Synthesis of 2-(3'bromo-3'- phenylallyl)indoles via znbr 2-mediated addition of bromomethylindoles/indolylmethylacetates with phenylacetylene

Article abstract of DOI:10.1080/00397911.2010.526282

An efficient synthesis of 2-(30-bromo-30-phenylallyl)indoles has been developed via direct carbon-carbon bond formation between bromomethylindoles/ indolylmethylacetates and phenylacetylene.Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.526282

Synthetic Communications1, 42: 526–533, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.526282
SYNTHESIS OF 2-(3’BROMO-3’-
PHENYLALLYL)INDOLES VIA ZnBr2-MEDIATED
ADDITION OF BROMOMETHYLINDOLES/
INDOLYLMETHYLACETATES WITH
PHENYLACETYLENE
Radhakrishnan Sureshbabu and
Arasambattu K. Mohanakrishnan
Department of Organic Chemistry, University of Madras, Guindy Campus,
Chennai, India
GRAPHICAL ABSTRACT
Abstract An efficient synthesis of 2-(30-bromo-30-phenylallyl)indoles has been developed via
direct carbon–carbon bond formation between bromomethylindoles/indolylmethylacetates
and phenylacetylene.
Keywords Allylindoles; bromomethylindole; coupling reaction; indolylmethylacetate
INTRODUCTION
In past decades, Lewis acid–promoted C-C bond-formation reactions by
addition of carbenium ions to alkenes have been extensively explored.[1] However,
compared with alkenes, the use of alkynes as electron-rich substrates to be attacked
by carbenium ions seems limited. Recently, iron-catalyzed reactions have been
explored for carbon–carbon couplings.[2] Kabalka et al. have synthesized stereo-
defined alkenyl halides using direct C-C bond formation of benzyl alcohols.[3] In
organic synthetic reactions, the scope and applications of alkenyl and aryl halides
have increased tremendously, because they can be easily converted into other valu-
able compounds. Moreover, halides play an important role in organic synthesis for
C-C and C-N bond formation by coupling using transition-metal catalysts.[4] Parti-
cularly, alkenyl halides are very useful substrates in a variety of chemical transforma-
tions.[5] Traditionally, vinyl halides are prepared from the corresponding carbonyl
Received July 15, 2010.
Address correspondence to Arasambattu K. Mohanakrishnan, Department of Organic Chemistry,
University of Madras, Guindy Campus, Chennai 600 025, India. E-mail: mohan_67@hotmail.com
526

SYNTHESIS OF 2-(30-BROMO-30-PHENYLALLYL)INDOLES
527
compounds with halogenated reagents,[6] under reflux or by Wittig[7] and Julia[8]
olefination reactions and other related reactions.[9] Kropp et al.[10] reported the syn-
thesis of alkenyl halides by the addition of hydrogen halide and alkyl halides to
alkynes in the presence of an excess amount of ZnCl2. Very recently Liu et al.[11]
and Jana et al.[12] reported an efficient and mild iron-mediated synthesis of alkenyl
halides via direct C-C bond formation of benzyl alcohols and aryl alkynes.
RESULTS AND DISCUSSION
In continuation of our work on studies related to the synthetic elaboration of
N-protected bromomethylindoles,[13] we herein report a simple and efficient method
for the preparation of phenylallylindoles by means of coupling of bromomethylin-
doles or indolylmethylacetates with phenylacetylene in the presence of ZnBr2. Easy
availability of various types of bromomethylindoles prompted us to explore ally-
lation of these compounds using phenylacetylene. Initially, the reaction of
2-(bromomethyl)-3-(phenylthio)-1-(phenylsulfonyl)-1H-indole 1a with phenylacety-
lene using 0.2 eq. of ZnBr2 in dry 1,2-dichloroethane at reflux for 12 h afforded
the expected allylbromo compound 2a as a mixture of E and Z (3:1) isomers in
59% yield (Scheme 1).
A list of various types of bromomethylindoles 1a–f employed for zinc bromide–
mediated phenylallylation with phenylacetylene, the respective allylindoles 2a–f, and
the yields obtained are summarized in Table 1.
3-Phenylthio-2-methylacetate 1a0 underwent a facile phenylallylation reaction
with phenylacetylene to afford the respective allyl indole 2a in comparatively lesser
yield as a mixture of E and Z isomers (entry 1). Similarly, the interaction of
3-bromo-2-bromomethyl=methylacetate indole 1b=1b0 with phenylacetylene furn-
ished the respective allylbromo compound 2b in moderate yield as an isomeric
mixture (entry 2). Surprisingly, when the same reaction was performed with 1b in
the presence of K2CO3, a single isomer of allylbromo compound 2b00 was obtained
in good yield (entry 3). The isomeric bromo compound 1c gave the corresponding
allylindole 2c in 57% yield as an isomeric mixture (entry 4).
The reaction of 3-vinylester-2-bromomethylindole=indolylmethylacetate 1d=1d0
with phenylacetylene gave the bromo compound 2d as E and Z isomers (entry 5).
Interaction of phenylacetylene with 3-bromomethylindole 1e led to the respective allyl
compound 2e in 51% yield (entry 6). It should be mentioned that the yields of the ally-
lated products 2a=2b=2d obtained from the corresponding bromo compounds 1a=1b=
1d are always better than the yields obtained from the respective methyl acetates 1a0=
1b0=1d0. Surprisingly, the isomeric bromo compound 1f upon reaction with phenyl
acetylene did not give the expected allylindole, but instead the cyclized product 2f
Scheme 1. Preparation of allyl indole 2a.

528
R. SURESHBABU AND A. K. MOHANAKRISHNAN
Table 1. Synthesis of 3-bromo-3-phenylallylindoles
Entry
Substrate
Condition
Allyl indole
Yield (%)a
59 (3:1)
51 (2:1)
1
2
3
64 (5:1)
57 (2:1)
61
4
57 (4:1)
54 (5:1)
40 (2:1)
5
6
51 (3:1)
(Continued )

SYNTHESIS OF 2-(30-BROMO-30-PHENYLALLYL)INDOLES
529
Table 1. Continued
Entry
Substrate
Condition
Allyl indole
Yield (%)a
7
50
aIsolated yield of the E=Z mixture; the E=Z ratios were determined using 1H NMR.
Scheme 2. Attempted preparation of allylindole 2g.
was isolated in 50% yield (entry 7). Obviously, the bromo compound 1f underwent a
smooth alkenylation reaction with phenylacetylene followed by intramolecular cycliza-
tion involving an elimination of ethyl bromide to afford the seven-member lactone 2f.
Finally, an attempt to form the corresponding allylindole 2g with indolylbromo com-
pound 1g having ketone functionality at the indole-3-position was unsuccessful; the
starting material was always recovered unchanged (Scheme 2).
In conclusion, we have achieved an efficient synthesis of 2-(30-bromo-30-
phenylallyl)indoles via direct C-C bond formation of bromomethylindoles=
indolylmethyl acetates and phenylacetylene using catalytic amount of ZnBr2.
Further studies to explore the synthetic utility of this reaction as well as allyindoles
are in progress in our laboratory.
EXPERIMENTAL
All melting points were uncorrected. Reagents were purchased from commer-
cial sources and used as received without purification. Solvents were dried by
standard procedures. Column chromatography was carried out on silica gel (grade
60, mesh size 230–400, Merck). Infrared (IR) spectra were recorded on a Shimadzu
Fourier transform (FT)–IR 8300 instrument. 1H and 13C NMR spectra were
recorded in CDCl3 using tetramethylsilane (TMS) as an internal standard on a
Bruker-300 spectrometer. Chemical shift values were quoted in parts per million
(ppm), and coupling constants were quoted in hertz (Hz). Chemical shift multiplici-
ties were reported as s ¼ singlet, d ¼ doublet, t ¼ triplet, q ¼ quartet, and
m ¼ multiplet. Mass spectra were recorded on a Jeol DX 303 HF spectrometer.
Elemental analyses were carried out on Perkin-Elmer series II 2400 (IIT Madras)
equipment. The required bromomethylindoles 1a–h were prepared from the corre-
sponding 2=3-methyl-N-phenylsulfonylindoles following the published procedure.[14]

530
R. SURESHBABU AND A. K. MOHANAKRISHNAN
Representative Procedure for the Phenylallylation of Bromomethyl
Indoles 1a–f
A mixture of substrate 1a–f (1.09 mmol), ZnBr2 (0.21 mmol), and phenylacety-
lene (1.3 mmol) in dry 1,2-dichloroethane (10 mL) were refluxed under a nitrogen
atmosphere for 12 h. It was then poured into crushed ice (100 g) containing few drops
of concentrated HCl and extracted with CHCl3 (2 ꢀ 20 mL). The combined extracts
were washed with water (10 mL) and dried (Na2SO4). Removal of solvent followed
by column chromatographic purification (silica gel, EtOAc-hexane 1:5) afforded the
allylated indoles 2a–f.
2-(3-Bromo-3-phenylallyl)-1-(phenylsulfonyl)-3-(phenylthio)-1H-
indole (2a)
1
Yield: 0.36 g (59%). IR (KBr): 1351 & 1176 (SO2Ph) cmꢁ1. H NMR (CDCl3,
300 MHz): d 8.19 (d, J ¼ 8.4 Hz, 1 H), 7.69 (d, J ¼ 7.5 Hz, 2 H), 7.58 (d, J ¼ 7.8 Hz,
1 H), 7.43–7.37 (m, 2 H), 7.31–7.24 (m, 4 H), 7.18–7.12 (m, 3 H), 7.01–6.94 (m,
3 H), 6.86–6.83 (m, 2 H), 6.69 (d, J ¼ 6.9 Hz, 1 H), 6.14 (t, J ¼ 6.9 Hz, 1 H), 4.23 (d,
J ¼ 6 Hz, 1 H), 3.92 (d, J ¼ 6.9 Hz, 1 H). 13C NMR (CDCl3, 75 MHz): 143.1, 142.9,
139.3, 138.6, 138.5, 138.3, 136.9, 136.8, 136.4, 136.2, 134.1, 130.8, 130.6, 129.8,
129.4, 129.1, 129.0, 128.9, 128.6, 128.5, 128.2, 128.1, 128.0, 127.7, 127.5, 127.0,
126.7, 126.6, 126.5, 126.4, 126.2, 126.0, 125.6, 125.5, 125.4, 124.5 (2C), 122.4,
120.1, 120.0, 115.2 (2C), 112.9, 112.3, 31.0, 28.8. MS (EI) m=z: 582 & 584 [Mþ þ Na].
Anal. calcd. for C29H22BrNO2S2: C, 62.14; H, 3.96; N, 2.50; S, 11.44%. Found: C,
62.39; H, 3.65; N, 2.76; S, 11.16%.
3-Bromo-2-(3-bromo-3-phenylallyl)-1-(phenylsulfonyl)-1H-indole (2b)
1
Yield: 0.79 g (64%). Mp: 86–88 ꢂC. IR (KBr): 1360 & 1172 (SO2Ph) cmꢁ1. H
NMR (CDCl3, 300 MHz): d 8.12 (dd, J ¼ 8.4 Hz, J ¼ 8.4 Hz, 1 H), 7.66 (d, J ¼ 8.1 Hz,
1 H), 7.55 (d, J ¼ 8.1 Hz, 1 H), 7.40–7.17 (m, 11 H), 6.23–6.16 (m, 1 H), 4.10 (d,
J ¼ 6.0 Hz, 1 H) 3.78 (d, J ¼ 6.9 Hz, 1 H). 13C NMR (CDCl3, 75 MHz): 138.2,
137.5, 137.4, 137.3, 135.0, 134.9, 133.9, 133.6, 133.0, 128.3 (2C), 128.2, 128.1,
128.0, 127.9, 127.7, 127.5, 127.2, 127.1, 126.5, 126.3, 125.3 (2C), 124.8, 123.4,
123.3, 121.4, 118.6, 118.5, 114.0, 113.9, 102.2, 102.0, 30.1, 27.9. MS (EI) m=z: 553
& 555 [Mþ þ Na]. Anal. calcd. for C23H17Br2NO2S: C, 52.00; H, 3.23; N, 2.64; S,
6.04%. Found: C, 51.71; H, 3.50; N, 2.39; S, 6.35%.
3-Bromo-2-(3-bromo-3-phenylallyl)-1-(phenylsulfonyl)-1H-indole
(2b’’)
Yield: 0.75 g (61%). Mp: 124–126 ꢂC. IR (KBr): 1360 & 1172 (SO2Ph) cmꢁ1. 1H
NMR (CDCl3, 300 MHz): d 8.18 (d, J ¼ 8.7 Hz, 1 H), 7.65 (d, J ¼ 8.1 Hz, 2 H),
7.54–7.49 (m, 4 H), 7.46–7.34 (m, 5 H), 7.32–7.27 (m, 2 H), 6.30 (t, J ¼ 6.9 Hz,
1 H), 3.87 (d, J ¼ 6.6 Hz, 2 H). 13C NMR (CDCl3, 75 MHz): 138.4, 135.9, 134.6,
134.0, 129.2, 128.9, 128.7, 128.2, 127.6, 127.3, 125.8, 124.4, 122.4, 119.6, 115.0,
103.1, 31.1, 29.0. MS (EI) m=z: 553 & 555 [MþþNa]. Anal. calcd. for

SYNTHESIS OF 2-(30-BROMO-30-PHENYLALLYL)INDOLES
531
C23H17Br2NO2S: C, 52.00; H, 3.23; N, 2.64; S, 6.04%. Found: C, 51.72; H, 3.50; N,
2.38; S, 6.35%.
2-Bromo-3-(3-bromo-3-phenylallyl)-1-(phenylsulfonyl)-1H-indole (2c)
Yield: 0.35 g (57%). Mp: 112–114 ꢂC. IR (KBr): 1362 & 1176 (SO2Ph) cmꢁ1. 1H
NMR (CDCl3, 300 MHz): d 8.26 (d, J ¼ 7.5 Hz, 1 H), 7.83 (d, J ¼ 7.5 Hz, 2 H), 7.51
(d, J ¼ 7.5 Hz, 1 H), 7.42–7.11 (m, 10 H), 6.14 (t, J ¼ 7.35 Hz, 1 H), 3.43 (d,
J ¼ 7.5 Hz, 1 H) 3.36 (d, J ¼ 7.5 Hz, 1 H). 13C NMR (CDCl3, 75 MHz): 138.2 (2C),
138.1, 137.4, 136.7, 134.1, 133.9, 132.4, 130.9, 129.5, 129.1, 129.0, 128.9, 128.8,
128.7, 128.4, 128.3, 127.6, 127.4, 127.1, 127.0, 126.4, 125.5, 125.2, 124.1, 124.0,
122.9, 122.6, 122.1, 118.9, 118.5, 115.4, 109.3, 109.2, 27.1, 26.1. MS (EI) m=z: 553
& 555 [Mþ þ Na]. Anal. calcd. for C23H17Br2NO2S: C, 52.00; H, 3.23; N, 2.64; S,
6.04%. Found: C, 51.70; H, 3.51; N, 2.39; S, 6.33%.
(2E)-Methyl 3-(2-(3-Bromo-3-phenylallyl)-1-(phenylsulfonyl)-1H-
indol-3-yl)acrylate (2d)
Yield: 0.67 g (54%). IR (KBr): 1700 (CO2Me), 1363 & 1172 (SO2Ph) cmꢁ1
.
1H-NMR (CDCl3, 300 MHz): d 8.22 (d, J ¼ 7.8 Hz, 1 H), 7.91 (d, J ¼ 16.2 Hz,
1 H), 7.72 (d, J ¼ 7.5 Hz, 3 H), 7.48–7.18 (m, 10 H), 6.46 (d, J ¼ 16.2 Hz, 1 H), 6.22
(t, J ¼ 6.1 Hz, 1 H), 4.18 (d, J ¼ 6 Hz, 2 H), 3.73 (s, 3 H). 13C NMR (CDCl3,
75 MHz): 167.5, 139.9, 139.1, 138.7, 137.0, 135.4, 134.1, 129.5, 128.7, 128.2, 127.6,
127.5, 127.4, 126.5, 126.4, 125.4, 124.5, 120.2, 119.4, 117.5, 115.1, 51.7, 30.2. MS
(EI) m=z: 558 & 560 [Mþ þ Na]. Anal. calcd. for C27H22BrNO4S: C, 60.45; H,
4.13; N, 2.61; S, 5.98%. Found: C, 60.72; H, 3.87; N, 2.92; S, 5.74%.
Ethyl 3-(3-Bromo-3-phenylallyl)-1-(phenylsulfonyl)-1H-
indole-2-carboxylate (2e)
1
Yield: 0.56 g (51%). IR (KBr): 1710 (CO2Me), 1372 & 1186 (SO2Ph) cmꢁ1. H
NMR (CDCl3, 300 MHz): d 7.95–7.91 (m, 1 H), 7.84–7.78 (m, 2 H), 7.54–7.40 (m,
3 H), 7.37–7.31 (m, 5 H), 7.22–7.17 (m, 3 H), 6.16–6.12 (m, 1 H), 4.40–4.29 (m,
2 H), 3.78 (d, J ¼ 6.6 Hz, 1 H), 3.44 (d, J ¼ 7.5 Hz, 1 H), 1.35–1.25 (m, 3 H). 13C
NMR (CDCl3, 75 MHz): 162.2, 162.1, 139.3, 138.1, 137.6, 137.4, 137.0, 136.9,
133.8 (2C), 130.1, 129.6, 129.3, 129.0, 128.9, 128.8, 128.7, 128.5, 128.4, 128.3,
128.2 (2C), 128.1, 127.6, 127.2 (2C), 127.1, 127.0, 126.9, 126.5, 125.9, 125.7, 124.3,
124.2, 124.1, 122.1, 120.9, 120.5, 120.4, 115.5 (2C), 62.3, 28.3, 26.1, 14.1, 14.0. MS
(EI) m=z: 546 & 548 [Mþ þ Na]. Anal. calcd. for C26H22BrNO4S: C, 59.55; H,
4.23; N, 2.67; S, 6.11%. Found: C, 59.27; H, 4.49; N, 2.37; S, 6.42%.
(Z)-6-phenylsulfonyl-3-phenyl-5H-oxepino[4,3,-b]indol-1(6H)-one (2f)
ꢂ
=
Yield: 0.56 g (51%). Mp: 214–216 C. IR (KBr): 1661 (C O), 1180 (C-O), 1378
& 1170 (SO2Ph) cmꢁ1. 1H NMR (CDCl3, 300 MHz): d 8.15 (d, J ¼ 8.7 Hz, 1 H), 8.02
(d, J ¼ 9 Hz, 1 H), 7.75 (d, J ¼ 8.1 Hz, 2 H), 7.56–7.50 (m, 3 H), 7.40 (t, J ¼ 7.8 Hz,
2 H), 7.31–7.24 (m, 5 H), 6.01 (t, J ¼ 7.0 Hz, 1 H), 4.08 (d, J ¼ 6.9 Hz, 2 H). 13C

532
R. SURESHBABU AND A. K. MOHANAKRISHNAN
NMR (CDCl3, 75 MHz): 160.3, 151.9, 145.8, 137.3, 134.6, 133.7, 132.8, 128.7, 128.0,
127.5, 126.6, 125.3, 124.7, 124.1, 120.2, 113.5, 110.1, 106.8, 28.6, 21.0. MS (EI) m=z:
438 [Mþ þ Na]. Anal. calcd. for C24H17NO4S: C, 69.38; H, 4.12; N, 3.37; S, 7.72%.
Found: C, 69.65; H, 3.86; N, 3.67; S, 7.47%.
ACKNOWLEDGMENTS
We thank the University Grants Commission (UGC), New Delhi (32-194=
2006=SR), for financial support. R. S. thanks UGC, New Delhi, for a senior research
fellowship. The authors thank the Department of Science and Technology
(DST-FIST) for a 300-MHz NMR facility.
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