Steric interactions in 2-substituted imidazoles. The molecular structure of 1-methyl-2-phenylimidazole and 1-methyl-4-phenylimidazole

The crystal and molecular structures are reported for two isomeric imidazoles: 1-methyl-2-phenylimida-zole ( 1 ) and 1-methyl-4-phenylimidazole ( 2 ). In molecule 2 the phenyl ring is rotated by 7.3° from the het-erocyclic plane due to steric interactions. The steric congestion is much more severe in 1 , with the adjacent methyl and phenyl substituents mininizing nonbonded interactions via a 32.3° rotation of the phenyl ring and a 0.159 Å displacement of the methyl carbon from the heterocyclic plane.     

The electron-impact mass spectra of 1,2-benzisoxazole N-oxides

The mass spectra of a new class of heterocyclic N-oxides are reported. It is proposed that the fragmentation processes proceed either by an initial deoxygenation, and or an impact-induced isomerisation, prior to fragmentation. Substituents on the carbocyclic ring normally confer stability on the m olecular ions, whereas those on the heterocyclic ring open up additional fragmentation modes. A phenyl group at the 3-position allows an intramolecular oxygen transfer prior to fragmentation.     

A Carbon-13 and nitrogen-15 NMR study of some nitrogen heterocycles

A number of phenyl-substituted nitrogen heterocycles and the corresponding 2,6-diethylphenyl derivatives were examined by 13 C nmr in order to verify that the heterocyclic ring system was the same in both series. The heterocyclic ring carbon shifts did not differ substantially between the phenyl and 2,6-diethylphenyl derivatives for all systems except one, where substantial differences were also seen in the ¹⁵N chemical shifts. Acetylated derivatives were prepared to confirm the cyclization mode in this latter case.     

N,O-Nucleoside Analogues: Metabolic and Apoptotic Activity

Two new families of N,O-nucleoside analogues containing the anthracene moiety introduced through the nitrosocarbonyl ene reaction with allylic alcohols were prepared. The core structure is an isoxazolidine heterocycle that introduces either atom either a phenyl ring or dimethyl moiety at the C3 carbon. Different heterobases were inserted at the position 5 of the heterocyclic ring. One of the synthesized compounds demonstrated a good capacity to induce cell death and an appreciable nuclear fragmentation was evidenced in treated cells.     

Reactions of 4-Hydrazinocarbonylmethylene 3-Arylsydnones and 3-(4-Hydrazinocarbonylphenyl)sydnones

The phenyl ring of 3-(4-hydrazinocarbonylphenyl)sydnone activates a hydrazinocarbonyl group to undergo a series of reactions for preparing heterocyclic compounds including oxadiazoles, dihydropyrroles, and pyrroles. However, the hydrazinocarbonyl group on the C(4)-CH₂ resists a cyclization under the same reaction conditions.     

Spectroscopic study of rigid-rod polymers. II. Protonation effect

The protonation of a heterocyclic rigid-rod polymer poly(p-phenylene benzbisthiazole) and its model compound has been studied by UV-visible and Raman spectroscopy. Because of the two nitrogens on the heterocyclic ring, spectroscopic features of unprotonated, half-protonated, and fully protonated structures have been identified. For the fully protonated molecule, there is also an increase in conjugation between the phenyl ring and the heterocyclic ring.     

Conformation of p-dimethylamino aza styryl dyes

Conformational studies on some p-dimethylamino β-aza and β-aza (with respect to the dimethylamino phenyl ring) styryl dyes derived from quinoline-4, quinoline-2, pyridine-4, pyridine-2, and benzothiazole-2 have been carried out using the quantum mechanical PCILO (perturbative configuration interaction using localized orbitals) method. These molecules may be considered as heterocyclic analogues of benzylidene anilines whose conformations have been studied extensively by both theoretical and experimental methods to explain the difference of their spectra from the isoelectronic benzylidene compounds. The results of the present studies show that the β-aza styryl dyes are nearly planar. In case of β-aza styryl dyes, although the phenyl ring is coplanar with the central atoms, there is a substantial twist of the heterocyclic ring. These results are explained in terms of CT -1 and CT -2 effects and are used as a possible explanation for the observed spectral and sensitization properties.     

Lithiation–alkylation of polystyrene occurs only on the ring

When polystyrene is treated with an organolithium reagent, the hydrogen on the para carbon is removed and this position may be alkylated with e.g., iodomethane or chlorotrimethylsilane. The metalation occurs directly at this position and does not involve metalation at the benzylic carbon with resonance distribution of the charge to the ring. The reaction of cumene with butyllithium also occurs by direct ring metalation and it is shown that these results are consistent with both the kinetic and the thermodynamic acidity of the hydrocarbons. © 1997 John Wiley & Sons, Inc.     

Synthesis,Structural Characterisation and Stereochemical Investigation of Chiral Sulfur‐Functionalised N‐Heterocyclic Carbene Complexes of Palladium and Platinum

Palladium and platinum complexes containing a sulfur‐functionalised N‐heterocyclic carbene (S‐NHC) chelate ligand have been synthesised. The absolute conformations of these novel organometallic S‐NHC chelates were determined by X‐ray structural analyses and solution‐phase 2D ¹H–¹H ROESY NMR spectroscopy. The structural studies revealed that the phenyl substituents on the stereogenic carbon atoms invariably take up the axial positions on the Pd‐C‐S coordination plane to afford a skewed five‐membered ring structure. All of the chiral complexes are structurally rigid and stereochemically locked in a chiral ring conformation that is either (R_s,S,R)‐λ or (S_s,R,R)‐δ in both the solid state and solution.     

The mass spectra of styryl sulfoxides and sulfones

A variety of rearrangement reactions have been documented in the gas phase ion chemistry of styryl sulfoxides and sulfones. The styryl group rearranges from sulfur to oxygen as evidenced by loss of SCH₃ from methyl styryl sulfoxide and loss of SOCH₃ from the corresponding sulfone. The resulting m/e 119 ion loses carbon monoxide in one fragmentation route and alternatively loses a hydrogen atom from the aromatic nucleus to produce the benzofuran molecule ion via an electrophilic aromatic ring closure reaction. Styryl sulfoxides lose both carbon monoxide and formyl radicals directly from their molecule ions, but the corresponding sulfones do not fragment in this manner. The mechanisms of the above reactions, as well as others, were investigated using substituent and deuterium labeling. The styryl group has been shown to migrate in preference to a phenyl or substituted phenyl group by investigation of the mass spectra of appropriate aryl styryl sulfoxides and sulfones.     

Novel crown ethers with pendant,proton-ionizable,chromogenic groups

Synthesis of fifteen new chromogenic crown ethers is described in which one phenyl group of diphenylamine has been substituted at the 2-, 4-, and 6-positions with strongly electron-withdrawing groups and the other phenyl group bears an oxymethyl crown ether substituent. Structural variations include the crown ether ring size, identity of the electron-withdrawing groups, acidity of the amine function, and positioning of the oxymethyl crown ether substituent ortho, meta, and para to the amine nitrogen. Preparation of a structurally-related chromogenic bis(crown ether) is also reported.     

Electron impact induced migrations of aryl groups in substituted 4(3H)-quinazolinones

Electron impact mass spectra of 2-diphenylmethyl-3-aryl-4(3H)-quinazolinones display ions arising from migrations of different aryl groups in the molecular and [M? H]⁺ ions. The most abundant ion due to rearrangement, [C₁₃H₉NO]^+˙, is formed by migration of a phenyl from the benzhydryl group onto N-1 and subsequent cleavage of the heterocyclic ring. Other rearrangements involve initial migration of the N-3 aryl group to the benzylic carbon. The mechanisms of migrations were elucidated by means of deuterium and ¹⁵N labelling and are supported by metastable spectra.     

Phosphine-oxazoline ligands with an axial-unfixed biphenyl backbone: the effects of the substituent at oxazoline ring and P phenyl ring on Pd-catalyzed asymmetric allylic alkylation

A novel kind of chiral phosphine-oxazoline ligands 3 with an axial-unfixed biphenyl backbone bearing different substituent on oxazoline ring and P phenyl ring was prepared. These ligands exist as a mixture of two diastereomers in equilibrium in solution. Upon coordinated to Pd(II), however, only one of the two possible kinds of diastereomer complexes with different axial chirality was formed. These compounds as chiral ligands were applied in Pd-catalyzed asymmetric allylic alkylation with high reaction activity and enantioselectivity. Meanwhile, the asymmetric catalytic behavior was affected obviously by the substituent at oxazoline ring and P phenyl ring. The best result, up to 92.3% ee and 99% yield, was obtained with the ligand 3c having two phenyl groups on P and a phenyl group on oxazoline ring in this asymmetric catalysis reaction.     

13C, 14N, 15N and 17O NMR spectra of nitropyrroles and nitroimidazoles

Carbon, nitrogen and oxygen NMR spectra of some nitro derivatives of pyrrole and imidazole have been investigated. The ¹³C chemical shifts of para-carbons and the ¹⁷O chemical shifts of the nitro group correlate qualitatively with the electron densities on these carbon and oxygen atoms, which in turn depend upon the degree of conjugation of the nitro groups with the heterocyclic ring. Conjugation of several nitro groups with the benzene ring is in most cases not impaired by mutual interactions and the ¹³C shifts show good additivity. Such additivity is much worse in pyrrole and imidazole derivatives. Taken together with the diamagnetic nature of these deviations from additivity, this leads to a possible conclusion about the less pronounced conjugation of the nitro groups with the heterocyclic ring in heterocyclic dinitro derivatives.     

Rearrangements and fragmentations of phenyl styryl sulfides: 2—labelling studies

Metastable molecular ions of phenyl styryl sulfides may decompose by loss of CH₃^˙, SH^˙, CHS^˙, C₆H₅^˙, C₆H₆ or C₇H₇^˙. Labelling with carbon-13 and deuterium gave information about the mechanisms of these reactions. It appears that extensive rearrangements occur prior to most of these fragmentations. In the case of phenyl β-styryl sulfide both phenyl groups and both vinyl carbon atoms are found in the C₇H₇ fragment in comparable amounts. For phenyl α-styryl sulfide this fragmentation leads more specifically to the loss of the S-phenyl group and the β-vinyl carbon atom. It was concluded that rearrangements occur, partly via symmetric diphenyl ethene sulfide structures, to benzyl phenyl thione ions, from which the fragmentation occurs. For the loss of CHS^˙ an earlier proposed mechanism was confirmed. From both compounds the S-phenyl ring can be lost as C₆H₅^˙ or C₆H₆ as well as the C-phenyl ring. Fragmentation occurs from one of the initial structures as well as from benzyl phenyl thione. Loss of CH₃^˙ is thought to occur after ring closure with formation of dihydrobenzo[b]thiophenes followed by ring opening by rupture of a C? S bond. While phenyl β-styryl sulfide shows a strong tendency towards isomerization to a symmetric structure like 1,2-diphenylethene sulfide, phenyl α-styryl sulfide easily rearranges in an electrocyclic reaction with formation of benzyl phenyl thione.     

Synthesis of new hetarylazoindole dyes from some 2-aminothiazole derivatives

A new series of heterocyclic disperse dyes were prepared by diazotization of some 2-aminothiazole derivatives and subsequent coupling with indole compounds. The dyes were characterized by UV-Vis, FT-IR, ¹H NMR, and mass spectra (LC-MS). Solvent effects on their visible absorption spectra were estimated. The color of the dyes is discussed with respect to the substituent therein. The effects of acids and bases on the visible absorption maxima of the dyes are also reported. Replacement of methyl group in the 4-position of the thiazole ring by phenyl group leads to red shift of the absorption maximum due to π-electron-donating properties of the phenyl group, while weak electron-withdrawing chlorine or bromine atom in the para-position of the phenyl group in the 2-amino-4-phenylthiazole fragment induce a small blue shift relative to 2-amino-4-phenylthiazole derivatives. Introduction of an electron-withdrawing 4-nitrophenylsulfonyl group into the thiazole ring produces bathochromic shift of the absorption maximum in all solvents. Published in Russian in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 4, pp. 592–599. The text was submitted by the authors in English.     

Synthesis of some 6-methoxyimidazo[1,2-b]pyridazines

The synthesis of 6-methoxyimidazo[1,2-b]pyridazine and its 2-methyl analog is reported. Carboxylic acids, esters and quaternary salts derived from this ring system are described and the heterocyclic system is shown to undergo the Mannich reaction. A condensation reaction of the 2-methyl group is reported and nuclear magnetic resonance (nmr) spectra and acidity constants of some imidazo[1,2-b]pyridazines are recorded.     

5‐Acetyl‐2‐amino‐6‐methyl‐4‐phenyl‐4H‐pyran‐3‐carbonitrile and 2‐amino‐5‐benzoyl‐6‐methyl‐4‐phenyl‐4H‐pyran‐3‐carbonitrile acetonitrile solvate

The syntheses, X‐ray structural investigations and calculations of the conformational preferences of the carbonyl substituent with respect to the pyran ring have been carried out for the two title compounds, viz. C₁₅H₁₄N₂O₂, (II), and C₂₀H₁₆N₂O₂·C₂H₃N, (III), respectively. In both mol­ecules, the heterocyclic ring adopts a flattened boat conformation. In (II), the carbonyl group and a double bond of the heterocyclic ring are syn, but in (III) they are anti. The carbonyl group forms a short contact with a methyl group H atom in (II). The dihedral angles between the pseudo‐axial phenyl substituent and the flat part of the pyran ring are 92.7 (1) and 93.2 (1)° in (II) and (III), respectively. In the crystal structure of (II), inter­molecular N—H⋯N and N—H⋯O hydrogen bonds link the mol­ecules into a sheet along the (103) plane, while in (III), they link the mol­ecules into ribbons along the a axis.     

Skeletal rearrangement of allyl acetates after protonation by chemical ionization

In the methane chemical ionization mass spectra of allyl phenylacetate and allyl phenoxyacetate the major reaction paths (>40%σ) involve skeletal rearrangements, which have no analogy in the corresponding, simpler electron-impact spectra. Substituent and deuterium labeling studies suggest a mechanism involving intramolecular substitution of the phenyl ring by the allyl group. Abstraction of hydrogen from the ortho position of the phenyl ring or from the rearranged allyl group is followed by expulsion of water and carbon monoxide.     

Intramolecular transacylation: fragmentation of protonated molecules via ion‐neutral complexes in mass spectrometry

An intramolecular transacylation reaction was observed in the mass spectrometry of molecules containing both benzoyl and carboxymethyl groups on an aromatic heterocyclic core. The reaction is triggered by a dissociative protonation on the heterocyclic ring at the atom (carbon or nitrogen) that bonds to the benzoyl group, leading to an intermediate ion‐neutral complex. The incipient benzoyl cation in the complex migrates to attack the carboxyl group of the neutral partner at the carbonyl or hydroxyl oxygen under thermodynamic or kinetic control, respectively. Elimination of benzoic acid followed by loss of carbon monoxide takes place as a result of the transacylation. Copyright © 2009 John Wiley & Sons, Ltd.