Electrospray tandem mass spectrometric study of ferrocene carbamate ester derivatives of saturated primary, secondary and tertiary alcohols

The fragmentation pathways and mechanisms for 27 ferrocene carbamate esters of saturated alkyl primary, secondary and tertiary alcohols were investigated using energy-resolved electrospray tandem mass spectrometry (ES-MS/MS). The mechanisms that control the formation and abundance of the three product ions common to all the derivatives, which appear at m/z 201, 227 and 245, were elucidated. Plotting the relative abundances of the three product ions versus a range of center-of-mass collision energies provided a graphical representation of the behavior of the fragmentation process that was directly comparable from compound to compound. As a result, it was possible to compare product ion spectra of the different derivatives to distinguish among different alcohol structural types. Straight-chain primary alcohols were easily distinguished from tertiary alcohols. Both of these structural types, including positional isomers, produced product ion spectra that were distinct from those of beta-branched primary alcohols, or acyclic secondary alcohols or cyclic secondary alcohols. The latter three alcohol types display similar product ion spectra and therefore cannot be distinguished from one another on the basis of these spectra alone.     

The mass spectra of some α-lactams

The electron-impact induced fragmentation of eight aziridinones has been studied by conventional as well as by high resolution mass spectrometry. All α-lactams exhibit a molecular ion. The major primary step, in the fragmentation, is the ejection of carbon monoxide from the molecular ion. Ions of the general formula R₁? NC and R₂R₃C?O were found in the mass spectra of all α-lactams investigated. A skeletal rearrangement to rationalize these ions is proposed. The fragmentation of the molecular ion is affected by the N-substituent. Exact mass measurement and specific deuterium labeling indicate the absence of McLafferty rearrangement from either the N- or C-substituent.     

Fragmentation of oxime and silyl oxime ether odd‐electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote α,β fragmentation

The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd‐electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC‐TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t‐butyldimethylsilyl oxime ethers with oxygen in a β‐position, the McLafferty rearrangement was accompanied by loss of the t‐butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β‐position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C–C bond breaking was the rate‐determining step. However, for both the oximes and t‐butyldimethylsilyl oxime ethers with oxygen at the β‐position, the hydrogen transfer step was rate limiting, whereas with a CH₂ group at the β‐position, the C–C bond breaking was again rate determining. n‐Propoxy‐acetaldehyde, bearing an oxygen atom at the β‐position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the (E)‐ and (Z)‐isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement. Copyright © 2012 John Wiley & Sons, Ltd.     

Mass spectrometry in structural and stereochemical problems—CXCIX Contrasting fragmentations of singly- and doubly-charged o-, m- and p-hydroxyalkylphenones and their trimethylsilyl ethers

High resolution mass spectrometry, metastable defocusing and deuterium labeling of trimethylsilyl (TMS) ethers have been used to study the electron-impact induced fragmentations of o-, m- and p-hydroxyalkylphenones and their TMS ether derivatives. These derivatives have proven useful in contrasting the fragmentation patterns of singly- and doubly-charged ions because of the competing fragmentations: α-cleavage and a McLafferty rearrangement from the ketone moiety and methyl cleavage from the TMS group. A proximity effect was responsible for a markedly increased methyl radical loss from the o-TMS ether. This fragmentation was minor with the m- and p-isomers. Significantly intense doubly-charged ions were formed from ketonic cleavage and by the loss of a TMS methyl radical. The sequence of fragmentation depended on the size of the alkyl group attached to the ketone carbonyl. There was no evidence found for a McLafferty rearrangement occurring from the doubly-charged molecular ion of the TMS ethers of the hydroxyalkylphenones but the rearrangement occurred from the doubly-charge molecular ion of bis-3-(1-oxopentyl)-4-hydroxy-phenyl-methane and, of course, from the singly charged [M]^+. The bis-p-hydroxyphenylmethane derivatives were studied in an effort to increase the intensity of the doubly-charged ions as it was expected that the charges would be separated by a longer distance.     

The influence of structural features on facile McLafferty-type, even-electron rearrangements in tandem mass spectra of carboxylate anions

In contrast to the well-described McLafferty rearrangement in odd-electron cations, relatively little has been reported on comparable rearrangements in even-electron ions, especially negative ions. This work reports a systematic study using tandem mass spectrometry (MS/MS) fragment ion spectra of carboxylate anions having a suitably acidic proton in the gamma position. The rearrangement process was studied in both ion trap and triple quadrupole mass spectrometers; characteristic enolate anions and stable neutral products were formed at low collision energies. The process has diagnostic and analytical potential in, for example, the analysis of peptides having C-terminal serine residues and of 3-hydroxy- or 3-aminocarboxylic acids in complex mixtures.     

Fast atom bombardment mass spectrometry of butyloxycarbonyl protected (BOC) amino acids

The positive and negative ion mass spectra of a series of BOC-protected amino acids have been obtained using an FAB source. The spectra show characteristic features including a McLafferty type of rearrangement.     

Destabilized carbenium ions. Secondary and tertiary α-acetylbenzyl cations and α-benzoylbenzyl cations

The secondary α-acetylbenzyl and α-benzoylbenzyl cations, as well as their tertiary analogues, have been generated in a mass spectrometer by electron impact induced fragmentation of the corresponding α-bromoketones. These ions belong to the interesting family of destabilized α-acylcarbenium ions. While primary α-acylcarbenium ions appear to be unstable, the secondary and tertiaiy ions exhibit the usual behaviour of stable entities in a potential energy well. This can be attributed to a ‘push-pull’ substitution at the carbenium ion centre by an electron-releasing phenyl group and an electron-withdrawing acyl substituent. The characteristic unimolecular reaction of the metastaible secondary and tertiary α-acylbenzyl cations is the elimination of CO by a rearrangement reaction involving a 1,2-shift of a methyl group and a phenyl group, respectively. The loss of CO is accompanied by a very large kinetic energy release, which gives rise to broad and dish-topped peaks for this process in the mass-analysed ion kinetic energy spectra of the corresponding ions. This behaviour is attributed to the rigid critical configuration of a corner-protonatei cyclopropanone derivative and a bridged phenonium ion derivative, respectively, for this reaction. For the tertiary α-acetyl-α-methylbenzyl cations, it has been shown by deuterium labelling and by comparison of collisional activation spectra that these ions equilibrate prior to decomposition with their ‘protomer’ derivatives formed by proton migration from the α-methyl substituent to the carbonyl group and to the benzene ring.     

Letter: electron impact mass spectrometry study of a series of substituted 5- aminoalkylmethyl-cytosines and their 1-N-(o-, m- and p-)bromobenzyl-substituted derivatives

The electron-impact mass spectra of 5-aminoalkylmethyl-substituted cytosine and of their 1-N- o-(m- or p-)bromobenzyl-substituted derivatives are discussed. The influence of 5-aminoalkyl and 1-N-bromobenzyl substituents on the mode of mass fragmentation occurring upon electron impact ionization was demonstrated. The fragmentation pathways of all compounds and the characteristic ions in these spectra originating from the McLafferty rearrangement, RDA reactions and simple cleavages are presented.     

The mass spectral behavior of β-bromoethyl benzoate and some of its ring substituted derivatives: Evidence for oxygen participation in the formation of [M ? Br]+

The expulsion of a bromine atom from the molecular ion of β-bromoethyl benzoate displays a kinetic behavior which resembles that of a rearrangement reaction. The two oxygen atoms in the resulting [M ? Br]⁺ ion become equivalent before or during the secondary decomposition of this ion, as shown by oxygen labeling. In addition, the primary ions generated from benzophenone ethylene ketal and acetophenone ethylene ketal by phenyl and methyl expulsion, respectively, undergo subsequent fragmentation in the same way as the [M ? Br]⁺ from β-bromoethyl benzoate. These results strongly indicate that the carbonyl-oxygen atom participates in the expulsion of bromine. The effect of ring substituents on the competing loss of bromine and McLafferty rearrangement is also discussed.     

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

The mass spectra of N-propyl and N-butyl barbitals show the loss of olefin radical (C_nH_2n-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.     

Mass spectra of some thionocarbamates

The mass spectra of a number O-alkylthionocarbamates with two, one or no alkyl substituents on the nitrogen atom are presented and discussed. The main primary fragmentation pathways are McLafferty rearrangements, including the double so-called protonated McLafferty rearrangement. Generally, rearrangement products give charge delocalized ions, which release hydroxyl or sulphydryl radicals and water or hydrogen sulphide, to produce stable fragments. It has been found that the position of alkyl moieties may be deduced on the basis of mass spectral fragmentation patterns.     

Electron-impact induced fragmentation of labeled 3-(m-nitrobenzoyl)-2-methylpropanoic acids : Evidence for a stepwise McLafferty rearrangement

The mass spectra of deuterium and ¹⁸O labeled 3-(m-nitrobenzoyl)-2-methylpropanoic acid reveal that the McLafferty rearrangement and the rearrangement process leading to protonated m-nitrobenzoic acid do not take place via a common intermediate. H-D exchange phenomena point to a stepwise McLafferty rearrangement.     

Mass spectrometry in structural and stereochemical problems—CCXXXV : A hidden hydrogen rearrangement in long chained anilides

The mass spectra of deuterium labeled hexananilides show an unexpected rearrangement of a hydrogen atom from C-4 with formation of a C₆H₇N⁺ ion. The decomposition of the McLafferty rearrangement ion in these compounds is shown to involve expulsion of the elements of ethynyl alcohol rather than ketene.     

Low–energy,low-temperature mass spectra. 8—The McLafferty rearrangement

The McLafferty rearrangement giving rise to the McLafferty ion, the complementary ion and a protonated molecule is discussed in the cases of the alkenes, alkanals, alkanones, alkanoic acids and alkanoic esters, in terms of the low-energy, low-temperature mass spectra. Based on the behaviour of the alkanals, which show a series of peaks corresponding to [M ? C_nH_2n]⁺˙, it is suggested that these also involve McLafferty rearrangements from rearranged distonic ions. The same processes are held to be responsible for similar series of peaks in the case of the alkenes and that of the amines. Where possible the energetics of the processes concerned are discussed. The low-energy mass spectra of the linear alkenes propene to octene and those of the octenes are reported.     

Aryl versus alkyl and vinyl migration in the mass spectra of sulphone derivatives

The 2-acetoxyethyl aryl sulphones were found to be unsuitable for a comparative study with the corresponding chlorides and alcohols as their predominant fragmentation involved cleavage of the aliphatic carbon-sulphur bond with charge retention on the aliphatic part of the molecule The expected McLafferty rearrangement to yield an intermediate vinyl species was only a minor fragmentation path. Indirect evidence for alkyl migration in the molecular ion was found in the spectrum of 2-acetoxyethyl 1-paranitro phenyl sulphone. The spectra of the aryl vinyl sulphones revealed intense [Aryl SO]+ ions resulting from preferred migration of the vinyl group from sulphur to oxygen.     

Facile synthesis of isocampholenic acids by the rearrangement of camphor derivatives

A variety of substituted isocampholenic acid derivatives were prepared by rearrangement of the camphor skeleton of a variety of tertiary alcohols derived from ketopinic acid. The reaction was highly reliable and retained the stereochemical information from the camphor scaffold. This rearrangement represents an efficient way to prepare synthetically useful isocampholenic acids. Mechanistic experiments show that a short-lived carbocation is implicated in the reaction.     

浅析质谱中的广义麦式重排

麦氏重排是对质谱分析中分子离子的重排反应提出的经验规则。对经典麦式重排的概念、裂解过程及其应用做进一步拓展,形成了广义麦式重排。在广义麦式重排中,γ-H的经典麦式重排是一步完成的六元环协同裂解,分子离子亦可通过六元环或五元环过渡态进行协同重排裂解,发生相应的γ-R、β-H(或R)的迁移,产生不同的碎片离子。这种广义麦氏重排在各种常见官能团化合物中均可发生,其在质谱解析和化合物结构研究中具有广泛应用。     

Reductive cyclization of o-nitrophenyl propargyl alcohols: facile synthesis of substituted quinolines

Reduction of secondary and tertiary o-nitrophenyl propargyl alcohols followed by acid-catalyzed Meyer-Schuster rearrangement gave 2-substituted and 2,4-disubstituted quinolines, respectively. Tertiary propargyl alcohols gave excellent yields of the quinoline derivative, while the yields of quinolines were slightly reduced when secondary propargyl alcohol derivatives were utilized.     

Mass-spectrometric investigation of ionophoric derivatives of benzo- and dibenzocrown ethers. 4. 4′-Acyl derivatives of benzo-15-crown-5 and benzo-18-crown-6

The mass spectra of 4-acyl derivatives of benzo-15-crown-5 and benzo-18-crown-6 were studied by the method of direct analysis of the daughter ions (DADI). It was established that the fragmentation of the M⁺ ions of the investigated compounds under electron impact proceeds via the scheme of the McLafferty rearrangement with the simultaneous elimination of ethylene oxide. The degree of occurrence of the McLafferty rearrangement increases with an increase in the length of the side chain.See [3] for communication 3.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 989–901, July, 1987.     

Massenspektrometrie und ihre Anwendung auf strukturelle und stereochemische Probleme CLXVII Langkettige Wasserstoffverschiebung in der Massenspektrometrie

The mass spectral fragmentation behavior of various α-substituted tetrahydrofuran derivatives and their deuterium labeled analogs has been examined. It was established that the ester and hydroxy derivatives undergo ring fragmentation associated with long range hydrogen transfer. The mass spectra of the deuterium labeled samples provided evidence that direct hydrogen transfer takes place from positions as far as thirteen or more carbon atoms away from the oxygen function in preference to the McLafferty rearrangement. Examination of the spectra of the deuterium labeled derivatives shed further light on the mechanism of the loss of water from the molecular ion and various fragment ions.