A Simple and Convenient Method for the Oxidation of Sulphides

Sulphides are efficiently oxidized to either sulphoxides or sulphones as required by treatment with acetonitrile/hydrogen peroxide and potassium carbonate in methanolic solution.     

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 water elimination from hydroxyamides. 2—N-acetyl- and N-benzoyl-4a-hydroxydecahydroquinoline and N-Methyl-4a-hydroxy-2-oxodecahydroquinoline

Electron impact induced water elimination from the metastable molecular ions of N-acetyl- and N-benzoyl-4a-hydroxydecahydroquinoline mainly follows a formal [1,2] elimination. The initiating and ratedetermining step in the reaction is the hydrogen rearrangement from C-8a onto the carbonyl group. The transferred hydrogen is subsequently lost, together with the hydroxyl group. The almost complete absence of H₂O loss from both diastereomers of N-methyl-4a-hydroxy-2-oxodecahydroquinoline confirms that the reaction can only proceed when the carbonyl group is able to function as ‘hydrogen carrier’ by occupying positions in the vicinity of both a hydrogen and the hydroxyl function.     

Site-specific hydrogen abstraction in the electrochemical oxidation of ketones

Electrochemical oxidation of saturated ketones involves specific abstraction from one γ hydrogen by the carbonyl oxygen in a six-membered ring cyclic transition state, reminiscent of the first chemical steps of the McLafferty rearrangement and Norrish type II photofragmentation.     

Hydrogen exchange in a six-membered ring transition state. Evidence for a stepwise McLafferty rearrangement

Mass spectra of 2-methyl-5-chloro-3-N-ethyl-benzothiazolium iodide and labeled counter-parts are discussed with particular emphasis on the loss of ethylene from the moiety produced by thermal elimination of HI from this salt. This process demonstrates the stepwise nature of a McLafferty rearrangement involving a hydrogen transfer to an olefinic carbon.     

The mass spectrum of isopropyl o-toluate

The formation of an [M + 1]⁺ ion and the fragmentation of isopropyl o-toluate have been investigated by the deuterium labelling technique and kinetic energy release measurements. The hydrogen atom involved in the [M + 1]⁺ ion formation does not originate from a specific part of the molecule, but from all parts. A small amount of hydrogen exchange between the secondary carbon atom in the isopropyl group and the carbon atoms in the tolyl group takes place prior to decomposition of the molecular ion into the m/z 136 ion by a McLafferty rearrangement. Either almost complete scrambling of the hydroxyl hydrogen atom and the methyl hydrogen atoms in tolyl group or an almost equilibrated exchange of the hydroxyl hydrogen atom with one of the remaining hydrogen atoms in tolyl group also takes place prior to the elimination of a water molecule from the intermediate m/z 136 ion.     

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.     

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.     

Studies in mass spectrometry—XV Mass spectra of sulphoxides and sulphones. The formation of C---C and C---O bonds upon electron impact

Although dialkyl sulphoxides and sulphones behave relatively simply upon electron impact, aromatic sulphoxides and sulphones show a pronounced tendency to undergo C---O bond formation, as evidence by a number of decomposition pathways which involve the elimination of carbon monoxide. For example, dibenzothiophene dioxide (XV) decomposes from its molecular ion by successive eliminations of carbon monoxide.     

Stereochemistries of electron impact induced hydrogen abstraction reactions proceeding through 6-membered transition states

The stereochemistries of nine electron-impact induced eliminations proceeding from derivatives of the cis-4-t-butyl system have been determined. The predominant cis elimination observed in every case is consistent with the substantial integrity of the cyclohexyl ring prior to fragmentation, and with a cyclic transition state for hydrogen abstraction. The stereochemistries of electron impact induced eliminations from 11 derivatives of the trans-4-t-butylcyclohexyl system exhibit a dichotomy. The predominatn trans stereochemistry observed in six electron impact induced eliminations, and the nonstereospecific electron impact induced dehydration of trans-4′-t-butylcyclohexyl-ethanol are consistent with nonconcerted elimination from a chair-like cyclohexyl ring. Conversely, the McLafferty rearrangement of trans-4′-t-butyl-cyclohexyl-2-propanone proceeds nonstereospecifically. trans-4-t-Butylcyclohexyl acetaldehyde, 2-methyl-3-(trans-4′-t-butylcyclohexyl)-1-propane and trans-4-t-butylcyclohexyl-S-methyl xanthate exhibit predominant cis McLafferty rearrangement stereochemistry. This result may be due to fragmentation through boat-like conformers in these compounds.     

A field ionization study of octan-2-one

H/D randomization is not observed within the octan-2-one-1,1,1,3,3-d₅ molecular ion at times less than 7 × 10^?10 sec following field ionization. Partial H/D randomization is observbed at 10^?6 to 10^?5 sec. It is deduced that the curves of microscopic rate constant k against internal excitation energy E for the reactions effecting randomization and for the McLafferty rearrangement intersect. The field ionization kinetics (FIK) for the McLafferty rearrangement in octan-2-one-1,1,1,3,3-d₅ are investigated over a time range extending from 10^?11 to 10^?6 sec. The maximum microscopic rate constant k for the reaction is estimated as 5 × 10¹⁰ sec^?1. It is suggested that FIK measurements with a double focusing mass spectrometer be made by sweeping the blade potential rather than by the alternative technique of sweeping the electric sector analyzer potential. The ‘normnal’ FI mass spectra of octan-2 one and octan-2-one and octan-2-one-1,1,1,3,3-d₅ are presented and discussed. It is proposed that the m/e 56 species in the ‘normal’ FI mass spectrum of octan-2-one represents a doubly charged ion formed by loss of oxygen in a surface process.     

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

The 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.     

Stereochemistry of organic sulphur compounds : Part. 17 conformational analysis of some erythro- and threo-thio-derivatives of β-fluorinated and β-oxygenated ethanes

The synthesis and conformational analysis of 2-fluoro, 2-hydroxy and 2-methoxy thioderivatives (thioethers, sulphoxides, sulphones and sulphonium salts) of 1,2-dimethyl and 1,2-diphenylethanes ($\text{erythro}$ and $\text{threo}$) are reported. Steric effects in thioethers and electrostatic interactions in sulphonium salts are the main factors determining the stability rotamers. A balance of these factors controls the conformational equilibria in sulphoxides and sulphones. Electronic interactions between the $\text{π}$-aromatic electrons or the unshared β-heteroatomic electrons and the sulphur atom are suggested to explain large differences in rotamer populations induced by changes in the carbon sketelon or in the relative configuration of the sulphinylic sulphur.     

On the 1,1-elimination of nitrous acid and the loss of hydroxyl from the molecular ion of the methyl ester of γ-nitrobutyric acid

It is shown by D labelling experiments that nitrous acid and hydroxyl, eliminated from the methyl ester of γ-nitrobutyric acid upon electron-impact, contain a γ-hydrogen atom to the extent of ～ 85% and ～ 100%, respectively. The loss of nitrous acid is the third case of a highly specific 1,1-elimination from a molecular ion^1a,1b reported hitherto and arguments are presented for the essential role played by carbomethoxy group in this reaction, i.e. it converts the nitro structure of the molecular ion to its acinitro structure. The results obtained further point to a stepwise character of the McLafferty rearrangement, at least for the present case.     

Fragmentation of the protonated stereoisomers of 5,7-Undecanediol and its bis(trimethylsilyl) ether under conditions of isobutane chemical ionization

The chemical ionization induced fragmentation behaviour of an open chain model compound, 5,7-undecanediol (meso and d,l stereoisomers), is studied and compared with that of previously studied alicyclic models, e.g. cyclohexanediols (cis and trans isomers). In close analogy to the latter, the major fragmentation processes are the one- and twofold loss of H₂O from the protonated molecules [MH]⁺. As borne out by appropriately deuterium labelled analogues, the first loss of H₂O is invariably a ‘clean’ heterolysis. The second loss is considerably more complex in that 1,3- and 1,4-elimination processes occur concurrently. While the 1,4-mode may be a direct elimination process in both types of compounds, the 1,3-mode is rearrangement induced in both. Whereas skeletal rearrangement (ring contraction) precedes this 1,3-elimination in the alicyclic models, hydrogen rearrangement (1,2-hydride shift) precedes it in the open chain model. No pronounced stereochemical effect is observed on the mechanistic course of the elimination processes, but is observed in their relative ease. The trimethylsilyl ethers of the stereoisomers of 5,7-undecanediol are also studied for analogous reactions, and are discussed in this context.     

Stereochemistry of the co-oxidation products of indene and thiophenol

The major primary product of the co-oxidation of indene and thiophenol is trans-2-phenylmercapto-1-indanyl hydroperoxide, which spontaneously rearranges to the two racemes of trans-2-phenylsulphinyl-1-indanol. Attack by the phenylmercaptyl radical on the indene molecule is, within 5 per cent, exclusively at the 2-position and subsequent addition of oxygen to the intermediate 2-phenylmercaptoindanyl radical is, within the same limits, exclusively a trans-addition.

The cis- and trans-2-phenylmercapto-1-indanols and the related sulphoxides and sulphones have been synthesised. Under certain conditions, the substitution reaction of thiophenate ion with indene bromohydrin is accompanied by migration of the hydroxyl group and production of trans-1-phenylmercapto-2-indanol; the sulphoxides and sulphone in this series are also described.     

Mass spectral fragmentation pathways in phthalate esters. A tandem mass spectrometric collision-induced dissociation study

A collision-induced dissociation study of a series of phthalate esters was carried out using a tandem BB mass spectrometer. Fragmentation pathways of the phthalates were determined in the electron impact mode. Two major daughter ions are formed, one by a McLafferty rearrangement and hydrogen transfer and the other by loss of an alkoxy radical Another major daughter ion, at m/z 149—which is the base peak in the electron impact mass spectra of most phthalate esters—is being formed through four alternate pathways.     

STUDIEN ZUM VORGANG DER WASSERSTOFFÜBERTRAGUNG 511 RANEY-NICKEL ALS REAGENS ZUR HYDRIERENDEN ABSPALTUNG DES CHALKOGENATOMS AUS SCHWEFEL-, SELEN- UND TELLUR-VERBINDUNGEN

Abstract

Thioäther, Sulfoxide, Sulfone, organische Selenide und Telluride (R – Y – R': R, R', = Alkyl, Aryl, Y=S, SO, SO₂, Se, Te) werden an der Grenzfläche von Raney-Nickel hydrogenolysiert. Als Spaltprodukte entstehen die Verbindungen RH und R'H sowie eine Grenzflächenverbindung (Raney-Nickel-Y) (Y=S, Se, Te; S auch aus Sulfoxiden und Sulfonen. Letztere entbinden ebenfalls mit Mineralsäuren YH₂).

Die Größe und ‘Struktur’ der Oberfläche des Raney-Nickels entscheidet über die Hydrogenolysekapazität. Bei einer technischen Raney-Nickel-Probe liegt bei Diphenylsulfid die Sättigungsausbeute an Benzol bei 1,1 mmol/g Raney-Nickel. Die Sättigungskapazität hängt ab von der Struktur von R bzw. R' und dem Heteroatom Y. (Auch Benzylether können–wenn auch langsamer als Thioether–zu Toluol aufgespalten werden). Auf Grund bekannter Daten werden Vorstellungen entwickelt, die ein Bild über den topochemischen Verlauf der Hydrogenolyse von R–Y–R' vermitteln sollen.

Vergiftetes Raney-Nickel ist im Gegensatz zum aktivierten Raney-Nickel nicht mehr oder nur stark vermindert zur Isomerisierung, Disproportionierung und zum H-D-Austausch befähigt. Bei diesen Prozessen spielt offenbar der im Nickel strukturgebundene Wasserstoff eine wichtige Rolle.

Some thioethers, sulphoxides, sulphones, organoselenium and tellurium compounds (R–Y–R' R, R'= Alkyl, Aryl, Y?S, SO, SO₂, Se, Te) have been reductively cleaved on the surface of Raney-nickel. The products comprise RH and R'H, and a surface bound material (Raney-nickel-Y) (Y?S, also from sulphoxides and sulphones, Se, Te. The latter may be released as YH₂ on treatment with mineral acids).

The area and ‘structure’ of the surface determines the hydrogenolysis capacity of the catalyst. For technical quality Raney-nickel, the limiting yield of benzene from diphenylsulphide is 1.1 mmol/g. The limiting yield is dependent on the structure of R and R' and also the nature of Y. (Benzyl ethers may also be reduced to toluene, if somewhat slower than the corresponding thioether).

Proposal for the processes possibly involved at the surface during hydrogenolysis of R–Y–R' are put forward in the basis of consideration of the available experimental data.

Poisoned Raney-nickel (in contrast to the activated catalyst) is practically inactive as agent for isomerization, disproportionation, or H–D exchange, and hence surface bound hydrogen clearly plays an important role in these processes.     

Elimination of water from the molecular ion of cycloheptanol

Under electron impact cycloheptanol decomposes by four fragmentation paths: (1) α-cleavage with subsequent losses of C¹-C⁵ fragments, (2) elimination of water, (3) loss of the hydrogen atom from C-1 and (4) loss of the hydroxyl group. The mechanism of water elimination was investigated by means of deuterium labelling. 1,4-Elimination of water predominates in cycloheptanol, with the stereospecific cis-1,3-elimination also being operative. The loss of water is preceded by extensive exchange of the hydroxyl hydrogen with those of the ring. This is attributed to a very facile transannular interaction of the hydroxyl group with the C-3 to C-6 positions that are made accessible due to conformational properties of the 7-membered ring. A kinetic model is proposed, describing migrations of the ring hydrogen atoms.     

Investigation of c ions formed by N‐terminally charged peptides upon collision‐induced dissociation

Peptide fragments such as b and y sequence ions generated upon low‐energy collision‐induced dissociation have been routinely used for tandem mass spectrometry (MS/MS)‐based peptide/protein identification. The underlying formation mechanisms have been studied extensively and described within the literature. As a result, the ‘mobile proton model’ and ‘pathways in competition model’ have been built to interpret a majority of peptide fragmentation behavior. However, unusual peptide fragments which involve unfamiliar fragmentation pathways or various rearrangement reactions occasionally appear in MS/MS spectra, resulting in confused MS/MS interpretations. In this work, a series of unfamiliar c ions are detected in MS/MS spectra of the model peptides having an N‐terminal Arg or deuterohemin group upon low‐energy collision‐induced dissociation process. Both the protonated Arg and deuterohemin group play an important role in retention of a positive charge at the N‐terminus that is remote from the cleavage sites. According to previous reports and our studies involving amino acid substitutions and hydrogen–deuterium exchange, we propose a McLafferty‐type rearrangement via charge‐remote fragmentation as the potential mechanism to explain the formation of c ions from precursor peptide ions or unconventional b ions. Density functional theory calculations are also employed in order to elucidate the proposed fragmentation mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.