A DFT-based theoretical investigation of the mechanism of the PtCl2-mediated cycloisomerization of allenynes

The mechanism of Pt(II)-catalyzed intramolecular cycloisomerization of allenyne systems has been extensively investigated by DFT calculations. Different mechanistic schemes have been proposed and discussed, including the Alder-ene reaction. The free energy results suggest that the kinetically preferred reaction pathway for precursors that are tri- and tetrasubstituted on the allene moiety should proceed by a five-step mechanism. This would involve formation of a platina(IV)cyclopentene intermediate by selective engagement of the external pi bond of the allene, which would undergo regioselective beta-H elimination from the equatorially disposed methyl group. A metal-induced H migration leads to a second octahedral Pt(IV)-chelate complex, which would yield the expected bicyclic system through an intramolecular migratory insertion step. Therefore, depending on the conformation of the initial eta(4)-reactant complex for trisubstituted patterns, two possible intermediates can be formed that would evolve through different paths. In these cases, the regio- and stereochemical outcomes predicted by the mechanistic scheme proposed agree with experimental data. Substituted precursors on the alkyne moiety follow a distinct, four-step, mechanism also involving an oxidative cyclometalation process to an octahedral Pt(IV) intermediate complex. Theoretical results reveal the kinetic preference for beta-H elimination from the allylic group rather than from the gem-dimethyl group, which should account for the observed regioselectivity.     

Spectroscopy and coordination chemistry of a new bisnaphthalene-bisphenanthroline ligand displaying a sensing ability for metal cations

A new fluorescent macrocyclic structure (L1) bearing two naphthalene units at both ends of a cyclic polyaminic chain containing two phenanthroline units was investigated with potentiometric and fluorescence (steady-state and time-resolved) techniques. The fluorescence emission spectra show the simultaneous presence of three bands: a short wavelength emission band (naphthalene monomer), a middle emission band (phenanthroline emission), and a long-wavelength band. All three bands were found to be dependent on the protonation state of the macrocyclic unit (including the polyaminic and phenanthroline structures). The existence of the long-wavelength emission band is discussed and is shown to imply that a bending movement involving the two phenanthroline units leads to excimer formation. This is determined by comparison with the excimer emission formed by intermolecular association of 1,10-phenanthroline. With ligand L1, excimer formation occurs only at pH values above 4. At very acidic pH values, the protonation of the polyamine bridges is extensive leading to a rigidity of the system that precludes the bending movement. The interaction with metal cations Zn(II) and Cu(II) was also investigated. Excimer formation is, in these situations, increased with Zn(II) and decreased with Cu(II). The long-emission band is shown to present a different wavelength maximum, depending on the metal, which can be considered as a characteristic to validate the use of ligand L1 as a sensor for a given metal.     

Theoretical reaction indices for dibenz[a,h]acridine

Huckel molecular orbital calculations and application of Seybold-Smith reactivity theory suggest that the Bay-Region diol epoxide derived from the 8,9,10,11-benzo ring of dibenz[a,h]acridine ( 1 ) is the same in reactivity as that of the 1,2,3,4-ring in agreement with experimental findings. The calculations dispute an earlier claim of regioselective reactivity at the 8,9,10,11-benzo ring.     

Rapid determination of ochratoxin A in cereals and cereal products by liquid chromatography

A new method based on extraction with octylsilica (C8) followed by liquid chromatography coupled with fluorescence detection (LC-FLD) was studied to determine ochratoxin A (OTA) from cereals and cereal products. Optimization of different parameters, such as type and amount of solid phase, type and volume of eluent and amount of sample were carried out. Recovery of OTA from rice samples spiked at 10 ng/g level was of 86% with relative standard deviation of 5%. The limits of detection and quantification of the proposed method were 0.25 and 0.75 ng/g, respectively. Furthermore, LC-FLD after of OTA methylation and liquid chromatography coupled to mass spectrometry with an electrospray interface were used for confirmation of OTA in all studied samples. The proposed method was applied to 62 samples of cereals and cereal products and the presence of OTA was found in seven samples.     

紧形变的一个正则值

给出了在K上的任意两个Banach空间之间，Fredholm映射在一个特定的开球中，至少有一个共同值的充分条件．结论的证明基于解析开拓法，并具有可构造性．