Shukla et al.
50 mL). Another recrystallization of the resulting solid from ethanol
C-C-C-C dihedral angle of 113°, and it also causes a bend
in the acetylenic triple bond by ∼19° compared to 12° in 4d.
Interestingly, however, the X-ray structure of 4f, with a
tetramethylene bridge akin to that in 4a, showed that it exists
in an asymmetric conformation in which one of the ethereal
groups lies perpendicular to the aromatic plane (i.e., dihedral
angle ∼ 76.7°, an elongated C(ar)-O bond, 1.382 Å, and a
contracted C-O-C angle of 114.5°), whereas the other ethereal
group lies in a quasi-trans-periplanar conformation (i.e., dihedral
angle of 34.4°, a shortened C(ar)-O distance, 1.364 Å, and a
more opened C-O-C angle of 118.2°). Importantly, the
asymmetric conformational arrangement in 4f leads to a more
linear central -CtC- triple bond; that is, it is bent by just
∼10° in comparison to that in 4a, where it is bent by ∼19°
(see Figure 1).
afforded 1a as a colorless crystalline solid. Yield: 28.7 g, 96%;
1
mp106-108 °C (lit.14 mp 84 °C); H NMR (CDCl3) δ 2.06 (m,
4H), 4.16 (m, 4H), 6.97 (m, 4H), 7.51 (m, 2H), 7.79 (dd, J ) 7.8,
1.9 Hz, 2H), 10.46 (s, 2H); 13C NMR (CDCl3) δ 26.0, 68.0, 112.5,
120.8, 124.9, 128.5, 128.6, 136.2, 161.3, 189.7. GC-MS: m/z 298
(M+), 298 calcd for C18H18O4. Anal. Calcd for C18H18O4: C, 72.47;
H, 6.08. Found: C, 72.45; H, 6.06. The yields and the spectral
data for 1b-f are summarized in the Supporting Information.
Preparation of Bridged cis/trans Stilbene 2a-h.15 General
Procedure. To chilled (∼0 °C) anhydrous tetrahydrofuran (1 L)
was added TiCl4 (30 mL, 270 mmol) dropwise with the aid of a
dropping funnel under an argon atmosphere. To the resulting
mixture were added Zn dust (22 g, 340 mmol) and dry pyridine (1
g, 13 mmol), and the resulting black suspension thus obtained was
warmed to room temperature and then refluxed for 2 h. A solution
of dialdehyde 1a (23.87 g, 80 mmol) in THF (500 mL) was added
dropwise to the above black reaction mixture during a course of
48 h while refluxing, and the resulting mixture was refluxed for an
additional 12 h. The resultant mixture was cooled to room
temperature and quenched with 10% aqueous K2CO3 (300 mL).
The organic layer was separated, and the aqueous suspension was
extracted with dichloromethane (5 × 150 mL) followed by diethyl
ether (3 × 100 mL). The combined organic layers were dried over
anhydrous MgSO4, filtered, and evaporated to afford a syrupy liquid
which was purified by flash chromatography on silica gel using a
1:9 mixture of ethyl acetate and hexanes to afford a mixture of
cis/trans stilbene 2a as a viscous oil.
In conclusion, the bending of the linear central -CtC- triple
bond as well as the twist angle between the mean planes of
aromatic rings in 4d, 4a, and 4f is controlled by the conforma-
tions of 11- and 12-membered rings, which in turn are largely
governed by the p-π conjugation among the aromatic rings
and the ethereal groups.12
Summary
In summary, a successful synthesis of various bridged
diarylacetylenes in multigram quantities has been accomplished
using high-yielding (classical) synthetic methods and readily
available starting materials. The structures of representative
bridged diarylacetylenes have been established by X-ray crystal-
lography. The structural analysis strongly indicates that the
conformations of 11- and 12-membered rings in various bridged
diarylacetylenes are largely controlled by the p-π conjugation
among the aromatic rings and the ethereal groups and that, in
turn, controls the bending of the linear triple bond as well as
the twist angle between the mean planes of aromatic rings.
The ready availability of bromo-substituted bridged diary-
lacetylenes 4a and 4f holds potential to be utilized for the
preparation of a variety of electro-active materials by modern
palladium- and nickel-catalyzed C-C bond-forming reactions.
We are presently exploring the preparation of polymeric
analogues of 4a and 4f and the hexaarylbenzene derivatives via
trimerization of various bridged diarylacetylenes for potential
applications in the emerging area of molecular electronics and
nanotechnology.
Note that attempts to separate cis/trans isomers of various
stilbenes, in most cases, were not successful. However, in most
cases, the integration of the peaks in the aliphatic region allowed
the ratios of the two isomers to be determined; however, the identity
of the isomers (i.e., whether it is cis or trans) could not be
established due to the extensive overlap of the signals in the
aromatic/olefinic region. Accordingly, the spectral data for various
bridged stilbenes in the aliphatic region are readily deconvoluted,
whereas in the aromatic/olefinic region, they are presented as ranges.
The number of signals in the 13C NMR spectra are in complete
agreement with the assignment that the bridged stilbenes exist as
isomeric (cis/trans) mixtures. 2a: Yield 20.2 g, 95% (ratio of
1
isomers I and II ) 2:1). Isomer I: H NMR (CDCl3) δ 1.76 (sym
m, 4H), 4.04 (sym m, 4H), 6.65-7.42 (aromatic/olefinic, 10H).
Isomer II: 2.01 (sym m, 4H), 4.11 (sym m, 4H), 6.65-7.42
(aromatic/olefinic, 10H); 13C NMR(CDCl3) δ 26.1, 26.2, 69.4, 71.9,
114.6, 118.3, 120.6, 122.9, 127.8, 128.2, 128.3, 129.0, 129.4, 130.0,
130.5, 155.5, 157.8. GC-MS: m/z 266 (M+), 266 calcd for
C18H18O2. The spectral data of 2b-f are summarized in the
Supporting Information. [Also, note that various stilbenes obtained
above were used in the next step without any further purification.]
Preparation of Bridged Diarylacetylenes. To a solution of a
cis/trans mixture of stilbene 2a (13.3 g, 50 mmol) in acetic acid
(50 mL) was added dropwise a solution of bromine (26.4 g, 165
mmol) in acetic acid (50 mL) at 22 °C. The reaction mixture was
stirred for 30 min and was poured into water (250 mL). The
resulting mixture was extracted with dichloromethane (3 × 100
mL), washed with 10% aqueous sodium bisulfite (2 × 50 mL),
and dried over anhydrous MgSO4. Evaporation of the solvent
afforded a quantitative yield of the corresponding tetrabromostilbene
3a, which was used in the next step without further purification as
follows. [Note that bromination of stilbenes 2b-e required only 1
equiv of bromine, while the bromination of 2f required 3 equiv of
bromine, and the reactions can be carried out in either acetic acid
or dichloromethane as solvent; see text.]
Experimental Section
Preparation of Bridged Dialdehydes 1a-f. Thus, following
closely a literature procedure,14 an addition of salicylaldehyde (24.4
g, 0.2 M) to an ethanolic solution (200 mL) of potassium hydroxide
(11.2 g, 0.2 mol) immediately resulted in a yellow precipitate of
the potassium salt of salicylaldehyde which dissolved upon further
refluxing. 1,4-Dibromobutane (20.5 g, 0.095 mol) was added
dropwise to the above reaction mixture, and the resulting mixture
was refluxed for additional 8-12 h. Upon cooling, the resulting
mixture just below the boiling point of ethanol and a rapid filtration
produced a clear solution which upon standing at room temperature
produced a mass of pale yellow crystals. The crystalline mass was
filtered and washed with a mixture of cold ethanol and water (1:1,
Dehydrobromination using Ethylene glycol/KOH. Tetrabro-
mostilbene derivative 3a (9 g, 0.015 mmol), ethylene glycol (30
mL), and KOH (5 g) were placed in a round-bottom flask fitted
(12) Compare Brizius et al. in ref 3.
(13) Hofslokken, N. U.; Skattebol, L. Acta Chem. Scand. 1999, 53, 258-
262.
(14) Simonis, U.; Walker, F. A.; Lee, P. L.; Hanquet, B. J.; Meyerhoff,
D. J.; Scheidt, R. J. Am. Chem. Soc. 1987, 109, 2659-2679.
(15) For a similar procedure, see: Rives, J. T.; Oliver, M. A.; Fronczek,
F. R.; Gandour, R. D. J. Org. Chem. 1984, 49, 1627-1634.
6128 J. Org. Chem., Vol. 71, No. 16, 2006