Tsuji-Trost Based Cascades: From Passerini Adducts of Cinnamaldehyde to Allylated Oxazolones.

The Passerini coupling of cinnamaldehyde derivatives affords allylic esters that may behave as both electrophiles and nucleophiles in Tsuji-Trost reactions. We present herein the interaction of the latter with methylallylcarbonate, leading to the formation of oxazolidine-diones. The efficiency of the process relies on the building up of a CO2 overpressure in the medium. A reaction mechanism highlighting the reversibility of the Tsuji-Trost reaction is proposed for the process.

Evidence for a Cooperative Mechanism Involving Two Palladium(0) Centers in the Oxidative Addition of Iodoarenes.

Oxidative addition of iodoarenes (ArI) to Pd0 ligated by 1-methyl-1H-imidazole (mim) in polar solvents leads to cationic arylpalladium(II) complexes [ArPd(mim)3 ]+ . Kinetic analyses evidence that this reaction is second order with respect to the concentration of Pd0 , and a mechanism involving the cooperative intervention of two Pd0 centers has been postulated to explain this finding. This unusual behavior is also observed with other nitrogen-containing ligands and it is general for iodobenzenes substituted with electron-donating or weakly electron-withdrawing groups. In contrast, bromoarenes and electron-poor iodoarenes display first-order kinetics with respect to Pd0 . Theoretical calculations performed at the density functional theory (DFT) level suggest the existence of mim-ligated ArI-Pd0 complexes, in which the iodoarene is bound to the metal in a halogen-bond-like fashion. Coordination weakens the C-I bond and facilitates the oxidative insertion of another Pd0 center across this C-I bond. This conceptually novel mechanism, involving the cooperative participation of two distinct metal centers, allows a full explanation of the experimentally observed kinetics.

Recovering Invisible Signals by Two-Field NMR Spectroscopy.

Nuclear magnetic resonance (NMR) studies have benefited tremendously from the steady increase in the strength of magnetic fields. Spectacular improvements in both sensitivity and resolution have enabled the investigation of molecular systems of rising complexity. At very high fields, this progress may be jeopardized by line broadening, which is due to chemical exchange or relaxation by chemical shift anisotropy. In this work, we introduce a two-field NMR spectrometer designed for both excitation and observation of nuclear spins in two distinct magnetic fields in a single experiment. NMR spectra of several small molecules as well as a protein were obtained, with two dimensions acquired at vastly different magnetic fields. Resonances of exchanging groups that are broadened beyond recognition at high field can be sharpened to narrow peaks in the low-field dimension. Two-field NMR spectroscopy enables the measurement of chemical shifts at optimal fields and the study of molecular systems that suffer from internal dynamics, and opens new avenues for NMR spectroscopy at very high magnetic fields.

Electrochemistry of acids on platinum. Application to the reduction of carbon dioxide in the presence of pyridinium ion in water.

A detailed cyclic voltammetric investigation of the reduction of moderately weak acids on platinum reveals that they are reduced in two steps: one involving the hydrated protons initially present at equilibrium and the second the reduction of the acid through its prior conversion into hydrated protons. The reduction of pyridinium ions (protonated pyridine) follows this reaction scheme as does any other acid of similar pK (e.g., acetic acid). Rather than being catalytically reduced, CO2 plays a similar role through its prior conversion to carbonic acid. No trace of methanol or formate could be detected upon preparative-scale electrolysis of CO2 on the same electrode in the presence of pyridinium ions.

Nanoparticle-electrode impacts: the oxidation of copper nanoparticles has slow kinetics.

The electrochemical oxidation of copper nanoparticles in aqueous solution was studied via their electrolysis upon impacting a carbon electrode held at a suitable anodic potential. The oxidations were found to be quantitative such that complete oxidation of the particle took place allowing their sizing. Experiments were performed in 1.0 M HNO(3) and in 1.0 M HNO(3)-0.1 M KCl. In the former case a two electron oxidation to Cu(2+) was seen at a formal potential of +0.11 V (vs. SCE). In the latter case two separate one-electron oxidations at -0.01 V and +0.26 V were seen. In addition, theoretical results were derived for the analysis of impact-charge vs. potential data for reversible and irreversible charge transfer kinetics for nanoparticle oxidation. This enabled the inference that overpotential is required for the oxidations and Butler-Volmer transfer coefficients to be determined. The latter are compared with literature data seen for macroscopic copper.

The charge transfer kinetics of the oxidation of silver and nickel nanoparticles via particle-electrode impact electrochemistry.

The electro-oxidation of silver and nickel nanoparticles in aqueous solution was studied via their collisions with a carbon electrode. The average charge passed per impact varies with electrode potential and was analysed to determine that AgNPs display an electrochemically fast ("reversible") one-electron oxidation, whilst the NiNPs exhibit slow ("irreversible") 2-electron kinetics. Kinetic parameters are reported.