Search results for "Proton"
showing 10 items of 5886 documents
Noncovalent assembly of functional groups on calix[4]arene molecular boxes
1997
Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well-defined box-like assemblies in the presence of two equivalents of 5,5-diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10-4M. This paper reports the first X-ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen-bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Fu…
Protonation and oxidation chemistry of a pentaethylcyclopentadienyl-containing molybdenum(IV) trihydride complex
2000
International audience; Compound CpEtMoCl4 (CpEt=η5-C5Et5) (1) can be transformed into CpEtMoH3(dppe) (2) and CpEtMoD3(dppe) (2-d3) [dppe=1,2-(diphenylphosphino)ethane] by reaction with LiAlX4 (X=H and D, respectively). The protonation and oxidation studies of these two compounds, in comparison with previously reported studies on (C5Me5) analogs, show important differences that may be attributed to a kinetic stabilization of the products, which is steric in nature. Protonation of 2 with HBF4 in acetonitrile affords [CpEtMoH4(dppe)]+ (3), which only slowly decomposes to [CpEtMoH2(MeCN)(dppe)]+ (4). Further protonation of the latter affords the monohydride species [Cp*MoH(dppe)(MeCN)2]2+ in t…
Informational rigidity in mesitylene-based calix[4]arenes adopting a 1,3-alternate conformation
2000
Abstract Two chiral derivatives of a mesitylene-based calix[4]arene known to exist in the 1,3-alternate conformation were prepared by the attachment of homochiral residues to the four exo -hydroxy groups. Thus, the enantiotopic protons of the central scaffold became diastereotopic, leading to a doubling of their 1 H NMR signals in one example. From the temperature independence of the NMR spectrum, a lower limit of 24.2 kcal/mol could be estimated for the barrier of ring inversion. MM3 calculations confirm the 1,3-alternate conformation as the energy minimum, and estimate a barrier of 25.7 kcal/mol for the 1,3-alternate-to-1,3-alternate* interconversion process. This high barrier is due to t…
Preparation and reactions of 1-methylpyrid-6-one-2-yl compounds of palladium(II) and platinum(II)
1984
Abstract The compounds trans -[MCl{(1-Me)C 5 H 3 (6-O)N- C 2 }(L) 2 ] (M = Pd, Pt; L = PPh 3 , PMe 2 Ph), can be prepared from the reaction of the corresponding 1-methyl-6-chloro-2-pyridylium cationic complexes, trans -[MCl{(1-Me)C 5 H 3 (6-Cl)N- C 2 }(L) 2 ]ClO 4 , with a mixture of acetic acid, ethanol, and triethylamine in the molar ratio M/MeCO 2 H/EtOH/NEt 3 of 1/3/3/4. The rate is slow compared to that of the 1-methyl-2-chloropyridinium cation under similar conditions, and is markedly affected by the steric and electronic effects of the trans -MCl(L) 2 unit. The novel 1-methylpyrid-6-one-2-yl derivatives have been characterized by conventional spectroscopic techniques and by reactions…
Formation of novel T-shaped NNN ligands via rare-earth metal-mediated Si-H activation.
2014
Reactions of silylamides [Ln{N(SiHMe2)2}3(thf)2] with sterically crowded terphenylamine DmpNH2 (Dmp = 2,6-Mes2C6H3 with Mes = 2,4,6-Me3C6H2) afforded via a template reaction the formation of a new tridentate ligand, and derived complexes of composition [LnN{SiMe2N(Dmp)}2] (Ln = Ce, Pr) were obtained. Usage of the even more bulky amine Ar*NH2 (Ar* = 2,6-Trip2C6H3 with Trip = 2,4,6-iPr3C6H2) yielded the free protonated ligand NH{SiMe2NH(Ar*)}2.
The proton complex of a diaza-macropentacycle: structure, slow formation, and chirality induction by ion pairing with the optically active 1,1'-binap…
2008
The protonation of a sterically crowded [N2S6] macropentacycle (1) with 1 equiv of CF3SO3H in CDCl3 is slow and gives the singly (oo(+) [1 x H](+)) and doubly (o(+)o(+) [1 x 2H](2+)) protonated forms as kinetic products, the i(+)o form of [1 x H](+) being the thermodynamic product. i(+)o [1 x H](+) is C3 helically chiral in the solid state and in solution. The barrier to racemization (DeltaG(double dagger)) of the [1 x H](+) propeller is71 kJ mol(-1). The ammonium proton is encapsulated in the tetrahedral coordination sphere provided by the endo (i) nitrogen bridgehead atom and the three proximal thioether sulfurs, which makes [1 x H](+) a proton complex. Use of the optically active acid (R…
ChemInform Abstract: Bioinspired Functional Analogs of the Active Site of Molybdenum Enzymes: Intermediates and Mechanisms
2015
Abstract Molybdenum(VI) complexes of the general type MoVI(L)2(E)O (L = 2-(aryl N CH)-pyrrolylato, E = O, NtBu) and their relevance as bioinspired functional analogs for molybdenum-containing oxotransferases are reviewed. All complexes are capable of transfering oxygen atoms to PR3 (forward oxygen atom transfer) giving OPR3 and phosphane molybdenum(IV) complexes MoIV(L)2(E)(PR3) (with a second equivalent PR3) via the transient phosphoryl complex MoIV(L)2(E)(OPR3) and the five-coordinate intermediate MoIV(L)2(E). Reactivity of MoIV(L)2(E) and the favored stereochemistry of products from excess PR3 MoIV(L)2(E)(PR3) depend on the steric demand of the chelate ligands (L)−, the π donor ligand E …
Bioinspired functional analogs of the active site of molybdenum enzymes: Intermediates and mechanisms
2015
Abstract Molybdenum(VI) complexes of the general type MoVI(L)2(E)O (L = 2-(aryl N CH)-pyrrolylato, E = O, NtBu) and their relevance as bioinspired functional analogs for molybdenum-containing oxotransferases are reviewed. All complexes are capable of transfering oxygen atoms to PR3 (forward oxygen atom transfer) giving OPR3 and phosphane molybdenum(IV) complexes MoIV(L)2(E)(PR3) (with a second equivalent PR3) via the transient phosphoryl complex MoIV(L)2(E)(OPR3) and the five-coordinate intermediate MoIV(L)2(E). Reactivity of MoIV(L)2(E) and the favored stereochemistry of products from excess PR3 MoIV(L)2(E)(PR3) depend on the steric demand of the chelate ligands (L)−, the π donor ligand E …
Gas-phase and solution basicities of some alkyl 2,6-dialkylphenyl ketones: A comparative analysis
2000
Abstract The gas-phase basicity of a number of alkyl 2,6-dialkylphenyl ketones (2,6-R2C6H3COR′) has been found to be almost insensitive to structural variations, as a result of a compensation of steric and electronic effects associated with the bulkiness and to the polarizability, respectively, of R and/or R′. On the contrary, the basicity in concentrated sulfuric acid undergoes, along the same series of compounds, a variation of nearly 8 pK units, as a consequence of steric inhibition of solvation of the protonated carbonyl as the main effect played by R and/or R′. The results in the condensed phase agree very nicely with recent findings relevant to some 4-substituted 2,6-dimethylacetophen…
A theoretical study of carbon-carbon bond formation by a Michael-type addition.
2012
A theoretical study of the Michael-type addition of 1,3-dicarbonyl compounds to α,β-unsaturated carbonyl compounds has been performed in the gas phase by means of the AM1 semiempirical method and by density functional theory (DFT) calculations within the B3LYP and M06-2X hybrid functionals. A molecular model has been selected to mimic the role of a base, which is traditionally used as a catalyst in Michael reactions, an acetate moiety to modulate its basicity, and point charges to imitate the stabilization of the negative charge developed in the substrate during the reaction when taking place in enzymatic environments. Results of the study of six different reactions obtained at the three di…