Computational characterization of the role of the base in the Suzuki-Miyaura cross-coupling reaction

The role of the base in the transmetalation step of the Suzuki-Miyaura cross-coupling reaction is analyzed computationally by means of DFT calculations with the Becke3LYP functional. The model system studied consists of Pd(CH=CH2)(PH3)2Br as the starting catalyst complex, CH2=CHB(OH)2 as the organoboronic acid, and OH- as the base. The two main mechanistic proposals, consisting of the base attacking first either the palladium complex or the organoboronic acid, are evaluated through geometry optimization of the corresponding intermediates and transition states. Supplementary calculations are carried out on the uncatalyzed reaction and on a process where the starting complex is Pd(CH=CH2)(PH3)2(OH). These calculations, considered together with available experimental data, strongly suggest that the main mechanism of transmetalation in the catalytic cycle starts with the reaction of the base and the organoboronic acid.     

Methodical aspects of studying the electroreduction of nitrate on modified single crystal Pt(hkl) + Cu electrodes

Technique of modification of basal faces Pt(hkl) by adatoms and epitaxial copper deposits is developed. Analysis of potentiostatic current transients of copper deposition/dissolution and atomic force microscopy showed that the activity of Pt(hkl) faces regarding the processes of copper nucleation and epitaxial growth increases in the sequence of Pt(111) < Pt(110) < Pt(100). The reaction of nitrate anion reduction is sensitive towards the surface structure, not only in the case of platinum, but also in the case of copper deposits (including a monolayer of adatoms). The highest process rate is observed for the Pt(100) electrode modified by a monolayer of adatoms or islands of bulk copper; nitrate reduction at the lowest rate occurs at Pt(111) + Cu electrodes. Structure-sensitive competitive adsorption of background electrolyte and nitrate anions is the factor that largely determines the kinetics of nitrate reduction on different faces of platinum single crystal and copper deposits.     

Brønsted Acid Catalyzed Morita–Baylis–Hillman Reaction: A New Mechanistic View for Thioureas Revealed by ESI‐MS(/MS) Monitoring and DFT Calculations

A Morita–Baylis–Hillman (MBH) reaction catalyzed by thiourea was monitored by ESI‐MS(/MS) and key intermediates were intercepted and characterized. These intermediates suggest that thiourea acts as an organocatalyst in all steps of the MBH reaction cycle, including the rate‐limiting proton‐transfer step. DFT calculations, performed for a model MBH reaction between formaldehyde and acrolein with trimethylamine as base and in the presence or the absence of thiourea, suggest that thiourea accelerates MBH reactions by decreasing the transition‐state (TS) energies through bidentate hydrogen bonding throughout the whole catalytic cycle. In the rate‐limiting proton‐transfer step, the thiourea acts not as a proton shuttle, but as a Brønsted acid stabilizing the basic oxygen center that is formed in the TS.     

Biomarker discovery by CE-MS enables sequence analysis via MS/MS with platform-independent separation

CE-MS is a successful proteomic platform for the definition of biomarkers in different body fluids. Besides the biomarker defining experimental parameters, CE migration time and molecular weight, especially biomarker's sequence identity is an indispensable cornerstone for deeper insights into the pathophysiological pathways of diseases or for made-to-measure therapeutic drug design. Therefore, this report presents a detailed discussion of different peptide sequencing platforms consisting of high performance separation method either coupled on-line or off-line to different MS/MS devices, such as MALDI-TOF-TOF, ESI-IT, ESI-QTOF and Fourier transform ion cyclotron resonance, for sequencing indicative peptides. This comparison demonstrates the unique feature of CE-MS technology to serve as a reliable basis for the assignment of peptide sequence data obtained using different separation MS/MS methods to the biomarker defining parameters, CE migration time and molecular weight. Discovery of potential biomarkers by CE-MS enables sequence analysis via MS/MS with platform-independent sample separation. This is due to the fact that the number of basic and neutral polar amino acids of biomarkers sequences distinctly correlates with their CE-MS migration time/molecular weight coordinates. This uniqueness facilitates the independent entry of different sequencing platforms for peptide sequencing of CE-MS-defined biomarkers from highly complex mixtures.     

Silyl,hydrido-silylene,or other bonding modes: some unusual structures of [(dhpe)Pt(SiHR2)]+ (dhpe = H2P-CH2-CH2-PH2; R = H,Me, SiH3, Cl,OMe, NMe2) and [(dhpe)Pt(SiR3)](+) (R = Me,Cl) from DFT calculations

DFT (B3LYP) calculations have been carried out in order to quantitatively evaluate the energies and stereochemistry of the accessible structures of [(dhpe)Pt(SiHR(2))](+) (dhpe = H(2)P-CH(2)-CH(2)-PH(2); R = H, CH(3), SiH(3), Cl, OMe, SMe, NMe(2)) and of [(dhpe)Pt(SiR(3))](+) (R = CH(3), Cl). A number of different isomers have been located. The expected terminal silyl or hydrido-silylene complexes are often not the most stable complexes. An isomer in which an H or an R group bridges a Pt=SiHR or Pt=SiR(2) bond is found to compete with the terminal silyl or hydrido-silylene isomers. In some cases, isomers derived from cleavage of a C-H bond and formation of a silene or disilene ligand are obtained. The structures of the platinum silyls differ from that of the equivalent alkyl complex, calculated for [(dhpe)Pt(CH(3))](+).     

Mechanistic and computational studies of the atom transfer radical addition of CCl4 to styrene catalyzed by copper homoscorpionate complexes

Experimental as well as theoretical studies have been carried out with the aim of elucidating the mechanism of the atom transfer radical addition (ATRA) of styrene and carbon tetrachloride with a Tp(x)Cu(NCMe) complex as the catalyst precursor (Tp(x) = hydrotrispyrazolyl-borate ligand). The studies shown herein demonstrate the effect of different variables in the kinetic behavior. A mechanistic proposal consistent with theoretical and experimental data is presented.     

Analysis of the Sulfonylurea Herbicide Metsulfuron-Methyl and Its Metabolites in the Soil of Cereal Crops. Comparative Analytical Chemistry of the Sulfonylureas

Abstract

For the analysis of metsulfuron-methyl in the crop soils with a sensitivity limit of 0.3 μg kg^?1 dry soil, in the soil extract metsulfuron-methyl was separated from its soil metabolites and the soil impurities by repeated thin-layer chromatographies (TLC). In the cleaned soil extract, diazomethane transformed metsulfuron-methyl 1 into N,N′ -dimethyl metsulfuron-methyl 2 (methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)methylamino]carbonyl]methylamino]sulfonyl]benzoate). In the gas-liquid chromatograph with detection by electron capture (GC-EC) and in the combined gas chromatograph-mass spectrometer (GC-MS), 2 was transformed into 1-dioxy-2-N-methyl-3-keto-1,2-benzisothiazole 3 which was measured by GC-EC with confirmation by GC-MS. The metsulfuron-methyl soil metabolites 2-sulfonamido-methylbenzoate 6, 1-dioxy-3-keto-1,2-benzisothiazole (saccharin) 7 and 2-sulfonamidobenzoic acid 8 were analyzed in the soil of winter wheat crops by a procedure similar to the one for metsulfuron-methyl. After their separation and purification in the soil extracts by TLC, 7 and 8 were methylated, and analyzed as 3 in the GC-EC and GC-MS apparatus where the generated 6 was quantitatively transformed into 3; 6 was analyzed as such with the GC and GC-MS apparatus wherein it was transformed into 3. The sensitivity limit for each metabolite was 0.3 μg of equivalents of metsulfuron-methyl kg^?1dry soil. The syntheses of the analysis standards of the metsulfuron-methyl derivatives 2 and 3, and of the metsulfuron-methyl metabolites 6, 7 and 8 are described. The transformation pathways of metsulfuron-methyl and of its derivatives are different from those of the pyridine-pyrimidine sulfonylurea herbicides flupyrsulfuron-methyl and rimsulfuron. The soil analysis of a sulfonylurea -by means of one of its transformation product- needs a previous study of the chemical reactivity of the sulfonylurea. This leads to the analysis procedures for the main soil metabolites of the sulfonylurea.     

Selective electrocatalysis of acetaldehyde oxime reduction on (111) sites of platinum single crystal electrodes and nanoparticles surfaces

The reduction of acetaldehyde oxime (AO) in acid medium on platinum surfaces is a structure sensitive reaction that takes place almost exclusively on (111) sites of Pt electrodes, and it is strongly inhibited on Pt(100) and Pt(110) surfaces. A study using stepped electrodes with (111) terraces and monoatomic steps either with (100) and (110) orientation shows that the activity of the electrode is also dependent on the terrace width, i.e., the wider the terrace is, the higher current density is recorded and the more positive the peak potential for AO reduction appears. Moreover, in the electrodes with (100) step sites, the reduction process appears at more negative potential than the electrodes with (111) step sites. Nanoparticles with some preferential orientations were also tested for the AO reduction reaction to check the presence of (111) ordered domains on the nanoparticles surface. Dedicated to Teresa Iwasita on the occasion of her retirement and for her contributions to Electrochemistry.     

Shape-dependent electrocatalysis: methanol and formic acid electrooxidation on preferentially oriented Pt nanoparticles

Reactivity towards methanol and formic acid electrooxidation on Pt nanoparticles with well characterised surfaces were studied and compared with the behaviour of single crystal electrodes with basal orientations. Polyoriented and preferential (100), (111) and (100)-(111) Pt nanoparticles were synthesised, cleaned preserving its surface structure, characterised and employed to evaluate the influence of the surface structure/shape of the Pt nanoparticles on these two relevant electrochemical reactions. The results pointed out that, in agreement with fundamental studies with Pt single crystal electrodes, the surface structure of the electrodes plays an important role on the reactivity of both oxidation processes, and thus the electrocatalytic properties strongly depend on the surface structure/shape of the nanoparticles, in particular on the presence of sites with (111) symmetry. These findings open the possibility of designing new and better electrocatalytic materials using decorated shape-controlled Pt nanoparticles as previously described with Pt single crystal electrodes.