Visible-light photocatalyzed synthesis of 2-aryl N-methylpyrroles, furans and thiophenes utilizing arylsulfonyl chlorides as a coupling partner

Article abstract of DOI:10.1016/j.tet.2016.03.087

Visible-light (Blue LED lamp: hν=425±15 nm) photocatalyzed cross-coupling reactions of arylsulfonyl chloride with N-methylpyrrole, furan, thiophene and their derivatives have been achieved in moderate to good yields at room temperature. A plausible mechan

Full text of DOI:10.1016/j.tet.2016.03.087

Accepted Manuscript
Visible-light photocatalyzed synthesis of 2-aryl N-methylpyrroles, furans and
thiophenes utilizing arylsulfonyl chlorides as a coupling partner
Palani Natarajan, Anu Bala, S.K. Mehta, K.K. Bhasin
PII:
S0040-4020(16)30229-0
10.1016/j.tet.2016.03.087
TET 27627
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 4 January 2016
Revised Date: 23 March 2016
Accepted Date: 25 March 2016
Please cite this article as: Natarajan P, Bala A, Mehta SK, Bhasin KK, Visible-light photocatalyzed
synthesis of 2-aryl N-methylpyrroles, furans and thiophenes utilizing arylsulfonyl chlorides as a coupling
partner, Tetrahedron (2016), doi: 10.1016/j.tet.2016.03.087.
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ACCEPTED MANUSCRIPT
Visible-light photocatalyzed synthesis of 2-aryl N-methylpyrroles, furans and
thiophenes utilizing arylsulfonyl chlorides as a coupling partner
Palani Natarajan*, Anu Bala, S. K. Mehta and K. K. Bhasin
Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab University,
Chandigarh - 160 014, India
pnataraj@pu.ac.in
Only for table of contents
Visible-light (Blue LED lamp: hν = 425±15 nm) photocatalyzed cross-coupling reactions of
arylsulfonyl chloride with N-methylpyrrole, furan, thiophene and their derivatives have been
achieved in moderate to good yields at room temperature. A plausible mechanism has been
proposed for the reaction. The arylation takes place at the C−H bond (C2−H or C5−H) next to the
heteroatom with a high regioselective manner! Moreover, arylsulfonyl chlorides utilized in this
study are inexpensive, environmentally benign and relatively more stable towards temperature,
moisture and air as compared to diazonium salts normally used as coupling partners in the
previously reported methods for the synthesis of heterobiaryls through visible-light
photocatalyzed conditions.
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Introduction
The visible-light-induced organic transformations developed by groups of MacMillan,1
Stephenson,2 Yoon3 and many others4 have been demonstrated as one of the most powerful tools
for the construction of carbon-carbon and carbon-heteroatom bonds in syntheses. Significant
energy efficiency, simple setup and mild reaction conditions are important features of this class
of reactions.1-4 Moreover, photoredox catalyzed reactions have been found numerous
applications in both industries and academics for the preparations of natural products,5 drugs6
and functional materials.7 All of the visible-light-induced formations of aryl-aryl and aryl-
heteroaryl bonds, known in time, have normally been realized utilizing aryl-/heteroaryl
diazonium salts as a coupling partner.1-8 For example, Konig and co-workers9 recently described
a visible-light-promoted eosin Y catalyzed direct C–H arylation of furans, thiophenes and N-
Boc-pyrroles with different aryl diazonium salts as a aryl radical source. Although diazonium
salts have been found very useful coupling partners to afford bi- and polyaryls, they suffer from
several disadvantages including unstable at ambient conditions, explosive in nature, cannot be
stored for longer periods of time even at low temperatures, expensive and being functionalized
are not commercially available.10 As a consequence, the extension of visible-light-induced cross-
coupling reactions utilizing readily available and environmentally benign aryl germ is highly
desirable.
Arylsulfonyl halides,11 in particular arylsulfonyl chlorides, are biocompatible, relatively cheap,
can be stored for longer periods of time under ambient conditions, wide variety of derivatives are
commercially available, provide only gaseous by-products (SO2 and HCl) and widely applicable
as electrophilic aryl partners in conventional transition metal-catalyzed cross-coupling
reactions12 to synthesize unsaturated carbonyl compounds, carboxylic acids, polyarenes, biaryls
and heterobiaryls.12f However, only few instances have hitherto been reported on the visible-light
photocatalyzed synthesis of biaryls or polyaryls using arylsulfonyl chlorides as a coupling
partner.13 For examples, Li and co-workers14 reported the sequential arylation of alkynes and
carbocyclization with benzylic C-H bonds using arylsulfonyl chlorides as aryl radical precursors.
The same group15 also described a synthesis of functionalized 10a,11-dihydro-10H-
benzo[b]fluorenes through tandem cyclization of 1,6-enynes with arylsulfonyl chlorides under
visible-light-promoted conditions.15 In these cases,14,15 authors proposed that single electron
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transfer (SET) from the excited-state photo-catalyst to arylsulfonyl chlorides provides an aryl
radical (vide infra) that subsequently reacts with unsaturated compounds to yield the products.
Herein, we demonstrate a novel protocol for the preparations of 2-aryl N-methylpyrrole, furan,
thiophene and their derivatives from arylsulfonyl chlorides and corresponding heteroarenes in
the presence of 3 mol% of [Ru(bpy)3]Cl2 under visible-light (Blue LED lamp: hν = 425±15 nm)
irradiation conditions, cf. Scheme 1. Moreover, a plausible mechanism has been proposed for the
coupling reaction (vide infra). Although synthesis of heterobiaryls using aryl diazonium salts as
aryl radical source has been reported,8,9 the arylsulfonyl chlorides employed synthesis of 2-aryl
N-methylpyrrole, furan, thiophene and their derivatives is not realized yet.
Scheme 1. Visible-light-induced desulfonylative coupling of arylsulfonyl chlorides with five-
membered heteroarenes reported in this work.
Results and Discussion
At the outset of this investigation, the reaction conditions were optimized for the C-H arylation
of N-methylpyrrole (HC1, 1.0 equiv.) with p-toluenesulfonyl chloride (AS1, 1.0 equiv.) in the
presence of K2CO3 (1.0 equiv.), fluorescent light irradiation and 3 mol% of [Ru(bpy)3]Cl2 as a
photoredox catalyst in CH2Cl2 at room temperature. After about 24 h, the desired 2-aryl-N-
methylpyrrole (AH1) was obtained in 23% yield (Table 1 and Entry 1). Encouraged by this
initial result, a series of trial experiments were performed to optimize the influences of radiation
source, solvents, base and catalyst quantity for the proposed reaction (Scheme 1). Interestingly,
the yield of AH1 enhanced to 39% when the reaction was performed with 5W blue LED lights
(hν = 425±15 nm, Table 1 and Entry 2). In contrast, the yield of AH1 was lowered to 10-20%
when reaction was irradiated with either 5W green LED lights or ambient lights (Table 1, Entries
3 and 4). However, no product was detected in the absence of the light source (Table 1 and Entry
5). Afterwards, the influence of solvents on the formation of AH1 was explored. The
investigation revealed that CH3CN was the most effective medium to promote the reaction with
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78% yield (Table 1 and Entry 6). While the use of other solvents such as hexane, heptane, diethyl
ether, glyme and CHCl3 resulted in lower yields (Table 1 and Entries 7-11) even after 24 h of
irradiation. A maximum yield of AH1 in CH3CN may be due to the greater solubility of both the
catalyst and substrates in that medium. We then went on to study the effect of base on the
reaction (Scheme 1). Indeed, a better yield of AH1 was obtained when Na2CO3 was used (Table
1 and Entry 12). Nevertheless, other inorganic/organic bases including Li2CO3, NaHCO3,
NaOAc, NaOH, K2CO3, t-BuOK, Cs2CO3 and Et3N afforded moderate to low yields of the
desired product (Table 1 and Entries 13-19). Hence, Na2CO3 was selected as a base for the
transformation outlined in Scheme 1. Moreover, optimization of amount of Na2CO3 revealed that
the excess base decreased the product yield (Table 1 and Entries 20-21), which perhaps
attributed to the direct reaction of a base with AS1.11 The influence of quantity of catalyst on the
product yield was also investigated. An increase in the catalyst amount from 3 mol% to 5 mol%,
the reaction time was obviously shortened with 94% yield of AH1 (Table 1 and Entry 22).
Further increase in the catalyst amount showed profound effect on neither in reaction time nor in
yield of the desired product (Table 1 and Entry 23). However, no product was observed in the
absence of catalyst [Ru(bpy)3]Cl2 (Table 1 and Entry 24). Subsequently, we investigated the
influence of atmosphere on the reaction. Under O2, the yield of AH1 decreased to 28%, whereas
under N2/Ar atmosphere the product yield increased up to 91-94% (Table 1, Entries 12 and 22).
Thus, the combinations of 3 mol% of [Ru(bpy)3]Cl2, 1 equiv. of Na2CO3 and 5W blue LED light
in CH3CN at room temperature for 12 h are the optimal conditions for this transformation, cf.
Scheme 1.16
Table 1. Selected results of screening the optimal conditions for the photocatalytic cross-
coupling of p-toluenesulfonyl chloride (AS1) with N-methylpyrrole (HC1)a
Entry
1
2
Solvent b
CH2Cl2
CH2Cl2
Base b
K2CO3
K2CO3
Time (h)
24
Yield (%) c
23
24
39d
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3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
CH2Cl2
CH2Cl2
CH2Cl2
CH3CN
hexane
heptane
diethyl ether
glyme
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
24
24
24
12
24
24
24
24
24
12
24
24
24
24
24
24
24
12
12
7
20e
<5f
0g
78
<5
<5
<10
≈20
≈20
92
44
53
13
<5
<10
<5
CHCl3
K2CO3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Na2CO3
Li2CO3
NaHCO3
NaOAc
NaOH
t-BuOK
Cs2CO3
(CH3CH2)3N
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
<10
83h
61i
94j
93k
0l
7
48
24
28m
a Unless stated otherwise all reactions were performed in a Schlenk tube with 1.0 equiv. of HC1,
1.0 equiv. of AS1, 1.0 equiv. of base and 3 mol% of [Ru(bpy)3]Cl2 in dry solvent under inert
atmosphere with the irradiation of a visible-light source.
b Solvents and bases were rigorously purified following the methods described in reference 17.
c Isolated yields.
d 5W blue LED lamp, hν = 425±15 nm, used.
e 5W green LED lamp, hν = 535±5 nm, used.
f 15W ambient lamp used.
g no irradiation source used.
h 1.5 mmol of Na2CO3 used
i 2.0 mmol of Na2CO3 used
j 5.0 mol% of [Ru(bpy)3]Cl2 used
k 10 mol% of [Ru(bpy)3]Cl2 used
l no catalyst, [Ru(bpy)3]Cl2, used
m Reaction performed under O2 atmosphere
Under the optimized conditions (Table 1), the substrate scope of the transformation (Scheme 1)
was investigated. The reaction between N-methylpyrrole (HC1) and various arylsulfonyl
chlorides (AS2-AS13) was looked at first and the results are summarized in Table 2. Notice that
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the reaction was significantly affected by the electronic effects of substituents in the arylsulfonyl
chlorides. The arylsulfonyl chlorides (AS1, AS3, AS4, AS5 and AS13) bearing electron-
donating groups showed slightly lower yields than those bearing electron-deficient groups (Cl,
Br, NO2, and COOCH3): probably due to the higher reduction potentials of latter precursors.11 In
addition, a strong dependence on the position of the substituents was observed, cf. Table 2 and
Entries 2, 4, 5, 9, 10 and 11. For example, the reaction with para-methyl substituted arylsulfonyl
chloride afforded higher yields than those of ortho-methyl substituted arylsulfonyl chloride
(Tables 1 and 2). This is likely attributed to the steric hindrance of the ortho-functionalized
arylsulfonyl chlorides (AS3, AS10, AS11 and AS12) as similar findings have been well
documented in the literatures.11, 12 Thus, a wide range of electronically and structurally diverse
arylsulfonyl chlorides and N-methylpyrrole can be cross-coupled efficiently with moderate to
good yields of desired products, cf. Tables 1 and 2. In addition, this transformation has been
found compatible to various functional groups including chloro, bromo, nitro and carboxylate
groups that generally advantageous for further functionalization.
Table 2. Direct C-H arylation of N-methylpyrrole (HC1) with different arylsulfonyl chlorides
(AS) a
Entry Ar b
1
Substrate Product c Yield d
b
(%)
AS2
AH2
83
H3C
2
AS3
AS4
AH3
AH4
64
72
3
4
OCH3
OCH3
AS5
AH5
67
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5
AS6
AH6
82
6
7
8
9
AS7
AS8
AS9
AH7
AH8
AH9
86
89
78
AS10
AH10
69
10
11
AS11
AS12
AH11
AH12
67
70
12
AS13
AH13
73
a Unless stated otherwise all reactions were performed in a Schlenk tube with 1.0 equiv. of HC1,
1.0 equiv. of AS, 1.0 equiv. of Na2CO3 and 3 mol% of [Ru(bpy)3]Cl2 in dry CH3CN under the
irradiation of 5W blue LED lamp, hν = 425±15 nm at room temperature for 12 h.
b Arylsulfonyl chlorides were distilled prior to use.16
c All products were characterized by 1H and 13C NMR spectroscopy (Supporting Information) in
comparison with known compounds from literature data.
d Isolated yield was calculated based on N-methylpyrrole.
Next, we varied the substituents on the N-methylpyrrole. Moreover, the direct arylation of other
readily available five membered heteroarenes such as furan, thiophene and their derivatives were
also examined (Table 3). Unsurprisingly, the desired C-H arylated products were obtained in
moderate to good yields (Table 3) regardless of the heterocyclic groups under the optimized
reaction conditions (Table 1). In fact, this is not a common feature with aryl diazonium salts8,9
a
well-known coupling partner used in the synthesis of heterobiaryls under photocatalyzed and
palladium mediated reactions.12f In general, the conventional transition metal-catalyzed direct
desulfitative arylation reactions happen at elevated temperatures (110-150 °C) and afford
diarylated products most often.12 We also observed an unprecedented selectivity for the
arylation, i.e., the C−H bond (C2−H or C5−H) next to the heteroatom was arylated with a high
7

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selectivity for all of the heteroarenes examined (Table 3). This regioselective arylation is a result
of resonance stabilization of intermediate radical generated by coupling of the aryl radical with
an electron-rich heterocycle, vide infra. Thus, the arylsulfonyl chlorides may be used as arylating
reagents for the synthesis of 2-aryl N-methylpyrrole, furan, thiophene and their derivatives under
visible-light photocatalyzed conditions.
Table 3. Direct C−H arylation of various five-membered heteroarenes with arylsulfonyl
chlorides a
a Unless stated otherwise all reactions were performed in a Schlenk tube with 1.0 equiv. of HC,
1.0 equiv. of AS, 1.0 equiv. of Na2CO3 and 3 mol% of [Ru(bpy)3]Cl2 in dry CH3CN under the
irradiation of 5W blue LED lamp, hν = 425±15 nm at room temperature for 12 h.
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The C–H arylation reported here seems to proceed through a radical pathway.11 Because the
irradiation of p-tolylsulfonyl chloride in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO, a radical trap) leads to aryl-TEMPO adduct in 26% yield (Supporting Information).
Likewise, addition of TEMPO to the photocatalytic reaction of AS1 with HC1 suppressed the
arylation process significantly (AH1 was obtained only about 17% yield). On the basis of these
results and literature reports,1-7,9,14,15 a plausible mechanism for the arylation of five-membered
heteroarenes has been proposed in Scheme 2. The irradiation of [Ru(bpy)3]Cl2 with a visible
light promotes metal-to-ligand charge transfer generating a photoexcited *[Ru(bpy)3]Cl2. A
single electron transfer from the excited *[Ru(bpy)3]Cl2 species to the arylsulfonyl chloride
affords [Ru(bpy)3]Cl3 and the phenyl radical (1, Scheme 2) by extrusion of SO2Cl.14,15,18 The
coupling of the phenyl radical with an electron-rich heterocycle to give a new radical
intermediate (2) that subsequently transformed into a carbocation intermediate (3, Scheme 2) by
oxidation by the [Ru(bpy)3]Cl3 (parent [Ru(bpy)3]Cl2 catalyst is regenerated). The unstable
carbocation intermediate (3) then undergoes deprotonation to give the desired heterobiaryl and
hydrogen chloride, cf. Scheme 2. The base (Na2CO3) employed in the reaction neutralizes the
hydrogen chloride generated as a byproduct in the reaction.
O
S
O
R
Cl
*[Ru(bpy)3]Cl2
SET
.
R
1
SO
+
2
R
[Ru(bpy)3]Cl2
[Ru(bpy)3]Cl3
H
X
.
R
SET
R
R
2
X
R
3
X
H
R
R
X
0.5 Na2CO3
0.5 CO2 + 0.5 H2O + NaCl
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Scheme 2. A plausible mechanism for the visible-light-induced desulfonylative coupling of
arylsulfonyl chlorides with five-membered heteroarenes. SET, single electron transfer.
As mentioned at the outset, arylsulfonyl halides have several advantages over aryldiazonium
salts in view of both economic and environmental standpoints. Moreover, heterobiaryls are
highly important structural motifs frequently found in natural products, pharmaceuticals (Figure
1) and functional materials.19 Thus, we believe that results reported here will find a bright future
in industrial applications for the direct C−H arylation of heteroarenes to synthesize heterobiaryls
and heteropolyaryls.
Figure 1. Few selected drug molecules containing heterobiaryl scaffold that is shown with a
circle.
Conclusion
In summary, a visible-light photocatalyzed direct desulfitative arylation of electron-rich
heteroarenes with arylsulfonyl chlorides is described. The method tolerates a variety of
functional groups on the phenyl ring of sulfonyl chlorides as well as heteroarenes and gives the
desired heterobiaryls in moderate to good yields at room temperature. Further work is in progress
to broaden the scope of this transformation to synthesize natural products and pharmaceuticals.
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Acknowledgments
Financial support from the Department of Science & Technology (DST), India through INSPIRE
Faculty Fellowship [IFA12-CH-62] is gratefully acknowledged. Also, we thank anonymous
reviewers for their insightful comments and suggestions.
Supplementary data
Supplementary data (general aspects, experimental characterization data for products and copies
of 1H and 13C NMR spectra for selected products) associated with this article can be found, in the
online version, at doi:00.0000/j.tet.00.
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(15) Deng, G.-B.; Wang, Z.-Q.; Xia, J.-D.; Qian, P.-C.; Song, R.-J.; Hu, M.; Gong, L.-B.;
Li, J.-H. Angew. Chem., Int. Ed. 2013, 52, 1535.
(16) General procedure for the synthesis of heterobiaryls: To an oven-dried Schlenk
tube equipped with a magnetic stir bar was charged with tris(2,2′-
bipyridyl)ruthenium(II) chloride (3 mol%), arenesulfonyl chloride (1.0 equiv.),
heteroarene (1.0 equiv.), Na2CO3 (1.0 equiv.) and dry acetonitrile. The mixture was
degassed by the freezepump-thaw procedure, and then the Schlenk tube was
irradiated under a 3W Blue LED bulb at a distance of 5 cm. After stirring at 25 °C for
12 h, the solvent was removed under reduced pressure and the residue was then
diluted with water. The aqueous solution was extracted (3 times) with ethyl acetate.
The combined organic phases were dried over Na2SO4 and filtered. The filtrate was
evaporated under reduced pressure to get the crude product, which was purified by
column chromatography using hexane-ethyl acetate mixtures. The identity and purity
1
of the product was confirmed by H and 13C NMR spectroscopy, cf. Supporting
Information.
(17) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory Chemicals, 6th ed.;
Butterworth-Heinemann, Oxford, 2009.
(18) (a) Nagib, D. A.; Macmillan, D. W. C. Nature 2011, 480, 224. (b) Houmam, A.;
Hamed, E. M. Phys. Chem. Chem. Pyhs. 2012, 14, 113.
(19) (a) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066; (b) Anthony, J. E.
Chem. Rev. 2006, 106, 5028; (c) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.;
́
Lemaire, M. Chem. Rev. 2002, 102, 1359; (d) Wang, C.; Dong, H.; Hu, W.; Liu, Y.;
Zhu, D. Chem. Rev. 2012, 112, 2208.
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