Copper-Catalyzed Three-Component System for Arylsulfenylation of Imidazopyridines with Elemental Sulfur

Article abstract of DOI:10.1021/acs.joc.6b01715

A one-pot three-component reaction for the regioselective synthesis of thioarylated imidazoheterocycles from aryl halides and elemental sulfur using copper(I) iodide as a catalyst has been developed. Reactions proceed with high efficiency and afford thioarylated imidazoheterocycles in good yields with broad functional group tolerance.

Full text of DOI:10.1021/acs.joc.6b01715

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Note
Copper-catalyzed three component system for
arylsulfenylation of imidazopyridines with elemental sulfur
Chitrakar Ravi, N Naresh Kumar Reddy, Venkatanarayana
Pappula, Supravat Samanta, and Subbarayappa Adimurthy
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01715 • Publication Date (Web): 23 Sep 2016
Downloaded from http://pubs.acs.org on September 24, 2016
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Copper-catalyzed three component system for arylsulfenylation of
imidazopyridines with elemental sulfur
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Chitrakar Ravi, N. Naresh Kumar Reddy, Venkatanarayana Pappula, Supravat Samanta and
Subbarayappa Adimurthy*
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Academy of Scientific & Innovative Research, CSIR–Central Salt & Marine Chemicals Research Institute, G.B.
Marg, Bhavnagar-364 002. Gujarat (INDIA). adimurthy@csmcri.org
ABSTRACT:
One-pot three-component reaction for the regioselective synthesis of thioarylated
imidazoheterocycles from aryl halides and elemental sulfur using copper (I) iodide as a catalyst
has been developed. Reactions proceed with high efficiency and afford thioarylated
imidazoheterocycles in good yields with broad functional group tolerance.
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Among bioactive azaheterocycles, particularly pyridine derivatives play a significant role in
medicinal chemistry.1 Since last decade a drastic developments have been achieved on the
synthesis of imidazopyridines, a special attention to imidazo[1,2-a]pyridines as these are
important class of natural products,2 and own substantial applications in pharmaceutical,
biological activities like antiviral, cytotoxic, antibacterial, fungicidal, anti-inflammatory
properties3 including GABA inhibitors.4 Imidazo[1,2-a]pyridine derivatives represent a
significant class of organic molecules with core entities in many of commercially available drugs
including alpidem, zolpidem, olprinone, minodronic acid, zolimidine, necopidem, saripidem and
optically active GSK812397 candidate (HIV infection).5-7
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These heterocyclic moieties also display excited-state of intramolecular proton transfer
properties,8 N-heterocyclic carbenes (NHC) in organometallic chemistry9 and also received
considerable attention as core ligands in the field of electronic devices (fig. S1).10 Hence,
numerous synthetic methods have been developed to access these molecules11 with
functionalization12 and through multicomponent reactions.13
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Scheme 1. Three component sulfenylation
Copper promoted Ullmann type condensations are fundamental reactions for the formation of
carbon–heteroatom (C-N, C-O, etc.) and carbon–carbon (C-C) bonds in organic synthesis. These
are the predominant building blocks for many privileged and biologically active structural
moieties in organic synthesis and in biochemistry.14 Synthesis of functionalized azaheterocycles
are the common approaches of biologically active N-heterocyclies.15 The C−S bond is one of the
most abundant chemical bonds exist in many organic molecules and bio-macromolecules.16
Hence, number of methods have been developed to access sulfenylated heterocycles mainly
using disulfides, thiols, sulfenyl chlorides, or sulfonyl hydrazines.17-19 Although, many organic
sulfenylating reagents are known, but, use of elemental sulfur as a source for C–S bond
formations are of significance interest in organic synthesis.20 Iodobenzene and elemental sulfur
are not being reported for sulfenylation of heterocycles. However, haloarenes are known for
arylations using transition metal catalysis.21 In continuation of our works on the synthesis and
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functionalization of azaheterocyclies,22 we report herein the selective sulfenylation of imidazo[1,
2-a]pyridines through one pot three component system using elemental sulfur and haloarenes as
thioarylation source with copper catalyst (Scheme 1).
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Table 1. Screening of reaction conditionsa
aReaction conditions: 0.25 mmol of 1a, 0.75 mmol of 2a, 0.75 mmol of 3a, CuI (20 mol %), DMF (1mL), 24 h,
130°C, argon balloon, isolated yields. b12 h. c Without sulfur. dO2 balloon.
Aryl iodides and elemental sulfur with mild bases such as metal carbonates/acetates yield
diaryldisulfides,23a however, strong bases such as metal hydroxides initially generate
diaryldisulfides, which in presence of copper generate diarylsulfane.23a To validate our present
approach on sulfenylation of imidazo[1, 2-a]pyridines via three component system, we initially
performed a reaction of 2-phenylimidazo[1,2-a]pyridine (0.25 mmol) 1a, elemental sulfur (0.75
mmol) S8, and 3a iodobenzene (0.75 mmol) in the presence of CuI (20 mol %) as catalyst, and
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Cs2CO3 as base under inert atmosphere (argon balloon) in toluene at 100 °C. Under these
conditions traces of desired product formation was observed (Table 1, entry 1). The desired
product was isolated in 23% yield, when DMF used as solvent instead of toluene (Table 1, entry
2). Surprisingly, the yield was increased to 57% when Na2CO3 was used as base (Table 1, entry
3). Then, the reaction was performed using different catalysts like CuBr, CuCl, Cu(OTf)2,
Ag(OTf)2, Zn(OTf)2, Fe(OTf)2, FeCl2 and FeCl3 but the yield was not improved (Table 1, entries
4-11). When the reaction temperature raised to130°C, gratifyingly, the yield was increased to
91% (Table 1, entry 12). By decreasing the catalyst loading (10 mol %) and reaction time (12 h),
the yield was also decreased (Table 1, entry 13 and 14). No reaction was observed with toluene
and NMP as solvents (Table 1, entry 15 and 16). The better reaction was observed only in DMF,
it may be due to the coordination of DMF with in-situ formed thiol derivative.23a A considerable
yield was obtained in DMSO however, not efficient (Table 1, entry 17). When the reaction was
conducted without sulfur, catalyst, base and oxygen atmosphere (instead of argon) under these
conditions no reaction and low yields were observed (Table 1, entries 18-21). Based on the
results obtained, the optimized conditions were set as: 0.25 mmol of 1a, 0.75 mmol of S8, 0.75
mmol of 3a, in 2.0 mL, at 130°C for 24 h for the present transformation.
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With the optimized conditions in hand (Table 1, entry 12), we investigated the substrate scope of
three component reactionsystem for the sulfenylation of imidazo[1, 2-a]pyridines 1 with aryl
iodides 3 and elemental sulfur (Scheme 2). Initially the reaction of 2-phenylimidazo[1,2-
a]pyridine and elemental sulfur (S8) were examined with iodoarenes. The presence of electron
rich and electron deficient groups at para- iodoarenes reacted smoothly under the optimized
conditions, and gave good to excellent yields (76-98%) of selective C-3 sulfenylated products
4a-e. Unfortunately no reaction was observed in the case of hydroxyl and aldehyde substituted
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iodoarenes 4f-g and traces of desired product was observed with 4-iodobenzamide 4h. Also,
meta/ortho iodoarenes reacted well and gave excellent yields of products 4i-k. Similarly, the
presence of different functional groups like electron releasing and electron with drawing groups
on the phenyl ring of imidazo[1, 2- a]pyridines were also reacted smoothly with iodoarenes and
gave the desired product 4l-q in 57-87% yield, and one of the products 4n was further confirmed
by single-crystal XRD (Figure 1).
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Scheme 2. Scope for three component coupling reactionsa
aReaction conditions: 0.25 mmol of 1, 0.75 mmol of S8, 0.75 mmol of 3, DMF (2mL), Na2CO3 (1.0 mmol), 24 h,
isolated yields.
It was found that, the electronic effects associated with electron-donating/withdrawing
substituents either on the imidazo[1, 2-a]pyridines. Under the same conditions, the reaction of
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(E)-2-styrylimidazo[1,2-a]pyridine and 2-(4-(methylsulfonyl)phenyl)imidazo[1,2-a]pyridine with
4-methyl iodobenzene and elemental sulfur gave the corresponding C-3 sulfenylated products 4r
and 4s in 18% and 56% yield respectively.
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Figure 1. Crystal structure of 4n.
However, only traces of product 4t formation was observed with 2-(thiophen-2-yl)imidazo[1,2-
a]pyridine under these conditions. Then the strategy was extended to 2-phenylimidazo[2,1-
a]isoquinoline derivatives. The presence of electron donating and withdrawing substituents
(methyl, chloro, bromo and nitro) on the 2-phenylimidazo[2,1-a]isoquinoline unit reacted
smoothly and yield the desired products 4u-w in 45-87% yields. The strategy was extended to
iodoheteroarenes like 2-iodopyridine in this case 77% yield of desired product 4z was isolated.
Scheme 3. Various halogen, heterocyclic sources and gram scale synthesis
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Further, we verified the reactivity of other haloarenes such as chlorobenzene 5a, fluoro benzene
5b, bromobenzene 5c, diphenyliodonium chloride 5d, and (diacetoxyiodo)benzene 5e under the
present reaction conditions (Scheme 3, eq.1). Under the optimized conditions, no reaction of 1a
was observed with 5a and 5b. The reaction of 1a with 5c, 5d, and 5e gave the desired
sulfenylated product 6a in 67%, 87% and 64% yield respectively. Compared to 5c and 5d, the 5a
and 5b are less reactive towards diphenyl disulfide intermediate formation through the oxidative
addition with metal. Also, the present reaction conditions were extended to various heterocycles
like indole and imidazole’s, unfortunately no desired product was observed (Scheme 3, eq. 2). To
confirm the feasibility of the process for scale up studies, we synthesized one of the molecule at
gram scale under the same optimized conditions. The reaction of 1 (1.067 g; 5.5 mmol), S8
(0.528 g; 16.5 mmol) and 3 (3.597 g; 16.5 mmol) was subjected and the desired product 4i was
obtained in 73% yield (1.263 g; Scheme 3, eq. 3).
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Scheme 4. Reactivity of haloarenesa
aReaction conditions: 0.25 mmol of 1, 0.75 mmol of S8, 0.75 mmol of 5, DMF (2 mL), Na2CO3 (1.0 mmol) 24 h,
isolated yields. bYield with 5b. cYield with 5c. dYield with 5d.
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Then with these haloarenes (Scheme 3), three component coupling reactions were extended to
different imidazoheterocycles to ascertain the scope of the present methodology (Scheme 4). The
reaction of 2-(p-tolyl)imidazo[1,2-a]pyridine and 2-(4-ethylphenyl)imidazo[1,2-a]pyridine with
haloarenes 5b-d, gave the corresponding sulfenylated products 6b-c with yields ranging from
40% to 84%. Further, the representative imidazopyridines having different substituent groups
(CH3, SCH3, SO2CH3, -Br and –CN) on the phenyl ring were subjected to these conditions with
haloarenes 5b-d, and obtained the corresponding sulfenylated products 6d-h in moderate to good
yields. Under these optimized conditions, the reaction of 1a with 4-bromobenzonitrile gave the
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desired sulfenylated product 6i in 72% yield.
Scheme 5. Control experiments
To understand the reaction mechanism, some selective and control experiments were performed
(Scheme 5). Principally, the reaction of 3-methylimidazo[1,2-a]pyridine 7 containing methyl
group at C-3 position and vacant at C-2 position was conducted with iodobenzene 3a and
elemental sulfur (S8), but the reaction does not yield the desired product 3-methyl-2-
(phenylthio)imidazo[1,2-a]pyridine 8 (Scheme 5, eq. a). This reaction indicates that the
regioselective sulfenylation takes place only at C-3 position of imidazopyridine unit. Further
study of the reaction was conducted by the addition of radical scavenger TEMPO under
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optimized conditions to determine whether the reaction proceeds via a radical pathway or an
ionic path; under these conditions the desired sulfenylated product 4e was isolated in 87% yield,
it indicates the reaction is not going through the radical pathway (Scheme 5, eq. b). When the
reaction of 1a was conducted with only diphenyl disulfide 9, under the optimized conditions,
70% of desired product 4e was obtained (Scheme 5, eq. c). It indicates, 9 may be generated in
situ under these conditions which triggers the reaction. Further, the same reaction was performed
with thiophenol 10, instead of 9, only 16% of desired product was isolated it supports the B and
C are the intermediates in the particular reaction mechanism (Scheme 5, eq. d).
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Scheme 6. Plausible reaction mechanism
Based on the literature reports23 and our above observations (Scheme 5), a plausible reaction
mechanism has been proposed (Scheme 6). Initially, sulfur powder reacts with the base
(Na2CO3) and generates metal sulfide A, and its reaction with haloarenes in presence of copper
by the oxidative addition generates another intermediate B. The intermediate B and C may exist
in equilibrium with each other. The intermediate B reacts with 1a and generates imidazolium
intermediate D.23b-e Finally, through reductive elimination give the desired sulfenylated product
4a through the elimination of thiophenol it will be further converted into di-m-tolylsulfane 13
after reaction with iodobenzene.23f
Conclusion:
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We have revealed a copper catalyzed an expeditious one-pot three component procedure for the
synthesis of sulfenylated imidazo[1, 2-a]pyridine and phenylimidazo[2,1-a]isoquinoline using
commercially available starting substrates. The method having broad substrate scope, with
variety of substituent groups on haloarenes as well as imidazopyridines moiety, and obtained
good to excellent yields of sulfinylated products.
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Experimental Section
General: All commercially available chemicals and reagents were used without any further
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purification unless otherwise indicated. H and C NMR spectra were recorded at 500, and 125
MHz, respectively. The spectra were recorded in CDCl3 as solvent. Multiplicity was indicated as
follows: s (singlet); d (doublet); t (triplet); m (multiplet); dd (doublet of doublets), etc. and
coupling constants (J) were given in Hz. Chemical shifts are reported in ppm relative to TMS as
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an internal standard. The peaks around delta values of H NMR (7.2), and C NMR (77.0) are
correspond to deuterated solvent chloroform respectively. Mass spectra were obtained using
electron impact (EI) HRMS (ESI-TOF) ionization method. Progress of the reactions was
monitored by thin layer chromatography (TLC). All products were purified through column
chromatography using silica gel 100-200 mesh size using hexane/ethyl acetate as eluent unless
otherwise indicated.
General procedure for 4a
A clean washed boiling tube equipped with a magnetic stir bar was charged with imidazo [1, 2-a]
pyridine 1a (0.0485 g, 0.25 mmol), elemental sulfur (S8) (0.024 g, 0.75 mmol), iodobenzene 3a
(0.165 g, 0.75 mmol) copper(I)iodide (0.0095 g, 0.050 mmol) Sodium carbonate (0.106 g,
1mmol) and DMF (2mL), the above mixture was stirred for 24h at 130°C temperature in Argon
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balloon atmosphere. After completion of the reaction, the mixture was poured into 10mL of
sodium bicarbonate solution. The product was extracted with ethyl acetate (10 mL × 3) and dried
with anhydrous Na2SO4. Removal of the solvent under reduced pressure, the left out residue was
purified through column chromatography using silica gel (20% EtOAc/hexane) to obtain 4a in
91 % yield (0.0721g).
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Characterization data
2-phenyl-3-(p-tolylthio)imidazo[1,2-a]pyridine (4a)2
(Eluent: 20% EtOAc/hexane); White solid; 91% yield (72.1 mg), Rf (n-hexane : ethyl acetate =
8:2) = 0.31. m.p.: 98-100°C; 1H NMR (500 MHz, CDCl3) δ 8.18 (d, J = 7.0 Hz, 1H), 8.14 (d, J =
7.5 Hz, 2H), 7.63 (d, J = 9.5 Hz, 1H), 7.34(t, J = 7.5 Hz, 2H), 7.27 (t, J = 7.5 Hz, 1H), 7.21 (t, J
= 8.0 Hz, 1H), 6.92 (d, J = 8.0 Hz, 2H), 6.82 (d, J = 8.5 Hz, 2H), 6.74 (t, J = 7.0 Hz, 1H), 2.15 (s,
3H). 13C NMR (125 MHz, CDCl3) δ 151.1, 146.9, 135.9, 133.3, 131.4, 130.1, 128.4, 128.3,
126.4, 125.7, 124.4, 117.5, 112.9, 106.7, 20.8.
3-((4-methoxyphenyl)thio)-2-phenylimidazo[1,2-a]pyridine (4b)5
(Eluent: 20% EtOAc/hexane); White solid; 97% yield (80.7 mg), Rf (n-hexane : ethyl acetate =
8:2) = 0.20. m.p.: 115-118°C; 1H NMR (500 MHz, CDCl3) δ 8.29 (q, J = 6.5 Hz, 3H), 7.70 (d, J
= 9.0 Hz, 1H), 7.44 (t, J = 7.5 Hz, 2H), 7.36 (t, J = 7.5 Hz, 1H), 7.27 (t, J = 7.5 Hz, 1H), 6.98 (d,
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J = 9.0 Hz, 2H), 6.81 (t, J = 7.5 Hz, 1H), 6.74 (d, J = 9.0 Hz, 2H) 3.67 (s, 3H). 13C NMR (125
MHz, CDCl3) δ 158.4, 150.6, 146.7, 133.3, 128.4, 128.2, 127.8, 126.3, 125.3, 124.3, 117.4,
115.0, 112.8, 107.6, 55.1.
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3-((4-fluorophenyl)thio)-2-phenylimidazo[1,2-a]pyridine (4c)
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(Eluent: 20% EtOAc/hexane); White solid; 98% yield (78.0 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.28. m.p.: 130-132°C; 1H NMR (500 MHz, CDCl3) δ 8.24 (q, J = 6.4 Hz, 3H), 7.71 (d, J
= 9.0 Hz, 1H), 7.45 (t, J = 7.5 Hz, 2H), 7.36 (t, J = 7.0 Hz, 1H), 7.29 (t, J = 7.0 Hz, 1H), 6.98-
6.95 (m, 2H), 6.88 (t, J = 8.5 Hz, 2H), 6.83 (t, J = 7.0 Hz, 1H). 13C NMR (125 MHz, CDCl3) δ
162.3 (d, JC-F = 244.6 Hz), 151.2, 146.9, 133.1, 129.9, 128.5, 128.3, 128.2, 127.5 (d, JC-F = 7.6
Hz),126.6, 124.1, 117.6, 116.5 (d, JC-F = 22.1 Hz),113.0, 106.4. HRMS (ESI-TOF) m/z: calcd for
C19H14N2FS: 321.0862, found: 321.0851.
4-((2-phenylimidazo[1,2-a]pyridin-3-yl)thio)benzonitrile (4d)
(Eluent: 20% EtOAc/hexane); White solid; 76% yield (61.5 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.14. m.p.: 192-195°C; 1H NMR (500 MHz, CDCl3) δ 8.18 (d, J = 7.0 Hz, 1H), 8.11 (d, J
= 7.5 Hz, 2H), 7.76 (d, J = 9.0 Hz, 1H), 7.46-7.40 (m, 4H), 7.36 (t, J = 7.0 Hz, 2H), 7.02 (d, J =
8.5 Hz, 2H), 6.90 (t, J = 7.0 Hz, 1H). 13C NMR (125 MHz, CDCl3) δ 152.1, 147.4, 142.4, 132.8,
128.8, 128.4, 128.1, 127.1, 125.3, 124.0, 118.3, 117.8, 113.4, 109.3,103.5. HRMS (ESI-TOF)
m/z: calcd for C20H14N3S: 328.0908, found: 328.0921.
2-phenyl-3-(phenylthio)imidazo[1,2-a]pyridine (4e)1
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(Eluent: 20% EtOAc/hexane); White solid; 90% yield (68.0 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.31. m.p.: 95-98°C; 1H NMR (500 MHz, CDCl3) δ 8.26(d, J = 7.0 Hz, 1H), 8.21(d, J =
7.5 Hz, 2H), 7.73(d, J = 9 Hz, 1H), 7.44(t, J = 7.5 Hz, 2H), 7.36 (t, J = 7.0 Hz, 1H), 7.31 (t, J =
7.0 Hz, 1H), 7.19(t, J = 7.5 Hz, 2H), 7.11 (t, J = 7.0 Hz, 1H), 7.00 (d, J = 7.5 Hz, 2H), 6.84 (t, J
=6.5, 1H). 13C NMR (125 MHz, CDCl3) δ 151.3, 147.0, 135.1, 133.2, 129.3, 128.5, 128.35,
128.30 126.5, 125.9, 125.4, 124.4, 117.6, 113.0, 106.2.
2-phenyl-3-(m-tolylthio)imidazo[1,2-a]pyridine (4i)
(Eluent: 20% EtOAc/hexane); White solid; 97% yield (77.0 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.31. m.p.: 83-85°C; 1H NMR (500 MHz, CDCl3) δ 8.12(d, J = 7.5 Hz, 3H), 7.60 (d, J =
9.0 Hz, 1H), 7.31 (t, J = 8.0 Hz, 2H), 7.23 (t, J = 7.5 Hz, 1H), 7.16 (t, J = 7.0 Hz, 1H), 6.94 (t, J
= 8.0 Hz, 1H), 6.80 (d, J = 8.5 Hz, 1H), 6.73 (s, 1H), 6.68 (t, J = 7.0 Hz, 1H) , 6.64 (d, J = 8.0
Hz, 1H), 2.08 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 151.1, 146.9, 139.2,134.7, 133.2, 129.1,
128.4, 128.26, 128.21, 126.8, 126.4, 125.8, 124.3, 117.4, 112,8, 106.2, 21.2. HRMS (ESI-TOF)
m/z: calcd for C20H17N2S: 317.1112, found: 317.1115.
2-phenyl-3-(o-tolylthio)imidazo[1,2-a]pyridine (4j)
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(Eluent: 20% EtOAc/hexane); White solid; 97% yield (77.7 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.28. m.p.: 130-132°C; 1H NMR (500 MHz, CDCl3) δ 8.17(q, J = 8.0 Hz, 3H), 7.72 (d, J
= 9.0 Hz, 1H), 7.40 (t, J = 7.0 Hz, 2H), 7.33 (t, J = 7.0 Hz, 1H), 7.27 (t, J = 7.5 Hz, 1H), 7.17 (d,
J = 7.5 Hz, 1H), 7.01 (t, J = 7.5 Hz, 1H), 6.87 (t, J = 7.5 Hz, 1H), 6.80 (q, J = 7.0 Hz, 1H) , 6.41
(d, J = 8.0 Hz, 1H), 2.49 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 151.5, 147.1, 134.7, 133.9,
133.2, 130.5, 128.4, 128.26, 128.23, 126.8, 126.4, 125.5, 124.3, 123.9, 117.5, 112.9, 105.4, 19.6.
HRMS (ESI-TOF) m/z: calcd for C20H17N2S: 317.1112, found: 317.1098.
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3-((3-bromophenyl)thio)-2-phenylimidazo[1,2-a]pyridine (4k)
(Eluent: 20% EtOAc/hexane); White solid; 95% yield (90.2 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.25. m.p.: 128-132°C; 1H NMR (500 MHz, CDCl3) δ 8.23(d, J = 6.5 Hz, 1H), 8.17(d, J =
9.0 Hz, 2H), 7.74 (d, J = 9.0 Hz, 1H), 7.43 (t, J = 7.5 Hz, 2H), 7.39-7.32 (m, 2H), 7.25 (d, J = 8.0
Hz, 1H), 7.18 (s, 1H), 6.88-6.83 (m, 3H). 13C NMR (125 MHz, CDCl3) δ 151.7, 147.2, 137.6,
133.0, 130.6, 129.1, 128.7, 128.4, 128.2, 127.9, 126.8, 124.2, 123.7, 123.3, 117.6, 113.2, 104.9.
HRMS (ESI-TOF) m/z: calcd for C19H14N2SBr: 381.0061, found: 381.0061.
2-(p-tolyl)-3-(p-tolylthio)imidazo[1,2-a]pyridine (4l)4
(Eluent: 20% EtOAc/hexane); White solid; 87% yield (71.6 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.31. m.p.: 138-141°C; 1H NMR (500 MHz, CDCl3) δ 8.23 (d, J = 6.5 Hz, 1H), 8.13 (d, J
= 8.0 Hz, 2H), 7.69 (d, J = 7.0 Hz, 1H), 7.27 (d, J = 7.5 Hz, 2H), 7.23 (d, J = 7.0 Hz, 2H), 6.99
(d, J = 8.5 Hz, 2H), 6.89 (d, J = 8.0 Hz, 2H), 6.79 (t, J = 6.5 Hz, 1H), 2.36 (s, 3H), 2.22 (s, 3H).
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13C NMR (125 MHz, CDCl3) δ 151.1, 146.8, 138.3, 135.8, 131.5, 130.4, 130.0, 129.0, 128.1,
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126,3, 125.6, 124.3, 117.3, 112.7, 106.3, 21.2, 20.7.
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(Eluent: 20% EtOAc/hexane); White solid; 85% yield (83.4 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.33. m.p.: 148-150°C; 1H NMR (500 MHz, CDCl3) δ 8.24 (d, J = 7.0 Hz, 1H), 8.13 (d, J
= 8.5 Hz, 2H), 7.68 (d, J = 9.0 Hz, 1H), 7.53 (d, J = 8.5 Hz, 2H), 7.27 (t, J = 7.5 Hz, 1H), 6.99
(d, J = 8.0 Hz, 2H), 6.87 (d, J = 8.0 Hz, 2H), 6.81 (t, J = 6.5 Hz, 1H), 2.21 (s, 3H). 13C NMR
(125 MHz, CDCl3) δ 149.7, 146.8, 136.0, 132.2, 131.4, 130.9, 130.1, 129.6, 126.6, 125.7, 124.3,
122.7, 117.4, 113.0, 106.9, 20.7.
2-(4-chlorophenyl)-3-(p-tolylthio)imidazo[1,2-a]pyridine (4n)2
(Eluent: 20% EtOAc/hexane); White solid; 80% yield (69.9 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.36. m.p.: 135-138°C; 1H NMR (500 MHz, CDCl3) δ 8.25 (d, J = 7.0 Hz, 1H), 8.19 (d, J
= 8.5 Hz, 2H), 7.69 (d, J = 9.0 Hz, 1H), 7.38 (d, J = 8.5 Hz, 2H), 7.28 (t, J = 7.0 Hz, 1H), 7.00
(d, J = 8.0 Hz, 2H), 6.88 (d, J = 8.5 Hz, 2H), 6.82 (t, J = 7.0 Hz, 1H), 2.22 (s, 3H). 13C NMR
(125 MHz, CDCl3) δ 149.7, 146.8, 136.0, 134.3, 131.8, 131.0, 130.1, 129.4, 128.4, 126.6, 125.7,
124.3, 117.4, 113.0, 106.9, 20.7.
7-methyl-2-(p-tolyl)-3-(p-tolylthio)imidazo[1,2-a]pyridine (4o)
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(Eluent: 20% EtOAc/hexane); White solid; 57% yield (48.9 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.30. m.p.: 132-135°C; 1H NMR (500 MHz, CDCl3) δ 8.11 (d, J = 8.0 Hz, 2H), 8.06 (s,
1H), 7.62 (d, J = 9.0 Hz, 1H), 7.23 (d, J = 8.0 Hz, 2H), 7.16 (d, J = 10.5 Hz, 1H), 7.02 (d, J = 8.5
Hz, 2H), 6.92 (d, J = 8.5 Hz, 2H), 2.37 (s, 3H), 2.29 (s, 3H), 2.25 (s, 3H). 13C NMR (125 MHz,
CDCl3) δ 151.1, 145.9, 138.2, 135.7, 131.9, 130.6, 130.0, 129.5, 129.0, 128.0, 125.5, 122.6,
122.1, 116.7, 105.7, 21.2, 20.8, 18.3. HRMS (ESI-TOF) m/z: calcd for C22H21N2S: 345.1425,
found: 345.1421.
2-(4-chlorophenyl)-7-methyl-3-(p-tolylthio)imidazo[1,2-a]pyridine (4p)4
(Eluent: 20% EtOAc/hexane); White solid; 70% yield (63.8 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.40. m.p.: 135-138°C; 1H NMR (500 MHz, CDCl3) δ 8.12 (d, J = 7.0 Hz, 1H), 8.08 (d, J
= 8.0 Hz, 2H), 7.47 (s, 1H), 7.23 (t, J = 8.0 Hz, 2H), 7.19 (t, J = 7.5 Hz, 2H), 7.11 (t, J = 7.5 Hz,
1H), 6.99 (d, J = 8.0 Hz, 2H), 6.67 (d, J = 7.0 Hz, 1H), 2.42 (s, 3H), 2.37 (s, 3H). 13C NMR
(125 MHz, CDCl3) δ 151.3, 147.4, 138.3, 137.7, 135.5, 130.5, 129.3, 129.0, 128.1, 125.8, 125.4,
123.5, 116.0, 115.5, 105.0, 21.36, 21.31.
4-(7-methyl-3-(p-tolylthio)imidazo[1,2-a]pyridin-2-yl)benzonitrile (4q)2
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(Eluent: 20% EtOAc/hexane); White solid; 69% yield (59.2 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.43. m.p.: 168-170°C; 1H NMR (500 MHz, CDCl3) δ 8.39 (d, J = 6.5 Hz, 2H), 8.15 (d, J
= 6.5 Hz, 1H), 7.68 (d, J = 6.5 Hz, 2H), 7.13 (d, J = 7.0 Hz, 1H), 7.01 (d, J = 8.0 Hz, 2H), 6.87
(d, J = 8.0 Hz, 2H), 6.79 (t, J = 7.0 Hz, 1H), 2.68 (s, 3H), 2.24 (s, 3H). 13C NMR (125 MHz,
CDCl3) δ 148.0, 147.3, 138.1, 136.2, 132.0, 130.8, 130.2, 128.6, 127.9, 125.7, 122.2, 119.0,
113.4, 111.3, 108.4, 20.7, 16.6.
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(E)-2-styryl-3-(p-tolylthio)imidazo[1,2-a]pyridine (4r)2
(Eluent: 20% EtOAc/hexane); White solid; 18% yield (13.3 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.20. m.p.: 114-116°C; 1H NMR (500 MHz, CDCl3) δ 8.20 (d, J = 7.0 Hz, 1H), 7.81 (d, J
= 16.0 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 7.5 Hz, 2H), 7.49 (d, J = 16.5 Hz, 1H), 7.37
(t, J = 7.5 Hz, 2H), 7.32 – 7.27 (m, 2H ), 7.02 (d, J = 8.0 Hz, 2H), 6.95 (d, J = 8.0 Hz, 2H),
6.82(t, J = 7.0 Hz, 1H), 2.25 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 149.9, 147.4, 137.0,
136.3, 132.6, 131.3, 130.1, 128.6, 128.0, 126.9, 126.5, 124.3, 118.2, 117.2, 112.6, 109.6, 20.8.
2-(4-(methylsulfonyl)phenyl)-3-(p-tolylthio)imidazo[1,2-a]pyridine (4s)
(Eluent: 20% EtOAc/hexane); White solid; 56% yield (54.7 mg), Rf (n-hexane: ethyl acetate =
5:5) = 0.26. m.p.: 174-176°C; 1H NMR (500 MHz, CDCl3) δ 8.48 (d, J = 8.5 Hz, 2H), 8.32 (d, J
= 7.0 Hz, 1H), 8.00 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 9.0 Hz, 1H), 7.37 (t, J = 7.5 Hz, 1H), 7.03
(d, J = 8.0 Hz, 2H), 6.92 (q, J = 6.5 Hz, 3H), 3.07 (s, 3H). 2.25 (s, 3H ). 13C NMR (125 MHz,
CDCl3) δ 148.5, 147.0, 139.7, 138.8, 136.4, 130.5, 130.2, 128.8, 127.3, 127.1, 125.8, 124.5,
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117.8, 113.5, 108.5, 44.4, 20.7. HRMS (ESI-TOF) m/z: calcd for C21H19N2O2S2: 395.0888,
found: 395.0877.
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(Eluent: 10% EtOAc/hexane); White solid; 78% yield (73.9 mg), Rf (n-hexane: ethyl acetate =
9:1) = 0.53. m.p.: 205-208°C; 1H NMR (500 MHz, CDCl3) δ 8.80 (d, J = 8.0 Hz, 1H), 8.16 (d, J
= 8.0 Hz, 2H), 8.10 (d, J = 7.5 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.68 (t, J = 7.5 Hz, 1H), 7.60
(t, J = 8.0 Hz, 1H), 7.25 (d, J = 7.5 Hz, 2H), 7.08 (d, J = 8.0 Hz, 1H) , 7.00 (d, J = 8.0 Hz, 2H),
6.93 (d, J = 8.0 Hz, 2H), 2.38 (s, 3H), 2.23 (s, 3H ). 13C NMR (125 MHz, CDCl3) δ 149.6,
144.8, 138.0, 135.8, 132.1, 130.7, 130.2, 130.1, 129.1, 128.8, 128.1, 128.0, 126.9, 125.7, 123.6,
121.2, 113.3, 108.4, 29.6, 21.3. HRMS (ESI-TOF) m/z: calcd for C25H21N2S : 381.1425, found:
381.1423.
2-(4-chlorophenyl)-3-(p-tolylthio)imidazo[2,1-a]isoquinoline (4v)
(Eluent: 10% EtOAc/hexane); White solid; 87% yield (86.6 mg), Rf (n-hexane: ethyl acetate =
9:1) = 0.60. m.p.: 209-212°C; 1H NMR (500 MHz, CDCl3) δ 8.78 (d, J = 8.0 Hz, 1H), 8.25 (d, J
= 8.0 Hz, 2H), 8.11 (d, J = 7.5 Hz, 1H), 7.74 (d, J = 7.5Hz, 1H), 7.69 (t, J = 7.0 Hz, 1H), 7.63 (t,
J = 7.5 Hz, 1H), 7.42 (d, J = 8.5 Hz, 2H), 7.12 (d, J = 7.5 Hz, 1H) , 7.02 (d, J = 8.0 Hz, 2H), 6.93
(d, J = 8.5 Hz, 2H), 2.25 (s, 3H ). 13C NMR (125 MHz, CDCl3) δ 148.2, 144.9, 136.1, 134.1,
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132.1, 131.6, 130.2, 129.3, 129.0, 128.5, 128.2, 126.9, 125.8, 123.5, 123.4, 121.1, 113.6, 109.1,
20.8. HRMS (ESI-TOF) m/z: calcd for C24H18N2SCl: 401.0879, found: 401.0868.
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(Eluent: 10% EtOAc/hexane); White solid; 62% yield (62.3 mg), Rf (n-hexane: ethyl acetate =
9:1) = 0.56. m.p.: 180-183°C; 1H NMR (500 MHz, CDCl3) δ 8.80 (d, J = 8.0 Hz, 1H), 8.33 (s,
1H), 8.19 (d, J = 8.0 Hz, 1H), 8.12 (d, J = 7.5Hz, 1H), 7.75 (d, J = 7.0 Hz, 1H), 7.69 (t, J = 7.5
Hz, 1H), 7.63 (t, J = 7.0 Hz, 1H), 7.38-7.32 (m, 2H) , 7.13 (d, J = 7.5 Hz, 1H), 7.02 (d, J = 8.0
Hz, 2H), 6.94 (d, J = 8.0 Hz, 2H), 2.25 (s, 3H ). 13C NMR (125 MHz, CDCl3) δ 147.8, 136.2,
135.4, 134.3, 131.5, 130.3, 130.2, 129.6, 129.0, 128.9, 128.8, 128.7, 128.6, 128.3, 128.1, 127.0,
126.1, 126.0, 123.6, 123.5, 121.1, 113.7, 109.6, 20.8. HRMS (ESI-TOF) m/z: calcd for
C24H18N2SCl: 401.0879, found: 401.0868.
2-(3-bromophenyl)-3-(p-tolylthio)imidazo[2,1-a]isoquinoline (4x)
(Eluent: 10% EtOAc/hexane); White solid; 63% yield (69.9 mg), Rf (n-hexane: ethyl acetate =
9:1) = 0.56. m.p.: 172-175°C; 1H NMR (500 MHz, CDCl3) δ 8.78 (d, J = 8.0 Hz, 1H), 8.48 (s,
1H), 8.23 (d, J = 7.5 Hz, 1H), 8.12 (d, J = 7.5Hz, 1H), 7.75 – 7.68 (m, 2H), 7.65 (t, J = 7.0 Hz,
1H), 7.49 (d, J = 7.0 Hz, 1H), 7.29 (t, J = 8.0 Hz, 1H), 7.13 (d, J = 7.0 Hz, 1H), 7.02 (d, J = 8.0
Hz, 2H), 6.94 (d, J = 7.5 Hz, 2H), 2.25 (s, 3H ). 13C NMR (125 MHz, CDCl3) δ 147.6, 144.9,
136.2, 135.6, 131.5, 131.0, 130.9, 130.3, 130.2, 129.8, 129.0, 128.8, 128.3, 127.0, 126.5, 126.0,
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123.6, 122.6, 121.1, 113.7, 109.7, 20.8. HRMS (ESI-TOF) m/z: calcd for C24H18N2S Br:
445.0374, found: 445.0394.
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(Eluent: 10% EtOAc/hexane); Yellow solid; 45% yield (46.1 mg), Rf (n-hexane: ethyl acetate =
9:1) = 0.40. m.p.: 215-220°C; 1H NMR (500 MHz, CDCl3) δ 9.15 (s, 1H), 8.73 (d, J = 8.0 Hz,
1H), 8.59 (d, J = 8.0 Hz, 1H), 8.12 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 7.0 Hz, 1H), 7.69 (d, J = 8.0
Hz, 1H), 7.66 (t, J = 7.0 Hz, 1H), 7.58 (t, J = 7.0 Hz, 1H), 7.52 (t, J = 7.0 Hz, 1H), 7.09 (d, J =
7.5 Hz, 1H), 6.96 (d, J = 8.5 Hz, 2H), 6.88 (d, J = 7.0 Hz, 2H), 2.17 (s, 3H ). 13C NMR (125
MHz, CDCl3) δ 148.5, 146.4, 145.0, 136.5, 135.4, 133.7, 131.1, 130.3, 129.2, 128.5, 127.0,
126.1, 123.6, 123.5, 123.0, 122.6, 121.0, 114.1, 110.5, 20.8. HRMS (ESI-TOF) m/z: calcd for
C24H18N3O2S : 412.1120, found: 412.1125.
2-phenyl-3-(pyridin-2-ylthio)imidazo[1,2-a]pyridine (4z)
(Eluent: 50% EtOAc/hexane); white solid; 77% yield (56.5 mg), Rf (n-hexane: ethyl acetate =
6:4) = 0.36. m.p.: 120-122°C; 1H NMR (500 MHz, CDCl3) δ 8.42 (d, J = 4.0 Hz, 1H), 8.31 (d, J
= 7.0 Hz, 1H), 8.19 (d, J = 7.5 Hz, 2H), 7.75 (d, J = 9.0 Hz, 1H), 7.43-7.39 (m, 3H), 7.36-7.27
(m, 2H), 7.03 (q, J = 5.0 Hz, 1H), 6.88 (t, J = 7.0 Hz, 1H), 6.63 (d, J = 8.0 Hz, 1H). 13C NMR
(125 MHz, CDCl3) δ 158.4, 151.3, 150.1, 147.2, 137.2, 136.9, 133.1, 128.5, 128.3, 126.7, 125.8,
124.4, 120.5, 119.3, 117.6, 113.0, 105.2. HRMS (ESI-TOF) m/z: calcd for C18H14N3S :
304.0908, found: 304.0907.
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3-(phenylthio)-2-(p-tolyl)imidazo[1,2-a]pyridine (6b)1
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(Eluent: 20% EtOAc/hexane); White solid; 84% yield (66.2 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.20. m.p.: 118-120°C; 1H NMR (500 MHz, CDCl3) δ 8.26 (d, J = 7.0 Hz, 1H), 8.12 (d, J
= 8.0 Hz, 2H), 7.73 (d, J = 6.0 Hz, 1H), 7.30 (t, J = 7.5 Hz, 1H), 7.25 (d, J = 8.0 Hz, 2H), 7.19
(t, J = 7.5 Hz, 2H), 7.13 (t, J = 7.0 Hz, 1H), 7.10 (d, J = 7.5 Hz, 2H), 6.83 (t, J = 7.0 Hz, 1H),
2.38 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 151.4, 147.0, 138.4, 135.2, 130.4, 129.3, 129.0,
128.1, 126.4, 125.9, 125.4, 124.3, 117.4, 112.8, 105.8, 21.2.
2-(4-ethylphenyl)-3-(phenylthio)imidazo[1,2-a]pyridine (6c)1
(Eluent: 20% EtOAc/hexane); White solid; 58% yield (47.4 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.26. m.p.: 98-100°C; 1H NMR (500 MHz, CDCl3) δ 8.26 (d, J = 6.0 Hz, 1H), 8.14 (d, J =
7.5 Hz, 2H), 7.72 (d, J = 9.0 Hz, 1H), 7.31 (t, J = 8.0 Hz, 1H), 7.27 (d, J = 8.0 Hz, 2H), 7.19 (t, J
= 7.5 Hz, 2H), 7.12 (t, J = 7.0 Hz, 1H), 7.00 (d, J = 8.0 Hz, 2H), 6.84 (t, J = 7.0 Hz, 1H), 2.70 (q,
J = 7.5 Hz, 2H), 1.25 (t, J = 8.0 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ 151.5, 147.0, 144.7,
135.2, 130.6, 129.3, 128.2, 127.9, 126.5, 125.9, 125.4, 124.4, 117.5, 112.9, 105.8, 28.6, 15.3.
6-methyl-2-phenyl-3-(phenylthio)imidazo[1,2-a]pyridine (6d)1
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(Eluent: 20% EtOAc/hexane); yellow solid; 47% yield (37.2 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.23. m.p.: 116-118°C; 1H NMR (500 MHz, CDCl3) δ 8.17 (d, J = 7.5 Hz, 2H), 8.06 (s,
1H), 7.64 (d, J = 9.0 Hz, 1H), 7.41 (t, J = 7.5 Hz, 2H), 7.36 (d, J = 7.5 Hz, 1H), 7.23 – 7.17 (m,
3H), 7.13 (t, J = 7.5 Hz, 1H), 7.00 (d, J = 7.0 Hz, 2H), 2.31(s, 3H). 13C NMR (125 MHz,
CDCl3) δ 151.2, 146.1, 135.5, 133.4, 129.7, 129.4, 128.4, 128.3, 128.2, 125.8, 125.3, 122.9,
122.1, 116.9, 18.3.
2-(4-(methylthio)phenyl)-3-(phenylthio)imidazo[1,2-a]pyridine (6e)1
(Eluent: 20% EtOAc/hexane); yellow solid; 59% yield (51.4 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.13. m.p.: 106-108°C; 1H NMR (500 MHz, CDCl3) δ 8.27 (d, J = 7.0 Hz, 1H), 8.17 (d, J
= 8.5 Hz, 2H), 7.72 (d, J = 9.0 Hz, 1H), 7.34 (q, J = 7.0 Hz, 3H), 7.21 (t, J = 7.5 Hz, 2H), 7.13 (t,
J = 7.5 Hz, 1H), 7.00 (d, J = 8.0 Hz, 2H), 6.86 (t, J = 6.5 Hz, 1H), 2.50 (s, 3H). 13C NMR (125
MHz, CDCl3) δ 150.8, 147.0, 139.2, 135.0, 129.9, 129.4, 128.5, 127.6, 126.6, 126.0, 125.5,
124.4, 117.5, 113.0, 105.9, 15.4.
2-(4-bromophenyl)-3-(phenylthio)imidazo[1,2-a]pyridine (6f)1
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(Eluent: 20% EtOAc/hexane); White solid; 80% yield (75.8 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.33. m.p.: 148-150°C; 1H NMR (500 MHz, CDCl3) δ 8.26 (d, J = 6.5 Hz, 1H), 8.11 (d, J
= 8.5 Hz, 2H), 7.71 (d, J = 9.0 Hz, 1H), 7.55 (d, J = 8.5 Hz, 2H), 7.33 (t, J = 7.0 Hz, 1H), 7.20 (t,
J = 7.5 Hz, 2H), 7.13 (t, J = 7.0 Hz, 1H), 6.98 (d, J = 7.5 Hz, 2H), 6.86 (t, J = 7.0 Hz, 1H). 13C
NMR (125 MHz, CDCl3) δ 150.1, 147.0, 134.7, 132.2, 131.5, 129.7, 129.4, 126.8, 126.1, 125.5,
124.4, 122.8, 117.6, 113.1, 106.4.
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2-(4-(methylsulfonyl)phenyl)-3-(phenylthio)imidazo[1,2-a]pyridine (6g)1
(Eluent: 40% EtOAc/hexane); White solid; 35% yield (33.6 mg), Rf (n-hexane: ethyl acetate =
5:5) = 0.26. m.p.: 188-190°C; 1H NMR (500 MHz, CDCl3) δ 8.46 (d, J = 6.5 Hz, 2H), 8.32 (d, J
= 7.0 Hz, 1H), 8.00 (d, J = 8.5 Hz, 2H), 7.76 (d, J = 9.0 Hz, 1H), 7.39 (t, J = 8.0 Hz, 1H), 7.23 (t,
J = 7.0 Hz, 2H), 7.16 (t, J = 7.5 Hz, 1H), 6.99 (d, J = 7.5 Hz, 2H), 6.93 (t, J = 6.5 Hz, 1H), 3.07
(s, 3H). 13C NMR (125 MHz, CDCl3) δ 148.8, 147.2, 139.8, 138.7, 134.3, 129.5, 128.9, 127.4,
127.3, 126.4, 125.6, 124.5, 117.9, 113.6, 107.9, 44.5.
4-(8-methyl-3-(phenylthio)imidazo[1,2-a]pyridin-2-yl)benzonitrile (6h)
(Eluent: 30% EtOAc/hexane); White solid; 93% yield (79.6 mg), Rf (n-hexane: ethyl acetate =
8:2) = 0.23. m.p.: 168-171°C; 1H NMR (500 MHz, CDCl3) δ 8.56 (s, 1H), 8.48 (d, J = 7.5 Hz,
1H), 8.16 (d, J = 7.0 Hz, 1H), 7.63 (d, J = 7.5 Hz, 1H), 7.52 (t, J = 7.5 Hz, 1H), 7.21 (t, J = 7.5
Hz, 2H), 7.17 (q, J = 5.0 Hz, 2H), 6.97 (d, J = 8.0 Hz, 2H), 6.84 (t, J = 7.0 Hz, 1H), 2.71 (s, 3H).
13C NMR (125 MHz, CDCl3) δ 148.3, 147.5, 135.0, 134.6, 132.4, 131.9, 131.6, 129.5, 129.3,
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129.1, 128.0, 126.3, 125.9, 125.6, 122.3, 118.9, 113.5, 112.6, 107.5, 16.6. HRMS (ESI-TOF)
m/z: calcd for C21H16N3S: 342.1065, found: 342.1057.
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(Eluent: hexane); liquid; Rf (n-hexane) = 0.43.; 1H NMR (500 MHz, CDCl3) δ 7.29 (d, J = 7.5
Hz, 4H), 7.17 (t, J = 7.5 Hz, 2H), 7.02 (d, J = 7.5 Hz, 2H), 2.30 (s, 6H). 13C NMR (125 MHz,
CDCl3) δ 138.8, 136.8, 128.8, 127.9, 124.5, 21.3.
Supporting Information:
Copies of NMR spectra for all compounds and HRMS spectra for new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for
compound 4n (CCDC- 1484511) can be obtained free of charge from the Cam-bridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ACKNOWLEDGMENT
CSIR-CSMCRI Communication No. 093/2016. C.R., N.N.K.R, P.V. and S.S are thankful
to AcSIR for Ph.D. enrollment and the “Analytical Discipline and Centralized
Instrumental Facilities” for providing instrumentation facilities and special thanks to Dr.
E. Suresh for providing the crystal data. P.V. and S.S. are also thankful to UGC, and
CSIR, New Delhi for their fellow-ships. We thank DST, Government of India
(
EMR/2016/000010), and CSIR-CSMCRI (OLP-087) for financial support.
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24. While our manuscript was under revision in JOC, the related work has been published by Huanfeng Jiang et.al
DOI: 10.1021/acs.joc.6b01428
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