Stereochemical applications of mass spectrometry 4. Energy-resolved study of the fragmentation of esters of maleic and fumaric acids

The fragmentation of the dimethyl and diethyl esters of maleic and fumaric acids have been studied as a function of the internal energy of the molecular ions using charge exchange techniques and metastable ion studies in combination with isotopic labelling. The dimethyl ester molecular ions show distinctive behaviours at both low and high internal energies, indicating that interconversion of the molecular ions does not occur. The fumarate molecular ion fragments by elimination of CH₂O and (CO₂ + CH₃) in the metastable ion time-frame, while the maleate ester fragments primarily by loss of CH₃O. At higher internal energies both molecular ions fragment primarily by loss of CH₃O but the fragment ion from the maleate ester shows a greater stability, presumably because it assumes the cyclic cationated maleic anhydride structure. The diethyl maleate and diethyl fumarate molecular ions show identical metastable ion characteristics; in addition the [COS]⁺· charge exchange mass spectra are very similar. These results indicate that low-energy molecular ions interconvert. At higher internal energies interconversion does not occur, and, although both moiecular ions fragment by loss of C₂H₅O, the resultsint fragment ions show different stabilities and fragmentation reactions.     

The effect of configuration of gas phase protonated ethenedicarboxylates on their low energy collision induced dissociation behaviour

Low energy collision induced dissociation (CID) spectra were measured by a triple stage quadrupole mass spectrometer for the [MH]⁺ ions of diethyl and dimethyl esters of maleic, fumaric, citraconic and mesaconic acids. A very high degree of stereospecificity was observed for the geometrically isomeric diethyl esters. The cis esters give rise to very abundant [MH? EtOH]⁺ and [MH? EtOH? C₂H₄]⁺ ions, while the trans isomers exhibit very abundant [MH? C₂H₄]⁺ and [MH? 2 C₂H₄]⁺ ions. The highly stereospecific processes indicate that the double bond configuration is retained in the protonated species under the conditions of the experiment.     

Effect of zinc chloride on thermal decomposition of polymethacrylic and polyacrylic esters

Zinc chloride reduces the rate of thermal and thermooxidative decompositions of polymethacrylic esters and increases the thermal decomposition rate of polyacrylic esters. Mechanisms of the thermal decompositions of polymethacrylates and polyacrylates in the presence of ZnCl₂ have been suggested.For thermal decomposition polymethacrylic esters, the rate of depolymerization decreases due to the formation of cycles in a polymer chain by reaction of Zinc chloride with neighbouring ester groups. For thermooxidative decomposition of polymethacrylates, ZnCl₂ decreases also the rate of initiation of depolymerization and causes decomposition of hydroperoxide groups by a heterolytic mechanism. The increase in thermal decomposition rate of polyacrylic esters results from the high activity of complexes of ester groups and ZnCl₂ in decomposition reactions with formation of alcohol and CO₂ through Cameron mechanism.     

Synthesis and properties of ethyl esters of some 1,2-dihydropyridine-3,5-dicarboxylic acids

The oxidation of diethyl 1-methyl- or 1-aryl-2,6-dimethyl-4-aryl-1,4-dihydropyridine-3,5-dicarboxylates with hydrogen peroxide in the presence of perchloric acid gave the perchlorates of the corresponding pyridinium ions, the reduction of which with NaBH₄ is a preparative method for the synthesis of diethyl esters of 1-substituted 2,6-dimethyl-1,2-dihydropyridine-3,5-dicarboxylic acids. Derivatives of the 5-carboxylic acid of the corresponding 1,2-dihydropyridine are formed by alkaline hydrolysis of these esters.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1225–1228, September, 1982.     

Metastable ions of methane and their use in calibrating a low mass scale

The decomposition of metastable positive ions of methane formed by electron bombardment is discussed. Some new collision-induced decompositions involving the formation of H⁺, H₂⁺˙ and H₃⁺ have been found in the ion kinetic energy spectrum as well as in the mass spectrum of methane. In the latter case, the ‘metastable peaks’ occur below mass 1 on the mass scale and can be used for mass calibration in this region of the mass spectrum.     

The mechanism of the loss of H2 and H2O from 1,4-cyclohexanediol anion and related compounds

Losses of H₂ and H₂O from the [M-H]^? alkoxide anions of 1,4-cyclohexanediol formed in OH^? negative chemical ionization were studied using synthetic deuterated analogues and labelling by hydrogen-deuterium exchange between neutrals in the ion source prior to ionization. Simple 1,2-H₂ elimination leading to an enolate ion does not occur, but stereospecific mechanisms are shown to give a ketolate intermediate daughter ion. Collisionally activated dissociation spectra of the resulting daughter ions are the same as those of the corresponding ions generated directly from ketols.     

Determination of the fragmentation mechanisms of organophosphorus ions by H2O and D2O atmospheric-pressure ionization tandem mass spectrometry II—dialkyl alkylphosphonate ions

Diethyl methylphosphonate (DEMP), diisopropyl methylpbosphonate (DIMP), diethyl isopropylphosphonate (DEIP) and diethyl ethylphosphonate (DEEP) were characterized by H₂O and D₂O atmospheric pressure ionization tandem mass Spectrometry (API-MS/MS). Collision-induced dissociation (CID)/fragmentation pathways included alkyl ions by direct cleavage, alkyl radical and water loss processes and McLafferty and McLafferty-type rearrangements by six- and five-membered ring transition states, respectively. D₂O API proved particularly useful in that certain decomposition pathways (i.e. water and methanol neutral losses) had a statistical distribution as to the loss of an acid deuteron and proton(s). This phenomenon was manifested by two pairs of ions in the D₂O API daughter-ion mass spectrum for each phosphonate compound (e.g. both m/z 79/80 and 65/66 for DEMP and DIMP). The observed ion intensity ratios for these pairs of ions served as guides in the determination of their predicted ion relative abundance ratios and CID decomposition pathways. Water neutral losses as opposed to ether and alcohol neutral losses were favored for most of the protonated organophosphonate molecular ion decomposition schemes.     

Solventless DBU-promoted Mannich-type reactions of α-amido p-tolylsulfones with diethyl fluoromalonates and diethyl malonates

Mannich-type reactions of α-amido p-tolylsulfones with diethyl fluoromalonates and diethyl malonates, respectively in presence of catalytic amount of DBU have been developed. A variety of α-amido p-tolylsulfones prepared from aromatic and aliphatic aldehydes reacts with diethyl fluoromalonates and diethyl malonates, respectively under mild reaction conditions to afford α-fluoro β-amino esters and β-amino esters in moderate to good yield.     

Mass spectrometry of π-complexes of transition metals : V. Dimeric (π-cyclopentadienyl)dicarboxylatovanadium complexes

Mass spectra of dimeric (π-cyclopentadienyl)dicarboxylatovanadium complexes [π-C₅H₅V(OOCR)₂]₂, have been studied. The principal paths of fragmentation of the trihaloalkyl derivatives involve the formation of VHal bonds, whereas VO bonds are formed during decomposition of acetato and formato complexes. An unusual increase in stability of dimeric and monomeric ions of the alkyl derivatives is observed in going from R = CH₃ to R = CF₃. With aromatic complexes, only ions containing a single metal atom occur.     

Determination of the fragmentation mechanisms of organophosphorus ions by H2O and D2O atmospheric-pressure ionization tandem mass spectrometry

Dimethylmethyl phosphonate (DMMP), dimethyl phosphite (DMPI), trimethyl phosphite (TMPI) and trimethyl phosphate (TMP) were investigated using H₂O and D₂O atmospheric-pressure ionization (API) tandem mass Spectrometry. All daughter ions could be explained by losses of one or a successive number of stable molecules as opposed to losses of radicals such as the hydride, methyl and methoxy species. Losses of neutral methanol and dimethyl ether and of protonated methanol and formaldehyde ions from all four organophosphorus pseudo-molecular ions were observed. The DMMP and DMPI MH⁺ pseudomolecular ions produced the losses of neutral C₂H₆ and water, respectively. Formaldehyde loss was not observed for the MH⁺ ions, but it was well represented in the decomposition pathways of daughter ions. The D₂O reagent gas highlighted the role of the ionizing proton/ deuteron in the various daughter ions, including m/z 95, 79, 65, 49, 33, 31 and 47. The last ion was found to be isobaric in that m/z 47 and 48 both appeared with similar abundances in the D₂O-API daughter ion mass spectra of TMPI and TMP.     

STUDY OF THE ALKALINE CLEAVAGE OF THE P[sbnd]C BOND IN PHOSPHINE OXIDES AND DERIVATIVES OF TRICHLOROMETHANEPHOSPHONATE

Abstract

A study of alkaline decomposition of several aromatic phosphine oxides containing p- and o-nitrobenzyl, and trichloromethyl as leaving groups, is reported. The property of the trichloromethyl group as leaving group, and the CCl^? ₃-group's further decomposition in the hydrolysis of diethyl and disodium trichloromethanephosphonates, have also been investigated.     

Vapor–liquid equilibrium of carbon dioxide with diethyl methylmalonate,diethyl ethylmalonate and diethyl n-butylmalonate at elevated pressures

Vapor–liquid equilibrium (VLE) data for binary mixtures of CO₂ with homologous esters of diethyl methylmalonate, diethyl ethylmalonate, and diethyl n-butylmalonate at 308.2, 318.2, and 328.2 K, respectively, over the pressure range 1.4–8.4 MPa were measured using a semi-flow apparatus. New gas solubility data for CO₂ in esters are presented, and the Henry’s law constants for CO₂ in these esters are evaluated by employing the Krichevsky–Ilinskaya (KI) equation. The VLE data were also correlated using the Soave–Redlich–Kwong and the Peng–Robinson equations of state (EOSs) with various types of mixing rules. It is shown that EOS with both the van der Waals mixing rules and the two adjustable parameters yield satisfactory correlation results.     

A concise synthesis of diethyl 1-(tert-butoxycarbonylamino)-1-alkenylphosphonates

An efficient completely diastereoselective synthesis of (Z)- and (E)- N-Boc 1-aminoalkenylphosphonic acid diethyl esters from easily available 5-substituted (2-thioxo-oxazolidin-4-yl)phosphonic acid diethyl esters has been developed.     

A concise synthesis of protected diethyl 1-amino-2-hydroxyalkylphosphonates

An efficient diastereoselective synthesis of 5-substituted (2-thioxo-oxazolidin-4-yl)phosphonic acid diethyl esters from metallated diethyl isothiocyanomethylphosphonate and aldehydes has been developed. The three-step transformation of oxazolidine-2-thione derivatives into N-Boc 1-amino-2-hydroxyalkylphosphonic acid diethyl esters is also described.     

Structural characterization of sulfonated azo dyes using liquid secondary ion mass spectrometry/tandem mass spectrometry

Eight monosulfonated and disulfonated azo dyes were analyzed using liquid secondary ion mass spectrometry/tandem mass spectrometry, in the negative ion mode, under low-energy collision conditions (110–150 eV). Many structurally characteristic fragment ions were obtained, several of which have not been reported previously using other mass spectrometric techniques. Among the structurally important ions observed were those due to loss of SO₂, SO₃ and various aromatic substituents, such as NO₂ and NHCOCH₃. Losses of N₂ were also proposed. In addition, product ions due to cleavage at the azo linkage were observed, and also SO₃^? and HSO₃^? ions. Several of the azo cleavage product ions detected did not contain sulfonate groups. Possible pathways for the formation of product ions are proposed. The structural information obtained should help to better characterize and identify sulfonated azo dyes in the future.     

Doubly-charged ethane ions: Solution to the dilemma of stability predicted by theory and instability observed in experiment

Potential energy curves have been computed for [C₂H₆]²⁺ ions and the results used to interpret the conspicuous absence of these ions in 2E mass spectra and in charge-stripping experiments. The energies and structures of geometry-optimized ground-state singlet and excited-state triplet [C₂H₆]²⁺ ions have been determined along with energies for different decomposition barriers and dissociation asymptotes. Although singlet and triplet [C₂H₆]²⁺ ions can exist as stable entities, they possess low energy barriers to decomposition. Vertical Franck-Condon transitions, involving electron impact ionization of ethane as well as charge-stripping collisions of [C₂H₆]⁺ ions, produce [C₂H₆]²⁺ ions which promptly dissociate since they are formed with energies in excess of various decomposition barriers. Appearance energies computed for doubly-charged ethane fragment ions are in accordance with experimental values.     

Copper-bipyridine-catalyzed enantioselective α-amination of β-keto esters

The Cu(OTf)₂-bipyridine-catalyzed, enantioselective, direct α-amination of β-keto esters and β-diketones with diethyl azodicarboxylate (DEAD) has been studied. Exceptionally high enantioselectivities of up to 99% ee were found for 1-indanone-based β-keto esters in particular even for substrates and reagents carrying conventional ester groups such as methyl and ethyl.     

Acyl Iodides in Organic Synthesis: V. Reactions with Carboxylic Acid Esters

Acyl iodides react with alkyl, alkenyl, and aralkyl esters derived from saturated, unsaturated, and aromatic mono- and dicarboxylic acids in the absence of a catalyst. The reaction involves cleavage of the OR bond and formation of organic iodide RI (including CH₂=CHI) and one or two symmetric carboxylic acid anhydrides. Phenyl acetate reacts with benzoyl iodide to give acetyl iodide and phenyl benzoate as a result of cleavage of the (O=)C–O bond. The reaction of diethyl fumarate with acetyl iodide is accompanied by cis– trans isomerization to afford maleic anhydride. In the reactions of acetyl iodide with diethyl oxalate and diethyl malonate, CO and CO₂ and CO₂ and polyketene are formed, respectively, in addition to ethyl iodide and acetic anhydride. Ethyl esters of strong organic acids, e.g., ethyl trihaloacetates, failed to react with acyl iodides under analogous conditions.     

Methylene chloride chemical ionization of fumaric and maleic acids and their esters

The chemical ionization (CI) mass spectrometry of fumaric and maleic acids and their esters with methylene chloride as reagent gas is described. The introduction of methylene chloride to the CI(CH₄) plasma led to the formation of new characteristic ions in addition to the protonation and the subsequent fragmentation, revealing diagnostic information on the configuration of geometrical isomers. The new characteristic ions have been found to arise from the addition of the reactant ion of methylene chloride, [CH₂Cl]⁺, to the substrates and, for higher dialkyl esters, from the further McLafferty rearrangement of the adduct ion [M + CH₂Cl]⁺.     

Studies of consecutive reactions of organic ions in a reversed geometry mass spectrometer

Over thirty consecutive reactions of a type m₁⁺ → m₂⁺ → m₃⁺ have been studied in a variety of organic compounds using a reversed geometry mass spectrometer. Two field free regions allow the separation of the two steps that make up the consecutive reaction sequence. Translational energy release measurements are used in the comparison of m₂⁺ ions formed as a result of a high energy process in the ion source with m₂⁺ ions formed as a product of a metastable decomposition. In some cases the structures of such ions have been found to be different. Examples have also been found where consecutive fragmentations of metastable ions do not occur although, when higher energy ions within the source are studied, the two-step reaction does take place. Furthermore, it has been found that a control over ion internal energy may be achieved by selecting portions of a peak due to the fragmentation of a metastable ion. Unimolecular reactions may then be used to study the reactivity of such ‘energy-selected’ ions; collision induced reactions can be used to study the structure of both the reactive and unreactive energy selected ions.