Mechanism of cycloaddition reactions between ketene and N-silyl-, N-germyl-, and N-stannylimines: a theoretical investigation

A theoretical study of the cycloaddition reactions of ketene and N-silyl-, N-germyl-, and N-stannylimines were performed at the B3LYP/6-311+G(d,p) theory level using the LANL2DZ effective core potential for Ge and Sn and taking into account the effect of diethyl ether solvent by means of the polarizable continuum model method. According to the obtained results the reaction between ketene and N-germylimine is a two-step process due to the effect of solvent, whereas the cycloaddition of ketene and N-silylimine follows a three-step mechanism because in this case the evolution of the electronic energy along the reaction coordinate predominates over the effect of solvent. For N-stannylimine the two- and three-step mechanisms are competitive. In all the cases the rate-determining barrier corresponds to the evolution of the azadiene intermediate. The cycloaddition of ketene and N-germylimine is kinetically the most favorable reaction of the three studied by us and can take place as a domino process. In the three cases the isomerization of the imine through the inversion at the nitrogen atom is easier than the formation of the azadiene intermediate so that the three processes would afford the trans-beta-lactam.     

Synthesis of 1,2-disubstituted-1,4,5,6-tetrahydropyrimidines

Cyclization of benzoyl derivatives of N-substituted trimethylenediamines with polyphosphoric acid or the corresponding ethyl ester, afforded 1, 2-disubstituted-1, 4, 5, 6-tetrahydropyrimidines. PPE was the more advantageous reagent. The excessive protonation of the starting material and the cleavage of the ether function produced by PPA explains the failure of this reagent to cause ring closure in some cases. Synthesis of the starting materials is described. Analytical and spectroscopic data of the new compounds are presented.     

Electrocatalytical properties of polymethylferrocenyl dendrimers and their applications in biosensing

The electrochemical characterization of polymethylferrocenyl dendrimers deposited onto a platinum electrode and their applications as hydrogen peroxide and glucose sensor are described. The redox dendrimers consist of flexible poly(propileneimine) dendrimer cores functionalised with octamethylferrocenyl units. Amperometric biosensors for glucose were prepared by immobilization of glucose oxidase onto these modified electrodes. The influence of the dendrimer generation and the thickness of the dendrimer layer, the effect of the substrate concentration, and the interferences and reproducibility on the response of the sensors were investigated.     

Efficient photochemical merocyanine-to-spiropyran ring closure mechanism through an extended conical intersection seam. A model CASSCF/CASPT2 study

A mechanism of the thermal and photochemical bleaching of merocyanine to spiropyran is proposed on the basis of CASSCF/CASPT2 calculations on the 6-(2-propenyliden)cyclohexadienone model system. Our results suggest that this photochemical transformation takes place in two steps. First, the initially pumped 1(pi-pi) S2 undergoes radiationless decay to 1(n-pi) S1 via an extended S2/S1 conical intersection seam that runs approximately parallel to the trans-to-cis isomerization coordinate, a few kilocalories per mole higher in energy. Thus, S2 --> S1 internal conversion is possible at all values of the S2 trans-to-cis reaction coordinate. Second, on the S1 potential energy surface, there is a barrierless ring closure reaction path from the S1 cis minimum that leads to a peaked S1/S0 conical intersection where the deactivation to the ground state takes place. The inertia of the moving nuclei then drives the system toward the ground-state minimum of the 2H-chromene product. Thus, the extended seam topology of the S2/S1 conical intersection and the coordinate of the branching space of the S1/S0 conical intersection are essential to explain the efficiency and high speed of this reaction.     

Stereoisomer discrimination in complexes of halogen-substituted difuranes and Li or Na cations

The theoretical study of the stereoisomer discrimination of the 2:1 homo- and heterochiral complexes between chiral 5,5'-dihalogen bifuranes and lithium or sodium cations has been carried out using DFT methods. To understand the chiral effect produced by the introduction of a second bifurane molecule, the 1:1 complexes also have been calculated. All the 2:1 heterochiral complexes computed showed a nonplanar configuration around the metallic cation, but in the case of the homochiral complexes, the dibromo- and dichloro-bifurane systems around Na+ were quasi-planar. The nature of the interactions established between cations and bifurane systems has been analyzed by means of AIM and NBO, and correlations between the electron density topological parameters with the O...M distance and with the orbital interaction energy have been found. Stereodiscrimination is observed favoring the heterochiral complexes except for the Na+ complexes with chloro and bromo substituents in which the homochiral forms are more stable. Stereodiscrimination values correlate with the difference in electron density at the bond critical point and orbital interaction energy between homo- and heterochiral systems.     

Sensory hybrid host materials for the selective chromo-fluorogenic detection of biogenic amines

Pyrylium-containing mesoporous materials have been used for the chromo-fluorogenic sensing of biogenic amines in an aqueous environment.     

Pd(0) supported onto monolithic polymers containing IL-like moieties. Continuous flow catalysis for the Heck reaction in near-critical EtOH

Long-term stable Pd(0) catalysts can be easily supported onto polymeric monoliths containing methyl-imidazole moieties and the corresponding reactors based on these materials can be applied for the continuous Heck reaction in near-critical EtOH.     

Catalytic properties of the dioxomolybdenum siloxide MoO2(OSiPh3)2 and its 2,2'-bipyridine adduct MoO2(OSiPh3)2(bpy)

The tetrahedral triphenylsiloxy complex MoO(2)(OSiPh(3))(2) (1) and its Lewis base adduct with 2,2'-bipyridine, MoO(2)(OSiPh(3))(2)(bpy) (2), were prepared and characterised by IR/Raman spectroscopy, and thermogravimetric analysis. Both compounds catalyse the epoxidation of cis-cyclooctene at 55 degrees C using tert-butylhydroperoxide (t-BuOOH) is decane as the oxidant, giving 1,2-epoxycyclooctane as the only product. The best results were obtained in the absence of a co-solvent (other than the decane) or in the presence of 1,2-dichloroethane, while much lower activities were obtained when hexane or acetonitrile were added. With no co-solvent, catalyst 1 (initial activity 272 mol x molMo(-1) x h(-1)for a catalyst:substrate: oxidant molar ratio of 1:100:150) is much more active than 2(initial activity 12 mol x molMo(-1) x h(-1)). The initial reaction rates showed first order dependence with respect to the initial concentration of olefin. With respect to the initial amount of oxidant, the rate order dependence for 1 (1.9) was higher than that for 2 (1.6).The dependence of the initial reaction rate on reaction temperature and initial amount of catalyst was also studied for both catalysts. The lower apparent activation energy of 1 (11 kcal x mol(-1)) as compared with 2 (20 kcal x mol(-1)) is in accordance with the higher activity of the former.