Alkene Loss Research Articles

Ion–neutral complex formation and hydrogen‐shifts in the alkene loss from ionized alkyl phenyl thioethers in the gas phase

AbstractThe mechanistic aspects of alkene loss from ionized ethyl‐and n‐propyl phenyl thioethers have been studied by use of deuterium labelling and tandem mass spectrometry. Loss of ethene from the molecular ion of ethyl phenyl thioether proceeds predominantly by a specific H‐shift from the β‐position of the ethyl group to the remaining part of the ion. In contrast, deuterium labelling reveals that elimination of propene from the molecular ions of the n‐propyl phenyl and n‐propyl 2,6‐dimethylphenyl thioethers involves three competing reactions: (i) a 1,2‐hydride shift assisted cleavage of the bond between the propyl group and the sulfur atom yielding an ion–neutral complex of a thiophenyoxy radical and a secondary propyl carbenium ion, which reacts further by proton transfer prior to dissociation; (ii) a partially reversible 1,5‐H shift from the β‐position to the 2‐ or 6‐position of the ring; and (iii) a 1,3‐H shift from the β‐position to the sulfur atom. The preference for transfer of a hydrogen atom from the β‐position is particularly pronounce for the reactions of the metastable molecular ions of the n‐propyl phenyl and n‐propyl 2,6‐dimethylphenyl thioethers. This indicates that the critical energies of the processes involving H‐shifts are lower than the critical energy for propene loss with intermediate formation of an ion–neutral complex. In addition to ethene elimination, ionized ethyl phenyl thioether expels a HS+· radical. This reaction is preceded by an interchange between the hydrogen atoms at the α‐methylene group and the hydrogen atoms at the 2‐ and 6‐positions of the ring as observed also for ionized methyl phenyl thioether.

The role of ion–neutral reorientation in the unimolecular reactivity of e‐ionized amines. The influence of the size of the fragments

AbstractDuring the unimolecular decomposition of an ion IN→I(ion) + N(neutral), the fragments may mutually reorient before decomposition, when the energy has reached the reorientation critical energy. After I‐N reorientation, I and N have only a very low probability of recombining into the original IN structure, but can only decompose (or occasionally follow a different reaction pathway, if thermochemically allowed). As a consequence, decompositions of metastable ions are irreversibly decided as soon as their corresponding I‐N reorientation threshold has been reached. It is shown, from the mass‐analysed ion kinetic energy of several series of homologous primary amines, that the competition between α‐cleavages (and the occasionally accompanying losses of alkanes) is governed by the relative I‐N reorientation–(rather than decomposition)–critical energies.When the neutral has no permanent dipole, the I‐N reorientation critical energy depends on the sizes of the fragments, especially that of the fragment ion. An interpretation is proposed related to the McAdoo and Hudson hypothesis.27

APCI low energy collision-induced dissociation fragmentation of protonated ortho silicates: McLafferty or ion-neutral complex rearrangement?

The fragmentation mechanism of tetraethyl ortho silicate and tetrapropyl ortho silicate was studied to determine if the consecutive alkene losses observed in their MS/MS daughter ion spectra were produced via a McLafferty rearrangement or by ion-neutral rearrangement mechanisms. The experiments were carried out using atmospheric pressure chemical ionization and low energy collision-induced dissociation. Deuterium labeling of the γ-position provided evidence that the rearrangement mechanism of the successive alkene losses proceeds predominantly via the ion-neutral complex mechanism. Because the energies imparted to the ions in this experiment are of the same order of magnitude as solution phase reactions, this mechanism may shed light on the formation of silica gels (e.g., aerogels) in that similar structures have been proposed as reaction intermediates in the polymerization of SiO 2.

Fragmentation of vinyl triflates by electron impact. Mechanism of an unusual double hydrogen atom transfer reaction

AbstractThe main fragmentation pathway of vinyl triflates in mass spectra involves the loss of CF3SO, followed by the loss of olefin and/or CO. The loss of TfOH takes place with fragmentation of a vinylic CH bond. TfOH ions are formed via regiospecific double hydrogen transfer from five‐ and six‐membered cyclic triflates.

Ion–neutral reorientation and unimolecular loss of alkanes from organic ions in the gas phase

AbstractThe loss of alkanes from ionized aliphatic alkanes, alcohols, ketones, ethers or amines takes place by a 1,2‐elimination involving a CC bond cleavage and the specific rearrangement of a hydrogen atom to the incipient radical. This reaction should normally occur within a limited distance between the reactant sites. This condition is accomplished with most probability during a concerted (or quasi‐concerted) mechanism, before the completion of the CC bond cleavage. The same reaction may also occur, however, in ion–neutral complexes, i.e. by a non‐concerted mechanism, after the CC bond cleavage is completed, provided that the probability is large enough for ion–neutral complexes to adopt suitable reactant configurations. This depends essentially on the proportion of possible reacting sites in the whole molecular ion.

Unimolecular reactions of isolated organic ions: The chemistry of the oxonium ions CH3CH2CH2CH2+O = CH2 and CH3CH2CH2CH = O+CH3

AbstractThe reactions of the metastable oxonium ions CH3CH2CH2CH2+O = CH2 and CH3CH2CH2 = O+ CH3 are reported and discussed. Both these isomers of C5H11O+ expel predominantly CH2O (75–90% of the metastable ion current), a moderate amount of C3H6 (5–15%), a minor amount of CH3OH (2–8%) and a very small proportion of H2O (0.5–3%). All these processes give rise to Gaussian metastable peaks. The kinetic energy releases associated with fragmentation of these oxonium ions are similar, but slightly larger for dissociation of CH3CH2CH2CH = O+CH3. The behaviour of labelled analogues confirm that the reactions of CH3CH2CH2CH = O+CH3 are closely related, but subtly different. Elimination of CH2O and C3H6 is intelligible by means of mechanisms involving CH3CH+CH2CH2OCH3. This open‐chain cation is accessible to CH3CH2CH2 +O = CH2 by a 1,5‐H shift and to CH3CH2CH2‐CH = O+CH3 by two consecutive 1,2‐H shifts (or, possibly, a direct 1,3‐H shift). The rates of these 1,2‐, 1,3‐ and 1,5‐H shifts are compared with one another and also with the rates of CH2O and C3H6 loss from each of the two oxonium ions. The 1,5‐H shift that converts CH3CH+CH2CH2OCH3 formed from CH3CH2CH2CH = O+ CH3 into CH3CH2CH2+O = CH2 prior to CH2O elimination is essentially unidirectional. In contrast, the corresponding step converting C5H11O+ ions generated as CH3CH2CH2CH2+O = CH2 into CH3CH+ CH2CH2OCH3 competes effectively with expulsion of CH2O and C3H6. The implications of the latter finding for the degree of concert in the hydrogen transfer and carbon‐carbon bond fission steps in alkene losses from oxonium ions via routes that are formally isoelectronic with the retro ‘ene’ pericyclic process are emphasized.

Gas-phase photodissociation of (.eta.5-cyclopentadienyl)Fe(CO)2R and (.eta.5-indenyl)Fe(CO)2R in a molecular beam: competitive loss of alkyl radical and alkene detected by vacuum-ultraviolet ionization and time-of-flight mass spectrometry

Molecular beam photofragmentation measurements on (cyclopentadienyl)(alkyl)iron compounds directly determine the relative importance of Fe-alkyl bond homolysis and β-hydride elimination. The UV photodissociation of CpFe(CO) 2 CH 2 CH 3 (1, Cp=η 5 -cyclopentadienyl), CpFe(CO) 2 CH 2 CH 2 CH 3 (6), and InFe(CO) 2 CH 2 CH 2 CH 3 (7, In=η 5 -indenyl) leads to extensive fragmentation under collision-free conditions, and each of the products carries away little translational energy. Fragmentation of CpFe(CO) 2 CH 2 CH 3 (1) produces CpFeH, CpFe, CpH, Cp, Fe, and .CH 2 CH 3 , as detected by vacuum-UV ionization and time-of-flight mass spectrometry

Unimolecular fragmentations of long‐chain aliphatic iminium ions

AbstractThe metastable ion characteristics of N‐alkyl‐N‐methylmethyleneiminium ions (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm H}_2 {\rm C}=\!=\mathop {\rm N}\limits^ + ({\rm CH}_3){\rm R} $\end{document}, R = n‐CmH2m + 1, m = 3, 4, 5, 6, 8, 10, 14, 18) are reported and discussed. For R = n‐propyl, alkene loss by onium reaction and alkene loss by McLafferty rearrangement occur, whereas for the higher homologues only the latter reaction is observed. As a result of 2H and 13C labelling experiments, the mechanism of alkene loss by γ‐H transfer and β‐cleavage does not change with increasing chain length and the iminium ions do not isomerize prior to decomposition, neither by H–D scrambling nor by carbon skeleton rearrangement. Whereas the sequence of elementary steps during fragmentation is not affected, the energetics of the reaction change as the chain length increases. Resulting from thermodynamic estimations, the enthalpy of reaction ΔHr, critical energy E0 and reverse critical energy E0r diminish markedly as R increases from n‐propyl to n‐octadecyl. The knowledge of the reaction energetics including kinetic energy release data allows information about partitioning of excess energy into internal and translational degrees of freedom to be deduced.

Low energy, low temperature mass spectra. Part 16. The predominance of peaks arising from single and double McLafferty rearrangements in the mass spectra of ketoximes

The partial mass spectra of 11 ketoximes [C n H 2 n+ 1(C m H 2m+ 1)C  NOH, ( n, m = 1–4)] recorded at an ionizing electron energy of about 12eV and a source temperature of about 350K are reported and discussed. Strong molecular ion signals are found in these spectra. The principal fragmentation pathway is loss of an alkene (C n−1 H n−2 or C m−1 H 2 m−2 ) via a process analogous to the McLafferty rearrangement. In suitable cases ( n, m ≥ 3), a second alkene [C m−1 H 2 m−2 ] is expelled from the [M — C n−1 H 2 n−2 ] + fragment ion. Metastable peaks are observed for both the primary and secondary McLafferty rearrangements Prominent [M — C n−1 H 2 n−2 ] + peaks are also observed in the spectra of the oximes derived from 3-methylbutan-2-one and 3,3-dimethylbutan-2-one, despite the fact that neither of these species contains a γ-hydrogen atom. A mechanism for this unexpected alkene loss is proposed in which a 1,2-CH 3 and 1,4-H shift lead to the ionized nitroso tautomers of the oximes, followed by γ-hydrogen transfer in the ionized nitroso form.

Gas-phase reactions of lanthanum dication with small alkanes and the photodissociation of LaC2H4n+ and LaC3H6n+ (n = 1 and 2)

The gas-phase reactivity of La{sup 2+} with C{sub 1}-C{sub 4} linear alkanes is investigated and compared to the analogous reactions of La{sup +}. La{sup 2+} is unreactive with methane and ethane but reacts with propane and butane to give dehydrogenation, alkane loss, and for butane charge-splitting reaction products. Interestingly, charge transfer is not observed in agreement with a simple curve-crossing model. Four ligated species, namely, LaC{sub 2}H{sub 4}{sup +}, LaC{sub 2}H{sub 4}{sup 2+}, LaC{sub 3}H{sub 6}{sup +}, and LaC{sub 3}H{sub 6}{sup 2+}, are photodissociated and the photodissociation thresholds are used directly, or together with information from ligand displacement reactions, to assign thermochemical values for these processes and metal ligand bond energies. The bonding properties of LaC{sub 2}H{sub 2}{sup +} and LaC{sub 2}H{sub 2}{sup 2+} are also investigated. A surprising outcome of this study is that the monocation is in some cases more strongly bound to these ligands than the dication. Theoretical calculations by Bauschlicher and co-workers are in good agreement with these results. 28 refs., 6 figs., 5 tabs.

In situ N‐phosphorylation of oligopeptides for fast atom bombardment mass spectrometry

AbstractPositive ion fast atom bombardment mass spectrometry (FABMS) of in situ N‐phosphorylated oligopeptides showed intense quasi‐molecular ions together with the successive alkene loss fragment ions, which afford multiple checks of the unequivocal reality of the relative molecular mass of the tested samples. More interesting, in a novel cleavage pattern only the N‐phosphoryl fragment ions gave intense peaks, the C‐terminal series ions being suppressed. For each of the N‐terminal ions, losses of alkenes also occur to provide multiple checks for the existence of these ions. The FABMS of the in situ N‐ phosphorylated oligopeptides might provide an easily accessible routine method for peptide sequencing.

Fe+‐Mediated Dehydrogenation of 2‐Propanol and Ethanol by 1,3‐Butadiene and Evidence for Interligand Hydrogen Exchange Processes

Gas‐phase experiments on the Fe+‐mediated oxidation of 2‐propanol and ethanol by 1,3‐butadiene are described. The reaction takes place at an atomic Fe+ centre, and labeling experiments uncover its specificity. While the gas‐phase reaction of Fe(η4‐C4H6)+ with 2‐propanol occurs at collision‐rate and the analysis of ligand‐binding energies, in principle, favour a catalytic cycle for the Fe+‐mediated dehydrogenation of 2‐propanol by 1,3‐butadiene, this conjecture is not born out experimentally due to complicating “side” processes. In addition to the specific interligand two‐hydrogen‐atom transfer from the alcohol to butadiene, the study of isotopomers reveals several hydrogen‐exchange processes preceding dehydrogenation, dehydration, and alkene loss from (C4H6)Fe+ (alcohol) complexes.

Alkane loss from collisionally activated alkylmethyleneimmonium ions

AbstractThe collision‐induced dissociation (CID) spectra of five alkylmethyleneimmonium ions (H2C‐N+R1R2, (a) R1 = R2 = C2H5, (b) R1 = n‐C3H7, R2 = H, (c) R1 = n‐C3H7, R2 = CH3, (d) R1 = n‐C3H7, R2 = C2H5, (e) R1 = R2 = n‐C3H7) are reported and discussed in terms of the mechanism of alkane loss. The most abundant alkane losses result from 2‐azaallylic bond cleavages within R1 and R2 leading to daughter ions of m/z 84. Ion d (R1 = n‐C3H7, R2 = C2H5) was chosen for a deuterium‐labelling study because it exhibited methane loss nearly free from interferences with other fragmentations. The methane lost consists to a great extent (95%) of the methyl moiety of R2. Whereas the methyl moiety obviously stays intact during the fragmentation process, the hydrogen additionally needed originates from all positions of R1 and the double‐bonded methylene in an approximately random distribution, suggesting extensive hydrogen migrations preceding the transfer step.

Alkene loss from metastable methyleneimmonium ions: Unusual inverse secondary isotope effect in ion‐neutral complex intermediate fragmentations

AbstractThe mechanism of propene elimination from metastable methyleneimmonium ions is discussed. The first field‐free region fragmentations of complete sets of isotopically labelled methyleneimmonium ions (H2C = \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm N}\limits^{\rm +} $\end{document}+R1R2: R1 = R2 = n‐C3H7; R1 = R2 = i‐C3H7; R1 = n ‐C3H7; R2 = C2H5; R1 = n‐C3H7; R2 = CH3; R1 = n‐C3H7; R2 = H) were used to support the mechanism presented. The relative amounts of H/D transferred are quantitatively correlated to two distinct mathematical concepts which allow information to be deduced about influences on reaction pathways that cannot be measured directly. Propene loss from the ions examined proceeds via ion‐neutral complex intermediates. For the di‐n‐propyl species rate‐determining and H/D distribution‐determining steps are clearly distinct Whereas the former corresponds to a 1,2‐hydride shift in a 1‐propyl cation coordinated to an imine moiety, the latter is equivalent to a proton transfer to the imine occurring from the 2‐propyl cation generated by the previous step. For the diisopropyl‐substituted ions which directly form the 2‐propyl cation‐containing complex, the rate‐determining hydride shift vanishes. The 2‐propyl cation‐containing complex can decompose directly or via an intermediate proton‐bridged complex. Competition of these routes is not excluded by the experimental results. Assuming a 2:1:3 distribution, a preference for the α‐ and β‐methylene of the initial n‐propyl chain as the source of the hydrogen transferred is detected for n‐propylimmonium ions containing a second alkyl chain R2. This preference shows a clear dependence on the steric influence of R2. During the transfer step isotopic substitution is found to affect the H/D distribution strongly. For the alternative route of McLafferty rearrangement leading to C2H4 loss, specific γ‐H transfer is observed.

Site‐Selectivity in the Fe+‐Mediated CH/CC Bond Activation of Aldimines Short Communication

AbstractIt is demonstrated for the first time that the site‐selectivity for the Fe+‐mediated CH bond activation of aldimines R1NCHR2 (R1, R2 = alkyl) involves the alkyl chain R1 by a factor ≥ 12 in comparison to the alkenyl part R2. This finding explains previous observations that dehydrogenation of intermediates formed by alkene loss from either R1 or R2 of R1NCHR2/Fe+ preferentially involves the alkyl part.

Pyrolysis gas chromatography/mass spectrometry of polyvinylsilanes. Mass spectrometric characteristics of alkanes and alkenes containing two or more triorganosilyl substituents

Pyrolysis gas chromatography/mass Spectrometry was applied to the investigation of homopolymers obtained from trimethyl-, n-pentyldimethyl-, n-octyldimethyl- and trideuteromethyldimethyl-vinylsilanes. The electron impact-induced fragmentation of alkanes, alkenes and alkadienes containing two or more corresponding silyl substituents and resulting from pyrolysis of polymers was elucidated. The most characteristic fragmentation patterns of trimethylsilyl-substituted compounds are due to the loss of HSi(CH3)3 and Si(CH3)4 from M+˙, the order of which depends on the presence or absence of a double bond in hydrocarbon chain. The same structural parameter determines the probability of the characteristic decomposition of n-alkyldimethylsilyl-substituted compounds through the loss of alkene and alkyl groups from the molecular ions.

Synthesis and characterization of mono-(pentamethylcyclopentadienyl)alkoxyscandium alkyl derivatives, (η 5-C 5Me 5)(OR)ScR′

Strategies for the syntheses of the title compounds are described. Cp*Sc(acac) 2 (CP* = (η 5-C 5Me 5); acac = acetylacetonate) is prepared by treatment of Sc(acac) 3 with Cp*MgCl·THF. Oligomeric [Cp*ScCl 2] n is prepared by treatment of Cp*Sc(acac) 2 with two equivalents of AlCl 3. Cp*(OR)ScCl (R = 2,4,6-C 6H 2(CMe 3) 3, 3,5-C 6H 3(CMe 3) 2) are obtained as their THF adducts via treatment of [Cp*ScCl 2] n with LiOR in THF or as their lithium chloride adducts via treatment of [Cp*ScCl 2] n with LiOR in toluene in the presence of PMe 3. Whereas the alkylation of Cp*{O-2,4,6-C 6H 2(CMe 3) 3}ScCl yields unstable products which appear to readily undergo metalation of an ortho tert-butyl group with loss of alkane, treatment of Cp*{O-3,5-C 6H 3(CMe 3) 2}ScCl with methyllithium yields an unreactive scandium methyl derivative, which is likely dimeric [Cp*(CH 3)Sc{μ-O-3,5-C 6H 3(CMe 3) 2}] 2. This methyl derivative proved to be unexpectedly inert toward olefins, 2-butyne and even Lewis bases such as pyridine and THF.

Evaluation of n-alkanes as digesta markers in dairy cows.

Recovery of hentriacontane (C31 alkane) in feces as influenced by amount consumed and level of dietary fat was examined in a 2 x 2 factorial study in a 4 x 4 Latin square. Treatments were 1) alfalfa hay:concentrate (70:30; AH); 2) AH + calcium soap (500 g/d); 3) grass hay:concentrate (70:30, GH); and 4) GH + calcium soap. Fat did not influence C31 recovery in feces. Fecal recovery (grams/day) decreased quadratically with increasing C31 intake. Compared with AIA, which was 100% recovered in feces, DM digestibility was estimated more accurately (P less than .05) by AIA than by C31. In a second experiment, site of loss of alkane in the intestinal tract was examined by dosing C32 into the rumen or duodenum. Recovery was lower with ruminal dosing, suggesting ruminal loss of the marker. Alkanes are potentially useful markers of particulate matter in the digestive tract; however, documentation of their behavior in a wider range of diets is needed.

Experimental evidence for .beta.-methyl migration in the course of Fe+-mediated alkane formation from silazanes in the gas phase

The Fe + -induced elimination of CH 3 CD 3 from CD 3 N(Si(CH 3 ) 3 ) 2 is best explained in terms of a β-methyl migration to the metal ion, followed by reductive elimination of ethane. The study of the reactions of bare Fe + with the analogous silazanes and silanes X(Si(CH 3 ) 3 ) 2 (X=NH, ND, CH 2 , CD 2 ) uncovers further remarkable gas-phase processes; these include a degenerate methyl migration preceding alkane loss

The fragmentation of ionized alkyl phenyl ethers

AbstractThe energetics of olefin loss from ionized alkyl phenyl ethers have been determined by ionization and appearance energy measurements. It is concluded that the reaction is governed by one or more of three features: (i) the strength of the bond between the phenoxy radical and the alkyl ion; (ii) the ease of isomerization of the alkyl ions, chiefly by H‐shifts therein, and (iii) the strength of the CH bond (primary, secondary and tertiary) involved in the H transfer to oxygen which precedes the olefin loss. The possible participation in the reaction of distonic ions and proton bound radical‐molecule pairs is also discussed.