Highly efficient tetrahydropyranylation of alcohols and phenols catalyzed by a new and reusable high-valent vanadium(IV) porphyrin

Article abstract of DOI:10.1016/j.crci.2011.09.009

In the present work, the catalytic activity of high-valent tetraphenylporphyrinatovanadium(IV) trifluoromethanesulfonate, [V ^IV(TPP)(OTf)₂], in the tetrahydropyranylation of alcohols and phenols with 3,4-dihydro-2H-pyran (DHP) is reported. This new electron-deficient V(IV) compound was used as a highly efficient catalyst for pyranylation of primary (aliphatic and benzylic), sterically-hindered secondary and tertiary alcohols with DHP. Tetrahydropyranylation of phenols with DHP was also performed to afford the desired THP-ethers. The chemoselectivity of this method was also investigated. The results indicated that primary alcohols are more reactive in the presence of secondary and tertiary alcohols and phenols. This catalyst was reused several times without loss of its activity.

Full text of DOI:10.1016/j.crci.2011.09.009

C. R. Chimie 14 (2011) 1095–1102
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´
Full paper/Memoire
Highly efficient tetrahydropyranylation of alcohols and phenols catalyzed
by a new and reusable high-valent vanadium(IV) porphyrin
*
*
S. Abdolmanaf Taghavi, Majid Moghadam , Iraj Mohammadpoor-Baltork ,
Shahram Tangestaninejad, Valiollah Mirkhani, Ahmad Reza Khosropour
Department of Chemistry, Catalysis Division, 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:
In the present work, the catalytic activity of high-valent tetraphenylporphyrinato-
Received 23 April 2011
vanadium(IV) trifluoromethanesulfonate, [VIV(TPP)(OTf)2], in the tetrahydropyranylation
of alcohols and phenols with 3,4-dihydro-2H-pyran (DHP) is reported. This new electron-
deficient V(IV) compound was used as a highly efficient catalyst for pyranylation of
primary (aliphatic and benzylic), sterically-hindered secondary and tertiary alcohols with
DHP. Tetrahydropyranylation of phenols with DHP was also performed to afford the
desired THP-ethers. The chemoselectivity of this method was also investigated. The results
indicated that primary alcohols are more reactive in the presence of secondary and tertiary
alcohols and phenols. This catalyst was reused several times without loss of its activity.
Accepted after revision 19 September 2011
Available online 4 November 2011
Keywords:
Vanadium porphyrin
Electron-deficient
Alcohols
Phenols
´
ß 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
THP ether
1. Introduction
compounds [30,31]. Due to the remarkable stability of
tetrahydropyranyl ethers under a variety of conditions
Despite several reports on the application of metallo-
porphyrins in the oxidation of organic compounds [1–8],
there are few reports on their applications as Lewis acid
catalysts. High-valent metalloporphyrins, in which the
metal is in its highest oxidation state, can be used as mild
Lewis acids. In this respect, electron-deficient porphyrins
of Cr(III), Fe(III), Sn(IV) and V(IV) have been used in organic
transformations [9–26].
Proctection of hydroxy compounds is of great impor-
tance in organic synthesism especially in the multi step
transformations [27–29]. Tetrahydropyranylation with
DHP, methoxymethylation with FDMA (formaldehyde
dimethyl acetal), trimethylsilylation with HMDS (hexam-
ethyldisilazane) and acetylation with Ac2O are the most
frequently used methods for protection of hydroxy
such as alkaline media, Grignard reagents, alkyl lithiums,
metal hydrides, oxidative reagents, and alkylating and
acylating reagents, tetrahydropyranylation has attracted
much attention in comparison with the others [30]. A
variety of catalysts including protonic and Lewis acid
catalysts such as p-toluene sulfonic acid (PTSA) [32],
bis[trimethylsilyl]sulfate [33], (CH3)3SiI [34], CuCl2 [35],
DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) [36],
Ru(CH3)3(triphosphine)](OTf)2 [37], I2 [38], AlCl3.6H2O
[39], In(OTf)3 [40], ZrCl4 [41], K5CoW12O40.3H2O [42],
ionic liquids [43], Bu4N+Br3À [44], and LiOTf [45], K-10 clay
[46], alumina impregnated with ZnCl2 [47], silica chloride
[48], AlCl3@polystyrene [49], silica-based sulfonic acid
[50,51], H6P2W18O62 [52], PdCl2(CH3CN)2 [53], vanadyl(IV)
acetate [54], BF3-etherate [55] and homogeneous and
heterogeneous [SnIV(TPP)(OTf)2] [56,57] have been devel-
oped for tetrahydropyranylation of alcohols and phenols.
Vanadium is required for normal health, and could act in
vivo either as a metal cation or as a phosphate analogue,
dependingontheoxidationstate, V(lV)orV(V), respectively.
* Corresponding authors.
E-mail addresses: moghadamm@sci.ui.ac.ir, imbaltork@sci.ui.ac.ir
(M. Moghadam).
1631-0748/$ – see front matter ß 2011 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2011.09.009

1096
S. Abdolmanaf Taghavi et al. / C. R. Chimie 14 (2011) 1095–1102
[VIV(TPP)(OTf)2]
THF
catalyst was filtered. The filtrates were washed with brine,
dried over Na2SO4 and concentrated under reduced
pressure to afford the crude product.
R
OH
+
O
RO
O
2.3. Catalyst recovery and reuse
Scheme 1. Tetrahydropyranylation of alcohols and phenols with DHP
catalyzed by [VIV(TPP)(OTf)2].
The reusability of the catalyst was checked in the
sequential pyranylation of benzyl alcohol. At the end of
each reaction, the solvent was evaporated, Et2O (10 mL)
was added and the catalyst was filtered. The catalyst was
used with fresh benzyl alcohol, DHP and THF.
In proteins, vanadium is a cofactor in an algal bromoper-
oxidase 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
3. Results and discussion
+
solution. These are V2+, V3+, VO2+ and VO2 (the oxidation
states 2, 3, 4, and 5, respectively) [58].
3.1. Investigation of catalytic activity of high-valent
[VIV(TPP)(OTf)2] in the tetrahydropyranylation of alcohol and
phenols with DHP
In the present work, highly efficient tetrahydropyr-
anylation of alcohols and phenols with 3,4-dihydro-2H-
pyran catalyzed by reusable electron-deficient tetraphe-
nylporphyrinatovanadium(IV) trifluoromethanesulfonate,
[VIV(TPP)(OTf)2], is reported (Scheme 1).
The preparation route for [VIV(TPP)(OTf)2] is shown in
Scheme 2. The [VIVO(TPP)], which can be obtained
commercially or prepared in laboratory, was converted
to [VIV(TPP)Cl2] with SOCl2, which in turn was converted to
[VIV(TPP)(OTf)2] with AgOTf. The catalytic activity of these
three catalysts was checked in the tetrahydropyranylation
of benzyl alcohol with DHP. The results showed that the
catalytic activity has the following order: [VIV(TP-
P)(OTf)2](100%) > [VIV(TPP)Cl2] (52%) > [VIVO(TPP)] (17%).
These data showed that introducing of OTfÀgroups has a
crucial effect on the catalytic activity of [VIV(TPP)(OTf)2].
This may be due to the electron withdrawing nature of
these non-coordinated ligands which in turn increases the
electron-deficiency of metal center and therefore, the
catalytic activity increases. Despite of high oxophilicity of
VIV species, [VIV(TPP)(OTf)2] is a stable catalyst and can be
used under air. Therefore, [VIV(TPP)(OTf)2] was chosen as
catalyst for tetrahydropyranylation of alcohols and phe-
nols. In this manner, the reaction parameters were
optimized in the reaction of benzyl alcohol with DHP. As
can be seen in Table 1, the best conditions are alcohol
(1 mmol), DHP (2 mmol), catalyst (1 mol%) and THF as
solvent. Under the optimized conditions a wide range of
alcohols including primary, secondary and tertiary alco-
hols were converted to their corresponding THP-ethers in
good to excellent yields and short reaction times. The
results, which are summarized in Table 2, showed that
2. Experimental
Chemicals were purchased from Fluka and Merck
chemical companies. 1HNMR spectra were recorded in
CDCl3 solvent on a Bruker-Avance 400 MHz spectrometer.
Infrared spectra were run on a Philips PU9716 or Shimadzu
IR-435 spectrophotometer. Elemental analysis was carried
out with a LECO, CHNS-932 instrument. UV-Vis spectra
were recoreded on a Shimadzu-160 spectrophotometer.
All analyses were performed on a Shimadzu GC-16A
instrument with a flame ionization detector using silicon
DC-200 or Carbowax 20 M columns. Tetraphenylpor-
phyrin, [VO(TPP)] and [V(TPP)Cl2] were prepared accord-
ing to the literature [59–61].
2.1. Preparation of tetraphenyporphyrinatovanadium(IV)
trifluoromethanesulfonate, [VIV(TPP)(OTf)2]
To a solution of V(TPP)Cl2 (0.97 g, 1 mmol) in THF
(50 mL), at 55 8C, AgCF3SO3 (0.54 g, 2 mmol) was added.
The solution was stirred at 55 8C for 30 min. The AgCl
precipitate was filtered through a 0.45 mM 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.
Visible spectrum: 456 (Soret), 585, 628 nm;
n(KBr):
Table 1
1030, 1168, 1224, 1298 cmÀ1 (belong to SO3 groups and
porphyrin ring); 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.
Optimization of reaction parameters in the tetrahydropyranylation of
benzyl alcohol (1 mmol) with DHP catalyzed by [VIV(TPP)(Otf)2] at room
temperature.
Entry
Catalyst
(mol%)
DHP
Solvent
Time
Yield
(%)a
(mmol)
(min)
2.2. General procedure for tetrahydropyranylation of alcohol
and phenols with DHP catalyzed by [VIV(TPP)(OTf)2]
1
2
3
4
5
6
7
8
0.5
1
2
2
2
1
2
2
2
2
THF
3
3
3
3
3
3
3
3
74
100
100
46
THF
2
THF
A solution of alcohol or phenol (1 mmol) and DHP
(2 mmol per OH group) in THF (0.5 mL) was prepared. The
[VIV(TPP)(OTf)2] (1 mol%) was added to this solution 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
1
THF
1
CH3CN
CH2Cl2
CHCl3
n-Hexane
69
1
58
1
35
1
19
a
GC yield.

S. Abdolmanaf Taghavi et al. / C. R. Chimie 14 (2011) 1095–1102
1097
O
N
N
N
N
V
SOCl2, CH2Cl2
Cl
V
N
N
N
N
Cl
THF, 55 oC
AgOTf, 0.5 h
OTf
N
N
N
V
AgCl +
N
OTf
Scheme 2. The preparation route for the catalyst.
Table 2
Tetrahydropyranylation of alcohols catalyzed by [VIV(TPP)(Otf)2] at room temperaturea.
Entry
1
Hydroxy compound
THP ether
Time (min)
3
Yield (%)b
100
95
94
95
93
CH2OH
CH2OTHP
2
3
4
5
2
2
3
2
CH2CH2CH2OTHP
CH2CH2OTHP
CH2CH2CH2OTHP
CH2CH2CH2OH
CH2OH
Br
Br
CH2OTHP
CH2OTHP
MeO
MeO
CH2OH

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S. Abdolmanaf Taghavi et al. / C. R. Chimie 14 (2011) 1095–1102
Table 2 (Continued )
Entry
6
Hydroxy compound
CH2OH
THP ether
CH2OTHP
OMe
Time (min)
5
Yield (%)b
97
OMe
7
8
4
4
97
94
t-Bu
CH2OTHP
t-Bu
CH2OH
CH2OH
Me
CH2OTHP
Me
9
8
100
OH
OH
OTHP
OTHP
10
11
8
8
94
85
OTHP
OH
Me
Me
OH
OTHP
12
4
94
OTHP
OH
OH
13
14
4
5
92
OTHP
100
15
16
8
6
91
85
OTHP
OH
CH3
C
CH3
H3C
CH3
H3C
C
CH3
OTHP
OH
OH
OTHP
17
18
8
85
OH
OTHP
Me
5
5
96
95
Me
OH
19
Me
Me
OTHP
a
Reaction conditions: alcohol (1 mmol), DHP (2 mmol), catalyst (1 mol%), THF (0.5 mL).
GC yield.
b
all primary, secondary and tertiary alcohols including
The ability of this catalyst in the tetrahydropyranyla-
tion of phenols was also investigated. The results showed
that this catalyst efficiently converted different phenols to
their corresponding THP-ethers in 3 to 5 min (Table 3).
Since the catalyst was active in the tetrahydropyrany-
lation of primary, secondary and tertiary alcohols as well as
aromatic, aliphatic and cyclic ones were converted
efficiently to their corresponding tetrahydropyranyl ethers
in 3 to 8 min. In the case of aromatic alcohols, the nature
of substituents has no significant effect on the product
yield.

S. Abdolmanaf Taghavi et al. / C. R. Chimie 14 (2011) 1095–1102
1099
Table 3
Tetrahydropyranylation of phenols with DHP catalyzed by [VIV(TPP)(Otf)2] at room temperaturea.
Entry
1
Phenol
THP ether
Time (min)
4
Yield (%)b
90
92
85
OTHP
OH
2
3
4
4
Cl
OH
OTHP
Cl
O2N
OH
O2N
H3C
OTHP
4
5
5
90
90
OTHP
H3C
OH
OTHP
OH
5
a
Reaction conditions: phenol (1 mmol), DHP (2 mmol), catalyst (1 mol%), THF (0.5 mL).
GC yield.
b
phenols, a set of competitive reactions was performed to
and 2-methyl-3-phenyl-2-propanol (tertiary alcohol), and
phenol was subjected to tetrahydropyranylation with DHP
(2 mmol) in the presence of [VIV(TPP)(OTf)2]. The results
indicated that primary alcohol was more reactive in the
investigate the chemoselectivity of this method. In this
manner, an equimolar mixture of benzyl alcohol
(primary alcohol) with benzhydrol (secondary alcohol)
Table 4
Selective pyranylation of alcohols and phenols catalyzed by [VIV(TPP)(Otf)2] in THFa.
Row
ROH
THP ether
Time (min)
Yield (%)b,c
90 (100)
CH2OTHP
CH2OH
1
3
30 (37)
OTHP
OH
90 (100)
24 (34)
CH2OTHP
Me
CH2OH
Me
2
3
Me
Me
OTHP
OH
92 (100)
CH2OH
CH2OTHP
3
3
3
47 (51)
OH
OTHP
90 (100)
CH2OTHP
OTHP
CH2OH
OH
4
14 (25)
a
Reaction conditions for a binary mixture: 1 mmole of each alcohol or phenol, DHP (2 mmol), catalyst (1 mol%), THF (0.5 mL).
b
c
GC yield.
Yields in the parentheses refer to the yields in the presence of 4 mmole of DHP.

1100
S. Abdolmanaf Taghavi et al. / C. R. Chimie 14 (2011) 1095–1102
Table 5
Comparison of the results obtained for the tetrahydropyranylation of benzyl alcohol catalyzed by [VIV(TPP)(Otf)2] with those obtained for the recently
reported catalysts.
Catalyst
OTHP
OH
Entry
Catalyst
Catalyst (mol%)
Temperature
Time (min)
Yield (%)
Ref.
1
[VIV(TPP)(Otf)2]
bis[trimethylsilyl]sulfate
CuCl2
1
R.T.
0
3
12
20
30
30
180
5
100
97
83
94
85
92
97
96
85
96
92
89
93
97
92
98
72
95
96
97
26
17
11
78
8
–
2
2
[22]
[24]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[41]
[42]
[43]
[44]
[45]
–
3
15
1
R.T.
R.T.
0
4
AlCl3.6H2O
5
In(Otf)3
0.5
5
6
ZrCl4
Reflux
R.T.
R.T.
R.T.
Reflux
R.T.
5
7
K5CoW12O40Á3H2O
Ionic liquidÀ/PPh3.HBr
n-Bu4N+Br3 (TBATB)
LiOTf
1
8
10
2.5
60
25 mg
–
240
60
150
5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
24
25
26
27
28
K-10 clay
Al2O3/ZnCl2
10
10
40
15
120
60
60
4
Silica chloride
AlCl3@PS
10 mg
15
2
R.T.
R.T.
R.T.
20
SiO2-SO3H
H6P2W18O62
PdCl2(CH3CN)2
Vanadyl (IV) acetate
[SnIV(TPP)(Otf)2]
[SnIV(TPP)(Otf)2]@CMP
CuCl2
1
10
27
1
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
R.T.
1
4
1
3
AlCl3.6H2O
1
3
–
ZrCl4
1
3
–
K5CoW12O40Á3H2O
LiOTf
1
3
–
1
3
–
H6P2W18O62
Vanadyl (IV) acetate
1
3
12
10
–
1
3
–
O
O
+
ROH
IV
IV
V
V
1
O
+
OR
H
R
IV
V
O
O
3
2
Scheme 3. The proposed mechanism for tetrahydropyranylation of alcohols and phenols with DHP catalyzed by [VIV(TPP)(OTf)2].

S. Abdolmanaf Taghavi et al. / C. R. Chimie 14 (2011) 1095–1102
1101
Fig. 1. UV-vis spectrum of: A: [VIV(TPP)(OTf)2] in tetrahydrofuran (1.0 Â 10À4 M, path length = 1.0 cm) at 25 8C and B: recovered [VIV(TPP)(OTf)2].
presence of secondary and tertiary ones, and phenol (Table
4). When the same reactions were carried out in the
presence of 4 mmole of DHP, the same results were
obtained.
4. Conclusion
In conclusion, electron-deficient [VIV(TPP)(OTf)2] can be
used as an efficient and reusable catalyst for tetrahydro-
pyranylation of alcohols and phenols with DHP. This
catalyst efficiently catalyzed the conversion of primary,
secondary and tertiary alcohols as well as phenols to their
corresponding THP-ethers. Short reaction times, good to
excellent yields and reusability of the catalyst are
noteworthy advantages of this catalytic system.
The results obtained by this catalytic system were
compared with some of those reported in the literature
(Table 5). As can be seen, this catalytic system is more
efficient than the others. In order to show the advantage
of this method, the tetrahydropyranylation of benzyl
alcohol was carried out in the presence of some of
available catalysts listed in Table 5 under the same
conditions described for [VIV(TPP)(OTf)2]. It was ob-
served that the [VIV(TPP)(OTf)2]/DHP catalytic system is
much more efficient than the others.
The actual mechanism is not clear at present. However,
based on a plausible mechanism, DHP is first activated by
V(IV) porphyrin to afford 1. Nucleophilic attack of alcohol
to 1 gives 2, which upon a proton transfer step produces
the final product 3 and releases the catalyst for the next
run (Scheme 3). Comparison of catalytic activity of
[VIV(TPP)(OTf)2] with Vanadyl(IV) acetate shows that
the presence of OTfÀgroups increases the interaction of
DHP with catalyst. This can be attributed to the high
electron-deficiency of V(IV) center in the presence of
OTfÀgroups.
Acknowledgement
We are thankful to the Center of Excellence of
Chemistry of University of Isfahan (CECUI)) for financial
support of this work.
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