Search results for "Reaction"

Polycondensed nitrogen heterocycles. Part 22. Pyrrolo[3,4-d]-1,2,3-triazines: A new ring system as potential antineoplastic agent

1989

Diazotization of the 3-aminopyrrole-4-carboxamides 6a,b under different conditions directly led to the new ring system pyrrolo[3,4-d]-1,2,3-triazine in excellent yields through an intramolecular coupling reaction. In all the cases it was impossible to isolate the intermediate 3-diazopyrroles.

Better understanding of the ring-cleavage process of cyanocyclopropyl anionic derivatives. A theoretical study based on the electron localization fun…

2005

[reaction: see text] Theoretical calculations at the B3LYP/6-31+G(d), MP2/6-31+G(d), and G3(MP2) levels have been carried out to understand the alternative reaction pathways (the cyclopropyl ring cleavage (RC) and the retrocycloaddition reaction (rCA)) of a constrained tricyanocyclopropyl anionic derivative. The more energetically favorable path is found to be the RC process, a formally "forbidden" rearrangement (Leiviers, M.; Tam, I.; Groves, K.; Leung, D.; Xie, Y.; Breslow, R. Org. Lett. 2003, 5, 19, 3407) yielding an allylic anion system via a concerted transition structure, in agreement with experimental outcomes. rCA is more energetically favorable along a two-stage mechanism, via an i…

Die acidolytische depolymerisation von trioxan in stationärer phase. 21. Mitt. über polyoxymethylene1

1964

Die Depolymerisation von Trioxan mit verschiedenen Phosphorsauren wurde im Temperaturbereich von 100–160°C untersucht. Sie fuhrt zu einem temperaturabhangigen Gleichgewicht zwischen Trioxan und Formaldehyd, das oberhalb 180°C weitgehend auf der Seite des Formaldehyds liegt. Zu einer schnellen Einstellung des Gleichgewichtes eignen sich Ortho- und Pyrophosphorsaure. Die bevorzugte Nebenreaktion ist die Bildung von Methylformiat; die Ausbeute an Methylformiat wird durch Erhohung der Reaktionszeit und der Katalysatoroberflache vergrosert. Das geschilderte Verfahren eignet sich zur Darstellung von monomerem Formaldehyd (Reinheitsgrad etwa 99,8%) im Laboratorium. Depolymerization of trioxane by …

Effect of pulsed electric field on Maillard reaction and hydroxymethylfurfural production

2020

Abstract As an emerging nonthermal processing technology, the application of pulsed electric field (PEF) has received increasing attention not only for its capability to inactivate enzymes and microorganisms or to enhance mass transfer during the extraction process but also for its potential to inhibit the formation of chemical contaminants during processing, such as the formation of hydroxymethylfurfural (HMF). HMF is a potential carcinogen to humans that can be produced via the Maillard reaction (MR). Compared with thermal treatment, PEF can minimize the extent of MR, reducing the contents of HMF. However, in some cases, MR and HMR may be promoted, closely associated with PEF conditions a…

Defects and Reactivity at Oxide Surfaces: Experimental Aspects of the Interaction of Hydrogen, Co And Co2 with the Nio{001} Surface

1989

This paper summarizes some previous or more recent experimental results on chemical interaction, at moderate temperatures, of H, H2, CO and CO2 with the surfaces of nickel oxide, mainly the NiO {001} surface. These results are in a general agreement with theoretical predictions. Nearly perfect surfaces of maximal valency oxides cannot react with molecules. Dissociation of molecules is often required for chemisorption. Conversely, chemical reaction is usually observed either when molecules are predissociated or the surface has defects with an energy level high enough to enable both dissociation and chemisorption. On a nearly perfect, “in situ” cleaved, NiO {001} surface, partially atomized h…

Toward an understanding of the acceleration of Diels-Alder reactions by a pseudo-intramolecular process achieved by molecular recognition. A DFT stud…

2007

The pseudo-intramolecular Diels−Alder (DA) reaction between a 2-substituted furan (1) and a N-maleimide derivative (2) has been analyzed using DFT methods. Formation of two hydrogen bonds between the appendages on furan and maleimide derivatives favors thermodynamically the formation of a molecular complex (MC1) through an efficient molecular recognition process. The large enthalpy stabilization associated with the molecular recognition overcomes the unfavorable activation entropy associated with the bimolecular process. As a consequence, the subsequent DA reaction is clearly accelerated through a pseudo-intramolecular process.

(E,E,E)-4,4′-Distyrylstilbenes – Synthesis, Photophysics, Photochemistry and Phase Behavior

2008

The fluorescence quantum yields F(n) of oligo[2,5-bis(propyloxy)-1,4-phenylenevinylene]s (1b) reach for the trimer (n = 3) in solution a maximum of 86 %, which is almost twice as high as for the monomer (n = 1) and the octamer or undecamer (n = 8, 11). The latter represent the convergence limit of 46 %. 4-Mono-, 3,4-di- or 3,4,5-trisubstituted terminal phenyl rings in the trimers 6a–f do not lead to calamitic or phasmidic liquid crystals; however, 2,3,4-trisubstitution with hexyloxy groups (6h) and even with OCH3 groups (6g) effects LC formation. Irradiation with energy-rich UV light provokes a crosslinking of the originally conjugated chains. In the case of 6c, a chemo- and stereoselective…

1975

In the polymerization of isobutene catalyzed by aluminium tribromide and in the polymerization of styrene catalyzed by trifluoromethanesulphonic acid it is supposed that an inactive catalyst monomer complex is formed which is in equilibrium with other active species. The monomer complexation of the catalyst explains the S-shaped conversion curves and the complex kinetics of the polymerization. The value of the equilibrium constant Km (complexation constant) expresses what part of the catalyst may take part in the polymerization process as an active initiating species. For the system isobutene/AlBr3, Km was found to be 0,62 1/mol. In the system styrene/CF3SO3H, the high value of the complexa…

Electrochemical oxidation of 2,4,5-triaryl-substituted pyrroles.II. Oxidative dimerization of 4,5-diphenyl-2-mesitylylpyrrole

1989

2,4,5-Triaryl-substituted pyrroles lead, upon chemical or electrochemical oxidation, to an intermediate β-β'-dimer, which, in the course of the reaction, undergoes further oxidation to a tetracyclic derivative. To improve the selectivity towards the uncyclized dimer the oxidation of a triarylpyrrole in which the ortho positions of the phenyl group in position 2 are hindered by the presence of methyl groups was attempted. The cyclization was hindered, but an α-β'-dimer was obtained as the major product. An unexspected isomeric α-β'-dimer, in which the mesitylyl group is shifted into the β position of the pyrrole ring which undergoes the oxidation, was obtained in minor amounts. Electroanalyt…

1976

The thermal degradation of poly(oxycarbonylpentamethylene), (poly-e-caprolactone), (3), was investigated at 220°C and 80 mm Hg under nitrogen. 3 was found to be thermally much more stable than poly(oxycarbonylethylene), (poly-β-propiolactone), (1), although it decomposed faster than poly(oxycarbonyl-1,1-dimethylethylene), (polypivalolactone), (2). The reaction was of the first order with a rate constant of k=6,2.10−4 min−1 and proceeded via a “zipper mechanism” to yield monomeric e-caprolactone. Der thermische Abbau von Poly(oxycarbonylpentamethylen), (Poly-e-caprolacton), (3), wurde bei 220°C im Stickstoffstrom bei 80 mm Hg untersucht. Dabei wurde gefunden, das 3 thermisch wesentlich stabi…