Characterization of dinitroporphyrin zinc complexes by electrospray ionization tandem mass spectrometry. Unusual fragmentations of beta-(1,3-dinitroalkyl) porphyrins

The zinc complexes of diaryl bis(p-nitrophenyl)porphyrins and beta-(1,3-dinitroalkyl)tetraphenylporphyrins were studied by electrospray ionization (ESI) tandem mass spectrometry (MS/MS). All porphyrins showed the protonated molecule under ESI conditions. The protonated molecules were induced to fragment and the corresponding ESI tandem mass spectra were analysed. Porphyrins with two p-nitrophenyl groups showed, as expected, characteristic fragmentations including either loss of one nitro group, as the major fragment of the tandem mass spectra, and loss of both nitro groups. In contrast, MS/MS of the beta-(1,3-dinitroalkyl)porphyrins provided interesting and unexpected results such as the absence (or in insignificant abundance) of the ions formed by loss of one nitro group. However, these porphyrins show an abundant fragment due to combined loss of the two nitro groups. Also, the typical beta-cleavage of the alkyl chain is not observed per se, only when combined with loss of HNO2 or *NO2. Instead, alpha-cleavage, with loss of the beta-pyrrolic substituent, is the most favourable process.     

Differentiation of isomeric substituted diaryl ethers by electron ionization and chemical ionization mass spectrometry

A series of isomeric substituted diaryl ethers, i.e., 2- or 4-NO2, 5- FC6H3OC6H4 (4-R), where R=H, COCH3, COOCH3, NO2, CHO, OCH3 etc., which comprise ortho and para isomers with respect to the position of the nitro group are studied under GC-EI-MS and CI-MS conditions. The EI mass spectra of ortho and para isomers show distinct fragment ions, where the [MOH]+ and [MOHO]+ ions specifically appeared in all spectra of the ortho isomers (I), whereas the para isomers (II) contain [MO]+ and [MNO]+ ions. The [MOHCO]+ and [MOHNO]+ ions in I, and [MNO2]+ ion in II are the other specific fragment ions observed but feasibility of these fragment ions are found to depend on the nature of the substituent (R). The substitution (R) effect is also clearly reflected in the formation of fragment ions due to sigma-cleavage process with or without hydrogen migration. Similar differences in the formation of specific fragment ions are also observed in ortho and para isomers of substituted aryl naphthyl ethers. The methane/CI of isomeric compounds resulted in the same set of fragment ions, but prominent differences are observed in the relative abundance of [MHNO]+, which is relatively higher in para isomers compared with corresponding ortho isomer.     

Unexpected fragmentation of beta-substituted meso-tetraphenylporphyrins induced by high-energy collisional activation

The protonated molecules and radical cations of meso-tetraphenylporphyrins with beta-pyrrolic substituents, when formed by fast atom bombardment (FAB) and subjected to high-energy collisions, give rise to unexpected fragment ions. The reaction involves hydrogen migration from the ortho position of the phenyl ring to the a atom of the substituent, with formation of an intramolecular, six-membered ring. The process is analogous to condensed-phase cyclizations described for the same type of compounds. The fragmentation requires the presence of a double bond in the substituent group attached to the pyrrolic ring. A rearrangement process involving anchimeric assistance by the phenyl group (analogous to an ortho effect) is proposed for the formation of these ions.     

Metallation effects on the thermal interconversion of atropisomers of di(orthomethylarene)-substituted porphyrins

A new series of meso-substituted diaryl free-base and metalloporphyrins have been prepared. Each arene has been substituted with both a methyl group in the ortho position and a formyl group in the meta position. Rotation of the arene units is prevented at room temperature due to the steric restrictions imposed by the flanking methyl groups at the porphyrin beta-pyrrolic positions on the methyl groups at the ortho position on the meso-substituted arene unit. This allowed the alpha alpha and alpha beta atropisomers of this porphyrin to be separated and characterised. X-Ray crystallographic determination of the structure of the free-base porphyrin revealed a very flat porphyrin core. Metallation resulted in the isolation and characterisation of the nickel, zinc and copper derivatives. The assignments of the alpha alpha and alpha beta isomers are confirmed by X-ray crystallographic determination of the structures of the Cu(II) analogues. The copper alpha alpha structure exhibits a very twisted porphyrin core, the copper alpha beta structure is also distorted, but to a lesser degree. The activation energy for rotation has been calculated for each of the 2H, Ni and Zn derivatives. The energy required to rotate the arene ring increases in the order Ni < Zn approximately 2H. No significant difference in the free energy of rotation was observed between experiments carried out with the alpha alpha and small alpha beta isomers.     

Investigations in the benzazole and naphthazole series

1-Benzazolyl-3-methyl-5-(p- or o-)tolylformazans (benzazolyl residues: 1-benzylbenzimidazole, benzothiazole, benzoxazole) have been synthesized and their structures have been studied by IR and UV spectroscopy. It has been found that in solutions the formazans are present in the open form and do not contain a chelate ring. Formazans containing an o-tolyl residue do not differ in respect of the structure of the formazan chain from their p-tolyl isomers, but in solutions of the complexes with nickel and copper the influence of the ortho methyl group is shown in the higher coloration of the complex of the ortho isomer than of the para isomer.     

Molecular structures of mononitroanilines and their thermal decomposition products

The molecular structures of 2-nitro, 3-nitro, and 4-nitroaniline and their internal rotational isomers were calculated by anab-initio method using HF/6-31G* basis set. The geometries were influenced by the nitro group's position. The perturbation of the amino group on the nitro group was observed in a 2-nitroaniline isomer having a molecular structure distinct from that of the other two isomers. Among them, 4-nitroaniline is the most stable one. Internal rotation tests of either the nitro or amino group of 3-nitro and 4-nitroaniline indicate that no significant deformations of the phenyl ring occurred after internal rotation; however, the internal rotational isomers of 2-nitroaniline differed from its original structure. Relatively easier internal rotation of the nitro group than the amino group and different C-NO₂ and C-NH₂ bonds indicate the bond-breaking message of nitroanilines. As products of explosives induced by thermal or shock are of interest, five products of 2-nitroaniline were selected to assess their geometries and energies. The above calculations revealed that these products are thermodynamically unfavorable.     

Nitration of some 2, 5-disubstituted 4-phenylpyridin es

2,5-Dimethyl-4-phenylpyridine is nitrated in the para position of the phenyl ring. Further nitration of the resulting nitro compound gives a mixture of isomeric dinitro derivatives. In the case of 4-phenylisocinchomeronic acid the nitro group enters the meta position of the phenyl ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii. No. 3, pp. 365–368, March, 1976.     

Orientation and mechanism of reactions of aromatic amines with sulfur monochloride

The reaction of m-substituted anilines with sulfur monochloride has been studied. It has been shown that cyclization with the formation of 1, 3, 2-benzothiazathiolium compounds takes place in the position para to the substituent. The introduction of methoxy groups into o-nitroanilines interferes with the displacement of the nitro group by the chlorine atom. A nucleophilic reaction mechanism has been proposed according to which the nitro group is displaced after the formation of the thiathioniaazine ring.     

Mass spectroscopic study of 2,5-dimethyl-4-benzoyl- and 2,5-dimethyl-4-benzylpyridine derivatives

The dissociative ionization of sixteen 4-benzoyl- and 4-benzylpyridine derivatives and their deuteroanalogs has been studied. An ortho effect, due to the benzoyl and benzyl radicals in the methyl group in the 5-position of the pyridine ring, has been detected. It has also been established that fragmentation of 4-benzoylpyridines substituted with a nitro group in the benzene ring leads to [M-OH]⁺ ions, due to the ortho effect, whereas fragmentation of 4-benzylpyridines leads to [M-C₆H₅R]⁺ ions. The probability of a given process depends on the position and nature of any substituent in the benzene ring; this makes it possible to identify different isomers in a given series of compounds.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 810–816, June, 1987.     

Effets de substituants et conformation de stilbenes

Simultaneous study of para nitrostilbenes by X-ray diffraction and ¹³C nuclear magnetic resonance shows a quantitative relationship between the effect of a para nitro substituent on the chemical shift of ethylenic carbon not adjacent to the substituted phenyl ring and the dihedral angle between this phenyl ring and the central double bond. The alternative introduction of substituents in each phenyl ring discloses at the position of carbon β a certain π-polarization which extends to the double bond and to an unsaturated group carried by carbon α.     

Investigations in the benzazole and naphthazole series

1-[1'-Benzylbenzimidazolyl]-5-halogenophenyl-3-methylformazans with the halogen (chlorine or bromine) in the para or ortho positions have been synthesized. In a comparative study of the reactions of the ortho and para halogen-containing isomers, steric hindrance due to a halogen in the ortho position of the phenyl radical has been observed in the formation of tetrazolium salts and nickel and copper complexes, and in the capacity of phototropic transformations.For part XXV, see [1].     

The nitrogen position effect on the selectivity of diazacrown ethers to metal ion

In this paper, the nitrogen position isomers of diaza-18-crown-6, diaza-15-crown-5, diaza-12-crown-4, and their complexes with Na⁺ ion are studied by the density functional theory. Their stable structures are identified by geometry optimization without strict for their geometry. The binding capability that reflects the selectivity of these diazacrown ethers to metallic ions is estimated by the binding energy. The position effects of nitrogen on the stability of these complexes are also investigated by the explicit natural bond orbital and atoms-in-molecule analysis. It is found that different nitrogen position in crown ring will result in different relative energy. To those diazacrown ethers under consideration, the calculation results show that 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane, 1,4-diaza-7,10,13-trioxacyclopentadecane, and 1,7-diaza-4,10-dioxacyclododecane exhibit higher metal binding selectivity than their corresponding nitrogen position isomers, respectively. Clearly, the position of nitrogen plays an important role in the selectivity of diazacrown ethers to metal ions in the system involved. This work will be help for the material design of ionic recognition and other related fields.     

Spectroscopic identification of oxonium and carbenium ions of protonated phenol in the gas phase: IR spectra of weakly bound C6H7O+ -L dimers (L = Ne, Ar, N2)

Structural isomers of isolated protonated phenol (C(6)H(7)O(+)) are characterized by infrared (IR) photodissociation spectroscopy of their weakly bound complexes with neutral ligands L (L = Ne, Ar, N(2)). IR spectra of C(6)H(7)O(+)-L recorded in the vicinity of the O-H and C-H stretch fundamentals carry unambiguous signatures of at least two C(6)H(7)O(+) isomers: the identified protonation sites of phenol include the O atom (oxonium ion, O-C(6)H(7)O(+)) and the C atoms of the aromatic ring in the ortho and/or para position (carbenium ions, o/p-C(6)H(7)O(+)). In contrast, protonation at the meta and ipso positions is not observed. The most stable C(6)H(7)O(+)-L dimer structures feature intermolecular H-bonds between L and the OH groups of O-C(6)H(7)O(+) and o/p-C(6)H(7)O(+). Extrapolation to zero solvation interaction yields reliable experimental vibrational frequencies of bare O-C(6)H(7)O(+) and o/p-C(6)H(7)O(+). The interpretation of the C(6)H(7)O(+)-L spectra, as well as the extrapolated monomer frequencies, is supported by B3LYP and MP2 calculations using the 6-311G(2df,2pd) basis. The spectroscopic and theoretical results elucidate the effect of protonation on the structural properties of phenol and provide a sensitive probe of the activating and ortho/para directing nature of the OH group observed in electrophilic aromatic substitution reactions.     

Solution NMR determination of the seating(s) of meso-nitro-etioheme-1 in myoglobin: implications for steric constraints to meso position access in heme degradation by coupled oxidation

The highly stereoselective cleavage of hemin in myoglobin by coupled oxidation has been attributed to steric barriers that leave more space near the alpha- than the other meso-positions. The steric barriers near meso positions in myoglobin have been investigated by establishing the thermodynamics and dynamics of possible seatings in the pocket of horse myoglobin of a four-fold symmetric etioheme I modified with a bulky nitro group at a single meso position. The cyanomet complex of this reconstituted myoglobin exhibits three sets of (1)H NMR resonances that are linked dynamically and occur in approximate populations ratios of 0.82:0.10:0.08. Two dimensional (1)H NMR has been used to assign the hemin and heme pocket resonances in the major isomer in solution and to determine that the hemin is oriented with the nitro group at the canonical gamma-meso position of native hemin. The dominance of this isomer is attributed to the solvent exposure of this portion of the hemin which stabilizes the highly polar nitro group. Using a combination of magnetization transfer among methyl groups of the three isomers due to "hopping" of the hemin about its normal, the assigned resonances of an isoelectronic, bis-cyano complex of meso-nitro-etioheme I, and the known essentially constant rhombic perturbation of heme pocket sites on the hyperfine shifts of heme methyl (Kolczak, U.; Hauksson, J. B.; Davis, N. L.; Pande, U.; de Ropp, J. S.; Langry, K. C.; Smith, K. M.; LaMar, G. N. J. Am. Chem. Soc. 1999, 121, 835-843); the two minor isomers are shown to place their bulky nitro group at the canonical delta-meso (8%) and alpha-meso positions (10%). The comparable population of the isomers with nitro groups at the hydrophobic alpha- and delta-meso positions dictates that, while the static crystal structure finds more room near the alpha-meso position, the deformation at minimal energetic expense near the alpha- and delta-meso positions is comparable. These results argue that factors other than simple steric influences control the selectivity of the ring cleavage in myoglobin.     

Synchronized motion and electron transfer of a redox-active rotor

We have constructed a series of copper complexes with asymmetric 4,6-substituted 2-pyrimidyl coordination units, which form two coordination isomers via rotation of the pyrimidine ring. Redox-active ferrocenyl moieties were introduced at the 4-positions of the pyrimidine as rotating units in these complexes. The stability and dynamics of the rotative interconversion could be tuned using the structure of the rotor to accommodate a large ferrocene unit within the inner space of the complex. Among these compounds, a complex with a ferrocenylvinyl substituent showed synchronized intramolecular electron transfer between the copper and the ferrocenyl moiety with rotational motion. That is, the oxidation state of the ferrocenyl unit depended on its position within a cyclic trajectory.     

Control of the site and potential of reduction and oxidation processes in pi-expanded quinoxalinoporphyrins

Quinoxalino[2,3-b]porphyrins are pi-expanded porphyrins, having a quinoxaline fused to a beta,beta-pyrrolic position of the porphyrin. They are used as components in systems proposed as molecular wires. Knowledge of their redox properties is of value in the design of electron- or hole-conduction systems. In particular, the location of the charge density in the radical anions of quinoxalinoporphyrins can be modulated by peripheral functionalization. New theoretical treatments of electrochemical potentials are developed that identify the site of reduction in both the anions and the dianions of 33 quinoxalinoporphyrins. These molecules include free-base and metallated macrocycles substituted on the quinoxaline with electron-withdrawing groups (NO2, Cl, Br) and/or electron-donating groups (NH2, OCH3). Spectroelectrochemistry, density-functional theory calculations, and substituent-parameter models are used to verify the analysis. Five distinct patterns are observed for the locations of the first and second reductions; some of these patterns involve delocalized charges. Nitroquinoxalinoporphyrins with the nitro groups at the 5- and 6-quinoxaline positions are found to have quite different properties owing to distortions caused by peri interactions that force the nitro group of the 5-nitro regioisomer out of conjugation. Charge localization on the nitroquinoxaline fragment is found for some molecules, and this is attributed to ion-pairing with the 0.1 M tetrabutylammonium perchlorate electrolyte used, leading to the verified prediction that electron-paramagnetic resonance spectra of these molecules taken without the electrolyte yield delocalized anions. These properties enable the control of conduction through molecular wires synthesised from quinoxalinoporphyrins.     

Unusual luminescence characteristics of aminobiphenyls

Analysis of fluorescence solvatochromic shifts of ortho, meta and para aminobiphenyls reveals that the change in dipolemoment of m-aminobiphenyl on excitation is more when compared to other isomers. This change is due to the resonance interaction of unsubstituted phenyl ring with -NH2 group at meta position in the excited singlet state. The fluorimetric titration curves of three aminobiphenyls are found to be different from each other. The stretched sigmoidal curves obtained for m-aminobiphenyl indicates that the rates of proton transfer in S1 state are comparable to the rates of fluorescence.     

Selective addition to iridium of aryl C-H bonds ortho to coordinating groups. Not chelation-assisted

Precursors of the pincer-ligated iridium species, (PCP)Ir, react with nitrobenzene or acetophenone at ambient temperature to give O,C-chelated complexes resulting from addition of an aryl C-H bond and coordination of a nitro or acetyl oxygen. The C-H additions appear to be completely regioselective for the position ortho to the functional group; however, structural characterization and low-temperature NMR studies demonstrate that the reaction does not proceed via coordination of the functional group followed by C-H addition. In the case of nitrobenzene, kinetic preference for the para and meta positions is demonstrated at low temperature. Addition occurs more slowly at the ortho position, without assistance by the functional group; the ortho-C-H addition product is then trapped by chelation.     

Characterisation of Plasmodium falciparum aspartic protease inhibition by piperidine derivatives

Piperidine derivatives are reported to exhibit a variety of pharmacological activities. In this article, synthesis and aspartic protease inhibitory activity of three nitrophenacyl derivatives of N-methyl-4-hydroxy piperidine are reported. Enzyme assays showed that the attachment of a nitro group in the benzene ring plays an important role in the inhibition of plasmepsin-II of Plasmodium falciparum. The compound 1-methyl-1-(4'-nitrophenacyl)-4-hydroxypiperidinium bromide (3), consisting of a nitro group at the para position, was the most active at the concentration of 1.0?μM. The activity of the compounds was evaluated through the observed orientation and diagrammatic representation of nitrophenacyl derivatives of 4-hydroxy piperidine.     

Photostability of hydrophobic amides of pyridinecarboxylic acid as copper extractants from chloride media

The photostability of N-alkyl and N,N-dialkylpyridine-3-carboxamide, and N-alkyl and N,N-dialkylpyridine-2-carboxamide, and their copper complexes were studied. The obtained results indicated the influence of light (UV and vis) on the stability of pyridinecarboxamides. The degree of photodegradation depended on the type of solvent, the presence of water, hydrochloric acid or air in the solution. Also, the structure of compounds (the number and position of the amides group, structure of the amides carbon chain) influenced amides photostability. Products of photofragmentation of amides groups and photosubstitution at the pyridine ring were identified. Influence of ions of copper(II) and chloride during the photodegradation of copper complexes with pyridinecarboxamide was analyzed.