Electron impact mass spectrometry of some 3-[3-(4-aryl)-1 ... › journals › jspec › 2000 ›...

Spectroscopy 14 (2000) 115–120 115IOS Press

Electron impact mass spectrometry of some3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acylarylaldehyde hydrazone derivatives

Lúcia Fernanda C.C. Leitea, Eliezer J. Barreiroa, Mozart N. Ramosb, João Bosco P. Silvab,Suely L. Galdinob and Ivan R. PittabaNúcleo de Pesquisas de Produtos Naturais & Laboratório de Avaliação e Síntese de SubstânciasBioativas, Faculdade de Farmácia, Universidade Federal do Rio de Janeiro, CEP 68006, Rio deJaneiro, BrasilE-mail: [email protected] Universidade Federal de Pernambuco, Cidade Universitária, CEP 50670-901, Recife, Brasil

Abstract. This report describes the results of a study of the electron impact mass spectrometry behaviour of 20 new com-pounds of a series of 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl arylaldehyde hydrazone derivatives. The most relevant possiblefragmentation patterns of this class of compounds under electron impact are outlined.

1. Introduction

The compounds 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl arylaldehyde hydrazone (1–20) were syn-thesized as isosteres of inhibitors of arachidonic acid cascade enzyme. Synthesis of the compounds dis-cussed herein have been described in a previous work [1].

2. Experimental

The mass spectra were measured on GC/VG Micromass 12 at 7 eV with computer-aided automaticrepresentations of spectra (NPPN & LASSBio). The fragmentations were described as a relation betweenthe unit of atomic mass and charge (m/z) and the relative abundance in percentage terms.

3. Results and discussion

All compounds from the series 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl arylaldehyde hydrazone (1–5) and 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] 2-furancarboxylaldehyde hydrazone (11–15) or 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] 2-thiophencarboxyaldehyde hydrazone (16–20) have similar mass spectra with arelatively simple fragmentation pattern (Tables 1–3) but exhibit characteristic behaviour which is usefulas a structure diagnostic for this heterocyclic system. These compounds have a characteristic fragmenta-tion process outlined in Fig. 1. Fragmentation patterns of this type were also found in compounds from

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116 L.F.C.C. Leite et al. / Mass spectrometry of some hydrazone derivatives

Fig. 1.

the series 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl 4-dimethylamino-benzaldehyde hydrazone (6–10),although with less intensity.

All compounds showed a variety of molecular ion intensity. The compounds 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl arylaldehyde hydrazones (1–5, 11–20), after electron impact provided byγ ruptureN–N of the molecular ions accompanied by loss of Ar–CN and McLafferty’s rearrangement generatesthe typical fragment (A, B), the base peak of (1, 3–5), compounds which can exist in two tautomericforms. The fragment (C), was observed by way of cleavage of bonds 1,5 and 3,4 in oxadiazole ring [2,3]. This peak (C) can subsequently provide the fragment (D) involving the expulsion of oxygen.

L.F.C.C. Leite et al. / Mass spectrometry of some hydrazone derivatives 117

Fig. 2.

The speciesm/z 90 (E) in compounds 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl arylaldehyde hydra-zones (1–10) results from a rearrangement and subsequent loss of CO and R substituent radical inparaposition from (C) due to rearrangement in the phenyl ring. The fragment (D) loses CN moiety accompa-nied by the expulsion of thepara substituent of the phenyl radical giving the ionm/z 77 (F).

The base peakm/z 133 (I), in the compounds 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acylp-dimethyl-aminephenyl hydrazone (7–10) is not formed by McLafferty’s rearrangement, seeming to be generated byβ scission of hydrazone bond as summarised in Fig. 2. This pattern of fragmentation has been describedfor substituted aroylhydrazone [4]. Another fragmentation pattern of 3-[3-(4-phenyl)-1,2,4-oxadiazole-5-yl] acyl p-dimethylaminephenyl hydrazone compound (6) is based on the C–N double cleavage of sidechain with loss of CHN2 and subsequent rearrangement giving the ionm/z 294 (100%). This degradationpattern differs from the analog derivatives. Principal fragmentations and intensities for these compounds(1–5 and 6–10) are given in Tables 1 and 2.


Table 1

Principal fragmentations and intensities of 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl ary-aldehyde hydrazone (1–5)

No Ionsm/z (I %)R M A-B C D E F

1 H 292 (25) 189 (100) 119 (30) 103 (85) 90 (46) 77 (40)2 CH3 306 (12) 203 (65) 133 (27) 117 (68) 90 (100) 77 (44)3 Br 370 (25) 267 (100) 197 (25) 181 (40) 90 (85) 77 (35)4 OCH3 322 (60) 219 (100) 149 (100) 133 (92) 90 (62) 77 (40)5 NO2 337 (35) 234 (100) 164 (8) 148 (23) 90 (77) 77 (33)

Table 2

Principal fragmentations and intensities of 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acylp-dimethyl-aminephenyl hydrazone (6–10)

No Ionsm/z (I %)R M G H I E F

6 H 335 (5) 189 (2) 147 (35) 133 (10) 90 (5) 77 (15)7 CH3 349 (40) 189 (22) 147 (18) 133 (100) 90 (11) 77 (12)8 Br 414 (26) 189 (23) 147 (22) 133 (100) 90 (12) 77 (12)9 OCH3 365 (46) 189 (22) 147 (23) 133 (100) 90 (17) 77 (13)

10 NO2 380 (50) 189 (18) 147 (20) 133 (100) 90 (13) 77 (10)

In the 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl 2-furancarboxyaldehyde hydrazone (11–15) and 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl 2-thiophencarboxyaldehyde hydrazone (16–20) (Fig. 3) these serieswere observed as the principal fragmentation processes, one very close to those observed in 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl benzaldehyde hydrazone derivatives (1–5). The ion K,m/z 80 couldbe the product of a cleavage of bond N=C in the side chain [2]. On the other hand, the ion J,m/z 93,could result from cleavage of the N–N bond [3] as illustrated in Fig. 3.

Principal fragmentations and intensities for these series (11–15 and 16–20) are given in Table 3.

L.F.C.C. Leite et al. / Mass spectrometry of some hydrazone derivatives 119

Fig. 3.

Table 3

Principal fragmentations and intensities of 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl 2-furancarb-oxyaldehyde hydrazone and 3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl 2-thiophencarboxyalde-hyde hydrazone derivatives

No Ionsm/z (I %)R X M A-B C D J K

11 H O 282 (40) 189 (33) 119 (14) 103 (100) 93 (15) 80 (45)12 CH3 O 296 (55) 203 (45) 133 (25) 117 (100) 93 (14) 80 (53)13 Br O 360 (45) 267 (54) 197 (21) 181 (85) 93 (37) 80 (100)14 OCH3 O 312 (45) 219 (22) 148 (87) 133 (100) 93 (12) 80 (37)15 NO2 O 327 (55) 234 (15) 164 (5) 148 (32) 93 (100) 80 (75)16 H S 298 (45) 189 (68) 119 (20) 103 (100) 109 (15) 96 (88)17 CH3 S 312 (57) 203 (95) 133 (60) 117 (88) 109 (25) 96 (100)18 Br S 376 (25) 267 (62) 197 (16) 181 (41) 109 (38) 96 (100)19 OCH3 S 328 (45) 219 (55) 148 (100) 133 (90) 109 (18) 96 (65)20 NO2 S 343 (30) 234 (7) 164 (3) 148 (15) 109 (100) 96 (70)


4. Conclusions

The following generalization can be made from the study by electron-impact mass spectrometry of the3-[3-(4-aryl)-1,2,4-oxadiazole-5-yl] acyl arylaldehyde hydrazone derivatives. The preferred fragmenta-tion results from theγ rupture N–N in the side chain of the molecular ions accompanied by McLafferty’srearrangement. The relative abundance of the peaks molecular ion M+2 for all compounds containingbromine was in agreement of the proposed structures.

References

[1] L.F.C.C. Leite, Núcleo de Pesquisas de Produtos Naturais (NPPN) & Laboratório de Avaliação e Síntese de SubstânciasBioativas (LASSBio), Ph.D. Dissertation, Universidade Federal do Rio de Janeiro, Brasil, 1996.

[2] A. Selva, L.F. Zerilli, B. Cavalleri and G.G. Gallo,Org. Mass Spectrom.6 (1972), 1347–1351.[3] A. Selva, L.F. Zerilli, B. Cavalleri and G.G. Gallo,Org. Mass Spectrom.9 (1974), 558–566.[4] D.G.I. Kingston, H.P. Tannenbaum, G.B. Baker, J.R. Dimmock and W.G. Taylor,J. Chem. Soc.(C) (1970), 2574–2577.

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