Synthesis of stilbene analogues by one-pot oxidation-Wittig and oxidation-Wittig-Heck reaction

Article abstract of DOI:10.1016/j.tetlet.2012.06.090

Synthesis of symmetrical (and unsymmetrical) stilbene derivatives is achieved by a combination of one-pot steps of Kornblum type oxidation of benzyl halide, its simultaneous in situ formation of phosphonium salt, and subsequently their Wittig reaction. In other variant it is oxidized to aldehyde, treated with ylide generated from phosphonium salt (CH₃PPh₃X) to give styrene, and subjected to Pd catalyzed Heck reaction with arylhalide to give stilbenes as the three-step one-pot sequence.

Full text of DOI:10.1016/j.tetlet.2012.06.090

Tetrahedron Letters 53 (2012) 4692–4696
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of stilbene analogues by one-pot oxidation-Wittig
and oxidation-Wittig–Heck reaction
⇑
Akeel S. Saiyed, Krupa N. Patel, Bola V. Kamath, Ashutosh V. Bedekar
Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of symmetrical (and unsymmetrical) stilbene derivatives is achieved by a combination of one-
pot steps of Kornblum type oxidation of benzyl halide, its simultaneous in situ formation of phosphonium
salt, and subsequently their Wittig reaction. In other variant it is oxidized to aldehyde, treated with ylide
generated from phosphonium salt (CH3PPh3X) to give styrene, and subjected to Pd catalyzed Heck reac-
tion with arylhalide to give stilbenes as the three-step one-pot sequence.
Received 24 May 2012
Revised 15 June 2012
Accepted 17 June 2012
Available online 29 June 2012
Ó 2012 Elsevier Ltd. All rights reserved.
We wish to dedicate this Letter to the fond
memory of Late Dr. S.S. Madhav Rao.
Keywords:
One-pot synthesis
Kornblum oxidation
Oxidation-Wittig
Oxidation-Wittig–Heck
Synthetic protocols in which more than one step is carried out
simultaneously or as one-pot process, offer a number of advanta-
ges to the chemists. Mainly the combination of operations results
in the lower overall consumption of reagents required for reac-
tion/work-up, reduction in the total reaction time, avoids purifica-
tion of unstable, toxic, or volatile intermediates, and often fulfill
some of the requirements of the alternative greener synthesis. In
recent decades several procedures have been developed for making
useful molecules and intermediates by adopting one-pot or dom-
ino or tandem synthetic schemes.1 Functional groups such as alde-
hydes and ketones are critical starting materials for building many
products by a number of important reactions. Several of these
starting materials are commercially available or can be easily syn-
thesized. Oxidation of 1o alcohols gives inherently unstable alde-
hydes and the strategy of their in situ one-pot reaction is an
attractive option.2 In this connection several reagents have been
applied for in situ oxidation of alcohols for subsequent wide vari-
ety of organic transformations (Scheme 1).3
reaction to efficiently access substituted stilbenes. Alkyl halides
can also be valuable precursors of aldehydes for further olefination
procedures. To the best of our knowledge such synthesis of sym-
metrical olefins by coupling of alkyl halides via its in situ generated
aldehyde by vanadium catalyzed oxidation of the ylide is the only
reported example.5
In this Letter we report our studies to access aromatic aldehydes
by in situ oxidation of benzyl halides and subject it to one-pot Wit-
tig reaction toward stilbenes. The approach in this direction is
based on the well known Kornblum oxidation6 of benzyl halides
to the corresponding aromatic aldehydes (Scheme 2). The combi-
nations of domino reactions with Kornblum type oxidation as
one of the steps are relatively rare.7 In the Approach A one mole-
cule of benzyl halide will undergo Kornblum oxidation to an alde-
hyde, the second one will form a phosphonium salt with triphenyl
phosphine.8 Both of them will subsequently combine to give a
symmetrical stilbene 1 as the Wittig product. Further modification
in Approach B will give unsymmetrical stilbene 2 when one equiv-
alent of a preformed phosphonium salt of a different benzyl halide
is added in the reaction mixture. The Approach C of this series
Similarly, substituted styrenes are not readily available due to
their tendency of polymerization, and hence are also good sub-
strates for investigations for one-pot in situ reactions. In our previ-
ous attempt we have prepared styrene from alkyl halides or
aldehydes by base mediated dehydrohalogenation or Wittig reac-
tion4 and subjected it to in situ palladium catalyzed one-pot Heck
further
O
one-pot
reactions
oxidation
R
H
R
OH
⇑
Corresponding author. Tel.: +91 0265 2795552.
E-mail address: avbedekar@yahoo.co.in (A.V. Bedekar).
Scheme 1. One-pot synthesis starting from alcohols.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.090

A. S. Saiyed et al. / Tetrahedron Letters 53 (2012) 4692–4696
4693
Table 1
O
Kornblum
oxidation
Examples of synthesis of symmetrical stilbenes as per Scheme 3
Wittig
X
H
% Y [Z:E]a
Ar
Ar
No.
1
ArCH2X
Stilbene Ar-CH@CH-Ar
reaction
Ar
Ar
C6H5CH2Cl
87 [45:55]
Ph3P, heat
3
Ar
X
PPh3X
Ar
F
phosphonium salt
formation
2
3
4
4-FC6H4CH2Br
4-ClC6H4CH2Br
4-BrC6H4CH2Br
86 [53:47]
60 [55:45]
63 [23:77]
F
4
Approach-A [Symmetrical Stilbenes]
Cl
Cl
O
5
Kornblum
oxidation
Br
X
H
Wittig
Ar
Ar
Br
reaction
Ar'
6
Ar
F
PPh3X
Ar'
X = Cl or Br
5
6
3-FC6H4CH2Cl
2-ClC6H4CH2Cl
75 [46:54]
58 [39:61]
Approach-B [Unsymmetrical Stilbenes]
7
F
Cl
Scheme 2. Synthesis of symmetrical/unsymmetrical stilbenes from benzyl halide
by one-pot oxidation-Wittig sequence.
8
Cl
NO2
PPh3 (0.5 eq.)
NaHCO3 (3.0 eq.)
Ar
7
2-NO2C6H4CH2Br
69 [46:54]
X
Ar
Ar
KI (0.1 eq.)
DMSO
9
NO2
140 oC, 40 h
a
Isolated yield. (Z:E) Ratio determined by H NMR.
Scheme 3. Conditions for the synthesis of symmetrical stilbenes (Approach A).
X
Ar
(1.0 eq.)
+
NaHCO3 (3.0 eq.)
KI (0.2 eq.)
R
involves in situ synthesis of styrene from aldehyde by Witting
reaction, followed by its Mizoroki–Heck reaction toward stilbenes.
The present work also explores the possibility of intramolecu-
lar version of oxidation-Wittig reaction to synthesize phenan-
threne molecule. In this approach a molecule is chosen where
the two benzyl bromide units are present on the biphenyl frame-
work for one to undergo oxidation while the second will give
phosphonium salt suitable for intramolecular Wittig olefination
reaction.
Ar'
R'
R
Ar
DMSO
140 oC, 40 h
PPh3X'
Ar'
(1.0 eq.)
R'
Scheme 4. Conditions for the synthesis of unsymmetrical stilbenes (Approach B).
furnish styrene as the intermediate for further coupling with dif-
ferently substituted aryl halide by a Pd mediated Mizoroki–Heck
reaction10 (Scheme 5; Approach (C). Easy availability of aryl halides
compared to benzyl halides may increase the usefulness of this ap-
proach. This novel combination of oxidation-Wittig–Heck reac-
tions is compatible with a base mediated reaction and quite
suitable for one-pot exploration.
This approach widens the scope of the present one-pot synthe-
sis of stilbene derivatives. A number of combinations are screened
to test the validity of this approach and results are summarized in
Scheme 6. As the stereochemistry of the final stilbene is deter-
mined primarily during the Mizoroki–Heck reaction step, as in
most of the cases with triphenylphosphine ligands, such as dppp,
the E isomer is predominantly formed in the cases investigated
(Table 3). Reaction conditions were optimized by performing pre-
liminary experiments and the selected conditions were found suit-
able. Longer reaction time furnished lower yield in the case of
some examples of this approach.
The concept is investigated by conducting the one-pot reaction
of benzyl chloride with half equivalent of Ph3P in dry DMSO, cata-
lytic amount of KI, and excess of NaHCO3 at 140 °C for 40 h. The
product stilbene was isolated from the reaction mixture by column
chromatography and the ratio of its Z:E isomer was established by
H NMR analysis (Scheme 3, Table 1). Encouraged by the reaction a
series of benzyl halides were subjected to the one-pot reaction se-
quence and the products were obtained in good yield, with consid-
erable amount of Z isomer as expected for the unstabilized Wittig
reagent.9 This reaction worked well with mostly benzylic halides,
although oxidation of aliphatic bromides did take place. A con-
trolled experiment with benzyl bromide and dodecyl bromide gave
stilbene 3 in 67% yield; no significant olefin containing dodecyl
unit was isolated, although dodecanal was detected by GC–MS of
the crude mixture.
The modified Approach B involves a one-pot Wittig reaction be-
tween a preformed phosphonium salt from one benzyl halide and
the aldehyde formed by the Kornblum oxidation from a different
benzyl halide. A variety of unsymmetrical stilbenes have been pre-
pared by this route (Scheme 4, Table 2).
However, both these approaches need the appropriate benzyl
halides to complete the synthesis. The third approach starts with
oxidation of benzyl halide to the aldehyde, then the Wittig reaction
with readily available one carbon phosphonium salt (CH3PPh3I) to
Certain conjugated molecules have very important applications
in material chemistry due to their ability to transport electrons
from one part to the other within its framework.11 Some of the
distyrylbenzenes (C6-C2-C6-C2-C6) have useful optical proper-
ties.11b The present procedure is investigated for the synthesis of
2,4-dimethoxy-1,4-bis(phenylethynyl)benzene 22 or its dichloro

4694
A. S. Saiyed et al. / Tetrahedron Letters 53 (2012) 4692–4696
Table 2
Synthesis of stilbenes as shown in Scheme 4
No.
ArCH2X and Ar0CH2PPh3X0
Stilbene Ar-CH@CH-Ar0
% Y [Z:E]a,b
1
C6H5CH2Cl and C6H5CH2PPh3Cl
84 [46:54]a
3
Br
2
3
4
5
4-BrC6H4CH2Br and 4-BrC6H4CH2PPh3Br
4-ClC6H4CH2Br and C6H5CH2PPh3Cl
4-BrC6H4CH2Br and C6H5CH2PPh3Cl
4-NO2C6H4CH2Br and C6H5CH2PPh3Cl
81 [31:69]a
73 [51:49]a
75 [47:53]a
65 [43:57]a
Br
6
Cl
10
11
Br
O2N
Br
12
R
6
7
4-BrC6H4CH2Br and 4-RC6H4CH2PPh3Br
C6H5CH2Cl and Ar0CH2PPh3Brd
54 [E]c
13
14
15
16
R = COOMe
87 [59:41]b
F
8
9
4-FC6H4CH2Br and Ar0CH2PPh3Brd
4-BrC6H4CH2Br and Ar0CH2PPh3Brd
79 [52:48]b
74 [51:49]b
Br
a
b
c
Isolated yield. (Z:E) Ratio determined by H NMR.
(Z:E) Ratio determined by HPLC analysis.
Mostly E isomer formed.
Ar0 = 2-Naphthyl.
d
O
Wittig
Kornblum
oxidation
Although the yield was moderate the simplicity of the reagents,
convenient reaction conditions, and high stereoselectivity for the
coupling reaction may make this a useful synthetic procedure for
such conjugated molecules.
reaction
X
H
Ar
Ar
CH3PPh3I
Heck
Ar'
Polyaromatic hydrocarbons are important class of compounds
and many derivatives find applications in material chemistry. Syn-
thesis of phenanthrene and its derivatives is achieved by one-pot
Suzuki-aldol condensation12a and Ullmann–Pinacol coupling12b
involving cyclization methodology. The present approach of
oxidation-Wittig reaction is extended to the intramolecular
version. In this attempt a sample of 2,20-dibromomethyl biphenyl
24 is subjected to partial oxidation of one ArCH2Br and phospho-
nium salt formation with the other, subsequent intramolecular
Wittig reaction leads to 25 in moderate yield (Scheme 8). Initial
experiments indicate better conversion with higher dilution to
favor intramolecular reaction with fewer side products.
In summary, we have developed three different variants of oxi-
dation-Wittig one-pot procedures for the efficient synthesis of
stilbenes with many possible substitution options. Some of the
stilbenes are synthesized by more than one approach. The scheme
involving Wittig reaction generally favors formation of Z isomer,
while the one involving Mizoroki–Heck reaction favored E isomer
of stilbenes. The Approach C involving in situ synthesis of styrene
gave the products in slightly less yield, probably due to unwanted
side reactions of the labile intermediates.13
Ar
Ar
Ar'
X'
Approach-C [Unsymmetrical Stilbenes]
Scheme 5. Synthesis of unsymmetrical stilbenes from benzyl halide by one-pot
oxidation-Wittig–Heck sequence.
X
Ar
+
+
Ar'
(1.0 eq.)
R
CH3PPh3I
(1.5 eq.)
R'
X'
(1.5 eq.)
NaHCO3 (6.0 eq.)
TBABr (0.2 eq.)
Pd(OAc)2 (1%)
dppp (2%)
Ar'
R
R'
Ar
DMSO
140 oC, 24 h
Scheme 6. Conditions for the synthesis of unsymmetrical stilbenes by (Approach
C).
analogue 23 (Scheme 7). Accordingly the bis-bromomethyl deriva-
tive was separately synthesized from 1,4-dimethoxybenzene by
the reaction of formaldehyde and hydrobromic acid.11c Double
oxidation-Wittig–Heck sequence on this compound furnished the
desired molecules 22 and 23 almost exclusively in the E form.
Acknowledgements
We wish to thank Gujarat Narmada Fertilizer Corporation
(GNFC), Bharuch for the financial assistance to this work.

A. S. Saiyed et al. / Tetrahedron Letters 53 (2012) 4692–4696
4695
Table 3
Examples of synthesis of stilbenes as shown in Scheme 6
No.
ArCH2X and Ar0X0
Stilbenes Ar-CH@CH-Ar0
% Y [Z:E]a,b,c
1
C6H5CH2Cl and C6H5I
75 [2:98]a
76 [9:91]a
73 [nd]
3
2
3
4
4-NO2C6H4CH2Br and C6H5I
4-NO2C6H4CH2Br and 4-NO2C6H4Br
4-FC6H4CH2Br and C6H5I
O2N
O2N
F
12
17
NO2
46 [10:90]b
18
NO2
5
6
2-ClC6H4CH2Cl and 4-NO2C6H4Br
C6H5CH2Cl and 3-NO2C6H4Br
50 [8:92]a
Cl
19
64 [12:88]a
20
NO2
7
8
ArCH2Brd and C6H5I
73 [7:93]c
44 [nd]
14
NO2
ArCH2Brd and 4-NO2C6H4Br
21
a
b
c
Isolated Yield. (Z:E) Ratio determined by H NMR.
(Z:E) Ratio determined by GC analysis.
(Z:E) Ratio determined by HPLC analysis.
2-Naphthyl. nd = not determined.
d
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
06.090.
CH3PPh3I (3.0 eq.)
NaHCO3 (12.0 eq.)
TBABr (0.2 eq.)
Pd(OAc)2 (2%)
dppp (4%)
OMe
References and notes
X'
Br
+
1. (a) Denmark, S. E.; Thorarensen, A. Chem. Rev. 1996, 96, 137–166; (b) Tietze, L.
F. Chem. Rev. 1996, 96, 115–135; (c) Muller, T. J. J. Metal Catalyzed Cascade
Reactions; Springer: Berlin, 2006. Vol. 19; (d) Ramon, D. J.; Yus, M. Angew.
Chem., Int. Ed. 2005, 44, 1602–1634; (e) Wasilke, J. O.; Obrey, S. J.; Baker, R. T.;
Bazan, G. C. Chem. Rev. 2005, 105, 1001–1020; (f) Dömling, A. Chem. Rev. 2006,
106, 17–89; (g) Nicolaou, K. C. Chem. Soc. Rev. 2009, 38, 2993–3009; (h)
Nicolaou, K. C.; Edmonds, D. J.; Bulger, P. G. Angew. Chem., Int. Ed. 2006, 45,
7134–7186; (i) Tietze, L. F.; Brasche, G.; Gericke, K. Domino Reactions in Organic
Synthesis; Wiley-VCH: Weinheim, 2006; (j) Climent, M. J.; Corma, A.; Iborra, S.
Chem. Rev. 2011, 111, 1072–1133.
Br
DMSO
140 oC, 24 h
R'
OMe
(1.5 eq.)
(2.0 eq.)
R'
OMe
2. (a) Taylor, R. J. K.; Reid, M.; Foot, J.; Raw, S. A. Acc. Chem. Res. 2005, 38, 851–869;
(b) Ekoue-Kovi, K.; Wolf, C. Chem. Eur. J. 2008, 14, 6302–6315.
22, R' = H, 50%, X' = I
23, R' = Cl, 40%, X' = Br
OMe
3. (a) Ireland, R. E.; Norbeck, D. W. J. Org. Chem. 1985, 50, 2198–2200; (b) Barrett,
A. G. M.; Hamprecht, D.; Ohkubo, M. J. Org. Chem. 1997, 62, 9376–9378; (c)
Shuto, S.; Niizuma, S.; Matsuda, A. J. Org. Chem. 1998, 63, 4489–4493; (d) Wei,
X.; Taylor, R. J. K. Tetrahedron Lett. 1998, 39, 3815–3818; (e) Blackburn, L.; Wei,
X.; Taylor, R. J. K. Chem. Commun. 1999, 1337–1338; (f) Crich, D.; Mo, X.-S.
Synlett 1999, 67–68; (g) Maiti, A.; Yadav, J. S. Synth. Commun. 2001, 31, 1499–
1506; (h) Dunlap, N. K.; Mergo, W.; Jones, J. M.; Carrick, J. D. Tetrahedron Lett.
2002, 43, 3923–3925; (i) MacCoss, R. N.; Balskus, E. P.; Ley, S. V. Tetrahedron
Lett. 2003, 44, 7779–7781; (j) Bressette, A. R.; Glover, L. C., IV Synlett 2004, 738–
740; (k) Shet, J.; Desai, V.; Tilve, S. Synthesis 2004, 1859–1863; (l) Crisostomo, F.
R. P.; Carrillo, R.; Martin, T.; Garcia-Tellado, F.; Martin, V. S. J. Org. Chem. 2005,
70, 10099–10101; (m) Vatèle, J.-M. Tetrahedron Lett. 2006, 47, 715–718; (n)
Davi, M.; Lebel, H. Org. Lett. 2009, 11, 41–44; (o) Lee, E. Y.; Kim, Y.; Lee, J. S.;
Park, J. Eur. J. Org. Chem. 2009, 18, 2943–2946; (p) Harris, G. H.; Graham, A. E.
Tetrahedron Lett. 2010, 51, 6890–6892; (q) Brioche, J.; Masson, G.; Zhu, J. Org.
Lett. 2010, 12, 1432–1435; (r) Ruiz, V. R.; Corma, A.; Sabater, M. J. Tetrahedron
2010, 66, 730.
R'
Scheme 7. Synthesis of unsymmetrical stilbenes (Approach B).
Br
Br
PPh3 (1.0 eq)
NaHCO3 (6.0 eq.)
KI (0.2 eq.)
DMSO (30 mL)
140 oC, 40 h
24 (0.2 g)
25 (70%)
Scheme 8. Synthesis of phenanthrene by intramolecular Approach A.
4. Saiyed, A. S.; Bedekar, A. V. Tetrahedron Lett. 2010, 51, 6227–6231.
5. Shi, M.; Xu, B. J. Org. Chem. 2002, 67, 294–297.
6. (a) Kornblum, N.; Jones, W. J.; Anderson, G. J. J. Am. Chem. Soc. 1959, 81, 4113–
4114; (b) Kornblum, N.; Powers, J. W.; Anderson, G. J.; Jones, W. J.; Larson, H.
O.; Levand, O.; Weaver, W. M. J. Am. Chem. Soc. 1957, 79, 6562-6562.
7. (a) Klein, T. A.; Schkeryantz, J. M. Tetrahedron Lett. 2005, 46, 4535–4538; (b)
Gao, M.; Yang, Y.; Wu, Y.-D.; Deng, C.; Cao, L.-P.; Meng, X.-G.; Wu, A.-X. Org.
Lett. 2010, 12, 1856–1859; (c) Gao, M.; Yang, Y.; Wu, Y.-D.; Deng, C.; Shu,
Supplementary data
Supplementary data (experimental procedures, characterisation
data and spectral data) associated with this article can be found, in

4696
A. S. Saiyed et al. / Tetrahedron Letters 53 (2012) 4692–4696
W.-M.; Zhang, D.-X.; Cao, L.-P.; She, N.-F.; Wu, A.-X. Org. Lett. 2010, 12,
4026–4029; (d) Adib, M.; Sheikhi, E.; Rezaei, N. Tetrahedron Lett. 2011, 52,
to give pure stilbene (0.148 g, 87%) (The ratio of Z:E isomers was determined to
be 45:55 by H NMR).
3191–3194.
Preparation of unsymmetrical stilbenes (Scheme 4):
8. Molander, G. A.; Ham, J.; Canturk, B. Org. Lett. 2007, 9, 821–824.
9. Kolodiazhnyi, O. I. Phosphorous ylides-chemistry and applications in organic
synthesis; Wiley-VCH: Weinheim, 1999.
10. (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581; (b) Heck,
R. F.; Nolly, J. P. J. Org. Chem. 1972, 37, 2320–2322.
11. (a) Li, C.-L.; Shieh, S.-J.; Lin, S.-C.; Liu, R.-S. Org. Lett. 2003, 5, 1131–1134; (b)
Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–656; (c) Shimizu,
M.; Hiyama, T. Chem. Asian J. 2010, 5, 1516–1531; (d) Thomas, R.; Varghese, S.;
Kulkarni, G. U. J. Mater. Chem. 2009, 19, 4401–4406; (e) Lin, C.-H.; Tsai, C.-M.;
Huang, G.-H.; Tao, Y.-T. Macromolecules 2006, 39, 557–568.
4-Bromo stilbenes (11): In a two-necked round bottom flask a mixture of benzyl
triphenylphosphonium chloride (0.389 g, 1.0 mmol), 4-bromobenzyl bromide
(0.25 g, 1.0 mmol), NaHCO3 (0.252 g, 3.0 mmol), and potassium iodide (0.033 g,
0.2 mmol) in dimethylsulfoxide (7 mL) was stirred at 140 °C under N2
atmosphere for 40 h. The cooled mixture was then poured in water (100 mL)
and extracted with dichloromethane (3 ꢀ 50 mL). The organic layer was washed
with water (2 ꢀ 20 mL), brine, and dried over anhydrous sodium sulfate. The
solution was concentrated under reduced pressure to obtain a viscous liquid,
which was purified by column chromatography on silica gel and petroleum
ether as eluent to give 4-bromo stilbenes (11) (0.193 g, 75%).
12. (a) Kim, Y. H.; Lee, H.; Kim, Y. J.; Kim, B. T.; Heo, J. N. J. Org. Chem. 2008, 73, 495–
501; (b) Lin, S.-Z.; You, T.-P. Tetrahedron 2008, 64, 9906–9910.
13. General procedure for the oxidation-Wittig reaction:
General Procedure for Oxidation-Wittig–Heck reaction (Scheme 6):
4-Nitro stilbene (12): A two-necked round bottom flask was charged with
iodobenzene (0.15 g, 0.735 mmol), p-nitrobenzyl bromide (0.238 g,
1.102 mmol), methyl triphenyl phosphonium iodide (0.448 g, 1.102 mmol),
sodium bicarbonate (0.370 g, 4.41 mmol), palladium acetate (0.00165 g,
0.0073 mmol), dppp (0.006 g, 0.014 mmol), tetrabutylammonium bromide
(0.0237 g, 0.073 mmol), and dimethyl sulfoxide (10 mL) under the nitrogen
atmosphere. This mixture was slowly heated to 140 °C and continued for 24 h.
The reaction mixture was quenched with water and extracted with ethyl acetate
(3 ꢀ 25 mL). The combined organic phase was washed with water (2 ꢀ 20 mL),
brine, and dried over anhydrous sodium sulfate. Solvent was removed in
vacuum and the crude product was purified by column chromatography on
silica gel to afford 4-nitro stilbene (0.126 g, 76.4%) as a pale yellow solid.
Preparation of symmetrical stilbenes (Scheme 3):
Stilbene (3): In a two-necked round bottom flask a mixture of benzyl chloride
(0.241 g, 1.906 mmol), triphenylphosphine (0.250 g, 0.953 mmol), NaHCO3
(0.480 g, 5.71 mmol), and potassium iodide (0.032 g, 0.192 mmol) in
dimethylsulfoxide (7 mL) was stirred at 140 °C under N2 atmosphere for 40 h.
The cooled mixture was then poured in water (100 mL) and extracted with
dichloromethane (3 ꢀ 50 mL). The organic layer was washed with water
(2 ꢀ 20 mL), brine, and dried over anhydrous sodium sulfate. The solution
was concentrated under reduced pressure to obtain a viscous liquid, which was
purified by column chromatography on silica gel and petroleum ether as eluent