Synthesis of New Alkynyl-Bridged 2,5-Disubstituted 1,3,4-Oxadiazoles

Article abstract of DOI:10.1055/s-0035-1560387

A synthesis of new alkynyl-derived 2,5-disubstituted 1,3,4-oxadiazoles through palladium/copper-catalyzed Sonogashira cross-coupling between oxadiazole-substituted phenyl bromides and various arylacetylenes is described. Investigation of the absorption and emission spectra of the target compounds indicates emission profiles in the near-blue and blue region and high luminescence intensities. The presented approach is very convenient for the synthesis of luminescent small-molecules or precursors of other complex derivatives that are useful in the preparation of OLEDs as electron-transporting components.

Full text of DOI:10.1055/s-0035-1560387

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2016, 48, 606–614
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Synthesis
Synthesis of New Alkynyl-Bridged 2,5-Disubstituted 1,3,4-Oxadi-
azoles
Anca Paun1
Codruta C. Paraschivescu1
Mihaela Matache*
Vasile I. Parvulescu*
University of Bucharest, Faculty of Chemistry,
Department of Organic Chemistry, Biochemistry,
and Catalysis, 90–92 Panduri Street, 050663
Bucharest, Romania
vasile.parvulescu@chimie.unibuc.ro
mihaela.matache@g.unibuc.ro
Received: 04.09.2015
Accepted after revision: 11.11.2015
Published online: 04.01.2016
to generate compounds 3 that exhibit emissions in the UV
region with very high fluorescence quantum yields (0.91
for 3a and 0.85 for 3b measured using quinine as reference).
Blue electrophosphorescent devices with high efficiencies
also resulted when used as host materials.10
DOI: 10.1055/s-0035-1560387; Art ID: ss-2015-t0519-op
Abstract A synthesis of new alkynyl-derived 2,5-disubstituted 1,3,4-
oxadiazoles through palladium/copper-catalyzed Sonogashira cross-
coupling between oxadiazole-substituted phenyl bromides and various
arylacetylenes is described. Investigation of the absorption and emis-
sion spectra of the target compounds indicates emission profiles in the
near-blue and blue region and high luminescence intensities. The pre-
sented approach is very convenient for the synthesis of luminescent
small-molecules or precursors of other complex derivatives that are
useful in the preparation of OLEDs as electron-transporting compo-
nents.
N
N
O
1
N
N
O
2a: R = H
2b: R = OMe
Key words 2,5-disubstituted 1,3,4-oxadiazoles, palladium-catalyzed
Sonogashira reaction, cross-coupling, ethynyl-bridged compounds,
electron-deficient heterocycles
R
N
O
N
The electron-transporting behavior of the 1,3,4-oxadi-
azole core has justified its utility in the manufacture of or-
ganic light-emitting diodes (OLEDs).2 In particular, the
chemistry of the 2,5-disubstituted 1,3,4-oxadiazoles3 has
evolved during the past two decades in an effort to synthe-
size and characterize new organic small molecules that ex-
hibit enhanced optoelectronic properties.
3a: R = H
3b: R = OMe
R
Scheme 1 Relevant examples of terminal or internal ethynyl-bridged
2,5-disubstituted 1,3,4-oxadiazoles (compounds 1 and 2) with im-
proved properties or precursors for compounds 3
On this ground, a number of interesting ethynyl-bridged
2,5-disubstituted 1,3,4-oxadiazoles were studied for their
luminescence,4 as fluorescent liquid crystals,5 selective sen-
sors for nitro compounds,6 or ligands for coordination poly-
mers with one-, two-, or three-dimensional structures.7 In-
fluence of the extended conjugation through the alkynyl
chain was also investigated for various oligoyne-bridged
2,5-diphenyl-1,3,4-oxadiazole molecules.8
In addition, alkyne-substituted 1,3,4-oxadiazoles like 1
(Scheme 1) found use as building block for the synthesis of
further π-extended functional molecules.9 Notably, internal
alkynes 2 were used in a Diels–Alder cycloaddition reaction
Hence, the design of new ethynyl-bridged 1,3,4-oxadi-
azoles bearing various combinations of electron-donating
or -withdrawing functional groups, both on the alkyne and
the oxadiazole core, is still challenging in order to obtain
molecules with improved optoelectronic properties.
The Sonogashira reaction11 has evolved as a very power-
ful tool for the Csp2–Csp bond formation and, consequently,
provides a suitable method for the synthesis of compounds
of type 14,9 and 2.10 Pd and Cu(I) are widely reported to cat-
alyze in tandem cross-coupling alkynylations of aryl io-
dides under very mild conditions (i.e., at room temperature
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and for short reaction times). Differently, the bromide ana-
logues display a lower reactivity.11b,c,12 This behavior is
mainly attributed to a lower rate of the oxidative addition
step, which may be enhanced by use of higher tempera-
tures or presence of electron-withdrawing groups that
could provide a more electrophilic character to the reacting
molecule. However, the bromides show significant advan-
tages over the iodides justified by their readily availability
and higher stability.
The oxadiazole heterocyclic ring is usually obtained14
through dehydrative cyclization of N,N′-diacylhydrazines,14b
oxidative cyclization of N′-acylhydrazones,14c or Huisgen re-
action of tetrazoles and acid chlorides.14d Among the nu-
merous methods described,14 our attention was turned to
the oxidative cyclization using hypervalent iodine reagents
due to the readily availability of the N′-acylhydrazones
from aldehydes and hydrazides, as well as the mild condi-
tions required for the oxadiazole ring closure.15
Based on this state-of-the art, the aim of the present
study was to design and prepare an extended series of
alkynyl-bridged 2,5-disubstituted 1,3,4-oxadiazoles 2 that
display enhanced photophysical properties and constitute
proper candidates as OLEDs precursors. The synthetic ap-
proach uses a simple and convenient procedure, having as
key intermediates 5-aryl-2-(4-bromophenyl)-1,3,4-oxadi-
azoles 4 (Scheme 2). Compounds 4, once synthesized, are
used as electrophiles in Sonogashira cross-couplings with
various alkynes, others than phenylacetylene.10
Viewed from the perspective of an oxadiazole-substi-
tuted aryl halide and considering a presumptive behavior of
the heterocycle core as an electron-deficient group, the 2,5-
disubstituted 1,3,4-oxadiazole bromides 4 would qualify as
excellent electrophilic partners in Sonogashira reaction for
the synthesis of the ethynyl-bridged 1,3,4-oxadiazoles.
Thus, our substrates 4 were synthesized first. We chose to
prepare the 5-aryl-2-(4-bromophenyl)-1,3,4-oxadiazoles 4
(Scheme 2) bearing a variety of unsubstituted or para-sub-
stituted aryls with electron-donating or -withdrawing
groups.
The 2,5-disubstituted 1,3,4-oxadiazole substrates 4a–f
were synthesized following a two-step procedure (Scheme
2). Condensation of 4-bromobenzaldehyde (5a), naphthyl
carbaldehydes 5b,c, and anthracene-9-carbaldehyde (5d)
with one of the corresponding hydrazides 6a–d yielded un-
der acidic catalysis (trifluoroacetic acid) the N′-acylhydra-
zones 7a–f in good to excellent yields (83–97%). Further
treatment of these compounds with bis(trifluoroacet-
oxy)iodobenze (PIFA) at room temperature in dichloro-
methane led to the oxadiazoles 4a–f in good to very good
yields (57–72%). All compounds were purified and charac-
terized by spectral analysis. The obtained data correspond-
ed with previously described reports for known com-
pounds.13c,15a,16
With the substrates 4a–f in hand, the cross-coupling ex-
periments, modifying previously described procedures for
Sonogashira couplings of aryl bromides, were conducted.17
Pd(dppf)Cl2 (5 mol%) was chosen as the palladium source,
considering the general advantages of a bulky diphosphine
ligand for aryl bromides cross-couplings,18 mainly to better
stabilize Pd(0) catalytic species over monodentate ligands.
This is highly desirable when inactivation of the catalyst
competes with the oxidative addition step of the cross-cou-
pling reaction.19 Furthermore, CuI (10 mol%) was used as
the co-catalyst, tetrahydrofuran as the polar aprotic sol-
vent, and triethylamine as the base. The reaction time was
kept at 12 hours for all assays, under an inert atmosphere,
using 1.5 equivalents of phenylacetylene.
Functional groups, such as methoxy or cyano, can be
easily grafted on the phenyl rings of the oxadiazole core, in
order to obtain compounds bearing opposite electronic ef-
fects. Moreover, the naphthyl and anthryl moieties were se-
lected as a result of the growing interest to synthesize com-
pounds with good electron transport properties and utility
in fabrication of OLEDs or polymers.13
H
N
N
H
Ar2
Ar2
N
N
PIFA, CH2Cl2
r.t., 12 h
TFA, CHCl3
reflux, 4 h
Ar1
Ar1
H2N
Ar2
N
Ar1
O
O
O
O
5a Ar1 = 4-BrC6H4
5b Ar1 = 1-naphthyl
5c Ar1 = 2-naphthyl
5d Ar1 = 9-anthryl
6a Ar2 = Ph
7a–f (83–97%)
4a–f
6b Ar2 = 4-MeOC6H4
6c Ar2 = 4-NCC6H4
6d Ar2 = 4-BrC6H4
N
N
N
N
N
N
O
O
O
CN
OMe
Br
Br
Br
Br
Br
Br
4a (61%)
4b (72%)
4c (57%)
N
N
N
N
N
N
O
O
O
4d (62%)
4e (70%)
4f (60%)
Scheme 2 Synthetic scheme for the 2,5-disubstituted 1,3,4-oxadiazole substrates 4a–f
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Performing the coupling reactions of substrates 4a–c at
room temperature (Scheme 3) furnished no target coupling
products. Increasing the temperature to solvent reflux gave
the products 2a–c in 70%, 83%, and 75% yield, respectively
(Scheme 3). One can note that the substituent in para-posi-
tion of the phenyl moiety has no significant influence on
the coupling efficiency. In addition, we remark that the
coupling reactions require higher temperature in order to
occur, suggesting that the electron-deficient oxadiazole
core does not sufficiently activate the bromides to undergo
couplings at room temperatures.11b,c,12 Very good to excel-
lent yields of products 2d–f (75–91%) (Scheme 3) were also
obtained in the case of the substrates bearing the naphthyl
and anthryl moieties.
The interest in more simple experimental protocols of
the Sonogashira cross-coupling, concomitant with preser-
vation of high product yields, has led to reports showing
experiments conducted under aerobic conditions and, con-
sequently, copper-free procedures,11c in order to reduce for-
mation of the homocoupled diyne side-product.20
We were tempted to see whether in our case, with our
particular substrates and using Pd(dppf)Cl2 the coupling re-
actions with phenylacetylene under aerobic conditions
would efficiently occur. The copper-free reaction of the me-
thoxy-substituted compound 4b was first performed using
1.5 equivalents of the alkyne in DMF at room temperature
(Table 1, entry 1), which gave no coupling product. Howev-
er, diphenylbutadiyne21 was isolated in 79% yield (see Sup-
porting Information). When the temperature was raised to
63 °C and an alkyne excess was used (3 equiv, entry 2), the
coupling product could be detected in 26% yield (as inferred
from the 1H NMR spectrum) as an unseparable mixture
with the starting material, along with the diyne side-
product. Adding copper iodide to the reaction mixture and
changing the solvent to THF (entry 3) yielded the coupling
product in 87%.
Further, when the unsubstituted compound 4a (Table 1,
entries 4 and 5) was treated in the same conditions using
only 1.5 equivalents of the alkyne the product was obtained
in 36% yield, while use of 3 equivalents of the alkyne led to
60% yield. In the case of the cyano-substituted substrate 4c
(entries 6 and 7), the target product 2c was isolated in only
47% yield using 3 equivalents of phenylacetylene, at reflux-
ing temperature of DMF.
We also performed the coupling reactions under aero-
bic conditions of the substrates 4d–f with phenylacetylene
and obtained moderate yields (Table 1, entries 8–11), irre-
spective of the solvent used in some cases.
Once the optimum reaction conditions were found, the
synthesis of a diverse-substituted series of new alkynyl-
bridged 1,3,4-oxadiazoles was successfully achieved under
inert conditions as shown in Scheme 4. One can notice that
the coupling reactions occurred in very good to excellent
yields for all our substrates and the alkynes used. The bro-
mo derivative 4b was used to synthesize compounds 2g–i
through coupling with 4-ethynyltoluene, 2-ethynylanisole,
and 4-ethynylbenzonitrile, respectively (60–95%) (Scheme
4). Moreover, alkynes bearing both electron-donating and
-withdrawing substituents such as the ones mentioned
above as well as 4-ethynylfluorobenzene were coupled
with the phenyl bromides bearing naphthyl and anthryl
substituted oxadiazoles, providing compounds 2j–m in
very good yields (64–93%) (Scheme 4).
Preliminary studies of the absorption and emission
properties of compounds 2a–m (see Schemes 3 and 4 for
the excitation and emission wavelengths, as well as the
Supporting Information for the excitation and emission
spectra of the compounds) indicate various profiles of the
optoelectronic features. For example, Figure 1 shows the
spectra profile of some selected compounds 2c,f,i,k,l having
a different substitution pattern. In general, compounds 2
have similar absorption profiles, with the excitation maxi-
N
N
N
N
Ph (1.5 equiv)
Br
O
Pd(dppf)Cl2 (5 mol%)
CuI (10 mol%), Et3N
THF, 12 h, Ar
O
Ar
Ar
4a–f
2a–f
N
O
N
N
N
O
N
N
O
2a: 0% (25 °C), 70% (63 °C)
ex = 320 nm, λem = 374 nm
Stokes shift = 54 nm, 4.51 x 10–3 cm–1
2b: 0% (25 °C), 83% (63 °C)
2c: 0% (25 °C), 75% (63 °C)
λ
λ
ex = 325 nm, λem = 383 nm
λex = 328 nm, λem = 388 nm
MeO
NC
Stokes shift = 58 nm, 4.66 x 10–3 cm–1
Stokes shift = 60 nm, 4.71 x 10–3 cm–1
N
N
N
N
N
N
O
O
O
2d: 91% (63 °C)
2e: 80% (63 °C)
2f: 75% (63 °C)
λ
ex = 336 nm, λem = 392 nm
λ
ex = 328 nm, λem = 392 nm
λ
ex = 258, 313 nm, λem = 360 nm
Stokes shift = 102, 47 nm,
Stokes shift = 56 nm, 4.25 x 10–3 cm–1
Stokes shift = 64 nm, 4.98 x 10–3 cm–1
4.17 x 10–3 , 10.98 x 10–3 cm–1
Scheme 3 Cross-coupling reaction of substrates 4a–f with phenylacetylene, structure of products 2a–f (yields of isolated products after column chro-
matography, absorption and emission experiments were performed in CHCl3: 2a: 5 × 10–7 M, 2b,d,e: 1 × 10–8 M, 2c,f: 1 × 10–7 M)
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Table 1 Coupling Reactions between Substrates 4a–f and Phenylacetylene under Aerobic Conditions
N
O
N
O
N
N
Ph
Br
Pd(dppf)Cl2 (5 mol%)
Et3N, solvent, 12 h, air
Ar
Ar
4a–f
2a–f
Entry
4
Solvent
Co-catalyst (mol%)
Temp (°C)
Alkyne (equiv)
Product
Yield (%)a
b
1
2
4b
4b
4b
4a
4a
4c
4c
4d
4e
4e
4f
DMF
DMF
THF
THF
THF
THF
DMF
THF
THF
DMF
THF
–
25
63
1.5
3
2b
2b
2b
2a
2a
2c
2c
2d
2e
2e
2f
–
–
26c
87
36
60
–
3
CuI (10)
CuI (10)
CuI (10)
CuI (10)
CuI (10)
CuI (10)
CuI (10
CuI (10)
CuI (10)
63
3
4
63
1.5
3
5
63
6
63
3
7
130
63
3
47
55
42
51
35
8
3
9
63
3
10
11
130
63
3
3
a Yields of isolated products after purification by column chromatography.
b Yield of the isolated diyne was 79%, no coupling product was detected.
c Yield calculated from the 1H NMR spectrum performed on unseparable mixture of the starting material and the coupling product.
ma between 320–340 nm, except the compounds bearing
the anthryl moiety, for which the most intense maximum
varies between 260–280 nm. The emission spectra indicate
high luminescence intensities at low concentrations, in the
nanomolar range, with emission maxima near the blue re-
gion, varying between 360 and 396 nm. One can note the
different behavior of the anthryl derivatives 2l and 2m,
which emit in the late blue-near green region and have
λem, max = 478 nm, unlike the unsubstituted compound 2f for
which λem, max = 360 nm. The Stokes shifts are moderate for
all compounds except 2l and 2m, which have very large val-
ues (218 nm for 2l and 198 nm for 2m).
R
N
N
N
O
R
N
Br
O
Ar
Ar
Pd(dppf)Cl2 (5 mol%)
CuI (10 mol%), Et3N
solvent, reflux, 12 h, Ar
4b,d–f
2g–m
N
O
N
N
N
O
N
N
CN
O
2g (95%)
2h (85%)
ex = 337 nm, λem = 392 nm
Stokes shift = 55 nm, 4.16 x 10–3 cm–1
2i (60%)
ex = 331 nm, λem = 396 nm
Stokes shift = 65 nm, 4.96 x 10–3 cm–1
MeO
λ
ex = 328 nm, λem = 385 nm
λ
λ
MeO
MeO
MeO
Stokes shift = 57 nm, 4.51 x 10–3 cm–1
N
N
N
O
N
F
O
2j (87%)
ex = 340 nm, λem = 392 nm
Stokes shift = 52 nm, 3.90 x 10–3 cm–1
2k (78%, DMF)
ex = 327 nm, λem = 383 nm
Stokes shift = 56 nm, 4.47 x 10–3 cm–1
MeO
λ
λ
N
N
N
N
F
O
O
2l (93%)
2m (64%)
λ
ex = 260 nm, λem = 478 nm
λ
ex = 280 nm, λem = 478 nm
Stokes shift = 218 nm, 17.54 x 10–3 cm–1
Stokes shift = 198 nm, 14.79 x 10–3 cm–1
Scheme 4 Synthesis of diverse-substituted ethynyl-bridged 1,3,4-oxadiazoles 2g–m. Yields of isolated products after purification by column chroma-
tography. Reactions were performed in refluxing THF as solvent, unless otherwise stated. Absorption and emission experiments were performed in
CHCl3: 2g–i,k,l: 1 × 10–7 M, 2j: 1 × 10–8 M, 2m: 5 × 10–7 M.
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pounds may also be used as building blocks in the synthesis
of new small molecules, more complex from a structural
point of view, in the attempt to generate enhanced opto-
electronic properties.
The solvents and reagents were purchased from commercial suppliers
and used without further purification. Anhydrous THF was distilled
from Na and benzophenone. Petroleum ether (PE) used refers to the
hydrocarbon mixture with a boiling range of 40–60 °C. Reactions con-
ducted under argon were performed in oven-dried Schlenk tubes us-
ing anhydrous solvent. The NMR spectra were recorded on Bruker
spectrometers operating at 300 MHz, 500, and 600 MHz for 1H and 75
MHz, 125 and 150 MHz, respectively, for 13C. Chemical shifts (δ) are
reported in parts per million (ppm) using residual solvent peak as in-
ternal reference. High-resolution mass spectra were recorded on a
ThermoScientific (LTQ XL Orbitrap) spectrometer using APCI tech-
nique and Orbital Ion Trap mass analyzer. TLC was performed on sili-
ca gel 60 coated aluminum F254 plates and preparative column chro-
matography was performed on Merck silica gel 60 (0.040–0.063 mm).
All plates were visualized by UV irradiation at 254 nm. Melting points
were determined in open capillary tubes using a Stuart SMP3 electric
melting point apparatus and are uncorrected. Hydrazides 6 were ei-
ther commercially available or synthesized from the corresponding
carboxylic acids, through esterification and further reaction with hy-
drazine hydrate.
Figure 1 Excitation (dotted lines) and emission (plain lines) spectra of
selected compounds 2 (performed in CHCl3 at 1 × 10–7 M)
Thus, a wide spectral range can be covered and the op-
toelectronic properties of the compounds can, therefore be
tuned, through selection of proper substituent both on the
oxadiazole and the alkyne cores. Another interesting obser-
vation would be that the moderate Stokes shifts are, howev-
er, accompanied by high luminescence intensities. Deeper
investigations of the luminescence behavior (such as the
quantum yields, etc.) may reveal some other interesting
features and these are currently under way.
N′-Acylhydrazones 7a–f; General Procedure
In conclusion, we have described synthesis of new
ethynyl-bridged 2,5-diaryl-1,3,4-oxadiazoles using a simple
three-step protocol, involving the synthesis of N′-acylhy-
drazones, their oxidative cyclization to form the oxadiazole
ring and, finally, the alkynylation reaction of the resulting
oxadiazole-aryl bromides. The target compounds were de-
signed and synthesized to contain various electron-donat-
ing or -withdrawing functional groups. The alkynylation
approach followed the Sonogashira coupling procedure be-
tween bromophenyl-1,3,4-oxadiazole derivatives and vari-
ous substituted arylacetylenes, bearing activating or deacti-
vating functional groups. The coupling reactions occurred
in good to excellent yields, indicating a convenient strategy
to synthesize new decorated 2,5-disubstituted 1,3,4-oxadi-
azoles as potential OLEDs precursors. The heterocyclic ring
behaves as a moderate electron-deficient group of aryl bro-
mides as suggested by the low reaction performance at
room temperature. The coupling reactions of the 1,3,4-oxa-
diazole substrates with phenylacetylene also occur in aero-
bic conditions in moderate to very good yields, using an
alkyne excess, along with the diyne side-product in consid-
erable amounts. Preliminary studies indicate that the target
internal oxadiazole alkynes are compounds that bear lumi-
nescent properties in the near-blue or blue region and have
high luminescence intensities. A systematic and more de-
tailed study regarding the influence of the structural par-
ticularities over the displayed luminescent properties may
reveal other interesting features. In addition, such com-
The corresponding aldehyde 5 (5 mmol) and the corresponding hy-
drazide 6 (5 mmol) were dissolved in CHCl3 (50 mL). To this solution
was added trifluoroacetic acid (2 drops) and the mixture was stirred
at reflux for 4 h and then at r.t. overnight. Evaporation of almost half
of the solvent volume led to massive precipitation of the product,
which was filtered, thoroughly washed with Et2O, and dried to yield
pure product 7.
N′-(4-Bromobenzylidene)benzohydrazide (7a)
Yield: 1.25 g (83%); white solid; mp 189–190 °C (Lit.16e mp 190–
191 °C); Rf = 0.38 (EtOAc–PE, 1:2).
1H NMR (500 MHz, CDCl3): δ = 11.91 (s, 1 H, NH), 8.46 (s, 1 H, CH=N),
7.87 (d, 3J = 8.4 Hz, 2 H, HAr), 7.77–7.73 (m, 4 H, HAr), 7.48–7.45 (m, 3
H, HAr).
13C NMR (125 MHz, CDCl3): δ = 162.2, 148.1, 134.2, 132.5, 131.5,
130.2, 129.7, 128.9, 127.1, 125.5.
N′-(4-Bromobenzylidene)-4-methoxybenzohydrazide (7b)
Yield: 1.55 g (93%); white solid; mp 207–209 °C; Rf = 0.31 (EtOAc–
PE, 1:2).
1H NMR (600 MHz, DMSO-d6): δ = 11.79 (s, 1 H, NH), 8.38 (s, 1 H, CH),
3
3
7.85 (d, J = 8.46 Hz, 2 H, HAr), 7.75 (d, J = 8.46 Hz, 2 H, HAr), 7.68 (d,
3J = 8.64 Hz, 2 H, HAr), 7.03 (d, 3J = 8.64 Hz, 2 H, HAr), 3.81 (s, 3 H,
OCH3).
13C NMR (150 MHz, DMSO-d6): δ = 166.3, 164.9, 148.0, 131.8, 131.5,
129.7, 128.8, 126.8, 126.6, 114.4, 55.3.
N′-(4-Bromobenzylidene)-4-cyanobenzohydrazide (7c)
Yield: 1.59 g (97%); white solid; mp 255–257 °C; Rf = 0.37 (EtOAc–
PE, 1:2).
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1H NMR (500 MHz, DMSO-d6): δ = 12.13 (s, 1 H, NH), 8.43 (s, 1 H,
CH=N), 8.06 (d, 3J = 8.5 Hz, 2 H, HAr), 8.03 (d, 3J = 8.5 Hz, 2 H, HAr), 7.70
(d, 3J = 8.5 Hz, 2 H, HAr), 7.67 (d, 3J = 8.5 Hz, 2 H, HAr).
2-(4-Bromophenyl)-5-(4-methoxyphenyl)-1,3,4-oxadiazole (4b)
Yield: 710 mg (72%); white solid; mp 158–160 °C (Lit.15a mp 154–
155 °C); Rf = 0.5 (EtOAc–PE, 1:4).
13C NMR (125 MHz, DMSO-d6): δ = 161.8, 147.5, 137.4, 133.4, 132.6,
131.9, 129.1, 128.5, 123.6, 118.3, 114.1.
4-[5-(4-Bromophenyl)-1,3,4-oxadiazol-2-yl]benzonitrile (4c)
HRMS (APCI+, Orbitrap): m/z [M
+
H]+ calcd for C15H11BrN3O:
Yield: 560 mg (57%); white solid; mp 227–229 °C (Lit.16d mp 220.3–
328.0080, 360.0060; found: 328.0107, 330.0087.
221.8); Rf = 0.32 (EtOAc–PE, 1:4).
HRMS (APCI+, Orbitrap): m/z [M
325.9924, 327.9903; found: 325.9932, 327.9904.
+
H]+ calcd for C15H9BrN3O:
4-Bromo-N′-(naphthalen-1-ylmethylene)benzohydrazide (7d)
Yield: 1.61 g (91%); white solid; mp 227–229 °C; Rf = 0.47 (EtOAc–
PE, 1:2).
2-(4-Bromophenyl)-5-(naphthalen-1-yl)-1,3,4-oxadiazole (4d)
1H NMR (300 MHz, DMSO-d6): δ = 12.01 (s, 1 H, NH), 9.10 (s, 1 H,
CH=N), 8.87 (d, 3J = 8.4 Hz, 1 H, HAr), 8.05–8.02 (m, 2 H, HAr), 7.95–7.92
(m, 3 H, HAr), 7.78 (d, 3J = 8.4 Hz, 2 H, HAr), 7.71–7.59 (m, 3 H, HAr).
Yield: 650 mg (62%); white solid; mp 148–149 °C (Lit.13c mp 139 °C);
Rf = 0.5 (EtOAc–PE, 1:4).
HRMS (APCI+, Orbitrap): m/z [M
+
H]+ calcd for C18H12BrN2O:
13C NMR (75 MHz, DMSO-d6): δ = 162.1, 148.1, 133.6, 132.5, 131.6,
351.0128, 353.0107; found: 351.0122, 353.0095.
130.7, 130.2, 129.8, 129.5, 128.9, 127.9, 127.4, 126.4, 125.6, 124.3.
+
H]+ calcd for C18H14BrN2O:
2-(4-Bromophenyl)-5-(naphthalen-2-yl)-1,3,4-oxadiazole (4e)
HRMS (APCI+, Orbitrap): m/z [M
353.0284, 355.0264; found: 353.0297, 355.0288.
Yield: 740 mg (70%); white solid; mp 169–170 °C; Rf = 0.55 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.60 (s, 1 H, HAr), 8.18 (d, 3J = 8.6 Hz, 1
H, HAr), 8.04 (d, 3J = 8.4 Hz, 2 H, HAr), 7.97 (d, 3J = 8.2 Hz, 2 H, HAr), 7.89
(d, 3J = 8.6 Hz, 1 H, HAr), 7.69 (d, 3J = 8.4 Hz, 2 H, HAr), 7.61–7.56 (m, 2
H, HAr).
13C NMR (125 MHz, CDCl3): δ = 165.1, 164.1, 134.9, 132.9, 132.6,
129.3, 128.9, 128.5, 128.2, 128.1, 127.5, 127.3, 126.6, 123.3, 122.9,
121.1.
4-Bromo-N′-(naphthalen-2-ylmethylene)benzohydrazide (7e)
Yield: 1.67 g (95%); white solid; mp 265–267 °C; Rf = 0.44 (EtOAc–
PE, 1:2).
1H NMR (300 MHz, DMSO-d6): δ = 12.02 (s, 1 H, NH), 8.60 (s, 1 H,
CH=N), 8.17 (s, 1 H, HAr), 8.03–7.93 (m, 4 H, HAr), 7.90 (d, 3J = 8.4 Hz, 2
H, HAr), 7.77 (d, 3J = 8.4 Hz, 2 H, HAr), 7.60–7.56 (m, 2 H, HAr).
13C NMR (75 MHz, DMSO-d6): δ = 162.2, 148.1, 133.8, 132.9, 132.5,
132.0, 131.6, 129.8, 128.9, 128.6, 128.4, 127.8, 127.2, 126.8, 125.6,
122.7.
HRMS (APCI+, Orbitrap): m/z [M
+
H]+ calcd for C18H12BrN2O:
351.0128, 353.0107; found: 351.0127, 353.0100.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C18H14BrN2O:
353.0284, 355.0264; found: 353.0313, 355.0294.
2-(Anthracen-9-yl)-5-(4-bromophenyl)-1,3,4-oxadiazole (4f)
Yield: 720 mg (60%); yellow solid; mp 257–259 °C (Lit.13c mp 261 °C);
Rf = 0.44 (EtOAc–PE, 1:4).
N′-(Anthracen-9-ylmethylene)-4-bromobenzohydrazide (7f)
Yield: 1.71 g (85%); yellow solid; mp 293–294 °C; Rf = 0.43 (EtOAc–
HRMS (APCI+, Orbitrap): m/z [M +
H]+ calcd for C22H14BrN2O:
PE, 1:2).
401.0284, 403.0264; found: 401.0278, 403.0255.
1H NMR (300 MHz, DMSO-d6): δ = 12.18 (s, 1 H, NH), 9.67 (s, 1 H,
3
Cross-Coupling Reaction Between Aryl Bromides Bearing the Sub-
stituted 1,3,4-Oxadiazole Moiety 4a–f and Arylacetylenes; General
Procedure
CH=N), 8.77–8.75 (m, 3 H, HAr), 8.17 (d, J = 8.3 Hz, 2 H, HAr), 7.98 (d,
3
3J = 8.3 Hz, 2 H, HAr), 7.83 (d, J = 8.3 Hz, 2 H, HAr), 7.69–7.57 (m, 4 H,
HAr).
13C NMR (75 MHz, DMSO-d6): δ = 162.1, 147.4, 132.5, 131.7, 130.9,
129.8, 129.7, 129.1, 127.3, 125.7, 125.6, 124.9, 124.9.
The corresponding aryl bromide 4a–f (0.125 mmol–0.166 mmol) was
dissolved in solvent (anhydrous THF or DMF for reactions performed
under an inert atmosphere, 1–3 mL). To the resulting solution the fol-
lowing were sequentially added: Pd(dppf)Cl2 (5 mol%), CuI (10 mol%),
Et3N (10 equiv), and the alkyne (1.5 equiv for reactions performed un-
der argon and 3 equiv for reactions performed under air, unless oth-
erwise stated). The reactions were left to stir for 12 h at solvent reflux
temperature, then the solvent was evaporated, and the residue was
purified by column chromatography on silica gel to afford the respec-
tive pure product 2.
HRMS (APCI+, Orbitrap): m/z [M
+
H]+ calcd for C22H16BrN2O:
403.0441, 405.0420; found: 403.0471, 405.0450.
2,5-Disubstituted 1,3,4-Oxadiazoles 4a–f; General Procedure
The corresponding N′-acylhydrazone 7 (3 mmol) and bis(trifluoro-
acetoxy)iodobenze (1.42 g, 3.3 mmol, 1.1 equiv) was dissolved in
CH2Cl2 (15 mL) and the resulting solution was stirred at r.t. overnight.
The solvent was removed under vacuum and the residue was purified
by column chromatography on silica gel to yield the respective pure
product 4.
2-Phenyl-5-[4-(phenylethynyl)phenyl]-1,3,4-oxadiazole (2a)10
Yield: 37.4 mg (70%); white solid; mp 194–196 °C; Rf = 0.59 (EtOAc–
PE, 1:4).
2-(4-Bromophenyl)-5-phenyl-1,3,4-oxadiazole (4a)
Yield: 550 mg (61%); white solid; mp 169–170 °C (Lit.16c mp 169–
170 °C); Rf = 0.62 (EtOAc–PE, 1:4).
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C22H15N2O: 323.1179;
found: 323.1170.
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612
A. Paun et al.
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Synthesis
2-(4-Methoxyphenyl)-5-[4-(phenylethynyl)phenyl]-1,3,4-oxadi-
azole (2b)10
13C NMR (75 MHz, CDCl3): δ = 165.6, 163.4, 132.5, 131.9, 131.7, 131.6,
131.2, 128.9, 128.6, 127.9, 127.2, 127.2, 125.9, 125.2, 123.4, 122.8,
117.2, 92.6, 88.7.
Yield: 44 mg (83%); white solid; mp 177–178 °C; Rf = 0.26 (EtOAc–
PE, 1:4).
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C30H19N2O: 423.1492;
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C23H17N2O2: 353.1285;
found: 423.1496.
found: 353.1285.
2-(4-Methoxyphenyl)-5-[4-(p-tolylethynyl)phenyl]-1,3,4-oxadi-
azole (2g)
4-{5-[4-(Phenylethynyl)phenyl]-1,3,4-oxadiazol-2-yl}benzonitrile
(2c)
Yield: 52.5 mg (95%); white solid; mp 181–183 °C; Rf = 0.25 (EtOAc–
PE, 1:4).
Yield: 40 mg (75%); white solid; mp 244–246 °C; Rf = 0.31 (EtOAc–
1H NMR (500 MHz, CDCl3): δ = 8.10 (d, 3J = 8.6 Hz, 2 H, HAr), 8.08 (d,
3J = 9.0 Hz, 2 H, HAr), 7.66 (d, 3J = 8.6 Hz, 2 H, HAr), 7.45 (d, 3J = 8.0 Hz, 2
H, HAr), 7.18 (d, 3J = 8.0 Hz, 2 H, HAr), 7.04 (d, 3J = 9.0 Hz, 2 H, HAr), 3.90
(s, 3 H, OCH3), 2.39 (s, 3 H, CH3).
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.27 (d, 3J = 8.6 Hz, 2 H, HAr), 8.14 (d,
3J = 8.6 Hz, 2 H, HAr), 7.85 (d, 3J = 8.6 Hz, 2 H, HAr), 7.70 (d, 3J = 8.6 Hz, 2
H, HAr), 7.58–7.56 (m, 2 H, HAr), 7.39–7.38 (m, 3 H, HAr).
13C NMR (125 MHz, CDCl3): δ = 164.8, 163.9, 162.6, 139.2, 132.2,
131.8, 129.4, 128.9, 126.9, 126.8, 123.4, 119.8, 116.5, 114.7, 92.7, 88.2,
55.6, 29.8.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C24H19N2O2: 367.1441;
found: 367.1437.
13C NMR (125 MHz, CDCl3): δ = 165.1, 163.3, 133.1, 132.5, 131.9,
129.1, 128.6, 127.8, 127.6, 127.5, 127.2, 122.8, 122.7, 118.0, 115.4,
93.0, 88.5.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C23H14N3O: 348.1131;
found: 348.1139.
2-(4-Methoxyphenyl)-5-{4-[(2-methoxyphenyl)ethynyl]phenyl}-
1,3,4-oxadiazole (2h)
2-(Naphthalen-1-yl)-5-[4-(phenylethynyl)phenyl]-1,3,4-oxadi-
azole (2d)
Yield: 49 mg (85%); white solid; mp 147–148 °C; Rf = 0.13 (EtOAc–
PE, 1:4).
Yield: 48.1 mg (91%); white solid; mp 121–123 °C; Rf = 0.35 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.10 (d, 3J = 8.8 Hz, 2 H, HAr), 8.08 (d,
3J = 9.0 Hz, 2 H, HAr), 7.70 (d, 3J = 8.8 Hz, 2 H, HAr), 7.52 (d, 3J = 8.5 Hz, 1
H, HAr), 7.35 (t, 3J = 8.5 Hz, 1 H, HAr), 7.04 (d, 3J = 9.0 Hz, 2 H, HAr), 6.97
(t, 3J = 8.5 Hz, 1 H, HAr), 6.93 (d, 3J = 8.5 Hz, 1 H, HAr), 3.94 (s, 3 H,
OCH3), 3.90 (s, 3 H, OCH3).
13C NMR (125 MHz, CDCl3): δ = 164.8, 163.9, 162.6, 160.2, 133.8,
132.4, 130.5, 128.9, 127.1, 126.8, 123.4, 120.7, 116.5, 114.7, 112.0,
110.9, 92.8, 88.9, 56.0, 55.6.
1H NMR (500 MHz, CDCl3): δ = 9.30 (d, 3J = 8.4 Hz, 1 H, HAr), 8.29 (d,
3J = 7.3 Hz, 1 H, HAr), 8.19 (d, 3J = 8.4 Hz, 2 H, HAr), 8.07 (d, 3J = 8.4 Hz, 1
H, HAr), 7.96 (d, 3J = 8.4 Hz, 1 H, HAr), 7.74–7.71 (m, 3 H, HAr), 7.64–7.61
(m, 2 H, HAr), 7.60–7.57 (m, 2 H, HAr), 7.40–7.37 (m, 3 H, HAr).
13C NMR (125 MHz, CDCl3): δ = 164.9, 163.9, 134.0, 132.9, 132.4,
131.9, 130.3, 128.9, 128.9, 128.6, 128.4, 127.1, 126.9, 126.4, 125.0,
123.4, 122.8, 120.6, 92.6, 88.7.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C26H17N2O: 373.1335;
found: 373.1339.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C24H19N2O3: 383.1390;
found: 383.1398.
2-(Naphthalen-2-yl)-5-[4-(phenylethynyl)phenyl]-1,3,4-oxadi-
azole (2e)
4-({4-[5-(4-Methoxyphenyl)-1,3,4-oxadiazol-2-yl]phe-
nyl}ethynyl)benzonitrile (2i)
Yield: 42.4 mg (80%); white solid; mp 162–164 °C; Rf = 0.38 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.65 (s, 1 H, HAr), 8.23 (dd, 3J = 8.6 Hz,
4J = 1.7 Hz, 1 H, HAr), 8.19 (d, 3J = 8.6 Hz, 2 H, HAr), 8.00 (d, 3J = 8.0 Hz, 2
H, HAr), 7.92 (d, 3J = 8.6 Hz, 1 H, HAr), 7.71 (d, 3J = 8.6 Hz, 2 H, HAr),
7.63–7.57 (m, 4 H, HAr), 7.40–7.38 (m, 3 H, HAr).
Yield: 16 mg (60%); white solid; mp 233–235 °C; Rf = 0.21 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.13 (d, 3J = 8.4 Hz, 2 H, HAr), 8.08 (d,
3
3J = 8.9 Hz, 2 H, HAr), 7.68 (d, J = 8.4 Hz, 2 H, HAr), 7.67–7.62 (m, 4 H,
HAr), 7.04 (d, 3J = 8.9 Hz, 2 H, HAr), 3.90 (s, 3 H, OCH3).
13C NMR (125 MHz, CDCl3): δ = 164.9, 163.7, 162.7, 133.2, 132.5,
132.3, 132.2, 128.9, 127.8, 126.9, 125.5, 124.4, 118.5, 116.3, 114.7,
92.9, 90.4, 55.6.
HRMS (ESI+, Orbitrap): m/z [M + H]+ calcd for C24H16N3O2: 378.1237;
found: 378.1243.
13C NMR (125 MHz, CDCl3): δ = 165.1, 164.5, 134.9, 133.0, 132.4,
131.9, 129.3, 129.0, 128.9, 128.6, 128.2, 128.2, 127.6, 127.3, 127.0,
123.4, 123.4, 122.8, 121.2, 92.6, 88.7.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C26H17N2O: 373.1335;
found: 373.1337.
2-{4-[(2-Methoxyphenyl)ethynyl]phenyl}-5-(naphthalen-1-yl)-
1,3,4-oxadiazole (2j)
2-(Anthracen-9-yl)-5-[4-(phenylethynyl)phenyl]-1,3,4-oxadiazole
(2f)
Yield: 49.6 mg (87%); white solid; mp 181–183 °C; Rf = 0.47 (EtOAc–
PE, 1:4).
Yield: 39.7 mg (75%); yellow solid; mp 193–195 °C; Rf = 0.52 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 9.30 (d, 3J = 8.4 Hz, 1 H, HAr), 8.29 (d,
3J = 7.6 Hz, 1 H, HAr), 8.18 (d, 3J = 8.4 Hz, 2 H, HAr), 8.07 (d, 3J = 8.4 Hz, 1
H, HAr), 7.95 (d, 3J = 8.4 Hz, 1 H, HAr), 7.75–7.71 (m, 3 H, HAr), 7.63–7.60
(m, 2 H, HAr), 7.53 (dd, 3J = 7.6 Hz, 4J = 1.7 Hz, 1 H, HAr), 7.58 (t, 3J = 7.6
Hz, 1 H, HAr), 6.98 (t, 3J = 7.6 Hz, 1 H, HAr), 6.94 (d, 3J = 8.4 Hz, 1 H, HAr),
3.95 (s, 3 H, OCH3).
1H NMR (300 MHz, CDCl3): δ = 8.71 (s, 1 H, HAr), 8.19 (d, 3J = 8.3 Hz, 2
H, HAr), 8.12–8.04 (m, 4 H, HAr), 7.71 (d, 3J = 8.3 Hz, 2 H, HAr), 7.60–7.56
(m, 6 H, HAr), 7.39–7.37 (m, 3 H, HAr).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, 606–614

613
A. Paun et al.
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Synthesis
13C NMR (125 MHz, CDCl3): δ = 164.9, 164.0, 160.3, 134.0, 133.8,
132.8, 132.4, 130.5, 130.3, 128.9, 128.6, 128.4, 127.4, 127.0, 126.9,
126.4, 125.0, 123.2, 120.7, 120.6, 92.8, 89.1, 56.0.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560387.
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p
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HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C27H19N2O2: 403.1441;
found: 403.1446.
References
2-{4-[(4-Fluorophenyl)ethynyl]phenyl}-5-(naphthalen-2-yl)-1,3,4-
oxadiazole (2k)
(1) These authors contributed equally to this work.
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Yield: 48.2 mg (78%); white solid; mp 194–196 °C; Rf = 0.32 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.65 (s, 1 H, HAr), 8.22 (dd, 3J = 8.5 Hz,
4J = 1.6 Hz, 1 H, HAr), 8.19 (d, 3J = 8.5 Hz, 2 H, HAr), 8.00 (d, 3J = 8.5 Hz, 2
H, HAr), 7.92 (d, 3J = 9.0 Hz, 1 H, HAr), 7.70 (d, 3J = 8.5 Hz, 2 H, HAr),
3
7.63–7.59 (m, 2 H, HAr), 7.58–7.54 (m, 2 H, HAr), 7.08 (t, J = 8.5 Hz, 2
H, HAr).
13C NMR (125 MHz, CDCl3): δ = 165.1, 164.4, 162.9 (d, 1JC,F = 249.0 Hz),
134.9, 133.8 (d, 3JC,F = 8.4 Hz), 133.0, 132.3, 129.3, 129.0, 128.2, 128.1,
127.6, 127.3, 127.0, 126.8, 123.5, 123.4, 121.2, 118.9 (d, 4JC,F = 3.8 Hz),
116.0 (d, 2JC,F = 22.0 Hz), 91.5, 88.4.
HRMS (APCI+, Orbitrap): m/z [M
+
H]+ calcd for C26H16FN2O:
391.1241; found: 391.1242.
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Bhanuprakash, K.; Rao, V. J.; Kamalasanan, M. N.; Srivastava, R.
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Chan, W. K. Macromolecules 2002, 35, 850.
2-(Anthracen-9-yl)-5-[4-(p-tolylethynyl)phenyl]-1,3,4-oxadiazole
(2l)
Yield: 50.7 mg (93%); yellow solid; mp 198–200 °C; Rf = 0.77 (EtOAc–
PE, 1:4).
1H NMR (500 MHz, CDCl3): δ = 8.71 (s, 1 H, HAr), 8.18 (d, 3J = 8.6 Hz, 2
H, HAr), 8.12 (d, 3J = 7.6 Hz, 2 H, HAr), 8.06 (d, 3J = 7.6 Hz, 2 H, HAr), 7.69
(d, 3J = 8.6 Hz, 2 H, HAr), 7.59–7.54 (m, 4 H, HAr), 7.46 (d, 3J = 8.0 Hz, 2
H, HAr), 7.19 (d, 3J = 8.0 Hz, 2 H, HAr), 2.39 (s, 3 H, CH3).
13C NMR (125 MHz, CDCl3): δ = 165.6, 163.4, 139.2, 132.4, 131.8,
131.7, 131.6, 131.2, 129.4, 128.9, 127.9, 127.5, 127.1, 125.9, 125.2,
123.2, 119.7, 117.3, 29.9.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C31H21N2O: 437.1648;
found: 437.1639.
(14) (a) For a review describing synthetic methods of 1,3,4-oxadi-
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Fernandez Lorenzo, J. G.; Filgueiras de Athayde-Filho, P. Mole-
cules 2012, 17, 10192. (b) For a recent example of dehydrative
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Ruan, Y. B.; Jiang, Q. Q.; He, W. B.; Jiang, Y. B. Chem. Eur. J. 2010,
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hydrazones using CAN, see: Dabiri, M.; Salehi, P.;
Baghbanzadeh, M.; Bahramnejad, M. Tetrahedron Lett. 2006, 47,
6983. (d) For an example of Huisgen reaction of tetrazole and
acid chlorides, see: Yang, C.-C.; Hsu, C.-J.; Chou, P.-T.; Cheng, H.
C.; Su, Y. O.; Leung, M. J. Phys. Chem. B 2010, 114, 756. (e) For
oxidative cyclisation using molecular iodine, see: Majji, G.;
Rout, S. K.; Guin, G.; Gogoi, A.; Patel, B. K. RSC Adv. 2014, 4, 5357.
(f) Yu, W.; Huang, G.; Zhang, Y.; Liu, H.; Dong, L.; Yu, X.; Li, Y.;
Chang, J. J. Org. Chem. 2013, 78, 10337.
2-(Anthracen-9-yl)-5-{[4-(p-fluorophenyl)ethynyl]phenyl}-1,3,4-
oxadiazole (2m)
Yield: 35.2 mg (64%); yellow solid; mp 209–211 °C; Rf = 0.43 (EtOAc–
PE, 1:6).
1H NMR (500 MHz, CDCl3): δ = 8.71 (s, 1 H, HAr), 8.19 (d, 3J = 8.3 Hz, 2
H, HAr), 8.11 (dd, 3J = 7.4 Hz, 4J = 2.4 Hz, 2 H, HAr), 8.05 (d, 3J = 7.9 Hz, 2
3
H, HAr), 7.69 (d, J = 8.3 Hz, 2 H, HAr), 7.59–7.54 (m, 6 H, HAr), 7.08 (t,
3J = 8.6 Hz, 2 H, HAr).
13C NMR (125 MHz, CDCl3): δ = 165.5, 163.4, 162.9 (d, 1JC,F = 249.0 Hz),
3
133.8 (d, JC,F = 8.4 Hz), 132.4, 131.73, 131.61, 131.18, 128.9, 127.9,
127.2, 127.0, 125.9, 125.2, 123.5, 118.9 (d, 4JC,F = 3.5 Hz), 117.2, 116.0
(d, 2JC,F = 22.0 Hz), 91.5, 88.4.
HRMS (APCI+, Orbitrap): m/z [M + H]+ calcd for C30H18N2OF:
441.1398; found: 441.1436
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Acknowledgment
Financial support from the National Research Council (CNCS) of Ro-
mania, Grant PN-II-PT-PCCA-2013-4-0291 (contract no. 203/2014) is
gratefully acknowledged.
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