Mesoporous and hexagonally ordered CuAl-SBA-15-catalyzed tandem C-C and C-O bond formation between phenols and allylic alcohols

Article abstract of DOI:10.1016/j.tetlet.2012.08.043

A novel mesoporous catalyst, CuAl-SBA-15, with a hexagonally ordered porous structure prepared via a soft-templating approach in a highly acidic medium is used for tandem C-C and C-O bond formation between phenols and allylic alcohols to afford a variety of dihydrobenzopyrans in good yields. The catalyst is also found to be highly active for the synthesis of vitamin E and can be recycled several times without significant loss of its activity.

Full text of DOI:10.1016/j.tetlet.2012.08.043

Tetrahedron Letters 53 (2012) 5656–5659
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Mesoporous and hexagonally ordered CuAl-SBA-15-catalyzed tandem C–C
and C–O bond formation between phenols and allylic alcohols
Shaji Varghese a,b,c, Chokkalinkam Anand , Dattatray Dhawale a,c, Ajayan Mano a,
a,c
a
b
a
Veerappan V. Balasubramanian , George Allen Gnana Raj , Samuthira Nagarajan ,
c
a,c,
⇑
Mohammad A. Wahab , Ajayan Vinu
a
MANA, WPI Research Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
Research and Development Centre, Bharathiar University, Coimbatore 641046, India
Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane 4072, QLD, Australia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel mesoporous catalyst, CuAl-SBA-15, with a hexagonally ordered porous structure prepared via a
soft-templating approach in a highly acidic medium is used for tandem C–C and C–O bond formation
between phenols and allylic alcohols to afford a variety of dihydrobenzopyrans in good yields. The cata-
lyst is also found to be highly active for the synthesis of vitamin E and can be recycled several times with-
out significant loss of its activity.
Received 5 July 2012
Revised 26 July 2012
Accepted 9 August 2012
Available online 17 August 2012
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords:
Chromans
Heterogeneous Catalyst
Mesoporous
Phenols
Tandem reactions
The dihydrobenzopyran (chroman) moiety is an important
highly efficient and stable, but are also eco-friendly. In addition,
the associated simplicity in work-up and recyclability of these
interesting catalysts make them attractive for various organic
transformations. Mesoporous catalysts such as faujasite zeolite
HSZ-360 have also been used for the synthesis of dihydrobenzopy-
rans from phenols and isoprene through a simple cyclization. How-
structural motif in both naturally occurring and biologically active
1
products such as vitamin E (
a
-tocopherol) and its derivatives, and
2
7
flavonoids. It is also found in pharmaceutical substrates such as
clusifoliol (Fig. 1). Dihydrobenzopyrans are generally prepared
3
by the cycloaddition of phenol derivatives with 1,3-dienes. These
reactions can be facilitated by either homogeneous or heteroge-
ever this process requires
a high temperature and most
4
neous Brønsted and Lewis acid catalysts. It has also been reported
importantly, the pores of the zeolites are too small for the synthe-
8
that dihydrobenzopyrans can be prepared from allylic alcohols
using Brønsted or Lewis acid catalysts. However, in most cases, a
stoichiometric amount of an acid catalyst is required,5 and offers
only a low to moderate yield of the required product with the for-
mation of by-products such as dihydrobenzofurans.5 Thus, there
is a need for the development of novel catalysts, which should be
stable, active, and environmentally friendly, for the straightfor-
ward synthesis of dihydrobenzopyrans.
sis of bulky molecules. To overcome these difficulties, we have
used extensively mesoporous aluminosilicate catalysts with well-
ordered porous structures and excellent physical characteristics
as catalysts for several organic transformations involving bulky
molecules, and found that they were highly efficient for the syn-
thesis of alkylated nitrogen heterocycles, substituted quinolines,
benzodiazepine and its derivatives, and triazolo indazolones.9
The introduction of different metal species into the framework of
the mesoporous silica matrix can tune the properties of the cata-
lysts. It has been reported that the acidity and the catalytic activity
of aluminosilicates can be enhanced or tuned by the simple addi-
tion of divalent metal ions such as Cu(II), Zn(II), or Fe(II) in the sil-
ica framework, as they generate highly acidic Lewis sites.10
,6
Recently, mesoporous materials have emerged as efficient cata-
lysts for selective organic transformations owing to their excellent
physical characteristics including high surface areas, large pore
volumes, and uniform pore size distribution.7
–10
They are not only
Here we propose to introduce Cu and Al sites to the hexagonally
ordered silica system as they can offer particular advantages in
catalysis over mono metal catalysts due to their cooperative nature
⇑
Corresponding author. Tel.: +61 7 3346 4122; fax: +61 7 3346 3973.
E-mail address: a.vinu@uq.edu.au (A. Vinu).
1
1
in tandem reactions. In addition, it is expected that these catalysts
0
040-4039/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.043

S. Varghese et al. / Tetrahedron Letters 53 (2012) 5656–5659
5657
Table 1
Optimization of the reaction of 2-naphthol (1a) with methylbut-3-en-2-ol (2a)a
HO
HO
Entry
Catalyst
Solvent
Yield (%)b
O
O
COOH
1
2
3
4
5
6
7
8
CuAl-SBA-15
CuAl-SBA-15
CuAl-SBA-15
CuAl-SBA-15
CuAl-SBA-15
AlSBA-15
cyclohexane
1,2-DCE
MeCN
86
76
16
74
61
72
88
—
trolox
vitamin E
Toluene
MeNO
2
Cyclohexane
Cyclohexane
Cyclohexane
OH
c
HO
CuAl-SBA-15
None
OH
O
-chroman-methanol
a
O
Reaction conditions: 2-naphthol (2 mmol), methylbut-3-en-2-ol (1 mmol),
solvent (5 mL), catalyst (30 mg), 85 °C, 3 h.
2
clusifoliol
b
Isolated yield after column chromatographic purification.
4
c
0 mg of catalyst was used.
Figure 1. Biologically active compounds and pharmaceutical substrates of
chromans.
the reaction. Interestingly, acetonitrile offered very low activity
with the lowest yield of 3a (16%) under similar reaction conditions.
Without any catalyst the reaction did not proceed. The effect of
the amount of catalyst on the synthesis of 3a was also investigated.
Increasing the catalyst loading did not make much difference to the
final yield of the product 3a (Table 1, entry 7). In order to check the
influence of the Cu and Al sites in the catalyst, the reaction was car-
ried out with pure AlSBA-15. To our delight, we found that the
activity of CuAl-SBA-15 was much higher than that of pure AlS-
BA-15, revealing the influence of the copper species in the alumi-
nosilicate framework of CuAl-SBA-15. Interestingly, the reaction
did not occur when pure CuSBA-15 was used as the catalyst. These
results reveal that the combination of both the acid and the redox
sites was extremely important to obtain high yields in this partic-
ular reaction.
can reduce the total mass of the solid support used for the reaction.12
In our ongoing research on mesoporous catalyst mediated organic
9
,13
transformations,
we found that ordered mesoporous silica
substituted with both Cu and Al in the silica framework catalyzed
the formation of dihydrobenzopyrans from phenol derivatives.
CuAl-SBA-15 was prepared using the polymeric triblock copoly-
mer, P123 (PEO20PPO70PEO20) as the structure-directing agent and
inorganic substrates such as tetraethyl orthosilicate, copper ni-
trate, and aluminum isopropoxide were used as the sources of Si,
Cu, and Al, respectively. The synthesis was carried out at a reaction
14
temperature of 100 °C in a highly acidic medium. The nSi/nCu and
Si/nAl ratios of the samples were calculated to be 20 and 7, respec-
tively. However, the nSi/(nCu+ Al) ratio of the final product after fil-
n
n
tration and calcination was found to be 16.38. The prepared CuAl-
SBA-15 catalyst was characterized by transmission electron
microscopy (TEM), XRD, HRSEM, EDX, and nitrogen adsorption iso-
therm experiments. The results revealed that the prepared catalyst
exhibited a well-defined hexagonally ordered porous structure
with a rod-like morphology and excellent physical parameters
The scope of the reaction was extended using different phenols
and the results are summarized in Table 2. The reactions were con-
1
5
ducted under the optimized conditions. Low yields were ob-
served for 4-chlorophenol and 1-naphthol which are mainly due
to the influence of steric effects and the electronic nature of these
compounds. When, 2,3,5-trimethylhydroquinone (Table 2, entry 8)
was subjected to the reaction with 2a, both dihydrobenzopyran
(3h) and prenylated 1,4-benzoquinone (3i) were formed in 52%
and 28% yields, respectively. However, allylic alcohol 2b (Table 2
entry 9) required a longer reaction time (16 h) for the formation
of vitamin E (3j) in 61% yield. The by-product of this reaction, phy-
tyl-trimethylbenzoquinone (3k) was also isolated and character-
ized. These results indicate that the methodology can be utilized
for the straightforward synthesis of commercially important vita-
min E and its analogues.
2
À1
such as high surface area (546 m g ), large pore volume
3
À1
(
1.1 cm g ), and uniform pore size distribution (9.9 nm) (see Sup-
plementary data Figs. 1S–4S). A highly intense peak together with
several well-ordered peaks at lower angles revealed the hexago-
nally-ordered pore system in the catalyst, which was similar to
that observed for pure SBA-15 (Fig. 1S). EDX and elemental map-
ping of the catalyst revealed a uniform distribution of Cu and Al
in the mesoporous matrix along the silica walls (Fig. 2S).
As this catalyst exhibited excellent physical characteristics to-
gether with a well-ordered porous structure, we tried to synthesize
dihydrobenzopyrans from phenols and allylic alcohols using this
novel catalyst (Scheme 1). In a typical reaction, 2-naphthol (1a)
was treated with methylbut-3-en-2-ol (2a) at reflux over CuAl-
SBA-15. When a mixture of 1a (2 mmol), 2a (1 mmol), and CuAl-
SBA-15 (30 mg) in cyclohexane (5 ml) was refluxed at 85 °C for
Interestingly, when the reaction was carried out at 50 °C with
1a and 2a for 3 h (Scheme 2), the Friedel–Crafts allylated interme-
diate, prenylated naphthol 3m was formed in a yield of 61%. Sub-
sequently, the intermediate 3m was subjected to further reaction
in the presence of the same catalyst at 85 °C and the product 3a
was formed within 30 min (Scheme 3), indicating that the reaction
proceeds through Friedel–Crafts alkylation followed by cyclization.
The recyclability of CuAl-SBA-15 was tested in the annulation of
1a and 2a. The catalyst was recovered simply by filtration and re-
used after washing with dichloromethane and being calcined at
500 °C for 4 h under an O2 atm. A yield of ca. 80% was achieved
even after the third run, showing only a small change in the activ-
ity of the catalyst. These results revealed that the catalyst was
highly robust in nature which is mainly due to the several layers
of the metallosilicate framework which stabilizes the mesostruc-
ture of the catalyst. In addition, the structural order of the catalyst
was completely retained even after the third cycle which should
enable reuse of the catalyst several times. From the view point of
stability, activity, and versatility, it can be envisaged that this novel
catalyst could replace existing hazardous and homogenous toxic
catalysts that are currently being used in industry for these types
3
h, product 3a was formed in 86% yield after column chromato-
graphic purification. The effect of the solvent on the yield of the fi-
nal product was also investigated and the results are shown in
Table 1. Cyclohexane and 1,2-dichloroethane were good solvents
leading to 86% and 76% yields of 3a, respectively, whereas other
solvents such as toluene and nitromethane were less suitable for
OH
CuAl-SBA-15
O
HO
cyclohexane,
reflux
1a
2a
3
a
Scheme 1. CuAl-SBA-15 catalyzed annulation of 2-naphthol with methylbut-3-en-
-ol.
2

5
658
S. Varghese et al. / Tetrahedron Letters 53 (2012) 5656–5659
Table 2
CuAl-SBA-15-catalyzed annulations of phenols and naphthols with allylic alcohols
a
Entry
ArOH
Alcohol
Producta
Time (h)
3
Yield (%)b
86
OH
HO2
a
1
O
3a
OH
HO2
a
2
3
4
5
6
4.5
5
81
78
73
70
56
O
3
b
OH
OH
HO2a
O
3
c
O
HO2
a
5
O3
d
O
OH
HO2
a
O 3
5
e
OH
Br
Cl
HO2
a
O
5.5
3
f
Br
OH
HO
2a
7
8
O
6
53
52
3
g
Cl
OH
OH
HO
HO2a
16
HO
O 3h
2
8
O
3
i
O
HO
9
HO
16
61
3
HO
2b
O
O
3
3
j
2
5
O
3
3
k
OH
HO2
O
a
1
1
0
1
6
5
51
28
3l
OH
HO
2c
OH
3m
4
7
O
3a
a
The products were characterized by 1H, 13C NMR and IR spectroscopy, and mass spectroscopy.
Yield refers to pure products after chromatography.
b
OH
CuAl-SBA-15
CuAl-SBA-15
OH
OH
O
50 oC, 3 h, cyclohexane
30 min, reflux
cyclohexane
HO
1
a
2a
3m
3m
3a
Scheme 2. CuAl-SBA-15-catalyzed prenylation of 2-naphthol.
Scheme 3. CuAl-SBA-15-catalyzed cyclization of prenylated naphthol.
of reactions. It is also expected that the linear arrays of mesopores,
as shown in the TEM images, help the diffusion of even bulky reac-
tants along the one-dimensional pore channels and allow access to
sites deep inside the pores and may further enhance the accessibil-
ity. These excellent features of this catalyst create scope for the
synthesis of fine chemicals.

S. Varghese et al. / Tetrahedron Letters 53 (2012) 5656–5659
5659
of a Future Fellowship. The authors also acknowledge the facilities,
and the scientific and technical assistance, of the Australian
Microscopy & Microanalysis Research Facility at the Centre for
Microscopy and Microanalysis, University of Queensland.
R
O
CuAl-SBA-15
OH
Supplementary data
CuAl-SBA-15
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.08.
043.
R
O
H
CuAl-SBA-15
Cooperative
Bronsted/Lewis Acidity
CuAl-SBA-15
OH
References and notes
OH
1
2
.
.
Schneider, C. Mol. Nutr. Food Res. 2005, 49, 7.
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R
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Soc. Jpn. 1995, 68, 2657; (e) Matsui, M.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1995,
Scheme 4. Plausible mechanism for the CuAl-SBA-15-catalyzed reactions.
6
8, 2663.
(a) Ismail, F. M. D.; Hilton, M. J.; Stefinovic, M. Tetrahedron Lett. 1992, 33, 3795;
b) Coman, S.; Wuttke, S.; Vimont, A.; Daturi, M.; Kemnitz, E. Adv. Synth. Catal.
008, 350, 2517.
5
.
(
2
Scheme 4 shows the probable pathway for this process. The
proposed mechanism is consistent with our experimental observa-
tions. The mechanism involves several steps: at first activation of
the allylic alcohol is induced by CuAl-SBA-15 which leads to the
6. (a) Adrio, L. A.; Hii, K. K. Chem. Commun. 2008, 2325; (b) Youn, S. W.; Eom, J. I. J.
Org. Chem. 2006, 71, 6705.
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1998, 301.
(a) Vinu, A.; Kalita, P.; Balasubramanian, V. V.; Oveisi, H.; Selvan, T.; Mano, A.;
Chari, M. A.; Subba Reddy, B. V. Tetrahedron Lett. 2009, 50, 7132; (b) Chari, M.
A.; Karthikeyan, G.; Naidu, T. S.; Sathyaseelan, B.; Zaidi, J. M.; Vinu, A.
Tetrahedron Lett. 2010, 51, 2629; (c) Chakravarti, R.; Kalita, P.; Selvan, S. T.;
Oveisi, H.; Balasubramanian, V. V.; Kantam, M. L.; Vinu, A. Green Chem. 2010,
formation of a p-allyl type complex followed by the Friedel–Crafts
9
.
reaction, preferably at the ortho position of the phenol. The high
ortho regioselectivity of the product arises from the close spatial
arrangement and the coordination of the metallosilicate frame-
work between the hydroxy and allyl groups, which leads to the for-
mation of an ortho-allylated intermediate (prenylated phenol).
Further co-ordination and intramolecular cyclization of this inter-
mediate lead to formation of the product dihydrobenzopyrans. It
should be mentioned that the cyclization step follows Markovni-
kov’s rule with the attack of oxygen at the most substituted allylic
1
2, 49; (d) Shobha, D.; Chari, M. A.; Mano, A.; Selvan, S. T.; Mukkanti, K.; Vinu,
A. Tetrahedron 2009, 65, 10608.
0. (a) Luan, Z.; Hatmann, M.; Zhao, D.; Zhou, W.; Kevan, L. Chem. Mater. 1999, 11,
1621; (b) Taguchi, A.; Schüth, F. Microporous Mesoporous Mater. 2005, 77, 1; (c)
Martín-Aranda, R. M.; Cˇ ejka, J. Top. Catal. 2010, 53, 141.
1. Yamada, Y.; Tsung, C. K.; Huang, W.; Huo, Z.; Habas, S. E.; Soejima, T.; Aliaga, C.
1
1
E.; Somorjai, G. A.; Yang, P. Nat. Chem. 2011, 3, 372.
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1
6
carbon.
Lett. 2008, 10, 4279.
In summary, we have demonstrated the preparation of a novel
mesoporous catalyst incorporating Cu and Al with a hexagonally-
ordered porous structure for the synthesis of dihydrobenzopyrans
through simple tandem C–C and C–O bond formation between var-
ious phenols and allylic alcohols. The catalyst was found to be
highly active, stable, and recyclable. Moreover, this catalyst affor-
ded commercially important vitamin E in good yield, and showed
much better activity than mono metal substituted mesoporous cat-
alysts and zeolites. We believe that this novel catalytic system
should create an excellent platform for the synthesis of a variety
of fine chemicals.
1
3. (a) Naidu, T. S.; Balasubramanian, V. V.; Mori, T.; Chari, M. A.; Zaidi, S. M. J.; Al-
deyab, S. S.; Subba Reddy, B. V.; Vinu, A. Synlett 2010, 2813; (b) Anand, C.;
Sathyaseelan, B.; Samie, L.; Beitollahi, A.; Pradeep Kumar, R.; Palanichamy, M.;
Murugesan, V.; Kenawy, E. R.; Al-Deyab, S. S.; Vinu, A. Microporous Mesoporous
Mater. 2010, 134, 87; (c) Varghese, S.; Nagarajan, S.; Benzigar, M. R.; Mano, A.;
Alothman, Z. A.; Gnana Raj, G. A.; Vinu, V. Tetrahedron Lett. 2012, 53, 1485.
14. Synthesis of the catalyst: 4 g of amphiphilic triblock copolymer, poly(ethylene
glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) was
dispersed in 30 ml of H O, and 120 g of 2 M HCl solution was added while
2
stirring. 8 g of TEOS was then added to the homogeneous solution with stirring.
The required quantities of aluminum isopropoxide (1.26 g) and copper nitrate
(0.52 g) were added. The resulting gel mixture was stirred continuously at
4
2
0 °C for 24 h and finally crystallized in a Teflon-lined autoclave at100 °C over
d. After crystallization, the solid product was filtered, washed with distilled
2
H O, and dried in air at room temperature. The material was calcined in static
Acknowledgments
air at 550 °C for 24 h to decompose the triblock copolymer and afford sky-blue
colored CuAl-SBA-15.
1
5. General procedure for the synthesis of dihydrobenzopyrans: A mixture of the
phenol (2 mmol), allylic alcohol (1 mmol), and CuAl-SBA-15 (30 mg) in
cyclohexane (5 ml) was refluxed for 3–4 h and the progress of the reaction
was monitored by TLC. The crude mixture was filtered and concentrated, and
the residue was purified by silica gel column chromatography to afford the
pure dihydrobenzopyran.
This work was financially supported by the Ministry of Educa-
tion, Culture, Sports, Science, and Technology (MEXT) under the
Strategic Program for Building an Asian Science and Technology
Community Scheme and World Premier International Research
Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT,
Japan. A.V. thanks the Australian Research Council for the award
16. Vito, V.; Ricci, J.; Martini, S. P.; Nava, P.; Carissan, Y.; Humbel, S.; Duñach, E. Eur.
J. Org. Chem. 2010, 6239.