Oxidative cyclization of N-acylhydrazones. Development of highly selective turn-on fluorescent chemodosimeters for Cu²⁺

Abstract

A series of N-acylhydrazones were synthesised and found to be “turn-on” fluorescent chemodosimeters for Cu²⁺. Among the tested transition metal ions such as Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Ni²⁺, a prominent fluorescence enhancement of up to 1000-fold was only observed for Cu²⁺ in acetonitrile (CH₃CN). This was indicated by an onset of unprecedented structured emission. Detailed experiments established that the highly Cu²⁺ selective fluorescence enhancement resulted from an oxidative cyclization by Cu²⁺of the originally nonfluorescent N-acylhydrazones into highly fluorescent rigid 1,3,4-oxadiazoles, n-dope type blocks in optoelectronic materials. The chemodosimeters can be applied to sense Cu²⁺ at nM levels in CH₃CN and sub-µM levels in neutral aqueous environments, despite a slower response in the latter case. It is expected that these redox-based chemodosimeters might be of general applicability.

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Introduction

N-Acylhydrazones have been widely employed in organic¹ and analytical² chemistry, mainly in terms of metal ligands.^2,3 It is known that N-acylhydrazones coordinate strongly with a variety of transition metal ions, forming complexes of varied biological and pharmaceutical activities. The development of sensitive and selective fluorescent chemosensors for biologically important metal ions is of intense current interest because these metal ions play important roles in living and environmental systems.⁴ Special attention has been focused on the design of fluorescent chemosensors for Cu²⁺ due to its essential yet toxic nature.⁵ We previously found that N-(p-dimethylaminobenzoyl)hydrazone (1, Scheme 1), bearing an intramolecular charge transfer (ICT) fluorophore, p-dimethylaminobenzamide, showed a highly selective fluorescence response toward Cu²⁺ in CH₃CN, despite similar absorption spectral variations being observed with other metal ions such as Pb²⁺, Zn²⁺, and Hg²⁺ too.⁶ This finding suggested that the N-acylhydrazone in this case might act not only as a ligand. Although several explanations were proposed, the exact mechanism, however, could not be clarified with just one molecule that also bears complicated excited-state ICT photophysics.⁶ In order to understand the responding mechanism, we decided to remove the ICT channel in 1 to simplify the excited-state photophysics and data rationalization. We therefore extended our investigation to a variety of N-benzoylhydrazones 2 and 3 bearing substituents X less electron-donating than p-NMe₂ in 1 (Scheme 1). A highly selective fluorescence response toward Cu²⁺ was again observed in both CH₃CN and CH₃CN–H₂O solutions. In particular, the observed enhanced emission is unprecedentedly structured in these polar solvents. Detailed experiments allowed us to establish that the enhanced fluorescence was due to an oxidative cyclization by Cu²⁺ of the originally nonfluorescent N-acylhydrazones into highly fluorescent rigid 1,3,4-oxadiazoles. N-Acylhydrazones were therefore shown to be a kind of redox-based “turn-on” fluorescent chemodosimeter for Cu²⁺, a new entrance to the active subject of “turn-on” fluorescent chemosensors for Cu²⁺, a strongly quenching paramagnetic species.^7–9 It should be pointed out that there have been several nice Cu²⁺ chemodosimeters with enhanced fluorescence signal outputs, which however follow hydrolysis⁸ or rearrangement reactions.⁹

Scheme 1 Molecular structures of N-acylhydrazones 1–4.

Results and discussion

N-Acylhydrazones 2 and 3 were facilely synthesized by a simple one-step reaction in ethanol of the corresponding N-benzoylhydrazine with 2-methoxybenzaldehyde and furan-2-carbaldehyde, respectively. 2a in CH₃CN exhibits three absorption bands centred at 287, 298, and 323 nm with respective molar absorption coefficients of 2.67 × 10⁴, 2.59 × 10⁴, and 2.93 × 10⁴ M⁻¹ cm⁻¹, indicative of the (π, π*) transition character. In the presence of Cu²⁺, the band at 323 nm is attenuated and a shoulder at ca. 365 nm is developed (Fig. 1a). Hg²⁺ and Pb²⁺ exert a similar effect, whereas other heavy transition metal ions such as Zn²⁺, Cd²⁺, and Ni²⁺ exert a minor influence on the absorption spectrum. 2b–d behave similarly in their absorption spectral response toward these metal ions (Fig. S1-S4†). 3, analogues of 2, exhibit spectral variation profiles (Fig. S5-S8†) similar to those of 2. 2 and 3 in CH₃CN emit extremely weak fluorescence. In the presence of Cu²⁺, however, an instant response was observed by a dramatic enhancement of up to 1000-fold, despite the well-known quenching character of Cu²⁺ (Fig. 1b, 2, S9, and S10†). The fluorescence response profiles of 2 and 3 toward Cu²⁺ were found to be simpler than that of 1,⁶ in that there was no emission band shift and no fluorescence quenching at higher Cu²⁺ concentration with 2 and 3 (Fig. 2). Removing the excited-state ICT channel in 1, much higher fluorescence enhancement for 2a and 3a by Cu²⁺ was observed (Fig. 2) than that for 1 which was ca.180-fold.⁶ Assays of the fluorescence response of 2 and 3 in CH₃CN toward a variety of other transition metal ions such as Pb²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Ni²⁺ indicated that a prominent fluorescence enhancement was again only observed for Cu²⁺ whereas the other transition metal ions tested exerted little influence (Fig. 2). This means that the fluorescence responses of 2 and 3 are highly selective for Cu²⁺, as is 1.⁶ The fluorescence enhancement factors (FEFs) of 2 and 3 by Cu²⁺ were found to be higher with increasing electron-donating ability of the substituent X and the FEFs of 3 were much higher than those of 2 (Fig. 2 and Table 1). It therefore appears that higher electron density at the hydrazone moiety and the relatively more exposed furan oxygen atom in 3 are important for higher FEFs. The fact that the fluorescence in CH₃CN of 4 (Scheme 1), a control molecule for 3a, does not show any response toward Cu²⁺, despite a substantial absorption variation (Fig. 3), nicely indicates that the vinyl proton =CH in 3a plays an essential role in the fluorescence enhancement of 3a by Cu²⁺.

Fig. 1 Absorption (a) and fluorescence (b) spectra of 2a (20 µM) in CH₃CN in the presence of increasing concentrations of Cu²⁺ (0–200 µM). The excitation wavelength was 267 nm, an isosbestic wavelength observed in absorption titrations.

Fig. 2 Plots of fluorescence enhancement factor (FEF, I/I₀) in CH₃CN versus concentration ratio of metal ion to 2 and 3. R = 2a–d or 3a–d, [2] = [3] = 10 µM.

Table 1 Absorption and fluorescence spectral parameters of 2 and 3, and 2 and 3 in the presence of 25 equivalents of metal ion in CH₃CN

	λ abs/nm	ε/10^4 M^−1 cm^−1	λ flu/nm	FEF ^a	Φ^b
a Fluorescence enhancement factor, the ratio of the intensity of 2 and 3 in the presence of 25 equivalents of metal ion to that in the absence of metal ion. b Fluorescence quantum yields of 2 and 3, and 2 and 3 in the presence of 25 equivalents of metal ion were measured using quinine sulfate as a standard (0.546 in 0.5 M H2SO4: Demas, J. N.; Crobys, G. A., J. Phys. Chem.1971, 75, 991–1024). The measurement errors were up to 50% and 15% for N-acylhydrazones and their metal complexes, respectively.
2a	287/298/323	2.67/2.59/2.93	369	—	0.0007
2a + Ni^2+	365	0.32	364	1.9	0.0008
2a + Cu^2+	365	0.50	358	280	0.34
2a + Zn^2+	365	0.10	360	5.7	0.0013
2a + Cd^2+	364	0.08	367	1.4	0.0009
2a + Hg^2+	368	0.56	364	1.5	0.0025
2a + Pb^2+	375	0.26	364	1.4	0.0012
2b	286/297/323	2.12/2.10/2.50	370	—	0.0008
2b + Ni^2+	365	0.25	358	1.1	0.0010
2b + Cu^2+	368	0.33	351	140	0.30
2b + Zn^2+	365	0.10	353	5.7	0.0013
2b + Cd^2+	366	0.04	370	1.1	0.0010
2b + Hg^2+	372	0.40	357	2.4	0.0018
2b + Pb^2+	375	0.39	355	1.4	0.0011
2c	286/298/323	1.92/1.92/2.37	370	—	0.0007
2c + Ni^2+	364	0.20	358	1.1	0.0011
2c + Cu^2+	370	0.30	355	55	0.20
2c + Zn^2+	364	0.05	358	1.9	0.0027
2c + Cd^2+	364	0.05	370	2.2	0.0012
2c + Hg^2+	361	0.47	357	1.7	0.0024
2c + Pb^2+	375	0.50	357	1.3	0.0015
2d	287/299/324	1.74/1.71/2.10	370	—	0.0008
2d + Ni^2+	364	0.23	364	1.1	0.0010
2d + Cu^2+	365	0.41	362	80	0.26
2d + Zn^2+	366	0.10	364	1.9	0.0014
2d + Cd^2+	367	0.03	373	2.2	0.0011
2d + Hg^2+	364	0.50	363	1.7	0.0042
2d + Pb^2+	375	0.75	364	1.3	0.0015
3a	254/310	1.01/3.75	370	—	0.0004
3a + Ni^2+	315/378	2.94/0.51	367	1.4	0.0016
3a + Cu^2+	296/345	2.66/1.55	366	1000	0.49
3a + Zn^2+	326	3.76	365	5.2	0.0038
3a + Cd^2+	328	4.06	381	1.5	0.0024
3a + Hg^2+	338	4.15	368	29	0.0032
3a + Pb^2+	333	3.69	367	1.1	0.0034
3b	238/308	1.02/3.37	371	—	0.0004
3b + Ni^2+	310/365	3.10/0.23	358	1.1	0.0032
3b + Cu^2+	327	1.71	361	260	0.36
3b + Zn^2+	313	3.23	364	1.3	0.0018
3b + Cd^2+	319	3.31	379	1.1	0.0025
3b + Hg^2+	333	3.43	364	8.1	0.0047
3b + Pb^2+	330	3.22	366	1.1	0.0025
3c	228/308	0.94/3.01	370	—	0.0005
3c + Ni^2+	309/365	3.03/0.24	372	1.1	0.0029
3c + Cu^2+	319	1.01	360	145	0.33
3c + Zn^2+	311	2.87	372	1.2	0.0030
3c + Cd^2+	314	2.95	385	1.1	0.0007
3c + Hg^2+	332	3.07	365	6.3	0.0047
3c + Pb^2+	330	2.74	366	1.1	0.0038
3d	236/309	1.27/3.26	371	—	0.0004
3d + Ni^2+	310/371	2.92/0.29	370	1.1	0.0024
3d + Cu^2+	319/376	1.31/0.12	368	136	0.29
3d + Zn^2+	311/374	3.11/0.06	369	1.0	0.0017
3d + Cd^2+	314	3.15	396	1.0	0.0010
3d + Hg^2+	334/390	3.11/0.18	371	2.9	0.0039
3d + Pb^2+	332	2.88	372	1.1	0.0026

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Fig. 3 Absorption spectra of 4 (10 µM) in CH₃CN in the presence of increasing concentrations of Cu²⁺ (0–200 µM). The excitation wavelength for acquiring fluorescence spectra given in the inset is an isosbestic wavelength of 274 nm.

It was surprising that an unexpected structured emission was observed for 2 and 3 in the presence of Cu²⁺ (Fig. 1b and 4 ). The trend shown in Fig. 4, that the emission becomes less structured while the FEF becomes lower with decreasing electron-donating ability of the substituent X, seems to suggest that the FEF value is related to the structured extent of the fluorescence spectrum. In order to test this correlation, an extended variety of N-acylhydrazones (5–12, Scheme 2) were prepared and their fluorescence response toward Cu²⁺ was monitored in CH₃CN. The fluorescence of 5–12 was similarly found to be enhanced by Cu²⁺, confirming that N-acylhydrazones could in general act as an excellent family of “chemosensors” for Cu²⁺. The structured extents of the emission spectra and the FEF values shown in Fig. 5, however, indicate that the apparent correlation between the structured extents and the FEF values reached in Fig. 4 does not hold in general, the rigidity of the ligand itself obviously contributing to the structured extent of the final emission spectrum. The unexpected structured emission and the high selectivity for Cu²⁺ therefore could not be simply attributed to Cu²⁺ coordination to N-acylhydrazones.

Fig. 4 Normalized fluorescence spectra of 2 and 3 in the presence of 25 equivalents of Cu²⁺ in CH₃CN at 25 °C.

Fig. 5 Normalized fluorescence spectra of 2c and 5–12 and FEF values in the presence of 25 equivalents of Cu²⁺ in CH₃CN at 25 °C.

Scheme 2 Structures of the extended variety of N-acylhydrazones.

The fluorescence emission of a ligand can in principle be affected not only by metal ion coordination, but by a metal ion involved reaction as well.¹⁰ The fact that the enhancement by Cu²⁺ of the fluorescence of 2 and 3 becomes higher when the substituent X is more electron-donating suggested that a redox reaction might occur. Indeed, it was reported that N-acylhydrazones underwent oxidative cyclization to 1,3,4-oxadiazoles by several oxidants.¹¹ Cu(ClO₄)₂ in CH₃CN could be an effective oxidant, since the Cu²⁺/Cu⁺ couple in CH₃CN reportedly has a high reduction potential due to the stabilization of Cu⁺ by solvent coordination.¹² We therefore hypothesized that Cu²⁺ acted as an oxidant in CH₃CN to result in an oxidative cyclization of N-acylhydrazones into 1,3,4-oxadiazoles. As no credible signals were detected in the cyclic voltammograms of the reported N-acylhydrazones in CH₃CN, we were unable to directly confirm this hypothesis on the basis of redox potentials data. Alternatively, crucial evidence supporting this hypothesis was obtained from the independent syntheses of 1,3,4-oxadiazoles by reactions of N-acylhydrazones 3 with Cu(ClO₄)₂ in CH₃CN. The oxidative cyclization products 1,3,4-oxadiazoles 13 (Scheme 3, Table S1†) were fully characterized by HRMS, ¹H NMR, and ¹³C NMR. Fluorescence excitation and emission spectra of the synthesized oxidative cyclization product 13a were found identical to those of 3a in the presence of 1.0 equivalent of Cu²⁺ (Fig. S11†). Obviously the observed fluorescence enhancement of 3a by Cu²⁺ was not caused by Cu²⁺ coordination but instead by the oxidative cyclization reaction. In order to confirm the role of Cu²⁺ in the reaction of N-acylhydrazones in CH₃CN, EPR experiments in CH₃CN at 100 K were carried out in which the Cu²⁺ concentration was made constant while the 3a concentration varied. It was found that the EPR signal of Cu²⁺ was indeed attenuated with increasing 3a concentration (Fig. 6). This points to the conversion of Cu²⁺ into diamagnetic Cu⁺ that can be stabilized in CH₃CN. A mechanism of the oxidative cyclization by Cu²⁺ was therefore suggested (Scheme 3), which forms the basis of this new kind of redox-based chemodosimeter for Cu²⁺.

Fig. 6 EPR spectra of 1.0 mM Cu²⁺ in CH₃CN at 100 K with increasing concentration of 3a.

Scheme 3 Proposed oxidative cyclization of N-acylhydrazones by Cu²⁺ in CH₃CN.

The absorption and fluorescence spectra of the oxidative cyclization product 13a in CH₃CN, hardly changed upon the addition of up to 25 equivalents Cu²⁺ (Fig. S12†). This explained the observed level-off of FEFs of 2 and 3 at higher Cu²⁺ concentration (Fig. 2). The observation that the absorption spectrum of 4 in CH₃CN undergoes substantial variation whereas it remains nonfluorescent in the presence of Cu²⁺ (Fig. 3) suggests that a simple coordination of Cu²⁺ to N-acylhydrazones in the ground state does not lead to an enhancement in the fluorescence of N-acylhydrazones. It was found that the fluorescence quantum yield of 13a (Φ = 0.725) in CH₃CN is ca.1800-fold that of 3a (Φ = 4 × 10⁻⁴). It is hence made clear that the dramatic fluorescence enhancement results fully from the oxidative cyclization by Cu²⁺ in CH₃CN of the nonfluorescent N-acylhydrazones which leads to the highly fluorescent rigid 1,3,4-oxadiazoles. The fluorescence response selectivity for Cu²⁺ is therefore due to its oxidation capability in CH₃CN, which makes it differ from the other metal ions tested. Hence the herein reported N-acylhydrazones do not act as metal coordinating chemosensors but instead fluorescent chemodosimeters for Cu²⁺.

The high fluorescence enhancement factor of 3a upon the addition of Cu²⁺ in CH₃CN shows great potential for application to the fluorescent sensing of Cu²⁺ at a low concentration level. The fluorescence of 3a (0.5 µM) in the presence of 0.5 equivalents of Cu²⁺ levels off rapidly within 2 min (Fig. S13†). Fluorescence titration of 3a (0.5 µM) shows a linear response toward Cu²⁺ over 25.0 nM to 0.25 µM in CH₃CN, with a detection limit of 3.5 nM (Fig. S14†). The fluorescent sensing of Cu²⁺ by 3a in H₂O–CH₃CN solutions was also tested. The optimal conditions obtained for the assay (Fig. S15-S17†) were to carry it out in a 20:80 (v/v) mixture of CH₃CN and H₂O at pH 7.2 (5 mM Tris-HCl buffer, 0.1 M KCl) after heating at 50 °C for 3 h. In this case the detection limit was 0.30 µM. The FEF was proportional to the Cu²⁺ concentration over 1.0–160 µM at a 3a concentration of 10 µM (Fig. 7). The fluorescence enhancement of 3a by Cu²⁺ was also found to be independent of the counter anions of the Cu²⁺ such as NO₃⁻, Cl⁻, AcO⁻, ClO₄⁻, and SO₄²⁻ in aqueous CH₃CN solutions (Fig. S18†).

Fig. 7 (a) Fluorescence spectra of 3a (10 µM) in a mixture of CH₃CN and Tris-HCl (5 mM, pH 7.2, 0.1 M KCl) aqueous buffer solution (20/80, v/v) in the presence of increasing concentrations of Cu²⁺ and (b) linear response curve. The excitation wavelength was 283 nm.

The fluorescence of 3a in 80% H₂O–CH₃CN (v/v) was found to be hardly altered by the other metal ions tested: Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Mg²⁺, Ca²⁺, and Ba²⁺. The selectivity for Cu²⁺ over these metal ions remains remarkably high and the FEF of Cu²⁺ is only slightly influenced by the addition of either 5 equivalents of each or all of the interference metal ions, Fig. 8. In aqueous solutions, however, the response reaction was substantially slowed down, likely due to the efficient hydration of Cu²⁺. Means of dehydrating Cu²⁺ and/or the stabilizing of Cu⁺ in aqueous solutions are expected to enhance the reaction to a reasonable level that might allow efficient aqueous phase assays. This is currently underway in this laboratory.

Fig. 8 (a) Fluorescence enhancement factors of 3a with individual ions (2.5 mM) and (b) Relative fluorescence responses to Cu²⁺ (100 µM) plus interference metal ion (500 µM) in a mixture of CH₃CN and Tris-HCl (5 mM, pH 7.2, 0.1 M KCl) aqueous buffer solution (20/80, v/v). “All” means the relative fluorescence response of 3a to Cu²⁺ (100 µM) plus all the interference metal ions tested, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Mg²⁺, Ca²⁺, and Ba²⁺ at 500 µM each.

Conclusions

Highly selective and dramatic enhancements by Cu²⁺ of the fluorescence in CH₃CN and CH₃CN–H₂O of a variety of N-acylhydrazones were observed and were shown to result from an oxidative cyclization by Cu²⁺ of the originally nonfluorescent N-acylhydrazones to the highly fluorescent rigid 1,3,4-oxadiazoles. In agreement with this conclusion was the structured emission from the CH₃CN solution of N-acylhydrazones and Cu²⁺. To the best of our knowledge, this is the first-set of reported chemodosimeters following an oxidative cyclization reaction that exhibits outstanding selectivity for Cu²⁺ with a dramatic fluorescence enhancement output. The chemodosimeters can be applied to sense Cu²⁺ at nanomolar levels in CH₃CN. Both high selectivity and sensitivity over other metal ions tested were obtained in a 20:80 (v/v) mixture of CH₃CN and Tris-HCl aqueous buffer solution, despite a much slower reaction rate. Although at the moment we are unable to set a structural limit for an N-acylhydrazone to be facilely oxidized by Cu²⁺, as all those reported here undergo efficient oxidation cyclization, we expect that this oxidative cyclization reaction could be taken as an alternative route to 1,3,4-oxadiazoles, efficient electron acceptors employed in optoelectronic materials.¹³ As redox reactions are easily made selective for a reductant or an oxidant and even made reversible, creating chemodosimeters following the redox reaction strategy illustrated here shall be of general applicability.

Experimental

Chemicals used for syntheses were commercially available. Solvents for spectral titrations were redistilled CH₃CN and deionized water.

Absorption and fluorescence spectra were recorded on Varian Cary 300 spectrophotometer and Hitachi F-4500 fluorescence spectrophotometer, respectively. Solutions were measured in a 1 cm quartz cell. Fluorescence quantum yields were measured using quinine sulfate as a standard (0.546 in 0.5 M H₂SO₄). ¹H NMR and ¹³C NMR were acquired on Bruker AV400 and Varian Unity⁺ 500 MHz NMR spectrometers. HRMS were obtained on a Micromass LCT spectrometer using methanol as the solvent. EPR experiments were carried out on a Bruker EMX-10/12 spectrometer.

All spectral titrations were carried out by keeping the sensor concentration constant while varying the metal ion concentration. Metal ions were used as their perchlorates. In the pH titration experiments, the solution pH was adjusted by dilute NaOH and HCl solutions that contained the same concentration of metal ion. Potassium chloride was employed to maintain solution ionic strength.

Preparation and characterization of 2–12 and 13.

2–12 were facilely synthesized from equimolar amounts of N-(substituted-benzoyl)hydrazine (2 mmol) with aldehyde or ketone (2 mmol) by a one-step reaction in ethanol, respectively. The mixture were refluxed for 3 h and then cooled down to room temperature. The crude products were isolated by filtration, and then recrystallized from absolute ethanol. New compounds were fully characterized by ¹H NMR, ¹³C NMR, and HRMS.

13 was synthesized from 3 (2 mmol) with 5 equivalents of Cu(ClO₄)₂ (10 mmol) in CH₃CN that were refluxed with stirring for 24 h, and then evaporated in vacuo. Aqueous ethylenediamine (1.0 mol L⁻¹× 25 mL) solution was added and the solution was then extracted by ethyl acetate (3 × 20 mL). The combined organic layer was thoroughly washed with water (3 × 20 ml), dried over anhydrous Na₂SO₄, and then concentrated under vacuum, then purified by column chromatography on silica gel with ethyl acetate-petroleum ether (1:5) as eluent. 13 was fully characterized by ¹H NMR, ¹³C NMR, and HRMS to confirm the occurrence of oxidative cyclization reaction.

N′-(2-Methoxybenzylidene)-4-ethoxybenzohydrazide (2a). ¹H NMR (500 MHz, DMSO-d₆, TMS): δ = 1.35 (t, 3H, J = 7.0 Hz), 3.87 (s, 3H), 4.09–4.13 (m, 2H), 7.02 (t, 3H, J = 8.0 Hz), 7.11 (d, 1H, J = 8.0 Hz), 7.41 (t, 1H, J = 7.5 Hz), 7.87 (d, 1H, J = 7.0 Hz), 7.92 (d, 2H, J = 8.5 Hz), 8.81 (s, 1H), 11.72 ppm (s, 1H); ¹³C NMR (125 MHz, DMSO-d₆, TMS): δ = 14.4, 55.6, 63.3, 111.7, 113.9, 120.6, 122.4, 125.2, 125.3, 129.4, 131.2, 142.4, 157.6, 161.1, 162.2 ppm; HRMS (ESI): m/z: calcd for C₁₇H₁₉N₂O₃: 299.1396 [M + H⁺]; found: 299.1395 [M + H⁺].

N′-(2-Methoxybenzylidene)-4-methylbenzohydrazide (2b). ¹H NMR (500 MHz, DMSO-d₆, TMS): δ = 2.38 (s, 3H), 3.87 (s, 3H), 7.03 (t, 1H, J = 7.5 Hz), 7.11(d, 1H, J = 8.5 Hz), 7.33 (d, 2H, J = 8.0 Hz), 7.42 (t, 1H, J = 7.5 Hz), 7.85 (d, 2H, J = 8.0 Hz), 7.88 (d, 1H, J = 8.5 Hz), 8.82 (s, 1H), 11.78 ppm (s, 1H); ¹³C NMR (125 MHz, DMSO-d₆, TMS): δ = 20.9, 55.5, 111.7, 120.6, 122.4, 125.4, 127.5, 128.8, 130.4, 131.3, 141.6, 142.9, 157.6, 162.7 ppm; HRMS (ESI): m/z: calcd for C₁₆H₁₇N₂O₂: 269.1290 [M + H⁺]; found 269.1295 [M + H⁺].

N′-(2-Methoxybenzylidene)benzohydrazide (2c). ¹H NMR (500 MHz, DMSO-d₆, TMS): δ = 3.87 (s, 3H), 7.04 (t, 1H, J = 7.5 Hz), 7.12 (d, 1H, J = 8.5 Hz), 7.43 (t, 1H, J = 7.5 Hz), 7.52 (t, 2H, J = 7.5 Hz), 7.59 (t, 1H, J = 7.0 Hz), 7.88 (d, 1H, J = 7.5 Hz), 7.93 (d, 2H, J = 7.5 Hz), 8.82 (s, 1H), 11.85 ppm (s, 1H); ¹³C NMR (125 MHz, DMSO-d₆, TMS): δ = 55.5, 111.6, 120.6, 122.3, 125.4, 127.5, 128.3, 131.4, 131.5, 133.3, 143.2, 157.7, 162.9 ppm; HRMS (ESI): m/z: calcd for C₁₅H₁₅N₂O₂: 255.1134 [M + H⁺]; found 255.1141 [M + H⁺].

N′-(2-Methoxybenzylidene)-4-chlorobenzohydrazide (2d). ¹H NMR (500 MHz, DMSO-d₆, TMS): δ = 3.87 (s, 3H), 7.03 (t, 1H, J = 7.5 Hz), 7.12 (d, 1H, J = 8.0 Hz), 7.43 (t, 1H, J = 7.5 Hz), 7.61 (d, 2H, J = 8.0 Hz), 7.88 (d, 1H, J = 8.5 Hz), 7.96 (d, 2H, J = 8.5 Hz), 8.81 (s, 1H), 11.90 ppm (s, 1H); ¹³C NMR (125 MHz, DMSO-d₆, TMS): δ = 55.5, 111.7, 120.6, 122.2, 125.4, 128.4, 129.4, 131.5, 132.0, 136.5, 143.5, 157.7, 161.8 ppm; HRMS (ESI): m/z: calcd for C₁₅H₁₄ClN₂O₂: 289.0744 [M + H⁺]; found 289.0754 [M + H⁺].

4-Ethoxy-N′-(furan-2-ylmethylene)benzohydrazide (3a). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.35 (t, 3H, J = 7.0 Hz), 4.11 (m, 2H), 6.64 (m, 1H), 6.91 (d, 1H, J = 3.2 Hz), 7.04 (d, 2H, J = 8.8 Hz), 7.85 (s, 1H), 7.87 (d, 2H, J = 8.8 Hz), 8.33 (s, 1H), 11.66 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 14.4, 63.3, 112.1, 113.1, 114.0, 125.1, 129.4, 136.9, 145.0, 149.5, 161.2, 162.4 ppm; HRMS (ESI): m/z: calcd for C₁₄H₁₅N₂O₃: 259.1083 [M + H⁺]; found 259.1077 [M + H⁺].

N′-(Furan-2-ylmethylene)-4-methylbenzohydrazide (3b). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 2.38 (s, 3H), 6.64 (s, 1H), 6.92 (s, 1H), 7.33 (d, 2H, J = 7.6 Hz), 7.81 (d, 2H, J = 7.6 Hz), 7.85 (s, 1H), 8.34 (s, 1H), 11.73 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 20.9, 112.1, 113.3, 127.5, 128.9, 130.4, 137.2, 141.7, 145.0, 149.4, 162.8 ppm; HRMS (ESI): m/z: calcd for C₁₃H₁₃N₂O₂: 229.0977 [M + H⁺]; found 229.0977 [M + H⁺].

N′-(Furan-2-ylmethylene)benzohydrazide (3c). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 6.65 (s, 1H), 6.94 (s, 1H), 7.53 (t, 2H, J = 6.4 Hz), 7.60 (t, 1H, J = 6.8 Hz), 7.86 (s, 1H), 7.90 (d, 2H, J = 7.2 Hz), 8.35 (s, 1H), 11.80 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 112.1, 113.4, 127.5, 128.4, 131.7, 133.3, 137.5, 145.1, 149.4, 163.0 ppm; HRMS (ESI): m/z: calcd for C₁₂H₁₁N₂O₂: 215.0821 [M + H⁺]; found 215.0819 [M + H⁺].

4-Chloro-N′-(furan-2-ylmethylene)benzohydrazide (3d). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 6.65 (m, 1H), 6.96 (d, 1H, J = 3.2 Hz), 7.62 (d, 2H, J = 8.4 Hz), 7.87 (s, 1H), 7.93 (d, 2H, J = 8.4 Hz), 8.34 (s, 1H), 11.86 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 112.1, 113.7, 128.5, 129.4, 132.0, 136.5, 137.7, 145.2, 149.3, 161.9 ppm; HRMS (ESI): m/z: calcd for C₁₂H₁₀ClN₂O₂: 249.0431 [M + H⁺]; found 249.0427 [M + H⁺].

4-Ethoxy-N′-(1-(furan-2-yl)ethylidene)benzohydrazide (4). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.35 (t, 3H, J = 7.0 Hz), 2.28 (s, 3H), 4.11 (m, 2H), 6.61 (m, 1H), 6.95 (s, 1H), 7.02 (d, 2H, J = 8.8 Hz), 7.80 (s, 1H), 7.86 (d, 2H, J = 8.8 Hz), 10.52 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 13.7, 14.5, 63.3, 111.2, 111.8, 113.8, 125.7, 129.9, 144.5, 145.1, 148.2, 151.8, 161.1 ppm; HRMS (ESI): m/z: calcd for C₁₅H₁₇N₂O₃: 273.1239 [M + H⁺]; found 273.1245 [M + H⁺].

N′-(2-Ethoxybenzylidene)-4-ethoxybenzohydrazide (5). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.34–1.41 (m, 6H), 4.09–4.15 (m, 4H), 7.00 (d, 1H, J = 7.6 Hz), 7.03 (d, 2H, J = 8.8 Hz), 7.09 (d, 1H, J = 8.0 Hz), 7.39 (t, 1H, J = 7.6 Hz), 7.88 (d, 1H, J = 7.6 Hz), 7.91 (d, 2H, J = 8.8 Hz), 8.80 (s, 1H), 11.75 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 14.5, 14.6, 63.3, 63.7, 112.7, 114.0, 120.6, 122.6, 125.2, 125.4, 129.5, 131.3, 142.5, 157.0, 161.2, 162.4 ppm; HRMS (ESI): m/z: calcd for C₁₈H₂₁N₂O₃: 313.1552 [M + H⁺]; found 313.1556 [M + H⁺].

N′-(2-Propoxybenzylidene)-4-ethoxybenzohydrazide (6). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.05 (t, 3H, J = 7.2 Hz), 1.36 (t, 3H, J = 7.2 Hz), 1.79 (m, 2H), 4.02 (t, 2H, J = 6.4 Hz), 4.11 (m, 2H), 6.99 (d, 1H, J = 7.2 Hz), 7.04 (d, 2H, J = 8.8 Hz), 7.08 (d, 1H, J = 8.4 Hz), 7.38 (m, 1H), 7.87 (s, 1H), 7.91 (d, 2H, J = 8.8 Hz), 8.80 (s, 1H), 11.76 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 10.5, 14.4, 22.0, 63.3, 69.4, 112.6, 113.9, 120.5, 122.6,125.3, 125.4, 129.5, 131.2, 142.4, 157.1, 161.1, 162.4 ppm; HRMS (ESI): m/z: calcd for C₁₉H₂₃N₂O₃: 327.1709 [M + H⁺]; found 327.1703 [M + H⁺].

N′-(2-Isopropoxybenzylidene)-4-ethoxybenzohydrazide (7). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.32 (d, 6H, J = 6.0 Hz), 1.36 (t, 3H, J = 6.8 Hz), 4.11 (m, 2H), 4.70 (m, 1H), 7.00 (t, 1H, J = 7.2 Hz), 7.03 (d, 2H, J = 8.8 Hz), 7.12 (d, 1H, J = 8.0 Hz), 7.37 (t, 1H, J = 8.0 Hz), 7.87 (d, 1H, J = 8.0 Hz), 7.91 (d, 2H, J = 8.8 Hz), 8.76 (s, 1H), 11.72 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 14.4, 21.8, 63.3, 70.4, 113.9, 114.5, 120.6, 123.6, 125.3, 125.6, 129.5, 131.1, 142.8, 156.0, 161.1, 162.3 ppm; HRMS (ESI): m/z: calcd for C₁₉H₂₃N₂O₃: 327.1709 [M + H⁺]; found 327.1705 [M + H⁺].

4-Ethoxy-N′-(thiophen-2-ylmethylene)benzohydrazide (8). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.35 (t, 3H, J = 6.8 Hz), 4.11 (m, 2H), 7.04 (d, 2H, J = 8.8 Hz), 7.14 (m, 1H), 7.45 (d, 1H, J = 3.2 Hz), 7.66 (d, 1H, J = 4.8 Hz), 7.88 (d, 2H, J = 8.8 Hz), 8.67 (s, 1H), 11.68 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 14.4, 63.3, 114.0, 125.1, 127.7, 128.6, 129.4, 130.5, 139.2, 142.2, 161.2, 162.3 ppm; HRMS (ESI): m/z: calcd for C₁₄H₁₅N₂O₂S: 275.0854 [M + H⁺]; found 275.0855 [M + H⁺].

N′-Benzylidene-4-ethoxybenzohydrazide (9). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.36 (t, 3H, J = 7.0 Hz), 4.09–4.14 (m, 2H), 7.05 (d, 2H, J = 8.8 Hz), 7.43–7.48 (m, 3H), 7.73 (d, 2H, J = 6.4 Hz), 7.91 (d, 2H, J = 8.8 Hz), 8.45 (s, 1H), 11.73 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 14.4, 63.3, 114.0, 125.2, 126.9, 128.7, 129.5, 129.8, 134.4, 147.0, 161.2, 162.5 ppm; HRMS (ESI): m/z: calcd for C₁₆H₁₇N₂O₂: 269.1290 [M + H⁺]; found 269.1289 [M + H⁺].

4-Ethoxy-N′-(2-methylpropylidene)benzohydrazide (10). ¹H NMR (400 MHz, CD₃CN): δ = 1.10 (d, 6H, J = 6.8 Hz), 1.38 (t, 3H, J = 7.0 Hz), 2.51–2.59 (m, 1H), 4.07–4.12 (m, 2H), 6.96 (d, 2H, J = 7.2 Hz), 7.51 (s, 1H), 7.77 (d, 2H, J = 8.4 Hz), 9.67 ppm (s, 1H); ¹³C NMR (400 MHz, DMSO-d₆, TMS): δ = 14.4, 19.6, 31.0, 63.2, 113.9, 125.4, 129.3, 155.9, 161.0, 162.2 ppm; HRMS (ESI): m/z: calcd for C₁₃H₁₉N₂O₂: 235.1447 [M + H⁺]; found 235.1446 [M + H⁺].

4-Phenyl-N′-(2-methoxybenzylidene)benzohydrazide (11). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 3.88 (s, 3H), 7.05 (t, 1H, J = 7.6 Hz), 7.13 (d, 1H, J = 8.4 Hz), 7.43 (m, 2H), 7.52 (t, 2H, J = 7.6 Hz), 7.76 (d, 2H, J = 7.6 Hz), 7.84 (d, 2H, J = 8.4 Hz), 7.91 (d, 1H, J = 6.8 Hz), 8.05 (d, 2H, J = 8.4 Hz), 8.86 (s, 1H), 11.92 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 55.6, 111.8, 120.7, 122.3, 125.4, 126.6, 126.8, 128.1, 128.2, 129.0, 131.5, 132.0, 139.0, 143.1, 157.7, 162.5 ppm; HRMS (ESI): m/z: calcd for C₂₁H₁₉N₂O₂: 331.1447 [M + H⁺]; found 331.1451 [M + H⁺].

N′-(2-Methoxybenzylidene)-2-naphthohydrazide (12). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 3.89 (s, 3H), 7.06 (t, 1H, J = 7.2 Hz), 7.13 (d, 1H, J = 8.4 Hz), 7.45 (t, 1H, J = 7.6 Hz), 7.64 (m, 2H), 7.92 (d, 1H, J = 7.6 Hz), 8.01–8.09 (m, 4H), 8.58 (s, 1H), 8.88 (s, 1H), 12.03 ppm (s, 1H); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 55.6, 111.8, 120.7, 122.3, 124.2, 125.4, 126.8, 127.6, 127.8, 127.9, 128.0, 128.3, 130.6, 131.5, 132.0, 134.2, 143.1, 157.7, 162.8 ppm; HRMS (ESI): m/z: calcd for C₁₉H₁₇N₂O₂: 305.1290 [M + H⁺]; found 305.1281 [M + H⁺].

2-(4-Ethoxyphenyl)-5-furan-2-yl-1,3,4-oxadiazole (13a). ¹H NMR (400 MHz, DMSO-d₆, TMS): δ = 1.37 (t, 3H, J = 7.0 Hz), 4.14 (m, 2H), 6.83 (m, 1H), 7.16 (d, 2H, J = 8.8 Hz), 7.43 (d, 1H, J = 3.6 Hz), 8.00 (d, 2H, J = 8.8 Hz), 8.08 ppm (d, 1H, J = 1.2 Hz); ¹³C NMR (100 MHz, DMSO-d₆, TMS): δ = 14.4, 63.5, 112.6, 114.4, 115.1, 115.2, 128.5, 138.6, 146.8, 156.4, 161.4, 163.1 ppm; HRMS (ESI): m/z: calcd for C₁₄H₁₃N₂O₃: 257.0926 [M + H⁺]; found 257.0927 [M + H⁺].

2-Furan-2-yl-5-p-tolyl-1,3,4-oxadiazole (13b). ¹H NMR (400 MHz, CDCl₃, TMS): δ = 2.44 (s, 3H), 6.62 (m, 1H), 7.23 (d, 1H, J = 3.2 Hz), 7.33 (d, 2H, J = 8.0 Hz), 7.67 (d, 1H, J = 1.2 Hz), 8.01 ppm (d, 2H, J = 8.0 Hz); ¹³C NMR (100 MHz, CDCl₃, TMS): δ = 21.7, 112.2, 113.9, 120.8, 127.0, 129.8, 139.6, 142.5, 145.6, 157.4, 164.2 ppm; HRMS (ESI): m/z: calcd for C₁₃H₁₁N₂O₂: 227.0821 [M + H⁺]; found 227.0822 [M + H⁺].

2-Furan-2-yl-5-phenyl-1,3,4-oxadiazole (13c). ¹H NMR (400 MHz, CDCl₃, TMS): δ = 6.63 (m, 1H), 7.24 (d, 1H, J = 3.6 Hz), 7.52 (s, 1H), 7.54 (m, 2H), 7.67 (d, 1H, J = 1.2 Hz), 8.13 ppm (m, 2H); ¹³C NMR (100 MHz, CDCl₃, TMS): δ = 112.2, 114.0, 123.5, 127.0, 129.0, 131.8, 139.4, 145.7, 157.4, 163.9 ppm; HRMS (ESI): m/z: calcd for C₁₂H₉N₂O₂: 213.0664 [M + H⁺]; found 213.0669 [M + H⁺].

2-(4-Chlorophenyl)-5-furan-2-yl-1,3,4-oxadiazole (13d). ¹H NMR (400 MHz, CDCl₃, TMS): δ = 6.63 (m, 1H), 7.25 (d, 1H, J = 3.6 Hz), 7.51(d, 2H, J = 8.8 Hz), 7.68 (t, 1H, J = 0.8 Hz), 8.06 ppm (d, 2H, J = 8.4 Hz); ¹³C NMR (100 MHz, CDCl₃, TMS): δ = 112.2, 114.3, 122.0, 128.2, 129.5, 138.1, 139.3, 145.8, 157.5, 163.1 ppm; HRMS (ESI): m/z: calcd for C₁₂H₈ClN₂O₂: 247.0274 [M + H⁺]; found 247.0272 [M + H⁺].

Acknowledgements

This work was supported by the NSFC of China (20425518, 20675069), NFFTBS (J0630429), and the Ministry of Education (MOE) of China.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Absorption and fluorescence spectral titration traces and NMR spectra for compounds 2–13. See DOI: 10.1039/b811612a

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