Organometallic compounds : XII. stereochemical interconversion reactions between 1,1′−bis(α-hydroxyalkyl)ferrocenes and 7-oxa[3]ferrocenophanes

Meso and racemic isomers of 1,1′-bis(α-hydroxyalkyl)ferrocene derivatives, and trans and cis isomers of 7-oxa[3]ferrocenophanes have been isolated. The configurations of these isomers have been determined both by spectroscopy and from their reaction behavior. It has been found that the ring-closure and ring-opening interconversion reactions between 1,1′-bis(α-hydroxyalkyl)ferrocenes and 7-oxa[3]ferrocenophanes are stereospecific. PMR spectra of the diols and ethers have been examined at 100 MHz in CF₃COOH, and the existence of α-[1′-(α-hydroxyalkyl)ferrocenyl]carbonium ions has been demonstrated in this acid. A mechanism for the various interconversion reactions is proposed on the basis of their stereospecific reactivity and from the PMR spectra of the carbonium ions.     

Pulse radiolysis study on the initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene

Initial processes of radiation-induced cationic polymerization of styrene and α-methylstyrene have been studied by means of microsecond pulse radiolysis. For styrene, absorption bands caused by the monomer cation radical St⁺? appear at 630 and 350 nm in a mixture of isopentane and n-butyl chloride at about ?165°C. In parallel with the decay of St⁺?, three absorption bands appear in the near-infrared (IR) region, and at 600 and 450 nm. The IR and 600 nm bands are assigned to the associated dimer cation radical St₂⁺?, and the 450 nm band to the bonded dimer cation radical St-St⁺?. The kinetic behavior of these species shows that reaction of St⁺? with styrene monomer forms both St₂⁺? and St-St⁺?. With the decay of St-St⁺?, another absorption band appears at 340 nm, and the lifetime of this band is relatively long. The 340 nm band may be due to carbonium ions of the growing polystyrene. For α-methylstyrene, the monomer cation radical (at 690 and 350 nm), the associated dimer cation radical (in the near-IR region and at 620 nm) and the bonded dimer cation radical (at 480 nm) behave in a manner similar to that of the corresponding styrene species. The absorption band caused by carbonium ions of growing poly(α-methylstyrene) appears at 340 nm.     

Unimolecular reactions of isolated organic ions: Some isomers of [C7H16]+˙

The slow unimolecular reactions of six isomers of [C₇H₁₆]^+˙ are reported and discussed. These results are interpreted in terms of dissociation via complexes of incipient carbonium ions and the appropriate associated radical. In some cases, rearrangement of the incipient carbonium ion precedes or accompanies decomposition; such isomerization generally favours alkyl radical loss, relative to elimination of the corresponding alkane.     

Isomerization of N-[α-(Alkylthio)alkyl]- and N-[α-(Arylthio)alkyl]benzotriazoles

The thermal isomerizations of N-[α-(alkylthio)alkyl]- and N-[α-(arylthio)alkyl]benzotriazoles have been investigated under N₂ atmospheres (i) in toluene, xylene, MeOH, or EtOH, in the presence of acid catalysts and (ii) in the absence of solvent. The sulfide isomerization rates depend on the number of H-atoms carried by the C-atom attached to the N-atom of the benzotriazole: tertiary (no hydrogen) > secondary (1 hydrogen) > primary (2 hydrogens). The results support an isomenzation mechanism involving a heterolytic N? C bond cleavage with formation of sulfonium/carbonium and benzotriazolate ions.     

Carbonium ion stabilization by (η5-cyclopentadienyl)dicarbonylnitrosylchromium; a 1H and 13C NMR study

the ¹H and ¹³C NMR spectra of a series of (η⁵-cyclopentadienyl)dicarbonylnitrosylchromium-substituted carbonium ions have been determined. These data suggest that the same mechanism is responsible for the stabilization of carbonium ions by (η⁵-cyclopentadienyl)dicarbonylnitrosylchromium as is operational in the analogous ferrocene and cymantrene complexes.     

Double-shift rearrangements of secondary ferrocenylcarbonium ions

In CF₃CO₂H solution, secondary ferrocenylcarbonium ions (Fc$\text{C}\text{+}$HCR¹R²R³) undergo a quantitative double-shift rearrangement to generate isomeric tertiary carbonium ions (Fc$\text{C}\text{+}$R¹CHR²R³).     

A predictive model for unimolecular reactions of organic ions

The slow unimolecular dissociations of six members of the [C_nH_2n-3]⁺ (n = 3-8) series of unsaturated carbonium ions are explained in terms of a potential surface approach together with some concepts of mechanistic organic chemistry. The occurrence of some dissociations is shown to be precluded because either the reacting configurations or product combinations are inaccessible at energies appropriate to metastable transitions. The approach permits correct predictions to be made concerning the shapes of metastable peaks for dissociations which occur without σ-bond formation in the final step. In particular, the observation of a composite peak, thus indicating two channels for reaction, for C₂H₄ loss from [C₇H₁₁]⁺ is naturally accommodated.     

Reactions of n-sulfinyl-p-toluenesulfonamide with alcohols

N-Sulfinyl p-toluenesulfonamide (1) reacted with triaryl- and diarylmethanols to give predominantly N- substiuted sulfonamides and SO₂ presumably via carbonium ion intermediates. When carbonium ion forming alchols, such as t-BuOH and Ph₂C(Me)OH, were used the predominant products were alkenes and p- toluenesulfonamide. Allytic alcohols afforded N- substiuted sulfonamides along with dienes andp-toluenesulfonamide. Alcohols which could not predictably give relatively stable intermediate carbonium ions, gave either dialkyl sulfites or dialkyl eithers, along with p-toluenesulfonamide. In one case, namely with 9-phenylfluorenol the 1:1 adduct with 1 (an amidosulfite) was isolated. A mechanism for the reactions is proposed.     

A carbon-13 NMR spectroscopic study of ferrocenylmethyl carbonium ions

The ¹³C NMR spectra of three ferrocenylmethyl carbonium ions and of the alcohols from which they were derived have been obtained. The chemical shifts and coupling constants of the carbonium ions are explicable in terms of a structure that has the positive charge extensively delocalized throughout the entire molecule, with a fulvenelike structure for the substituted cyclopentadienyl ring.     

Polycondensation of benzyl chloride and its derivatives: A study of the reaction at different temperatures

The polycondensation reactions of benzyl chloride, α-chloroethylbenzene, and benzhydryl chloride in the presence of SnCl₄ or AlCl₃ as catalysts have been investigated in the temperature range between +80° and ?135°C. Polycondensations of benzyl chloride and α-chloroethylbenzene are quite similar in the reaction kinetics and are thought to occur by the same displacement mechanism. Polycondensation of benzhydryl chloride, however, seems to involve the formation of benzhydryl carbonium ions. At low temperatures linear polymers tend to be formed, in contrast with branched polymers produced at room temperature. Steric effects are found to play a major role in protecting polymers from branching at lower temperatures. Polybenzyl polymers are found to be less linear than poly(-α-methylbenzyl), even when prepared at ?135°C.     

Application of semi-empirical calculations of 13C chemical shifts and 1J(CH) coupling constants to the analysis of carbonium ion structures

¹³C chemical shifts and ¹³CH coupling constants for classical and non-classical structures of some typical carbonium ions have been calculated by the method of summation over excited states in the INDO approximation using a special parameterization. Comparison of the data obtained with the experimental values shows that the calculation of the chemical shifts and spin-spin coupling constants is superior to energy calculations, and provides a useful means for the study of the structure of carbonium ions.     

Carbocations and Electrophilic Reactions

The general concept of carbocations encompasses all cations of carbon containing compounds which can be differentiated into two distinct classes: trivalent (“classical”) carbenium ions and pentacoordinated (“nonclassical”) carbonium ions. In this paper the preparation and structural study (by NMR, IR, Raman and ESCA spectroscopy) of stable carbenium ions and carbonium ions are discussed. As is well known, trivalent carbenium ions play an important role in electrophilic reactions of π- and n-donor systems. Similarly, pentacoordinated carbonium ions are the key to electrophilic reactions of σ-donor systems (single bonds). The ability of single bonds to act as σ-donor lies in their ability to form carbonium ions via triangular two electron, three-center bond formation. Consequently there seems to be no principle difference between the electrophilic reactions of π- and σ-bonds except that the former react more easily, even with weak electrophiles, whereas the latter necessitate more severe conditions. The role of carbocations in electrophilic reactions of π- and σ-donor systems is discussed.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

Dissociation of Multisubunit Protein–Ligand Complexes in the Gas Phase. Evidence for Ligand Migration

The results of collision-induced dissociation (CID) experiments performed on gaseous protonated and deprotonated ions of complexes of cholera toxin B subunit homopentamer (CTB₅) with the pentasaccharide (β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp (GM1)) and corresponding glycosphingolipid (β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp-Cer (GM1-Cer)) ligands, and the homotetramer streptavidin (S₄) with biotin (B) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (Btl), are reported. The protonated (CTB₅ + 5GM1)ⁿ⁺ ions dissociated predominantly by the loss of a single subunit, with the concomitant migration of ligand to another subunit. The simultaneous loss of ligand and subunit was observed as a minor pathway. In contrast, the deprotonated (CTB₅ + 5GM1)^n- ions dissociated preferentially by the loss of deprotonated ligand; the loss of ligand-bound and ligand-free subunit were minor pathways. The presence of ceramide (Cer) promoted ligand migration and the loss of subunit. The main dissociation pathway for the protonated and deprotonated (S₄ + 4B)^n+/– ions, as well as for deprotonated (S₄ + 4Btl)^n– ions, was loss of the ligand. However, subunit loss from the (S₄ + 4B)ⁿ⁺ ions was observed as a minor pathway. The (S₄ + 4Btl)ⁿ⁺ ions dissociated predominantly by the loss of free and ligand-bound subunit. The charge state of the complex and the collision energy were found to have little effect on the relative contribution of the different dissociation channels. Thermally-driven ligand migration between subunits was captured in the results of molecular dynamics simulations performed on protonated (CTB₅ + 5GM1)¹⁵⁺ ions (with a range of charge configurations) at 800 K. Notably, the migration pathway was found to be highly dependent on the charge configuration of the ion. The main conclusion of this study is that the dissociation pathways of multisubunit protein–ligand complexes in the gas phase depend, not only on the native topology of the complex, but also on structural changes that occur upon collisional activation.

Mass spectra of some (Z)-α-Phenyl-β-(2-thienyl)acrylonitriles

The mass spectra of some (Z)α-(4-R′-phenyl)-β-(2-thienyl-5-R)acrylonitriles (R = H, CH₃, Br; R′ = H, CH₃O, CH₃, Cl, NO₂) at 70 eV are reported. Mass spectra exhibit pronounced molecular ions. The compound's where R = H, and CH₃ are characterized by the occurrence of a strong [M - H]⁺ peak. Moreover, in all the compounds a m/z 177 peak occurs. In the compounds where R = H, [M - HS]* and [M - CHS]* ions are present except the nitroderivatives. Where R = CH₃, [M - HS]⁺ ion occurs.     

Local structure distortion and spin Hamiltonian parameters for Cr4+ ions in Cr4+:α-Al2O3 crystals

The relations between the spin Hamiltonian (SH) parameters and crystal structure of Cr⁴⁺:α-Al₂O₃ crystals have been established. On the basis of this, the SH parameters including zero-field splitting parameter D and Zeeman g-factors (g_|| and g_⊥) for Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals, taking into account the spin–spin (SS), spin-other-orbit (SOO) and orbit–orbit (OO) magnetic interactions in addition to the spin–orbit (SO) magnetic interaction, are theoretically investigated using complete diagonalization method (CDM). The theoretical results are in excellent agreement with the experimental ones when the upper three O^2? ions rotate 0.94° toward [1 1 1] axis and the lower three O^2? ions rotate 0.92° toward it. Hence, the local structure distortion effect plays an important role in explaining the spectroscopic properties of Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals. This study shows that for Cr⁴⁺:α-Al₂O₃ the contributions arising from SS, SOO, and OO interactions to the zero-field splitting (ZFS) parameter D are appreciable, whereas those to g_|| and g_⊥ are quite small.     

Loss of Internal Backbone Carbonyls: Additional Evidence for Sequence-Scrambling in Collision-Induced Dissociation of y-Type Ions

It is shown that y-type ions, after losing C-terminal H₂O or NH₃, can lose an internal backbone carbonyl (CO) from different peptide positions and yield structurally different product fragment ions upon collision-induced dissociation (CID). Such CO losses from internal peptide backbones of y-fragment ions are not unique to a single peptide and were observed in four of five model peptides studied herein. Experimental details on examples of CO losses from y-type fragment ions for an isotopically labeled AAAAHAA-NH₂ heptapeptide and des-acetylated-α-melanocyte-stimulating hormone (dα-MSH) (SYSMEHFRWGKPV-NH₂) are reported. Results from isotope labeling, tandem mass spectrometry (MSⁿ), and ion mobility-mass spectrometry (IM-MS) confirm that CO losses from different amino acids of m/z-isolated y-type ions yield structurally different ions. It is shown that losses of internal backbone carbonyls (as CID products of m/z-isolated y-type ions) are among intermediate steps towards formation of rearranged or permutated product fragment ions. Possible mechanisms for generation of the observed sequence-scrambled a-“like” ions, as intermediates in sequence-scrambling pathways of y-type ions, are proposed and discussed. ?      

Model quantum chemical studies of the electronic structure and aspects of reactivity of diol epoxides of aromatic hydrocarbons

This paper offers a unified presentation of the main ring conformers of the diol expoxides, triol carbonium ions, and tetrols related to the “bay-region” benzo ring of carcinogenic metabolites of PAH and brings forward quantitative information through ab initio SCF computations about their relative energies. It substantiates and evaluates the energy of the syn epoxide-OH4 hydrogen bond and, on the contrary, refutes the significance of an O ?H? O or an O^? ?H? O bond in the triol carbonium ions. It provides an explanation for the similar rates of acid-catalyzed hydrolysis of the syn and anti diol expoxides. It evaluates the stabilization of the triol carbonium ions due to the presence of a neighboring double bond and accounts for the spontaneous opening of protonated epoxides. Finally it accounts for cis hydrolysis of the syn diol epoxide and trans hydrolysis of the anit diol epoxide under acid conditions.     

Structural characterization of xylo-oligosaccharides from corncob residues

The structure of xylo-oligosaccharides (XOSs) derived from corncob residues was elucidated by means of FT-IR, GC-MS, ESI-CID-MS/MS, and 2D-NMR. GC-MS analysis confirmed that the partially methylated acetyl alditol derivatives of XOS were 1,5-Ac₂-2,3,4-Me₃-xylitol and 1,4,5-Ac₃-2,3-Me₂-xylitol. The C- and Y-type ions from glycosidic bond cleavage and ^0,2A (?60) and ^0,2A-H₂O (?78) ions from cross-ring cleavage of XOS were produced in ESI-CID-MS/MS. Both results indicated that the XOSs were (1→4)-linked xylans with a degree of polymerization from 2 to 7. The anomeric configuration and glycosidic linkage of XOSs were determined by 2D-NMR. Eventually, it was suggested that the structure of these XOSs was β-Xyl-(1→4)-[β-Xyl-(1→4)]_n-α/β-Xyl (n = 0–5) without other substituents.     

Specific Cα-C Bond Cleavage of β-Carbon-Centered Radical Peptides Produced by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

Radical-driven dissociation (RDD) of hydrogen-deficient peptide ions [M???H?+?H]^·+ has been examined using matrix-assisted laser dissociation/ionization in-source decay mass spectrometry (MALDI-ISD MS) with the hydrogen-abstracting matrices 4-nitro-1-naphthol (4,1-NNL) and 5-nitrosalicylic acid (5-NSA). The preferential fragment ions observed in the ISD spectra include N-terminal [a]?+?ions and C-terminal [x]⁺, [y?+?2]⁺, and [w]⁺ ions which imply that β-carbon (Cβ)-centered radical peptide ions [M???Hβ?+?H]^·+ are predominantly produced in MALDI conditions. RDD reactions from the peptide ions [M???Hβ?+?H]^·+ successfully explains the fact that both [a]⁺ and [x]⁺ ions arising from cleavage at the Cα-C bond of the backbone of Gly-Xxx residues are missing from the ISD spectra. Furthermore, the formation of [a]⁺ ions originating from the cleavage of Cα-C bond of deuterated Ala(d3)-Xxx residues indicates that the [a]⁺ ions are produced from the peptide ions [M???Hβ?+?H]^·+ generated by deuteron-abstraction from Ala(d3) residues. It is suggested that from the standpoint of hydrogen abstraction via direct interactions between the nitro group of matrix and hydrogen of peptides, the generation of the peptide radical ions [M???Hβ?+?H]^·+ is more favorable than that of the α-carbon (Cα)-centered radical ions [M???Hα?+?H]^·+ and the amide nitrogen-centered radical ions [M???H_N?+?H]^·+, while ab initio calculations indicate that the formation of [M???Hα?+?H]^·+ is energetically most favorable.

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