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

The 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.     

Electron impact mass spectrometry of nuphar alkaloids. II—Thiohemiaminals

The appearance of m/e 228 and 176 ions, useful in ascertaining the presence of the hemiaminal hydroxyl, are discussed for six thiaspirane-type hemiaminals isolated from Nuphar luteum and three epimeric pairs of α-thiohemiaminals derived from (–)-deoxynupharidine. Other groups of ions useful in ascertaining stereochemical differences of the sulfur moiety in α-thiohemiaminals are discussed for the same compounds. One such group of ions is comprised of 248, 246 and 231; a second is 192 and 191.     

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

Using 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.     

Hydrogen migrations in mass spectrometry. VI—the chemical ionization mass spectra of substituted benzoic acids and benzyl alcohols

The H₂ and CH₄ chemical ionization mass spectra of a series of series of substituted benzoic acids and substituted benzyl alcohols have been determined. For the benzoic acids the major fragmentation reactions of the protonated molecule involve elimination of H₂O or elimination of CO₂, the latter reaction involving migration of the carboxylic hydrogen to the aromatic ring. For the benzyl alcohols the major fragmentation reactions of [MH]⁺ involve loss of H₂O or CH₂O, analogous to the CO₂ elimination reaction for the benzoic acids. It is shown that the CO₂ and CH₂O elimination reactions occur only when a conjugated aromatic ring system is present, and that for the carboxylic acid systems, methyl groups and, to a lesser extent, phenyl groups are capable of migrating. The only discernible effect of substituents on the fragmentation of [MH]⁺ is an enhancement of the H₂O loss reaction in the benzoic acid system when an amino, hydroxyl, or halogen substituent is ortho to the carboxyl function. This ‘ortho’ effect, which differs in scope from that observed in electron impact mass spectra, is attributed to an intramolecular catalysis by the ortho substituent of the 1,3 hydrogen migration in the carbonyl protonated acid followed by H₂O elimination. Apparently, this route is favoured over the direct elimination of H₂O from the carbonyl protonated acid, since the latter has a high activation energy barrier because of unfavourable orbital symmetry restrictions.     

Studies in mass spectrometry. II—A common intermediate in the primary fragmentation process of α-phenylmethylenecylopropane

The variation of Z = [(F)⁺]/[(M)^+·] vs electron energy for the loss of H^· and CH₃^· from the molecular ion of 2-phenylmethylenecyclopropane (I) and α-phenylmethylenecyclopropane (II) suggests that they both rearrange to a common intermediate before fragmentation occurs. The structure of the intermediate is discussed with regard to the xperimental values found for the Z ratios and to the mass spectra of deuterium labelled compounds.     

Hydrogen transfer in the retro diels–alder fragmentation of oxygen-containing heterocyclic compounds. II—Chromones

Mass Spectra of unsubstituted, 2-methyl-, 3-methyl and 2,3-dimethylchromones were examined. These compounds showed [RDA]⁺˙ and [RDA + H]⁺ ions as characteristc ions, together with [M? H]⁺,[M? CO]⁺˙,[M? CHO]⁺ and [RDA? CO]⁺˙ ions. Based on deuterium labelling experiments and measurement of metastable peaks by the ion kinetic energy defocusing technique, the origin of transferred hydrogen in the [RDA + H]⁺ ion was clarified. The mechanism of the [RDA + H]⁺ ion formation is discussed.     

Hydrogen migrations in mass spectrometry. IV—formation of [C6H7]+ in the chemical ionization mass spectra of alkylbenzenes

Using specific deuterium labelling the mechanisms of the olefin elimination reactions leading to formation of [C₆H₇]+ in the H₂ and CH₄ chemical ionizatin mass spectra of ethylbenzene and n-propylbenzene (and to [C₂H₅C₆H₆]+ in the CH₄ chemical ionization mass spectra) have been investigated. The results show that the reaction does not occur by specific migration of H from the β position of the alkyl group to the benzene ring. For ethylbenzene 23–29% of the migrating H originates from the α-position, while for n-propylbenzene H migration from all propyl positions is observed in the approximate ratio, position 1:position 2:position 3=0.30:0.22:0.48. It is proposed that the results can be explained on the basis of competing H migration from each alkyl position involving cyclic transition states of different ring sizes, rather than by H randomization within the alkyl chain.     

Intramolecular single and double hydrogen migrations in the mass spectra of substituted tetrahydro-1,3,2-oxazaphosphorin-2-oxides

The mass spectral fragmentation of substituted tetrahydro-1,3,2-oxazaphosphorin-2-oxides occurs by the cleavage of ring bonds. The ions due to simple cleavage, single and double hydrogen migration, seem to be triggered by C O bond cleavage of the oxazaphosphorin ring. The variations in the relative abundance of ions arising due to similar fragmentation modes have been found to depend on the nature of the substituent and the stability of the particular fragment. The single hydrogen transfer process is supported by metastable ion and shift techniques.     

Mass spectrometry in structural and stereochemical problems. CCLVI—electron impact induced fragmentation of juvenile hormone analogs

Complete high resolution mass spectra of several analogs of the juvenile hormones have been analyzed with the aid of our recently developed INTSUM program and mass spectra of deuterated analogs. Representative compounds chosen for study included the biologically interesting (2E, 4E)-3,7,11-trimethyl-2,4-dodecadienoates (1,2), (2E,4E)-11-methoxy-3,7,-11-trimethyl-2,4-do-decadienoates (3,4) and three analogs (5-7) of the trans-trans-farnesoate skeleton. These compounds display characteristic fragmentations which can be used to detect the presence of the compounds in complex mixtures using mass fragmentography or aid in the structure elucidation of related compounds.     

Mass spectrometry in structural and stereochemical problems—CLVIII electron impact promoted fragmentation of triphenylmethyl ethers

The triphenylmethyl (trityl) moiety is frequently used for the protection of alcohols but the mass spectra of such trityl ethers have hitherto escaped scrutiny. It has now been found that triphenylmethyl derivatives of primary alcohols yield abundant molecular ions which permit the determination of the isotopic purity of the parent alcohol. Upon electron impact the triphenyl-methyl entity directs the fragmentation of trityl ethers as demonstrated by a detailed study of n-pentyl trityl ether and its deuterated analogs. Ions formed by migration of phenyl groups were observed in the mass spectra of the trityl ethers investigated as well as in the spectrum of triphenyl-methane itself.     

Application of mass spectrometry to the structural investigation of triterpenes. II—3,4-A-seco-triterpenoids

The mass spectral behaviour of a series of naturally occurring and synthetic 3,4-A-seco-triterpenes has been examined and fragmentations characteristic of the 3,4-A-seco function observed. Deuterium labelling has been used to investigate the origins of their characteristic fragments.     

Mass spectrometry of condensed N-heterocycles: II—A novel ortho effect in the electron impact induced fragmentation of some C-(2-nitrophenyl)-azoles and -azoloazines

Heterocycles containing an integrated 2-nitrobenzaldehyde imine moiety produce ions of m/z 134 with high abundance under electron impact. As shown by kinetic energy release measurements these ions have the same structure in all the cases studied, identical to m/z 134 formed from the reference compound 2-nitrobenzoylpiperidide. The mechanism of formation of m/z 134 most probably involves isomerization of its precursor ion to give a spiro intermediate, from which m/z 134 arises either synchronously or after a second isomerization. Both alternatives formally represent an oxygen transfer from the nitro group to the imino carbon via a 5-membered transition state.     

Negative ions in organic mass spectrometry. II—Negative ion mass spectra of simple thio and dithio esters.

The 70 eV negative ion mass spectra of some simple aromatic and aliphatic thio and dithio esters are discussed. A characteristic fragmentation process, a rearrangement analogous to the nitro-nitrite conversion of aromatic nitro compounds, is observed for aromatic thio esters.     

Stereochemical applications of mass spectrometry. II—Chemical ionization mass spectra of isomeric dicarboxylic acids and derivatives

The H₂ and CH₄, chemical ionization mass spectra of the cis dicarboxylic acids, maleic and citraconic acid, show much more extensive loss of H₂O from [MH]⁺ than the trans isomers, fumaric acid and mesaconic acid. Similarly, esters of maleic acid show a much more facile loss of ROH (R=alkyl or phenyl) from [MH]⁺ than do esters of fumaric acid. Similar differences are observed in the chemical ionization mass spectra of the isomeric phthalic and isophthalic acids and derivatives, where the ortho isomers show more extensive fragmentation of [MH]⁺ than the meta isomers. The facile fragmentation of [MH]⁺ for the cis and ortho isomers is attributed to ROH elimination involving interaction between the two carboxylate functions and forming the stable cyclic anhydride structure for the fragment ion. By contrast ROH elimination from [MH]⁺ for the trans and metu isomers requires a symmetry-forbidden [1,3]-H migration in the carboxyl protonated species and cannot lead to the cyclic anhydride structure. The chemical ionization mass spectra of cis and trans cyclohexane-1,2-dicarboxylic acids are essentially identical and show extensive fragmentation of the [IMH]⁺ ion. Experiments using deuterium labelling show extensive carboxyl group interactions for both isomers. The chemical ionization mass spectra of maleanilic and phthalanilic acids and of the related anhydrides and imides also are reported, as are the electron impact mass spectra of diphenyl maleate, diphenyl fumarate, diphenyl phthalate, maleanilic acid and phthalanilic acid.     

Mass spectrometry in structural and stereochemical problems—CLXVI. The electron impact promoted fragmentation of some aliphatic 1,2-glycols

The mass spectra of nineteen aliphatic 1,2-glycols containing a variety of substituents have been recorded and mechanistic rationalizations are presented for the more important degradation processes observed. Fragmentation of the hydroxylated carbon-carbon bond in these compounds occurred to an appreciable degree often accompanied by the transfer of a hydroxylic hydrogen atom.     

Laser Desorption electron impact mass spectrometry: Studies of ion radical fragmentation mechanisms and hydrogen/deuterium rearrangement in time-resolved experiments

Laser desorption of neutral molecules followed by electron impact ionization is shown to produce gaseous ion radicals that undergo fragmentation analogous to that observed with electron impact and field ionization. Spectra were measured of heptanal and four deuterium-labeled analogs regenerated from bisulfite derivatives, and compared with electron impact, chemical ionization and field ionization spectra published previously. Lifetimes of desorbed neutral molecules and of ions were found to influence the fragmentation process as reflected in isotope patterns. This is explained on the basis of internal energies.     

One and two hydrogen atom migrations in the retro-diels-alder fragmentation of norbornene and bicyclo [2.2.2] octene derivatives under electron impact

Norbornene derivatives with one or two carbonyl-containing substituents at positions 5 and 6 (anhydrides, esters, amides, cyclic ketones) undergo an electron impact induced rerro-Diels-Alder fragmentation accompanied by the migration of one hydrogen atom (RDA + H) giving rise to the [dienophile + H]⁺ ions. Bicyclo[2.2.2]octene derivatives substituted at C(5) and C(6) by a ring with two carbonyl groups (anhydrides, imides, diketones) undergo an RDA fragmentation accompanied by the transfer of two hydrogen atoms (RDA + 2H). Bicyclo[2.2.2]octene 5,6-diesters undergo both the RDA + H and RDA + 2H fragmentations, and the relative abundance of the resulting [dienophile + H]⁺ and [dienophile + 2H]⁺˙ ions is strongly affected by configuration.     

New fragmentation pathways in the electron impact mass spectrometry of derivatized pyrano-1,3-diphenylprop-2-enones

The fragmentation patterns of closely related chalcones, cinnamoylchromans and cinnamoylchromenes, are reported to be strikingly different. The mass spectra of the first group show peaks typical of the fragmentation of simple chalcones balanced by additional fragmentation routes competing effectively with the typical chalcone fragmentation. For the other group with the introduced double bond the fragmentation is considerably changed. Initial loss of a methyl group gives rise to formation of the base peak in three of four examples. The [M – CH₃]⁺ ion decomposes further, eliminating a styrene yielding the m/z 187 ion. This process may be rationalized as a retro-Diels–Alder fragmentation of a flavanone formed on intramolecular rearrangement of the molecular ion.     

Studies in mass spectrometry—XV: Retro-diels-alder fragmentation and double hydrogen migration in some polycyclic diketones

In contrast to adducts I of bi-1-cycloalken-1-yls and p-benzoquinone, their reduction products II do not exhibit a double hydrogen migration from δ positions accompanying a ‘retro-Diels-Alder’ type fragmentation. An ordinary retro-Diels-Alder fragmentation was found to take place, with charge retention in the diene portion of the molecule. A double hydrogen migration has been detected in II leading to m/e 112 ion c₂, which differed, however, from that in I in charge retention and in the origin of the migrating hydrogen atoms. Adducts III of di-1-cycloalken-1-yls and naphthoquinone behave similarly to II. They exhibit relatively low abundance ions a, however, due to a double hydrogen migration from δ positions, similarly to I. The origin of the migrating hydrogen atoms have been determined by deuterium labelling. Mechanistic suggestions are presented to explain the observed facts.     

Electron impact induced hydrogen migrations in organic molecules: IV—novel hydrogen transfers in alkyl o-methoxybenzoates

Simultaneous hydrogen transfers—one from the methoxy group and the other from the alkyl group—to both the oxygen atoms of the ester function result in the formation of a common ion at m/z 152 in the alkyl o-methoxybenzoates on electron impact. Expulsion of the formyl radical from this ion leads to a fragment resembling the protonated benzoic acid. Another novel feature in these compounds is the loss of H₂O from the [M? R]⁺ ion which arises through an ortho effect during a secondary fragmentation process.