A. N. Kashin

NC-palladacycles as highly effective cheap precursors for the phosphine-free Heck reactions

Eight cyclopalladated complexes of the formula [Pd2(μ-L)2(NC)2] (L=OAc, Cl; NC=cyclometalled N donor: o-(2-pyridyl)phenyl, o-(2-pyridyloxy)phenyl, o-(2-pyridylmethyl)phenyl, o-(N,N-dimethylaminomethyl)phenyl, 8-quinolylmethyl and others) and a six-membered palladacycle with OC coordination (ligand related to 2-acetoamido-4-nitrophenyl), are highly efficient catalysts for the Heck arylation of olefins (styrene, ethyl acrylate) by aryl halides (iodobenzene, bromobenzene, 4-bromoacetophenone). These catalysts are air stable, easy to obtain from a vast number of readily available nitrogen containing molecules, are generally much cheaper than phosphine-ligated palladacycles, but as or more efficient than the latter. Turnover numbers (ton) of up to 4 100 000 and turnover frequencies (tof) up to 530 000 are achieved in the reaction of iodobenzene with ethyl acrylate. Bromobenzene undergoes the Heck reaction (ton=400–700; tof=5–30) in the presence of the promoter additive Bu4NBr. The palladacycles are likely to operate in a common phosphine-free Pd(0)/Pd(II) catalytic cycle, while the differences between various types of palladacycle precursors are accounted for by the kinetics of the catalyst preactivation step.

Structure and solvent effects in organomercuric electrochemistry. Polarographical scale of CH bond acidities

Abstract A number of organomercuric compounds, R2Hg and RHgClO4, with different R groups have been studied by the polarographic method. Both the half-wave potential, E 1 2 , and the transfer coefficient, α, are shown to be necessary for judging the CHg bond strength from the polarographic data. These data for R2Hg were used for the construction of a polarographic scale of CH bond acidities.

Acidities of some fluorine substituted CH acids

Abstract Acidities of some fluorinated hydrocarbons, R F H, have been estimated by polarographic method. Polarographic data on electrochemical reduction of (R F ) 2 Hg have been correlated with rate constants of hydrogen isotopic exchange in R F H in methanol.

Palladium-catalyzed arylation of sulfonyl CH-acids

Abstract A method for the palladium-catalyzed monoarylation of a series of functionalized sulfones by aryl halides is described. The reaction proceeds in the presence of 2 mol% of Pd 2 dba 3 ·CHCl 3 (dba=dibenzylideneacetone), PPh 3 and NaH as a base, only with relatively strong CH-acids and gives monoarylated products in moderate to high yields.

Exchange of organomercury compounds with mercury metal : II. Dismutation of organomercury radicals on the surface of mercury☆

The galvanostatic method has been applied to the study of the dismutation of organomercury radicals formed by the electrochemical reduction of organomercury salts on the mercury surface. The relative stabilities of organomercury radicals have been found to follow the series AlkHg· >ArHg·, CH2CHHg· ? PhCH2Hg·. The lifetimes on the mercury surface have been determined as 10−2 to 5 x 10−2 sec for AlkHg·, 1 x 10−4 to 2 x 10−4 for ArHg· and CH2CHHg.·, and < 10(su−4) sec for PhCH(in2)- Hg·. The CHg bond in radicals on the mercury surface is more stable than that in radicals generated under homogeneous conditions. This has been attributed to the stabilising effect of the metal.

Exchange of organomercury compounds with mercury metal : I. Diethylmercury and mercury metal. The application of the galvanostatic technique to radical reactions on the metal surface

Abstract By means of a galvanostatic method it has been shown that diethylmercury reacts with mercury metal to form organomercury radicals, a conclusion based on the similarity between the charging curves obtained for solutions of diethylmercury and ethylmercury bromide. RHg· radicals have been suggested as the intermediates in the exchange of organomercury compounds with mercury metal.

Some processes occuring in graphite furnaces used for electrothermal atomic absorption spectrometry in the presence of organic chemical modifiers

Abstract Under stabilized-temperature platform furnace (STPF) conditions ascorbic acid does not significantly affect the peak areas for lead and tin and increases the peak area 1.37-fold for gallium. The appearance times for all three elements increase in the presence of ascorbic acid. The thermal decomposition of a number of organic chemical modifiers in an argon atmosphere was studied by means of thermal and elemental analysis. It was shown that the ascorbic acid melt wets lead and gallium oxides better than the glucose melt. By the use of an electron microprobe it was shown that after drying and ashing at 500°C of a solution containing ascorbic acid, lead and gallium, only 7% and 14% of the corresponding metal was retained on the surface of the carbon residue. For glucose the values were 25% and 30%, respectively. It is proposed that the increase in appearance time for the analytes studied in the presence of ascorbic acid is a result of their diffusion from the carbon residue at thte atomization stage. The sensitivity enhancement in the presence of ascorbic acid, which was observed earlier for lead, tin and a number of other elements, was caused by the substantial changes in the composition of the gas phase in the electrothermal analyser under non-STPF conditions.

Structure and solvent effects in organometallic electrochemistry correlation of CH-acid acidity with parameters of polarographical reduction of organomercury compounds

Abstract The possibilities for a correlation of the polarographical reduction parameters of organometallic compounds with the pKa-values of CH-acids are discussed. It is concluded that the correlation is possible if the electron transfer coefficients for all organometallic compounds of the given series at the same potential are of equal value and if the value of the transfer coefficient depends only upon electrode potential and not on the structure of the organometallic compound.

The Direct Non‐Perturbing Leaching Test in the Phosphine‐Free Suzuki–Miyaura Reaction Catalyzed by Palladium Nanoparticles

The aryl halides normally unreactive in the reaction with phenylboronic acid in the presence of a palladium nanoparticle catalyst were shown to become involved, if a more reactive aryl iodide is simultaneously participating in the process. The involvement phenomenon can be interpreted as a direct evidence of the formation of smaller and more reactive palladium nanoparticles through leaching from the initial nanocatalyst. The involvement test was applied both to free unsupported Pd nanoparticles and supported species, thus exhibiting essentially the same character of evolutions of various nanomaterials during the catalytic transformation associated with leaching of reactive species from the initial particles. The system allows for concurrent processing of less reactive aryl bromides and even chlorides together with much more reactive aryl iodides. The interpretation of the observed phenomena and implications for practical realization of nanoparticle catalysts are discussed.

Mechanism of electrophilic substitution with participation of ion pairs in organometallic chemistry (SE2 ion pair mechanism)

Abstract A new mechanism S E 2 ion pair) has been suggested for the reaction of organotin compounds with electron-withdrawing groups (R = 9-Me-fluorenyl, indenyl, PhC C etc.). It has been shown that some RSnMe 3 compounds may undergo ionization or dissociation in polar solvents. In low-polar solvents optically active indenyl derivatives of tin racemize slowly but the rate of racemization is increased by addition of hexamethylphosphoric triamide. This result was explained by the loss of configuration when the ion pair R − Sn + Me 3 reverts to the covalent form.