Isomerization and fragmentation of methyl benzofuran ions and chromene ions in the gas phase

The rearrangement of the molecular ions of the isomeric 2- and 3-methyl benzofurans (1 and 2), 2H-chromene (3) and 4H-chromene (4) has been studied as a further example of the isomerization of oxygen-heteroaromatic radical cations via a ring expansion/ring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1⁺˙?4⁺˙ fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect k_H/k_D, which arise from a rate-determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the behaviour of alkyl benzene ions, a unidirectional isomerization of the methyl benzofuran ions by ring expansion takes place but no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.     

Unimolecular reactions of isolated organic ions: Some isomers of [C7H16]+˙

The slow unimolecular reactions of six isomers of [C₇H₁₆]^+˙ are reported and discussed. These results are interpreted in terms of dissociation via complexes of incipient carbonium ions and the appropriate associated radical. In some cases, rearrangement of the incipient carbonium ion precedes or accompanies decomposition; such isomerization generally favours alkyl radical loss, relative to elimination of the corresponding alkane.     

The mass spectrometric fragmentation of n-heptane

The electron impact fragmentation of n-heptane has been investigated using ¹³C labelled derivatives. A mechanism is proposed for the loss of alkyl radicals where the cleavage of a C? C bond is coupled with the rearrangement of a hydrogen atom, thus yielding a secondary alkyl ion that eventually fragments further by a subsequent loss of olefin. For alkyl ions with less than six carbon atoms this consecutive pathway is in competition with formation directly from the molecular ion. The consecutive pathway contributes about 15% to the intensity of the alkyl ions with four and five carbon atoms and 80% for smaller ions. The electron energy dependence was studied.     

Hydrogen rearrangements of ionized propanoic acid and isomeric ions

Ionized propanoic acid and its enol isomer lose H^˙ in both normal and metastable decompositions. These C₃H₆O₂ ions lose their distinctness prior to undergoing metastable decomposition in the mass spectrometer. Deuterium labeling indicated that this results from the transfer of a β-hydrogen to oxygen in the propanoic acid ion followed by rapid exchange of hydrogen and deuteriums between the α- and β-carbons and slower exchange between the carbons and the oxygens. 3- and 5-Membered ring hydrogen rearrangements are preferred over 4-membered ring transfers. It is concluded that 1,2-hydrogen shifts following removal of hydrogen from alkyl chains may sometimes cause site-specific hydrogen rearrangements to appear non-site specific.     

VUV Photoionization and Dissociation of Tyramine and Dopamine: the Joint Experimental and Theoretical Studies

Photon induced dissociation investigations of neutral tyramine and dopamine are carried out with synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. At low photon energy, only molecular ions are measured by virtue of near-threshold photoionization. While increasing photon energy to 11.7 eV or more, four distinct fragment ions are obtained for tyramine and dopamine, respectively. Besides, the ioniza-tion energies of tyramine and dopamine are determined to be 7.98±0.05 and 7.67±0.05 eV by measuring the photoionization efficiency curves of corresponding molecular ions. With help of density function theory calculations, the detailed fragmentation pathways are es-tablished as well. These two molecular cations have similar aminoethyl group elimination pathways, C₇H₈O₂^+·(m/z=124) and C₇H₈O^+·(m/z=108) are supposed to be generated by the McLafferty rearrangement via γ-hydrogen (γ-H) shift inducing β-fission. And CH₂NH₂⁺ is proposed to derive from the direct fission of C7-C8 bond. Besides, the McLafferty rear-rangement and the C7-C8 bond fission are validated to be dominant dissociation pathways for tyramine and dopamine cations.     

The mass spectra of diaziridinones

The electron-impact induced fragmentation of four N,N′-di-t-alkyl-substituted diaziridinones (I to IV) has been studied by both conventional and high resolution mass spectrometry. All diaziridinones exhibit weak molecular ions. Ejection of an alkyl isocyanate, corresponding to the N-alkyl substituent, from the molecular ion, is a dominant and general fragmentation process. Isocyanate-type odd-electron fragment ions occur only in III and IV (where at least one R group is phenyl) and are of low abundance. Elimination of a hydrocarbon radical from the tertiary alkyl substituents is observed in all compounds investigated. McLafferty rearrangement with elimination of a neutral alkene occurs in all compounds. Abundant even-electron hydrocarbon ions corresponding to the mass of the N-alkyl substituent are prevalent. The complete absence of elimination of carbon monoxide is noted. Loss of oxygen from the [M ? RCH²]+ species has been confirmed by accurate mass measurement. Several remarkable rearrangement reactions have been uncovered by high resolution studies and deuteration experiments.     

Studies in negative ion mass spectrometry. VII—Negative ion mass spectra of copper (II) β-diketonate complexes

Electron capture processes in a series of copper (II) β-diketonate complexes of formula Cu[R¹COCHCOR²]₂ (where R¹ is an alkyl, perfluoroalkyl or aryl group and R² either an alkyl or aryl group) have been examined. Molecular anions, ligand ions and some novel rearrangement ions have been observed with these compounds. Relative intensities of fragment ions were dependent on the substituents R¹, R² as well as the electron energy and compound pressure in the ion source. By operating the mass spectrometer at compound pressures of c. 4×10^?6 Torr and higher, reproducible negative ion mass spectra (free from any significant ion-molecule contributions) have been obtained for all compounds of the series.     

Valency changes of sulphur in the mass spectra of sulphones

The aryl substituent has been shown to affect the fragmentation and abundance of skeletal rearrangement ions in the spectra of aryl sulphonyl chlorides and sulphonic acids. The spectra of 2-hydroxy and 2-chloroethyl aryl sulphones contain [aryl SO]⁺ ions, suggesting that alkyl migration has competed successfully with aryl migration to the electron deficient oxygen atom.     

Tandem mass spectrometry of lithium-attachment ions from polyglycols

A detailed study has been carried out of the fast atom bombardment tandem mass spectrometry (MS/MS) behavior of lithium-attachment ions from three glycol polymers: linear poly(ethylene glycol), linear poly(propylene glycol), and an ethoxylated fatty alcohol. Collisional activation was carried out in the “collision octapole” of a BEoQ hybrid mass spectrometer at a translational energy of 50 eV, with collision gas air. It was found that [M + Li]⁺ ions provide a number of advantages as precursors for practical MS/MS analysis as compared to the use of [M + H]⁺ or [M + Na]⁺ ions. First, [M + Li]⁺ ions are much more intense than the corresponding [M + H]⁺ ions. Second, [M + Li]⁺ ions dissociate to lithiated organic fragments with reasonable efficiency, which is not the case with [M + Na]⁺ precursors. Third, product ions are generally formed over the entire mass range for low molecular weight polyglycols. The most intense product ions are lithiated, linear polyglycol oligomers. These ions are formed via internal hydrogen transfer reactions which are facilitated by lithium (charge-induced). Two series of less intense product ions are formed via charge-remote fragmentations involving l,4-hydrogen elimination. A fourth product ion series consists of lithiated radical cations; these form via homolytic bond cleavages near chain ends. Overall, MS/MS analysis of [M + Li]⁺ polyglycol ions proved to be quite useful for chemical structure elucidation.     

Destabilized carbenium ions: α-imidoyl carbenium ions and the electron impact mass spectra of α-haloaldimines and α-haloketimines

A series of α-chloro- and α-bromoketimines compounds (1-9) with different substituents at the α-position and at the imino group has been investigated by electron impact mass spectrometry as possible precursors of the correspondingly substituted α-imidoyl carbenium ion, an important class of destabilized carbenium ions. The main fragmentation of the molecular ions of compounds, 1-9 in the ion source corresponds to an α-cleavage at the imino group; however, fragment ions are also formed by loss of the α-halo substituent. These fragment ions correspond at least formally to α-imidoyl carbenium ions. Their further reactions in dependence on the type of substituents at the imino group and at the α-C atom, were studied by mass-analysed ion kinetic energy and collisional activation mass spectrometry. The results agree with the initial formation of destabilized α-imidoyl carbenium ions but indicate an easy rearrangement of these ions in the presence of suitable alkyl substituents by 1,2- and 1,4-hydrogen shifts to more stable isomers.     

The mass spectra of alkyl phenyl tellurides

The mass spectra of several alkyl phenyl tellurides, C₆H₅TeR (R = CH₃, CD₃, C₂H₅, n-C₃H₇, i-C₃H₇ and n-C₄H₉) have been studied with special emphasis on the fragmentation patterns involving cleavage of the alkyl and aryl tellurium–carbon bonds. Each compound exhibited intense parent ions. The rearrangement ions [C₆H₆Te]⁺? and [C₆H₆]⁺? were found in the spectra of phenyl ethyl and higher tellurides. Two other rearrangement ions [HTe]⁺ and [C₇H₇]⁺ were observed in the spectrum of each compound. Examination of the mass spectrum of phenyl methyl-d₃ telluride demonstrated that the [HTe]⁺ ions derive hydrogen from the phenyl group.     

Rhodium(III)-Catalyzed Asymmetric [4+1] and [5+1] Annulation of Arenes and 1,3-Enynes: A Distinct Mechanism of Allyl Formation and Allyl Functionalization

We report chiral Rh^III cyclopentadienyl-catalyzed enantioselective synthesis of lactams and isochromenes through oxidative [4+1] and [5+1] annulation, respectively, between arenes and 1,3-enynes. The reaction proceeds through a C−H activation, alkenyl-to-allyl rearrangement, and a nucleophilic cyclization cascade. The mechanisms of the [4+1] annulations were elucidated by a combination of experimental and computational methods. DFT studies indicated that, following the C−H activation and alkyne insertion, a Rh^III alkenyl intermediate undergoes δ-hydrogen elimination of the allylic C−H via a six-membered ring transition state to produce a Rh^III enallene hydride intermediate. Subsequent hydride insertion and allyl rearrangement affords several rhodium(III) allyl intermediates, and a rare Rh^III η⁴ ene-allyl species with π-agostic interaction undergoes SN₂′-type external attack by the nitrogen nucleophile, instead of C−N reductive elimination, as the stereodetermining step.     

Abstraction of β-hydrogen vs. alkyl groups in reactions of dialkylzinc compounds and bis(oxazolinyl)borane

An ambiphilic bis(oxazolinyl)borane proligand and zinc dialkyls react via alkyl group transfer or β-hydrogen abstraction. The latter process is favored by formation of a bis(oxazolinyl)borane-zinc adduct that positions a β-hydrogen in the proximity of the Lewis acid center.     

Electron impact mass spectrometry of some geminal bis-sulphones

The behaviour under electron impact of six 1,1-bis(benzenesulphonyl)cyclioalkanes and of 1-phenyl-1-(benzensesulphaonyl)cyclopropane has been studied in detail with the aid of exact mass measurements, linked scans for metastable ion analysis, collisional spectroscopy and kinetic energy release measurements. The molecular ions of these compounds undergo a sulphone-sulphinate rearrangement with alkyl group migration on oxygen, in analogy with what is found for simple monosulphones and, in general, their fragmentation resembles that of mono-sulphonyl compounds. Loss of SO₂ from the molecular ion is observed for all substrates, but only in the case of 1, 1-bis(benzenesulphonyl)cyclopropane is this followed by loss of the second SO₂ unit; the first loss of SO₂ is probably accompanied by rearrangement since the fragmentation pattern of [M ? SO₂]⁺˙ ions from this compound is different than that of the isobaric molecular ions of 1-phenyl-1-(benzenesulphonyl)cyclopropane.     

The loss of a methyl radical and the retro diels—Alder reaction in the electron impact mass spectrometry of 2-cyclohexen-1-ol and related compounds

The electron impact mass spectra of 2-cyclohexen-1-ol and of several of its ²H and ¹³C labelled analogues show that the molecular ions lose a methyl radical by a completely different means from the mechanism described previously. Moreover, the retro Diels–Alder reaction also proceeds in a non-classical way; in addition to the elimination of an olefinic molecule from unrearranged molecular ions, a second more important route implies a formal 1,3 allylic rearrangement prior to the retro Diels–Alder reaction. The mass spectra of a series of alkyl substituted homologues show that the competition between the two processes is closely related to the size of the olefinic moiety that is expelled.     

Mass spectra of alkenyl methyl ethers

Electron impact ionization mass spectra of numerous alkenyl methyl ethers C_nH_2n-1OCH₃ (n = 3–6) recorded under normal (4 kV, 70 eV, 175°C) and low-energy, low-temperature (8 kV, 12 eV, 75 °C) conditions are reported. The influence of the position and stereochemistry of the double bond on the dissociation of ionized alkenyl methyl ethers is discussed. The mechanisms by which these ethers fragment after ionization have been further investigated using extensive ²H-labelling experiments and by studying the energy dependence of the reactions. Ethers of allylic alcohols show spectra that are distinct from those of the isomeric species in which the double bond is separated by one or more sp³ carbon atoms from the carbon atom carrying the methoxy group. Three principal primary fragmentations are observed. The most common process, especially for ionized ethers of allylic alcohols, is loss of an alkyl group. This reaction often occurs by simple α-cleavage of radical-cations of the appropriate structure; however, alkyl groups attached to either end of the double bond are also readily lost. These formal β- and γ-cleavages are explained in terms of rearrangements via distonic ions and, at least in the case of γ-cleavages, ionized methoxycyclopropanes. Ionized homoallyl methyl ethers tend to eliminate an allylic radical, particularly at high internal energies, with formation of an oxonium ion (CH₃ ⁺O?CH₂ or CH₃ ⁺O?CHCH₃). The ethers of linear pentenols and hexenols show abundant [M - CH₃OH]⁺? ions in their spectra, especially when a terminal methoxy group is present Methanol loss also takes place from ionized ethers of allylic alcohols in which there is a Δ-hydrogen atom; this process is significantly favoured by cis, rather than trans, stereochemistry of the double bond.     

The synthesis of N-substituted ketimines from oxazirdines

N-Unsubstituted diaryl, alkyl aryl and dialkyl ketimines are prepared in good yield at ambient temperature by the reaction of KOBu^t on oxaziridines having an α-hydrogen atom on the N-alkyl substituent; the mechanism and stereochemistry of the reaction are discussed.     

Reactivity of tert-butoxy radicals towards thiols,alkyl sulfides,and alkyl disulfides

Rate constants for the reactions of tert-butoxy radicals (generated by the thermal decomposition of di-tert-butylperoxyoxalate) with several sulfur containing compounds have been measured at 310 K in benzene. Hexanethiol (k = 6.5 × 10⁷M^?1s^?1) reacts considerably faster than alkyl sulfides and disulfides. For these compounds the reaction rate constants are slightly dependent on the α-hydrogen type, changing (when it is expressed per hydrogen atom) only a factor 5 for sulfides and 3 for disulfides when the α-hydrogen is changed from primary (methyl) to tertiary (isopropyl). The data obtained are compared to those found for the deactivation of the benzophenone triplet. Values of k_tert-butoxy/k_benzophenone range from ca 10^?3 (di-tert-butyl disulfide) to 7.5 (hexanethiol). The results obtained are rationalized in terms of bond strength, steric hindrance, and charge transfer contributions to the critical configuration energies.     

Ion-molecule reactions in alkyl cyanides: Identification of reactive hydrogen in precursor ions

The relative abundance of [M + H]⁺ ions in the spectra of different nitriles depends on the nature of the nitrile. A new method for the identification of ion-molecule reactions has been applied, by determining the [M + D]⁺ ion intensity with respect to the [M + H]+ ion intensity in the spectra of partially deuteriated alkyl cyanides. This intensity ratio is correlated with the hydrogen-deuterium content of the suspected primary ions. In addition not only the reacting primary ions, but also the reactive hydrogen atom in the primary ion could be indicated. There is clear evidence that the proton attached to the nitrogen atom in the H₂C?C?N⁺˙? H rearrangement ion is transferred to the nitrile molecule.     

Intramolecular rearrangements of silenes II . Synthesis of 1-methyl-1-hydro-1-silaacenaphthene from 1-methyl-1-naphthyl-1-silacyclobutane via [4 → 2 + 2]thermocyclodecomposition — transient silene rearrangement sequence. Novel 1,4-hydrogen shift from aryl carbon to sp2 silicon of the silicon-carbon double bond

A clean rearrangement of 1-naphthyl-1-methylsilene generated by 1-naphthyl-1-methyl-1-silacyclobutane gas-phase [4 → 2 + 2]-thermocyclodecomposition results in 1-methyl-1-silacenaphthene. It involves a novel 1,4-hydrogen shift from carbon of an aromatic nucleus to an sp² silicon of the silicon-carbon double bond.