Copper acetate-DMSO promoted methylthiolation of arenes and heteroarenes

Article abstract of DOI:10.1021/acs.joc.5b00443

An unprecedented copper acetate-DMSO promoted methylthiolation of arenes and heteroarenes in the presence of air has been developed. The reaction is highly regioselective under the directing group influence of pyridine and pyrimidine functional units and gives the thiomethylated product in moderate to high yields.

Full text of DOI:10.1021/acs.joc.5b00443

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Article
Copper acetate-DMSO promoted methylthiolation of arenes and heteroarenes
Poonam Sharma, Sandeep Rohilla, and Nidhi Jain
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00443 • Publication Date (Web): 20 Mar 2015
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Copper acetate-DMSO promoted methylthiolation of arenes and
heteroarenes
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Poonam Sharma, Sandeep Rohilla and Nidhi Jain*
Department of Chemistry, Indian Institute of Technology, Delhi 110016
ABSTRACT
An unprecedented copper acetate-DMSO promoted methylthiolation of arenes and heteroarenes
in the presence of air has been developed. The reaction is highly regioselective under the
directing group influence of pyridine and pyrimidine functional units and gives the
thiomethylated product in moderate to high yields.
Key words: methylthiolation; directed C-H activation; heteroaryl; copper acetate
*Corresponding author e-mail: njain@chemistry.iitd.ac.in
INTRODUCTION
Substantial progress has been made in the utilization of directing-group assisted C-H
activation reactions catalyzed by a range of metals including Ru, Rh, Pt, and Pd.1,2 Controlling
selectivity and identifying inexpensive metal catalysts for these reactions is a challenge worth
undertaking. Since copper is a relatively economical metal to work with, copper catalyzed C-H
activation has gained tremendous momentum in recent times.3 Copper promoted C-H
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functionalization either under free-radical4 or under oxidative conditions5 has been successfully
exploited in a variety of reactions including peroxidation,6 alkylation,7 acetoxylation,
halogenation, cyanation and amination,8 to name a few.9 In our efforts directed toward
development of copper catalysed C-H functionalization reactions,10 we discovered copper
acetate-mediated direct synthesis of aryl and heteroaryl sulphide structural motifs with DMSO as
a simple, cheap and easy-to-handle methylthiolation reagent.
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Owing to the importance of methylthioethers in many pharmaceutically active
compounds and advanced materials,11 the development of simple and facile methods for their
synthesis is a highly desirable goal for chemists. While the traditional methods involve the use of
dimethyl disulphide as the methylthiolating reagent (Figure 1, a),12 there are two recent reports
which demonstrate DMSO as a convenient reagent for preparing methyl thioethers.13 In this
context, Gao et al. have used Lewis acids [Ag(I), Ni(II), Fe(II)] along with Cu(OAc)2 for
methylthiolation of heterocycles (Figure 1, b);13a while Qing et al. disclosed a CuF2-K2S2O8
mediated methylthiolation of 2-phenylpyridine.14 Both protocols reported using DMSO,
however, suffer from limitations. While one involves the use of Lewis acid for enhancing the C2
C-H bond acidity, the second one demands drastic conditions such as the use of large amount of
expensive catalyst, strong oxidant, and long reaction time. As a solution to the current shortfalls,
we report for the first time a generalized DMSO mediated methylthiolation protocol that works
for both aryl and heteroaryl C-H bonds (Figure 1, c).15 The reaction is assisted by inexpensive
copper acetate in the presence of air as the oxidant. Our method is distinctly different from the
one reported earlier by Qing et al. in which they very categorically state the inactivity of copper
acetate; use excess of CuF2 (4 equiv), and require K2S2O8 (2 equiv) as a very specific oxidant for
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the reaction. Further, they demonstrate thiomethylation of aromatic sp2 C-H bonds only with the
reaction time prolonging to 72 hours.
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Figure 1. An overview of previous methylthiolation methods vs. our approach
REULTS AND DISCUSSION
The methylthiolation reported herein was discovered accidently during our endeavours of
carrying out copper acetate-promoted arylation of 2-phenylpyridine (1a) using sodium p-
toluenesulfinate as the arylating agent and DMSO as the solvent (Scheme 1).
Scheme 1. Reaction of 1a with sodium p-toluenesulfinate
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While we did not obtain the desired arylated product, we found the reaction to be very
clean, giving a single product in 60% yield after 12 hours with no trace of side impurities. On
spectroscopic characterization, the product was found to be 2-(2-(methylthio)phenyl)pyridine
(2a). We repeated the reaction without adding sodium p-toluenesulfinate, but found the same
result with 64% yield of 2a. To confirm these initial observations, we decided to extend the
reaction conditions to 2-naphthylpyridine (1b) as the substrate. We were excited to find that with
2.0 equiv of copper acetate and DMSO as the solvent, 2-(3-(methylthio)naphthalen-2-yl)pyridine
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(2b) was obtained as a single product in 89% yield at 125 C (Table 1). We also noticed that
unlike previous reports, no oxidized products or dimethylthiolated products were formed.14 The
high monoselectivity is most likely due to the binding of the thiomethyl group and the nitrogen
in the product to Cu(II) (taken in an equivalent amount), which prevents further reaction.9a
Certain of the findings, we next moved on to optimize the reaction with respect to
catalyst, time and temperature, taking 1b as the substrate. Lowering the equivalents of Cu(OAc)2
to 0.5 and 0.8 resulted in reduced yield of the product (Table 1, entry 1) while with 1.0 equiv, a
similar yield as with 2.0 equiv was obtained. Therefore, it was decided to carry out the remaining
optimizations with 1.0 equiv of the copper catalyst. A variety of copper(II) salts such as CuF2,
Cu(OTf)2 and CuCl2 were screened but were found to be inactive. This was in contrast to the
previous methylthiolation reported by Qing et al. in which they demonstrate CuF2 as the best
catalyst. Copper (I) salts such as CuBr and CuI were ineffective as well (Table 1, entries 2 and 7)
as no product was seen with CuI, and only 10% of the desired product was isolated with CuBr.
Since the reaction appeared to be a copper(II) promoted C-H activation, we investigated the
reaction with copper(I) salts in the presence of various oxidants such as benzoquinone, K2S2O8
and AgOAc with a view to generate the active Cu(II) species in-situ. While no product was
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isolated with CuBr-BQ and CuBr-K2S2O8, 40% product 2b was obtained when 1.0 equiv of
AgOAc was used as the oxidant with CuBr (Table 1, entry 3-6). A similar trend was observed
with CuI-oxidant combinations as well (Table 1, entry 8-11). The yield with CuI-AgOAc,
however was much higher than CuBr-AgOAc system (Table 1, entry 5, 6, 10, 11). No reaction
was observed in the absence of copper salt, emphasizing that the mediation of Cu(II) is
necessary, and also pointed out towards the observed ortho-selectivity via coordination of Cu(II)
to the N atom of the pyridine in the substrate. Addition of base was found to be detrimental to the
reaction. While the product yield dropped to 30% with K2CO3, only traces were seen with KOH
(Table 1, entry 12, 13) apparently due to the formation of inactive Cu2O as seen in PXRD
(supplementary). With organic bases such as triethylamine (TEA) and diisopropylethylamine
(DIPEA), lower yields of 2b were obtained (Table 1, entry 14, 15). The effect of temperature on
product yield was also monitored. On decreasing the temperature to 110 °C from 130 °C, no
product was seen; while increasing the temperature to 140 °C resulted in similar yield of the
product (Table 1, entry 16). Addition of the ligands did not help the reaction either. With ethyl
acetoacetate, yield dropped to 70% while with ethylenediamine, no product was formed (Table 1,
entry 17). Addition of oxidants such as K2S2O8, BQ and O2 quenched the reaction completely by
deactivating the Cu(OAc)2 catalyst. It was also noticed that air was necessary to drive the
reaction as under nitrogen atmosphere, lower conversions were obtained. Further, the optimum
reaction time was found to be 12-18 hours. Although the substrate did not show 100%
conversion in this time, extending it further to 24 hours led to complete decomposition of the
thiomethylated product.
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Thus, with optimized conditions in hand, we next moved on to examine the scope of
thiomethylation on various 2-aryl pyridines (1a-k) synthesized via Suzuki coupling of the
corresponding boronic acids with 2-chloropyridine16,17 (Table 2).
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Table 1. Optimization of methylthiolation of 2-(naphthalen-2-yl)pyridine
Cu salt
(1.0 equiv)
oxidant
(1.0 equiv)
entry
yieldb(%)
1
2
Cu(OAc)2
CuBr
-
-
90c, 58d, 46e
10
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4
CuBr
CuBr
BQ
-
-
K2S2O8
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CuBr
AgOAc
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CuBr
AgOAc (2 equiv)
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CuI
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CuI
K2S2O8
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CuI
BQ
-
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12f
13g
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17
CuI
AgOAc
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CuI
AgOAc (2 equiv)
85
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
-
-
-
-
-
30
trace
65
54
0j, 82k
70l, 0m
-
b
c
a2-(Naphthalen-2-yl)pyridine 1b (1.0 equiv; 0.05 mmol); HPLC yield of 2b; Cu(OAc)2 (1.0
d
e
f
g
equiv), Cu(OAc)2 (0.8 equiv), Cu(OAc)2 (0.5 equiv), K2CO3 (1.0 equiv), KOH (1.0 equiv),
i
hReaction with 1.0 equivalent of triethylamine (TEA), Reaction with 1.0 equivalent of
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diisopropylethylamine (DIPEA), reaction carried at 110 C; reaction carried at 140 C;
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m
lethylacetoacetate (1.0 equiv), ethylenediamine (1.0 equiv). BQ = benzoquinone.
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9
In general, with 2-naphthylpyridine substrates, yield of the thiomethylated products (2b
and 2c) was higher than the corresponding 2-phenylpyridine substrates. This may be attributed to
the lower aromaticity and more localized π-electrons in naphthalene compared to benzene, which
enables a better coordination with copper. Further, it was found that 2-phenylpyridine derivatives
bearing both electron-donating as well as electron-withdrawing substituents on the benzene ring
underwent facile monomethylthiolation to yield the desired products (2d-k) in 42-87% yield. In
an unusual finding though, we found that thiolation occurred at the more hindered ortho-position
of 2-(3-methylphenyl)pyridine to yield 2j in 67% yield. 2-(2’,5’Dimethoxyphenyl)pyridine gave
the lowest yield of the product (2i, 42%) due to conformational restrictions. Reaction of 1a was
also explored with an unsymmetrical arylalkyl sulfoxide so as to understand the selectivity of
thiomethylation versus thioarylation. We found that only the thiomethylated product 2a was
formed (Table 2), and thioarylation did not take place at all. Further, with CuCl2 as the catalyst, a
chlorinated product, 2-chloro phenylpyridine (2aa) was isolated in 50% yield. A similar CuCl2
mediated chlorination reaction has been reported previously.9a,18
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Table 2. Substrate scope with pyridine directed methylthiolationa
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b
aMolar ratios: Substrates (1a-k) 1.0 equiv, Cu(OAc)2 1.0 equiv, DMSO (5.0 vol.). Reaction of
1a with methyl phenyl sulfoxide, cReaction carried out with CuCl2.
Taking this protocol further to pyrimidine as a directing group, a library of
methylthiolated derivatives 4a-f were prepared from their respective 2-pyrimidyl aryl
compounds 3a-f (Table 3) in moderate yields. The yields with pyrimidine directing groups (4a-
f) in general were lower than with the corresponding pyridine analogs (2a-k). As expected, 2-(3-
methylphenyl)pyrimidine with a methyl substituent at the meta-position, underwent thiolation
exclusively at the less-hindered ortho-site to yield 4c in 58% yield.
Table 3. Pyrimidine directed methylthiolationa
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aMolar ratios: Substrates (3a-f) 1.0 equiv, Cu(OAc)2 1.0 equiv, DMSO (5.0 vol.).
The scope of methylthiolation on benz-fused aza-heterocycles 5a-e was also investigated.
It was found that the directing groups (pyridine and pyrimidine) favoured thiomethylation at the
C-2 carbon in both indole and benzimidazole derivatives, yielding the corresponding products
6a-e in 38-64% yield (Table 4). To examine whether it was the directing group that influenced
the outcome of thiomethylation; the reaction of N-methyl benzimidazole under the optimized
conditions was attempted. As expected, no thiomethylation took place, and the starting material
was recovered along with some unidentified decomposed products.
Table 4. Methylthiolationa of benzofused aza-heterocycles
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b
aMolar ratios: Substrates (5a-e) 1.0 equiv, Cu(OAc)2 1.0 equiv, DMSO (5.0 vol.). Molecular
structure of compound 6a obtained from single crystal x-ray analysis.19 Orteps for C, N and S
were set at 40% probability (see Supporting Information for details about crystallographic data).
Further, methylthiolation using deuterated DMSO was carried out. With 1b as the
substrate, deuterated thiomethyl derivative 2bb was obtained in 82% yield, confirming that the
sulphur atom was being transferred from DMSO (Scheme 2).
Scheme 2. Methylthiolation of 2-(naphthalen-2-yl)pyridine 1b using deuterated DMSO
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Further, to verify the absence of free radical species in the reaction; quenching studies
with TEMPO were performed. As anticipated, the reaction remained uninfluenced, suggesting
that it does not go via a free radical path. Next, to get some more insight into the mechanism, the
isotope effect was examined using deuterated 2-phenylpyridine (7) as the substrate. As shown in
Scheme 3, an intramolecular competition took place and a kinetic hydrogen isotope effect
(K.H.I.E.) of 1.53 was observed, suggesting that the ortho-C-H bond cleavage is involved in the
rate determining step.
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Scheme 3. Methylthiolation of deuterated 2-phenylpyridine
As an important observation it was seen that as the reaction proceeded, a red powder
precipitated out of solution. This was identified as Cu2O from PXRD20 data (see Supporting
Information); and was believed to be the by-product of Cu(OAc)2 mediated oxidation of
methanethiol to dimethyl disulfide21 (Scheme 4). Thermal decomposition of DMSO to yield
methanethiol and dimethyl disulfide is a known phenomenon (Figure 2)22 and in this case
formation of dimethyl disulfide was confirmed by GC-MS analysis (see Supporting
Information), which showed its molecular ion peak in the reaction mixture after one hour of
reaction time.
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Figure 2. Thermal decomposition of DMSO
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Based on these observations, we believe that the reaction is initiated by coordination of A
(generated in situ and verified by GC-MS) to 1a to yield the complex B, which also explains the
observed ortho-selectivity.23 This is followed by metallation and deprotonation to yield C with
the elimination of acetic acid. Cu(OAc)2 promoted oxidation of Cu(II) to high valent Cu(III)
species D takes place next, which is followed by reductive elimination to yield the product 2a
and Cu(I), which gets reoxidized to Cu(II) under air atmosphere and enters back into the catalytic
cycle. The involvement of methanethiol and dimethyl disulfide in the reaction was verified
further by two cross-over experiments (Scheme 5) a) in which butanethiol (2.0 equiv) was added
to the reaction mixture, and b) in which diethyl disulfide (1.0 equiv) was added under the
optimized reaction conditions. It was found that in the first case, 40% thiobutyl derivative (9)
and 10% thiomethyl product (2k) were formed. In the second case, only the thioethyl derivative
(10) was formed, and no traces of 2k were seen. The preferred formation of 9 and 10 over 2k in
the competing experiments suggested that dibutyldisulfide and diethyldisulfide were the actual
thioalkylating agents. Since they were directly available in these reactions, it allowed faster
kinetics due to which 2k was seen only as a minor product. The GC-MS data along with these
cross-over experiments confirmed that the reaction goes via an in-situ formation of dimethyl
disulfide, which through coordination with Cu(II) leads to the subsequent thiomethyl transfer
reaction. Previous thiomethylation protocol with CuF2-K2S2O8/DMSO system proposes an
altogether different mechanism involving a sulfonium ion intermediate for methyl transfer; and
dimethyl sulfate and potassium methyl sulfate as the possible by-products. However, no
experimental evidence was provided to certify the same.
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Scheme 4. Plausible mechanism for methylthiolation
Scheme 5. Cross-over experiment study
CONCLUSION
In conclusion, we have developed a method for direct, auxiliary-assisted
monothiomethylation of sp2 C-H bonds of both arenes and heteroarenes. No oxidized products
or dimethylthiolated products as reported earlier with DMSO14 are formed by this method. The
reaction employs stoichiometric Cu(OAc)2 and DMSO at elevated temperatures. The use of
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inexpensive copper acetate and air as the oxidant are significant advantages that allow an
increased usefulness of the reaction. The protocol shows high generality, excellent selectivity
toward ortho C-H bonds, as well as good functional group tolerance. Through mechanistic
studies, we prove that copper acetate assisted thiomethylation with DMSO involves an in-situ
generation of dimethyl disulfide, which on complexation with copper acetate furnishes the actual
thiomethylating species.
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EXPERIMENTAL SECTION
General remarks
All the reactions were executed in anhydrous solvents under air atmosphere in one hour oven
dried glassware at 100 °C. All reagents and solvents were of pure analytical grade. Thin layer
chromatography (TLC) was performed on 60 F254 silica gel, pre-coated on aluminium plates and
revealed with either a UV lamp (λmax = 254 nm) or iodine vapors. The products were purified by
column chromatography on silica gel 230-400 mesh. 1HNMR spectra were measured on 300 and
400 MHz spectrometers using CDCl3 as the solvent with tetramethylsilane (TMS) as the internal
standard at room temperature. Chemical shifts are given in δ (ppm) relative to TMS. The
coupling constants (J) are given in Hz. ESI-MS was performed on high resolution mass
instrument using quadrupole-TOF mass analyzer, and PXRD was taken on powder X-ray
diffractometer.
Representative procedure for synthesis of 2-arylpyridines (1a-k)
2-bromopyridine (1 g, 6.33 mmol), phenylboronic acid (771 mg, 6.33 mmol), potassium
carbonate (1.75 g, 6.33 mmol), Pd(OAc)2 (14.2 mg, 0.06 mmol), PPh3 (332 mg, 1.26 mmol) were
taken in a round bottom flask containing DMF: H2O (30.0 mL, 1:1). The reaction mixture was
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stirred for 12 hours at 130 °C, and monitored through TLC. After completion of reaction, it was
extracted with ethyl acetate and concentrated at reduced pressure. The product was purified by
column chromatography on silica gel using hexane: EtOAc (95:5), and 1a was obtained in 94%
(922 mg) yield.24
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Representative procedure for synthesis of 2-arylpyrimidines (3a-f)
2-chloropyrimidine (1 g, 8.73 mmol), phenylboronic acid (1.06 g, 8.73 mmol), potassium
carbonate (2.41 g, 17.46 mmol), Pd(OAc)2 (20 mg, 0.09 mmol) , PPh3 (458 mg, 1.74 mmol) were
taken in a round bottom flask containing DMF: H2O (30.0 mL, 1:1). The reaction mixture was
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stirred for 12 h at 130 C. Reaction was monitored through TLC. After completion of reaction,
reaction mixture was extracted with ethyl acetate and concentrated under reduced pressure. The
product 3a was purified through the column chromatography on silica gel using hexane: EtOAc
(95:5), and isolated in 90% (1.23 g) yield.24
Representative procedure for synthesis of 2-pyrimidylindoles (5a-c)
Indole (1.0 g, 8.54 mmol) was placed in a 100 mL two-necked reaction flask, and flushed with
nitrogen. DMF (25 mL) and NaH (819 mg, 34.1 mmol) were added. After stirring at room
temperature for 30 min, 2-chloropyrimidine (1.46 mg, 12.8 mmol) was added, and the contents
were stirred at 150 °C for 24 hours. Reaction was monitored through TLC. After completion of
reaction, reaction mixture was extracted with ethyl acetate and concentrated at reduced pressure.
The product 5a was purified through the column chromatography on silica gel using hexane:
EtOAc = 90:10) and isolated in 97% (1.62 g) yield.17a
Representative procedure for synthesis of 2-pyrimidinebenzimidazoles (5d-e):
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Benzimidazole (1 g, 8.46 mmol), CuI (160 mg, 10 mol%), TBAF (25 mL, 25.3 mmol) were
dissolved in dry distilled DMF (20 mL) in a 100 mL round bottom flask, and flushed with
nitrogen for 10 minutes. 2-chloro pyrimidine (961 mg, 8.4 mmol) and 1,10 Phenanthroline (330
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mg, 20 mol%) were added to the mixture, and the contents were stirred at 140 C for 18 hours
under nitrogen atmosphere. Reaction was monitored through TLC. After completion of reaction,
reaction mixture was extracted with ethyl acetate and concentrated at reduced pressure. The
product 5d was purified through the column chromatography on silica gel using hexane: EtOAc
= 90:10) and obtained in 93% (1.54 g) yield.17b
General procedure for methylthiolation of 1a-k, 3a-f, 5a-e and 7:
Substrate (1.0 equiv) and Cu(OAc)2 (1.0 equiv) were taken in a 10 mL tube sealed with a Teflon
lined cap. To it DMSO (5 vol.) was added, and the contents were stirred for 12-18 hours at 125
oC. In a typical procedure, 1a (50 mg, 0.32 mmol) and Cu(OAc)2 (57.9 mg, 0.32 mmol) were
taken in a 10 mL tube sealed with a Teflon lined cap; and to it DMSO (0.25 mL) was added. The
reaction did not go to completion even after stirring for 24 hours and showed the formation of
decomposed products on prolonged reaction time with complete loss of the desired
thiomethylated product. Therefore as monitored by TLC, after 12-18 hours the contents were
extracted with ethyl acetate, dried over anhydrous sodium sulphate and concentrated at reduced
pressure. The product was purified through the column chromatography on silica gel using
hexane: EtOAc (95:5).
Physical properties and characterization data of the synthesized compounds.
2-(2-(methylthio)phenyl)pyridine (2a): Colourless liquid, yield 64% (42 mg), Rf (0.48) in
1
hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.74 (d, J = 2.7 Hz, 1H), 7.80-7.75 (m,
1H), 7.59-7.57 (m, 1H), 7.46-7.43 (m, 1H), 7.40-7.35 (m, 2H), 7.31-7.23 (m, 2H), 2.41 (s, 3H);
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13C{1H}NMR (75 MHz, CDCl3): δ 158.4, 149.1, 139.6, 137.4, 136.1, 129.9, 128.9, 126.0, 124.9,
124.2, 122.2, 16.4; MS (ESI) m/z 202 [M+H]+; HRMS Calculated for C12H12NS+ 202.0685;
Found: 202.0689 [M + H]+.
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2-(2-chlorophenyl)pyridine (2aa): Colourless liquid, yield 50% (31 mg), Rf (0.50) in hexane:
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EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.72 (d, J = 3 Hz, 1H), 7.84-7.74 (m, 1H), 7.66
13
(d, J = 9 Hz, 1H), 7.61-7.58 (m, 1H), 7.49-7.47 (m, 1H), 7.42-7.33 (m, 3H); C{1H}NMR (75
MHz, CDCl3): δ 156.9, 149.5, 139.2, 135.8, 132.1, 131.5, 130.1, 129.6, 127.0, 124.9, 122.4; MS
(ESI) m/z 190 [M+H]+; HRMS Calculated for C11H9ClN+ 190.0418; Found:190.0416 [M+H]+.
2-(3-(methylthio)naphthalen-2-yl)pyridine (2b): Pale yellow solid, yield 90% (52 mg), Rf
1
(0.44) in hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.68 (d, J = 3.9 Hz, 1H), 7.81
(s, 1H), 7.77-7.70 (m, 3H), 7.60-7.56 (m, 2H), 7.44-7.30 (m, 2H), 7.25-7.18 (m, 1H), 2.42 (s,
13
3H); C{1H}NMR (75 MHz, CDCl3): δ 158.3, 149.2, 138.0, 136.2, 135.6, 133.6, 131.0, 129.2,
128.0, 126.9, 126.5, 125.3, 124.4, 123.7, 122.3, 16.4; MS (ESI) m/z 252 [M+H]+; HRMS
Calculated for C16H14NS+ 252.0841; Found: 252.0848 [M + H]+.
2-(3-(methylthio)naphthalen-2-yl)pyridine-d3 (2bb): Yellow solid, yield 82% (51 mg), Rf
1
(0.48) in hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.69 (d, J = 3.9Hz, 1H), 7.81
(s, 1H), 7.76-7.70 (m, 3H), 7.58 (d, J = 6.6 Hz, 2H), 7.48-7.32 (m, 2H), 7.26-7.18 (m, 1H) ;
13C{1H}NMR (75 MHz, CDCl3): δ 158.5, 149.4, 138.1, 136.4, 135.8, 133.8, 131.1, 129.4, 128.2,
127.1, 126.7, 125.7, 124.5, 123.7, 122.5, 29.8; MS (ESI) m/z 255 [M+H]+; HRMS Calculated
for C16H11D3NS+ 255.1030; Found: 255.1038 [M+H]+.
2-(2-(methylthio)naphthalen-1-yl)pyridine (2c): Pale yellow solid, yield 76% (47 mg), Rf
(0.44) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.86 (d, J = 4.5 Hz, 1H), 7.93-
7.84 (m, 3H), 7.56 (d, J = 8.7 Hz, 1H), 7.48-7.38 (m, 4H), 7.36 (d, J = 1.2, 1H), 2.48 (s, 3H);
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13C{1H}NMR (75 MHz, CDCl3): δ 157.7, 149.9, 136.6, 136.3, 134.7, 132.5, 131.5, 129.0, 128.0,
126.8, 126.1, 125.2, 125.1, 124.3, 122.5, 16.8; MS (ESI) m/z 252 [M+H]+; HRMS Calculated for
C16H14NS+ 252.0841; Found: 252.0811 [M + H]+.
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2-(2-(methylthio)-4-nitrophenyl)pyridine (2d): Yellow solid, yield 89% (54 mg), Rf (0.36) in
hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.69(d, J = 3 Hz, 1H), 8.08 (s, 1H), 7.99
(d, J = 9, 1H), 7.75 (t, J = 9, 1H), 7.53 (d, J = 9 Hz, 2H), 7.28 (t, J = 6 Hz, 1H), 2.43 (s, 3H);
13C{1H}NMR (75 MHz, CDCl3): δ 155.0, 148.3, 147.0, 143.3, 139.6, 135.3, 129.4, 122.9, 122.0,
118.6, 118.2, 14.9; MS (ESI) m/z 247 [M+H]+; HRMS Calculated for C12H11N2O2S+ 247.0536;
Found: 247.0544 [M + H]+.
3-(methylthio)-4-(pyridin-2-yl)benzonitrile (2e): White solid, yield 67% (42 mg), Rf (0.40) in
hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.67 (d, J = 4.2 Hz, 1H), 7.76-7.71 (m,
13
1H ), 7.51-7.42 (m, 4H), 7.29-7.25 (m, 1H), 2.45 (s, 3H); C{1H}NMR (75 MHz, CDCl3): δ
171.1, 156.5, 149.5, 143.1, 140.1, 136.5, 130.4, 128.5, 128.0, 124.0, 123.0, 118.5, 112.9, 16.2;
MS (ESI) m/z 227 [M+H]+; HRMS Calculated for C13H11N2S+ 227.0637; Found: 227.0631 [M +
H]+.
3-(methylthio)-4-(pyridin-2-yl)benzaldehyde (2f): Colourless liquid, yield 58% (36 mg), Rf
(0.48) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 10.07 (s, 1H), 8.77 (d, J = 3.9
Hz, 1H), 7.85-7.82 (m, 2H), 7.75-7.71 (m, 1H), 7.62 (d, J = 7.8 Hz, 2H), 7.38-7.35 (m, 1H),
2.50 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 190.7, 156.1, 148.4, 145.1, 143.6, 138.6, 135.4,
129.5, 125.7, 124.6, 123.1, 121.9, 15.1; MS (ESI) m/z 202 [M+H]+; HRMS Calculated for
C13H12NOS+ 230.0634; Found: 230.0646 [M + H]+.
2-(4-fluoro-2-(methylthio)phenyl)pyridine (2g): Colourless liquid, yield 56% (35 mg), Rf
(0.32) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.72 (t, J = 2.1 Hz, 1H), 7.79-
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7.74 (m, 1H), 7.55-7.52 (m, 1H), 7.44-7.34 (m, 1H), 7.30-7.28 (m, 1H), 7.02 (d, J = 9.9, 1H),
6.92 (t, J = 8.4, 1H), 2.41 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 163.7, 160.4, 156.5, 148.2,
139.3 (d, J = 8.0 Hz), 135.2, 134.2, 130.5 (d, J = 8.8 Hz), 123.1, 121.2, 111.3 (d, J = 24.4 Hz),
110.5 (d, J = 21.3 Hz), 15.1; MS (ESI) m/z 220 [M+H]+; HRMS Calculated for C12H11FNS+
220.0591; Found: 220.0588 [M + H]+.
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2-(4-methoxy-2-(methylthio)phenyl)pyridine (2h): Colourless liquid, yield 76% (47 mg), Rf
(0.72) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.72 (d, J = 4.5 Hz, 1H), 7.78-
7.72 (m, 1H), 7.57 (d, J = 7.8, 1H), 7.44-7.39 (m, 1H), 7.29-7.23 (m, 2H), 6.90 (d, J = 2.4, 1H),
3.89 (s, 3H), 2.42 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 160.1, 158.2, 149.1, 138.9, 136.0,
132.4, 131.2, 124.1, 121.7, 112.3, 109.6, 55.4, 16.4; MS (ESI) m/z 227 [M+H]+; HRMS
Calculated for C13H14NOS+ 232.0791; Found: 232.0789 [M+H]+.
2-(3,6-dimethoxy-2-(methylthio)phenyl)pyridine (2i): Red liquid, yield 42% (26 mg), Rf (0.40)
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in hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.71 (d, J = 6.00 Hz, 1H), 7.75 (t, J
= 6.9 Hz, 1H ), 7.28 (s, 1H), 7.05 (s, 1H), 6.92 (s, 2H) 3.92 (s, 3H) 3.67 (s, 3H ) 2.20 (s, 3H);
13C{1H}NMR (75 MHz, CDCl3): δ 157.5, 156.9, 156.8, 154.3, 151.5, 149.0, 135.7, 125.3, 122.0,
111.9, 111.6, 56.5, 18.54; MS (ESI) m/z 262 [M+H]+; HRMS Calculated for C14H16NO2S+
262.0896; Found: 262.0907 [M+H]+.
2-(3-methyl-2-(methylthio)phenyl)pyridine (2j): Colourless liquid, yield 67% (43 mg), Rf
(0.36) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.64 (d, J = 3.6, 1H), 7.67 (t, J
= 7.5, 1H), 7.50 (d, J = 7.8 Hz, 1H), 7.20 (d, J = 7.8, 3H), 7.11 (d, J = 8.1, 1H), 2.28 (d, J = 2.4
Hz, 6H); 13C{1H}NMR (75 MHz, CDCl3): δ 158.2, 149.2, 140.1, 135.9, 135.1, 133.5, 130.8,
129.8, 127.2, 124.3, 122.0, 17.0; MS (ESI) m/z 227 [M+H]+; HRMS Calculated for C13H14NS+
216.0841; Found: 216.0844 [M+H]+.
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2-(4-methyl-2-(methylthio)phenyl)pyridine (2k): Colourless liquid, yield 73% (46 mg), Rf
(0.36) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.62 (d, J = 4.2, 1H), 7.65-7.62
(m, 1H), 7.47 (d, J = 7.5 Hz, 1H), 7.26 (d, J = 7.5, 1H), 7.18-7.14 (m, 1H), 7.07 (s, 1H), 6.97 (d,
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J = 6.9 Hz, 1H), 2.32 (s, 3H), 2.30 (s, 3H); C{1H}NMR (75 MHz, CDCl3): δ 158.6, 157.6,
149.2, 138.9, 137.1, 136.1, 130.0, 126.9, 126.0, 124.3, 122.1, 21.5, 16.64; MS (ESI) m/z 216
[M+H]+; HRMS Calculated for C13H14NS+ 216.0841; Found: 216.0847 [M+H]+.
2-(2-(methylthio)phenyl)pyrimidine (4a): Yellow solid, yield 58% (38 mg), Rf (0.68) in
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hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.92 (d, J = 7.2 Hz, 2H), 8.07-8.04 (m,
1H), 7.49-7.38 (m, 2H), 7.30 (d, J =1.5, 1H), 7.27-7.24 (m, 1H), 2.48 (s, 3H); 13C{1H}NMR (75
MHz, CDCl3): δ 165.8, 156.6, 139.5, 136.5, 130.8, 130.2, 125.5, 124.3, 118.8, 16.6; MS (ESI)
m/z 216 [M+H]+; HRMS Calculated for C11H11N2S+ 203.0637; Found: 203.0647 [M+H]+.
2-(4-fluoro-2-(methylthio)phenyl)pyrimidine (4b): Colourless liquid, yield 48% (30 mg), Rf
(0.72) in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.88 (d, J = 4.8 Hz, 2H), 8.18-
8.13 (m, 1H), 7.24 (t, J = 5.1 Hz, 1H), 7.09-7.05 (m, 1H), 6.99-6.96 (m, 1H), 2.47 (s, 3H);
13C{1H}NMR (100 MHz, CDCl3): δ 164.0 (d, J = 41 Hz), 161.8, 155.5 (d, J = 2.0 Hz), 141.9 (d,
J = 8.0 Hz), 134.4, 131.8 (d, J = 9.0 Hz), 117.6 (d, J = 3.0 Hz), 111.1 (d, J = 25.0 Hz), 110.1 (d,
J = 22.0 Hz), 15.6; MS (ESI) m/z 221 [M+H]+; HRMS Calculated for C11H10FN2S+ 221.0543;
Found: 221.0543 [M+H]+.
2-(3-methyl-2-(methylthio)phenyl)pyrimidine (4c): Colourless liquid, yield 58% (37 mg), Rf
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(0.70) in hexane: EtOAc (80:20); H NMR (400 MHz, CDCl3): δ 8.88 (d, J = 4.8 Hz, 2H), 7.85
(s, 1H), 7.30-7.22 (m, 3H), 2.44-2.36 (s, 6H); 13C{1H}NMR (100 MHz, CDCl3): δ 164.9, 155.6,
135.5, 134.9, 133.2, 130.3, 131.1, 130.0, 124.9, 117.7, 21.8, 15.8; MS (ESI) m/z 217 [M+H]+;
HRMS Calculated for C12H13N2S+ 217.0793; Found: 217.0803 [M+H]+.
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2-(4-methoxy-2-(methylthio)phenyl)pyrimidine (4d): Pale yellow solid, yield 67% (42 mg), Rf
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(0.68) in hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.83 (d, J = 4.8Hz, 2H), 8.14
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3H), 2.46 (s, 3H); C{1H}NMR (75 MHz, CDCl3): δ 165.3, 161.1, 156.4, 141.7, 132.5, 128.9,
118.1, 111.6, 108.9, 55.4, 16.6; MS (ESI) m/z 233 [M+H]+; HRMS Calculated for C12H13N2OS+
233.0743; Found: 233.0754 [M+H]+.
2-(2-(methylthio)naphthalen-1-yl)pyrimidine (4e): Yellow solid, yield 72% (44 mg), Rf (0.48)
in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 9.00 (d, J = 4.5 Hz, 2H), 7.93 (d, J =
8.7 Hz, 1H), 7.86, (d, J = 7.5 Hz, 1H), 7.60 (d, J = 8.7 Hz, 1H), 7.42-7.39 (m, 3H), 7.23-7.28 (m,
1H), 2.49 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 166.8, 157.3, 136.0, 134.7, 131.8, 131.7,
129.5, 128.1, 127.1, 125.5, 125.2, 124.8, 119.5, 17.3; MS (ESI) m/z 253 [M+H]+; HRMS
Calculated for C15H13N2S+ 253.0794; Found: 253.0797 [M+H]+.
2-(3-(methylthio)naphthalen-2-yl)pyrimidine (4f): Yellow solid, yield 64% (39 mg), Rf (0.48)
in hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.84 (d, J = 4.2 Hz, 2H), 8.43 (s, 1H),
7.81 (d, J = 8.1 Hz, 1H), 7.72 (d, J = 8.1 Hz, 1H), 7.60 (d, J = 14.7 Hz, 1H), 7.45 (t, J = 7.8 Hz,
1H), 7.39-7.28 (m, 1H), 7.20 (d, J = 4.8 Hz, 1H), 2.97 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3):
δ 165.9, 156.8, 136.6, 135.3, 134.3, 131.0, 130.6, 128.6, 127.6, 126.5, 125.6, 123.7, 119.0, 17.0;
MS (ESI) m/z 253 [M+H]+; HRMS Calculated for C15H13N2S+ 253.0794; Found: 253.0803
[M+H]+.
2-(methylthio)-1-(pyrimidin-2-yl)-1H-indole (6a): Pale yellow solid, yield 53% (33 mg), Rf
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(0.72) in hexane: EtOAc (70:30); H NMR (300 MHz, CDCl3): δ 8.70 (d, J = 4.5 Hz, 2H), 8.42
(d, J = 8.4 Hz, 1H), 7.42 (d, J = 6.3, 1H), 7.18-7.12 (m, 2H), 7.05 (t, J = 4.8, 1H), 6.37 (s, 1H),
2.46 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 157.8, 138.0, 137.0, 130.0, 124.5, 122.4, 122.3,
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118.8, 116.8, 114.6, 104.4, 18.0; MS (ESI) m/z 242 [M+H]+; HRMS Calculated for C13H12N3S+
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242.0746; Found: 242.0750 [M+H]+.
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2-(methylthio)-5-nitro-1-(pyrimidin-2-yl)-1H-indole (6b): Yellow solid, yield 64% (38 mg),
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Rf (0.68) in hexane: EtOAc (70:30); H NMR (300 MHz, CDCl3): δ 8.84 (d, J = 4.5Hz, 2H),
8.56 (d, J = 9.3 Hz, 1H), 8.39 (d, J = 1.8, 1H), 8.12-8.08 (m, 1H), 7.28-7.25 (m, 1H), 6.54 (s,
1H), 2.55 (s, 3H); C{1H}NMR (75 MHz, CDCl3): δ 158.0, 157.2, 143.4, 142.6, 140.0, 129.8,
13
118.0, 117.6, 114.8, 114.7, 104.3, 17.9; MS (ESI) m/z 287 [M+H]+; HRMS Calculated for
C13H11N4O2S+ 287.0597; Found: 287.0593 [M+H]+.
2-(methylthio)-1-(pyrimidin-2-yl)-1H-indole-3-carbaldehyde (6c): Yellow solid, yield 38%
1
(23 mg), Rf (0.36) in hexane: EtOAc (70:30); H NMR (300 MHz, CDCl3): δ 10.54 (s, 1H),
8.96 (d, J = 4.8 Hz, 2H), 8.42-8.39 (m, 1H), 7.83-7.80 (m, 1H), 7.44-7.38 (m, 3H), 2.59 (s, 3H);
13C{1H}NMR (75 MHz, CDCl3): δ 187.9, 158.7, 156.5, 145.0, 137.3, 125.5, 124.2, 122.1, 121.6,
119.6, 112.4, 22.9; MS (ESI) m/z 270 [M+H]+; HRMS Calculated for C14H12N3OS+ 270.0696;
Found: 270.0694 [M+H]+.
2-(methylthio)-1-(pyridin-2-yl)-1H-benzo[d]imidazole (6d): Colourless liquid, yield 56% (35
mg), Rf (0.68) in hexane: EtOAc (70:30); 1H NMR (300 MHz, CDCl3): δ 8.63(d, J = 0.9 Hz, 1H)
7.96-7.93 (m, 1H), 7.76 (d, J = 7.8 Hz, 1H), 7.61(d, J = 8.1, 1H), 7.50-7.47 (m,1H), 7.43-7.39
(m, 1H), 7.30-7.23 (m, 2H), 2.55 (s, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 153.3, 149.7, 149.4,
143.9, 138.9, 135.8, 123.0, 123.0, 122.7, 119.0, 118.5, 110.2, 15.3; MS (ESI) m/z 242 [M+H]+;
HRMS Calculated for C13H12N3S+ 242.0746; Found: 242.0748 [M+H]+.
2-(methylthio)-1-(pyrimidin-2-yl)-1H-benzo[d]imidazole (6e): White solid, yield 56% (35
mg), Rf (0.72) in hexane: EtOAc (70:30); 1H NMR (300 MHz, CDCl3): δ 8.85 (d, J = 4.8 Hz, 2H)
8.45-8.47 (m, 1H), 7.74-7.71 (m, 1H), 7.36-7.30(m, 2H), 7.26-7.22(m, 1H), 2.28 (s, 3H);
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13C{1H}NMR (75 MHz, CDCl3): δ 157.9, 156.6, 154.9, 143.9, 134.6, 123.7, 122.9, 118.1, 117.7,
114.9, 16.4; MS (ESI) m/z 216 [M+H]+; HRMS Calculated for C12H11N4S+ 243.0700; Found:
243.0699 [M+H]+.
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2-(2-Deutero)phenylpyridine (7): Colourless liquid, yield 90% (210 mg), Rf (0.84) in hexane:
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EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.59 (d, J = 4.5 Hz, 1H), 7.89 (d, J = 7.2 Hz,
1H), 7.64-7.60 (m, 2H), 7.40-7.28 (m, 3H), 7.12-7.07 (m, 1H), 13C{1H}NMR (75 MHz, CDCl3):
δ 157.5, 149.7, 139.4, 136.8, 129.0, 128.8, 128.7, 126.9, 126.3, 122.1, 120.6; MS (ESI) m/z 157
[M+H]+; HRMS Calculated for C11H9DN+ 157.0871; Found: 157.0863 [M+H]+.
Mixture of compounds 2a and 8: Colourless liquid, yield 56% (36 mg), Rf (0.48) in hexane:
EtOAc (80:20); 1H NMR (300 MHz, DMSO): δ 8.66 (d, J = 3.9 Hz, 1H), 7.90-7.85 (m, 1H), 7.59
(d, J = 7.8 Hz, 1H), 7.43-7.36 (m, 3H), 7.28-7.26 (m, 1H), 2.38 (s, 3H); 13C{1H}NMR (75 MHz,
CDCl3): δ 158.4, 149.2, 139.6, 137.4, 136.1, 129.9, 128.9, 126.0, 124.9, 124.8, 124.2, 122.2,
16.4; MS (ESI) m/z 157 [M+H]+; HRMS Calculated for C12H11DNS+ 203.0748; Found:
203.0748 [M+H]+.
2-(2-butylthio)-4-methylphenylpyridine (9): Colourless liquid, yield 40% (30 mg), Rf (0.49) in
hexane: EtOAc (80:20); 1H NMR (300 MHz, CDCl3): δ 8.63 (d, J = 4.5 Hz, 1H), 7.66 (t, J = 7.5
Hz, 1H), 7.50 (d, J = 7.8 Hz, 1H), 7.28 (d, J = 7.5 Hz, 1H), 7.17 (t, J = 7.2 Hz, 2H), 7.00 (d, J =
=7.8 Hz, 1H), 2.71 (t, J = 7.2, 2H), 2.32 (s, 3H), 1.51-1.41 (m, 2H), 1.32-1.27 (m, 2H), 0.79 (t, J
= 7.2 Hz, 3H); 13C{1H}NMR (75 MHz, CDCl3): δ 158.7, 149.1, 138.6, 138.3, 135.8, 135.5,
130.2, 129.3, 126.5, 124.7, 121.9, 33.6, 30.9, 22.1, 21.4, 13.7; MS (ESI) m/z 258 [M+H]+;
HRMS Calculated for C16H20NS+ 258.1311; Found: 258.1323 [M+H]+.
2-(2-ethylthio)-4-methylphenylpyridine (10): Colourless liquid, yield 60% (34 mg), Rf (0.47)
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in hexane: EtOAc (80:20); H NMR (300 MHz, CDCl3): δ 8.69 (d, J = 4.5 Hz, 1H), 7.74-7.68
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(m, 1H), 7.56 (d, J = 7.8 Hz, 1H), 7.36 (d, J = 7.8 Hz, 1H), 7.26-7.21 (m, 2H), 7.07(d, J = 7.8
Hz,1H) 2.81 (q, J = 7.5 Hz, 2H), 2.38 (s, 3H), 1.22(t, 7.5 Hz, 3H); 13C{1H}NMR (75 MHz,
CDCl3): δ 158.6, 149.1, 138.6, 138.1, 135.8, 135.2, 130.2, 128.9, 126.5, 124.5, 121.9, 29.7, 27.7,
21.3, 13.9; MS (ESI) m/z 230 [M+H]+; HRMS Calculated for C14H16NS+ 230.0998; Found:
230.1009 [M+H]+.
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ACKNOWLEDGMENT
The authors thank Council of Scientific and Industrial Research (CSIR), India (RP02948) for
financially supporting this work and DST-FIST for funding the ESI-HRMS facility at IIT Delhi.
PS and SR thank University Grants Commision (UGC) and CSIR, respectively for their graduate
fellowships.
Supporting Information
1
Copies of HNMR, 13CNMR and HR-MS for all the synthesized compounds, ORTEPs and
associated X-ray crystallographic data for 6a (CIF) and PXRD for various copper salts has been
included. This material is available free of charge via the Internet at http://pubs.acs.org.
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