Hydroarylation of alkynes and alkenes through alumina-sulfuric acid catalyzed regioselective C–C bond formation

Article abstract of DOI:10.1016/j.tetlet.2019.03.006

A highly atom-efficient synthetic protocol for hydroarylation of terminal-aryl alkynes and styrene through the regioselective C–C bond formation via the electrophilic addition of naphthols and substituted phenols has been developed using alumina-sulfuric acid as a heterogeneous supported solid acid catalyst. This methodology shows excellent regioselectivity and affords the desired product in good to excellent yield. The heterogeneous catalyst can also be recycled efficiently without much loss of activity.

Full text of DOI:10.1016/j.tetlet.2019.03.006

Accepted Manuscript
Hydroarylation of alkynes and alkenes through alumina-sulfuric acid catalyzed
regioselective C-C bond formation
Amit Pramanik, Avishek Ghatak, Sagar Khan, Sanjay Bhar
PII:
S0040-4039(19)30207-2
https://doi.org/10.1016/j.tetlet.2019.03.006
TETL 50647
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 January 2019
27 February 2019
3 March 2019
Please cite this article as: Pramanik, A., Ghatak, A., Khan, S., Bhar, S., Hydroarylation of alkynes and alkenes
through alumina-sulfuric acid catalyzed regioselective C-C bond formation, Tetrahedron Letters (2019), doi: https://
doi.org/10.1016/j.tetlet.2019.03.006
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Graphical Abstract
Hydroarylation of alkynes and alkenes
through alumina-sulfuric acid catalyzed
regioselective C-C bond formation
Leave this area blank for abstract info.
Amit Pramanik, Avishek Ghatak, Sagar Khan and Sanjay Bhar*
OH
OH
13 examples
Yield 85-93%
Alumina-sulfuric acid
Ar1
Ar1
H
+
toluene, reflux, 4h
R1
OH
R1
OH
4 examples
Alumina-sulfuric acid
toluene, reflux, 4h
Ph
+
Ph
Yield 87-89%
R2
R2

1
Tetrahedron Letters
Hydroarylation of alkynes and alkenes through alumina-sulfuric acid catalyzed
regioselective C-C bond formation
Amit Pramanik,a Avishek Ghatak,b Sagar Khanc and Sanjay Bharc,*
a Department of Chemistry, Taki Government College, Taki, North 24 Pgs, Pin-743 429, India
b Department of Chemistry, Dr. A.P.J Abdul Kalam Govt. College, Newtown, Kolkata- 700156, India
c Department of Chemistry, Organic Chemistry Section, Jadavpur University, Kolkata- 700 032, India
e-mail: sanjaybharin@yahoo.com; sanjay.bhar@jadavpuruniversity.in
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A highly atom-efficient synthetic protocol for hydroarylation of terminal-aryl alkynes and
styrene through the regioselective C-C bond formation via the electrophilic addition of
naphthols and substituted phenols has been developed using alumina-sulfuric acid as a
heterogeneous supported solid acid catalyst. This methodology shows excellent regioselectivity
and affords the desired product in good to excellent yield. The heterogeneous catalyst can also
be recycled efficiently without much loss of activity.
Available online
Keywords:
Catalysis
Regioselection
2009 Elsevier Ltd. All rights reserved.
supported reagents/reactions
Sulfuric acid and derivatives
(HSZ-360) in 1,2-dichlorobenzene,8b FeCl3 in DCM,8c Bi(OTf)3
in cyclohexane,8d GaCl3 in toluene,8e different bimetallic catalysts
1. Introduction
Catalytic hydroarylation of alkenes and acetylenes is one of
the fascinating and useful tools for C-C bond forming reaction as
they lead to the construction of the core structure of a number of
biologically active compounds such as isocombretastatin A-4 (iso
like Fe-Al-MCM-41 in cyclohexane,8f Fe-Al-KIT-5 in
cyclohexane,8g iron containing mesoporous aluminosilicate,8h
In(OTf)3 under microwave irradiation8i as well as organometallic
complexes like Ir(I) diphosphine complexes,9a Re2(CO)10 in
toluene9b and many others. Most of the aforesaid reagents suffer
from one or more drawbacks like lower selectivity, over-
alkylation, large amount of salt formation, moisture-sensitivity,
toxicity, involvement of metals, prolonged reaction time and
utilization of toxic organic solvents. Although iron catalysts are
comparatively less toxic but they often require long reaction
time, activating groups and protection and de-protection steps.10
Therefore, development of a cost-effective, operationally simple,
eco-compatible transition metal-free efficient catalytic system for
the hydroarylation reaction is of great demand in the perspective
of present environmental scenario.
CA-4),1
avrainvilleol,
haplopappin,
papaverine,
and
phenprocoumone (Figure 1).2 Diarylmethanes are widely used for
the preparation of fluorenyl-based electroactive and photoactive
oligomers and polymers.3 In addition, these 1,1-disubstituted
alkenes are frequently used as key starting materials in the
synthesis of fine chemicals,4a-4c agrochemicals,4d polymers,5 and
pharmaceutically important compounds.6
HO
N(iPr)2
OH
H
O
O
OH
Br
OMe
Br
OH
OH
O
Me
OH
Avrainvilleol
Iso CA-4
Detrol
In recent years, organic reactions using heterogeneous acid
catalysts have gained much interest11 and often found as better
alternatives than the similar conventional homogeneous
protocols12 because of cost effectiveness, operational simplicity,
reusability and environmental benefits. Alumina–sulfuric acid13
has been reported in recent past but not exploited much in
organic synthesis. As a part of our ongoing research programme
to unravel novel catalytic attributes of alumina-sulfuric acid,14 we
disclose herein a convenient protocol for the synthesis of novel
1,1-diarylalkene and 1,1-diarylalkane moieties from naphthols
and differently substituted phenols with a wide substrate scope
using alumina-sulfuric acid as an easily accessible,
heterogeneous and reusable transition metal-free solid acid
catalyst.
HO
O
O
OMe
OH
O
N
HO
O
O
O
O
OMe
OH
O
Phenprocoumone
Papaverine
Haplopappin
Figure1: Some important biologically active 1,1-
and 1,1-diarylalkanes
diarylalkenes
The Friedel-Crafts reaction7 and hydroarylation8-9 of olefins
and alkynes are commonly employed for the construction of the
aforesaid molecular frameworks. In recent times, various metal-
promoted hydroarylation protocols of alkenes and alkynes have
been reported using SnCl4/Bu3N in acetonitrile,8a Y-type zeolite

2
Tetrahedron Letters
2. Results and discussion
Table 1. Optimization of reaction condition
When 2-naphthol 1a (1.5 mmol) was treated with
phenylacetylene 2a (1 mmol) in a specified solvent in the
presence of alumina-sulfuric acid under ambient atmosphere,
hydroarylated product 3a was obtained (Scheme 1). The reaction
was carried out with different solvents, temperatures and
catalysts to obtain the optimum condition. The results are
summarised in Table 1.
Entrya
Catalyst
Solvent
Temp.
(oC)
rt
Time
(h)
10
Conversion
Yield of
3ac (%)
NRd
of 2ab (%)
1
2
3
None
None
Toluene
Toluene
Toluene
0
0
0
120
rt
10
NRd
Alumina–sulfuric
acid
20
trace
OH
4
5
Alumina–sulfuric
acid
Toluene
Toluene
Toluene
EtOH
80
120
120
80
8
75
79
100
5
67
69
OH
Catalyst
Ph
H
+
Alumina–sulfuric
acid
3
Solvent, Time
Temperature
1a
2a
3a
6
Alumina–sulfuric
acid
4
91
Scheme 1. Reaction between 2-naphthol and phenylacetylene
7
Alumina–sulfuric
acid
10
10
10
10
10
10
10
10
10
10
LRd
LRd
LRd
LRd
NRd
NRd
NRd
LRd
30
There was no reaction either at room temperature or at
elevated temperature (120 C) in the absence of any catalyst, the
o
8
Alumina–sulfuric
acid
H2O
100
153
189
rt
9
unreacted substrates were isolated intact (Table 1, entries 1 and
2). When alumina-sulfuric acid was used as a catalyst at room
temperature, a trace amount of product was isolated after 20h
(Table 1, entry 3). But when the reaction was carried out at 80oC
the yield of the isolated product 3a was increased to 67% (Table
1, entry 4). Extent of conversion and isolated yield of the product
were further improved when the reaction was carried out under
reflux (120oC) (Table 1, entries 5 and 6). This reaction was
extremely sluggish in protic polar solvents (ethanol and water,
Table 1, entries 7 and 8) as well as aprotic polar solvents (DMF
and DMSO, Table 1, entries 9 and 10) even under reflux. It also
failed in DCM, CH3CN and MeNO2 at ambient temperature but
occurred at refluxing condition albeit with moderate conversions
(Table 1, entries 11–16). Thus, toluene, an aromatic non-polar
solvent, has come out as the best choice as a medium for this
reaction.
9
Alumina–sulfuric
acid
DMF
7
10
11
12
13
14
15
16
Alumina–sulfuric
acid
DMSO
MeCN
DCM
10
0
Alumina–sulfuric
acid
Alumina–sulfuric
acid
rt
0
Alumina–sulfuric
acid
CH3NO2
MeCN
DCM
rt
0
Alumina–sulfuric
acid
82
5
Alumina–sulfuric
acid
52
53
29
In order to establish the catalytic efficacy of the alumina-sulfuric
acid, the coupling reaction between 1a and 2a was carried out
using a wide variety of catalysts in toluene at 120 C. HY-zeolite
Alumina–sulfuric
acid
CH3NO2
80
15
o
17
18
19
20
HY Zeolite
Neutral Al2O3
Conc. H2SO4
Conc. H2SO4 on
Al2O3
Toluene
Toluene
Toluene
Toluene
120
120
120
120
4
4
4
4
0
0
NRd
NRd
--e
and neutral alumina failed to afford the hydroarylated product 3a
(Table 1, entries 17–18). Although a little amount of conversion
of 2a was noted in the presence of conc. H2SO4 (Table 1, entry
19) but the desired product 3a could not be isolated probably due
to acid-catalyzed polymerization the 2a and 3a. Even conc.
H2SO4 supported on neutral Al2O3 (Table 1, entry 20) could not
promote the reaction to a satisfactory level. Some other silica gel-
supported acids (Table 1, entries 21–23) afforded 3a in poor
yields due to various side reactions like polyalkylation and
polymeric decomposition. Only NaHSO4 – SiO2 afforded 59% of
3a (Table 1, entry 23). The reaction between 1a and 2a totally
failed to produce any product in the presence of chlorosulfonic
acid alone without alumina support. Thus the importance
alumina-sulfuric acid as an effective supported Bronsted acid
catalyst for this hydroarylation reaction in terms of stability and
catalytic activity was firmly established.
95
50
6
21
22
23
HClO4 – SiO2
PPA – SiO2
NaHSO4 – SiO2
Toluene
Toluene
Toluene
120
120
120
4
4
4
33.5
13.7
72.9
26
7
59
aReaction condition: 2-naphthol 1a (1.5 mmol), phenylacetylene 2a (1 mmol),
b
catalyst (100 mg), solvents (5 ml).
Percentage of conversion of
phenylacetylene 2a was calculated by 1H NMR (300 MHz). c Isolated yield of
the product. d unreacted substrates were isolated almost exclusively.
e the polymerized materials.
Table 2. Effect of the catalyst loading towards the model
reaction
The effect of catalyst loading was also studied (Table 2), where
the optimum amount of alumina-sulfuric acid (with H+
concentration of 3x10―4 molgm―1 of the support14b) was found to
be 100 mg per 1 mmol of 2a. Highest conversion of 2a and
maximum yield of 3a were achieved using 1a and 2a in the molar
ratio of 1.5:1. Therefore, optimum condition for the present
protocol has been laid down in Scheme 2.
Entrya
Amount of catalyst (mg)
Isolated yield (%)
1
2
3
4
50
75
63
74
91
90
100
150
a
Reactions condition: 1a (1.5 mmol), 2a (1 mmol), and alumina-sulfuric
acid in dry toluene at 120oC.

3
OH
The heterogeneity of the catalyst was established with the help
OH
of “hot filtration method” as depicted by Lempers and Sheldon.16
β-Naphthol and phenylacetylene were taken for this experiment.
When 37% conversion of the substrate was noted after 1h,
catalyst was taken out by simple filtration under hot condition to
avoid re-adsorption of leached sulphur (if any) at room
temperature on the catalyst surface. The “catalyst free” filtrate
was then kept under optimized reaction conditions. After 18 h,
further progress of the reaction was not observed at all. Hence,
the heterogeneity of the catalyst was conclusively proved.
Alumina-sulfuric acid
Ph
H
+
Toluene
120 oC, 4h
1a
1.5 equivalent
2a
1.0 equivalent
3a
Yield = 91%
Scheme 2. Optimized reaction between 2-naphthol (1a) and
phenylacetylene (2a) ( Reactions condition: 1a (1.5 mmol), 2a (1
mmol), and alumina-sulfuric acid (100 mg ).
After each cycle, the reaction mixture was cooled to ambient
temperature, and the catalyst was separated by simple filtration.
Recovered catalyst was successively washed with toluene,
ethanol and dried at 130oC for 1 h. The catalyst was recycled
successively up to eight runs without significant loss of its
catalytic activity (Figure 2). Thus the alumina-sulfuric acid has
been found to be more effective and efficient compared to many
recently reported laboriously prepared catalysts, for example,
sulfonic acid functionalized hyperbranched poly (ether sulfone)
[SHBPES] grafted on carbon black15 in terms of operational
simplicity and recyclability.
The said reactions are highly atom-efficient (atom
economy 100%) and generate no by-product as the waste. The
present method involves alumina-sulfuric acid as an efficient and
recyclable catalyst and toluene as the reaction medium, which
has greater acceptance than many aromatic and other
conventional organic solvents in terms of safety and
environmental issues. Thus the present method bodes for eco-
compatibility in terms of atom-economy, reaction medium,
minimal waste generation and maximum utilization of renewable
resources through efficient recovery and recycling of both the
catalyst as well as the solvent.
100
In order to explore the scope and limitation of the present
protocol, a wide variety of naphthols 1a–1b and phenols 1c–1k
were reacted with substituted arylacetylenes and styrene under
optimized reaction condition (Scheme 2) in order to obtain the
hydroarylated products 3, as listed below.
80
60
40
20
Compounds prepared by the alumina-sulfuric acid-
catalyzed protocol
0
Fresh Cycle-1 Cycle-2 Cycle-3 Cycle-4 Cycle-5 Cycle-6 Cycle-7 Cycle-8
OH
OH
OH
OH
OH
Number of Cycle
Figure 2. Recycling of the alumina-sulfuric acid
Ph
Me
F
The comparative profile of the atom composition (Table 3)
obtained from the EDX spectra (Supporting information) of
freshly prepared and recycled alumina-sulfuric acid provided
further support to substantiate the efficacy and recyclability of
the present catalyst. Extremely marginal loss of sulphur from the
supported catalyst during the reaction and recycling process was
noted. So it can be concluded that sulphur containing -SO3H
moiety remained covalently anchored and firmly immobilized
with the alumina support throughout the entire process.
(3a)8e (4h, 91%)
(3c)8e (4h, 93%)
(3d) (4h, 90%)
(3b)8e (4h, 89%)
(3e) (4h, 87%)
OH
OH
OH
OH
OH
Me
(3j)8f (4h, 88%)
(3f)8e (4h, 89%)
(3g)8e (4h, 88%)
(3i)8h (4h, 89%)
(3h)8e (4h, 87%)
Me
OH
OH
OH
OH
OH
Me
OH
I
Me
Cl
Br
OMe
(3o)8h (4h, 85%)
(3l)17c (4h, 87%)
(3n)8h (4h, 86%)
(3k)17b (4h, 88%)
(3m)8h (4h, 90%)
(3p) (4h, 85%)
OH
Table 3. Comparative profile of the atom compositions
S
according to EDX measurements
S
S
S
Support
Element
Weight% Atomic%
Total
100.0
(3q)17d (4h, 86%)
(3r)17a (3h, 89%)
(3s)17e(3h, 88%)
Alumina – sulfuric acid
(freshly prepared)
O K
Al K
S K
54.69
44.20
1.11
67.14
32.18
0.68
a Isolated yield of the purified product fully characterized spectroscopically
Hydroarylation of 2-naphthol 1a and 1-naphthol 1b with
differently substituted arylacetylenes moieties 2a, 2c-2e and
styrene 2b furnished the desired products 3a-3g in good to
excellent yield, where regioselectivity was governed in
accordance to the fixation of double bonds in the naphthol
moieties. Interesting regioselectivity was observed in case of
phenol. In spite of the availability of both ortho- and para-
positions, the reaction took place exclusively at the sterically
more demanding ortho-position with >98% of conversion and the
product 3h was obtained with 87% of yield. para-Substituted
phenol 1d reacted with 2a and 2b to produce the expected ortho-
arylated products 3i and 3j with 89% and 88% of yields
respectively. Exclusive ortho-arylation occured in 2-
methylphenol (1e) where sterically more accessible para-position
was left unaffected and the corresponding products 3k and 3l
were obtained with 88% and 87% yields respectively. When two
Alumina – sulfuric acid
(after first recycling)
O K
54.66
67.16
100.0
Al K
44.23
32.16
S K
O K
Al K
S K
O K
Al K
S K
O K
Al K
S K
O K
Al K
S K
1.11
54.69
44.20
1.11
54.18
44.73
1.09
54.07
44.91
1.02
54.62
45.37
0.01
0.68
67.14
32.18
0.68
66.53
32.81
0.66
66.13
33.22
0.65
67.00
33.00
0.00
Alumina – sulfuric acid
(after recycling five times)
100.0
100.0
100.0
100.0
Alumina – sulfuric acid
(after recycling six times)
Alumina – sulfuric acid
(after recycling eight times)
Neutral alumina

4
Tetrahedron Letters
electron-donating groups, namely, OH and OMe were present
mutually at para-positions in the substrate 1f, 2a reacted
exclusively at the ortho-position with respect to the OH group
and 3m was produced with 90% of yield. Similar trends were
also observed with halogen-substituted phenols (1g-1i) and the
products (3n-3p) were obtained with good yield without affecting
the substituents. 3-Ethynylthiophene (2c) (with electron-rich
heterocycle) reacted smoothly with 1a to produce 3q in 86%
yield but 2-ethynylpyridine (2d) (with electron-deficient
heterocycle) failed to react with 1a under the present protocol.
The ortho-disubstituted-para-unsubstituted substrate, namely, 2,
6-dimethoxyphenol 1j failed to react with 2a even after
prolonged reaction (10h). Thus, the exclusive ortho-selectivity of
the present protocol was convincingly established. A plausible
reaction pathway has been speculated (Figure 3). Presumably, the
reaction was initiated by the initial polarization of the carbon-
carbon multiple bond by the protic hydrogen of alumina-sulfuric
acid with the development of incipient electron-deficiency at the
α–carbon (A). This seemed to be crucial because no conversion
was noted with the alkyne 2d with electron-withdrawing
heterocyclic moiety. Subsequent intermolecular reaction of A
with phenolic compounds might occur through a six-membered
cyclic transition state involving the phenolic-OH moiety leading
to B with exclusive ortho-substitution, followed by aromatization
through tautomerization. The exact mechanism of the present
protocol is neither unambiguously nor conclusively established.
3. Conclusion
In summary, we have developed a highly regioselective, atom
efficient, environmentally benign transition metal-free synthetic
protocol using alumina-sulfuric acid as a reusable heterogeneous
solid acid catalyst for the hydroarylation of arylacetylenes and
styrene with differently substituted phenols and naphthols to
prepare
a series of important 1,1-diarylalkenes and 1,1-
diarylalkanes within a reasonable reaction time. There was no
occurrence of any by-product due to polyalkylation, dimerization
and polymerization of styrene and phenylacetylenes as well as
the hydroarylated products.
Acknowledgements
Infrastructural support from DST-FIST programme and
financial assistance from DST-PURSE-programme and UGC-
CAS-II programme in Chemistry at Jadavpur University are
gratefully acknowledged. The authors express sincere gratitude to
Ms. S. Nandy and Dr. K. K. Chattopadhyay of Jadavpur
University, Mr. N. Dutta and Mr. A. K. Ghosh of Indian
Association for the Cultivation of Science and Ms. P. Das of
Indian Institute of Chemical Biology for necessary assistance.
References and notes
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HO3S

HO3S
OH
HO3S

Ar
Ar
A
Ar
H
O
OH
O
HO3S
Ar
Ar

H
Ar
B
Figure 3: Speculated reaction pathway
Unlike phenols, thiophenol 1k reacted with both
phenylacetylene 2a and styrene 2b under the present reaction
condition but did not afford to the hydroarylated products, rather
nucleophilic addition across the multiple bonds took place to
produce the bis- and mono-S-alkylated products 3r and 3s with
89% and 88% yields respectively. This observation was in
accordance to the HSAB (Hard Soft Acid Base) principle, where
sulphur atom of 1k, being a relatively softer nucleophile,
preferentially reacted with the C-2 (relatively softer electrophilic
centre) of both 2a and 2b. As a result, 2a initially produced the
mono-S-alkylated product through nucleophilic addition of 1k
across the acetylenic linkage which on further nucleophilic
addition with the second molecule of 1k yielded the bis-S-
alkylated product 3r. Similarly, 2b afforded the mono-S-
alkylated product 3s. Alumina-mediated thia-Michael reactions
between thiols and conjugated enones as well as ynones have
earlier literature precedence.18 It is important to note that the site-
selectivity with reference to the nucleophile as well as the
arylalkyne/arylalkene in the nucleophilic addition reaction (3r
and 3s) is totally opposite in comparison to the earlier
observations (3a – 3q).
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1903, 26, 4012. (c) Calloway, N. O. Chem. Rev. 1935, 17, 327-
392 (d) Olah, G. A. In Friedel-Crafts Chemistry; Wiley: New
York, 1973. (e) Olah, G. A.; Krishnamurti, R.; Prakash, G. K. S.
In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I.,
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Supplementary Material
Representative experimental procedure, EDX spectra of alumina-
sulfuric acid and the spectral data of the hydroarylated
compounds have been furnished as Supporting information.
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Click here to remove instruction text...
Title: Hydroarylation of alkynes and alkenes
through alumina-sulfuric acid catalyzed
regioselective C-C bond formation
Graphical Abstract
Authors: Amit Pramanik, Avishek Ghatak,
Sagar Khan and Sanjay Bhar
Highlights
 regioselective C-C bond formation
 recyclable heterogeneous supported solid
acid catalyst
OH
O
Alumina-sulfuric acid
Ar1
H
+
toluene, reflux, 4h
R1
OH
 no occurrence of any by-product
 minimal waste generation
 eco-compatibility
Alumina-sulfuric acid
toluene, reflux, 4h
+
Ph
R2
Hydroarylation of alkynes and alkenes
through alumina-sulfuric acid catalyzed
regioselective C-C bond formation
Leave this area blank for abstract info.
———
Amit Pramanik, Avishek Ghatak, Sagar Khan and Sanjay Bhar*
 Corresponding author. Tel.: +918697179547; e-mail:
sanjaybharin@yahoo.com, sanjay.bhar@jadavpuruniversity.in

6
Tetrahedron Letters