Yves Van Haverbeke

Old reagents, new results: Aromatization of Hantzsch 1,4-dihydropyridines with manganese dioxide and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone

Abstract Hantzsch 1,4-dihydropyridines are readily oxidized by manganese dioxide or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in dichloromethane at room temperature. With manganese dioxide loss of the 4-substituent, in addition to aromatization, occurs when it is a secondary alkyl or a benzylic group and sonication significantly reduces the reaction times. In contrast, loss of the 4-substituent is never observed when DDQ is the oxidative species.

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone, a mild catalyst for the formation of carbon-nitrogen bonds

Abstract 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) catalyzes the preparation of N-tetrahydropyranylbenzazoles, 2-substituted 1,3-diphenylimidazolidines, and N-(arylmethylene)benzene-1,2-diamines. In the latter case, use of one equivalent of DDQ provides a novel one-pot method for the synthesis of 1 H -benzimidazoles.

Potassium permanganate, a versatile reagent for the aromatization of Hantzsch 1,4-dihydropyridines

Abstract A variety of Hantzsch esters have been oxidized with potassium permanganate. The structure of the final products dramatically depends on the nature of the 4-substituent and on the experimental conditions.

Applications in gaseous ion and neutral chemistry using a six-sector mass spectrometer

Abstract A tandem mass spectrometer of EBEEBE geometry (B = magnetic sector; E = electric sector), designed for fundamental studies of gaseous ions and neutral reactive molecules, is described. The production of neutrals in the instrument can be performed by neutralization of fast ion beams during the flight or by flash-vacuum pyrolysis within the ion source. Examples involving ionization of selected precursor ions and pyrolysis are illustrated.

QUATERNARY AMMONIUM SALT-ASSISTED SYNTHESIS OF EXTENDED π-SYSTEMS FROM METHYLDIAZINES AND AROMATIC ALDEHYDES1

4-Methyl- and 4,6-dimethylpyrimidines, methyl- and 2,5-dimethylpyrazines, as well as 3-methylpyridazine readily react with aromatic aldehydes in a hot solution of sodium hydroxide in the presence of a catalytic amount of a quaternary ammonium salt and in the absence of any organic solvent to yield the corresponding condensation products.

Ion–molecule reaction of pyridine with CS3 radical cations: experimental evidence for the production of pyridine N-thioxide distonic ions

Chemical ionization of pyridine using carbon disulphide as the reagent gas leads to the formation of new distonic ions, pyridine N-thioxide radical cations, 2+•. The origin of these ions is unambiguously attributed to the reaction of CS3+• ions with neutral pyridine owing to the use of a new hybrid mass spectrometer combining sectors and an r.f.-only quadrupole collision cell floated at a voltage similar to the accelerating voltage of the ions. Collisional activation (CA) of appropriate reference ions (the molecular ions of isomeric mercaptopyridines, 4–6+•) demonstrates the actual structure of the ions, while neutralization–reionization experiments indicate the stability of the corresponding neutral dipole in the gas phase. Several reactions were performed in the quadrupole collision cell with molecules recognized as excellent trapping reagents of distonic ions: dimethyl disulphide, dimethyl diselenide and nitric oxide. The 2+• ions react with nitric oxide generating NOS+ ions; this reaction is not observed for the reference 4–6+• ions. Although the transfer of thiomethyl radicals or selenomethyl radicals is observed for all the radical cations, the resulting [2(4–6)+S(Se)]+ cations are clearly differentiated by CA. It is also shown that the transfer of S+• to perdeuterated pyridine is a specific reaction of the distonic 2+• ions.

Ultrasound-promoted benzylation of arenes in the presence of zinc chloride mixed with a K10 clay

Abstract Using K10 clay-supported zinc chloride, Friedel-Crafts benzylations proceed readily at room temperature. Significant improvements of the rates can be obtained when the reaction mixtures are sonicated.

Pyridine N-selenide radical cations: synthesis, characterization, neutralization, and ion–molecule reactions in the gas phase

Abstract Cyanogen N-selenide radical cations, NCCNSe·+, react efficiently with neutral pyridine producing the elusive pyridine N-selenide radical cations. The structure of these new distonic ions has been established by high energy collisional activation (CA), neutralization–reionization mass spectrometry, and associative ion–molecule reactions with nitric oxide, methyl isocyanide, pyridine-d5, dimethyl disulfide, and dimethyl diselenide. These experiments have been performed in a new hybrid mass spectrometer having the sectors–quadrupole–sectors configuration.

An Unexpected Effect of the Nature of the Collision Gas in Collisional Activation Mass Spectrometry

Radical-cations a-c with the structure HN=C=C=X (X=O, NH, S respectively) have been studied by collisional-activation (CA) mass spectrometry using different target gases (helium, oxygen and nitrogen). An unusual effect of the nature of the collision gas has been noted for oxygen which induces a very intense loss of a nitrogen atom for ions a and b, but not for ion c. The replacement of the NH hydrogen by a methyl group suppresses this effect Oxygen therefore appears to induce an isomerization of ions a and b into nitrene isomers a′ and b′ (N - CH=C=X). The use of nitrogen, which gives essentially the same CA spectra as helium, eliminates the possibility of an effect based on the center-of-mass collision energy. G2(MP2) calculations indicate that the nitrene radical-cations lie close in energy to the cumulene ions when X=O or NH.

Surfactant-assisted organic reactions in water. Effect of ultrasound on condensation reactions between active methylene compounds and arylaldehydes.

A series of quaternary ammonium salts has been tested as phase transfer agents to promote condensation reactions in an aqueous solution of sodium hydroxide in the absence of any organic solvent. Methyltrioctylammonium chloride (Aliquat 336) emerges as the most efficient catalyst. Sonication of the reaction media has a poor but positive kinetic effect.