Heterogeneous thiocyanation of benzylic alcohols and silyl and THP ethers, and deprotection of silyl and THP-ethers by [PCl3-n(SiO 2)n] (silphos)

Article abstract of DOI:10.1080/10426500903419145

Silicaphosphite (silphos), [PCl_3-n(SiO₂) _n], as a heterogeneous phosphorous compound, catalyzes the thiocyanation of benzylic alcohols and silyl and THP ethers in the presence of I₂ and NH₄SCN in refluxing CH₃CN. The produced silphos oxide byproduct can be easily separated by a simple filtration. Silphos is also used for the efficient and selective deprotection of silyl and THP-ethers to their corresponding alcohols. Copyright

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Heterogeneous Thiocyanation of
Benzylic Alcohols and Silyl and THP
Ethers, and Deprotection of Silyl and
THP-Ethers by [PCl3-n(SiO2)n] (Silphos)
N. Iranpoor a , H. Firouzabadi a , H. Bahador a & A. Jamalian b
a Chemistry Department, College of Sciences, Shiraz University,
Shiraz, Iran
b Medicinal & Natural Products Chemistry Research Centre, Shiraz
University of Medical Sciences, Shiraz, Iran
Version of record first published: 25 Aug 2010.
To cite this article: N. Iranpoor , H. Firouzabadi , H. Bahador & A. Jamalian (2010): Heterogeneous
Thiocyanation of Benzylic Alcohols and Silyl and THP Ethers, and Deprotection of Silyl and THP-
Ethers by [PCl3-n(SiO2)n] (Silphos), Phosphorus, Sulfur, and Silicon and the Related Elements, 185:9,
1972-1978
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Phosphorus, Sulfur, and Silicon, 185:1972–1978, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903419145
HETEROGENEOUS THIOCYANATION OF BENZYLIC
ALCOHOLS AND SILYL AND THP ETHERS, AND
DEPROTECTION OF SILYL AND THP-ETHERS
BY [PCl3-n(SiO2)n] (SILPHOS)
N. Iranpoor,1 H. Firouzabadi,1 H. Bahador,1 and A. Jamalian2
1Chemistry Department, College of Sciences, Shiraz University, Shiraz, Iran
2Medicinal & Natural Products Chemistry Research Centre, Shiraz University
of Medical Sciences, Shiraz, Iran
Silicaphosphite (silphos), [PCl3-n(SiO2)n], as a heterogeneous phosphorous compound, cat-
alyzes the thiocyanation of benzylic alcohols and silyl and THP ethers in the presence of I2
and NH4SCN in refluxing CH3CN. The produced silphos oxide byproduct can be easily sep-
arated by a simple filtration. Silphos is also used for the efficient and selective deprotection
of silyl and THP-ethers to their corresponding alcohols.
KEYWORDS Alcohol; deprotection; silyl ether, silphos; thiocyanation; THP-ether
INTRODUCTION
The preparation of alkyl halides and thiocyanates from alcohols through the Mit-
sunobu reaction has been widely studied with tertiary phosphines.1–3 The use of PPh3/Br2/
SCN−, phenylene alkylphosphite/Br2/AgSCN4, or in situ–generated PPh3(SCN)25 is well
studied. The difficulty of removing the phosphine oxide byproduct is usually the major
disadvantage encountered with these systems. Heterogeneous phosphorous-containing
reagents such as diphenyl(2-pyridyl)phosphine,6 (p-dimethylaminophenyl)-diphenyl
phosphine,7 1,2-bis(diphenylphosphino)ethane (diphos),8 tris[4-(1H,1perfluorooctyloxy-
phenyl)-phosphine,9 and 4-diphenyl phosphanyl benzoic acid 2-trimethylsilanylethylester
(DPPBE),10 or the use of crosslinked11 or non-crosslinked polystyrene12 and PEG-bound
reagents13 carrying a phosphine group have been applied to carry on the reactions under
heterogeneous conditions for easy removal of the phosphine oxide byproduct.
Recently, we have introduced a cheap and easily prepared heterogeneous phosphite
reagent, silphos [PCl3-n(SiO2)n], and applied it in various synthetic transformations.14 The
use of silphos in these synthetic methods offers the possibility of easy removal of silphos
oxide by filtration.
The ease of preparation and stability of tetrahydropyranyl (THP) and trimethylsilyl
(TMS) groups under a variety of reaction conditions, and also their easy removal, offer
Received 22 July 2009; accepted 14 October 2009.
Address correspondence to N. Iranpoor and H. Firouzabadi, Chemistry Department, College of Sciences,
Shiraz University, Shiraz 71454, Iran. E-mail: iranpoor@susc.ac.ir; firouzabadi@susc.ac.ir
1972

HETEROGENOUS THIOCYANATION AND DEPROTECTION
1973
great applicability for their use in protection of hydroxyl groups in multistep synthesis. A
wide range of catalysts and reagents have been used for deprotection of tetrahydropyranyl
ethers and trimethylsilyl ethers.15–19 In comparison, the reports on selective deprotection
reactions and also deprotection thiocyanation are rare, and as far as we know, there is
no report on selective thiocyanation of these compounds.3,5a–b In this article, we report an
efficient and selective method for the conversion of benzylic alcohols and their trimethylsilyl
and tetrahydropyranyl ethers to their corresponding thiocyanates or isothiocyanates by
using silphos/I2/NH4SCN reagent system. We also report on the efficient use of silphos for
deprotection of TMS- and THP-ethers and also highly selective deprotection of TMS-ethers
to their corresponding hydroxyl compounds in the presence of THP-ethers.
RESULTS AND DISCUSSION
In order to avoid the problems of using homogeneous phosphorous compounds, and
in continuation of our work on thiocyanation reactions,3–5,19,20 we used the heterogeneous
silica-based phosphorous compound (silphos) in conjunction with ammonium thiocyanate
and iodine for thiocyanation of benzylic alcohols and their protected derivatives (Scheme 1).
(R=1o )
R-SCN
silphos
R-OX
R-SCN + RNCS, (R=2o)
major
I2/NH4SCN/CH3CN
reflux
X= H, THP, TMS
silphos = [PCl3-n(SiO2)n]
R = benzylic
Scheme 1
The use of Br2 or compounds carrying electrophilic halogens, such as N-
bromosuccinimide (NBS) or N-chloro succinimide (NCS), for this conversion was
also studied. We first optimized the reaction conditions using 4-methoxybenzyl alco-
hol as a model compound. The optimized conditions for the reaction of silphos and
ROH/I2/NH4SCN with stoichiometry of 1/1.2/2.8 are shown in Table I.
It was observed the reaction of 4-methoxybenzyl alcohol with silphos/I2/NH4SCN
was completed after 45 min and produced 4-methoxybenzyl thiocyanate in quantitative
Table I Conversion of 4-methoxybenzyl alcohol (1.0 mmol) to 4-methoxy benzyl thio-
cyanate using silphos (1.2 g) in the presence of NH4SCN (2.8 mmol) and different source
of halogens (1.2 mmol) in refluxing acetonitrile
Entry
Halogen compounds
Conversiona %
Time
1
2
3
4
I2
100
10
5
45 min
30 h
30 h
Br2
NBS
NCS
0
30 h
aConversion yield is based on GC analysis using n-octane as internal standard.

1974
N. IRANPOOR ET AL.
yield. The use of Br2, NBS, and NCS instead of I2 is not successful for this transformation.
The results are shown in Table I.
When we applied the optimized reaction conditions to other alcohols, it was observed
that this reagent system is more efficient for benzylic alcohols (Scheme 1, Table I), and the
reaction of nonbenzylic alcohols and deactivated benzylic alcohols do not go to completion.
Using this method, primary benzylalcohols are converted to their corresponding alkyl
thiocyanates without the formation of any isothiocyanates, but as is the case with other
reagent systems, the secondary ones give also some minor amounts of isothiocyanates as a
side product. The results are shown in Table II.
We then studied the possibility of applying this selective reagent system to THP-
and TMS-ethers. The results obtained from this study (Table II) show that the combination
of silphos/I2/NH4SCN in the same stoichiometry as used for alcohols provides the most
suitable reaction conditions for the quantitative conversion of only protected benzylic
alcohols to their corresponding thiocyanates in refluxing acetonitrile.
The order of addition of reagents is also important. The mixture of silphos and I2 is
first prepared in acetonitrile and NH4SCN is added to this mixture followed by the addition
of alcohol or the protected alcohol.
We also prepared a wide variety of structurally different THP-22 and TMS-ethers23
according to the procedures in the literature and studied their deprotection to their cor-
responding alcohols using silphos [(SiO2)n PCl3-n]. We optimized the conditions for the
reaction of benzyltrimethylsilyl ethers and found that the reaction in the presence of silphos
(1.2 g) and silyl ether (1 mmol) proceeds at room temperature in CH3CN (5 mL). The
desired alcohol was produced in high yield almost immediately (Table III). In order to
show the effect of silphos on this reaction, we replaced silphos with the same amount
of silica gel for the deprotection of benzyltrimethylsilyl ether in acetonitrile. This reac-
tion, after 5 h, showed the formation of trace amounts of alcohol. In comparison with
the reaction of silphos, which completes almost immediately, the efficient role of silphos
Table II Conversion of benzylalcohols, and silyl and THP ethers to thiocyanates or isothiocyanates
Entry
Substrate
Time
Conversion
RSCN/RNCSa
% Isolated yielde
1
2
3
4
5
6
7
8
9
4-MeOC6H4CH2OH
2-MeOC6H4CH2OH
C6H5CH(OH)CH3
(C6H5)2CHOH
2-MeC6H4CH2OH
C6H5CH2OH
45 min
2 h
7 h
14 h
12 h
24 h
35 h
35 h
50 min
9 h
100
100
100
80
100/0
100/0
70/30b
60/20
100/0
100/0
100/0
65/25b
100/0
65/35d
100/0
70/10d
90
92
85
88
90
90
—
—
90
85
88
82
90
100
40c
30c
100
100
100
100
C6H5CH2CH2OH
CH3(CH2)5CH(OH)CH3
4-MeOC6H4CH2OSi(Me)3
C6H5CH(OSi(Me)3)CH3
2-MeOC6H4CH2OTHP
C6H5CH(OTHP)CH3
10
11
12
10 h
12 h
aGC and NMR yield using n-octane as internal standard.
bYield% of thiocyanates and isothiocyanates were determined by NMR analysis.
cGc yield.
dThe reaction was done with 2 equivalents of silphos.
eAll the products are known compounds and were identified by comparison of their physical and spectral data
with those of authentic samples.1,2

HETEROGENOUS THIOCYANATION AND DEPROTECTION
1975
Table III Conversion of silyl ethers to alcohols after 2 min
Silphos
−−−−−−−−−→
ROSiMe3
ROH
CH3CN, r.t
Entry
R
Conversion (%)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
p-Methoxybenzyl
benzyl
100
100
100
100
100
100
100
100
100
100
90
85
90
90
92
88
88
95
92
90
1-Phenyl-ethyl
4-Nitrobenzyl
4-Chlorobenzyl
1,1-Diphenylmethyl
2-Phenyl-ethyl
1-Adamantyl
2-Cyclohexylethyl
2-Octyl
10
is well demonstrated. The recyclability of silphos was also examined. In the reaction of
benzyltrimethylsilyl ether, the catalyst was filtered and washed with ether and dried. After
drying in a vacuum oven (40◦C), it was reused for the same reaction. This process was
repeated five times, and the recovered silphos was found to have almost the same reactivity.
We observed that deprotection of different THP-ethers with silphos as a heteroge-
neous reagent occurs in acetonitrile at reflux conditions. We optimized the conditions for
cleavage of the THP-ethers to obtain the parent hydroxyl compounds. The results of the
optimization reactions show that silphos (1.2 g) in refluxing acetonitrile (5 mL) converts
benzyl tetrahydropyranyl ether (1.0 mmol) to benzyl alcohol (90%) after 40 min. In compar-
ison to the silphos, SiO2 is not effective for this deprotection. The results for the reactions
of THP-ethers with this reagent showed that this system is suitable for the conversion
of 1◦, 2◦, 3◦, and benzylic THP-ethers to their corresponding alcohols (Table IV). Various
types of benzylic THP-ethers with electron-donating and electron-withdrawing groups were
converted to their corresponding alcohols in the presence of silphos with excellent yields at
room temperature. Our studies showed that this method is also applicable for deprotection
of nonbenzylic THP-ethers (Table IV).
Table IV Conversion of THP ethers to alcohols
Silphos
−−−−−−−−−−−→
ROTHP
ROH
Conversion
CH3CN, reflux
Entry
R
Time (min)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
4-Methoxybenzyl
Benzyl
40
100
85
90
70
90
80
90
100
90
100
100
100
100
100
100
100
100
100
100
90
88
92
90
85
88
85
90
92
90
1-Phenyl-ethyl
4-Nitrobenzyl
4-Chlorobenzyl
1,1-Diphenylmethyl
2-Phenyl-ethyl
1-Adamantyl
2-Cyclohexylethyl
2-Octyl
10

1976
N. IRANPOOR ET AL.
Table V Selective reactions of different binary mixtures of THP and silyl ethers
Entry
Substrate
PhCH2OSiMe3
Time
1 min
Conversion
% Isolate yielda
1
2
3
4
5
100
0
100
0
100
0
80
20
100
0
85
0
90
0
92
0
75
7
PhCH2OTHP
4-MeOPhCH2OSiMe3
4-MeOPhCH2OSi(i-Pr)3
4-MeOPhCH2OSiMe3
4-MeOPhCH2OSi(Me)2t-Bu
PhCH2OTHP
4-MeOPhCH2OSi(i-Pr)3
PhCH2OTHP
PhCH2OSi(Me)2t-Bu
1 min
1 min
100 minb
100 minb
88
0
aGC yield using n-octane as internal standard.
bReflux condition.
In comparison with THP-ethers, the deprotection reactions of TMS-ethers with
silphos were found to be much quicker, and occurred almost immediately to give their
corresponding alcohols (Table V).
Deprotection reactions of silyl ethers with silphos were found to be highly se-
lective in the presence of THP-ethers. In a binary mixture of benzylsilyl ethers and
benzyl-THP-ethers, the silyl ethers were converted quantitatively to their corresponding
alcohols, while the THP-ethers remained intact (Table V). Excellent selectivity was also
observed for the conversion of TMS-ethers in the presence of bulky trialkylsilyl ethers
such as t-butyldimethylsilyl (TBS) and triisopropylsilyl ethers (Table V). Similarly, this
method showed excellent selectivity for the conversion of tTHP-ethers in the presence of a
t-butyldimethylsilyl (TBS) and triisopropylsilyl ethers to the corresponding alcohols.
In addition to the selective nature of silphos for the conversion of only active benzylic
alcohols and their protected derivatives to their corresponding thiocyanates, silphos is also
very efficient for deprotection reactions. In Table VI, the present method for deprotection
of trimethylsilyl ethers was compared with some of the reported methods in the literature.
In conclusion, the use of silphos provides a very selective method for thiocyanation of
only active benzylic alcohols. In comparison, none of the reported methods for this transfor-
mation has shown any selectivity between alcohols. The use of silphos offers the possibility
Table VI Comparison of this method with some of the methods in the literature
Entry
Substrates
Reagent or catalystref
Condition
Time
Isolated yield%
1
2
3
4
5
6
7
8
9
4-MeOC6H4CH2OTMS
4-MeOC6H4CH2OTMS
Ph2CHOTMS
Ph2CHOTMS
Ph2CHOTMS
4-NO2C6H4CH2OTMS
4-NO2C6H4CH2OTMS
4-ClC6H4CH2OTHP
4-ClC6H4CH2OTHP
4-MeOC6H4CH2OTHP
4-MeOC6H4CH2OTHP
Silphos
rt
r.t
rt
rt
r.t
rt
rt
reflux
rt
2 min
40 min
2 min
1.5 h
2 h
2 min
2 h
70 min
3 h
90
95
88
90
95
90
90
85
90
90
85
[bmim]Cl21
Silphos
[bmim]Cl21
Fe(NO3)3.9H2O/Fe(HSO4)3
Silphos
[bmim]Cl21
22
22
Silphos
Fe(NO3)3.9H2O/Fe(HSO4)3
Silphos
β-CD23
10
11
reflux
60◦C
40 min
9 h

HETEROGENOUS THIOCYANATION AND DEPROTECTION
1977
of efficient deprotection of TMS- and THP-ethers to their corresponding alcohols. In ad-
dition, the highly selective use of silphos for the high yielding deprotection of TMS-ethers
in the presence of THP-ethers and in the presence of more bulky trialkylsilyl ethers can
be considered as strong points in favor of using this compound. In addition, the ease of
separation of silphos oxide in the thiocyanation reactions and silphos in the deprotection
reactions can be regarded as another advantage of using this heterogeneous method.
EXPERIMENTAL
Chemicals were either prepared in our laboratories or were purchased from chemical
companies. The purity determination of the products was accomplished by GC on a Shi-
madzu model GC-14A instrument or by TLC on silica gel polygram SIL G/UV 254 plates.
The IR spectra were recorded on a Shimadzu FTIR-8300 spectrophotometer. The NMR
spectra were recorded on a Bruker Avance DPX-250 MHz spectrometer. All products are
known compounds, and their analytical and spectral data were compared with those of
authentic samples.
Typical Procedure for the Conversion of 4-Methoxybenzyl Alcohol
or 4-Methoxybenzyl TMS-Ether to 4-Methoxybenzyl Thiocyanate
To a flask containing a stirring mixture of silphos (1.2 g) and I2 (0.3 g, 1.2 mmol)
in refluxing dry acetonitrile (5 mL), NH4SCN (0.21 g, 2.8 mmol) was added. Then 4-
methoxybenzyl alcohol (0.13 g, 1 mmol) or 4-methoxybenzyl trimethylsilyl ether (0.21 g,
1 mmol) was added. Monitoring of the reaction by TLC and GC analysis showed the
completion of the reaction after 45–50 min. After evaporation of the solvent on a rotary
evaporator, the obtained solid phase was washed by diethyl ether (3 × 15 mL). The combined
organic solution was washed with saturated solution of sodium thiosulfate (10 mL) to
remove the unreacted iodine. The organic layer was separated and dried with anhydrous
Na2SO4. After filtration, the solvent was evaporated under vacuum. The product was
purified on a short column of silica gel using n-hexane-ethyl acetate (2:1) as eluent to give
the pure 4-methoxybenzyl thiocyanate (90–92%). IR ( SCN) in CCl4 2158 cm−1; 1H NMR
(CDCl3) δ (ppm) = 3.2 (3H, s), 3.9 (2H, s), 7.1 (4H, m); 13C NMR (CDCl3) δ (ppm) =
159.5, 132.8, 127.5, 115.9, 111.2, 38.8.
Typical Procedure for the Conversion of Benzyltetrahydropyranyl Ether
or Benzyltrimethylsiliyl Ether to Benzyl Alcohol in the Presence of Silphos
Silphos (1.2 g) and benzyltetrahydropyranyl ether (0.19 g, 1 mmol) or ben-
zyltrimethylsiliyl ether (0.18 g, 1 mmol) and dry acetonitrile (5 mL) were placed in a round
bottom flask. The reaction benzyltetrahydropyranyl ether was refluxed for 40 min, and the
reaction of benzyltrimethylsiliyl ether was stirred at room temperature for 1 min. TLC
monitoring showed the completion of the reaction. The reaction mixture was filtered and
washed by ethanol (50 mL). The solvent was evaporated on a rotary evaporator. Column
chromatography of the crude mixture on silica-gel using n-hexane-ethyl acetate (3:1) as
eluent gave pure benzyl alcohol in 88% (0.095 g) and 85% (0.092 g) yields, respectively.

1978
N. IRANPOOR ET AL.
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