Silica-supported KHSO4: An efficient system for activation of aromatic terminal olefins

Article abstract of DOI:10.1055/s-0030-1259041

Potassium hydrogen sulfate adsorbed on chromatography-grade silica gel activates electron-rich aromatic terminal olefins towards nucleophilic attack at the benzylic position by alcohols. Temperature plays a crucial role and facilitates suppressing nucleophilic reaction in favor of dimerization of the terminal olefin. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1259041

2908
LETTER
Silica-Supported KHSO4: An Efficient System for Activation of Aromatic
Terminal Olefins
Efficient
System
a
for Activa
t
m
ion of Aromatic Terminal
N
O
lefins ath Das, Kuladip Sarma, Madan Gopal Pathak, Amrit Goswami*
Synthetic Organic Chemistry Division, North-East Institute of Science & Technology (A Constituent Establishment of CSIR, New Delhi),
Jorhat, Assam 785006, India
Fax +91(376)2370011; E-mail: amritg_2007@rediffmail.com
Received 21 July 2010
R2
R2
Abstract: Potassium hydrogen sulfate adsorbed on chromatogra-
phy-grade silica gel activates electron-rich aromatic terminal ole-
fins towards nucleophilic attack at the benzylic position by
alcohols. Temperature plays a crucial role and facilitates suppress-
ing nucleophilic reaction in favor of dimerization of the terminal
olefin.
KHSO4-SiO2
dimerized
product (trace)
OR3
+
R3OH, 60–115 °C
R1
R1
2 (major)
1
R1 = H, OH, OMe, OBn, OCOMe
R2 = H, Me
3 = Me, Et, i-Pr, Bu,
Key words: alkenes, alkoxylation, dimerization, potassium hydro-
gen sulfate, silica gel
R
C6H5(CH2)4, C6H5CH(Me)
Scheme 1
Activation of terminal alkenes is a challenging task in or-
ganic chemistry for preparation of industrially important
higher alkenes, lubricants and detergents through homo-
and co-dimerization.1 Effective activation of alkenes by
transition metals iron,2 copper,3 titanium on montmorillo-
nite support,4 zirconium,5 ruthenium,1 rhodium,6 palladi-
um,7–9 gold and platinum catalysts,10 sometimes in the
presence of co-catalysts11 has been reported for formation
of C–C or C–heteroatom bonds. Use of proton-exchanged
montmorillonite as a solid Brønsted acid has been report-
ed to be another extremely effective catalyst for direct ad-
dition of alkenes and alcohols to active methylene of 1,3-
dicarbonyl compounds.12 Direct formation of C–C or C–
O bonds by coupling of an alkene with another or with an
alcohol through functionalization of the C=C bond at the
benzylic position is thus a very attractive reaction. The
problem in the use of late transition metals in many pro-
cesses is their high cost and toxicity. On the other hand,
zeolites and other polymeric solid supports suffer from
limitations of pore-size dependency and requirement for
large quantities. Therefore achieving an alternative per-
spective on this conversion is definitely highly appealing.
Thus silica-supported KHSO4 and NaHSO4 have been de-
veloped for cleavage of TBDMS ethers14 and esters,15 ac-
etal preparation15 and as dehydrating agents.16 We have
observed a novel property of this system and herein we
present our experimental results for activation of aromatic
terminal alkenes leading to cross-coupling with alcohols
by nucleophilic attack on the benzylic carbon atom and
also dimerization (Scheme 1).
Deprotection17 of 4-acetoxystyrene by silica-supported
KHSO4 in methanol at its reflux temperature revealed that
the alkenic double bond was attacked by methanol in the
benzylic position to yield 1-(4-hydroxyphenyl)-1-meth-
oxyethane in good yield (entry 1, Table 1). The reaction
product was accompanied by a trace amount of head-to-
tail dimerized product of 4-hydroxystyrene. Gratifyingly
similar results were observed with a range of styrenes
having electron-releasing group (entries 2–4, Table 1) on
the benzene ring. Styrenes having electron-withdrawing
groups on the benzene ring such as Cl or NO2 showed dis-
appointing results.
Intrigued by the unusual mode of this reaction, a system-
atic investigation of the reaction of styrene derivatives
was carried out with a range of higher alcohols in the pres-
ence of silica-supported KHSO4 at 80–115 °C (entries 5–
9, 13 and 14, Table 1). Depending upon the alcohol, a spe-
cific reaction temperature (80–115 °C, Table 1) was re-
quired for nucleophilic insertion at the benzylic carbon of
the substrate to occur (10–73%) along with the parallel
dimerization reaction to give 4 (15–56%, Table 1). In the
reaction of 4-methoxystyrene in a toluene–methanol (4:1)
solvent system, it was observed that the dimerization re-
action predominated over nucleophilic insertion with in-
crease of reaction temperature (Figure 1). The reaction
did not proceed with a tertiary alcohol; in the case of a sec-
ondary alcohol it was found to be sluggish (entries 7 and
9, Table 1).
Although triflic acid adsorbed unmodified chromato-
graphic silica has been studied as effective recyclable sys-
tem for alkylation of b-dicarbonyl compounds with
alcohols or olefins,13 formation of C–C or C–heteroatom
bond through activation of alkenes by readily available
and cheap alkali metal salts has not been explored. When
the surface area of such metal salts is increased by sup-
porting on a solid matrix, it constitutes an attractive
heterogenous catalyst system with improved activity and
selectivity.
SYNLETT 2010, No. 19, pp 2908–2912
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x
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x
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0
Advanced online publication: 10.11.2010
DOI: 10.1055/s-0030-1259041; Art ID: D19910ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Efficient System for Activation of Aromatic Terminal Olefins
2909
Table 1 Reaction of Different Styrene Derivatives with Different Alcohols using KHSO4–SiO2 System
Entries Substrates
Solvent
Temp (°C) a-Alkoxylated
Dimerized Time Yieldc (%)
productb
(A)
productb
(B)
(h)
A
B
–
OMe
1
MeOH
MeOH
MeOH
MeOH
EtOH
70
70
70
70
80
N.O.d
N.O.d
N.O.d
4
3
3
3
3
3
84
AcO
HO
OMe
OMe
OMe
OEt
2
87
82
65
20
–
HO
HO
3
–
BzO
BzO
MeO
MeO
4
30
45
MeO
5
4
MeO
O
6
4-phenyl-1-butanol
115
4
4
45
50
MeO
MeO
MeO
MeO
MeO
7
2-PrOH
115
115
115
4
4
4
5
5
5
11
15
10
55
56
35
O
MeO
8
n-octanol
OC8H17
MeO
9
1-phenylethanol
O
Ph
MeO
OMe
10
toluene–MeOH (4:1)
115
4
3
25
61
MeO
MeO
11
DCE
80
–
4
4
3
3
–
–
63
65
MeO
12
toluene
115
–
MeO
OEt
13
EtOH
80
3
3
10
25
Synlett 2010, No. 19, 2908–2912 © Thieme Stuttgart · New York

2910
R. N. Das et al.
LETTER
Table 1 Reaction of Different Styrene Derivatives with Different Alcohols using KHSO4–SiO2 System (continued)
Entries Substrates
Solvent
Temp (°C) a-Alkoxylated
Dimerized Time Yieldc (%)
productb
(A)
productb
(B)
(h)
A
B
14
n-BuOH
80
3
4
73
15
OC4H9
15
16
toluene
DCE
115
80
–
–
3
3
3
3
–
–
66
64
17
18
19
MeOH
toluene
DCE
70
5
5
5
3
3
3
26
–
55
59
58
OMe
115
80
–
–
–
a The reactions were run with olefin (8–10 mmol) for 3–6 h.20,
b The products were characterized from their respective IR, NMR, GC and mass spectroscopic data and by comparison with literature data.21
c Isolated yields of products.
d The products were not observed (N.O.).
R1
R1
3
(tail-to-tail)
(major when R1, R2 = H)
R2
KHSO4-SiO2
toluene
115 °C
R1
R1
4
R1
1
(head-to-tail)
(major when R1 = OMe, R2 = H )
R1 = H, OMe
R2 = H, Me
Figure 1 Effect of temperature on dimerization of 4-methoxysty-
rene in toluene–MeOH (4:1) solvent system
R1
R1
5
(head-to-tail)
(major when R1 = H, R2 = Me )
However, in aprotic solvents like dichloroethane and tol-
uene, dimerization reaction of 4-methoxystyrene occurred
exclusively to give 63–65% yield (Scheme 2; entries 10–
12, Table 1) of the head-to-tail dimerized product 4 along
with a trace amount of another dimerized product. Such a
product was reported18 to be formed from styrene on reac-
tion with trifluoroacetoxy nickel hydride in 40 hours.
Scheme 2
rene gave 1-phenyl diethylether (10%) and tail-to-tail
dimerized product 3 (25%) having a terminal double bond
(Scheme 2; entry 13, Table 1), which is reported19 to be
formed from styrene on reaction with cyclopalladated tet-
rafluoroborate from N-(p-tolyl)-p-nitrobenzalidimine at
100 °C over 19 hours. Surprisingly, styrene on refluxing
with n-butanol in the presence of silica-supported KHSO4
at 80 °C for three hours gave 73% of n-butyloxy-1-phe-
Styrene itself did not undergo any reaction when refluxed
in methanol in the presence of silica-supported KHSO4.
However, when heated in ethanol at its boiling point, sty-
Synlett 2010, No. 19, 2908–2912 © Thieme Stuttgart · New York

LETTER
Efficient System for Activation of Aromatic Terminal Olefins
2911
nylethanol and only 15% of tail-to-tail dimerized product
3 (Scheme 1; entry 14, Table 1). On the other hand sty-
rene or a-methylstyrene on heating with silica-supported
KHSO4 in dichloroethane or in toluene, gave 58–66%
yields of tail-to-tail and head-to-tail dimerized products 3
and 5. respectively (Scheme 2; entries 15, 16, 18, and 19,
Table 1). a-Methylstyrene on heating in methanol at its
reflux temperature in the presence of KHSO4–SiO2 also
gave 26% of 1-methyl-1-phenylmethoxyethane (entry 17,
Table 1) and 55% of head-to-tail dimerized product 5
(Scheme 2). It is postulated that the absence of any elec-
tron-releasing group on the benzene ring facilitates tail-to-
tail dimerization over head-to-tail dimerization. The role
of the electron-releasing substituent in both the nucleo-
philic addition and the dimerization is presumably to sta-
bilize the carbonium ion generated in the benzylic
position by potassium bisulfate facilitating the attack by
nucleophilic methoxy group or by another styrene mole-
cule. This is not possible in the case of substrates having
electron-withdrawing groups.
Acknowledgment
We gratefully acknowledge Dr. P. G. Rao, Director, NEIST, Jorhat
for providing the facilities to carry out this work. The authors also
gratefully acknowledge Dr. J. C. S. Kataky, Head, Synthetic Orga-
nic Chemistry Division for his help with carrying out this work and
CSIR, India for the funding.
References and Notes
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Chem. 2010, 12, 45. (b) Zaccheria, F.; Psaro, R.; Ravasio,
N. Tetrahedron Lett. 2009, 50, 5221.
The reaction was studied with other sodium and potassi-
um salts such as NaHSO4, K2SO4, KCl and KF. However,
compared to KHSO4 all other salts showed inferior or no
catalytic activity (Figure 2). The detailed assignment of
the 1H NMR spectra of 3–5 and comparison with the liter-
ature data established their structures (Scheme 2).
(4) Motokura K., Fujita N., Mori K., Mizugaki T., Ebitani K.,
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K.; Kaneda, K. Angew. Chem. Int. Ed. 2006, 45, 2605.
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Green Chem. 2010, 12, 1049.
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Lett. 2004, 33, 1146. (b) Kumar, R. S.; Nagarajan, R.;
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Figure 2 Effect of change of metal catalyst on conversion of 4-
methoxystyrene to both types of products
(15) Sartori, G.; Balini, R.; Bigi, F.; Bosica, G.; Maggi, R.; Righi,
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(20) (a) Catalyst Preparation: KHSO4 (20 g, 144 mmol) was
dissolved in distilled H2O (100 mL) and silica gel (25 g, 60–
120 mesh) was added. The soaked mixture was thoroughly
mixed and dried in a hot oven at 150 °C for 24 h to give a
free flowing powdery solid. The dried solid mixture was
then stored in a vacuum desiccator. (b) Typical
Experimental Procedure for Alkoxylation:
In conclusion, we have described a KHSO4-catalyzed ac-
tivation of styrenes which involves either nucleophilic at-
tack by alkoxide or dimerization through head-to-tail or
tail-to-tail coupling. Although several reports have ap-
peared for such reactions using transition metal catalysts,
the solid-supported acidic salt mediated catalyzed reac-
tions described in this communications represent the first
report of C–C and C–O functionalization. The advantages
of the method are that it is a one-pot process, noncorro-
sive, the catalyst can be easily recovered for recycling and
the procedure is simple.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
4-Methoxystyrene (1 g, 7.5 mmol) was added slowly to a
round-bottomed flask containing KHSO4–SiO2 (100 mg)
and 4-phenyl-1-butanol (5 mL). The mixture was then stirred
Synlett 2010, No. 19, 2908–2912 © Thieme Stuttgart · New York

2912
R. N. Das et al.
LETTER
for 3 h at 115 °C to give 1-(4-methoxyphenyl)-1-(4-phenyl-
1-butoxy)ethane (0.96 g, 45%) as a liquid. 1H NMR (300
MHz, CDCl3; Me4Si): d = 1.33 (d, J = 6.4 Hz, 3 H,
liquid. 1H NMR (300 MHz, CDCl3; Me4Si): d = 1.40 (d, J =
6.5 Hz, 3 H, CH3CH), 3.56 (q, J = 6.5 Hz, 1 H, CH3CHCH),
3.79 (s, 6 H, 2 × PhOCH3), 6.23–6.36 (m, 2 H, CH=CH),
6.80–6.87 (m, 4 H, 4 × ArH), 7.17–7.29 (m, 4 H, 4 × ArH).
13C NMR (75 MHz, CDCl3; Me4Si): d = 21.5 (CH3CH), 41.7
(CH3CH), 55.3 (2 × OCH3), 113.9 (2 × ArC), 127.3, 127.6 (2
× CH=CH), 127.8, 128.2, 128.4, 129.9, 130.5, 133.5, 138.0,
157.9, 158.8 (10 × ArC). MS: m/z = (EI) 268 [M+]. Anal.
Calcd for C18H20O2: C, 80.59; H, 7.46. Found: C, 80.54; H,
7.49.
CH3CHOCH2), 1.58 (m, 4 H, PhCH2CH2), 2.51 (m, 2 H,
OCH2CH2CH2), 3.20 (t, J = 6.4 Hz, 2 H, OCH2CH2), 3.73 (s,
3 H, PhOCH3), 4.24 (q, J = 6.4 Hz, 1 H, CH2OCHCH3), 6.80
(d, J = 6.8 Hz, 2 H, 2 × MeOAr-m-H), 7.06–7.21 (m, 7 H,
7 × ArH). 13C NMR (75 MHz, CDCl3; Me4Si): d = 24.2
(CH3CHOCH2), 28.1, 29.6, 35.8, 55.3 (4 × CH2), 68.3
(CH3CHOCH2), 113.8 (OCH3), 125.7, 127.4, 128.3, 128.5,
136.2, 142.6, 158.9 (12 × ArC). MS: m/z (EI) = 284 [M+].
Anal. Calcd for C19H24O2: C, 80.28; H, 8.45. Found: C,
80.32; H, 8.41. (c) Typical Procedure for Dimerization: A
solution of 4-methoxystyrene (1 g, 7.5 mmol) in toluene (2
mL) was added slowly to a round-bottomed flask containing
KHSO4–SiO2 (100 mg) in toluene (10 mL). The mixture was
then stirred for 3 h at 115 °C to give the head-to-tail dimmer,
1,3-di-(4-methoxyphenyl)-1-butene (0.65 g, 65%) as a semi-
(21) (a) Fujii, Y.; Furugaki, H.; Tamura, E.; Yano, S.; Kita, K.
Bull. Chem. Soc. Jpn. 2005, 78, 456. (b) Zaccheria, F.;
Psaro, R.; Ravasio, N. Tetrahedron Lett. 2009, 50, 5221.
(c) Murphy, J. A.; Khan, T. A.; Zhou, S.; Thomson, D. W.;
Mahesh, M. Angew. Chem. Int. Ed. 2005, 44, 1356.
(d) Zhang, Y.; Sigman, M. S. Org. Lett. 2006, 8, 5557.
(e) Minai, M.; Higashii, T. Eur. Patent Appl. EP 288297,
1988.
Synlett 2010, No. 19, 2908–2912 © Thieme Stuttgart · New York