Solution-phase photochemical transformation of 2-aroylbenzofurans: Addition-elimination mechanism

Article abstract of DOI:10.1007/s11164-014-1544-6

An efficient conversion of 2-aroylbenzofurans into arylbenzofurylmethanols through photoreduction has been described. In these molecules, the photo transformation occurred through the initial addition of solvent followed by elimination involving γ-hydroge

Full text of DOI:10.1007/s11164-014-1544-6

Res Chem Intermed
DOI 10.1007/s11164-014-1544-6
Solution-phase photochemical transformation
of 2-aroylbenzofurans: addition–elimination
mechanism
•
•
•
Pooja Jindal Geeta Sharma Rita Arora
Ramesh C. Kamboj
Received: 30 October 2013 / Accepted: 15 January 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract An efficient conversion of 2-aroylbenzofurans into arylbenzofuryl-
methanols through photoreduction has been described. In these molecules, the photo
transformation occurred through the initial addition of solvent followed by elimi-
nation involving c-hydrogen abstraction. The photoproduct distribution markedly
depends upon the substituent present on the benzofuran moiety.
Keywords 2-Aroylbenzofurans ꢀ Photoreduction ꢀ c-Hydrogen abstraction ꢀ
Addition–elimination
Introduction
Synthetic organic photochemistry—chemistry of the electronic excited states of a
molecule—differs from ground state chemistry and affords a convenient approach
to complex structures that are difficult to obtain by traditional synthetic methods.
The photochemistry of O-heterocycles is a less developed area as compared to
carbon aromatics [1–4], such as benzene, naphthalene, and anthracene derivatives.
Numerous types of conjugated O-heterocyclic compounds [5, 6] play important
roles in both natural and artificial systems due to their particular photochemical and
photophysical properties. From synthetic and mechanistic perspectives, benzofuran
derivatives [7, 8] showing very interesting photochemical and photophysical
properties are among the most extensively investigated organic compounds.
Benzofuran [9] and its derivatives are widely distributed in nature, particularly
among the plant monarchy. The benzofuran derivatives hold importance in
molecular biology such as in the treatment of osteoporosis [10, 11], as anesthetic
agents [12, 13], inhibitors of platelets aggregations [14], inhibitors of xanthine
P. Jindal ꢀ G. Sharma ꢀ R. Arora ꢀ R. C. Kamboj (&)
Department of Chemistry, Kurukshetra University, Kurukshetra 136119, Haryana, India
e-mail: rckamboj2012@hotmail.com
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P. Jindal et al.
oxidase [15], antiarrhythmics [16, 17], anti-HIV agents [12], agonists of benzodi-
azepines [18], and as inhibitors of ATPase [19]. Some of these derivatives have also
found applications as sedatives [20], agrochemicals [21], cosmetics [22], and dyes
[23, 24], and as the building blocks of optical brighteners [25]. In addition to the
biological properties, benzofuran derivatives have fascinating photo-luminescent
properties due to their conjugated p-system [26, 27]. Moreover, some benzofuran
derivatives have been used as light collectors in photovoltaic cells [28, 29].
In the light of the enormous interest in terms of versatile photochemistry and other
applications as above, in this paper, we endeavored to study the photochemistry of
some benzoyl derivatives of benzofuran. The salient feature of these substrates is their
extended conjugation, as in these compounds the benzofuran ring is substituted with
the benzoyl group at the 2-position and with a CH3 group at the 3-position on the
benzofuran moiety. The main focus is to explore the photolytic behavior of these
specially designed 2-aroylbenzofurans with the objectives: (1) to observe the
intramolecular c-H abstraction by C=O from the suitably positioned 3-CH3 group on
the benzofuran moiety, (2) to analyze the direct intermolecular hydrogen transfer for
photoreduction from the solvent to the C=O group of the substrate, (3) to examine the
other photochemical reactions like the addition of solvent, etc. to the substrates as
these have the chromophores as double bonds, (4) to analyze the possibility of the
photoreduction of the carbonyl chromophore, (5) to explore the ring photochemistry of
benzofurans, and (6) to investigate the formation of dimeric products.
Experimental
General
1
The nuclear magnetic resonance H NMR (300 MHz) and proton-decoupled 13C
NMR (75.4 MHz) spectra were recorded on a Bruker spectrometer using TMS as
internal standard. The infrared (IR) spectra were scanned in KBr pellets on a
MB3000 FT-IR with Horizon MBTM FTIR software from ABB Bomen. The mass
spectral data was obtained on a Waters Q-TOF Micromass (LC–MS) spectrometer.
Melting points were determined in open capillaries and are uncorrected. The
photoirradiation of the deaerated solution of substrates was carried out using a Pyrex
filter under nitrogen atmosphere from a 125 W Hg lamp. The columns for
chromatographic separation were packed in petroleum ether with silica gel and were
eluted with petroleum ether and ethyl acetate mixture (99:1).
Synthesis of 2-aroylbenzofurans
Synthesis of 2-(4-bromobenzoyl)-3-methylbenzofuran 3a A mixture of o-hydroxy-
acetophenone (1.36 g, 0.01 mol), a-bromoketone (2.62 g, 0.01 mol), Na2CO3
(2.12 g, 0.02 mol) and n-Bu4N?I- (0.01 mol) was mixed in a mortar with pestle.
This mixture was transferred to a round-bottom flask containing 10.0 ml water and
then refluxed on a heating mantle. After completion of the reaction (30 min, TLC),
the reaction mixture was cooled at room temperature. The brown-colored solid thus
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Photochemical transformation of 2-aroylbenzofurans
obtained was filtered, dried, and purified by column chromatography on silica gel
(100–200 mesh) using petroleum ether/ethyl acetate as eluent mixture (petroleum
ether:EtOAc::99:1) to remove the coloured impurities that furnished the pure white
product.
The other 2-aroylbenzofurans 3b–e and 3a0–e0 were also synthesized following a
similar procedure by condensing the respective o-hydroxyacetophenones with
a-bromoketones.
2-(4-Bromobenzoyl)-3-methylbenzofuran 3a Yield 2.35 g (75 %), white solid, mp
80–82 °C; mmax (cm-1): 1643 (C=O); dH (CDCl3): 8.03 (d, 2H, Jo = 8.4 Hz, H-200,
600), 7.74–7.70 (m, 1H, H-7), 7.68–7.53 (m, 4H, H-4, 6, 300, 500), 7.39–7.37 (m, 1H,
H-5), 2.68 (s, 3H, 3-CH3); dC (CDCl3): 184.49 (C=O), 154.17, 147.89, 136.38,
132.12, 131.53, 130.33, 129.01, 128.39, 127.64, 123.37, 121.45, 112.14, 10.01.
Mass (m/z) ES?: 316.7, 314.7 (M?), 184.9, 182.9, 159.1 (100), 156.9, 154.9; Anal.
Calcd. for C16H11BrO2: C, 60.98; H, 3.52. Found: C, 60.93; H, 3.47.
2-(4-Bromobenzoyl)-5-chloro-3,6-dimethylbenzofuran 3b Yield 2.78 g (77 %),
white solid, mp 141–142 °C; mmax (cm-1): 1643 (C=O); dH (CDCl3): 7.94 (d, 2H,
Jo = 8.1 Hz, H-200, 600), 7.64 (d, 2H, Jo = 8.1 Hz, H-300, 500), 7.40 (s, 1H, H-4), 7.24
(s, 1H, H-7), 2.58 (s, 3H, 3-CH3), 2.50 (s, 3H, 6-CH3); dC (CDCl3): 184.15 (C=O),
152.93, 147.58, 137.02, 136.37, 131.64, 131.27, 130.13, 128.37, 128.22, 126.84,
121.16, 113.78, 21.23, 10.03. Mass (m/z) ES?: 367.0, 365.0, 363.0 (M?), 209.0,
207.0 (100), 184.9, 182.9, 156.9, 154.9; Anal. Calcd. for C17H12BrClO2: C, 56.15;
H, 3.33. Found: C, 56.19; H, 3.36.
2-(4-Bromobenzoyl)-3,5-dimethylbenzofuran 3c Yield 2.69 g (82 %), white solid,
mp 96–98 °C; mmax (cm-1): 1,636 (C=O); dH (CDCl3): 8.01 (d, 2H, Jo = 8.4 Hz,
H-200, 600), 7.68 (d, 2H, Jo = 8.4 Hz, H-300, 500), 7.50 (d, 1H, Jm = 1.5 Hz, H-4), 7.43
(d, 1H, Jo = 8.4 Hz, H-7), 7.34 (dd, 1H, Jo = 8.4 Hz, Jm = 1.5 Hz, H-6), 2.66 (s,
3H, 3-CH3), 2.52 (s, 3H, 5-CH3); dC (CDCl3): 184.56 (C=O), 152.83, 148.23,
136.62, 133.07, 131.58, 131.31, 130.04, 129.20, 127.59, 127.33, 120.98, 111.74,
21.36, 10.03; Mass (m/z) ES?: 330.7, 328.7 (M?), 173.0 (100), 184.9, 182.9, 156.9,
154.9; Anal. Calcd. for C17H13BrO2: C, 62.03; H, 3.98. Found: C, 62.08; H, 3.93.
5-Bromo-2-(4-bromobenzoyl)-3-methylbenzofuran 3d Yield 3.14 g (80 %), white
solid, mp 126–128 °C; mmax (cm-1): 1,643 (C=O); dH (CDCl3): 7.99 (d, 2H,
Jo = 8.4 Hz, H-200, 600), 7.86 (d, 1H, Jm = 1.8 Hz, H-4), 7.69 (d, 2H, Jo = 8.4 Hz,
H-300, 500), 7.61 (dd, 1H, Jo = 8.7 Hz, Jm = 1.8 Hz, H-6), 7.44 (d, 1H, Jo = 8.7 Hz,
H-7), 2.64 (s, 3H, 3-CH3); dC (CDCl3): 184.37 (C=O), 152.87, 148.85, 136.18,
131.71, 131.35, 131.28, 131.05, 128.01, 126.43, 124.19, 116.62, 113.77, 9.91. Mass
(m/z) ES?: 396.8, 394.8, 392.8 (M?), 238.9, 236.9 (100), 184.9, 182.9, 156.9,
154.9; Anal. Calcd. for C16H10Br2O2: C, 48.77; H, 2.56. Found: C, 48.72; H, 2.51.
2-(4-Bromobenzoyl)-5-chloro-3-methylbenzofuran 3e Yield 2.72 g (78 %), white
solid, mp 106–108 °C; mmax (cm-1): 1,643 (C=O); dH (CDCl3): 8.03 (d, 2H,
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Jo = 8.4 Hz, H-200, 600), 7.69–7.68 (m, 3H, H-4, 300, 500), 7.48–7.47 (m, 2H, H-6, 7),
2.64 (s, 3H, 3-CH3); dC (CDCl3): 184.38 (C=O), 152.51, 149.04, 136.19, 131.70,
131.28, 130.44, 129.27, 128.72, 128.00, 126.60, 121.02, 113.36, 9.92. Mass (m/z)
ES?: 353.2, 351.2, 349.2 (M?), 195.1, 193.1 (100), 184.9, 182.9, 156.9, 154.9;
Anal. Calcd. for C16H10BrClO2: C, 54.97; H, 2.88. Found: C, 54.90; H, 2.81.
2-(4-Chlorobenzoyl)-3-methylbenzofuran 3a0 Yield 1.89 g (70 %), white solid, mp
100–102 °C; mmax (cm-1): 1,643 (C=O). dH (CDCl3): 8.09 (d, 2H, Jo = 8.4 Hz, H-200,
600), 7.74 (dd, 1H, Jo = 7.8 Hz, Jm = 1.5 Hz, H-7), 7.58–7.50 (m, 4H, H-4, 6, 300, 500),
7.40–7.35 (m, 1H, H-5), 2.69 (s, 3H, 3-CH3). dC (CDCl3): 184.39 (C=O), 154.28,
148.06, 139.02, 136.10, 131.22, 129.13, 128.88, 128.41, 127.44, 123.44, 121.49,
112.21, 10.03; Mass (m/z) ES?: 273.2, 271.2 (M?), 159.1 (100), 141.1, 139.1, 113.1,
111.1; Anal. Calcd. for C16H11ClO2: C, 70.99; H, 4.10. Found: C, 70.93; H, 4.02.
5-Chloro-2-(4-chlorobenzoyl)-3,6-dimethylbenzofuran 3b0 Yield 2.16 g (68 %),
white solid, mp 140 °C; mmax (cm-1): 1,643 (C=O); dH (CDCl3): 8.06 (d, 2H,
Jo = 8.4 Hz, H-200, 600), 7.69 (s, 1H, H-4), 7.51 (d, 2H, Jo = 8.4 Hz, H-300, 500), 7.43
(s, 1H, H-7), 2.63 (s, 3H, 3-CH3), 2.54 (s, 3H, 6-CH3); dC (CDCl3): 184.13 (C=O),
152.91, 148.52, 139.13, 136.99, 135.92, 131.18, 130.11, 128.66, 128.36, 126.80,
121.15, 113.77, 21.24, 10.04; Mass (m/z) ES?: 322.2, 321.2, 319.2 (M?), 209.2,
207.2 (100), 141.1, 139.1, 113.1, 111.1; Anal. Calcd. for C17H12Cl2O2: C, 63.97; H,
3.79. Found: C, 63.93; H, 3.75.
2-(4-Chlorobenzoyl)-3,5-dimethylbenzofuran 3c0 Yield 2.04 g (72 %), white solid,
mp 116 °C; mmax (cm-1): 1,636 (C=O). dH (CDCl3): 8.01 (d, 2H, Jo = 8.1 Hz, H-200,
600), 7.53–7.50 (m, 3H, H-4, 300, 500), 7.43 (d, 1H, Jo = 8.4 Hz, H-7), 7.34 (d, 1H,
Jo = 8.4 Hz, H-6), 2.66 (s, 3H, 3-CH3), 2.51 (s, 3H, 5-CH3). dC (CDCl3): 184.37
(C=O), 152.82, 148.27, 138.91, 136.19, 133.06, 131.21, 130.01, 129.20, 128.59,
127.27, 120.97, 111.73, 21.35, 10.02; Mass (m/z) ES?: 287.3, 285.3 (M?), 173.0
(100), 141.1, 139.1, 113.1, 111.1; Anal. Calcd. for C17H13ClO2: C, 71.71; H, 4.60.
Found: C, 71.75; H, 4.63.
5-Bromo-2-(4-chlorobenzoyl)-3-methylbenzofuran 3d0 Yield 2.26 g (65 %), white
solid, mp 98–100 °C; mmax (cm-1): 1,643 (C=O); dH (CDCl3): 8.07 (d, 2H,
Jo = 8.4 Hz, H-200, 600), 7.85 (d, 1H, Jm = 1.8 Hz, H-4), 7.60 (dd, 1H, Jo = 8.4 Hz,
Jm = 1.8 Hz, H-6), 7.52 (d, 2H, Jo = 8.4 Hz, H-300, 500), 7.44 (d, 1H, Jo = 8.4 Hz,
H-7), 2.63 (s, 3H, 3-CH3); dC (CDCl3): 184.10 (C=O), 152.83, 148.87, 139.30,
135.72, 131.30, 131.19, 131.03, 128.71, 126.37, 124.16, 116.61, 113.75, 9.91; Mass
(m/z) ES?: 353.2, 351.2, 349.2 (M?), 239.1, 237.1 (100), 141.1, 139.1, 113.1,
111.1; Anal. Calcd. for C16H10BrClO2: C, 54.97; H, 2.88. Found: C, 54.94; H, 2.82.
5-Chloro-2-(4-chlorobenzoyl)-3-methylbenzofuran 3e0 Yield 2.22 g (73 %), white
solid, mp 118 °C; mmax (cm-1): 1,643 (C=O); dH (CDCl3): 8.07 (d, 2H, Jo = 8.1 Hz,
H-200, 600), 7.70 (s, 1H, H-4), 7.54–7.45 (m, 4H, H-6, 7, 300, 500), 2.64 (s, 3H, 3-CH3);
dC (CDCl3): 184.18 (C=O), 152.49, 149.07, 139.30, 135.75, 131.19, 130.44, 129.26,
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Photochemical transformation of 2-aroylbenzofurans
128.71, 128.68, 126.54, 121.01, 113.34, 9.91; Mass (m/z) ES?: 308.2, 307.2, 305.2
(M?), 195.1, 193.1 (100), 141.1, 139.1, 113.1, 111.1; Anal. Calcd. for C16H10Cl2O2:
C, 62.97; H, 3.30. Found: C, 62.92; H, 3.23.
Photolysis of 2-aroylbenzofurans
2-(4-bromobenzoyl)-3-methylbenzofuran 3a A 1.0 mM deoxygenated solution of
2-aroylbenzofuran 3a in dry methanol was irradiated in a Pyrex glass vessel under
nitrogen atmosphere for 90 min with a 125 W Hg vapor lamp. Progress of the
reaction was monitored by tlc. The photolysate was chromatographed over a column
of silica gel (100–200 mesh) and the column was eluted with increasing proportion
of ethylacetate in petroleum ether-ethylacetate system yielding a photoproduct 5a
along with some starting compound 3a.
The other 2-aroylbenzofurans 3b–e and 3a0–e0 were also photolysed by similar
procedure yielding their respective photoproducts 5b–e, 4a0–e0 and 5a0–e0 along
with some starting compound.
Compound 5a Yield 0.11 g (35 %), white solid, mp 104–106 °C; mmax (cm-1):
3,263 (OH); dH (CDCl3): 7.52–7.23 (8H, m, H-4, 5, 6, 7, 200, 300, 500, 600), 6.03 (1H, d,
J = 5.4 Hz, H-10), 2.57 (1H, d, J = 5.4 Hz, –OH), 2.28 (3H, s, 3-CH3); dC
(CDCl3): 154.09, 151.38, 139.85, 131.62, 129.71, 128.07, 124.67, 122.52, 121.83,
119.56, 112.64, 111.25, 68.12, 7.89; Mass (m/z) ES?: 318.3, 316.3, 301.0, 299.0
(100); Anal. Calcd. for C16H13BrO2: C, 60.59; H, 4.13. Found: C, 60.46; H, 4.20.
Compound 5b Yield 0.12 g (32 %), white solid, mp 118–120 °C; mmax (cm-1):
3256 (OH); dH (CDCl3): 7.52 (1H, s, H-4), 7.49 (2H, d, Jo = 8.4 Hz, H-200, 600), 7.35
(2H, d, Jo = 8.4 Hz, H-300, 500), 7.28 (1H, s, H-7), 5.99 (1H, d, J = 5.4 Hz, H-10),
2.50 (1H, d, J = 5.4 Hz, –OH), 2.46 (3H, s, 6-CH3), 2.22 (3H, s, 3-CH3); dC
(CDCl3): 152.51, 151.48, 139.93, 132.00, 131.59, 129.78, 128.06, 125.89, 121.78,
119.40, 112.41, 110.76, 68.11, 21.31, 7.88; Mass (m/z) ES?: 367.0, 365.0, 363.0,
351.0, 349.0, 347.0 (100); Anal. Calcd. for C17H14BrClO2: C, 55.84; H, 3.86.
Found: C, 55.73; H, 3.93.
Compound 5c Yield 0.07 g (23 %), white solid, mp 62–63 °C; mmax (cm-1): 3,354
(OH); dH (CDCl3): 7.51 (2H, d, Jo = 8.4 Hz, H-200, 600), 7.36–7.28 (4H, m, H-300, 500, 4,
7), 7.12 (1H, dd, Jo = 8.4 Hz, Jm = 2.4 Hz H-6), 5.99 (1H, d, J = 4.5 Hz, H-10), 2.59
(1H, d, J = 4.5 Hz, –OH), 2.47 (3H, s, 3-CH3), 2.46 (3H, s, 5-CH3); dC (CDCl3):
152.36, 151.43, 139.29, 133.55, 132.00, 129.66, 128.43, 127.65, 125.75, 119.23,
112.76, 110.36, 76.79, 68.55, 21.31, 7.85; Mass (m/z) ES?: 331.0, 329.0, 315.0, 313.0
(100); Anal. Calcd. for C17H15BrO2: C, 61.65; H, 4.56. Found: C, 61.52; H, 4.44.
Compound 5d Yield 0.17 g (44 %), white solid, mp 88–90 °C; mmax (cm-1): 3,328
(OH); dH (CDCl3): 7.62 (1H, d, Jm = 2.1 Hz, H-4), 7.42–7.33 (5H, m, H-200, 300, 500,
600, 6), 7.26 (1H, d, Jo = 8.7 Hz, H-7), 6.01 (1H, d, J = 5.4 Hz, H-10), 2.50 (1H, d,
J = 5.4 Hz, –OH), 2.22 (3H, s, 3-CH3); dC (CDCl3): 152.83, 152.74, 138.93,
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133.89, 131.76, 128.32, 128.49, 127.69, 127.53, 122.41, 115.64, 112.53, 112.20,
76.63, 68.02, 7.91; Mass (m/z) ES?: 395.1, 393.1, 379.1, 377.1 (100); Anal. Calcd.
for C16H12Br2O2: C, 48.52; H, 3.05. Found: C, 48.60; H, 3.11.
Compound 5e Yield 0.18 g (51 %), creamish solid, mp 56–58 °C; mmax (cm-1):
3,263 (OH); dH (CDCl3): 7.52 (1H, d, Jm = 2.1 Hz, H-4), 7.49–7.28 (5H, m, H-200,
600, 300, 500, 7), 7.25 (1H, dd, Jm = 2.1 Hz, Jo = 8.4 Hz, H-6), 6.00 (1H, d,
J = 4.5 Hz, H-10), 2.65 (1H, d, J = 4.5 Hz, –OH), 2.24 (3H, s, 3-CH3); dC
(CDCl3): 153.03, 152.42, 138.92, 133.83, 131.03, 128.71, 128.13, 127.63, 124.72,
119.24, 112.31 112.12, 68.01, 29.58, 7.61; Mass (m/z) ES?: 353.0, 351.0, 349.0,
337.0, 335.0, 333.0 (100); Anal. Calcd. for C16H12BrClO2: C, 54.65; H, 3.44.
Found: C, 54.71; H, 3.28.
Compound 4a0 Yield 0.05 g (15 %), white solid, mp 88–90 °C; mmax (cm-1): 1,690
(C=O); dH (CDCl3): 8.25 (2H, d, Jo = 8.7 Hz, H-200, 600), 7.51 (2H, d, Jo = 8.7 Hz,
H-300, 500), 7.24–7.20 (2H, m), 7.07–6.98 (2H, m), 3.66 (1H, q, J = 7.2 Hz, H-3), 3.33
(3H, s, –OCH3), 1.01 (3H, d, J = 7.2 Hz, 3-CH3); dC (CDCl3): 193.04 (C=O), 156.18,
140.43, 133.06, 131.08, 130.98, 129.11, 128.53, 123.98, 122.07, 114.46, 110.78,
52.08, 47.47, 17.21; Mass (m/z) ES?: 274.1, 272.1 (M?-OCH3); Anal. Calcd. for
C17H15ClO3: C, 67.44; H, 4.99. Found: C, 67.21; H, 4.64.
Compound 5a0 Yield 0.11 g (41 %), white solid, mp 74–76 °C; mmax (cm-1):
3,263 (OH). dH (CDCl3): 7.53–7.23 (8H, m, H-4, 5, 6, 7, 200, 300, 500, 600), 6.04 (1H, d,
J = 5.1 Hz, H-10), 2.58 (1H, d, J = 5.1 Hz, –OH), 2.28 (3H, s, 3-CH3); dC
(CDCl3): 154.08, 151.43, 139.31, 133.70, 129.72, 128.67, 127.74, 124.68, 122.53,
119.58, 112.63, 111.27, 68.05, 7.91; Mass (m/z) ES?: 271.1, 269.1, 257.1, 255.1
(100); Anal. Calcd. for C16H13ClO2: C, 76.46; H, 4.80. Found: C, 76.33; H, 4.69.
Compound 4b0 Yield 0.47 g (12 %), yellow solid, mp 202–204 °C; mmax (cm-1):
1,690 (C=O); dH (CDCl3): 8.22 (2H, d, Jo = 8.7 Hz, H-200, 600), 8.02 (2H, d,
Jo = 8.7 Hz, H-300, 500), 7.40 (1H, s, H-4), 7.22 (1H, s, H-7), 3.64 (1H, q,
J = 7.2 Hz, H-3), 3.33 (3H, s, –OCH3), 2.30 (3H, s, 6-CH3), 1.00 (3H, d,
J = 7.5 Hz, 3-CH3); dC (CDCl3): 192.45 (C=O), 156.27, 141.24, 133.81, 132.19,
131.34, 131.02, 129.21, 128.48, 124.23, 114.28, 111.82, 52.21, 47.36, 29.69, 17.05;
Mass (m/z) ES?: 322.0, 320.0 (M?-OCH3); Anal. Calcd. for C18H16Cl2O3: C,
61.55; H, 4.59. Found: C, 61.30; H, 4.29.
Compound 5b0 Yield 0.1 g (35 %), white solid, mp 78–80 °C; mmax (cm-1): 3,356
(OH); dH (CDCl3): 7.47 (1H, s, H-4), 7.41 (2H, d, Jo = 8.4 Hz, H-200, 600), 7.35 (2H,
d, Jo = 8.4 Hz, H-300, 500), 7.27 (1H, s, H-7), 6.00 (1H, d, J = 5.4 Hz, H-10), 2.53
(1H, d, J = 5.4 Hz, –OH), 2.46 (3H, s, 6-CH3), 2.23 (3H, s, 3-CH3); dC (CDCl3):
152.78, 152.08, 139.11, 133.79, 132.54, 128.98, 128.81, 128.71, 127.69, 119.47,
112.97, 112.16, 68.02, 20.79, 7.90; Mass (m/z) ES?: 324.1, 322.1, 320.1, 307.1,
305.1, 303.1 (100); Anal. Calcd. for C17H14Cl2O2: C, 63.57; H, 4.39. Found: C,
63.38; H, 4.20.
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Photochemical transformation of 2-aroylbenzofurans
Compound 4c0 Yield 0.04 (10 %), yellow gummy mass; mmax (cm-1): 1,690
(C=O); dH (CDCl3): 8.25 (2H, d, Jo = 8.7 Hz, H-200, 600), 7.50 (2H, d, Jo = 8.7 Hz,
H-300, 500), 7.05–6.98 (2H, m, H-4, 6), 6.94 (1H, d, Jo = 7.8 Hz, H-7), 3.61 (1H, q,
J = 7.5 Hz, H-3), 3.32 (3H, s, –OCH3), 2.34 (3H, s, 5-CH3), 0.99 (3H, d,
J = 7.5 Hz, 3-CH3); dC (CDCl3): 193.16 (C=O), 154.07, 140.38, 133.10, 132.11,
131.09, 130.95, 129.09, 128.92, 124.52, 114.53, 110.35, 68.17, 52.02, 47.55, 17.19;
Mass (m/z) ES?: 287.1, 285.1 (M?-OCH3); Anal. Calcd. for C18H17ClO3: C, 68.25;
H, 5.41. Found: C, 68.10; H, 5.21.
Compound 5c0 Yield 0.08 g (28 %), white solid, mp 68–70 °C; mmax (cm-1): 3,364
(OH); dH (CDCl3): 7.42 (2H, d, J = 8.4 Hz, H-200, 600), 7.35–7.30 (4H, m, H-300, 500, 4,
7), 7.11 (1H, dd, Jo = 8.4 Hz, Jm = 1.2 Hz, H-6), 6.02 (1H, d, J = 5.4 Hz, H-10),
2.53 (1H, d, J = 5.4 Hz, –OH), 2.46 (3H, s, 3-CH3), 2.25 (3H, s, 5-CH3); dC (CDCl3):
152.48, 151.52, 139.38, 133.63, 132.00, 129.76, 128.64, 127.73, 125.88, 119.41,
112.40, 110.76, 68.02, 21.34, 7.92; Mass (m/z) ES?: 285.1, 283.1, 271.1, 269.1 (100);
Anal. Calcd. for C17H15ClO2: C, 71.20; H, 5.27. Found: C, 71.08; H, 5.14.
Compound 4d0 Yield 0.08 (18 %), yellow gummy mass; mmax (cm-1): 1690 (C=O);
dH (CDCl3): 8.23 (2H, d, Jo = 8.7 Hz, H-200, 600), 7.52 (2H, d, Jo = 8.7 Hz, H-300, 500),
7.36–7.31 (2H, m, H-4, 6), 6.94 (1H, d, Jo = 8.1 Hz, H-7), 3.64 (1H, q, J = 7.2 Hz,
H-3), 3.32 (3H, s, –OCH3), 0.99 (3H, d, J = 7.2 Hz, 3-CH3); dC (CDCl3): 192.40
(C=O), 155.35,140.67,133.45, 132.76, 131.40,131.02,129.22, 127.11, 114.90,113.69,
112.44, 52.23, 47.36, 17.08; Mass (m/z) ES?: 354.0, 352.0, 350.0 (M?-OCH3); Anal.
Calcd. for C17H14BrClO3: C, 53.50; H, 3.70. Found: C, 53.32; H, 3.41.
Compound 5d0 Yield 0.16 g (49 %), white solid, mp 60–62 °C; mmax (cm-1): 3325
(OH); dH (CDCl3): 7.63 (1H, d, Jm = 1.8 Hz, H-4), 7.43–7.34 (5H, m, H-200, 300, 500,
600, 6), 7.28 (1H, d, Jo = 8.7 Hz, H-7), 6.02 (1H, d, J = 5.4 Hz, H-10), 2.52 (1H, d,
J = 5.4 Hz, –OH), 2.24 (3H, s, 3-CH3). dC (CDCl3): 152.83, 152.81, 138.93,
133.89, 131.76, 128.76, 128.49, 127.69, 122.41, 115.64, 112.74, 112.20, 68.02,
7.83; Mass (m/z) ES?: 353.0, 351.0, 349.0, 337.0, 335.0, 333.0 (100); Anal. Calcd.
for C16H12BrClO2: C, 54.65; H, 3.44. Found: C, 54.38; H, 3.24.
Compound 4e0 Yield 0.08 (20 %), white solid, mp 108–110 °C; mmax (cm-1): 1,690
(C=O); dH (CDCl3): 8.23 (2H, d, Jo = 8.7 Hz, H-200, 600), 7.51 (2H, d, Jo = 8.7 Hz,
H-300, 500), 7.21 (1H, d, Jm = 2.1 Hz, H-4), 7.19–7.17 (1H, m, H-6), 6.97 (1H, d,
Jo = 8.4 Hz, H-7), 3.63 (1H, q, J = 7.2 Hz, H-3), 3.32 (3H, s, –OCH3), 1.00 (3H, d,
J = 7.2 Hz, 3-CH3); dC (CDCl3): 192.45 (C=O), 156.27, 141.24, 133.81, 131.02,
130.18, 129.21, 125.18, 128.48, 124.23, 115.02, 111.82, 52.21, 29.69, 17.05;Mass(m/z)
ES?: 310.1, 308.1, 306.1 (M?-OCH3); Anal. Calcd. for C17H14Cl2O3: C, 60.55; H,
4.18. Found: C, 60.28; H, 3.69.
Compound 5e0 Yield 0.16 g (55 %), creamish solid, mp 62–64 °C; mmax (cm-1):
3,263 (OH); dH (CDCl3): 7.47 (1H, d, Jm = 2.1 Hz, H-4), 7.42–7.28 (5H, m, H-200,
600, 300, 500, 7), 7.24 (1H, dd, Jo = 8.7 Hz, Jm = 2.1 Hz, H-6), 6.01 (1H, d,
123

P. Jindal et al.
J = 4.5 Hz, H-10), 2.58 (1H, d, J = 4.5 Hz, –OH), 2.42 (3H, s, 3-CH3); dC
(CDCl3): 153.04, 152.44, 138.99, 133.88, 131.17, 128.75, 128.21, 127.69, 124.82,
119.31, 112.30, 112.25, 68.07, 7.82; Mass (m/z) ES?: 305.1, 303.1, 301.1, 293.1,
291.1, 289.1 (100); Anal. Calcd. for C16H12Cl2O2: C, 62.56; H, 3.94. Found: C,
62.38; H, 3.64.
Results and discussion
The substrates, 2-aroylbenzofurans 3a–e and 3a0–e0, were synthesized by the phase
transfer catalyzed Rap–Stoermer [30–36] type cyclocondensation between o-
hydroxyacetophenones 1a–e and a-bromoketones 2a and 2b using water as solvent,
Na2CO3 as a mild base, and n-Bu4N?I- (Scheme 1) by developing a green protocol.
The choice of aqueous media for chemical reactions has gained much attention due
to its environmentally friendly impact and low cost of chemical processes. Using
water as the reaction medium offers a number of advantages such as being cheap,
readily available, non-toxic, non-flammable, and safe for use. Water often increases
the reactivity or selectivity [37, 38] of the reaction that could be unattainable in
organic solvents. In aqueous media, the work-up procedure becomes simple and the
product can be easily isolated. The structures of these photo-labile substrates 3a–e
and 3a0–e0 were found to be consistent with their spectral parameters (IR, 1H NMR,
13C NMR, and Mass; see ‘‘Experimental’’).
The IR spectrum of compound 3a (mp 80–82 °C) showed a strong absorption at
1
1,643 cm-1 for the carbonyl group. The compound 3a in its H NMR (300 MHz,
CDCl3) showed the most downfield signal at d 8.03(dd, Jo = 8.4 Hz, Jm = 1.5 Hz),
which could be ascribed to H-200 and H-600 due to the deshielding effect of the C=O
group. The proton H-7 produced a multiplet between d 7.74 and 7.70. The four
protons H-4, -6, -300, and -500 gave an unresolved multiplet between d 7.68 and 7.53.
The spectrum exhibited a multiplet in the region d 7.38–7.33 for proton H-5. The
3-CH3 group protons were found to be located at d 2.68 as a singlet. The structure of
3a was further confirmed by the 13C NMR spectrum. The molecular ion peak in the
mass spectrum of 3a was located at m/z 314.7/316.7, but the base peak appeared at
159.1 (100 %). The structures of other benzofuran derivatives, 3b–e and 3a0–e0 were
ascertained likewise.
The methanolic solution of the benzofuran derivatives 3a–e and 3a0–e0 was
photolysed with Pyrex-filtered UV-light from a 125 W Hg lamp under nitrogen
atmosphere (Scheme 2) that resulted in the formation of photoproducts 5a–e, 5a0–e0,
and 4a0–e0.
1
The structures of photoproducts were established by their IR, H NMR, 13C
NMR, and mass spectra data (vide experimental). The photoproduct 5a in its IR
spectrum showed absorption at 3,263 cm-1 which indicated the presence of the
1
–OH group. In the H NMR spectrum (300 MHz, CDCl3) of 5a, the eight protons
H-4, -5, -6, -7, -200, -300, -500, and H-600 gave unresolved multiplet between d 7.52 and
7.23. The proton H-10 appeared as a doublet at d 6.03 (J = 5.4 Hz). The –OH proton
gave a doublet at d 2.57 (J = 5.4 Hz). A sharp singlet integrating for three protons
at d 2.28 was assigned to the 3-CH3 group. The D2O shake of the photoproduct 5a
123

Photochemical transformation of 2-aroylbenzofurans
7
4
1
R1
R
R1
R
OH
O
O
O
O
2
H2O, Na2CO3
Br
1'
1''
CH3
n-Bu4N+I-, reflux
3
3''
R2
CH3
R2
5''
1 (a-e)
3 (a-e), 3 (a'-e')
2a, 2b
3a, 3a'; R = H, R1 = H
1a; R = H, R1 = H
3a, 3b'; R = Cl, R1 = CH3
3c, 3c'; R = CH3, R1 = H
3d, 3d'; R = Br, R1 = H
3e, 3e'; R = Cl, R1 = H
2a, 3 (a-e); R2 = Br
2b, 3 (a'-e'); R2 = Cl
1b; R = Cl, R1 = CH3
1c; R = CH3, R1 = H
1d; R = Br, R1 = H
1e; R = Cl, R1 = H
Scheme 1 Synthesis of 2-aroylbenzofuran derivatives
further confirmed the –OH group. The structure of photoproduct 5a was further
corroborated by its proton decoupled 13C NMR spectra and mass spectrum. In the
mass spectrum, the molecular ion peak did not appear due to its low intensity, but
the base peak appeared at 299.0/301.0 (100 %). The direct photo irradiation of the
isolated intermediates 4a0–e0 also furnished the alcohols 5a0–e0.
The nature of the substituents onto the benzofuran ring had anoticeable effect on the
photoproduct formation from these aroylbenzofurans (Table 1). The benzofuran(s),
having electron-withdrawing groupd (which decrease the electron density on the
benzofuran ring), furnished the photoproduct in higher yields while the benzofu-
ran(s) with electron-releasing groups (which increase the electron density) gave the
photoproducts in lower yields. The only assignable reason for this observation may be
the participation in product formation of p–p* excitation as its energy is lowered more
than that of n–p* excitation [39, 40], a case of state switching.
Solvent also have an appreciable effect on photoproduct formation (Table 1)
which was determined by photoirradiating these benzofuran derivatives in different
solvents like methanol, ethanol, isopropyl alcohol, and toluene, separately. The
polar protic solvents: methanol, ethanol, and isopropyl alcohol offered a favorable
environment for the phototransformation of the target benzofurans 3a–e. The yield
of the all the photoproducts 5a–e and 5a0–e0 was in the order isopropanol [ eth-
anol [ methanol. When these substrates were photoirradiated in toluene, no
photoproduct was detected, which rules out the possibility of direct intermolecular
H-transfer from the solvent to the carbonyl group for photoreduction.
The phototransformations of target aroylbenzofurans 3 to photoproducts 5 can be
rationalized to occur through two familiar mechanistic routes: route 1 and route 2
(Scheme 3). Route 1 is the simple direct intermolecular H-transfer from the solvent
to the carbonyl group to furnish the alcohols 5 just like the photoreduction of
benzophenones. But, here, the formation of the additional products, i.e. 1,2-diols
(the products of reductive coupling) and the tertiary alcohols as found in the
photoreduction of benzophenones, were not observed. This rules out the existence
of the above proposed mechanism, i.e. route 1 for the phototransformation of these
substrates into alcohols 5. The route 2 mechanism is the addition–elimination
process. There is an initial addition of methanol, the solvent, to the double bond of
123

P. Jindal et al.
7
4
1
R1
R
H3C
O
O
1
7
7
2
R1
R
R1
R
O
OH
O
O
O
hν
1'
1''
3
1'
1'
MeOH
1''
1''
3''
R2
3
3
CH3
4
4
H
3''
3''
R2
CH3
CH3
5''
5''
5''
R2
5 (a-e), 5 (a'-e')
3 (a-e) 3 (a'-e')
,
4 (a'-e')
Scheme 2 Photolysis of 2-aroylbenzofurans
Table 1 Effect of substituents and solvent on the photoproducts formation on the photolysis of com-
pounds 3a–e and 3a0–e0
Photoproduct
Yield (%)
Substituents
Solvent
R
R1
Methanol
Ethanol
Isopropyl alcohol
Toluene
5a
H
H
35
32
23
44
51
41
35
28
49
55
15
12
10
18
20
38
36
27
49
54
43
40
33
55
59
–
42
39
35
62
60
50
43
41
64
67
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
5b
5c
Cl
CH3
H
CH3
Br
Cl
5d
5e
H
H
5a0
5b0
5c0
5d0
5e0
4a0
4b0
4c0
4d0
4e0
H
H
Cl
CH3
H
CH3
Br
Cl
H
H
H
H
Cl
CH3
H
–
–
CH3
Br
Cl
–
–
H
–
–
H
–
–
the benzofuran ring leading to the formation of intermediate 4. Then, the carbonyl
group of the aroyl moiety present in the additional product 4 gets excited upon
photoirradiation and abstracts hydrogen from –OCH3 through Norrish type-II c-H
abstraction, which results in formation of 1,4-biradical. The elimination of HCHO
from the 1,4-biradical gives the enol that upon 1,3-hydrogen shift furnishes the final
product 5 (Scheme 3). This mechanism gets support from the fact that the
intermediates 4a0–e0 produced by the addition of solvent on 3a0–e0 have been
isolated when the methanol was used as the solvent and also by the formation of
5a0–e0 from the direct irradiation of isolated intermediate 4a0–e0. The intermediates
like 4a0–e0 could not be isolated in other alcohols, i.e. ethanol and isopropanol
because of the rapid ease of H-abstraction from the –O–CH2-CH3 and
–O-CH(CH3)2 groups of the addition product to give the 1,4-biradical intermediate
of higher stability. This is further corroborated by the yield of the photoproduct
123

Photochemical transformation of 2-aroylbenzofurans
O
H2
C
R1
R
O
H
O
O
O
Route 2
R1
R2
hν/CH3OH
CH3
CH3
H
R
3
R2
4
Route 1
hν
hν
.
.
.
CH2
OH
.
.
R1
O
O
O
O
.
.
R
R1
H
CH3
R2
CH3
1,4-biradical
R
R2
R''-H
-HCHO
O
OH
.
R1
R
OH
O
R1
H
R2
H3C
CH3
R
R2
Ketyl Radical
1,3-Hydrogen
Shift
R''-H
H-abstraction
HO
O
H
R1
R2
CH3
R
5
Scheme 3 Probable mechanism of product 5 formation from 2-aroylbenzofurans 3 by photolysis
which was found to be in the order: isopropanol [ ethanol [ methanol. Hence, the
addition–elimination mechanism seems to be the most preferred and plausible
reaction pathway for the formation of these photoproducts.
In conclusion, the 2-aroylbenzofurans gave the arylbenzofurylmethanols very
efficiently through photoreduction. In the photolysis of 3, no ring photochemistry
was observed and, also, no photoproduct whatsoever corresponding to the
intramolecular H-abstraction by C=O from the 3-methyl group on the benzofuran
moiety was furnished. The variation in substituents at the benzofuran ring has a
pronounced effect on the yields of the photoproducts.
123

P. Jindal et al.
Acknowledgments Financial support as junior research fellowship by the University Grant Commis-
sion (UGC), New Delhi is gratefully acknowledged for accomplishing this work.
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