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High Energy Barrier (high + energy_barrier)
Selected AbstractsInteractions of Cationic Palladium(II)- and Platinum(II)-,3 -Allyl Complexes with Fluoride: Is Asymmetric Allylic Fluorination a Viable Reaction?EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 7 2006Lukas Hintermann Abstract The complex cations [M(,3 -R2All)(PPFPz{3- tBu})]+ (M = PdII, R2All = 1,3-diphenylallyl, 1,3-dicyclohexylallyl, indenyl; M = PtII, R2All = 1,3-diphenylallyl; PPFPz-{3- tBu} = 3- tert -butyl-1-{1-[2-diphenylphosphanyl-ferrocenyl]ethyl}-1H -pyrazole)have been prepared as salts with PF6, or SbF6,. They have been characterized by NMR spectroscopy in solution and by X-ray crystallography in the solid state. Their reactions with sources of nucleophilic and "naked" fluoride have been investigated by multinuclear NMR spectroscopy. The PdII complexes did not undergo any nucleophilic substitution with concomitant release of allyl fluorides. The dicyclohexylallyl fragment was released as a 1,3-diene by elimination, but with other allyl complexes nonspecific decomposition reactions predominated. The complex [Pt(,3 -1,3-Ph2C3H3)(PPFPz{3- tBu})]PF6 underwent an anion exchange with Me4NF to give [Pt(1,3-Ph2C3H3)(PPFPz{3- tBu})]F which existed as a mixture of interconverting allyl isomers in solution at ambient temperature. For the bromide salt, [Pt(,3 -1,3-Ph2C3H3)(PPFPz{3- tBu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt0 reacted with bromo-1,3-diphenylprop-2-ene to give [Pt2(,-Br)2(,3 -1,3-Ph2All)2] and precursors of Pd0 underwent oxidative additions with bromo- and fluoro-1,3-diphenyl-2-propene to give 1,3-diphenylallyl complexes of PdII. Therefore, the nucleophilic attack of fluoride on the allyl fragment of PdII complexes is endergonic, and the high energy barrier of this step is difficult to overcome in a catalytic allylic fluorination reaction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Comprehensive Analysis of DNA Strand Breaks at the Guanosine Site Induced by Low-Energy Electron AttachmentCHEMPHYSCHEM, Issue 1 2010Jiande Gu Prof. Dr. Abstract To elucidate the role of guanosine in DNA strand breaks caused by low-energy electrons (LEEs), theoretical investigations of the LEE attachment-induced CO ,-bonds and N-glycosidic bond breaking of 2,-deoxyguanosine-3,,5,-diphosphate (3,,5,-dGMP) were performed using the B3LYP/DZP++ approach. The results reveal possible reaction pathways in the gas phase and in aqueous solutions. In the gas phase LEEs could attach to the phosphate group adjacent to the guanosine to form a radical anion. However, the small vertical detachment energy (VDE) of the radical anion of guanosine 3,,5,-diphosphate in the gas phase excludes either CO bond cleavage or N-glycosidic bond breaking. In the presence of the polarizable surroundings, the solvent effects dramatically increase the electron affinities of the 3,,5,-dGDP and the VDE of 3,,5,-dGDP,. Furthermore, the solvent,solute interactions greatly reduce the activation barriers of the CO bond cleavage to 1.06,3.56 kcal,mol,1. These low-energy barriers ensure that either C5,O5, or C3,O3, bond rupture takes place at the guanosine site in DNA single strands. On the other hand, the comparatively high energy barrier of the N-glycosidic bond rupture implies that this reaction pathway is inferior to CO bond cleavage. Qualitative agreement was found between the theoretical sequence of the bond breaking reaction pathways in the PCM model and the ratio for the corresponding bond breaks observed in the experiment of LEE-induced damage in oligonucleotide tetramer CGTA. This concord suggests that the influence of the surroundings in the thin solid film on the LEE-induced DNA damage resembles that of the solvent. [source] A New Family of Homoleptic Ir(III) Complexes: Tris-Pyridyl Azolate Derivatives with Dual PhosphorescenceCHEMPHYSCHEM, Issue 11 2006Yu-Shan Yeh Blue-emitting complexes: Iridium complexes (see figure) exhibit dual phosphorescence. The blue phosphorescence at room temperature, attributed to TILCT,TLLCT conversion, occurs via a high energy barrier (,6.9 kcal,mol,1) resulting from large-amplitude motions, such as partial twisting of the chelate groups which flip the orbital configuration and consequently results in dual phosphorescence. [source] A Proposed Mechanism for the Reductive Ring Opening of the Cyclodiphosphate MEcPP, a Crucial Transformation in the New DXP/MEP Pathway to Isoprenoids Based on Modeling Studies and Feeding ExperimentsCHEMBIOCHEM, Issue 3 2004Wolfgang Brandt Dr. Abstract Experimental and theoretical investigations concerning the second-to-last step of the DXP/MEP pathway in isoprenoid biosynthesis in plants are reported. The proposed intrinsic or late intermediates 4-oxo-DMAPP (12) and 4-hydroxy-DMAPP (11) were synthesized in deuterium- or tritium-labeled form according to new protocols especially adapted to work without protection of the diphosphate moiety. When the labeled compounds MEcPP (7), 11, and 12 were applied to chromoplast cultures, aldehyde 12 was not incorporated. This finding is in agreement with a mechanistic and structural model of the responsible enzyme family: a three-dimensional model of the fragment L271,A375 of the enzyme GcpE of Streptomyces coelicolor including NADPH, the Fe4S4cluster, and MEcPP (7) as ligand has been developed based on homology modeling techniques. The model has been accepted by the Protein Data Bank (entry code 1OX2). Supported by this model, semiempirical PM3 calculations were performed to analyze the likely catalysis mechanism of the reductive ring opening of MEcPP (7), hydroxyl abstraction, and formation of HMBPP (8). The mechanism is characterized by a proton transfer (presumably from a conserved arginine 286) to the substrate, accompanied by a ring opening without high energy barriers, followed by the transfer of two electrons delivered from the Fe4S4cluster, and finally proton transfer from a carboxylic acid side chain to the hydroxyl group to be removed from the ligand as water. The proposed mechanism is in agreement with all known experimental findings and the arrangement of the ligand within the enzyme. Thus, a very likely mechanism for the second to last step of the DXP/MEP pathway in isoprenoid biosynthesis in plants is presented. A principally similar mechanism is also expected for the reductive dehydroxylation of HMBPP (8) to IPP (9) and DMAPP (10) in the last step. [source] | ||||||||||