Calcium(II) catalyzed regioselective dehydrative cross-coupling reactions: Practical synthesis of internal alkenes and benzopyrans

Article abstract of DOI:10.1016/j.tetlet.2016.11.027

A simple and operationally easy protocol for the regioselective synthesis of internal alkenes through a dehydrative cross-coupling and direct coupling procedure has been described using the environmentally benign Ca(II) catalyst. Several alkenes and alcohols underwent the coupling reaction under solvent-free conditions in a short time to produce the desired alkenes. This method is further extended to show the application in the synthesis of novel benzopyran molecules through a cascade strategy.

Full text of DOI:10.1016/j.tetlet.2016.11.027

Accepted Manuscript
Calcium(II) Catalyzed Regioselective Dehydrative Cross-Coupling Reactions:
Practical Synthesis of internal alkenes and benzopyrans
Srinivasarao Yaragorla, Abhishek Pareek, Ravikrishna Dada, Abdulrahman I.
Almansour, Natarajan Arumugam
PII:
S0040-4039(16)31483-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.027
TETL 48313
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
22 September 2016
20 October 2016
6 November 2016
Please cite this article as: Yaragorla, S., Pareek, A., Dada, R., Almansour, A.I., Arumugam, N., Calcium(II)
Catalyzed Regioselective Dehydrative Cross-Coupling Reactions: Practical Synthesis of internal alkenes and
benzopyrans, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.11.027
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Graphical Abstract
Calcium(II) Catalyzed Regioselective Dehydrative Cross Coupling Reactions:
Practical Synthesis of internal alkenes and benzopyrans
Srinivasarao Yaragorla,* Abhishek Pareek, Ravikrishna Dada, Abdulrahman. I. Almansour and Natarajan
Arumugam

1
Tetrahedron Letters
Calcium(II) Catalyzed Regioselective Dehydrative Cross-Coupling
Reactions: Practical Synthesis of internal alkenes and benzopyrans
Srinivasarao Yaragorla,a Abhishek Pareek,a Ravikrishna Dada,a Abdulrahman I. Almansourb and Natarajan Arumugamb
aDepartment of Chemistry, Central University of Rajasthan, Bandarsindri, NH-8, Ajmer-distt., Rajasthan, 305817, India Tel: +91-1463-
288535 Fax: +91-1463-238722. bDepartment of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451,
Saudi Arabia.
E-mail: srinivasarao@curaj.ac.in; ysriict@gmail.com.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple and operationally easy protocol for the regioselective synthesis of internal
alkenes through a dehydrative cross-coupling and direct coupling procedure has been
described using the environmentally benign Ca(II) catalyst. Several alkenes and
alcohols underwent the coupling reaction under solvent-free conditions in a short time
to produce the desired alkenes. This method is further extended to show the
application in the synthesis of novel benzopyran molecules through a cascade strategy.
Available online
Keywords:
Dehydrative cross coupling reaction
Calcium catalysis
Internal alkenes
Benzopyrans
Green synthesis
2009 Elsevier Ltd. All rights reserved.
Cascade synthesis
Carbon-carbon bond formation by direct coupling of alkenes with
other partners (alcohols/alkenes) is attracting many of the
synthetic chemists owing to the atom efficiency and
environmentally benign as water is the only byproduct.1
However, the major challenge in this method is to activate the
hydroxyl group which is not known to be a good leaving group.2
One of the oldest direct cross coupling methods between 9-
fluorenol and 9-alkylidene fluorenes has been reported 65 years
ago.3 Since then many transition metals catalyzed reactions have
been developed.4-6. For example recently, Yamamoto et al.5a
reported a Pd(II) catalyzed cross coupling of alkenes with
alcohols and Ji et al.5b reported an efficient reaction with TfOH
but the solvent was 1,2-dibromoethane. Liu et al.5c reported
another interesting reaction of alcohols with alkenes in
dichloromethane using FeCl3 and stoichiometric amount of
TsOH. Similarly many reports are available for the transition
metal catalyzed dimerization of alkenes, such as Co(II)/Zn,6a
bonds (sp3-sp2) starting from easily available alcohols and
alkenes using environmentally benign and highly abundant
calcium salt as the green catalyst.
Figure 1. Possible direct/dehydrative cross-coupling reactions.
Pd(OAc)2/diazonium
salt/BmimPF6,6b
Pd(OAc)2-HFIP,6c
As described in the Figure 1 (equation b), we commenced the
synthesis of but-2-ene-1,1,3-triyltribenzene (3a) from benzhydrol
(1a) and  -methylstyrene (2a) through a dehydrative coupling
reaction. 1.0 mmol of 1a and 1.0 mmol of 2a were stirred in 1,
Pd(OAc)2/TFA6d and many others.7 Nevertheless, most of these
direct cross-coupling/homo-coupling methods are inevitably
associated with some of the problems such as the use of
expensive and readily-non available catalysts, use of
stoichiometric amounts of catalysts, halogenated solvents etc.
Hence there is still a high demand for the development of user-
friendly and environmentally friendly methods to make sp3-sp2 C-
C bonds (Figure 1) from sp3-sp3 or sp3-sp2 or sp2-sp2 C-C bond
forming reactions. In this context herein, we report a user-
2-dichloroethane at rt for
8
h
using
5
mol% of
Ca(OTf)2/Bu4NPF68 and isolated the product 3a but only in 28%
yield. However encouraged by this result we investigated several
other conditions for the better synthesis of 3a (Table 1). When
the above reaction was performed under reflux conditions (90
oC), the reaction yielded 3a in 65% after 6 h (entry 2, Table 1).
Increasing the catalyst loading from 5 to 10 mol% influenced the
friendly and a solvent free method for the formation of new C-C
———
 Corresponding author. Tel.: +91-463-238535; fax: +91-463-238722; e-mail: Srinivasarao@curaj.ac.in

2
yield (76%) but not to a great extent (entry 3, Table 1). When 1a
and 2a were heated at 90 oC with 10 mo% of catalyst and
additive under neat conditions, the reaction gave the encouraging
yield of 86% in 1.5 h (entry 4, Table 1). Further neither the raise
o
o
in temperature (110 C) nor the lowering in temperature (70 C)
could change the chemical yield of 3a (entries 5, 6, Table 1).
However the yield of 3a was decreased to 60% at 50 oC (entry 7).
o
These observations suggested us that 70 C could be the ideal
temperature for the synthesis of 3a under neat conditions. To
confirm the role of catalyst, the reaction was performed in the
absence of catalyst (entry 8) and found that no reaction was
initiated. Neither the Ca(OTf)2 (catalyst) nor the Bu4NPF6
(additive) alone could furnish the better yield of 3a (entries 9, 10)
as it was well observed by us and others that Bu4NPF6 will help
in solubilizing the Ca(II) salts for the enhanced reactivity.9
Ca(ClO4)2 and Mg(OTf)2 proved to be less efficient catalysts
(entries 11, 12) in furnishing the 3a. The reaction with 10 mol%
of FeCl3 gave a reasonable yield of 75% in 2 h. Among the
thirteen different conditions tried (Table 1), entry 6 was found to
be the better condition for the dehydrative cross coupling of 1a
and 2a (Scheme 1).
Table 1. Optimization study for the dehydrative coupling reaction
between benzhydrol 1a and 1-methyl styrene 2aa
Entry
Catalyst (mol%)
Solvent, Temperatureb, Yield
Time
DCE, rt, 8 h
(%)c
28
65
76
86
86
86
60
nr
1
2
3
Ca(OTf)2/Bu4NPF6 (5)
Ca(OTf)2 /Bu4NPF6 (5)
Ca(OTf)2 /Bu4NPF6 (10)
Ca(OTf)2/Bu4NPF6 (10)
Ca(OTf)2/Bu4NPF6 (10)
Ca(OTf)2/Bu4NPF6 (10)
Ca(OTf)2/Bu4NPF6 (10)
--------
Ca(OTf)2 (10)
Bu4NPF6 (10)
Ca(ClO4)2/Bu4NPF6 (10)
Mg(OTf)2//Bu4NPF6 (10)
FeCl3 (10)
DCE, 90 oC, 6 h
DCE, 90 oC, 6 h
neat, 90 oC, 1.5 h,
neat, 110 oC, 1.5 h
neat, 70 oC, 1.5 h
neat, 50 oC, 6 h
neat, 70 oC, 12 h
neat, 70 oC, 8 h
neat, 70 oC, 8 h
neat, 70 oC, 6 h
neat, 70 oC, 5 h
neat, 70 oC, 2 h
4
5
6d
7
8
9
10
11
12
13
30
45
55
74
75
aStoichiometric amounts of 1a and 2a were used. bOil bath
temperature. cIsolated yields. dOptimum conditions. nr=no reaction.
rt=room temperature. DCE=1,2-dichloroethane
Having the optimum conditions in hand, we aimed to see the
substrate scope of the method with various alcohol and alkene
partners in the coupling reaction (Table 2). Under the standard
conditions (entry 6 Table 1), styrene derivative 3b was
synthesized from styrene (2b) and benzhydrol (1a) (Table 2,
entry 2) in 82% yield after 1 h. Cyclic alkene, indene (2e) gave 2-
benzyhydryl-indene 3e in 91% yield in 30 minutes. Similarly
benzylic alcohol 1b reacted with alkene 2a and 2b to furnish the
alkenes 3f and 3g in 73%, 82% yields respectively (entries 6, 8).

Driven by the Ca(II) catalyzed dehydrative cross coupling of
alkenes with alcohols, we extended the scope with two different
alcohols to take part in the reaction to furnish alkene derivatives
as shown in the Figure 1, equation-a. However there could be a
self-dimerization possible in these reactions and hence to avoid
this, we chose benzhydrol (1a) as one of the alcohol partner. 1a
was treated with other -activated alcohols such as 2c, 2d and 1b
and furnished the respective alkenes 3c, 3d and 3b in good
yields. The homo dimerization of alcohols and alkenes (Table 2)
was also investigated. Alcohol 1b underwent a dimerization to
form the alkene 3g in in 91% yield after 30 min. Similarly
benzylic alcohols 2c, 2d, 2f, and 1c took part in the dimerization
to yield the corresponding alkenes in good yields (entries 11, 14,
15 and 12, Table 2). Alkene homo-coupling was also observed
when -methylstyrene (2a) alone was subjected under standard
conditions. The reaction gave a mixture of 3j1 and 3j210 in 95%
yield. Other alkenes 2b and 2e gave the respective alkene
derivatives 3g and 3l in good yields. The other possible
combination to make the internal alkenes is the direct cross
coupling between two different alkenes (Figure 1, equation –c),
which was also pursued by treating the indene (2e) with  -
methylstyrene (2a)13 and isolated 2-(2-phenylpropan-2-yl)-1H-
indene 3m in 74% yield after 2 h (entry 18). All these
experimemts proved Ca(OTf)2 as an alternative and a green
catalyst for the dehydrative cross coupling of alcohols-alkenes,
alcohols-alcohols and direct coupling of alkenes to furnish the
alkenes readily.
benzopyrans 5b in 78% after 2.5 h. Similarly, -methylstyrene
(2a) furnished the 2-quaternary centered benzopyrans 5c, 5d, 5e
and 5f in good yields (Table 3).
A plausible mechanism for this cascade synthesis of benzopyrans
is described in the Scheme 3. Benzylic cation 4a reacts with the
alkene and furnish the intermediate 4aa, which further undergo a
cascade oxa-cyclisation to yield the desired product 5. Parallelly,
4ab could be formed by the dehydrative cross coupling of 4 with
alkene, which will undergo a hydroalkoxylation reaction which
was known before by Niggemann et al.11 to furnish the
compound 5 via the formation of 4ab, 4ac and 4ad.
The plausible mechanism for the Ca(OTf)2 catalyzed dehydrative
coupling of alcohols and alkenes is described in the Scheme 2.
Initially Ca(II) promotes the formation of benzylic cation 2fa
from 1-indanol (2f). This benzylic cation 2fa would undergo
elimination (E1) to produce the indene 2e. In the absence of
foreign body, a homo-coupling will take place between 2fa and
2e to produce 2fb (path A), which would further experience
elimination to yield a stable alkene 3l. Alternatively, in presence
of another reactive partner (1a), 2e would participate in a
dehydrative coupling reaction (path B) to form the cation 1ab,
which yields a stable alkene 3e after elimination.
After a successful demonstration of Ca(II) catalyzed green
approach (Table 2) for the coupling reaction of alcohols-alkenes
(entries 1, 2, 5, 6 and 8), alcohols-alcohols (entries 3, 4, 7 and 9),
alkenes-alkenes (entry 18) and homo-coupling of alcohols
(entries 10, 11, 12, 14 and 15) and alkenes (entries 13, 16 and 17)
to form a new C-C bonds (sp3-sp2), we were interested to extend
this method towards the synthesis of novel benzopyran
derivatives12 using a cascade C-C bond formation followed by C-
O bond formation in one pot (Scheme 3).11 As proposed styrene
2b reacted with 2-(hydroxy(phenyl)methyl)phenol (4a) in
All compounds were fully characterized by 1H NMR, 13C
NMR, Mass and IR.13
In summary, we have developed a simple and user-friendly
green synthetic procedure for the synthesis of various internal
alkenes through a dehydrative cross coupling/homo-dimerization
of alcohols and alkenes. The reaction also showed its utility to
furnish the benzopyrans molecules through a dehydrative cross-
coupling/oxa-cyclisation strategy. Solvent free condition, broad
substrate scope, regioselective alkene formation and atom-
economy are some of the highlights of this methodology.
o
presence of 10 mol% Ca(OTf)2/Bu4NPF6 in DCE at 70 C and
furnished the benzopyrans 5a in 76% after 3 h (Table 3). 2b also
reacted with 2-(hydroxy(p-tolyl)methyl)phenol (4b) to yield the

4
Venkataiah, B. Tetrahedron Lett. 2004, 45, 2275; (c)
Acknowledgment
Nishiwaki, N.; Kamimura, R.; Shono, K.; Kawakami, T.;
Nakayama, K.; Nishino, K.; Nakayama, T.; Takahashi, K.;
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(a) Tsukada, N.; Sato, T.; Inoue, Y. Chem. Commun. 2003,
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K. Chem. Commun. 2011, 5031; (c) Tarlani, A.; Riahi, A.;
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Am. Chem. Soc. 2010, 132, 6880; (e) Liu, Z.-Q.; Zhang,
Y.; Zhao, L.; Li, Z.; Wang, J.; Li, H.; Wu, L.- M. Org.
Lett. 2011, 13, 2208; (f) Dai, J.; Wu, J.; Zhao, G.; Dai, W.-
M. Chem. Eur. J. 2011, 17, 8290.
(a) Dada, R.; Singh, G.; Pareek, A.; Kausar, S.; Yaragorla,
S. Tetrahedron Lett. 2016, 57, 3739; (b) Yaragorla, S.;
Dada, R.; Pareek, A.; Singh, G. RSC Adv. 2016, 6, 28865;
(c) Yaragorla, S.; Saini, P. L.; Pareek, A.; Almansour, A.
I.; Arumugam, N. Tetrahedron Lett. 2016, 57, 2034; (d)
Pareek, A.; Dada, R.; Rana, M.; Sharma, A.K.; Yaragorla,
S. RSC Adv. 2016, 6, 89732; (e) Yaragorla, S.; Singh, G.;
Dada, R. Tetrahedron Lett. 2016, 57, 591; (f) Yaragorla,
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Haubenreisser, S.; Niggemann, M. Adv. Synth. Catal.
Two of the authors AP and RD thank Central University of
Rajasthan for the fellowships. The authors acknowledge the
Deanship of Scientific Research at King Saudi University for the
Research grant RGP-026
7.
8.
9.
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General Experimental Procedure (3a - 3m): A mixture of
alcohol 1a (100 mg, 0.54 mmol), alkene 2a (43 mg, 0.54
mmol), Ca(OTf)2 (18.3 mg, 0.05 mmol), Bu4NPF6 (21 mg,
o
0.05 mmol) was heated under solvent free condition at 70 C
for 1.5 h until complete consumption of the starting material
as monitored by TLC. After the completion of the reaction the
mixture was purified by flash column chromatography
(petroleum ether) to afford the desired product 3a. Typical
procedure for the construction of benzopyran (5a – 5f): A
mixture of 2-hydroxy alcohol 4 (1 equiv.), alkene 2a or 2b
(1.2 equiv.) and Ca(OTf)2 (0.1 equiv.), Bu4NPF6 (0.1 equiv.)
was stirred in 1,2-dichloroethane at 70 oC for 1.5 h until
complete consumption of the starting material as monitored
by TLC. After completion, the reaction mixture was diluted
with water and extracted into dichloromethane. Combined
organic layers were washed with brine solution, dried over
anhydrous Na2SO4, evaporated under reduced pressure and the
crude product was purified by column chromatography to
obtain desired product 5. (Please refer the supporting
information for the spectral data and copies of the spectra.)
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Highlights of the Work:
 Highly Versatile and Regioselective method for the synthesis of internal alkenes
 Unique method for dehydrative cross-coupling/homo-dimerization/direct cross couplings
 Successfully extended to the synthesis of novel Benzopyrans
 Solvent-Free; Step, Atom Economy and environmentally benign catalyst
 Large Substrate Scope, High Yields; User-Friendly Protocol