1,720,963 research outputs found
Stereoselective Epoxidation of cis-3,4-Disubstituted-(CH2X)-Cyclobutenes with Dimethyldioxirane and Peroxy Acids. Experimental and Computational Evidence for a Syn-Orienting Electrostatic Effect
No full textThe epoxidation reactions of a series of cis-3,4-disubstituted-(CH2X)-cyclobutenes with dimethyldioxirane (DMD) and mClPBA have been investigated with both reagents. A remarkable syn diastereoselectivity in the formation of the epoxide has been obsd. for substrates bearing electron withdrawing substituents
Selectivity of Purine Alkylation by a Quinone Methide. Kinetic or Thermodynamic Control?
No full textThe alkylation reaction of 9-methyladenine and 9-methylguanine (as prototype substrates of deoxyadenosine
and -guanosine), by the parent o-quinone methide (o-QM), has been investigated in the gas phase and in aqueous solution, using density functional theory at the B3LYP/6-311+G(d,p) level
Vanadium and niobium mixed-oxide catalysts obtained via sol-gel: preparation and catalytic behaviour in oxidative dehydrogenation of propane
No full texto-Quinone Methide as Alkylating Agent of Nitrogen, Oxygen and Sulfur Nucleophiles. The Role of H-bonding and Solvent Effects on the Reactivity through a DFT Computational Study
No full textThe reactivity of the alkylating agent o-quinone methide (o-QM) toward NH3, H2O, and H2S, prototypes of nitrogen-, oxygen-, and sulfur-centered nucleophiles, has been studied by quantum chemical methods in the frame of DF theory (B3LYP) in reactions modeling its reactivity in water with biological nucleophiles
Transition Structures for One Step Nonconcerted Oxygen Insertion Mechanism of Oxidation of Alkanes with Trifluoroperoxyacetic acid.
No full textHigh level calcns. (UB3LYP/6-31G*) on the oxygen insertion by trifluoroperoxyacetic acid (TFPA) into methane and isobutane C-H bonds strongly supports the viability of a one step nonconcerted mechanism. That is, the O-H bond forms first and the C-O bond formation commences afterwards, but only one first order saddle point, exhibiting high diradical character and strong polarization, is present on the energy profile of the min. energy path. No minima corresponding to intermediates were located
Novel Pathways for Oxygen Insertion into Unactivated C-H Bonds by Dioxiranes. Transition Structures for Stepwise Routes via Radical Pairs and Comparison with the Concerted Pathway.
No full textThe oxygen insertion into C-H bonds (of methane, isobutane, and acetone) by dioxiranes (parent dioxirane and dimethyldioxirane) to give alcs. was studied with the DFT theory, using both restricted and unrestricted B3LYP method
Planar Transition Structures in the Epoxidation of Alkenes. A DFT Study on the Reaction of Peroxyformic Acid with Norbornene Derivatives.
No full textWe studied, with the RB3LYP/6-311+G(d,p) method, the mechanism of peroxyformic acid epoxidation of norbornene, norbornadiene, tetramethylethene, and anti- and syn-sesquinorbornenes. The transition structures (TSs) for the reaction of tetramethylethene and norbornene show a perfect
spiro geometry (the peroxy acid plane is perpendicular to the CdC bond axis) with synchronous bond formation. Also three out of the four TSs of the norbornadiene reaction are spiro-like, but the highly asynchronous syn,endo-TS has a planar-like geometr
Facial Selectivity in Epoxidation of 2-Cyclohexen-1-ol with Peroxy Acids. A Computational DFT Study
No full textWe addressed the mechanism of epoxidation of 2-cyclohexen-1-ol by locating all the transition structures (TSs) for the reaction of peroxyformic acid (PFA) with both pseudoequatorial and pseudoaxial cyclohexenol conformers (five TSs for each conformer) and, for purpose of comparison,
also those for the PFA epoxidation of cyclohexene. Geometry optimizations were performed at the B3LYP/6-31G* level, energies refined with single point B3LYP/6-311+G**// B3LYP/6-31G* calculations and solvent effects introduced with the CPCM method
Competition between Peroxy Acid Oxygens as Hydrogen Bond Acceptors in B3LYP Transition Structures for Epoxidations of Allylic Alcohols with Peroxyformic Acid
No full textThe transition structures (TSs) for the epoxidation of 2-propen-1-ol and 2-cyclobuten-1-ol with peroxyformic acid have been located with the B3LYP method using three basis sets (i.e., 6-31G*, 6-311G**, and 6-311+G**). Syn attacks on these alcohols by peroxy acid lead to syn TSs in which
hydrogen bonding is operative. The allylic OH group always acts as hydrogen-bond donor while either the carbonyl oxygen or the peroxo oxygens of the peroxy acid can play the role of hydrogenbond acceptors. The Sharpless qualitative TS model for peroxy acid epoxidation of allyic alcohols is discussed in the light of computational data
The effect of sol-gel promoters on the characteristics of mixed V-Nb oxides and their catalytic properties in propane oxidative dehydrogenation
No full textHydrolytic sol–gel procedures are implemented for the preparation of mixed Nb–V oxide systems, the main objective being the development of potential catalysts for the oxidative dehydrogenation (ODH) of propane. The effects of various Nb precursors (alkoxide or chloride), different acid promoters and/or complexing agents (HCl, HNO3, oxalic acid and citric acid) and the influence of the Nb/V ratio on the hydrolytic sol–gel preparation are investigated. Several 1:1 Nb/V systems are prepared. It is found that the addition of citric acid as complexing agent stabilises the solution of Nb and V precursors after the addition of water, favouring the direct formation of a gel. A homogeneous and well-interdispersed 1:1 Nb–V system is obtained by this method. In all the other cases, precipitation of a yellow solid occurs, leading after calcination to heterogeneous systems, in which the presence of segregated V2O5 is detected.
The hydrolytic preparation of systems with higher amount of Nb, 9:1 Nb/V and 4.5:1 Nb/V, does not require the addition of a complexing agent like citric acid. Sols containing higher amounts of Nb precursor are stable and turn rapidly into gels. In both 9:1 Nb/V and 4.5:1 Nb/V samples, there is no evidence of crystalline phases containing vanadium. Only the presence of crystalline Nb2O5 is detected by XRD. The effects of sol–gel additives and niobium amount on the morphological and catalytic properties are investigated. The 4.5:
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