Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen–II: Methoxygruppenwanderungen und Ringfragmentierungen in den Massenspektren der Methylather cyclischer Polyalkohole

The mass spectra of methyl ethers of cyclopentane-, cyclohexane- and cycloheptanediols contain a characteristic m/e 75 peak of ions of the structure ⊕CH(OCH₃)₂, which are formed by 1,3-, 1,4- or 1,5-migration of a methoxy group. The formation of these ions does not depend on the stereochemistry of the original molecule; probably the methoxy group migration occurs after α-cleavage of the molecular ion in an open chain radical ion: In the case of cycloalkanes with three or more methoxy groups the m/e 75 rearrangement ions give rise to one of the most prominent peaks of the mass spectra. Chemical bonds, terminating at a carbinol-C-atom, are weaker than normal. The presence of several carbinol-C-atoms in the molecular ions of cyclic polymethoxy compounds therefore favours fragmentations which differ from those of simple derivatives of cyclanols and which are typical of the position of the methoxy groups.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen.—I Methoxygruppenwanderungen in den Massenspektren der Methyläther von linearen und verzweigten aliphatischen Polyalkoholen

The formation of 1,1-dimethoxy-alkyl rearrangement ions in the mass spectra of methyl ethers of linear diols, 1,2,3-triols and of derivatives of pentaglycerol and pentaerythritol has been investigated by deuterium labelling and mass measurements. Methoxy group migrations do not occur, or at least only to a small amount, in the mass spectra of the diol-dimethyl ethers. The mass spectra of methyl ethers of 1,2,3-triols exhibit characteristic peaks of the rearrangement ions +CH(OCH₃)₂ and +CR(OCH₃)₂. These ions arise by a 1,3-migration of a methoxy group, probably during a one step degradation of the molecular ion to give a molecule methyl alkenyl ether and a H-atom or alkyl radical as neutral fragments. Large peaks of the rearrangement ion +CH(OCH₃)₂ are observed in the mass spectra of compounds of the following type: A radical ion, formed by loss of HY from the molecular ion, rearranges by methoxy group migration in the mass spectra of these substances. Rearrangement by migration of hydroxy and acetoxy groups are also observed, but no migration of a Cl-atom.     

Induktive und sterische Effekte bei der BECKMANN-CHAPMAN-Umlagerung von Alkyl-ketoxim-pikryläthern

The rates of the BECKMANN -CHAPMAN rearrangement of the ketoximpicryl-ethers 1a–1k in 1, 4-dichlorobutane are correlated with parameters describing the bulk of the substituted groups R and R′. From regression analyses it is concluded that the relative rate constants are mainly controlled by steric contributions in the ground state, such as geminal interactions of R with R′ and vicinal interactions of R′ with O? X, and less by inductive effects. Conformational equilibria between (Z)- and (E)-isomers of N-alkyl-N-picryl-amids have been studied by NMR. in several solvents.     

Zum Mechanismus Massenspektrometrischer Fragmentierungsreaktionen—IV: Zur Bildung von Phenonium-Ionen bei der Elektronenstoß-Fragmentierung von ß-Phenyläthylbromiden

The apparently equal binding of all 9 H-atoms, demonstrated with the aid of deuterium labeling by N. M. M. Nibbering et al.,³ in [C₈H₉]⁺-ions generated from β-phenylethyl bromides has been confirmed. This result has been explained by a cyclooctatrienyl structure (a) of the ions. A comparison of the heats of formation of [C₈H₉]⁺-ions of different origin reveals, however, that this is not true for [C₈H₉]⁺-ions of β-phenylethyl bromide in the ground state. As a result of a study of the influence of substituents at the phenylring of β-phenylethyl bromide on the intensities of the [M ? Br]⁺-ions, R. H. Shapiro et al.⁴ proposed a phenonium structure (c) for the [C₈H₉]⁺-ions. The substituent effect on intensities and AP of [M ? CH₂Br]⁺- and [M ? Br]⁺-ions of β-phenylethyl bromides is quite different and can be explained much better by a ‘classical’ structure of the [C₈H₉]⁺- ions. This structure can also be deduced by a comparison of the IP and AP of [M ? Br]⁺-ions of benzyl-, β-phenylethyl- and γ-phenylpropyl-bromide.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen: XIII—Einfluß der Dissoziationsenergie auf intramolekulare aromatische Substitutionen in den Molekül-Ionen von N,N-Dimethyl-N′-phenylformamidinen

The effect of the dissociation energy of the C? X bond (X = H, F, Cl, Br, I) on the formation of benzimidazolium ions (b) by elimination of X from the molecular ions of ortho-substituted N,N-dimethyl-N′-phenylformamidines (I to V) has been investigated. No simple relation is observed between the intensities of ions b and the dissociation energy. Furthermore, the appearance potentials of ions b are not greatly affected by the dissociation energy, although differences of about 2.5 eV are expected for a simple cleavage reaction. The behaviour of the molecular ions of I to V is in accord with a two step addition-dissociation mechanism [M]⁺· → a → b, and the highest activation energy is required in the first addition step. Similar mechanisms are known for aromatic substitution reactions in the condensed phase, but have not been observed for mass spectrometric fragmentations. The detection of additional kinetic energy T in the reaction products by an analysis of the metastable transitions [M]⁺· → b corroborates the proposed mechanism.     

Zum Mechanismus Massenspektrometrischer Fragmentierungsreaktionen—V: Substituenteneffekte auf Intramolekulare Substitutionen des Phenylrestes in den Massenspektren von N,N-Dimethyl-N′-Phenylformamidinen

The mass spectrum of N,N-dimethyl-N′-phenyl-formamidine (I) contains a large peak due to [M ? H]+-ions. As is shown by deuterium labelling, one of the ortho hydrogen atoms of the phenyl group is lost. The same result has been observed for the corresponding fragmentation of thioformanilide (V). This can be explained by the formation of benzimidazolium-ions (a). The effect of substituents at the phenyl group on the intensities and AP of these ions and on the IP of the molecular ions has been investigated. A mechanism of the cyclization reaction is proposed.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen: XI—Einfluß von Substituenten auf die Bildung cyclischer Fragment-Ionen in den Massenspektren von N,N-Dimethyl-N′-2-chlorphenylformatmidinen und 2-Chlorformaniliden

Molecular ions of N,N-dimethyl-N′-2-chlorophenylformamidines (III) and 2-chloroformanilides (IV) lose a chlorine atom to give benzimidazolium and benzoxazolium ions, respectively. As with N,N-dimethyl-N′-phenylformamidines (I), a linear relationship exists between the Hammett σ-constants and the effect of substituents on the ionisation potentials of substituted III and IV. In contrast to this, the appearance potentials of the cyclic fragment ions of III and IV cannot be easily related to polar effects of substituents; these effects are similar for the cyclic fragment ions of I, III and IV however. Furthermore, the intensities of these ions are influenced in the same direction by substituents in the mass spectra of I, III and IV, and are strongly reduced by electron donating substituents in the para position. The formation of cyclic fragment ions in the mass spectra of I, III and IV therefore occurs by the same mechanism.     

Zum Mechanismus der Imidazol-Katalyse der Cyclo-trimerisation und cis-trans Umlagerung von Maleinsäure-Derivaten

A plausible explanation of the reaction mechanism is given based on addition intermediates with zwitter ion structure.     

Zum Mechanismus massenspektrometrischer Fragmentierungsreaktionen: XII-Einfluß einer sterischen Hinderung der Mesomerie bei der Fragmentierung von N,N-Dimethyl-N′-4-Biphenylformamidinen

The formation of cyclic [M ? H]⁺ ions (a) in the mass spectra of N,N-dimethyl-N′-arylformamidines is suppressed by electron donating groups in the aryl group para to the formamidine group, due to the participation of resonance structures of type c. This effect is also observed in the mass spectra of N,N-dimethyl-N′-4-biphenylformamidines (III) substituted in the 4′-position. By substitution in the 2-, 2′-, 6- and 6′- positions of III, the formation of resonance structure c′ in the molecular ions is so difficult that these derivatives show the normal fragmentations. This is observed with two methyl groups in the 2- and 2′- positions of III.     

Zum mechanismus Massenspektrometrischer Fragmentierungsreaktionen–III: Stereospezifische Reaktionen in den Massenspektren der Methyläther cyclischer Polyalkohole

The mass spectra of stereoisomers of polymethoxy cycloalkanes depend on the geometry of the molecular ions. The magnitude of the stereochemical effect is influenced by the stability of the cyclic molecular ions. Due to energetically favourable ring-fragmentations the effect is cancelled by vicinal methoxy substituents and diminished by a methyl group next to a methoxy substituent. Stereochemically controlled fragmentations are the eliminations of a methoxy group in the form of a methanol or formaldehyde molecule from the molecular ions. By an investigation of di- and trimethoxy cyclohexanes, specifically labelled with deuterium, it is shown that both reactions are initiated by a transfer of an H-atom from a carbinol-C-atom to an O-atom of a methoxy group. Whether or not these energetically favourable reactions will occur depends on how close the H- and O-atoms involved can approach each other in the possible conformations of the molecular ion. The stereochemical control of the fragmentation of 1,3-dimethoxy cyclopentane, containing a more or less fixed five membered ring is small, that of dimethoxy cycloheptanes with a flexible seven membered ring is of comparable magnitude as the steric effect in the mass spectra of cyclohexane derivatives.     

Zum Mechanismus der Photochemie von 2-Alkylindazolen in wässerigen Lösungen

Mechanistic studies on the photochemistry of 2-alkylindazoles in aqueous solutions. The photochemistry of 2-alkylindazoles 1 in aqueous solutions is rather complex, the relative yields of different products being dependent on the pH-value of the irradiated solution: In neutral or basic solutions (pH > 7) as well as in most of the organic solvents isomerization to 1-alkyl-benzimidazoles 2 takes place. In dilute sulfuric acid (pH 2–4) this reaction is suppressed and the dihydro-azepinones 3 and 4 are formed. Irradiation in strongly acid solutions (pH < 1) yields the o-amino-acetophenones 5 (Scheme 1). The relative quantum yields of the photoproducts 2–5 have been measured as a function of the pH-value of the irradiated solution (Fig. 1). A comparison of these yields with the protonation equilibrium of the indazole in its first excited singlet state (pK = 2.8) suggests that 2 and 3 are both photoproducts of the neutral indazole molecule, whereas 4 as well as 5 are formed from the protonated indazole. The rearrangement of the indazole 1 to the benzimidazole 2 proceeds via an intermediate 6 , which can be produced in high concentrations by monochromatic irradiation of 1 at low temperatures. The thermal reactivity of this intermediate in dilute sulfuric acid could be investigated: At pH 8 the only product is the benzimidazole 2 . With decreasing pH-value increasing amounts of 3 are formed and at pH < 4 the formation of 2 is completely suppressed, the only product being the azepinone 3 . Thus, 3 is a solvolysis product of the intermediate 6 (Scheme 2). The most probable primary product of singlet indazolium is the nitrenium ion 7 . From this intermediate the formation of 5 can proceed in well-known thermal reactions. The formation of 4 is possibly due to a further protonation equilibrium nitrenium-nitrene. The nitrene 7 can be converted into the azepinone 4 via the azirine 8 (Scheme 3). The pK-values of different indazoles and intermediates are listed in the Table.     

Massenspektroskopische Fragmentierungsreaktionen. X—Zum Mechanismus der Methyl- und Äthylabspaltung bei benzanellierten 6- und 7-gliedrigen heterocyclen

It is demonstrated that alkyl elimination from 6- and 7-membered benzoheterocyclics usually occurs via several competing mechanisms, the prevalence and nature of which depend upon the heteroatom.     

Massenspektroskopische Fragmentierungsreaktionen—II: Ein den ‘Ortho-Effekt’ vortäuschender Zerfall von Diphenylmethanderivaten

It is shown that in general it is not possible to differentiate between diphenyl methane derivatives (and compounds of analogous structure) substituted in o-position to the CH₂-bridge and those carrying an m- or p-substituent by resorting solely to the ‘ortho-effect’ of the former. A rationale for this apparent anomaly is given.     

Zum Mechanismus der photochemischen Umwandlung von 2-Alkyl-indazolen in 1-Alkyl-benzimidazole. II. Photophysikalische und photochemische Primärprozesse

Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. II. Primary photochemical processes and photophysical deactivation. In the previous paper [1] the structure of the intermediate in the photochemical indazole-benzimidazole-isomerization was discussed ( 3 in Scheme 1). In this communication experiments concerning the photochemical primary processes and photophysical deactivation of 2-alkyl-indazoles ( 1 ) are described. The quantum yield of the rearrangement 1 → 2 (Φ_R) decreases with decreasing temperature while the fluorescence quantum yield (Φ_F) increases and finally reaches a constant value ( ≠ 1) (Fig.10). This behaviour is inconsistent with the mechanism shown in Scheme 2. Photoreaction and fluorescence are both quenched, but not to the same extent, by freon 113 (Fig. 2). In addition the Stern-Volmer-plots are not linear. These observations are best explained by assuming the existence of two excited states in equilibrium (Scheme 3). The mechanism in Scheme 3 correctly explains the quenching experiments and the temperature dependence of Φ_R and Φ_F if the Arrhenius law holds for the two rate constants k_sx and k_R. However, for a quantitative calculation of Φ_R, an additional branching of the reaction pathway must be postulated (Scheme 4). Two-dimensional drawings of hypothetical potential energy surfaces of the ground state and the first excited singlet state yielding a qualitative picture of the reaction and deactivation pathways of the discussed molecule are given in Fig. 15 a and b.     

Zum Mechanismus der photochemischen Umwandlung von 2-Alkyl-indazolen in 1-Alkyl-benzimidazole. I. Struktur und Reaktivität eines Zwischenproduktes

Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. I. Structure and reactivity of an intermediate. 2-Alkyl-indazoles ( 1 ) undergo photochemical isomerization to 1-alkyl-benzimidazole via previously unknown intermediates 3 (Scheme 1). In the present paper the structure and reactivity of these intermediates are discussed. Low-temperature irradiation (?60°) of 1 b with 300 nm light gives 3 b in quantitative yield. 3 b is transformed during warm-up to 1 b and 2 b (UV.-evidence). The formations of 1 and 2 show the same temperature dependence but their ratio is found to be temperature-independent. In contrast to the above behaviour, low-temperature irradiation with 250 nm light of 3 b yields 1 b only (no 2 b ). These findings are consistent with the proposed reaction mechanism 2 c in Scheme 2. On the basis of spectroscopic properties and the described reaction pathways, it appears that the most suitable structure for intermediate 3 is a 7,8-diaza-tricyclo[4.3.0.0^7,9]nona-2,4,6(10)-trien ( 9 ). In Scheme 4 the reaction pathway for the iudazole-benzimidazole-rearrangement is summarized.