Alkene Loss Research Articles

Beitrag zum Mechanismus der stossinduzierten Fragmentierungen von quaternären Stickstoffverbindungen. 2. Mitteilung über stossinduzierte Fragmentierungen von felddesorbierten Kationen

Mechanistic studies on the collision‐induced fragmentations of quaternary ammonium ionsThe collision‐induced fragmentation behaviour of field desorbed quarternary ammonium ions has been investigated. A main reaction of these ions is the cleavage of the NC bond accompanied by hydrogen rearrangement, i.e. alkane loss from the tetraalkyl substituted ammonium ions of the iodides 1, 2 and 3, respectively, Deuterium labelling indicates that the hydrogen transfer to the leaving group occurs to the extent of about 80% from the α‐position and about 20% from the other positions of an alkyl group. Pronounced heterolytic cleavage of the NC bond is observed in the benzyl substituted ammonium ion of 4. The β‐phenylethyl substituted ammonium ion of 5 shows a homobenzylic heterolysis, possibly yielding the phenonium ion j.

The mass spectrometric fragmentation of n‐heptane

AbstractThe 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.

Collisional Activation Spectra of Quaternary Phosphonium and Tertiary Sulfonium Cations Produced by Field Desorption

The collisional activation spectra of a variety of tetraalkyl phosphonium and trialkyl sulfonium ions generated by field desorption are reported. The fragmentation behaviour of the onium ions is compared with that of ammonium ions, reported earlier [4] and the corresponding open shell amine, phosphine and sulfide molecular ions. Cleavage of the heteroatom -carbon bond, with or without hydrogen rearrangement (i.e. alkyl and alkane loss) leads to prominent fragment ions in all onium ions. Pronounced heterolytic cleavage of this bond was only observed in sulfonium ions. No direct ɑ-cleavage of the cation is observed, however, ɑ-cleavage can occur in a second step after elimination of a substituent. This process leads to the base peak with ammonium ions, occurs less frequently in sulfonium ions and cannot be observed unequivocally in phosphonium ions. In addition, the collisional activatiou spectra of phenylalkyl phosphonium ions and chlorine containing quaternary phosphonium ions are discussed. It is demonstrated that collisional activation in conjunction with field desorption is a useful technique for the structure elucidation of onium cations

Hydrogen migrations in mass spectrometry. V—loss of olefin from n‐propyl esters following chemical ionization

AbstractThe sources of the migrating hydrogen in the elimination of propylene from the protonated and ethylated n‐propyl acetate and n‐propyl benzoate molecules have been determined by studying the CH4 and H2 chemical ionization mass spectra of esters specifically deuterated in the propyl group. It is shown that the migrating hydrogen originates from C‐1 ( ∼ 27%), C‐2 ( ∼ 23%) and C‐3 ( ∼ 50%) of the propyl group, independent of ester and mode of ionization. It is argued that the observed reaction involves specific competing H‐migration reactions from each propyl position to the ether oxygen in a keto‐protonated (ethylated) ester molecule.

The mass spectrum of 1‐heptyl iodide

AbstractThe mass spectra of twelve 13C labeled compounds have been measured in order to investigate the fragmentation of the 1‐heptyl iodide molecular ion. A mechanism is proposed for each reaction leading to the ions containing the iodine atom. The loss of olefin from the heptyl ion derived from the 1‐heptyl iodide proceeds via extensive rearrangement of the linear ion. A non‐linear intermediate structure obtained via protonated substituted cyclopropane is suggested to explain the major fragmentation reactions of the heptyl ion.

Unexpected course of olefin loss from some alkylcyclopentanone ions. Gaseous ion structure by ion cyclotron resonance spectrometry, mass‐analyzed ion kinetic energy spectrometry and collision‐induced dissociation

AbstractIon cyclotron resonance results show that the ions formed by single and by double McLafferty rearrangement in 2‐ethyl‐5‐propylcyclopentanone have neither keto nor enol structures. Collision‐induced dissociations confirm that these ions are structurally distinct from the keto ions formed directly by electron impact upon the corresponding neutral molecules. It is suggested that the major reaction path for olefin loss from 2‐ethyl‐5‐propylcyclopentanone and from 2‐ethylcyclopentanone involves ring opening followed by hydrogen transfer to carbon in the alkene elimination step. Only in metastable ions is there evidence for the occurrence of the normal McLafferty rearrangement. The techniques mentioned in the title, together with conventional low and high resolution mass spectrometry, have been used to characterize the sometimes complex mixtures of cyclic and acyclic ions formed from cyclopentanone and some of its alkyl derivatives. Use of a number of different techniques of ion structure characterization allowed corroboration of particular results by quite distinct methods and it also allowed the effects of ion internal energy and lifetime upon structure to be partly elucidated.

Hydrogen migrations in mass spectrometry. I—The loss of olefin from phenyl‐n‐propyl ether following electron impact ionization and chemical ionization

AbstractUsing specifically labelled compounds we have made a detailed study of the source of the hydrogen transferred in the elimination of C3H6 from the molecular ion of phenyln‐propyl ether following electron impact ionization and from the protonated (and ethylated) molecule following chemical ionization. The migrating hydrogen originates from all three positions of the npropyl group but not in the ratio expected for randomization of the alkyl hydrogens prior to transfer. The source of the migrating hydrogen is similar for both electron impact ionization and chemical ionization, indicating that the factors governing the rearrangement are the same for both modes of ionization. From a comparison of the results for labelled 2,6‐dimethyl phenyl n‐propyl ethers with the results for the unsubstituted ether it is concluded that hydrogen transfer occurs only to the ether oxygen and not to the phenyl ring. A two‐step mechanism involving a set of competing reversible hydrogen transfer reactions followed by CO bond cleavage is proposed to explain the results.

The mass spectra of some N‐substituted barbitals and their trimethylsilyl derivatives

AbstractThe mass spectra of N‐propyl and N‐butyl barbitals show the loss of olefin radical (CnH2n‐1) in analogy to structurally similar molecules such as N‐alkyl succinimides and 3‐alkyl uracils. Trimethylsilylation of the N‐substituted barbitals suppresses this fragmentation and loss of olefin via apparent McLafferty rearrangement from the even‐electron ion, [M – 15]+, becomes significant. The trimethylsilyl derivatives of N‐allyl barbital and N‐phenyl barbital show an unusually facile elimination of the appropriate isocyanate from the molecular ion.

Ligand displacement reactions. Part V. The mechanisms of displacement of bidentate olefins from olefin(tetramethyl-1,4-benzoquinone)-nickel(0) complexes by trimethyl phosphite

Tributylphosphine reacts with the complexes olefin(tetramethyl-1,4-benzoquinone)nickel (olefin = cyclooctatetraene or cyclo-octa-1,5-diene) to give bis(tributylphosphine)(tetramethyl-1,4-benzoquinone)nickel. With trimethyl phosphite the olefin complexes give tetrakis(trimethyl phosphite)nickel and, although intermediates cannot be detected, it is shown that displacement of the olefin precedes that of the quinone. The rate laws found for the reactions of trimethyl phosphite (P) with various olefin complexes (C) are for cyclo-octa-1,5-diene, rate ={k1[C][P]+k2[C][P]2}/(1 +k3[P]); for cyclo-octatetraene and norborna-2,5-diene, rate =k1[C][P]+k2[C][P]2; for endo-dicyclopentadiene rate =k1[C][P]. The reactions of trimethyl phosphite with bis(tetramethyl-1,4-benzoquinone)nickel and of tributylphosphine with the cyclo-octa-1,5-diene complex have rate =k1[C][P]. The results are accommodated by a mechanism which involves initial bimolecular reaction of (C) and (P) to form an intermediate (A) in which the olefin is monodentate. Intermediate (A) can undergo unimolecular reversion to (C) and (P), unimolecular loss of olefin, or bimolecular reaction with (P) with loss of olefin. Subsequent reactions with trimethyl phosphite are fast. Stationary state theory applied to [A] gives a rate equation of the same form as that found for reaction of the cyclo-octa-1,5-diene complex with trimethyl phosphite. The rate laws for the other complexes are limiting forms of this equation. Primary rate constants are derived and the rate of initial reaction of (P) with (C) is shown to depend on olefin in the order: (tetramethyl-1,4-benzoquinone) cyclo-octa-1,5-diene < endo-dicyclopentadiene < norborna-2,5-diene and cyclo-octatetraene.

Electron‐impact induced fragmentation of alkylthiopyridines. The extent of ring nitrogen involvement

AbstractThe fragmentation patterns obtained upon electron impact of 2‐, 3‐, and 4‐alkylthio‐pyridines were examined to determine the extent that the ring nitrogen is involved. The effect of the size of the alkyl group, as well as the ring position of the sulfide on the fragmentation of the methyl‐, ethyl‐, n‐propyl‐, n‐butyl‐ and t‐butylthiopyridines, is discussed. The mass spectrum of 2‐n‐octylthiopyridine is also recorded.The molecular ions decomposed with rupture of the bonds α, β, γ, and δ from the hetero‐aromatic ring. The molecular ions also exhibited the loss of alkenes via a number of mechanisms, notably the loss of Cn‐1 H2n from the alkyl group of 2‐alkylthiopyridines by a transition involving the rupture of a C‐C bond γ to the ring and the transfer of a proton on a carbon ϵ to the ring. The role of the pyridine sp2 nitrogen in this and other fragmentations is discussed. An elimination of HS radical is also observed in a number of these alkylthiopyridines.

Formation of [M — C2H4]+· in O‐d1‐butyric acid

AbstractThe mass spectrum of CH3CH2CH2COOD shows no observable loss of C2H3D. This indicates for this compound that either the McLafferty rearrangement is concerted, or that the second step (olefin loss) is very fast in comparison to the reverse transfer of hydrogen back to the methylene radical.

Mass-Spectral Fragmentation of Phenyl-n-Alkanes

The fragmentation mechanisms for the 1-phenyl and 2-phenyl alkanes were found to differ from those having substituted two or more carbon atoms from the terminal carbon atom. The latter series of compounds have two competing primary fragmentation reactions involving loss of either alkyl group. The loss of the smaller alkyl group has the lower appearance potential, but the competing reaction to lose the larger alkyl group increases in yield more rapidly at the higher electron energies, and it has greater intensity at 70 eV. The ratio of intensities of these two ions correlates with the relative position of the phenyl group from the center of the chain for phenyl alkanes of different carbon numbers. Subsequent decomposition of these primary ions is by successive loss of olefins of C3 through C6. The 2-phenyl compounds differ in that subsequent skeletal decomposition of the CH3C+HC6H5 is less probable, thus making this ion the major ion instead of C7H7+. The 1-phenyl alkanes decompose directly to C7H7+ for the lower-carbon-number compounds, while the larger molecules form toluene ion followed by hydrogen-atom loss to C7H7+ and other fragmentation reactions. Partial isomerization to the 4-phenyl compounds is suggested. Attempts to explain the higher yield of the higher-energy primary path for the internally substituted compounds within the framework of current theoretical models were unsuccessful. It is suggested that the usual method of applying the quasiequilibrium assumption is not valid for molecules of this size and degree of localization of charge and energy.