Organic Syntheses, Vol. 79, p. 52
Submitted by Saumitra Sengupta and Subir K. Sadhukhan
1.
Checked by Kimberly Savary and Edward J. J. Grabowski.
1. Procedure
A.4-[(Bromophenyl)azo]morpholine.
In a
500-mL Erlenmeyer flask equipped with a
magnetic
stirring bar are placed
4-bromoaniline
(15.0 g, 87 mmol, Note
1) and
6 N hydrochloric acid
(HCl), 36.4 mL, 210 mmol) and the mixture
is warmed on a
water bath to make a clear solution. It is cooled
to 0°C to produce a heavy precipitate. A solution of
sodium
nitrite (6.30 g, 91 mmol) in water
(10 mL) is added dropwise over 10 min. Stirring is continued at 0°C for 20 min (Note
2), and
morpholine
(8.3 g, 9.0 mL, 96 mmol) is
added dropwise to the above solution over 10 min. Water (100 mL) is added followed
by the dropwise addition of
10% aqueous
sodium bicarbonate solution (130 mL)
(Note
3). After the solution is stirred for a further hour, the
precipitated solid is filtered, washed with water and dried in air. The solid is dissolved
in
hot light petroleum (60-80 fraction) (80 mL)
and treated with
activated charcoal (1.5 g).
The mixture is filtered while hot and the filtrate concentrated to ca. 40 mL. Upon
cooling to room temperature, shiny crystals of the pure triazene are obtained. The
mother liquor is concentrated to give a second crop of crystals (combined yield
20.3 g,
85%,
Notes
4,
5).
Btrans-4,4'-Dibromostilbene. A
500-mL
round-bottomed flask equipped with a
magnetic stirring bar
is charged with the above
triazene (14.3 g,
53 mmol) and
methanol
(125 mL). The stirred solution is cooled to 0°C and
40% tetrafluoroboric acid (HBF4,
23 mL, 106 mmol) is added dropwise over
10 min. After the addition is complete, the
ice bath is removed
and the reaction brought to room temperature. It is stirred for an additional 10 min
(Note
6) and
palladium acetate
[Pd(OAc)2, 0.12 g, 0.53 mmol] is
added followed by the dropwise addition of a solution of
vinyltriethoxysilane
(4.94 g, 5.5 mL, 26 mmol)
in
methanol(10 mL).
A second lot of
Pd(OAc)2 (0.12 g,
0.53 mmol) is added and stirring continued for a further
30 min at room temperature (Note
7). The mixture is warmed to
40°C for 20 min and finally heated under reflux for 15 min (Note
8).
The solution is concentrated under reduced pressure to half its volume and water (150
mL) is added. The precipitated solid is filtered, washed with water and dried in air.
It is then boiled with
toluene (125
mL) and filtered while hot. The filtrate is concentrated to ca.
70 mL, warmed to
70°C and light petroleum (30 mL)
is added. Upon cooling to room temperature, the product crystallizes (
4.20 g). Concentration of the mother liquor gives an additional
crop (
0.70 g) of the product (combined
yield
4.90 g,
46.5%, Note
9).
2. Notes
2. At this point a clear solution is obtained.
3.
Caution: Vigorous evolution of carbon
dioxide occurs. The product began to precipitate. The checkers
used
1 N sodium hydroxide (NaOH, approximately
120 mL to pH≈7 at 0-10°C) to effect neutralization
and avoid difficulties with
carbon dioxide evolution.
4.
Caution: All 1-aryl triazenes are toxic and direct
hand contact should be avoided. The checkers used
hexane
for the recrystallization.
5. The product showed the following physical data:
mp 87-88°C (lit.
2
mp 89.5-90°C); IR (CHCl
3) cm
−1: 1100, 1210,
1345, 1430, 1480, 3010;
1H NMR (300 MHz,
CDCl
3) δ: 3.76-3.87 (AA'BB', 8 H), 7.29-7.48
(AA'BB', 4 H); Anal. Calcd for C
10H
12BrN
3O:
C, 44.47; H,4.44; N,15.56. Found: C, 44.22; H, 4.38; N, 15.50.
6. The solution became clear at this stage.
7. Some of the product, as it formed, precipitated.
8. Inferior yields were obtained when the reaction is heated directly
under reflux without being held at 40°C for ca. 20 min.
9. The product showed the following physical data:
mp 214–215°C (lit.
3
mp 215–216°C);
1H NMR (300 MHz, CDCl
3) δ:
7.01 (s, 2 H), 7.35 (d, 4 H, J = 6.9), 7.47 (d, 4
H, J = 6.9).
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the
Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Stilbenes are an important class of chromophores that have found various applications
in molecular photonics and optoelectronics.
4 Symmetrical
trans-stilbenes are classically prepared via oxidative dimerization of benzylic halides,
which usually require strongly basic conditions and high reaction temperatures.
5
More recent methods include McMurry-coupling of aromatic aldehydes,
6 Wittig
condensations and double Heck reactions of haloarenes with
ethylene.
7
The latter procedure, however, is inconvenient since it requires a measured amount
(half equivalent) of
ethylene gas under
autoclave
conditions. Moreover, a double Heck reaction of aryl bromides and iodides with
ethylene
invariably produces some of the undesired
1,1-diarylethylene
regioisomer (up to 20%). The
stilbene synthesis described here
is based on the submitters' recent report on double Heck reactions of arenediazonium
salts with
vinyltriethoxysilane (Scheme 1).
8 The procedure has been suitably modified so that the
arenediazonium salt is generated in situ from 1-aryltriazene, thus avoiding direct handling of
the diazonium salt.
9

Synthesis of symmetrical stilbenes via a double Heck reaction of arenediazonium
salts with
vinyltriethoxysilane is attractive on several counts:
Ready availability of starting materials (anilines are more readily available in a
diverse substitution pattern than are aryl halides and aldehydes); Mild reaction conditions
that tolerate a number of functional groups (e.g., NO
2, CO
2R);
Strategic use of
vinyltriethoxysilane as a cheap and easily handled
ethylene equivalent; And operational simplicity (alcoholic reaction medium, ligandless
Pd-catalyst, no additives). Most significantly, by virtue of the superior Heck reactivity
of the diazonium nucleofuge over bromide (and even iodides),
10,11 the present methodology allows for a facile
synthesis of 4,4'-dibromo- (and 4,4'-diiodo-) stilbenes, which would otherwise
be difficult to synthesize (impossible for the diiodo derivative) via the double Heck
reaction of haloarenes with ethylene. Recently, the submitters have also described
the double Heck reaction of bisarenediazonium salts with
vinyltriethoxysilane
for the synthesis of poly(1,4-biarylenevinylenes).
12
The dibromo- (and diiodo-) stilbenes are potentially useful monomers for the synthesis
of poly(
p-phenylenevinylenes) via poly-Heck or poly-Suzuki coupling reactions.
13
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
Vinyltriethoxysilane: ALDRICH: Triethoxyvinylsilane:
Silane, triethoxyvinyl- (8); Silane, ethenyltriethoxy-
(9); (78-08-0)
trans-4,4'-Dibromostilbene: Stilbene,
4,4'-dibromo-, ( E)- (8); Benzene, 1,1'-(1,2-ethenediyl)bis[4-bromo-,
( E)- (9); (18869-30-2)
4-Bromoaniline: Aniline, p-bromo-
(8); Benzenamine, 4-bromo- (9); (106-40-1)
4-[(4-Bromophenyl)azo]morpholine: Morpholine,
4-[(4-bromophenyl)azo]- (14); (188289-57-8)
Sodium nitrite: Nitrous acid, sodium salt
(8, 9); (7632-00-0)
Morpholine (8, 9); (110-91-8)
Tetrafluoroboric acid: Borate (1-), tetrafluoro-,
hydrogen (8, 9); (16872-11-0)
Palladium acetate: Acetic acid, palladium(2+)
salt (8, 9); (3375-31-3)
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