Home About us Contact
|
Home About us Contact | ||
|
|
||
Hydrogen Migration (hydrogen + migration)
Selected AbstractsThermal Cyclization of N-[2-(2-Propenyl)-1-naphthyl]ketenimines: Intramolecular Diels,Alder Reaction versus [1,5] Hydrogen Migration.CHEMINFORM, Issue 47 2005Benzo[h]quinolines., Synthesis of Dibenz[b, h]acridines No abstract is available for this article. [source] Reactions of the Aluminum(I) Monomer LAl [L = HC{(CMe)(NAr)}2; Ar = 2,6- iPr2C6H3] with Imidazol-2-ylidene and Diphenyldiazomethane.EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 20 2004A Hydrogen Transfer from the L Ligand to the Central Aluminum Atom, Formation of the Diiminylaluminum Compound LAl(N=CPh2) Abstract The solid-state reaction of LAl and imidazol-2-ylidene at elevated temperature (120 °C) yielded the aluminum monohydride N -heterocyclic carbene adduct [{HC[C(CH2)NAr] (CMeNAr)}AlH-{CN(R)C2Me2N(R)}] [R = iPr (1), Me (2)]. Compounds 1 and 2 have been characterized by spectroscopic (IR, and 1H and 13C NMR), mass spectrometric, and elemental analyses, and 1 was further characterized by X-ray structural analysis. These experimental data indicate that the Al,H bond is formed by hydrogen migration from one of the methyl groups of the ,-diketiminato ligand backbone. The reaction of LAl with two equivalents of diphenyldiazomethane afforded the diiminylaluminum compound LAl(N=CPh2)2 (3), while an excess of diphenyldiazomethane resulted in the formation of Ph2C=N,N=CPh2. This suggests that Ph2C=N,N=CPh2 is initially generated and then reacts further by oxidative addition to yield 3. The X-ray structural analysis reveals that compound 3 contains the shortest Al,Niminyl bond among those with a four-coordinate aluminum center. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Quantum chemical study of penicillin: Reactions after acylationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 10 2007Rui Li Abstract The density functional theory methods were used on the model molecules of penicillin to determine the possible reactions after their acylation on ,-lactamase, and the results were compared with sulbactam we have studied. The results show that, the acylated-enzyme tetrahedral intermediate can evolves with opening of ,-lactam ring as well as the thiazole ring; the thiazole ring-open products may be formed via ,-lactam ring-open product or from tetrahedral intermediate directly. Those products, in imine or enamine form, can tautomerize via hydrogen migration. In virtue of the water-assisted, their energy barriers are obviously reduced. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Electrospray tandem mass spectrometry of lexitropsinsRAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 12 2001M. Rosário M. Domingues Several compounds, representative of the class of lexitropsins, were analyzed by electrospray tandem mass spectrometry. The study of the fragmentations of the protonated molecular species ([M,+,H]+) and of selected fragment ions allowed proposals for the main fragmentation pathways of compounds of this type. The interpretation of the fragmentation pathways of these compounds was complicated because of intramolecular hydrogen migration. In order to better understand the fragmentation pathways, the MS/MS/MS spectra of several compounds, and the MS/MS and MS/MS/MS spectra of the deuterated compounds, were obtained. Accurate mass measurements helped elucidate the structures of smaller fragment ions. Low-energy collision-induced decomposition (CID) tandem mass spectrometry of lexitropsins with electrospray ionization has proven to be a good method for the structural characterization and identification of this class of compounds. Main fragmentation pathways occur by cleavage of the peptide bond followed by the elimination of the substituted pyrrole ring, and their elucidation will facilitate structural characterization of new lexitropsins. Copyright © 2001 John Wiley & Sons, Ltd. [source] Theoretical study of protonated xylenes: ethene elimination and H,C-scrambling reactionsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2004Bjřrnar Arstad Abstract Quantum chemical calculations have been carried out to investigate various unimolecular rearrangements that can take place in protonated gas-phase xylenes. Hydrogen and methyl group ring migrations were investigated. The barriers for hydrogen migrations are lower than the barriers for methyl group migrations. Mechanisms for ring expansion to seven-membered rings, and for contraction to five-membered rings were studied. Both of these mechanisms can eventually lead to ethene elimination. The most favourable ring expansion step goes through a 1,3-hydrogen shift from a methyl group onto the arenium ring, forming a protonated methylcycloheptatriene. Interconversions between various ring-expanded forms have been investigated. Re-contraction can lead to an ethylbenzenium ion that could subsequently split off ethene. Alternatively, the xylenium ion can contract to a five-membered ring. The immediate product is a bicyclic ion (bicyclo[3.1.0]hexane skeleton) that can rearrange further to give an ethylbenzenium ion, or the five-ring system can split off ethene, and be converted into a cyclopentadienyl ion that can isomerize into a benzenium ion. Stable structures and transition states are calculated both at the B3LYP/cc-pVTZ//B3LYP/6-311G(d,p) and at the MP2/cc-pVTZ//MP2/6-31G(d) levels. The energies needed for ring expansion or ring contraction are not very different, and the calculations suggest that both reaction paths are possible, but the energy needed for actually splitting off an ethene molecule is lower along the expansion path. Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||