High-valent tin(IV) porphyrin: An efficient and reusable catalyst for tetrahydropyranylation of alcohols and phenols under mild conditions

Article abstract of DOI:10.1016/j.ica.2010.01.017

In this paper, rapid and highly efficient tetrahydropyranylation of alcohols and phenols with 3,4-dihydro-2H-pyran (DHP) in the presence of catalytic amounts of high-valent tin (IV) tetraphenylporphyrinato trifluoromethanesufonate, [Sn^IV(TPP)(OTf)₂] is reported. In this catalytic system, primary, secondary and tertiary alcohols as well as phenols were converted to their corresponding tetrahydropyranyl ethers (THP-ethers) in excellent yields and short reaction times at room temperature. It is noteworthy that this method can be used for chemoselective tetrahydropyranylation of primary alcohols in the presence of secondary and tertiary alcohols and phenols. The catalyst was reused several times in the protection reactions without loss of its catalytic activity.

Full text of DOI:10.1016/j.ica.2010.01.017

Inorganica Chimica Acta 363 (2010) 1523–1528
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Inorganica Chimica Acta
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High-valent tin(IV) porphyrin: An efficient and reusable catalyst for
tetrahydropyranylation of alcohols and phenols under mild conditions
a,
a
Majid Moghadam a,b, , Shahram Tangestaninejad , Valiollah Mirkhani ,
*
*
Iraj Mohammadpoor-Baltork a, Shadab Gharaati a
a Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
b Department of Nanotechnology Engineering, University of Isfahan, Isfahan 81746-73441, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 June 2009
Received in revised form 5 January 2010
Accepted 13 January 2010
Available online 28 January 2010
In this paper, rapid and highly efficient tetrahydropyranylation of alcohols and phenols with 3,4-dihydro-
2H-pyran (DHP) in the presence of catalytic amounts of high-valent tin (IV) tetraphenylporphyrinato tri-
fluoromethanesufonate, [SnIV(TPP)(OTf)2] is reported. In this catalytic system, primary, secondary and
tertiary alcohols as well as phenols were converted to their corresponding tetrahydropyranyl ethers
(THP-ethers) in excellent yields and short reaction times at room temperature. It is noteworthy that this
method can be used for chemoselective tetrahydropyranylation of primary alcohols in the presence of
secondary and tertiary alcohols and phenols. The catalyst was reused several times in the protection reac-
tions without loss of its catalytic activity.
Keywords:
Alcohol
Phenol
Tetrahydropyranyl ether
3,4-Dihydro-2H-pyran
High-valen tin(IV) porphyrin
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Electron-deficient metalloporphyrins have been used as mild
Lewis acids catalysts [26–31]. Recently, chromium and iron por-
The protection of hydroxyl function in alcohols and phenols is a
common and versatile event in multistep organic synthesis. Sev-
eral methods such as acetylation, trimethylsilylation, methoxyme-
thylation and tetrahydropyranylation have been reported for
masking of hydroxyl groups [1]. Amongst them, tetrahydropyrany-
lation is more frequently used due to the remarkable stability of
tetrahydropyranyl ethers under a variety of conditions such as
alkaline media, Grignard reagents, alkyl lithiums, metal hydrides,
oxidative reagents and alkylating and acylating reagents [2]. A
variety of catalysts including protonic and Lewis acid catalysts
have been introduced for tetrahydropyranylation of alcohols and
phenols such as p-toluene sulfonic acid (PTSA) [3], bis[trimethyl-
silyl]sulfate [4], (CH3)3SiI [5], CuCl2 [6], DDQ [7], Ru(CH3)3(tri-
phos)](OTf)2 [8], I2 [9], AlCl3Á6H2O [10], In(OTf)3 [11], ZrCl4 [12],
phyrins have been used for organic transformations. Chromium(III)
tetraphenylporphyrinato chloride, Cr(tpp)Cl, has been applied for
regioselective [3,3] rearrangement of aliphatic allyl vinyl ethers
and for Claisen rearrangement of simple aliphatic allyl vinyl ethers.
Iron(III)tetraphenylporphyrinato triflate, Fe(tpp)OTf, has been used
for rearrangement of
a,b-epoxy ketones into 1,2-diketones and
chromium(III)tetraphenylporphyrinato triflates, Cr(tpp)OTf, has
been reported for highly regio- and stereoselective rearrangement
of epoxides to aldehydes [32–35].
Recently, we have reported the use of tin(IV)tetraphenylpor-
phyrinato perchlorate [36,37], tin(IV)tetraphenylporphyrinato
trifluoromethanesulfonate [38–41], and tin(IV)tetraphenylporphy-
rinato tetrafluoroborate [42–44] in organic synthesis.
In this paper, we wish to report a highly efficient method for tet-
rahydropyranylation of alcohols and phenols with DHP catalyzed
by high-valent [SnIV(TPP)(OTf)2] at room temperature (Scheme 1).
K5CoW12O40Á3H2O [13], ionic liquids [14], Bu4N+Br3 [15], and
À
LiOTf [16], K-10 clay [17], alumina impregnated with ZnCl2 [18],
silica chloride [19], AlCl3@PS [20], silica-based sulfonic acid
[21,22], H6P2W18O62 [23], PdCl2(CH3CN)2 [24] and vanadyl(IV) ace-
tate [25] have been developed for this purpose.
2. Experimental
Chemicals were purchased from Merck chemical company. 1H
NMR spectra were recorded in CDCl3 solvent on a Bruker AM
80 MHz or a Bruker AC 500 MHz spectrometer using TMS as an
internal standard. Infrared spectra were run on a Philips PU9716
or Shimadzu IR-435 spectrophotometer. All analyses were per-
formed on a Shimadzu GC-16A instrument with a flame ionization
* Corresponding authors. Tel.: +98 311 7932712; fax: +98 311 6689732 (M.
Moghadam).
E-mail addresses: moghadamm@sci.ui.ac.ir (M. Moghadam), stanges@sci.ui.ac.ir
(S. Tangestaninejad).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.017

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M. Moghadam et al. / Inorganica Chimica Acta 363 (2010) 1523–1528
Table 2
OTf
The effect of different solvents on the tetrahydropyranylation of benzyl alcohol with
DHP catalyzed by [SnIV(TPP)(OTf)2].a
N
N
N
IV
Sn
Entry
Solvent
Time (min)
Yieldb (%)
N
1
2
3
4
5
THF
4
4
4
4
4
96
72
64
38
10
OTf
CH3CN
CH2Cl2
CHCl3
R
OH
+
THF
O
O
RO
n-Hexane
a
Scheme 1.
Reaction conditions: benzyl alcohol (1 mmol), catalyst (1 mol%), DHP (2 mmol),
solvent (0.5 mL).
b
Isolated yield.
Table 1
The catalytic activity of different tin(IV) porphyrin catalysts in the tetrahydropyr-
anions such as OTf, ClO4À, TFA (trifluoroacetate), ClÀ, BrÀ and FÀ
were checked for tetrahydropyranylation of benzyl alcohol. The
results (Table 1) showed that [SnIV(TPP)(OTf)2] and [SnIV(TPP)(-
ClO4)2] are more efficient than the others. Since [SnIV(TPP)(ClO4)2]
is potentially explosive, therefore, [SnIV(TPP)(OTf)2] was chosen as
catalyst.
anylation of benzyl alcohol.a
Entry
Catalyst
Time (min)
Yieldb (%)
1
2
3
4
5
6
[SnIV(TPP)(OTf)2]
[SnIV(TPP)(ClO4)2]
[SnIV(TPP)(TFA)2]
[SnIV(TPP)F2]
4
4
4
4
4
4
94
88
51
30
22
17
[SnIV(TPP)Cl2]
In order to choose the reaction media, different solvents such as
dichloromethane, chloroform, n-hexane, acetonitrile and tetrahy-
drofuran (THF) were used and the best results were obtained in
THF (Table 2). In order to check the possibility of tetrahydrofurany-
lation with THF, as reported in the literature [47,48], the reaction of
benzyl alcohol with THF was carried out in the absence of DHP and
the results showed that no tetrahydrofuranyl ether was produced.
Therefore, [SnIV(TPP)(OTf)2] was used as catalyst for tetrahydro-
pyranylation of different alcohols with DHP at room temperature.
The optimized conditions which obtained for tetrahydropyranyla-
tion of benzyl alcohol were alcohol, DHP and catalyst in a molar ra-
tio of 100:200:1. Under these optimized conditions, a wide variety
of primary, secondary and tertiary alcohols were subjected to tet-
rahydropyranylation with DHP in the presence of catalytic
amounts of [SnIV(TPP)(OTf)2] and at room temperature. The results,
which are summarized in Table 3, showed that all primary, second-
ary and tertiary alcohols including aromatic, aliphatic and cyclic
ones were converted efficiently to their corresponding tetrahydro-
pyranyl ethers in 3–6 min. In the case of aromatic alcohols, the
nature of substituents has no significant effect on the tetrahydro-
pyranylation yield.
A plausible mechanism for these reactions is that DHP is first
activated by tin(IV) porphyrin to afford 1. Nucleophilic attack of
alcohol to 1 gives 2 which upon proton transfer produces final
product 3 and releases the catalyst for the next run (Scheme 2).
The high catalytic activity of [SnIV(TPP)(OTf)2] catalyst in the
tetrahydropyranylation of alcohols, prompted us to explore its cat-
alytic activity in the tetrahydropyranylation of phenols. The tetra-
hydropyranylation of different phenols was carried out under the
same reaction conditions which applied for alcohols and the corre-
sponding THP-ethers were obtained in high yields and short reac-
tion times (Table 4).
[SnIV(TPP)Br2]
a
Reaction conditions: benzyl alcohol (1 mmol), catalyst (1 mol%), DHP (2 mmol),
THF (0.5 mL).
b
Isolated yield.
detector using silicon DC-200 or Carbowax 20 M columns and n-
decane was used as internal standard. The tetraphenylporphyrin
was prepared and metallated according to the literature [45,46].
The catalyst, [SnIV(TPP)(OTf)2], was prepared as reported previ-
ously [38].
2.1. General procedure for the tetrahydropyranylation of alcohols and
phenols
A mixture of alcohol or phenol (1 mmol), [SnIV(TPP)(OTf)2]
(10 mg, 0.01 mmol) and DHP (2 mmol) in THF (1 mL) was prepared
and stirred at room temperature for appropriate time (Table 1). The
progress of the reaction was monitored by GC and TLC. After com-
pletion of the reaction, the solvent was evaporated, n-hexane
(10 mL) was added and the catalyst was filtered. The filtrates were
concentrated under reduced pressure to afford the crude product,
which was confirmed by 1H NMR and IR spectral data.
3. Results and discussion
3.1. Tetrahydropyranylation of alcohols and phenols with DHP
catalyzed by [SnIV(TPP)(OTf)2]
Metalloporphyrins are widely used as redox catalysts, but there
are few reports on their catalytic activity as Lewis acids [26–44].
Since high-valent [SnIV(TPP)(OTf)2] is a mild Lewis acid, its cata-
lytic activity was investigated in the tetrahydropyranylation of
alcohols and phenols. In order to show the high catalytic activity
of this catalyst, different metalloporphyrins were applied as
catalysts in the tetrahydropyranylation of benzyl alcohol with
DHP. The catalytic activity of these catalysts in this reaction was
appeared in the following order: [SnIV(TPP)(OTf)2] (96%) >>
[FeIII(TPP)(OTf)] (53%) > [MnIII(TPP)(OTf)] (31%) > [SnIV(TPP)Cl2]
(22%) > [FeIII(TPP)Cl] (13%) > [MnIII(TPP)Cl] (7%) > [Cu(TPP)] (4%).
In the case of tin porphyrins, it is clear that introducing of OTf
instead of Cl has increased the electron deficiency of [SnIV(TP-
P)(OTf)2] which in turn increases its catalytic activity. The presence
of OTf anion is necessary for catalytic activity of tin(IV) porphyrin.
To stress this point, tin(IV) tetraphenylporphyrin bearing different
3.2. Catalyst reusability
The reusability of the [SnIV(TPP)(OTf)2] catalyst was studied
using multiple tetrahydropyranylation of 4-chlorobenzylalcohol
with DHP. At the end of the reaction, the solvent was evaporated,
n-hexane was added, the catalyst was filtered and used in the next
run. The results showed that after reusing the catalyst for several
times (four consecutive runs was checked), no change was ob-
served in its catalytic activity. In another experiment, at the end
of the reaction, fresh alcohol and DHP were added to the reaction
mixture. Execution of the reaction showed that the alcohol was
completely converted to its corresponding THP-ether.

M. Moghadam et al. / Inorganica Chimica Acta 363 (2010) 1523–1528
1525
Table 3
Tetrahydropyranylation of alcohols catalyzed by [SnIV(TPP)(OTf)2] at room temperature.a
Entry
Hydroxy compound
THP ether
Time (min)
4
Yieldb (%)
1
96
CH2OH
CH2OTHP
2
3
4
5
3
3
5
6
95
96
97
60
CH2CH2OH
CH2CH2OTHP
CH2CH2CH2OTHP
CH2OTHP
CH2CH2CH2OH
CH2OH
Br
Br
O2N
CH2OH
CH2OH
O2N
CH2OTHP
CH2OTHP
6
4
4
4
4
4
96
96
95
97
94
F
F
7
MeO
CH2OTHP
CH2OTHP
MeO
t-Bu
CH2OH
CH2OH
8
OMe
OMe
9
CH2OH
CH2OH
t-Bu
CH2OTHP
CH2OTHP
10
Me
Me
OH
OTHP
11
12
13
5
4
4
94
94
92
OH
OTHP
OTHP
OTHP
OH
OH
Me
Me
14
15
16
17
4
4
6
6
94
95
95
96
OH
OTHP
OTHP
OH
CH3
CH3
H3C C CH3
H3C
C
CH3
OH
OH
OTHP
OTHP
18
6
93
a
Reaction conditions: alcohol (1 mmol), DHP (2 mmol), catalyst (1 mol%), THF (0.5 mL).
Isolated yield.
b

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M. Moghadam et al. / Inorganica Chimica Acta 363 (2010) 1523–1528
Table 4
2
Tetrahydropyranylation of phenols with DHP catalyzed by [SnIV(TPP)(OTf)2] at room
temperature.a
1.8
1.6
1.4
1.2
Entry Phenol
THP ether
Time Yieldb
(min) (%)
1
5
5
5
5
90
91
70
93
OTHP
OH
1
0.8
0.6
0.4
0.2
2
Cl
OTHP
Cl
OH
3
O2N
OH O2N
OTHP
0
300
400
500
600
700
800
4
Wavelength (nm)
H3C
OH H3C
OTHP
Fig. 1. UV–Vis spectrum of [SnIV(TPP)(OTf)2]: (A) before reaction and (B) after
reaction.
OTHP
OH
5
5
92
The results indicated that primary alcohols are more reactive in
the presence of secondary and tertiary alcohols and phenols.
In order to show the efficiency and applicability of the present
method, the catalytic activity of this catalyst was compared with
some of those reported previously in the literature. The results,
which are summarized in Table 6, show that this catalyst is supe-
rior in terms of reaction time, product yield, catalyst amount or
catalyst reusability. It is also important to note that the catalytic
activity of [SnIV(TPP)(OTf)2] is similar to K5CoW12O40Á3H2O [13],
but the later is not reusable, while the former is reusable without
loss of its catalytic activity.
a
Reaction conditions: phenol (1 mmol), DHP (2 mmol), catalyst (1 mol%), THF
(0.5 mL).
b
Isolated yield.
The nature of the recovered catalyst was monitored by its UV–
Vis spectrum Fig. 1, in which no change was observed in its spec-
trum and since the catalyst retained its activity in the protection
reaction, it seems that the recovered catalyst is porphyrin triflates
and not porphyrin alkoxide. This point was also confirmed by low
catalytic activity of porphyrin alkoxide.
4. Conclusion
The chemoselectivity of the presented method was investigated
in a set of competitive reactions which was conducted between
primary or secondary and tertiary alcohols or phenols (Table 5).
In conclusion, [SnIV(TPP)(OTf)2], which has been introduced
as a stable tin (IV) compound, is able to catalyze the reactions
O
O
+
ROH
IV
IV
Sn
Sn
1
O
+
OR
H
R
IV
O
O
Sn
3
2
Scheme 2.

M. Moghadam et al. / Inorganica Chimica Acta 363 (2010) 1523–1528
1527
Table 5
Selective pyranylation of alcohols and phenols catalyzed by [SnIV(TPP)(OTf)2] in THF.a
Row
1
ROH
THP ether
Time (min)
4
Yieldb (%)
88
28
92
25
CH2OH
OH
CH2OTHP
OTHP
2
4
CH2OH
CH2OTHP
Me
OH
Me
Me
Me
OTHP
3
4
4
4
92
51
90
12
CH2OH
CH2OTHP
OH
OTHP
CH2OTHP
OTHP
CH2OH
OH
a
Reaction conditions for a binary mixture: 1 mmol of each alcohol or phenol, DHP (2 mmol), catalyst (1 mol%), THF (0.5 mL).
Isolated yield.
b
Table 6
Comparison of the results obtained for the tetrahydropyranylation of benzyl alcohol catalyzed by SnIV(TPP)(OTf)2 with those obtained by the recently reported catalysts.
Catalyst
OTHP
OH
Entry
Catalyst
Catalyst (mol%)
Temperature
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
SnIV(TPP)(OTf)2
bis[trimethylsilyl]sulfate
CuCl2
1
2
15
1
0.5
5
R.T.
0
R.T.
R.T.
0
Reflux
R.T.
R.T.
R.T.
Reflux
R.T.
5
R.T.
R.T.
R.T.
20
4
12
20
30
30
180
5
240
60
150
5
10
10
40
15
120
60
60
96
97
83
94
85
92
97
96
85
96
92
89
93
97
92
98
72
95
–
[4]
[6]
AlCl3Á6H2O
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[23]
[24]
[25]
In(OTf)3
ZrCl4
(K5CoW12O40Á3H2O)
1
Ionic liquid/PPh3 HBr
10
2.5
60
25 mg
–
10 mg
15
2
À
9
n-Bu4N + Br3 (TBATB)
10
11
12
13
14
15
16
17
18
LiOTf
K-10 clay
Al2O3/ZnCl2
Silica chloride
AlCl3@PS
SiO2–SO3H
H6P2W18O62
PdCl2(CH3CN)2
Vanadyl (IV) acetate
1
10
27
R.T.
R.T.
that [SnIV(TPP)Cl2] is not able to catalyze them. Different pri-
mary and secondary as well as tertiary alcohols and phenols
efficiently converted to their corresponding THP-ethers in the
presence of catalytic amounts of [SnIV(TPP)(OTf)2] at room tem-
perature. Short reaction times, easy preparation of catalyst, high
product yield, reusability of catalyst and applicability for both
alcohols and phenols are the advantages of this catalytic
system.

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M. Moghadam et al. / Inorganica Chimica Acta 363 (2010) 1523–1528
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