Ethynyl π-extended 2,5-diphenyl-1,3,4-oxadiazoles and 2-phenyl 5-(2-thienyl)-1,3,4-oxadiazoles: Synthesis, X-ray crystal structures and optical properties

Article abstract of DOI:10.1039/b407698m

2-(4-tert-Butylphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole 1 reacts with a series of heteroaryl iodides under standard Sonogashira cross-coupling conditions (Pd[PPh₃]₂Cl₂, Cul, triethylamine, THF) to yield products 2a-g i

Full text of DOI:10.1039/b407698m

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A R T I C L E
Ethynyl p-extended 2,5-diphenyl-1,3,4-oxadiazoles and 2-phenyl 5-
(2-thienyl)-1,3,4-oxadiazoles: synthesis, X-ray crystal structures and
optical properties†
Gregory Hughes, David Kreher, Changsheng Wang, Andrei S. Batsanov and Martin R. Bryce*
Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
E-mail: m.r.bryce@durham.ac.uk
Received 20th May 2004, Accepted 8th September 2004
First published as an Advance Article on the web 7th October 2004
2-(4-tert-Butylphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole 1 reacts with a series of heteroaryl iodides under
standard Sonogashira cross-coupling conditions (Pd[PPh3]2Cl2, CuI, triethylamine, THF) to yield products 2a–g in
40–79% yields (heteroaryl = 2-pyridyl, 3-pyridyl, 4-pyridyl, 2-pyrazyl, 5-bromo-2-pyrimidyl, 2-thienyl and 3-thienyl,
respectively). Compound 2f was lithiated followed by electrophilic iodination (BuLi, perfluorohexyl iodide) to give
3, which by a two-step sequence gave the terminal ethynylthienyl derivative 5. Conversion of 5 into the terminal
ethynylaldehyde derivative 7, via acetal derivative 6, proceeded in high yield. Starting from 2-iodo-5-methoxy-
carbonylthiophene, a five-step sequence afforded 2-(4-tert-butylphenyl)-5-(4-ethynylthienyl)-1,3,4-oxadiazole 13
(13% overall yield). Reactions of 13 gave terminal pyridyl, pyrazyl, pyrimidyl and thienyl derivatives, analogous
to those obtained from 1. Two-fold reaction of 13 with 2,5-diiodothiophene gave the bis(ethynylthienyl)thiophene
derivative 15 (30% yield). Solution UV-Vis absorption and photoluminescence spectra establish that replacement of
the phenyl ring in the 2,5-diphenyl-1,3,4-oxadiazole series 2a–g by a thienyl ring [i.e. the 2-phenyl-5-(2-thienyl)-1,3,4-
oxadiazole series 14a–g] leads to a red shift in the lowest energy band in both the absorption spectra and emission
spectra. The X-ray crystal structures of compounds 2d, 2g, 5 and 14d·CHCl3 reveal that the molecular structures are
approximately planar although there are substantial differences in the conformations.
Introduction
Results and discussion
2,5-Diphenyl-1,3,4-oxadiazole derivatives have been widely
studied in diverse areas of chemistry. In particular, due to
the electron-deficient properties of the oxadiazole ring, their
luminescence properties and their good thermal and chemical
stabilities,1 a range of derivatives (e.g. low molecular weight
monomers,2 star-shaped oligomers3 and polymers4) have been
used as emissive materials and/or electron-transporting/hole-
blocking compounds in organic light emitting devices (OLEDs).5
Furthermore, many 1,3,4-oxadiazole derivatives are biologically
active and continue to find applications in medicinal chemistry.6
This combination of properties ensures that new functionalised
derivatives are of considerable interest.
Very recently, the first 2,5-diphenyl-1,3,4-oxadiazole
derivative possessing an alkyne substituent, compound 1,
was reported independently by Cha et al.3 and by ourselves.7
From a synthetic viewpoint, the ethynyl substituent offers
unprecedented scope for functionalisation reactions which will
extend the p-conjugation at the periphery of the framework.
Compound 1 is, therefore, a very attractive building block in the
lightof thecurrentinterestinconjugationof aryl/heteroarylrings
through sp hybridised carbon linkages,8 and the study of ethynyl
derivatives of arenes and heteroarenes as “molecular wires”.9 In
this article we report the synthesis of the novel thienyl analogue
13 and describe organometallic cross-coupling reactions at the
terminal alkyne positions of 1 and 13 to obtain heteroaryl-
functionalised derivatives possessing extended p-electron
conjugation. The optical absorption and photoluminescence
properties of the products are reported, along with four X-ray
crystal structures.
Synthesis
Compound 1 reacted with a series of heteroaryl iodides under
standard Sonogashira conditions10 [Pd(PPh3)2Cl2, triethylamine,
CuI, THF] to afford products 2a–g in 40–79% yields (Scheme 1).
Electron-deficient heterocycles (pyridyl, pyrazyl and pyrimidyl:
a–e) and electron-rich thienyl derivatives (f, g) reacted similarly.
Compound 2f (obtained in 79% from 1) was lithiated followed
by electrophilic iodination (BuLi, perfluorohexyl iodide)11 to
give 3 in 83% yield (Scheme 2). This two-step sequence from 1
was more efficient than the direct reaction of 1 with 2,5-diiodo-
thiophene, which gave 3 in 33% yield.
The terminal iodo substituent in compound 3 enabled further
functionalisation reactions (Scheme 2). Sonogashira reaction
with 2-methyl-3-butyn-2-ol gave product 4 (70% yield), which
was deprotected under standard basic conditions12 with loss
† Electronic supplementary information (ESI) available: crystal
structure data; full synthetic details and characterisation data for
compounds 2a–g, 3–7, 14a–g and 15; UV-Vis absorption and photo-
luminescence spectra for compounds 2g and 14g; packing diagrams for
compounds 2d, 2g, 5 and 14d·CHCl3. See http://www.rsc.org/suppdata/
ob/b4/b407698m/
Scheme 1 Reagents and conditions: i, iodoheterocycle, CuI,
Pd[PPh3]2Cl2, THF, NEt3, reflux.
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 3 6 3 – 3 3 6 7
3 3 6 3

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Scheme 2 Reagents and conditions: i, n-BuLi, THF, C6F13I, −78 °C to 20 °C; ii, 2-methyl-3-butyn-2-ol, CuI, Pd[PPh3]2Cl2, THF, NEt3, reflux;
iii, NaOH, toluene, reflux; iv, triethyl orthoformate, ZnI2, 140 °C; v, Amberlyst-15, acetone–water, 20 °C.
the ion exchange resin Amberlyst-15 in acetone–water14 to give
aldehyde 7 (88% yield).
of acetone to give the new terminal alkyne derivative 5 (79%
yield). One reason for synthesising 5 was that we thought that
the low nucleophilicity of the acetylide anion of 1 towards
carbonyl groups was due to the strongly electron-withdrawing
effect of the oxadiazole ring.7a The insertion of an additional
electron-rich thiophene ring should offset this effect and the
nucleophilicity of the acetylide anion of 5 would thereby be
increased compared to that of 1. However, attempted func-
tionalisation of the alkyne group of 5 by deprotonation (with
NaH, DBU, LDA or tert-BuLi) and subsequent reaction with
4-ethoxybenzaldehyde or DMF gave only recovered starting
material in high yield in each case, with no substitution product
being detected. Similar to compound 1,7a functionalisation of 5
using triethyl orthoformate proceeded smoothly in the presence
of zinc iodide13 to yield 6 (89% yield) which was hydrolysed with
To explore the effect of replacing the alkynylphenyl
moiety of compound 1 with an alkynylthienyl moiety, a series
of novel 2-phenyl-5-(2-thienyl)-1,3,4-oxadiazole derivatives
was synthesised as shown in Scheme 3.15 The readily avail-
able thiophene derivative 816 was converted into the hydrazide
derivative 9. Reaction of 9 with tert-butylbenzoyl chloride
gave the intermediate diacylhydrazine compound 10, which
was not purified. The crude product 10 was cyclodehydrated
under the standard conditions for the formation of 1,3,4-oxa-
diazoles17 using phosphorus oxychloride, to give the 2-phenyl-
5-(2-thienyl)-1,3,4-oxadiazole system 11. By analogy with the
reactions of 3 (Scheme 2), Sonogashira reaction of 11 with
2-methyl-3-butyn-2-ol gave 12, from which the target alkyne
Scheme 3 Reagents and conditions: i, hydrazine hydrate, MeOH, reflux; ii, 4-tert-butylbenzoyl chloride, pyridine, reflux; iii, POCl3, reflux;
iv, 2-methyl-3-butyn-2-ol, CuI, Pd[PPh3]2Cl2, THF, NEt3, reflux; v, NaOH, toluene, reflux; vi, iodoheterocycle (2,5-diiodothiophene for 15), CuI,
Pd[PPh3]2Cl2, THF, NEt3, reflux.
3 3 6 4
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 3 6 3 – 3 3 6 7

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Table 1 Absorption and steady state emission spectra at 20 °C in
DCM
13 was obtained (13% overall yield from compound 8). Cross-
coupling reactions analogous to those shown in Scheme 1, using
the corresponding iodoheterocycle, gave the terminal heteroaryl
derivatives 14a–g in 33–65% yield. Two-fold reaction of 13 with
2,5-diiodothiophene gave the p-extended bis(ethynylthienyl)-
thiophene derivative 15 (30% yield).
Compound
kmax(abs)
kmax(PL) (PLQY)a
2a
2b
322
319
320
323
325
335
325
352
348
351
353
361
355
347
400
354, 374
354, 373
375
380 (24%)
384
389 (10%)
361, 376 (28%)
386, 408
385, 408
388, 409
396, 415 (22%)
420
407, 425
395, 413
452, 480
2c
X-Ray crystal structures of compounds 2d, 2g, 5 and 14d
2d
2e
The molecular structures of 2d, 2g, 5 and 14d, shown in Fig. 1,
can be described as approximately planar. The deviations of all
non-hydrogen atoms (except the methyl carbons) from their
mean plane are small [average 0.22 (2d), 0.21 (2g), 0.11 (5) and
0.15 Å (14d)] compared to the total lengths of the molecules [ca.
22.5 (2d, 2g), 25.4 (5) and 23.2 Å (14d)]. Largely, this planarity
is due to the molecules having few degrees of conformational
freedom, as they comprise a number of planar rings on a rigid
‘rod’. However, the rings can rotate, and there are substantial
differences in the conformations. The asymmetric unit of struc-
ture 2d comprises two molecules. In each of them, both benzene
rings are nearly coplanar (within 1.5 to 4.8°) with the oxadiazole
ring, while the angle between benzene ring B and the pyrazine
ring is 42.5 or 39.3°. In molecule 2g the oxadiazole ring is in-
clined by 24.7 and 18.1° (in the opposite sense) to the benzene
rings A and B, while the latter is inclined to the thiophene ring
by 3.5°. The corresponding interplanar angles in molecule 5 are
6.8, 14.3 and 11.0°, respectively. The acetylenic H atom does not
participate in any hydrogen bond. Compound 14d crystallises as
a monosolvate, with the asymmetric unit comprising one mole-
cule of 14d and one of CHCl3, linked by a weak hydrogen bond
Cl3C–HN(2) (HN 2.48 Å for the idealised C–H distance of
1.08 Å). The dihedral angles between benzene and oxadiazole
rings are 13.6°; oxadiazole and thiophene 6.1°; thiophene and
pyrazine 18.5°.
2f
2g
14a
14b
14c
14d
14e
14f
14g
15
a Standards were quinine sulfate (U = 0.577 in 0.1 M H2SO4) and
b-carbolene (U = 0.60 in 0.5 M H2SO4); excited at 350 nm.
14d·CHCl3 comprises double layers of molecules 14d separated
by single layers of chloroform molecules.
Optical absorption and emission properties
The UV-Vis absorption and photoluminescence properties in
dichloromethane solution for the two series of compounds
2a–g and 14a–g, 15 are collated in Table 1. Fig. 2 shows the
absorption and emission spectra for selected compounds. The
Stokes shift in kmax values for all the compounds is in the range
50–80 nm, which agrees with known diphenyl-1,3,4-oxadiazole
derivatives18 and diaryl(heteroaryl)ethynes where there is a rela-
tively small conformational change upon photoexcitation.8a An
interestingtrendisthatreplacementof aphenylring(compounds
2a–g) by the thienyl ring (compounds 14a–g) leads to a red shift
in the lowest energy band in both the absorption and emission
spectra. This can be explained by a “push-pull effect” of the
conjugated electron-donating thiophene ring and the electron-
accepting oxadiazole ring lowering the HOMO–LUMO gap.19
In the absorption spectra, this shift is within the range 21–36 nm.
In the PL spectra the red shift which occurs on replacing phenyl
with thienyl is remarkably similar for all the analogues [Dkmax(em)
34–37 nm]. An additional feature of the data in Table 1 is a
further red shift in the absorption and emission peaks of 15
compared to 14f [Dkmax(abs) 45 nm; Dkmax(em) 55 nm], which
is consistent with extended p-conjugation through the central
bis(ethynylthiophene) unit of 15.20 Photoluminescence quantum
yield (PLQY) values were obtained for compounds 2d, 2f, 2g and
14d and fall within the range 10–28% (Table 1). These values are
considerably lower than those reported for model compounds
which lack the ethynyl bond, viz. 2,5-diphenyl-1,3,4-oxadiazole
The crystal packing diagrams are included in the
Supplementary Information.† The motifs of 2g and 5 (and their
lattices) are similar, with stacks of molecules running in the y
direction. The structure of 2d has a herringbone motif, while
Fig. 2 UV-Vis absorption and photoluminescence spectra (excitation
at 355 nm) in dichloromethane, 20 °C: compounds 2d and 14d (top) and
compounds 14f and 15 (bottom).
Fig. 1 X-Ray molecular structures of 2d, 2g, 5 and 14d·CHCl3.
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 3 6 3 – 3 3 6 7
3 3 6 5

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(89%) and 2-(4-biphenyl)-5-phenyl-1,3,4-oxadiazole (83%) in
non-polar solvents,21 and are consistent with the data obtained
for a series of simpler di(aryl/heteroaryl)ethynyl derivatives.22
was added in one portion and the solution turned orange, then
red and brown within 15 min of the addition. The mixture was
stirred at 20 °C for 12 h to yield a yellow suspension. Additional
THF (5 cm3) was added and the suspension was heated at reflux
for 5 h. The mixture was evaporated in vacuo and the residue was
chromatographed on a silica column to yield the product.
Cyclic voltammetry
Cyclic voltammograms of 2d and 14d, representing each of
the two series of analogues, were recorded (Fig. 3). These
two compounds had similar redox behaviour featuring a
reversible one-electron reduction and another irreversible
two-electron reduction wave. We assume that the wave at lower
reductionpotentialwasduetothereductionof themostelectron-
deficient oxadiazole ring of the molecule, and the irrversible wave
at higher reduction potential was attributed to the reduction of
the triple bond. Due to the reduced HOMO–LUMO gap from
the introduction of the thiophene unit, the reduction of 14d is
ca. 100 mV less negative than its phenylene analogue 2d.
Compound 9
Compound 816 (10.0 g, 37.3 mmol) was dissolved in methanol
(70 cm3). Hydrazine hydrate (7.2 cm3, 149 mmol) was added and
the mixture was heated under reflux overnight. The solvent was
partially evaporated in vacuo and the resulting yellow solid was
isolated by filtration and recrystallised from methanol to yield 9
as yellow crystals (4.60 g, 46%), mp 146–147 °C. dH (DMSO-d6):
4.44 (s, 2H), 7.34 (d, 1H, J = 4.0 Hz), 7.37 (d, 1H, J = 4.0 Hz),
9.77 (s, 1H). dC (DMSO-d6): 82.43, 129.09, 137.72, 144.08,
160.02. MS (EI) m/z (%): 268 (M+, 100). Anal. for C5H5IN2OS:
calcd. C, 22.40; H, 1.88; N, 10.45. Found C, 22.35; H, 1.87; N,
10.49%.
Compound 11
Compound 9 (7.50 g, 27.9 mmol) was dissolved in pyridine
(20 cm3) and 4-tert-butylbenzoyl chloride (5.50 g, 27.9 mmol)
was added. The mixture was stirred at 20 °C under Ar for 1 h
then refluxed for 1 h. The pyridine was removed in vacuo and
methanol (100 cm3) was added to the insoluble white residue.
This mixture was boiled for 2 min (to extract impurities), cooled,
and the solid (compound 10) was isolated by suction filtration
and thoroughly dried under vacuum overnight. The anhydrous
10 was mixed with phosphorus oxychloride (75 cm3) and the
mixture was refluxed for 3 h. The excess POCl3 was removed by
vacuum distillation. The viscous residue was crystallised from
chloroform–ethanol, yielding 11 as pale yellow plates (6.04 g,
53%), mp 170–172 °C. dH (CDCl3): 1.37 (s, 9H), 7.33 (d, 1H,
J = 4.0 Hz), 7.46 (d, 1H, J = 4.0 Hz), 7.52 (d, 2H, J = 8.4 Hz),
8.00 (d, 2H, J = 8.4 Hz). dC (CDCl3): 31.08, 35.09, 79.61, 120.60,
126.08, 126.79, 130.67, 1311.10, 137.99, 155.55, 159.44, 164.16.
MS (EI) m/z (%): 410 (M+, 100). Anal. for C16H15IN2OS: calcd.
C, 46.84; H, 3.69; N, 6.83. Found C, 46.58; H, 3.62; N, 6.73%.
Fig. 3 Cyclic voltammograms of compounds 2d (upper) and 14d
(lower) in DMF solution containing 0.1 M NBu4PF6. The potentials
were relative to Ag/Ag+, using a Pt disk (U = 1.6 mm) as the work-
ing electrode and a Pt wire as the counter electrode, and scanned at
100 mV s−1.
Conclusions
We have achieved efficient functionalisation of the terminal
ethynyl carbon of compound 1 by cross-coupling reactions with
a series of heteroaryl iodides using Sonogashira methodology.
The first ethynyl derivative of 2-phenyl-5-(2-thienyl)-1,3,4-
oxadiazole and its cross-coupled derivatives are reported: a
five-step sequence from the readily available 2-iodo-5-methoxy-
carbonylthiophene gives 2-(4-tert-butylphenyl)-5-(4-ethynyl-
thienyl)-1,3,4-oxadiazole 13 in 16% overall yield. Subsequent
functionalisation of 13 proceeds smoothly by analogy with 1.
Spectroscopic studies in solution establish that replacement
of the phenyl ring (i.e. the 2,5-diphenyl-1,3,4-oxadiazole series
2a–g) by a thienyl ring (i.e. the 2-phenyl-5-thienyl-1,3,4-oxadia-
zole series 14a–g) leads to a significant red shift in the lowest en-
ergy band in both the absorption spectra and emission spectra.
Conjugation is extended further in the bis(ethynylthienyl)thio-
phene derivative 15. X-Ray crystal structure analyses reveal
that the p-systems of compounds 2d, 2g, 5 and 14d adopt pre-
dominantly planar conformations. This work clearly establishes
that the alkynes 1 and 13 are synthetically viable and versatile
building blocks for the construction of extended functional p-
electron systems with interesting optoelectronic and structural
properties.
Compound 12
Compound 11 (5.0 g, 12.2 mmol), 2-methyl-3-butyn-2-ol
(2.6 cm3, 26.7 mmol), Pd[PPh3]2Cl2 (0.86 g) and CuI (0.23 mg)
were added to a mixture of THF–Et3N (100 cm3, 1:1 v/v),
stirred at 20 °C overnight and refluxed for 3 h, followed by
chromatography on a silica column (eluent DCM–diethyl ether,
95:5 v/v). The product was recystallised from ethanol–H2O,
yielding 12 as golden needles (3.2 g, 72%), mp 161–163 °C. dH
(CDCl3): 1.41 (s, 9H), 1.69 (s, 6H), 2.30 (s, 1H), 7.20 (d, 1H,
J = 3.5 Hz), 7.53 (d, 2H, J = 8.0 Hz), 7.66 (d, 1H, J = 4.0 Hz),
8.01 (d, 2H, J = 8.0 Hz). dC (CDCl3): 31.08, 31.16, 65.74, 74.59,
100.59, 120.62, 125.70, 126.08, 126.80, 127.32, 129.20, 132.64,
155.55, 159.87, 164.26. MS (EI) m/z (%): 366 (M+, 100). Anal.
for C21H22N2O2S: calcd. C, 68.82; H, 6.05; N, 7.64. Found: C,
68.61; H, 6.00, N, 7.54%.
Compound 13
Compound 12 (2.62 g, 7.15 mmol) was dissolved in dry
toluene (40 cm3). Sodium hydroxide powder (0.34 g, freshly
ground from pellets) was added and the mixture was refluxed
under Ar until the reaction had gone to completion (ca.
40 min – tlc monitoring). The solvent was removed in vacuo
and the residue was dissolved in DCM (15 cm3), and chromato-
graphed on a silica column (eluent DCM–diethyl ether, 98:2
v/v). The product was recystallised from ethanol–H2O, yielding
13 as golden needles (1.54 g, 72%), mp 165–166 °C. dH (CDCl3):
1.36 (s, 9H), 3.51 (s, 1H), 7.31 (d, 1H, J = 3.5 Hz), 7.53 (d, 2H,
J = 8.0 Hz), 7.67 (d, 1H, J = 3.5 Hz), 8.01 (d, 2H, J = 9.0 Hz).
Experimental
General details are the same as those reported previously.2b
Cross-coupling reactions: general procedure
A mixture of the alkyne (1.0 mmol), the halo compound
(1.0 mmol), CuI powder (20 mg) and Pd[PPh3]2Cl2 (70 mg) in
THF (15 cm3) was stirred at 20 °C for 5 min to afford a yellow
solution (CuI remained undissolved). Triethylamine (5 cm3)
3 3 6 6
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 3 6 3 – 3 3 6 7

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Table 2 Crystal data
Compound
2d
2g
5
14d
Formula
Formula weight
T/K
C24H20N4O
380.44
120
C24H20N2OS
384.48
120
C26H20N2OS
408.50
120
C22H18N4OS·CHCl3
505.83
120
Symmetry
Space group
a/Å
b/Å
c/Å
a/°
b/°
monoclinic
P21/a (# 14)
22.881(3)
6.314(1)
28.860(4)
90
109.86(1)
90
3921.4(10)
8
triclinic
P1(# 2)
6.1665(5)
6.9748(5)
22.943(3)
83.55(1)
84.09(1)
83.67(1)
970.45(16)
2
triclinic
P1(# 2)
6.345(2)
6.985(2)
23.672(8)
94.24(1)
91.96(1)
91.93(1)
1045.0(6)
2
monoclinic
P21/c (# 14)
6.1798(4)
26.851(8)
13.650(1)
90
92.26(1)
90
2263.2(7)
4
c/°
V/Å3
Z
l/mm−1
0.08
0.18
0.18
0.52
Reflns. collected
Unique reflns.
Rint
44484
9004
0.073
5530
0.050
0.154
18140
5663
0.021
5010
0.037
0.108
13306
4788
0.040
3407
0.045
0.137
26697
5191
0.076
3576
0.035
0.081
Reflns. F2 > 2r(F2)
R[F2 > 2r(F2)]
wR(F2), all data
L.-O. Palsson, M. R. Bryce, A. P. Monkman and I. D. W. Samuel,
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p. 521; (b) K. Sonogashira, J. Organomet. Chem., 2002, 653, 46.
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dC (CDCl3): 31.08, 35.10, 75.85, 84.30, 120.59, 126.09, 126.28,
126.41, 126.83, 129.05, 133.71, 155.60, 159.76, 164.35. MS (EI)
m/z (%): 308 (M+, 100). Anal. for C18H16N2OS: calcd. C, 70.10;
H, 5.23; N, 9.08. Found C, 69.93; H, 5.22; N, 8.93%.
X-Ray crystallography
Single-crystal diffraction experiments (Table 2) were carried out
on a Bruker 3-circle diffractometer with a SMART 6000 CCD
area detector, using graphite-monochromated Mo–Ka radiation
(k = 0.71073 Å) and Cryostream (Oxford Cryosystems) open-
flow N2 cryostats. The structures were solved by direct methods
and refined by full-matrix least squares against F2 of all data,
using SHELXTL software.23 Non-H atoms were refined with
anisotropic displacement parameters; H atoms in 2g and 14d
were refined in anisotropic approximation; in 2d and 5 methyl
groups were treated as rigid bodies; other H atoms in ‘riding’
model. Structure 5 shows an insignificant (5%) contribution
of another conformer, differing by a 180° rotation of the thio-
phene ring around the C(6)–C(7) bond. Full crystallographic
data, excluding structure factors, have been deposited at the
Cambridge Crystallographic Data Centre, CCDC nos. 239476–
239479 for 2d, 2g, 5 and 14d, respectively.
12 D. E. Ames, D. Bull and C. Takunda, Synthesis, 1981, 364.
13 A. Gorgues, Ann. Chim., 1972, 7, 373.
14 G. M. Coppola, Synthesis, 1984, 1021.
15 The parent 2-phenyl-5-(2-thienyl)-1,3,4-oxadiazole system is known:
F. N. Hayes, B. S. Rogers and D. G. Ott, J. Am. Chem. Soc., 1955,
77, 1850. During the course of our work, 2-furyl-5-phenyl-1,3,4-
oxadiazole derivatives where synthesised (ref. 6) using different
methodology from that in Scheme 3.
Acknowledgements
We thank the EPSRC for funding this work and Karen S.
Findlay for assistance with the PLQY measurements.
16 M. D’Auria and G. Mauriello, Tetrahedron Lett., 1995, 36, 4883.
17 (a) A. Hetzheim and K. Möchel, Adv. Heterocycl. Chem., 1966, 7,
183; (b) J. Hill, Comprehensive Heterocyclic Chemistry, ed. A. R.
Katritzky and C. W. Rees, Pergamon Press, Oxford, 1984, vol. 6,
p. 427.
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O r g . B i o m o l . C h e m . , 2 0 0 4 , 2 , 3 3 6 3 – 3 3 6 7
3 3 6 7