The structure of ionized 1,5 hexadiene in the gas phase

The structure of ionized 1,5-hexadiene, prepared by charge transfer between 1,5-hexadiene and CS2+*, is examined using energy-resolved collision-induced dissociation (CID). By comparing the product distributions and product appearance curves with those of authentic low-energy C6H10+* ions, it is determined that 1,5-hexadiene cation spontaneously rearranges to cyclohexene cation in the gas-phase. The proposed mechanism for formation of cyclohexene cation in the gas phase is analogous to that determined for this process under matrix isolation conditions, where it proceeds via a Cope rearrangement to the cyclohexane-1,4-diyl cation, followed by isomerization to cyclohexene cation. It is shown that electron ionization (EI) of 1,5-hexadiene gives a different molecular ion than is obtained upon chemical ionization (CI). The energy-resolved CID mass spectrum for the EI product is consistent with what would be obtained for a mixture of low energy ion isomers.     

Obtaining structural information on perfluorocarbons by mass spectrometry II-charge-exchange method

Structural and molecular ion information on perfluorocarbons (PFCs) can be obtained by charge-exchange reactions of nitrogen or oxygen radical cations with PFCs; such information usually cannot be obtained by the electron impact or negative ion chemical ionization methods. Charge-exchange reactions occurred when 0.5-1.0 Torr (1 Torr as 133.3 Pa) of nitrogen, air or other gases was introduced into the chemical ionization source of a mass spectrometer, with electron energies ranging from 55 to 230 eV. Changing the repeller voltages and electron energies had little effect on the appearance of charge-exchange mass spectra of PFCs. However, when air or nitrogen was introduced in the direction opposite to sample flow, more intense molecular ions and/or higher mass fragment ions were obtained at the cost of a small loss of sensitivity. The charge-exchange mass spectrum of perfluoro-1,3-dimethyladamantane is very similar to the high-energy collisionally induced dissociation spectrum of its molecular radical cation. This technique provides an important validation tool without the use of tandem mass spectrometry for the structure determination of PFC isomers.     

Tandem mass spectrum of a farnesyl transferase inhibitor--gas-phase rearrangements involving imidazole

Compound 1 (1-(3-chlorophenyl)-4-[1-(4-cyanobenzyl)imidazolylmethyl]-2-piperazinone hydrochloride) is a farnesyl transferase inhibitor intended for treatment of cancer. A detailed analysis of the electrospray ionization mass spectrometry and tandem mass spectrometry data of protonated 1 shows that in the gas phase, upon collision-induced dissociation, this ion undergoes complicated rearrangement and fragmentation. These processes include a novel two-step rearrangement. The first step involves a gas-phase intramolecular S(N)2 reaction that forms an intermediate. The second step consists of three competitive rearrangement/fragmentation pathways of the intermediate, giving rise to protonated 2, protonated methylene-imidazole, and a distonic methylimidazole radical cation. Deuterated 1 was studied under the same experimental conditions, and the results strongly support the proposed two-step rearrangement process. It is noted that the unique structure of 1, especially the imidazole ring of 1, plays a critical role in the rearrangement of protonated 1.     

A PhSeCl-mediated allylic oxidation of prenyl moiety: a convenient method for the construction of 3-isopenten-2-ol unit

A phenylselenenyl chloride (PhSeCl)-mediated allylic oxidation to give allylically rearranged alcohol has been developed. A possible mechanism for the present reaction is generation of allylic selenide from prenyl moiety via [1,3]-sigmatropic rearrangement, followed by oxidation and [2,3]-sigmatropic rearrangement to afford 3-isopenten-2-ol.     

3,3]-Sigmatropic shifts of N-allylhydrazones: quantum chemical comparison of concerted and radical cation pathways

N-Allylhydrazones are reported to undergo an elaborate [3,3]-sigmatropic shift/N2 extrusion sequence. Both concerted and radical cation pathways for the [3,3]-sigmatropic shift of several N-allylhydrazones were investigated using B3LYP/6-31+G(d,p) calculations. It was discovered that, assuming facile formation of the N-allylhydrazone radical cation, the rearrangement takes place through a series of low barrier steps energetically preferred to the concerted alternative available to neutral N-allylhydrazones. Subsequent N2 extrusions forming corresponding homoallyl radicals were found to be extremely facile.     

A Non-Covalent Dimer Formation of Quaternary Ammonium Cation with Unusual Charge Neutralization in Electrospray-Ionization Mass Spectrometry

Specific and nonspecific non-covalent molecular association of biomolecules is characteristic for electrospray-ionization mass spectrometry analysis of biomolecules. Understanding the interaction between two associated molecules is of significance not only from the biological point of view but also gas phase analysis by mass spectrometry. Here we reported a formation of non-covalent dimer of quaternary ammonium denatonium cation with +1 charge detected in the positive ion mode electrospray ionization mass spectrometry analysis of denatonium benzoate. Hydrogen deuterium exchange of amide and carbon-bonded hydrogens revealed that charge neutralization of one denatonium cation is the consequence of amide hydrogen dissociation. DFT (Density Functional Theory) calculations proved high thermodynamic stable of formed dimer stabilized by the short and strong N..H-N hydrogen bond. The signal intensity of the peak characterizing non-covalent dimer is low intensity and does not depend on the sample concentration. Additionally, dimer observation was found to be instrument-dependent. The current investigation is the first experimental and theoretical study on the quaternary ammonium ions dimer. Thus the present study has great significance for understanding the structures of the biomolecules as well as materials.     

Gas-phase generation and electronic structure investigation of chlorosulfanyl thiocyanate, ClSSCN

The chlorosulfanyl thiocyanate molecule, ClSSCN, was generated in the gas phase through heterogeneous reaction of SCl2 on the surface of finely powdered AgSCN for the first time. The reaction products were detected and characterized in situ by ultraviolet photoelectron and photoionization mass spectrometry. The molecular geometry and electronic structures of ClSSCN were investigated by a combination of PES experiment and theoretical calculations with the density functional theory and ab initio methods. It was found that the outermost electrons of ClSSCN reside in the Cl-S antibonding pi orbital, predominantly localized on the sulfur atom, and the experimental first vertical ionization potential of ClSSCN is 10.20 eV. The dominant fragment SSCN+ in the mass spectrum indicates that the ClSSCN cation prefers the dissociation of the Cl-S bond.     

Asymmetric organocatalysed [1,3]-sigmatropic rearrangements

The first organocatalysed enantioselective [1,3]-sigmatropic O- to N-rearrangement reactions are presented. The reactions take place under regio- and enantioselective control, and are catalysed by cinchona alkaloids. Two reactions have been developed the first one is the rearrangement of imidates to amides, while the other rearrangement occurs from carbamates to amines via a decarboxylation. Both transformations give nitrogen protected beta-amino acid derivatives as the product. These novel asymmetric organocatalysed [1,3]-sigmatropic O- to N-rearrangement reactions provide a reliable and efficient synthetic method for obtaining enantioenriched beta-amino acid derivates in good yield from racemic starting materials.     

Catalytic Asymmetric [ 2,3 ] -Sigmatropic Rearrangement of Sulfur Ylides Generated from Carbenoids

The catalytic asymmetric reactions of oxygen or sulfur ylides generated from carbenoids have attracted consider able attention in recent years. High enantioselectivities have been achieved in the 1,3-dipolar cycloaddition reactions and [ 2,3 ]-sigmatropic rearrangement of oxygen ylides. In contrast to the oxygen ylide, the corresponding catalytic asymmetric reaction of sulfur ylide is less developed. Compared to oxygen ylides, the sulfur ylides are more stable and experimental evidence supports a free ylide rather than a metal-bound ylide as reaction intermediate. That means the enantio-control must be in the step of the ylide formation. If the subsequent reaction such as [ 2,3 ]-sigmatropic rearrangement or 1,3-dipolar cycloaddition is a concerted process and is faster than the racemization of the chiral ylide intermediate, then the catalytic asymmetric sulfur ylide reaction will be possible.     

1-Alkynyl Disulfides: Their Characterization and Their 1-Thiapropargyl — 3-Thiaallenyl Rearrangements

The synthesis of novel 1-alkynyl disulfides 1 is described together with their 1-thiapropargyl — 3-thiaallenyl rearrangement and a [1,3]-sigmatropic shift of 1-alkynyl amino disulfides, followed by rearrangement of the aminothio-substituted thioketenes so formed.     

Semiconductor-catalyzed photocycloreversion,valence isomerization and [1,3]-sigmatropic rearrangement

The semiconductor-catalyzed photochemical [2+2]cycloreversion of n-methylquinolone dimer, valence isomerization of hexamethyl(Dewar)benzene, and [1,3]-sigmatropic rearrangement of 2,2-bis(4-methoxyphenyl)-1-dideuteriomethylenecyclopropane gave N-methylquinolone, hexamethylbenzene, and 2,2-bis(4- methoxyphenyl)-3,3-dideuterio-1-methylenecyclopropane, respectively.     

Nouvelles synthèses de systèmes bicycliques hétéroaromatiques. I. Cycloaddition [4 + 2]π de la diméthyl-2,7 diméthylthio-3,5 triazépine-1,2,4

The cycloaddition of 3,5-dimethylthio-1,2,4-triazepine (1) to various dienophiles has been studied. 4-Phenyl-1,2,4-triazoline-3,5-dione, dimethyl acetylene dicarboxylate and meleic anhydride react with I to give the [4+2]π cycloadduct. The heterocyclic products were found to be formed via 1,3-sigmatropic rearrangement of this cycloadduct.     

Vinylphosphirane-phospholene rearrangements: pericyclic [1,3]-sigmatropic shifts or not?

The conversion of "free" and Cr(CO)(5)-complexed 2-vinylphosphiranes into 3-phospholenes via [1,3]-sigmatropic shifts was studied with density functional theory and compared with the corresponding hydrocarbon system, that is, the vinylcyclopropane-cyclopentene rearrangement. All three systems behave similarly with subtle but important differences. No intermediate was found on any of the potential energy surfaces. 2-Vinylphosphiranes have smaller rearrangement barriers than vinylcyclopropane, and those carrying the Cr(CO)(5) group have still smaller ones. The rearrangement of both anti- and syn-2-vinylphosphiranes occurs in a concerted pericyclic manner with inversion of configuration at the migrating phosphorus, requiring, respectively, 29.3 and 36.7 kcal/mol, much in contrast to the 44.6 kcal/mol demanding diradical-like process for the hydrocarbon analogue. Epimerization at the phosphorus center (syn right arrow over left arrow anti) requires approximately 32.0 kcal/mol and occurs in a single step, reflecting a diradical-like ring opening-ring closure process that can occur in both a clockwise and counterclockwise fashion. Complexation of the phosphorus center by Cr(CO)(5) results in the substantial stabilization of reagents and products and further reduces the barriers for rearrangement. The anti isomer has the lowest barrier for the [1,3]-shift (DeltaE = 20.5 kcal/mol), which is slightly less than that needed for P-epimerization and for conversion of the syn isomer, both of which are nonpericyclic processes. When a P-phenyl group is introduced, the diradical-like conversion of the syn isomer is favored over the anti isomer, in agreement with experimental reports. The influence of torquoselectivity is discussed for the rearrangements of these structures with their heavy substituents. The origin of the stabilization rendered by the Cr(CO)(5) group and its influence on the [1,3]-conversion are also analyzed. The DFT activation energies for the diradical-like [1,3]-sigmatropic shifts were verified with a multireference method.     

Kombination von Feldionen- und Elektronestoß-Massenspektren zur Struckturbestimmung,Demonstriert am Beispiel von Monoterpenen

The field ionization and electron impact mass spectra of some monoterpenes are compared. Some general rules applicable also to other substances are derived, summarizing the new structural information that can be obtained by combination of field ionization and electron impact data. The general conclusions are: (1) By comparison of FI and EI mass spectra one can recognize which of the most intense peaks in the EI mass spectra are originating from rearrangement or multistip dissociation processes on the one hand, or from simple direct bond rupture on the other hand. (2) The molecular weight of each substance containing C, H, O, N atoms (or some of them) can be determined unambiguously by field ionization, even if there is no parent peak detectable by electron impact. (3) Extremely strong metastable peaks in the FI mass spectrum are indicative of the splitting off of a polar group from a highly branched carbon atom, under rearrangement of the molecular ion. (4) Ions of about double the molecular weight are often found in the FI mass spectra if the molecule contains carbonyl or hydroxyl groups, or conjugated multiple bonds in aliphatic compounds.     

Competing Diels-Alder reactions of activated nitroethylene derivatives and [3,3]-sigmatropic rearrangements of the cycloadducts

Diels-Alder reaction of nitroethylene derivatives with cyclohexa-1,3-diene afforded three pericyclic products some of which could be converted to others via a new [3,3]-sigmatropic rearrangement or via a Claisen rearrangement.     

Gas phase substitution reactions by radical cations Part 5: Reaction of the CC ring-opened oxirane radical cation with 1,2-, 1,3- and 1,4-dichlorobenzene

Under conditions of chemical ionization in the high pressure source of a mass spectrometer, the α-distonic CC ring-opened oxirane radical cation transfers a methylene group to 1,2-, 1,3- and 1,4-dichlorobenzene. The structures of the M + 14]^·+ product ions have been established by collisionally induced dissociation of these ions compared with reference ions and application of principal component analysis. 1,2-Dichlorobenzene yields 80% 2-chlorobenzyl chloride, 5% 2,3-dichlorotoluene and 15% 3,4-dichlorotoluene. The [M + 14]^·+ ions from 1,3-dichlorobenzene are 64–67% 3-chlorobenzyl chloride, 27–28% 2,6-dichlorotoluene and 7% 2,4- or 3,5-dichlorotoluene. From 1,4-dichlorobenzene mainly 4-chlorobenzyl chloride is formed, together with some 2,5-dichlorotoluene. In this case there is also an unidentified contribution, probably by 1,4-dichlorocycloheptatriene ions. Possible formation of distonic product ions does not occur in the cases of 1,2- and 1,3-dichlorobenzene, and from 1,4-dichlorobenzene it is considered to be unlikely.     

[3,3]-Sigmatropic rearrangement of allyl and 2-butenyl 1-naphthyl sulfides

The [3,3]-sigmatropic rearrangement of alkenyl 1-naphthyl sulfides in solutions with various polarities was investigated at 138–190 °C. The reaction proceeds through the formation of 2-alkenyl-1-naphthalene thiols, which subsequently undergo cyclization to compounds of the 2,3-dihydronaphtho[1,2-b]thiophene and naphtho[1,2-b]dihydrothiopyran series. 2-Butenyl 1-naphthyl sulfide, in addition to its passing directly through a [3,3]-sigmatropic rearrangement, to a considerable extent undergoes a prior [1,3]-sigmatropic rearrangement, which ultimately leads to the formation of four cyclic products. The kinetic parameters of the rearrangement of the sulfides were determined. The more negative entropies of activation constitute evidence for the high symmetry of the transition state.Translated from Khimiya Geterotskilicheskikh Soedinenii, No. 5, pp. 611–614, May, 1981.     

Microstructural study of a nitroxide-mediated poly(ethylene oxide)/polystyrene block copolymer (PEO-<Emphasis Type="Italic">b</Emphasis>-PS) by electrospray tandem mass spectrometry

Electrospray ionization tandem mass spectrometry has been used to characterize the microstructure of a nitroxide-mediated poly(ethylene oxide)/polystyrene block copolymer, called SG1-capped PEO-b-PS. The main dissociation route of co-oligomers adducted with lithium or silver cation was observed to proceed via the homolytic cleavage of a C-ON bond, aimed at undergoing reversible homolysis during nitroxide mediated polymerization. This cleavage results in the elimination of the terminal SG1 end-group as a radical, inducing a complete depolymerization process of the PS block from the so-formed radical cation. These successive eliminations of styrene molecules allowed a straightforward determination of the PS block size. An alternative fragmentation pathway of the radical cation was shown to provide structural information on the junction group between the two blocks. Proposed dissociation mechanisms were supported by accurate mass measurements. Structural information on the SG1 end-group could be reached from weak abundance fragment ions detected in the low m/z range of the MS/MS spectrum. Amongst fragments typically expected from PS dissociation, only beta ions were produced. Moreover, specific dissociation of the PEO block was not observed to occur in MS/MS, suggesting that these rearrangement reactions do not compete effectively with dissociations of the odd-electron fragment ions. Information about the PEO block length and the initiated end-group were obtained in MS(3) experiments.     

Generation of arylnitrenium ions by nitro-reduction and gas-phase synthesis of <Emphasis Type="Italic">N</Emphasis>-Heterocycles

Nitro-reduction by the vinyl halide radical cation CH2 = CH-X+* (X = Cl or Br) converts nitroaromatics into arylnitrenium ions, significant intermediates in carcinogenesis, and the present study reports on the scope and regioselectivity of this versatile reaction. The reaction is general for different kinds of substituted nitroaromatics; para/meta substitutents have little effect on the reaction while ortho substitutents result in low yields of arylnitrenium ions. The phenylnitrenium ion PhNH+ can be generated by chemical ionization (CI) of nitrobenzene using 1,2-dichloroethane as the reagent gas or by atmospheric pressure chemical ionization (APCI) of 1,2-dichloroethane solution doped with nitrobenzene. The chemical reactivities of the arylnitrenium ions include one-step ion/molecule reactions with nucleophiles ethyl vinyl ether and 1,3-dioxolanes, respectively, involving the direct formation of new CN bonds and synthesis of indole and benzomorpholine derivatives. The indole formation reaction parallels known condensed phase chemistry, while the concise morpholine-forming reaction remains to be sought in solution. The combination of collision-induced dissociation (CID) with novel ion/molecule reactions should provide a selective method for the detection of explosives such as TNT, RDX and HMX in mixtures using mass spectrometry. In addition to the reduction of the nitro group, reduction of methyl phenyl sulfone PhS(O)2Me to the thioanisole radical cation PhSMe+* occurs using the same chemical ionization reagent 1,2-dichloroethane. This probably involves an analogous reduction reaction by the reagent ion CH2 = CH-Cl+*.