Search results for "Reaction intermediate"
showing 10 items of 35 documents
Benzo-Fused Periacenes or Double Helicenes? Different Cyclodehydrogenation Pathways on Surface and in Solution
2019
Controlling the regioselectivity of C-H activation in unimolecular reactions is of great significance for the rational synthesis of functional graphene nanostructures, which are called nanographenes. Here, we demonstrate that the adsorption of tetranaphthyl- p-terphenyl precursors on metal surfaces can completely change the cyclodehydrogenation route and lead to obtaining planar benzo-fused perihexacenes rather than double [7]helicenes during solution synthesis. The course of the on-surface planarization reactions is monitored using scanning probe microscopy, which unambiguously reveals the formation of dibenzoperihexacenes and the structures of reaction intermediates. The regioselective pl…
Photochemistry of 1,2,4-oxadiazoles. A DFT study on photoinduced competitive rearrangements of 3-amino- and 3-N-methylamino-5-perfluoroalkyl-1,2,4-ox…
2006
The photoinduced competitive rearrangements of 5-perfluoroalkyl-3-amino(N-alkylamino)-1,2,4-oxadia- zoles have been investigated by DFT calculations and UV-vis spectroscopy. The observed product selectivity depends on the number of hydrogen atoms present in the amino moiety and involves two or three possible routes: (i) ring contraction-ring expansion (RCRE), (ii) internal-cyclization isomerization (ICI), or (iii) C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC). UV absorption and fluorescence spectra of the reactants, and vertical excitation energy values, calculated by time dependent DFT, support the involvement of a neutral singlet excited state in the photoexcitation process. …
The interplay of crystallization kinetics and morphology during the formation of SnO2 nanorods: snapshots of the crystallization from fast microwave …
2011
A microwave-assisted reaction pathway to rutile SnO2nanorods was investigated. The microwave-treatment significantly reduces the reaction time compared to standard hydro-/solvothermal techniques. By moving the overall process into a shorter time slot, the growth and crystal formation during the reaction could be monitored via snapshots by trapping the intermediates through quenching. To gain a better insight into the template-free growth of one-dimensional (1D) nanostructures, a parameter-dependent (various temperatures/pressures and times were investigated) study was carried out. For all materials, the phase purity and crystallite sizes were determined by X-ray powder diffraction (XRD). Th…
Structural insight into the reaction mechanism of Pd-catalyzed nitrile hydration: Trapping the [Pd(H2O)4]2+ cation through a supramolecular complex
2016
Abstract Four new bis(oxamato)palladate(II) complexes of formula (n-Bu4N)2[Pd(2,4,6-Me3pma)2]·2CH3CN (1), (n-Bu4N)2[Pd(2,4,6-Me3pma)2]·2CH3CONH2 (2) and (n-Bu4N)4[Pd(H2O)4][Pd(4-Xpma)2]3·2CH3CONH2 with X = Br (3) and Cl (4) (2,4,6-Me3pma = N-2,4,6-trimethylphenyloxamate, 4-Brpma = N-4-bromophenyloxamate, N-4-chlorophenyloxamate and n-Bu4N+ = tetra-n-butylammonium) have been obtained and characterized by single crystal X-ray diffraction. All of them contain bis(oxamato)palladate(II) anions and bulky n-Bu4N+ cations, but compounds 3 and 4 have also the out of the ordinary [Pd(H2O)4]2+ inorganic cation. Acetonitrile and appealing acetamide are present as lattice molecules in compounds (1) and …
The influence of aluminium cations on electrocarboxylation processes in undivided cells with Al sacrificial anodes
2005
Abstract The influence of Al cations on the electrochemical carboxylation of acenaphthylene ( 1 ), benzophenone ( 2 ), 6-methoxy-2-acetonaphthone ( 3 ), and benzyl chloride ( 4 ) has been investigated in dimethylformamide at a glassy carbon cathode. The Al 3+ ions were either added at the beginning of the experiment or were released from the anode during electrocarboxylation. It has been found that when Al cations are added to the reaction medium, they strongly influence the reduction process, in most cases leading to a complete change of the mechanism. For example, in the case of 1 , the radical anion ( 1 − ) formed upon 1e − reduction rapidly reacts with Al 3+ to give an Al(III) adduct, w…
An unprecedented, bridged dihydrogen complex of a cofacial metallodiporphyrin and its relevance to the bimolecular reductive elimination of hydrogen
1990
Recently, the authors characterized the first metalloporphyrin dihydrogen complex, Os(OEP)(H{sub 2}), and proposed that a similar ruthenium porphyrin dihydrogen complex, Ru(OEP)(H{sub 2}), is involved as an intermediate in the catalytic H/D isotopic exchange between water and hydrogen. They now report that treatment of a metal-metal-bonded cofacial ruthenium porphyrin dimer with a sterically bulky ligand in the presence of hydrogen gas has yielded the first known complex containing a dihydrogen ligand bound between two metals. Such a bridged dihydrogen complex is proposed as an intermediate in the bimolecular elimination of dihydrogen from two metalloporphyrin hydrides.
Evidence of a 1,4-dipole intermediate in the reaction of 1-phenyl-4-vinylpyrazole with ptad
1985
Abstract 1-Phenyl-4-vinylpyrazole reacts with 1-phenyl-1,2,4-triazoline-3,5-dione in methylene chloride to form an unstable azetidine (4). An intermediate 1,4-dipole is probably involved since, when the reaction is conducted in acetone, a 1:1:1 adduct (3) is isolated.
Arynic species II
1986
Abstract o-Benzyne and its 4-methyl, 4-chloro and 4-bromo-derivatives were generated in the thermal decomposition of two new kinds of polymer-bound precursors: 1(2-carboxyaryl)triazenes and 2-carboxyaryl-sulphonates. New kinds of trapping polymers for these elusive species are also presented.
Heterogeneous photocatalytic degradation of nitrophenols
2003
The photocatalytic degradation of 2-, 3- and 4-nitrophenol has been investigated in oxygenated aqueous suspensions containing TiO2. The organic reaction intermediates have been determined by high performance liquid chromatography. The results indicate the complete mineralisation of the substrates and the formation of both nitrate and ammonium ions. The degradation pathways involve a rapid opening of the aromatic ring followed by a slower oxidation of the aliphatic compounds. The hydroxyl radicals are responsible for the primary attack of the nitrophenols with formation of dihydroxynitrobenzenes. The ring hydroxylation occurs in the positions activated by the contemporaneous presence of phen…
Metallocene sulfide chemistry of niobium and tantalum: new insights into the formation of niobium-polysulfido complexes and synthesis, structure, and…
1992
The previously described reaction of Cp'2NbH3 (Cp' = t-BuC5H4) with S8 was investigated with regard to the kinetic stabilization of as yet elusive intermediates during the formation of the polysulfide complex Cp'4Nb2S9 (1). Therefore, the permethylated complex Cp*2NbS3H (2; Cp* = C5Me5) and the Ta complex Cp'2TaS2H (3) were synthesized in a related reaction and structurally investigated. Despite the problematic localization of the hydride atom during X-ray crystallographic studies, the molecules can be described in a proper manner: Characteristic of 2 and 3 are M(eta(2)-S2) cores to which either an SH (2) or a H (3) ligand is attached. Complex 3 is the first known tantalocene sulfide. It re…