Mass spectrometry of heterocyclic compounds. V—Substituent effects on the fragmentation pathways of 3,5-diphenyl-1,2,4-oxadiazole derivatives

The 70 eV mass spectra of a number of derivatives of 3,5-diphenul-1,2,4-oxadiazole substituted either in the 5-phenyl (fifteen) or in the 3-phenyl (four) position, have been studied using exact mass measurements and metastable determinations by the defocusing technique. The substituent effects on the heterocyclic cleavage are not very important for electron-withdrawing and weak electron-donating groups. The main cleavage is the formal retro 1,3-dipolar cycloaddition with the positive charge retained by the C₇H₅NO (orYC₇H₄NO in the case of the 3-substituted derivatives) fragment specifically containing the 3-phenyl, as oreviously observed for 3,5-diphenyl-1,2,4-oxadiazole using labelling experiments. Two other minor primary processes leading to benzoyl and nitrile ions, both containing the 5-phenyl, become important in the case of the electron-donating substituted compounds. Correlation of the abundance of these fragmednts and of the molecular ions with σ⁺ (para) Brown constants is discussed. Proximity effets are shown by some ortho derivatives.     

Mass spectrometry of hop compounds—II: Mass spectra of related oxygen heterocyclic compounds

Mass spectra have been determined of a number of phloroglucinol derivatives and closely-related compounds encountered in chemical studies of hop components. Initial fission involving acyl, alkyl or alkenyl side-chains gives predominantly even-electron ions which then fragment their oxygen heterocyclic rings by elimination of neutral molecules, whilst gem-dialkenyl centres show preference for loss of an alkenyl group. The fragment ions induced by electron-impact may be correlated to a useful extent with molecular structure.     

Mass spectrometry of heterocyclic compounds—III. Electron-impact-induced fragmentation of 1,2-benzisothiazole and some 3-substituted derivatives

The mass spectra of fifteen 1,2-benzisothiazoles are reported; their fragmentation patterns have been investigated by labelling experiments, high resolution mass measurements and defocused metastable ion detection. The parent compound of the series, 1,2-benzisothiazole, eliminates HCN after partial inter-ring hydrogen scrambling. The extent of scrambling has been determined for normal daughter ions produced at different electron beam energies and also for metastable daughter ions, and is compared with the data reported for benzothiazole. Some 3-substituted derivatives show unusual fragmentation patterns. The mechanisms of these processes are under further investigation.     

Mass spectrometry in structural and stereochemical problems—CLXXIV:1 Electron-impact induced fragmentation of some aryl- and alkylsulfonylthioureas

The mass spectra of five aryl (I to V) and four alkylsulfonylthioureas (VI toIX) have been recorded and mechanistic rationalizations are suggested for their principal fragmentation processes. The aryl analogs exhibited peaks in their mass spectra corresponding to skeletal rearrangements with elimination of SO₂ from their molecular ions but this fragmentation was absent in those alkylsulfonylthioureas (VI to IX) examined.     

Mass spectra of organoarsenic compounds—II: Electron-impact-induced fragmentation of 2-substituted-1,2,2-dioxarsenanes

The electron-impact-induced fragmentation of 2-substituted 1,3,2-dioxarsenanes has been studied. The main fragmentation modes have been determined with the use of high resolution mass measurements and by application of the metastable defocusing technique. The predominant fragmentation for the 2-alkyl-1,3,2-dioxarsenanes proceeds via the loss of the 2-substituent from the molecular ion. In the case of 2-phenyl-1,3,2-dioxarsenanes elimination of the phenyl group competes with the formation of a C₆H₅ As ion as well as loss of aldehyde from the molecular ions.     

Mass spectrometry of onium compounds—II. Carnitine and O-acyl derivatives

Carnitine and its O-acyl derivatives undergo two major pyrolytic reactions in the mass spectrometer. Firstly elimination of water from carnitine or acid from acylcarnitine takes place followed by intramolecular displacement and formation of crotonyl lactone and trimethylamine. Secondly intramolecular displacement occurs with formation of a substituted γ-lactone and trimethylamine. For the lower acid derivatives only the elimination pathway is important. For carnitine and higher derivatives both processes are important. The electron induced fragmentations of the major pyrolysis products are relatively simple. The most important pathways for the major components are discussed. The pyrolytic composition and mode of fragmentation are characteristic and can be used for identification purposes.     

Stepwise retro 1,3-dipolar cycloaddition induced by electron. Impact on 3,5-diphenyl-1,2,4-oxadiazole

The retro 1,3-dipolar cycloaddition induced by electron impact on 3,5-diphenyl-1,2,4-oxadiazole is interpreted as a two-step process on the basis of the energetics and kinetics of the fragments [C₇H₅NO]⁺., [C₆H₅CN]⁺. and [C₆H₅CO]⁺.     

Mass spectrometry of heterocyclic compounds. XV—The mechanism of the fragmentation under electron impact of 3-p-NH2-phenyl-5-phenyl-1,2,4-oxadiazole. A CNDO/2 treatment

Molecular orbital calculations using the semi-empirical CNDO/2 method were carried out on the molecular ion of 3-p-NH₂-phenyl-5-phenyl-1,2,4-oxadiazole, whose structure had been determined by X-ray diffraction. The calculated diatomic interaction energy values are consistent with the mechanistic proposals made previously in terms of the quasi-equilibrium theory concerning the cleavage of the heterocycle ring after electron impact.     

Mass spectra of 1,2,4-triazoles—II: Alkyl 1,2,4-triazoles

The mass spectra of monomethyl 1,2,4-triazoles contain fragment ions produced by specific cleavage of the heterocyclic ring. A major fragmentation from many molecular ions involves the elimination of HCN, but loss of N₂ is either very small or completely absent. No N or H scrambling occurs within the triazole ring system, as evidenced by labelling studies. The loss of a hydrogen atom from the molecular ions of 3-alkyl-1,2,4-triazoles (alkyl ? C₂H₅) originates from hydrogens attached to the β carbon and nitrogen atoms.     

The chlorinalion of 3,5-dimethyl-1,2,4-oxadiazole

The clilorination of 3,5-dimethyl-1,2,4-oxadiazole has been studied using both photo and thermal initiation. It has been found that photoinitiation allows reaction at both the 3- and 5-methyl group, the former being preferred by a factor of 3-4 at moderate (up to 75° ) temperature. The difference in reactivity is related to a difference in electron availability at the two reaction sites. The thermally initiated chlorination occurs almost exclusively at the 5-methyl group. Overall, 3,5-dimethyl oxadiazole is much less active than toluene or cyclo-liexane toward chlorination.     

Mass spectrometry of heterocyclic compounds VIII–electron-impact-induced fragmentation of 1,2-diphenyl-pyrazolidine-3,5-dione and some 4-substituted derivatives

The mass spectra of 1,2-diphenyl-pyrazolidine-3,5-dione and twenty-one 4-substituted derivatives are reported. Their fragmentation patterns have been studied by deuterium labelling, exact mass measurements, metastable studies by the defocusing technique and low energy spectra. Hydrogen rearrangements from the 4-position of the heterocycle and/or from the ß-position of the 4-substituent groups, lead to the main primary fragment ions [C₁₂H₁₁N₂]⁺ (m/e 183) as shown by the metastables. The 4,4-d₂ derivative shows an appreciable isotope effect even for molecular ions decomposing in the ion source. By comparison with the metastable abundances of competitive reactions, the molecular ions (m/e 252) of the 4-unsubstituted compound appear to be structurally different from the corresponding m/e 252 fragment ions formed from 4-derivatives by the loss of 4-substituent with H rearrangement. If only vinylic or aromatic hydrogen atoms are present, primary cleavage of the heterocyclic ring occurs with loss of OH·, C₃O₂ and C₃HO₂. Important rearrangements leading to elimination of C₆H₆N and C₆H₇N are typical for unsaturated substituents on position four having allylic hydrogen atoms. Fragment ions, identical to molecular ions of some compounds discussed here, are obtained by electron-impact and/or thermal decompostion of some complex compounds containing more than one 1,2-diphenyl-pyrazolidine-3,5-dione system. The [C₆H₅N₂]⁺ (m/e 105) and [C₆H₅]⁺ (m/e 77) ions are common fragments of all the title compounds. Any hydrogen scrambling reactions between phenyl and heterocycle or 4-substituent groups can be excluded.     

Mass spectrometry of onium compounds—XI: Ionization potentials of hydroxy and mercapto pyridines

Ionization potential measurements of the volatile species from the three isomers of the tautomeric hydroxyl and mercapto pyridines show that these are in the hydroxy and mercapto forms rather than appearing as lactams and betaines. The corresponding N-methyl lactams and S- and O-methyl ethers were also studied for comparative purposes.     

Mass spectrometry of onium compounds—XVII: Anilinium carboxylates. orthoeffect in anthranilic acid derivatives

The N,N,N-trimethylanilinium carboxylates are transmethylated to the isomeric esters before evaporation in the mass spectrometer. Pyroltic reactions and electron-impact fragmentations are elucidated by means of deuterio analogues. The orthoisomer behaves anomalously in that initial expulsion of a methyl radical is important. Deuteration has shown this reaction to take place along two distinct routes, either from the nitrogen or from the ester.     

Electron-impact induced fragmentation of dimethanonaphthalene-type compounds

The mass spectra of endo,endo-1,2,3,4,4a,5,8,8a-octahydro-1,4:5,8-dimethanonaphthalen-anti-2-01 (Ia) and its endo,exo-2-ol (IIa) and endo,exo-10-ol (IIIa) isomers have been examined. Fragmentation schemes have been constructed on the basis of metastable transition measurements, accurate mass determination and deuterium substitution. A presence of the double bond in the molecule and a favourable hydrogen atom orientation in the vicinity of the double bond play an important role in the ion degradation process.     

Mass spectrometry of onium compounds—XX: Ionisation potential in structural assignment of some gaseous molecules

The ionisation potentials for a series of thiazolo[3,2-a]pyridinium-3-oxides are reduced by about 1.7 to 1.8eV on introduction of an 8-OH group. The gaseous species from the former compounds are ascribed a mesoion, structure while the hydroxy-derivatives are rearranged to non charged molecules.     

Mass spectrometry: XI—Electron impact induced fragmentation of some amides of F-acids and their deuterated homologues

The mass spectrometry of perfluoro compounds (F-alkyl compounds) has seldom been the subject of systematic studies. Fluorocarbons excepted, only a few mass spectra of such compounds have been analysed and corresponding fragmentations correlated. In this paper we report the mass spectra of 19 amides of perfluoro acids (R_FCONHR) with R=benzyl, 2-phenylethyl, 2-phenylpropyl, 3-phenylpropyl, 2-(N-phenylamino)ethyl, and R_F=F-methyl, n-perfluoropropyl, n-perfluoropentyl and n-perfluoroheptyl. These compounds exhibit quite different behaviour from their hydrocarbon homologues under electron impact (for instance no [R_FCO]⁺ fragment was found). Specific deuterium labelling and high resolution measurements have been used to show typical rearrangements and to establish the fragmentation routes.     

Mass spectrometry of hop compounds—III. Mass spectra of substituted 1,3-cyclopentanediones derived from hop constituents

The mass spectra of isohumulones (IV, V), tetrahydroisocohumulones (VI, VII), tetrahydroisohumulones (VIII) neohydroisocohumulones (XI, XII) cohumulinic acid (III), lupuloxinic acid (XV), humulinone (XIV) and related compounds are described. Ions which appear to be diagnostic for particular structures are discussed.     

Mesoionic compounds—XIX: Mass spectral fragmentation patterns for some five-membered systems containing oxygen,nitrogen and sulfur

Mesoionic compounds of the s-triazole, 1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,2,3-thiadiazole, 1,3-thiazole and 1,3-dithiole systems all underwent deep-seated decomposition on electron-impact to small stable ions, determined to a large extent by the nature of the peripheral heteroatoms and the exocyclic substituent. In some instances fragmentation most probably involved an open-chain isomer of the cyclic system.     

Thermal fragmentation of 3,5-diaryl-1,2,4-oxadiazoles

3,5-Diaryl-1,2,4-oxadiazoles have been shown to undergo thermal fragmentation, and the pathway for the transformation is discussed.