Nickel(II) chloride hexahydrate catalyzed reaction of aromatic aldehydes with 2-mercaptoethanol: Formation of supramolecular helical assemblage of the product

Article abstract of DOI:10.1016/j.tetlet.2013.08.070

Various aromatic aldehydes on reaction with 2-mercaptoethanol provided an unanticipated product, bis(2-hydroxyethyl)dithioacetals (3) as the major product along with the expected product 1,3-oxathiolanes (4) in the presence of 0.05 equiv amount of nickel(

Full text of DOI:10.1016/j.tetlet.2013.08.070

Tetrahedron Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Nickel(II) chloride hexahydrate catalyzed reaction of aromatic
aldehydes with 2-mercaptoethanol: formation
of supramolecular helical assemblage of the product
Rajibul A. Laskar a, Naznin A. Begum a, Mohammad Hedayetullah Mir b, Md. Rumum Rohman c,
Abu T. Khan d,
⇑
a Bio-Organic Chemistry Lab, Department of Chemistry, Visva-Bharati, Santiniketan 731 235, India
b Department of Chemistry, Aliah University, DN 41 Salt Lake, Kolkata 700 091, India
c Department of Chemistry, North-Eastern Hill University, Shillong 793 022, Meghalaya, India
d Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Various aromatic aldehydes on reaction with 2-mercaptoethanol provided an unanticipated product,
bis(2-hydroxyethyl)dithioacetals (3) as the major product along with the expected product 1,3-oxathiol-
anes (4) in the presence of 0.05 equiv amount of nickel(II) chloride hexahydrate (NiCl2Á6H2O) under
solvent-free conditions. Products 3c and 3e exhibit an interesting hydrogen-bonded infinite supra-molec-
ular helical structure.
Received 24 June 2013
Revised 15 August 2013
Accepted 19 August 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Aromatic aldehyde
2-Mercaptoethanol
Bis(2-hydroxyethyl)dithioacetal
1,3-Oxathiolane
Nickel(II) chloride hexahydrate (NiCl2Á6H2O)
Supramolecular helix
1,3-Dithiane derivatives derived from aromatic aldehydes have
been extensively used as valuable building blocks in the synthesis
of natural products1 as well as non-natural2 products. Sometimes
aldehydic functionality is protected as 1,3-oxathiolane, because
of its easy removal at the later stage. Moreover, 2-mercaptoethanol
is cheaper as compared to 1,2-ethanedithiol or 1,3-propanedithiol.
Eliel first demonstrated the usefulness of chiral oxathioacetals de-
Sc(OTf)3,15 In(OTf)3,16 Pr(OTf)4,17a and Y(OTf)4.17b Some of them
have some drawbacks like prolonged reaction time and poor
yields,6,7 requirement of an expensive metal triflate catalyst,15–17
high catalyst loading,6,7,13 and use of volatile organic solvents
namely acetonitrile and dichloromethane. Besides, some of these
catalysts have to be prepared prior to use.11 Though all these meth-
ods are quite useful for the protection of the carbonyl functionality
as oxathioacetals, there is still a need to find out better alternatives
which might work using cheaper and readily available catalyst,
easy to be handled, and environmentally benign reaction
conditions.
rived from (+)pulegone for enantioselective synthesis of tertiary
a-
hydroxy aldehyde,
a
-hydroxy acid, and glycols.3,4 1,3-Dithianes
and 1,3-oxathiane can both serve as carbanion equivalent used
for carbon–carbon bond forming reactions.1–4 Over the years,
numerous synthetic methods have been developed for the protec-
tion of the carbonyl group as dithioacetals as well as oxathioace-
tals.5 1,3-Oxathiolanes are typically prepared from the
corresponding carbonyl compounds by reacting with 2-mercap-
A few years ago, we had reported nickel(II) chloride hexahy-
drate (NiCl2Á6H2O) as
a useful catalyst for chemoselective
dithioacetalization of aldehydes.18a Later on, we had also shown
that a catalytic amount of NiCl2Á6H2O and 1,2-ethanedithiol is a
good combination for the cleavage of THP or TBS ethers.18b Very
recently, we have further demonstrated its usefulness for the syn-
thesis of highly substituted pyrroles.18c All these successful results
prompted us to examine whether NiCl2Á6H2O could be used as a
catalyst for the protection of aldehydes/ketones as 1,3-oxathiol-
anes. In this paper we would like to report the behavior of aromatic
aldehydes with 2-mercaptoethanol in the presence of NiCl2Á6H2O
(Scheme 1).
.
6
toethanol using equimolar amounts of Lewis acid, for e.g., BF3 OEt2
and ZnCl2.7 Recently some more methods have also been reported
in the literature for the protection of the carbonyl groups as 1,3-
oxathiolanes by utilizing various catalysts, viz ZrCl4,8 TMSOTf,9 TIP-
SOTf,10 OATB,11a BDMS,11b HClO4–SiO2,11c LiBF4,12 NBS,13 TBAB,14
⇑
Corresponding author.
E-mail address: atk@iitg.ernet.in (A.T. Khan).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.070
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R. A. Laskar et al. / Tetrahedron Letters xxx (2013) xxx–xxx
OH
O
S
.
NiCl2 6H2O
S
R1
(0.05 equiv.)
R1
OH
+
CHO
S
3a-o
4a-o
HS
OH
2
Major
Minor
R1
1a-o
[Catalyst]6-17
O
S
R1
4a-o
Scheme 1.
Figure 1. (a) Crystal structure of 3c (CCDC No. 913095).28 (b) Depiction of an individual helical strand displaying the O–HÁ Á ÁO hydrogen bonds viewed along a axis.
When the reaction of 4-methoxybenzaldehyde (1c) with
unusual product formation by employing TMSCl as catalyst. How-
ever, lack of a generalized protocol and the use of an expensive cat-
alyst are the demerits of the above protocols.
Next, we carried out the reaction using 2 equiv of 2-mercap-
toethanol due to recovery of unreacted aldehyde from the reaction
mixture. To further optimize the reaction conditions, several trial
reactions were performed in the presence of NiCl2Á6H2O by
employing different mol % of the catalyst as shown in Table 1 (en-
tries 1–6). It was noted that 5 mol % of Ni(II) catalyst was sufficient
for the complete consumption of aldehyde.
To verify the efficacy of other nickel salts, we have examined the
similar reactions with nickel(II) nitrate hexahydrate, nickel(II) ace-
tate dihydrate, and nickel(II) carbonate tetrahydrate and the results
are shown in Table 1. It was observed that nickel(II) nitrate hexahy-
drate gives the products in the ratio 3:2 along with 45% unreacted
aldehyde. On the other hand, nickel(II) acetate dihydrate and nick-
el(II) carbonate tetrahydrate afforded 1,3-oxathiolane derivatives
4c exclusively. From these observations, we may conclude that
nickel(II) chloride hexahydrate is more effective for the unusual
product formation as compared to other nickel(II) salts.
2-mercaptoethanol (1:1) was carried out in the presence of
1 mol % of NiCl2Á6H2O under solvent-free conditions, it gave the
unusual product, bis(2-hydroxyethyl)dithioacetal derivative (3c)
along with the expected product 1,3-oxathiolane (4c) in the ratio
6:4 in 85% overall yield with a recovery of 42% unreacted aldehyde.
Product 3c is relatively more polar in nature and easily separable.
In IR spectrum, compound 3c gives characteristic peaks at
3300 cmÀ1and no carbonyl peak appears for the aldehydic group.
The unusual product 3c was further confirmed from 1H NMR spec-
trum in which benzylic hydrogen appears at d 5.02 whereas in
1,3-oxathiolane (4c) it comes at d 5.99. Further, product 3c gives
a diacetate derivative 5 on acetylation, which was also character-
ized from IR, 1H NMR spectra, and elemental analysis. Moreover,
it shows as S,S-acetal having two free hydroxyl groups in single-
crystal X-ray crystallography data (Fig. 1). It is well-known that
the reaction of aromatic aldehydes with 2-mercaptoethanol usu-
ally gives 1,3-oxathiolane derivatives (4) as the sole product using
different catalysts.6–17 It was found in the literature that only a few
methods are known for unusual product formation using SiO2–
20
NaHSO419, Ce(OTf)3, and TMSCl21 as catalysts. In 2006, we have
After optimizing the reaction conditions,22 benzaldehyde and
other aromatic aldehydes containing electron-donating as well as
electron-withdrawing substituents in the ring (1a–j) were exam-
ined in the presence of a catalytic amount of NiCl2Á6H2O under
identical reaction conditions. The ratio of the products of 3 and 4
as well as overall yield is summarized in Table 2. From these obser-
vations, we have noted that the electron-rich aromatic aldehydes
demonstrated the protection of various carbonyl compounds as
1,3-oxathiolane derivatives (4) using silica supported perchloric
acid.11c However, no unusual product formation was observed like
the silica supported sodium hydrogen sulfate. Using Ce(OTf)3 cata-
lyst, only product 3e was reported from 4-chlorobenzaldehyde.
Similarly, only nitro containing aromatic aldehydes provided the
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R. A. Laskar et al. / Tetrahedron Letters xxx (2013) xxx–xxx
3
Table 1
Optimization of reaction conditions using various Ni(II) salts as catalysta
Entry
Catalyst used
Molar ratio 1&2
Catalyst (mol %)
Reaction time (min)
Aldehyde consumed
Yield %b(over all)
Product ratioc (3c:4c)
1
2
3
4
5
6
9
7
8
No catalyst
NiCl2Á6H2O
NiCl2Á6H2O
NiCl2Á6H2O
NiCl2Á6H2O
NiCl2Á6H2O
Ni(NO3)2Á6H2O
Ni(OAc)2Á2H2O
NiCO3Á4H2O
1:1
1:1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
—
1
1
2
3
5
5
5
5
2 days
30
30
30
30
15
30
30
30
0
0
85
87
89
91
98
82
76
88
—
6:4
7:3
7:3
8:2
9:1
3:2
0:100
0:100
58
45
55
65
100
55
68
46
a
b
c
The reactions were carried out with 4-methoxybenzaldehyde (2.0 mmol) and 2-mercaptoethanol (4.0 mmol).
Yield based on 4-methoxybenzaldehyde consumed.
The ratio of the products was determined from 1H NMR spectra from the integration of benzylic hydrogen of the products.
Table 2
NiCl2Á6H2O catalyzed formation of bis(2-hydroxyethyl)dithioacetals (3) and 1,3-oxathiolanes (4)a
CHO
.
OH
OH
O
S
NiCl2 6H2O
(0.05 equiv.)
S
R1
+
HS
OH
+
R1
S
3a-o
solvent-free / rt
R1
1a-o
2
4a-o
Entry
Aldehyde(1)
Time (min)
Product
(3:4)
Over all % yieldb
3
4
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
Benzaldehyde (1a)
4-Methylbenzaldehyde (1b)
4-Methoxybenzaldehyde (1c)
15
10
15
15
15
15
20
45
35
35
35
20
35
20
25
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
8:1
8:1
9:1
8:1
7:1
7.5:1
5:1
5.5:1
5:1
4.5:1
8:1
89
90
98
94
90
85
88
89
84
87
93
90
86
88
85
2,4-Dimethoxybenzaldehyde (1d)
4-Chlorobenzaldehyde (1e)
4-Bromobenzaldehyde (1f)
4-Fluorobenzaldehyde (1g)
4-Nitrobenzaldehyde (1h)
3-Nitrobenzaldehyde (1i)
2-Nitrobenzaldehyde (1j)
2-Naphthaldehyde (1k)
Cinnamaldehyde (1l)
2:1
3:1
1:1
1:1
2-Furfural (1m)
n-Butylaldehyde (1n)
Cyclohexanecarboxaldehyde (1o)
a
The reactions were carried out with different aldehyde (1.0 mmol) and 2-mercaptoethanol (2.0 mmol).
Isolated yield.
b
(1b–1d) give comparatively better yield of the dithioacetals
(3b–d). Likewise, 2-naphthaldehyde (1k), cinnamaldehyde (1l),
and 2-furfuralaldehyde (1m) also provided the desired bis(2-
hydroxyethyl)dithioacetals (3k–m) along with the 1,3-oxathiolane
derivatives under similar reaction conditions. Similarly, the
aliphatic aldehydes such as n-butylaldehyde (1n) and cyclohex-
anecarboxaldehyde (1o) on reaction with 2-mercaptoethanol un-
der identical reaction conditions provided both the products in
1:1 ratio as shown in Table 2. All the products 3a–o and 4a–o were
fully characterized by recording 1H NMR, 13C NMR spectra, and
their elemental analyses.
To verify the selectivity between aldehyde and ketone, we have
carried out a reaction using 1:1 molar ratio of benzaldehyde (1a)
and cyclohexanone (6) in the presence of 5 mol % NiCl2Á6H2O. We
had recovered the unreacted cyclohexanone after the reaction,
which was confirmed from 1H NMR spectrum (Scheme 2). It indi-
cates that the reaction is highly chemoselective for the protection
of the aldehyde group, which may be due to the reactivity differ-
ence between aldehyde and ketone toward the nucleophile, but
in case of aryl aldehyde and alkyl aldehyde no special selectivity
has been found.
To examine the compatibility of TBS or THP protecting groups,
we have carried out the reaction of TBS and THP ether of 4-
hydroxybenzaldehyde (7a and 7b) under identical reaction condi-
tions separately as shown in Scheme 3. Unfortunately, both TBS
and THP groups were cleaved under the present experimental con-
ditions (Scheme 3).
To understand the formation of the product, we presumed that
aromatic aldehydes first react with 2-mercaptoethanol to form 1,3-
oxathiolanes, which react further with another molecule of 2-
mercaptoethanol to provide the unexpected products 3. To verify
this, we performed a reaction with 1,3-oxathiolane 4a with an-
other equivalent of 2-mercaptoethanol in the presence of the same
catalyst under similar reaction conditions. We did not observe any
formation of product 3a after 4.0 h (Scheme 4). From this study, it
can be concluded that the unusual product (3) formation may not
be going through the opening of the normal product (4). We also
did not observe the conversion of bis(2-hydroxyethyl)dithioacetal
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4
R. A. Laskar et al. / Tetrahedron Letters xxx (2013) xxx–xxx
OH
S
S
CHO
O
OH
S
O
(2 eq.)
HS(CH2)2OH
+
1a
+
4a
3a, 75%
, 10%
.
15 min.
NiCl2 6H2O
O
6, (Unreacted)
6
Scheme 2.
OH
OH
O
S
S
HS
OH
CHO
RO
HO
+ HO
.
NiCl2 6H2O
(0.05 equiv.)
S
9
8
7a
, R = TBS
68%
60%
17%
15%
7b, R = THP
Scheme 3.
Table 3
OH
OH
S
Recyclability of the catalyst NiCl2Á6H2Oa
.
S
NiCl2 6H2O
HS(CH2)2OH,
No. of
cycles
Amount of used
Amount of recovered
Reaction
time (min)
Yieldb
(%)
S
NiCl2Á6H2O (mg)
NiCl2Á6H2O (mg)
O
4 h
( 0% )
0
1
2
3
4
48
46
44
41
39
46
44
41
39
36
15
15
15
15
15
98
96
95
93
92
Scheme 4.
(3c) into 1,3-oxathiolane derivative (4c) even after one year keep-
ing at room temperature, which was observed by Papernaya et al.,
in case of compound 3j.21b
a
The first reaction was carried out with 4-methoxybenzaldehyde (4 mmol) and
2-mercaptoethanol (8 mmol) and successive reactions were performed depending
upon the amount catalyst recovered using aldehyde and 2-mercaptoethanol (1:2)
ratio.
It was reported earlier that NiCl2Á6H2O on reaction with 1,2-eth-
anethiol gives HCl in the medium along with polymeric nickel
complexes Ni(es)n, which might promote the formation of
products.23 We assumed that hydrated nickel(II) chloride might
react with 2-mercaptoethanol to generate HCl in the reaction med-
ium. Consequently, we have executed a reaction between 4-
methoxybenzaldehyde and 2-mercaptoethanol using dry HCl,
which was generated in situ from NaCl and Concd H2SO4. In the
present study, we have isolated 1,3-oxathiolane derivative 3c
exclusively. From all these experimental results, we may put for-
ward a plausible mechanism as depicted in Scheme 5. We believe
b
Isolated yield.
that 2-mercaptoethanol on reaction with NiCl2Á6H2O may provide
nickel(II) compound (X) and dil HCl, which catalyzes the deprotec-
tion of TBS and THP ethers as well as formation of some amount of
1,3-oxathiolane derivatives 4. While doing the reaction, the pH of
the solution was found to be 4–5. Interestingly, green colored
NiCl2Á6H2O can be recovered from the reaction mixture at the
Cl
.
+ H2O
HCl
Ni
x
+
Ni
Cl2 6H2O
+
HS
Ni
OH
S
OH
Cl
S
Ar
OH
OH
ArCHO
H
OH
Ar
1
OH
HO
SH
S
H
2
HO
SH
A
2
OH2
Ar
OH
HS
OH
S
H
Cl
Path A
Ar
S
B
OH2
.
Ni
S
NiCl2 6H2O
- H2O
-
H
Cl
OH
Path B
-HO(CH2)2SH
OH
S
O
Ar
H
Ar
OH
OH
H
S
3
S
4
Scheme 5. Plausible mechanism for the formation of products.
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R. A. Laskar et al. / Tetrahedron Letters xxx (2013) xxx–xxx
5
Figure 2. (a) Crystal structure of 3e (CCDC No. 944760) .28 (b) Depiction of an individual helical strand.
left-handed are obtained in equal amount as racemic mixture by
using chiral ligands.27 Here, in both 3c and 3e the ligands
being achiral the helices exhibit right- and left-handedness in 1:1
ratio.
The crystal structure analysis28 showed that 3c and 3e formed
3D hydrogen-bonded network structure via inter-molecular
hydrogen bonding through O–HÁ Á ÁO–H bond (HÁ Á ÁO = 1.93 and
2.05 Å, OÁ Á ÁO = 2.741 and 2.793 Å, <O–HÁ Á ÁO = 174 and 158° in 3c
and HÁ Á ÁO = 1.88 and 1.91 Å, OÁ Á ÁO = 2.697 and 2.718 Å, <O–
HÁ Á ÁO = 179 and 167° in 3e) as shown in Figs. 1 and 2.
This hydrogen bonding interaction leads to the formation of
helical supramolecular motif. The pitches of helices are 7.561 and
7.960 Å, respectively, corresponding to the b axis of the unit cell
length.
The unit cell packing diagram shows that both the helices make
their direction right-handed and left-handed in 1:1 ratio (Fig. 3).
In conclusion, we have reported the formation of bis(2-
hydroxyethyl)dithioacetal derivatives using NiCl2Á6H2O as a cata-
lyst. The protection mode discussed here is distinctly different
from the conventional method of formation of 1,3-oxathiolane
derivatives. Moreover, our protocol is a general one to provide
unexpected products from aromatic aldehydes with electron-
donating as well as electron-withdrawing substituents on the ring.
It is worth mentioning that our method is useful in the selective
protection of the aldehyde group in the presence of a keto group.
This method is anticipated to have applicability for the selective
protection of various aldehydes owing to its efficiency, cost effec-
tiveness, and recyclability of the catalyst. Finally, an interesting
supramolecular helical assembly of the unexpected products has
been established.
Figure 3. View of the alternating helical strands in 3c along crystallographic a- axis.
end of the reaction. We presume that aromatic aldehydes react
with 2-mercaptoethanol in the presence of dil HCl to give interme-
diates either A or B. The intermediate A is stabilized with nickel(II)
compound X, which reacts with another molecule of 2-mercap-
toethanol to form bis(2-hydroxyethyl)dithioacetals 3. On the other
hand, intermediate B provided conventional 1,3-oxathiolanes 4. It
is noteworthy to mention that nickel(II) acetate dehydrate and
nickel(II) carbonate tetrahydrate do not provide the unusual prod-
uct at all and the detailed mechanism is under investigation, which
will be reported in full paper.
For recovery of the catalyst,24 the reaction between 4-methoxy-
benzaldehyde (4 mmol) and 2-mercaptoethanol (8 mmol) was
done in the presence of 0.20 equiv amount of NiCl2Á6H2O under
identical reaction conditions. After completion of the reaction,
5 mL dichloromethane was added into it and the green colored
NiCl2Á6H2O catalyst was separated out from the reaction mixture.
Then, the catalyst was filtered, dried, and recycled for another four
consecutive cycles as shown in Table 3.
The representative compounds 3c and 3e have been observed to
have serendipitous infinite helical conformation via hydrogen
bonding interactions. Infinite helical structural motif has created
a great deal of interest because of its similarities in biological sys-
tems as well as enantioselective catalysis.25,26 Generally, supramo-
lecular helical motifs have been assembled from flexible chiral or
achiral ligands. When chiral ligands are used, usually enantiomeric
chiral helices have been obtained. However, both right- and
Acknowledgements
A.T.K. acknowledges the DST for providing single XRD-facility to
the Department of Chemistry, IIT Guwahati. R.A.L. and N.A.B. thank
the DST-FIST and UGC-SAP programs of the Department of Chem-
istry, Visva-Bharati for providing instrument facilities. We are
grateful to the referees for their valuable comments and
suggestions.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.08.
070.
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6
R. A. Laskar et al. / Tetrahedron Letters xxx (2013) xxx–xxx
dichloromethane was washed with water, brine solution and dried over
anhydrous Na2SO4. Finally, dichloromethane was removed in rotary
References and notes
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were first eluted followed by compounds
3 during chromatographic
separation. Spectral data of compound 3c: White solid, mp: 88 °C;IR (KBr):
3308 cmÀ1(–OH group); 1H NMR (400 MHz, CDCl3): d 2.12 (t, 2H, J = 6.0 Hz, OH,
D2O exchangeable), 2.66–2.73 (dt, 2H, J = 6.0 Hz, J = 14.0, SCH2), 2.80–2.85 (dt,
2H, J = 6.4 Hz, J = 14.0 Hz, –SCH2À), 3.68–3.74 (q, 4H, J = 6.0 Hz, –OCH2À), 3.78
(s, 3H, OCH3), 5.02 (s, 1H, Ar–CH–), 6.85 (d, 2H, J = 8.4 Hz, Ar–H), 7.36 (d, 2H,
J = 8.8 Hz, Ar–H); 13C NMR (100 MHz, CDCl3): d 35.64 (2C), 52.69, 55.49, 61.48
(2C), 114.26 (2C), 129.04 (2C),132.04, 159.56. Anal. Calcd for C12H18O3S2: C,
52.53; H, 6.61; S, 23.37. Found: C, 52.65; H, 6.34; S, 23.25.
23. Rauchfuss, T. B.; Roundhill, D. M. J. Am. Chem. Soc. 1975, 97, 3386.
24. Typical procedure for the recycling of the catalyst: After completion of the
reaction, dichloromethane was added to the reaction mixture to separate out
the NiCl2Á6H2O catalyst from the reaction mixture as it is not soluble in organic
solvent. Then it was washed with same solvent to eliminate any organic
impurity and dried off to retrieve the catalyst. The same procedure was
repeated after each reaction cycle to isolate and reuse the NiCl2Á6H2O as the
catalyst.
15. Karimi, B.; Ma’mani, L. Synthesis 2003, 2503.
16. Kazahaya, K.; Hamada, N.; Ito, S.; Sato, T. Synlett 2002, 1535.
17. (a) De, S. K. Synthesis 2004, 2837; (b) De, S. K. Tetrahedron Lett. 2004, 45, 2339.
18. (a) Khan, A. T.; Mondal, E.; Sahu, P. R.; Islam, S. Tetrahedron Lett. 2003, 44, 919;
(b) Khan, A. T.; Islam, S.; Choudhury, L. H.; Ghosh, S. Tetrahedron Lett. 2004, 45,
9617; (c) Khan, A. T.; Lal, M.; Bagdi, P. R.; Basha, R. S.; Saravanan, P.; Patra, S.
Tetrahedron Lett. 2012, 53, 4145.
25. (a) Constable, E. C. Comprehensive Supramolecular Chemistry In
; Elsevier
Science: Oxford, 1996; vol. 9,; (b) Prins, J.; Huskens, J.; deJong, F.;
L
Timmerman, P.; Reinhoudt, D. N. Nature 1999, 398, 498.
26. Leong, W. L.; Vittal, J. J. Chem. Rev. 2011, 111, 688.
27. De Brabander, H. F.; Poucke, L. C. V.; Eeckhaut, Z. Inorg. Chim. Acta 1972, 8, 3.
28. Crystal Data: compound 3c: orthorhombic space group P212121, a = 5.5442(6),
19. Azarifar, D.; Forghaniha, A. J. Chin. Chem. Soc. 2006, 53, 1189.
20. Kumar, A.; Rao, M. S.; Rao, V. K. Aust. J. Chem. 2010, 63, 135.
21. (a) Papernaya, L. K.; Shatrova, A. A.; Albanov, A. I.; Klyba, L. V.; Levkovskaya, G.
G. Russ. J. Org. Chem. 2011, 47, 300; (b) Papernaya, L. K.; Shatrova, A. A.;
Albanov, A. I.; Klyba, L. V.; Levkovskaya, G. G. ARKIVOC 2012, vi, 173.
22. Typical experimental procedure: To a stirred mixture of aromatic aldehyde (1
b = 7.5613(9),
q
GOF = 1.042 for I >2
c = 32.258(4) Å,
a
= b =
= 0.388 mmÀ1, T = 296(2) K, R1 = 0.0484, wR2 = 0.1336,
(I), Mok -Ray (k = 71,073 ppm). Crystal Data: compound
c
= 90°,
V = 1352.3(3) Å3,
Z = 4,
calcd = 1.348 g cmÀ3
,
l
r
a
3e: orthorhombic space group P212121, a = 5.2581(3), b = 7.9604(4),
c = 31.262(1) Å,
l
a
= b =
c
= 90°, V = 1308.5(1) Å3, Z = 1,
q
calcd = 1.878 g cmÀ3
,
mmol) and 2-mercaptoethanol (140
l
L, 2.0 mmol) was added NiCl2.6H2O (12
= 0.236 mmÀ1
,
T = 296(2) K, R1 = 0.0438, wR2 = 0.1071, GOF = 1.075 for
I
mg, 0.05 mmol) at room temperature and stirring was continued for 10-45 min
at the same temperature. After completion of the reaction as indicated by TLC,
5 mL of dichloromethane was added into the above reaction mixture to
separate out the catalyst as it was insoluble in it. Then, dichloromethane was
transferred into a separating by decanting carefully and the catalyst was
>2
r(I), Mok -Ray (k = 71,073 ppm). Crystallographic data have been
a
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 913095 and 944760. A copy of
the data can be obtained free of charge from CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK or e-mail: deposit@ccdc.cam.ac.uk.
washed again with another
5 mL of dichloromethane. The combined
Please cite this article in press as: Laskar, R. A.; et al. Tetrahedron Lett. (2013), http://dx.doi.org/10.1016/j.tetlet.2013.08.070