Electron impact induced ortho effects in 2-nitro-substituted aromatic sulphides containing 2-pyridyl, 2-(5-methylthio-1,3,4-thiadiazolyl) and/or 2-(4-methyl-1,3,4-thiadiazolinyl-5-thione) moieties

The most significant mass spectral features of 14 title compounds are discussed with the aid of deuterium labelling experiments. The decomposition patterns of these compounds are strongly affected by several competing ortho effects, due to the interaction of the nitro function(s) with neighbouring electron-poor N-heterocycles. Very intense polycyclic ions are produced via addition-elimination reactions by loss of simple radicals (H˙, OH˙, NO₂˙) from the molecular ion, followed by the ejection of neutral molecules (HNO₂,CH₃SCN or CH₃NCS). In addition, primary or secondary intramolecular oxygen transfers, preceded or not by hydrogen migration, from the nitro group to the imino carbon via spirocyclic intermediates, are generally observed. Minor skeletal rearrangements, triggered by single or multiple intramolecular oxygen transfer to the bridgehead sulphur atom, followed by SO or SO₂ ejection, are also noticed.     

The mass spectra of some bis[5-(2-mercapto-1,3,4-thiadiazolyl)]methyl and aryl derivatives

The general fragmentation patterns of five selected title compounds are described. The most significant mass spectral features arise from the interaction of aryl substituents with the adjacent heterocyclic ring. The decomposition of the compounds bearing an ortho nitro group in the S-aryl moieties is strongly affected by a prominent “ortho effect”.     

Ortho effects in organic molecules on electron impact: XI—the interesting ortho effect of the methoxy group in o-methoxy aromatic thioamides

It has been noticed that the major part of the loss of ?H from the molecular ion of most of the o-methoxythioamides results from an ortho effect of the methoxy group. Comparison of the MIKE spectra of the [M? SH]⁺ of 1-(2-methoxyphenylthioxomethyl)piperidine and 1-(2-methoxyphenylthioxomethyl)pyrrolidine with the MIKE spectra of [M? SH]⁺ of the corresponding unsubstituted compounds, reported earlier, indicated two parallel pathways for the formation of [M? SH]⁺ in the o-methoxy compounds. In the first pathway, as has been noticed in thioamides in general, the loss of ?H involves the migration of either the α-hydrogen in the amine moiety or the hydrogen attached to nitrogen. In the second pathway, the migration of a hydrogen from the o-methoxy group to the sulphur atom followed by ejection of SH from the molecular ion leads to a stable cyclized ion. Interesting secondary fragmentations as a consequence of this ortho effect have also been noticed.     

Ortho effect of the nitro group in the fragmentation of 1-phenyl-3-nitrophenyl-2-pyrazolines

Electron impact mass spectra and mass-analysed ion kinatic energy spectra of the title compounds are discussed. The characteristic fragment ions of the ortho isomer are shown by high resolution and kinetic energy release measurements to involve the transfer of hydrogen to the nitro group and oxygen from the nitro group to the pyrazoline carbon.     

Solvation Dependences of Isotropic Hyperfine Interaction Constants and out-of-Plane Distortions of ortho-Substituted Nitrobenzene Radical Anions

For a number of ortho-substituted nitrobenzene radical anions (RAs) generated in DMF and its binary mixtures with water, it is shown that for radical anions with a substituent of minor effective volume in one ortho position to the nitro group, the dependences of the isotropic hyperfine interaction (ihfi) constants on the mole fraction of water are S-like and dictated by the medium composition and the concerted out-of-plane rotational and pyramidal distortions of the nitro group of the radical anion. The S-like shape of the solvation dependences of the ihfi constants is dictated by the dominant rotational distortions of the nitro group. For most radical anions with two ortho substituents or with one ortho substituent with a large effective volume, the S-like dependences are not observed, and the values of the nitrogen ihfi constants depend on the dominant pyramidal distortion of the nitro group. For the 2-tert-butylnitrobenzene radical anion in water, the nitrogen ihfi constant is a_N=25.62 G, which is typical of nitroaliphatic radical anions. This effect is explained based on the pyramidal structure of the nitro group in the case of its large rotation angles.     

Ortho effects in organic molecules on electron impact. Part 15. Oxygen transfers from the nitro group to both sulphur and olefinic double bonds in allyl o-nitrophenyl sulphide

An oxygen transfer from the nitro group to the C?C group, followed by a simple cleavage, afford intense fragments corresponding to o-nitrosothiophenol at m/z 139 and o-nitrosothiophenoxy cation at m/z 138 during mass spectral fragmentations of allyl o-nitrophenyl sulphide. Further, a concerted double oxygen transfer from the nitro group to the sulphur is proposed for the ejection of ?O₂H from the molecular ion of this compound, leading to the quinolinium cation at m/z 130. These processes are supported by the high-resolution data, collision-induced dissociation linked-scan spectra and chemical evidence.     

Cycloreversal reaction and ortho interactions in 2-aryl-4H-3, 1-benzoxazin-4-ones on electron impact

A cycloreversal reaction, leading to aroyl cations, is the major process in 2-aryl-4H-3,1-benzoxazin-4-ones under electron impact conditions. The ortho interaction of the methoxy and the nitro groups in the 2-phenyl moieties in these compounds present the most abundant ions at m/z 119 and 134, respectively, in their mass spectra as a result of the transfer of a hydrogen atom from the former and an oxygen atom from the latter to the imine nitrogen of the heterocycle. The ion structures and the mechanisms for the proposed fragmentations are based on high-resolution data, B/E and B²/E linked-scan spectra, collision-activated decomposition–B-/E linked-scan spectra and deuterium labelling.     

Ortho effects in organic molecules on electron impact. 14—Concerted and stepwise ejections of SO2 and N2 from N-arylidene 2-nitrobenzenesulphenamides

Unexpected ortho interaction of the nitro group has been noticed during the mass spectral fragmentations of N-arylidene 2-nitrobenzenesulphenamides, where the molecular ions expel SO₂ and N₂ both in concerted and stepwise processes. Loss of a hydrogen or the substituent from this fragment leads to a very abundant ion in all the compounds studied. Based on chemical evidence and linked-scan studies, a 1,2-phenylenetropylium cation structure has been postulated for the [M–SO₂–N₂–H/substituent]⁺ ion.     

Ortho effects–I: Fragmentation mechanisms in some ortho-substituted nitroarenes

The mass spectra of o-nitrobenzoic acid, o-nitroanisole, o-nitrosobenzoic acid, o-nitrobenzamide, o-nitrobenzyl alcohol and o-nitrosobenzaldehyde have been studied. Fragmentation mechanisms are proposed for the above compounds; their elucidation was aided by isotopic labeling with D and O¹⁸. Two ‘ortho-effects’ are discussed; one involving H atom transfer between substituents and the other migration of an atom or group to a charge carrying vacant ortho position. The importance of nitro to nitrite conversion in molecular and fragment ions is discussed.     

Mass spectral fragmentation pathways in some dinitroaromatic compounds studied by collision-induced dissociation and tandem mass spectrometry

A collision-induced dissociation study of a series of dinitroaromatic compounds was carried out using a tandem BB mass spectrometer. Fragmentation pathways were determined in the electron impact mode. Loss of NO₂˙ from the molecular ion was observed In most of the investigated compounds. In some compounds loss of NO₂˙ occurred only after loss of OH˙. In other compounds it was not observed at all because of competitive processes, such as loss of NO˙, CO₂, CH₂O, C₂H₄ or H₂O. Loss of NO˙ was a major decomposition pathway, forming ‘dished peaks’ in some of the compounds having a nitro group ortho to a phenyl group, indicating a release of kinetic energy associated with the decomposition. Loss of OH˙ due to an ‘ortho effect’ occurred in compounds where a nitro group was ortho to a group containing a labile hydrogen, but was not observed when competitive processes such as loss of NO˙, NO₂˙ or H₂O occurred. ‘Nitro to nitrite’ isomerization was suggested to explain the decarboxylation process in 2,4- and 2,5-dinitrobenzoic acid and the loss of COH₂ in 2,4-dinitroanisole.     

Ortho effects in organic molecules on electron impact: 21—Oxygen transfers from the ortho nitro group to sulphur in N-aryl-2-nitrobenzenesulphenamides and phenyl-2-nitrophenyl disulphide

Fragment ions arising as a result of oxygen transfers from the nitro group to sulphur have been noticed in N-aryl-2-nitrobenzenesulpbenamides and phenyl-2-nitrophenyl disulphide. In the case of the former a double oxygen transfer to the sulphur has been noticed in the molecular ion whilst a single oxygen transfer to the β-sulphur atom and a double oxygen transfer to the α-sulphur atom have been observed in the latter. The proposed fragmentations are confirmed by high-resolution data, B/E linked-scan spectra and chemical substitution.     

Ortho effects in organic molecules on electron impact: 18—Novel hydrogen transfer from the methoxy group to acetylenic carbon in 2-methoxyphenylacetylene and 2-methoxydiphenylacetylenes

The transfer of a hydrogen from the ortho methoxy group to the acetylenic carbon in 2-methoxyphenylacetylene under electron impact conditions affords the rearranged molecular ion corresponding to 2H-1-benzopyran. Similar processes leading to cyclic products are also noticed in 2-methoxydiphenylacetylenes. The ion structures and the mechanism of fragmentations are established through high-resolution data, collision-induced decomposition, mass-analysed ion kinetic energy spectra, B/E linked scan spectra, D-labelling and chemical substitution.     

Chemical ionization mass spectra of mononitroarenes

The H₂ and CH₄ chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H₂ chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH₄ CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH ? H₂O]⁺ fragments are observed. When the substituent is electron-releasing both ortho and para substituted nitrobenzenes show abundant [MH? OH]⁺ fragment ions while meta substituted compounds show abundant loss of NO and NO₂ from [MH]⁺. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the vitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron-attracting the chemical ionization mass spectra of isomers are very similar except for the H₂O loss reaction for ortho compounds.     

Mass spectrometry of imidazole-4(5)-carboxaldehyde and some 1-methyl and nitro derivatives

The mass spectra of imidazole-4(5)-carboxaldehyde, its two 1-methyl derivatives, 4(5)-nitroimidazole, 5(4)-nitroimidazole-4(5)-carboxaldehyde and 1-methyl-5-nitroimidazole-4-carboxaldehyde are presented and discussed in comparison with those of other imidazole-carboxaldehydes and nitroimidazoles earlier reported. The imidazole-carboxaldehydes and their 1-methyl derivatives exhibit the characteristic fragmentation of aromatic aldehydes, and differences between the isomers can be observed. The nitroimidazoles show the fragmentation typical of aromatic nitrocompounds. In the o-nitroimidazole-carboxaldehydes, the typical losses of aldehydes do not occur, but primary ortho effects between the formyl and nitro groups give rise to important fragmentation routes. In their 1-methyl derivatives, the presence of the methyl group adjacent to the nitro group originates additional double and secondary ortho effects. For some of these transformations, fragmentation mechanisms are proposed.     

A study of the ortho effects and rearrangement processes in the mass spectra of 2-arylaminothiazine and -thiazoline derivatives

The behaviour of the title compounds under electron impact has been investigated using low and high resolution mass spectrometry and partial ¹⁵N and D labelling. A number of ions produced by rearrangement were observed. Abundant ions are formed by intramolecular ortho substitution reactions (cyclizations), as demonstrated by energetic and kinetic considerations and by studying the decomposition pathways of these ions. The ortho substitution processes involve loss or rearrangement of an ortho group of the aromatic ring.     

Spectrométrie de Masse d'Hétérocycles Azotés. VIII–Comportement sous l'Impact Electronique de Nitroarylazoles

The mass spectra of nitrophenylimidazoles and nitrophenylpyrazoles have been examined in order to establish whether neighbouring ortho-ortho′ substituents have an appreciable influence on the fragmentation patterns. For compounds having an ortho nitro group on the heterocycle, specific effects are observed. Isotope effects observed with deuterated derivatives confirm this and establish that in the case of o-nitroimidazoles the 2-H proton adjacent to both nitrogen atoms is involved in the loss of OH. Fragmentation of each compound is specific and study could eventually provide a satisfactory means for structure determination.     

Synthesis and Antibacterial Activity of Sulfur‐linked Bis and Tris Heterocycles

The bis and tris heterocycles‐benzoxazolyl/benzothiazolyl/benzimidazolyl quinazolines linked by sulfur and/or nitrogen were prepared from 2,4‐dichloroquinazoline and benzazolyl‐2‐thiol/benzazolyl‐2‐amine and studied their antibacterial activity. The nitro‐substituted sulfur‐linked bisbenzothiazolylquinazoline (12f) , bisbenzimidazolylquinazoline ( 13f ), and nitro‐substituted sulfur and nitrogen‐linked bisbenzothiazolylquinazoline ( 15f ) were found to be potential antibacterial agents against Staphylococcus aureus .      

The gas enhanced negative ion mass spectra of polychloroanisoles

Methane or a methane–oxygen mixture was used as an enhancement gas to obtain negative ion mass spectra of polychloroanisoles. Dichloroanisoles did not react with oxygen but the more highly chlorinated anisoles did. Compounds with hydrogen ortho to the methoxy group had [M? 1]^? ions, while others gave . The fragment arose through loss of an ortho chlorine and amethyl hydrogen. The loss of HCl followed by oxygen displacement of a remaining ortho or para chlorine produced [M? 55]^? ions; the para position was the preferred site of displacement. Another ion-molecule reaction with oxygen leads to [M? CH₂Cl]^?. The fragmentations resemble those of chlorinated aromatics such as the polychlorodibenzodioxins.     

Novel Ortho effects detected in the fast atom bombardment mass spectra of substituted bisaryl phosphate antagonists of platelet-activating factor

A series of substituted bisaryl phosphate compounds, (R¹CH₂)⁺ ArOP = O(O^?)(OArR²R₃), was analyzed and characterized by fast atom bombardment mass spectrometry. Abundant fragment ions were observed and correlated with the proposed structures. From fragmentation pattersn, ‘ortho effect’ reactions were demonstrated to have occurred when the phosphoryl oxygen reacted with the (CH₂R¹)⁺ and C?O(OCH₃) substituents in the ortho position, relative to the phosphate group, and displaced the R¹ and OCH₃ groups, respectively, to produce phosphorus containing six-membered rings fused to the aryl moiety. When the (CH₂R¹)⁺ substituents were in the meta position relative to the phosphate group, the ‘ortho effect’ reactions were not observed. However, when the C?O(OCH₃) substituent was in the meta position relative to the phosphate group, an abundant fragment ion containing a five-membered phosphate ring fused to the aryl ring was detected with the original phosphoryl oxygen ortho to both the phosphate oxygen and a formyl group, formed from the original C?O(OCH₃) substituent. All other fragmentations not involving the ‘ortho effect’ reactions were nearly identical for the different structural isomers of the substituted bisaryl phosphate compounds.     

Organische Schwefelverbindungen—IV: Elektronenstossinduzierte Redoxreaktion von ortho-Nitrothiobenzoesäure-S-p-tolylester

The electron-impact fragmentation of S-p-tolylester of nitrothiobenzoic acid leads to a previously unknown rearrangement in the case of the ortho compound. This consists of an intra molecular redoxreaction with oxygen transfer from the o-nitro group onto the sulphur of the thiol ester group. The position of the nitro group is not only an essential influence on the fragmentation pattern, but also of dramatic importance in determining the lifetime of the molecular ion.