Time (min) Yield (%)b
1
2
3
4
5
6
7
8
4-ClC6H4CH2OH
4-NO2C6H4CH2OH
2-MeC6H4CH2OH
4-MeOC6H4CH2OH
PhCH(OH)Ph
PhCH(OH)C(O)Ph
PhCH2CH2OH
PhCH2CH2CH2OH
4-ClC6H4CH2OSiMe3
4-NO2C6H4CH2OSiMe3
2-MeC6H4CH2OSiMe3
4-MeOC6H4CH2OSiMe3
PhCH(OSiMe3)Ph
PhCH(OSiMe3)C(O)Ph
PhCH2CH2OSiMe3
PhCH2CH2CH2OSiMe3
20
75
10
8
15
75
14
15
95
60
92
95
90
90
92
89
9
90
75
OH
OH
OSiMe3
OSiMe3
10
30
87
11
12
45
70
90
92
OH
OH
OSiMe3
OSiMe3
13
14
15
PhCH2C(OH)Me2
Ph3COH
PhCH2C(OSiMe3)Me2
Ph3COSiMe3
60
60
120
0
0
10
OSiMe3
OH
16
2-MeC6H4CH2OH + Ph3COH 2-MeC6H4CH2OSiMe3 + Ph3COSiMe3
10
100c + 0c
aProducts were characterized by their physical data, comparison with authentic samples, and IR and NMR
spectroscopy, and also by the conversion of silyl ethers to their corresponding alcohols.19, 22–24, 31, 32
bIsolated yield.
cConversion.

644
F. SHIRINI ET AL.
Table II Deprotection of trimethylsilyl ethersa
Entry
Substrate
Product
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
4-ClC6H4CH2OSiMe3
2-ClC6H4CH2OSiMe3
2-BrC6H4CH2OSiMe3
2-MeC6H4CH2OSiMe3
4-Me3CC6H4CH2OSiMe3
2-MeOC6H4CH2OSiMe3
4-MeOC6H4CH2OSiMe3
4-NO2C6H4CH2OSiMe3
2-NO2C6H4CH2OSiMe3
PhCH(OSiMe3)CH3
4-ClC6H4CH2OH
2-ClC6H4CH2OH
2-BrC6H4CH2OH
2-MeC6H4CH2OH
4-Me3CC6H4CH2OH
2-MeOC6H4CH2OH
4-MeOC6H4CH2OH
4-NO2C6H4CH2OH
2-NO2C6H4CH2OH
PhCH(OH)CH3
3
2
2
1
1
2
2
3
4
2
2
3
2
3
2
90
92
95
90
90
92
87
95
90
92
90
90
85
90
95
PhCH(OSiMe3)Ph
PhCH(OSiMe3)C(O)Ph
PhCH2CH2OSiMe3
PhCH(OH)Ph
PhCH(OH)C(O)Ph
PhCH2CH2OH
PhCH2CH2CH2OSiMe3
PhCH2CH2CH2OH
OSiMe3
OH
16
17
3
3
90
85
OSiMe3
OSiMe3
OH
OH
18
19
2
3
90
85
OSiMe3
OH
OSiMe3
OH
aProducts are characterized by IR, NMR, elemental analysis, and comparison with authentic samples.
bIsolated yield.
performed in a mixture of MeOH and H2O (2:1, v/v) at room temperature in good to high
yields [Scheme 4, Table S1 (Supplementary Materials online)].
BTPTB
R1R2CH(OTMS)
R1R2CO
MeOH : H2O (2:1, v/v) / r.t.
Scheme 4
Our studies revealed that the reaction consists of two steps: deprotection of TMS
ethers and oxidation of the produced alcohols. On the basis of these results and the liter-
ature,30 the mechanism of the reaction can be suggested, which is shown in Scheme S2
(Supplementary Materials online).
In conclusion, we have reported the application of benzyltriphenylphosphonium tri-
bromide in the efficient trimethylsilylation of alcohols, as well as in the deprotection and
oxidation of silyl ethers. Relatively short reaction times, high yields of the products, and
easy workup are important advantages of this method, which make this procedure a useful
and attractive addition to the currently available methods.

BTPTB AS A VERSATILE REAGENT
645
EXPERIMENTAL
Chemicals were purchased from Fluka, Merck, and Aldrich Chemical Companies.
Yields refer to isolated products. Products were characterized by their physical constants,
comparison with authentic samples, and IR and NMR spectroscopy.19, 22–24, 31, 32
Preparation of BTPTB
A 0.5 M solution (1 mL) of Br2 in MeOH was added dropwise to a solution of
benzyltriphenylphosphonium bromide (0.3 mmol, 0.13 g) in EtOH (5 mL) with stirring in
an ice bath. The mixture was stirred for 1 h, then filtered. The yellow solid residue was
washed with distilled water, dried at room temperature, and recrystallized from CH3CN to
give BTPTB in 85% yield.
Trimethylsilylation of Alcohols Catalyzed by BTPTB
A mixture of the substrate (1 mmol), HMDS (2 mmol, 0.32 g), and BTPTB (0.01
mmol, 0.006 g) in CHCl3 (2 mL) was stirred under reflux conditions. The progress of
the reaction was monitored by TLC. Upon completion, a saturated solution of sodium
bicarbonate (5 mL) was added, and the product was extracted with CHCl3 (2 × 10 mL).
The organic layer was separated, washed with water, and dried over MgSO4. The solvent
was removed to give crude TMS ether. Further purification proceeded by bulb-to-bulb
distillation under reduced pressure or recrystallization to afford pure silyl ether.
Deprotection of Trimethylsilyl Ethers Catalyzed by BTPTB
A mixture of the substrate (1 mmol) and BTPTB (0.025 mmol, 0.015 g) in MeOH
(2 mL) was stirred at room temperature. The progress of the reaction was monitored by
TLC. After completion of the reaction, the solvent was evaporated and Et2O (5 mL) was
added. The mixture was filtered and the solid residue was washed with Et2O (5 mL). The
filtrate was washed with saturated solution of NaHCO3 and H2O and dried over MgSO4.
Evaporation of the solvent followed by column chromatography on silica gel gave the
corresponding alcohols in good to high yields.
Oxidation of Trimethylsilyl Ethers Using BTPTB
BTPTB (2 mmol, 1.2 g) was added to a solution of the substrate (1 mmol) in
MeOH:H2O (2:1, v/v) (3 mL). The mixture was stirred at room temperature. After comple-
tion of the reaction, most of the solvent was evaporated, and Et2O (5 mL) was added. The
mixture was filtered and the solid residue was washed with Et2O (5 mL). The filtrate was
washed with a saturated solution of NaHCO3 and H2O and dried over MgSO4. Evaporation
of the solvent followed by column chromatography on silica gel gave the corresponding
carbonyl compounds in good to high yields.
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