Nickel catalyzed intramolecular oxidative coupling: synthesis of 3-aryl benzofurans

Article abstract of DOI:10.1039/d0ra03071f

Recent research has been focused on the transition metal-catalyzed reactions. Herein we have developed nickel-catalyzed synthesis of 3-aryl benzofurans fromortho-alkenyl phenolsviaintramolecular dehydrogenative coupling. Notably, simple O₂gas served as an oxidant, without using any sacrificial hydrogen acceptor. The strategy enabled the synthesis of 3-aryl benzofurans in good to excellent yields.

Full text of DOI:10.1039/d0ra03071f

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Nickel catalyzed intramolecular oxidative coupling:
synthesis of 3-aryl benzofurans†
Cite this: RSC Adv., 2020, 10, 22264
Sakshi Aggarwal, Dasari Srinivas, Chinnabattigalla Sreenivasulu
*
and Gedu Satyanarayana
Received 5th April 2020
Accepted 27th May 2020
Recent research has been focused on the transition metal-catalyzed reactions. Herein we have developed
nickel-catalyzed synthesis of 3-aryl benzofurans from ortho-alkenyl phenols via intramolecular
dehydrogenative coupling. Notably, simple O2 gas served as an oxidant, without using any sacrificial
hydrogen acceptor. The strategy enabled the synthesis of 3-aryl benzofurans in good to excellent yields.
DOI: 10.1039/d0ra03071f
rsc.li/rsc-advances
furomollugin II, amiodarone III, Iantheran A IV, viniferifuran V
and pterolinus A VI (Fig. 1).3
Introduction
Due to their wide occurrence and interesting biological
properties, numerous reports have disclosed the synthesis of
benzofurans.4–22 In this context, most of the reports mainly
centered on the synthesis of 2,3-diaryl benzofurans, either via
intermolecular annulation of ortho-halophenols with olens or
via intramolecular annulation of ortho-vinyl phenols promoted
by Lewis acids/oxidants/some strong acids/bases.5–9 All of them
Benzofuran is a heterocyclic compound made up of benzene
ring fused with a furan ring and a prominent structural motif
that constitutes naturally occurring compounds, pharmaceuti-
cals, photosensitizers and molecules of biological relevance.1–3
Some of the biologically active compounds containing benzo-
furan skeleton are fused tricyclic compound (R7000) I,
Fig. 1 Representative examples of benzofurans of biological relevance.
Department of Chemistry, Indian Institute of Technology, Kandi, Sangareddy,
Hyderabad, 502 285, Telangana, India. E-mail: gvsatya@iith.ac.in; Fax: +91 40
2301 6003/32; Tel: +91 40 2301 6033
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03071f
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Scheme 1 Representative approaches for the synthesis of 3-aryl benzofurans.12,14,20a,20b,22a,22b,22d
require stoichiometric amounts of acid/oxidant/base. Arcadi and an oxidant. The required ortho-vinyl phenols have prepared
et al. described a rst palladium-catalyzed synthesis of 2,3- from the reaction of phenols and terminal arylacetylenes using
diaryl benzofuran via intramolecular cyclization of ortho- Friedel–Cras alkenylation induced by a suitable Lewis acid.
alkynes substituted phenols.6a Subsequently, a reasonable With the available ortho-alkenyl phenols, it is set for the opti-
number of reports have appeared on the synthesis of 2,3-dia- mization study to achieve 3-aryl benzofurans. Thus, initially,
rlybenzofurans using transition metal catalysts like Au, Ir, Rh, ortho-alkenyl phenol 1c was chosen as the model compound for
Cu, Pd and Fe etc.10 The best synthetic route to accomplish the preparation of 3-aryl benzofuran 2c. Various screening
benzofuran could be the intramolecular cyclization from ortho- conditions (i.e., by varying ligand, additive, oxidant, reaction
alkenyl phenol, unfortunately, this protocol requires a stoi- time and solvent etc.) have explored to nd the best-optimized
chiometric amount of sacricial hydrogen acceptor-like DDQ.8b conditions and the outcomes are summarised in Table 1. To
Notably, recently, the oxidative C–H functionalization of ortho- begin with, the reaction was performed with Ni(acac)2 (5 mol%),
alkenyl phenols to generate benzofurans has been accom- 1,10-phenanthroline (10 mol%) and DMF as solvent under inert
plished using some transition metals without the need of any conditions (nitrogen atmosphere) at 140 ꢀC for 48 h. The ex-
sacricing hydrogen acceptor.9,22b,22d Most of the earlier reports pected 3-aryl benzofuran 2c was obtained albeit in moderate
are devoted to the formation of 2,3-diaryl benzofurans/2-aryl yield along with
a
minor side product (2-hydroxy-5-
benzofurans, whereas the synthesis of 3-aryl benzofurans methylphenyl)(phenyl)methanone 3c (Table 1, entry 1). Even
from ortho-alkenyl phenols was scarcely explored.20,22 Some of switching to PPh3 as the ligand, furnished the product 2c in
the representative examples of the previous study versus the more or less same yield (Table 1, entry 2). Interestingly, under
present protocol is described in Scheme 1.
the same reaction conditions (i.e. Table 1, entry 2), but with
We have been interested in the ambitious catalytic nature of molecular O2 as the oxidant, there was a drastic change in the
late transition metals.23 Recently, we have developed a synthesis yield of 2c along with the minimal amount of ketone 3c (Table 1,
of 2,3-diaryl benzofuran by using phenols and internal alky- entry 3). However, with additive TEMPO and with PPh3 under
nes11a and also reported a synthesis of 2H/4H-chromenes from open air at 140 ꢀC for 48 h, improved the yield of 2c to 80%
phenols with terminal alkynes (aryl/alkyl) using Lewis acidic (Table 1, entry 4). Performing the reaction with TEMPO/PPh3
conditions.11b With this background of phenols and the alkyne and under an oxygen atmosphere, gave the product 2c in the
chemistry. We intended to develop nickel catalyzed oxidative same yield but with a reduced amount of time 36 h (Table 1,
cross coupling reactions. Herein, we describe an efficient entry 5). On the other hand, the other additives (DDQ, Oxone,
method to cyclize ortho-alkenyl phenols to give benzofurans K2S2O8 and TBHP/H2O, under oxygen atmosphere) in the same
facilitated by Ni(acac)2 and O2 as an oxidant.
solvent DMF were not active improve the yields of 2c (Table 1,
entries 6 to 9). The reaction was inferior in solvents, such as
H2O and ortho-xylene, under oxygen atmosphere (Table 1,
entries 10 & 11), while in DMSO under an oxygen atmosphere,
gave 2c in 70% yield (Table 1, entry 12). To our delight, the
reaction in solvent DMA under oxygen atmosphere afforded the
product 2c in 81% yield (Table 1, entry 13). When the reaction
Results and discussion
To begin with, it was contemplated that 3-aryl benzofuran can
be achieved from ortho-vinyl phenols using intramolecular
oxidative coupling feasible by means of a suitable metal catalyst
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Table 1 Screening conditions for the formation of 3-aryl benzofurans 2c from 1ca,e
Yield 2c
(%)
Entry
Catalyst (mol%)
Ligand (10 mol%)
Additives (mol%)
Oxidant
Solvent (mL)
Time (h)
Temp. (ꢀC)
b
1
2
3
4
5
6
7
8
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (2)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
NiCl2 (5)
1,10-Phen
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
1,10-Phen
1,10-Phen
—
—
—
—
—
DMF (1.5)
DMF (1.5)
DMF (1.5)
DMF (2)
48
36
36
48
36
36
36
48
48
48
36
36
36
36
36
40
40
72
72
72
72
48
36
36
72
72
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
140
120
80
40c,d
42
b
—
O2
Open air
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
Open air
O2
70d
80d
80d
TEMPO (10)
TEMPO (10)
DDQ (10)
DMF (2)
c
DMF (1.5)
DMF (1.5)
DMF (1.5)
DMF (1.5)
H2O (1)
o-Xylene (1)
DMSO (1.5)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
CH3CN (2)
CH3CN (2)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
DMA (1)
CH3CN (2)
CH3CN (2)
—
OXONE (10)
K2S2O8 (10)
TBHP in H2O (10)
TEMPO (10)
TEMPO (10)
TEMPO (10)
TEMPO (10)
TEMPO (10)
TEMPO (10)
TEMPO (10)
—
30c,d
34c,d
48c,d
9
c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
18
19
—
c
—
70c
81d
70d
64d
60d
45
O2
—
—
TEMPO (5)
DDQ (5)
Open air
Open air
Open air
O2
O2
O2
O2
Open air
Open air
60d
50d
c
PPh3
PPh3
PPh3
PPh3
PPh3
—
K2CO3 (2 equiv.)
K2CO3 (2 equiv.)
TEMPO (10)
TEMPO (10)
TEMPO (10)
TEMPO (5)
DDQ (5)
—
c
—
50d,e
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
Ni(acac)2 (5)
30c
c
—
140
140
60d
50d
—
a
All reactions were carried out using ortho-alkenyl phenol 1c (83 mg, 0.4 mmol), Ni(acac)2 [0.008 mmol (2 mol%) to 0.02 mmol (5 mol%)], ligand
(0.04 mmol, 10 mol%). b Reaction was conducted under nitrogen atmosphere. c Very less conversion was observed. d The by-product 3c was formed
(up-to 5–20% yields). e Some other volatile by-products also formed along with 2c, which were not isolable.
conducted using 2 mol% of the catalyst Ni(acac)2, under an
Now to test the scope and applicability of the strategy, best-
oxygen atmosphere, afforded the product 2c but with slightly optimized conditions (Table 1, entry 15) applied to different
decreased yield (Table 1, entry 14). Replacing the ligand PPh3 ortho-alkenyl phenols 1a–w. This protocol was found to be quite
with 1,10-phenanthroline in the open air and an oxygen atmo- successful and afforded the corresponding 3-aryl benzofurans
sphere, gave the product 2c in 64% and 60% yields, respectively 2a–w, good to very good yields (Table 2). For example, the
(Table 1, entries 15 & 16). The reaction without ligand and reaction was amenable with electron-donating groups like Me,
additive, under oxygen atmosphere, afforded 2c in 45% Et, OMe substituents on phenol moiety of ortho-alkenyl phenols
moderate yield (Table 1, entry 17). On the other hand, treatment (Table 2). The reaction also found smooth with the a-naphthol,
of 1c either with TEMPO or DDQ as additive without ligand and b-naphthol, and 6-bromo-2-naphthol derived alkenols (Table 2).
in the open air in CH3CN as a solvent found to be slightly Moreover, the reaction was obedient with the electron donating
inferior (Table 1, entries 18 & 19). While using K2CO3 as the base (Me and OMe) and the partial electron withdrawing substitu-
instead of additive TEMPO, in the open air and in the presence ents (F and Br) on the ring of terminal alkynes of ortho-alkenyl
of oxygen atmosphere showed no progress indicating the phenols (Table 2). All synthesized 3-aryl benzofurans 2a–w, are
importance of TEMPO to initiate the reaction (Table 1, entries characterized by spectrometric data (1H-NMR and 13C-NMR and
20 & 21). On the other hand, by employing NiCl2 as a catalyst, Mass Spectrometry) and also with the earlier literature.
only a 50% yield of the product 2c was obtained (Table 1, entry However, it is essential to note that even alkenylation reaction
ꢀ
22). Further, the reaction at reduced temperatures 120 C and did not proceed when electron withdrawing groups such as
80 ꢀC, showed little conversion and no conversion, respectively CHO and NO2 anked to the phenol moiety. And it can be
(Table 1, entries 23 & 24).
justied based on the fact that electron withdrawing functional
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Table 2 Scope for the formation of 3-aryl benzofurans 2a–wa,b
a
Reactions were carried out using ortho-alkenyl phenols 1a–w (79.0–110.0 mg, 0.4 mmol), Ni(acac)2 (5.2 mg, 0.02 mmol, 5 mol%), PPh3 (10.5 mg,
0.04 mmol, 10 mol%), TEMPO (6.5 mg, 0.04 mmol) at 140 ꢀC under oxygen atmosphere. b Yields in the parenthesis are isolated yields of products.
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Scheme 2 Plausible mechanism for the formation of 3-aryl benzofurans 2 from 1.
groups retard the Friedel–Cras alkylation/alkenylation reac- This methodology found viable for accomplishing several
tions. Similarly, the attempt made to synthesis ortho-alkenyl different 3-arylbenzofuran derivatives in good to very good yields.
phenols were not successful with meta-amino functionality on
the aromatic ring of the alkyne. This could be due to the more
reactive nature of the amino group under Lewis acid conditions.
Experimental section
Moreover, aliphatic alkynes could not make compatible to give
ortho-alkenylation products as well.
IR spectra recorded on a Bruker Tensor 37 (FT-IR) spectro-
1
photometer. H-NMR spectra recorded on Bruker Avance 400
(400 MHz) spectrometer at 295 K in CDCl3; chemical shis (d
in ppm) and coupling constants (J in Hz) reported in standard
Plausible mechanism
fashion with reference to either internal standard tetrame-
Though the exact mechanism is not very certain at this stage,
thylsilane (TMS) (dH ¼ 0.00 ppm) or CHCl3 (dH ¼ 7.25 ppm).
based on the present observations and literature reports,12,20,23 we
have attempted to propose a plausible reaction mechanism as
depicted in Scheme 2. Initially, Ni(II) could combine with phos-
phine ligand PPh3 converted into its reduced Ni(0)-catalyst, which
may act as an active catalyst. On the other hand, in an independent
path, ortho-alkenyl phenol 1 would be transformed into its oxy-
radical A under probable catalytic oxidative conditions of
TEMPO/O2 system. Now coupling of oxy-radical A with Ni(0)-
catalyst would lead to the formation of intermediate B. Subse-
quently, intramolecular p-complexation with olenic double bond
followed by olenic C–H activation could generate a six-membered
oxa-nickelacycle C via the removal of H-radical. Finally, reductive
elimination of catalyst from C gives 3-aryl benzofuran 2 and
regenerates the Ni(0)-catalyst. Thus, completes the catalytic cycle.
13C-NMR spectra recorded on Bruker Avance 400 (100 MHz)
spectrometer at RT in CDCl3; chemical shis (d in ppm) are
reported relative to CDCl3 [dC ¼ 77.00 ppm (central line of the
triplet)]. In the 13C-NMR, the nature of carbons (C, CH, CH2
and CH3) was determined by recording the DEPT-135 spectra,
and is given in parentheses and noted as s ¼ singlet (for C),
d ¼ doublet (for CH), t ¼ triplet (for CH2) and q ¼ quartet (for
1
CH3). In the H-NMR, the following abbreviations were used
throughout: s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet, m
1
¼ multiplet. The assignment of signals conrmed by H, 13C
CPD and DEPT spectra. High-resolution mass spectra (HR-MS)
were recorded on an Agilent 6538 UHD Q-TOF using multi-
mode source. Reactions were monitored by TLC on silica gel
coated on alumina plate or glass plate using a mixture of
petroleum ether and ethyl acetate as eluents. Reactions carried
out under oxygen atmosphere.
Conclusion
Materials
In summary, we have established a nickel-catalyzed synthesis of
3-aryl benzofurans via intramolecular oxidative cyclization of All solvents distilled before using; petroleum ether with
ꢀ
ortho-alkenyl phenols. Notably, simple oxygen served as the sole a boiling range of 60 to 80 C, dichloromethane (DCM), ethyl
oxidant and precludes the use of sacricial hydrogen acceptor. acetate, dry DMA (boiling range 160 to 170 ꢀC; with purity 99%)
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were purchased from Sigma Aldrich
commercial sources used. Acme's silica gel (100–200 mesh) at 140 C for 36 h for the product 2f formation. Purication of
&
locally available (6.3 mg, 0.04 mmol) and allowed the reaction mixture to stirred
ꢀ
used for column chromatography.
the crude material by silica gel column chromatography
(petroleum ether/ethyl acetate, 100 to 99 : 01), furnished the
product 2f (60 mg, 68%) as pale yellow viscous liquid. [TLC
control (petroleum ether/ethyl acetate 99 : 01), Rf(1f) ¼ 0.10,
Rf(2f) ¼ 0.60, UV detection]. 1H NMR (CDCl3, 400 MHz): d ¼ 7.47
(s, 1H, Ar–O–CH), 7.46–7.30 (m, 5H, Ar–H), 7.19 (s, 1H, Ar–H),
GP (general procedure for the synthesis of 3-aryl benzofurans)
To an oven dry Schlenk tube was equipped with a magnetic stir
bar, were added Ni(acac)2 (5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04
mmol), TEMPO (6.3 mg, 0.04 mmol), ortho-alkenyl phenols 1a–w
(79–110.0 mg, 0.4 mmol), and DMA (1 mL). Then a balloon lled
with O2 was attached to the Schlenk tube. The reaction mixture
stirred at 140 ꢀC for 24 to 36 h. TLC monitored the progress of the
reaction. The reaction mixture was then cooled to room tempera-
ture and extracted by using ethyl acetate (3 ꢁ 20 mL). The organic
layers were washed with saturated NH4Cl solution, dried by
Na2SO4 and then ltered. Evaporation of the solvent(s) under
reduced pressure and renement of the crude mixture by silica gel
column chromatography (petroleum ether/ethyl acetate), gave the
3-aryl benzofurans (68–88%) as semi-solid or liquid.
6.84 (s, 1H, Ar–H), 2.44 (s, 3H, CH3), 2.21 (s, 3H, CH3) ppm. 13
C
NMR (CDCl3, 100 MHz): d ¼ 155.9 (s, Ar–C), 141.4 (d, Ar–O–CH),
134.6 (s, Ar–C), 133.1 (s, Ar–C), 131.4 (s, Ar–C), 130.1 (d, 2C, 2 ꢁ
Ar–CH), 128.0 (s, 2C, Ar–CH), 127.4 (d, Ar–CH), 125.9 (d, Ar–CH),
123.5 (s, Ar–C), 123.2 (s, 1Ar–C), 109.4 (d, Ar–CH), 21.4 (q, CH3)
19.7 (q, CH3) ppm. HR-MS (ESI+) m/z calculated for
[C16H18OFN]+ ¼ [M + NH4]+: 240.1383; found 240.2062.
5-Fluoro-3-(p-tolyl)benzofuran (2n).
5-Ethyl-3-phenylbenzofuran (2d).
GP was carried out with 1n (90 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO
(6.3 mg, 0.04 mmol) and allowed the reaction mixture to stirred
at 140 ꢀC for 36 h for the product 2n formation. Purication of
the crude material by silica gel column chromatography
(petroleum ether/ethyl acetate, 100 to 99 : 01), furnished the
product 2n (68 mg, 75%) as pale yellow viscous liquid. [TLC
control (petroleum ether/ethyl acetate 99 : 01), Rf(1n) ¼ 0.10,
GP was carried out with 1d (89 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO (6.3 mg,
0.04 mmol) and allowed the reaction mixture to stirred at 140 ꢀC
for 36 h for the product 2d formation. Purication of the crude
material by silica gel column chromatography (petroleum ether/
ethyl acetate, 100 to 99 : 01), furnished the product 2d (71 mg,
80%) as pale yellow viscous liquid. [TLC control (petroleum ether/
ethyl acetate 99 : 01), Rf(1d) ¼ 0.10, Rf(2d) ¼ 0.60, UV detection]. 1H
NMR (CDCl3, 400 MHz): d ¼ 7.77 (s, 1H, Ar–O–CH), 7.70–7.57 (m,
3H, Ar–H), 7.56–7.44 (m, 3H, Ar–H), 7.43–7.28 (m, 1H, Ar–H), 7.27–
7.13 (m, 1H, Ar–H), 2.79 (q, 2H, J ¼ 7.6 Hz, Ar–CH2CH3), 1.31 (t, 3H,
J ¼ 7.6 Hz, Ar–CH2CH3) ppm. 13C NMR (CDCl3, 100 MHz): d ¼
154.3 (s, Ar–C), 141.5 (d, Ar–O–CH), 139.2 (s, Ar–C), 132.3 (s, Ar–C),
128.9 (d, 2C, 2 ꢁ Ar–CH), 127.5 (d, 2C, 2 ꢁ Ar–CH), 127.3 (d, Ar–
CH), 126.5 (s, Ar–C), 124.8 (d, Ar–CH), 122.1 (s, Ar–C), 119.0 (d, Ar–
CH), 111.4 (d, Ar–CH), 29.01 (t, Ar–CH2), 16.44 (q, Ar–CH2–
CH3) ppm. HR-MS (ESI+) m/z calculated for [C16H14OK]+ ¼ [M + K]+:
261.0676; found 261.0859.
1
Rf(2n) ¼ 0.60, UV detection]. H NMR (CDCl3, 400 MHz): d ¼
7.78 (s, 1H, Ar–O–CH), 7.54–7.41 (m, 4H, Ar–H), 7.28 (d, 2H, J ¼
7.8 Hz, Ar–H), 7.05 (td, 1H, J ¼ 9.1, 2.45 Hz, Ar–H), 2.41 (s, 3H,
Ar–CH3) ppm. 13C NMR (CDCl3, 100 MHz): d ¼ 160.6 (s, Ar–C),
158.3 (s, Ar–C), 152.0 (s, Ar–C), 142.7 (d, Ar–O–CH), 137.5 (s, Ar–
C), 129.8 (d, 2C, 2 ꢁ Ar–CH), 128.5 (s, Ar–C), 127.5 (s, Ar–C),
127.4 (s, Ar–C) 127.2 (d, 2C, 2 ꢁ Ar–CH), 122.5 (s, Ar–C), 122.4 (s,
Ar–C), 112.4 (d, Ar–CH), 112.3 (d, Ar–CH), 112.1 (d, Ar–CH),
106.2 (d, Ar–CH), 105.9 (d, Ar–CH), 21.23 (q, Ar–CH3) ppm. HR-
MS (ESI+) m/z calculated for [C15H10F]+ ¼ [(M + H) + (–H2O)]+:
209.0761; found 209.0765.
5-Methyl-3-(m-tolyl)benzofuran (2q).
4,6-Dimethyl-3-phenylbenzofuran (2f).
GP was carried out with 1q (89 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO
(6.3 mg, 0.04 mmol) and allowed the reaction mixture to stirred
at 140 ꢀC for 36 h for the product 2q formation. Purication of
GP was carried out with 1f (89 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO
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the crude material by silica gel column chromatography the crude material by silica gel column chromatography
(petroleum ether/ethyl acetate, 100 to 99 : 01), furnished the (petroleum ether/ethyl acetate, 100 to 99 : 01), furnished the
product 2q (70 mg, 79%) as pale yellow viscous liquid. [TLC product 2t (74 mg, 75%) as pale yellow viscous liquid. [TLC
control (petroleum ether/ethyl acetate 99 : 01), Rf(1q) ¼ 0.10, control (petroleum ether/ethyl acetate 99 : 01), Rf(1t) ¼ 0.10,
1
Rf(2q) ¼ 0.60, UV detection]. H NMR (400 MHz, CDCl3): d ¼ Rf(2t) ¼ 0.60, UV detection]. 1H NMR (400 MHz, CDCl3) d ¼ 7.77
7.73 (s, 1H Ar–O–CH), 7.61 (s, 1H, Ar–H), 7.47–7.33 (m, 4H, Ar– (s, 1H, Ar–O–CH), 7.49–7.43 (m, 2H, Ar–H), 7.41 (dd, J ¼ 3.4,
H), 7.23–7.08 (m, 2H, Ar–H), 2.47 (s, 3H, Ar–CH3), 2.44 (s, 3H, 1.9 Hz, 1H, Ar–H), 7.32 (ddd, J ¼ 9.8, 2.4, 1.6 Hz, 1H, Ar–H), 7.24
Ar–CH3) ppm. 13C NMR (CDCl3, 100 MHz): d ¼ 154.2 (s, Ar–C), (d, J ¼ 2.6 Hz, 1H, Ar–H), 7.11–7.03 (m, 1H, Ar–H), 6.98 (dd, J ¼
141.4 (d, Ar–O–CH), 138.6 (s, Ar–C), 132.4 (s, Ar–C), 132.1 (s, Ar– 8.9, 2.6 Hz, 1H, Ar–H), 3.88 (s, 3H, OCH3) ppm. 13C NMR (100
C), 128.8 (d, Ar–CH), 128.2 (d, Ar–CH), 128.1 (d, Ar–CH), 126.6 (s, MHz, CDCl3) d ¼ 163.2 (d, Jc–f ¼ 294 Hz, Ar–CF), 156.38 (s, Ar–C),
Ar–C), 125.7 (d, Ar–CH), 124.6 (d, Ar–CH), 122.0 (s, Ar–C), 120.2 150.76 (s, Ar–C), 142.57 (d, Ar–CH), 134.3 (d, Jc–f ¼ 8 Hz, Ar–C),
(d, Ar–CH), 111.2 (d, Ar–CH), 21.5 (q, Ar–CH3), 21.5 (q, Ar– 130.5 (d, Jc–f ¼ 9 Hz, Ar–CH), 126.57 (s, Ar–C), 123.02 (d, Jc–f
¼
CH3) ppm. HR-MS (ESI+) m/z calculated for [C16H18NO]+ ¼ [M + 2 Hz, Ar–CH), 121.4 (s, Ar–C), 114.2 (d, Jc–f ¼ 21 Hz, Ar–CH),
NH4]+: 240.1383; found 240.1373.
114.2 (d, Jc–f ¼ 21 Hz, Ar–CH), 113.50 (d, Ar–CH), 112.31 (d, Ar–
CH), 102.65 (d, Ar–CH), 56.03 (q, OCH3) ppm. HR-MS (ESI+) m/z
calculated for [C15H13FKO3]+ ¼ [(M + K) + (–H2O)]+: 263.0269;
found 263.0273.
5-Ethyl-3-(m-tolyl)benzofuran (2r).
1-(3-Fluorophenyl)naphtho[2,1-b]furan (2u).
GP was carried out with 1r (94 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO
(6.3 mg, 0.04 mmol) and allowed the reaction mixture to stirred
GP was carried out with 1u (105 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO
(6.3 mg, 0.04 mmol) and allowed the reaction mixture to stirred
at 140 ꢀC for 24 h for the product 2u formation. Purication of
the crude material by silica gel column chromatography
(petroleum ether/ethyl acetate, 100 to 99 : 01), furnished the
product 2u (82 mg, 88%) as pale yellow viscous liquid. [TLC
control (petroleum ether/ethyl acetate 99 : 01), Rf(1u) ¼ 0.10,
Rf(2u) ¼ 0.60, UV detection]. 1H NMR (400 MHz, CDCl3) d ¼ 7.86
(d, J ¼ 8.2 Hz, 1H, Ar–H), 7.80 (d, J ¼ 8.0 Hz, 1H, Ar–H), 7.63 (d, J
¼ 9.0 Hz, 1H, Ar–H), 7.55 (d, J ¼ 9.8 Hz, 2H, Ar–H), 7.37–7.29 (m,
2H, Ar–H), 7.25 (t, J ¼ 7.5 Hz, 2H, Ar–H), 7.18 (d, J ¼ 9.6 Hz, 1H,
Ar–H), 7.09–6.98 (m, 1H, Ar–H) ppm. 13C NMR (100 MHz,
CDCl3) d ¼ 162.8 (d, Jc–f ¼ 245 Hz, Ar–C), 153.2 (s, Ar–C), 141.8
(d, Ar–CH), 135.3 (d, Jc–f ¼ 8 Hz, Ar–C), 130.8 (s, Ar–C), 130.0 (d,
Jc–f ¼ 9 Hz, Ar–CH), 129.0 (d, Ar–CH), 128.1 (s, Ar–C), 126.1 (d, Jc–
ꢀ
at 140 C for 36 h for the product 2r formation. Purication of
the crude material by silica gel column chromatography
(petroleum ether/ethyl acetate, 100 to 99 : 01), furnished the
product 2r (71 mg, 76%) as pale yellow viscous liquid. [TLC
control (petroleum ether/ethyl acetate 99 : 01), Rf(1r) ¼ 0.10,
Rf(2r) ¼ 0.60, UV detection]. 1H NMR (400 MHz, CDCl3): d ¼ 7.74
(s, 1H, Ar–O–CH), 7.62 (d, 1H, J ¼ 0.1 Hz, Ar–H), 7.51–7.41 (m,
3H, Ar–H), 7.39–7.33 (m, 1H, Ar–H), 7.21–7.15 (m, 2H, Ar–H),
2.77 (q, 2H, J ¼ 7.3 Hz, Ar–CH2CH3), 2.44 (s, 3H, CH3), 1.28 (t,
3H, J ¼ 7.6 Hz, Ar–CH2CH3) ppm. 13C NMR (CDCl3, 100 MHz):
d ¼ 154.3 (s, Ar–C), 141.5 (d, Ar–O–CH), 139.1 (s, Ar–C), 138.6 (s,
Ar–C), 132.2 (s, Ar–C), 128.8 (d, Ar–CH), 128.2 (d, Ar–CH), 128.2
(d, Ar–CH), 126.6 (s, Ar–C), 124.7 (d, Ar–CH), 124.6 (d, Ar–CH),
122.2 (s, Ar–C), 119.0 (d, Ar–C), 111.3 (d, Ar–C), 29.0 (t, CH2),
21.5 (q, CH3), 16.5 (q, CH3) ppm. HR-MS (ESI+) m/z calculated
for [C17H19NO]+ ¼ [M + NH4]+: 253.1461; found: 254.2219.
3-(3-Fluorophenyl)-5-methoxybenzofuran (2t).
¼ 5 Hz, Ar–CH), 125.6 (d, Jc–f ¼ 3 Hz, Ar–CH), 124.5 (d, Ar–CH),
f
123.3 (d, Jc–f ¼ 2 Hz, Ar–C), 123.2 (d, Ar–CH), 120.3 (s, Ar–C),
116.8 (d, Jc–f ¼ 22 Hz, Ar–CH), 114.8 (d, Jc–f ¼ 21 Hz, Ar–CH),
112.6 (d, Ar–CH) ppm. HR-MS (ESI+) m/z calculated for
[C18H12FO]+ ¼ [M + H]+: 263.0867; found 263.0862.
Conflicts of interest
There are no conicts of interest to declare.
GP was carried out with 1t (98 mg, 0.4 mmol) using Ni(acac)2
(5.2 mg, 0.02 mmol), PPh3 (10.5 mg, 0.04 mmol), TEMPO
Acknowledgements
(6.3 mg, 0.04 mmol) and allowed the reaction mixture to stirred We are grateful to the Department of Science and Technology-
ꢀ
at 140 C for 24 h for the product 2t formation. Purication of Science and Engineering Research Board (DST-SERB) [No.
22270 | RSC Adv., 2020, 10, 22264–22272
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EMR/2017/005312], New Delhi, for nancial support. C. B. S
thanks to UGC and D. S thanks to DST-SERB for the research
fellowship.
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