Computational study of the transmetalation process in the Suzuki-Miyaura cross-coupling of aryls

The transmetalation step of the Suzuki-Miyaura cross-coupling between aryl groups is analyzed by means of DFT calculations with the Becke3LYP functional. The halide considered is Ph-Br, and the organoboronic acid is Ph-B(OH)₂. The model catalyst is Pd(PH₃)₂, and the base, OH⁻. The transmetalation is considered to start from the Pd(Ph)(PH₃)₂Br complex, the product of the oxidative addition. The results are compared with those of a previous study on the analogous reaction with vinyl groups, and it is shown that the reaction mechanism is very similar.     

Stereoselective synthesis of enynes by nickel-catalyzed cross-coupling of divinylic chalcogenides with alkynes

(Z,Z)- and (E,E)-divinylic selenides and telurides undergo direct coupling with terminal alkynes in the presence of a nickel/CuI catalyst at room temperature to give (Z)- and (E)-enyne systems in good yields and with complete retention of configuration.     

Aziridine sulfides and disulfides as catalysts for the enantioselective addition of diethylzinc to aldehydes

Chiral aziridine sulfides and disulfides were synthesized from readily available and inexpensive R-cysteine by a Mitsunobu reaction; their application in the addition of diethylzinc to aldehydes provides secondary alcohols with up to 99% ee and S-configuration.     

Novel organometallic building blocks for molecular crystal engineering. Part 4. Synthesis and characterization of mono- and bis-amido derivatives of [Co(III)(eta5-C5H4COOH)2]+ and their utilization as ligands

The synthesis and structural characterization of the hexafluorophosphate salts of the substituted bis-amido molecular complexes [Co(III)(eta5-C5H4CONHC4H3N2)2]+ (1), [Co(III)(eta5-C5H4CONHCH2C5H4N)2]+ (2), [Co(III)(eta5-C5H4CON(C5H4N)2)2]+ (3), and of the amido-carboxyl complexes [Co(III)(eta5-C5H4CON(C5H4N)2)(eta5-C5H4COOH)]+ (4), and [Co(III)(eta5-C5H4CONHC2N3(C5H4N)2)(eta5-C5H4COOH)]+ (5) are reported. The pyridyl and pyrazine substituted amido ligands on the sandwich cores have been chosen because they allow both coordination to metal centres and participation in hydrogen bonding. The hydrogen bonding interactions established by the family of complexes in the solid state has been investigated. The utilization of complex 5 for the preparation of the complex of complexes[Cd(NO3)2{Co(III)(eta5-C5H4CONHC2N3(C5H4N)(C5H4NH))(eta5-C5H4COOH)}2]6+ (6) is reported as a first example of the potential of the substituted mono-and bis-amides as ligands. The isolation and structural characterization of the carbonyl chloride cation [Co(III)(eta5-C5H4COCl)2]+ (7) as its tetrachloro cobaltate anion salt is also described.     

Synthesis,Reactivity, and Fluxional Behaviour of [Ir2Rh2(CO)12], and Crystal Structure of [Ir2Rh2(CO)8(norbornadiene)2]

The synthesis of [Ir₂Rh₂(CO)₁₂] ( 1 ) by the literature method gives a mixture 1 /[IrRh₃(CO)₁₂] which cannot be separated using chromatography. The reaction of [Ir(CO)₄]^? with 1 mol-equiv. of [Rh(CO)₂(THF)₂]⁺ in THF gives pure 1 in 61% yield. Crystals of 1 are highly disordered, unlike those of its derivative [Ir₂Rh₂(CO)₅(μ₂-CO)₃(norbornadiene)₂] which were analysed using X-ray diffraction. The ground-state geometry of 1 in solution has three edge-bridging CO's on the basal IrRh₂ face of the metal tetrahedron. Time averaging of CO's takes place above 230 K. The CO site exchange of lowest activation energy is due to one synchronous change of basal face, as shown by 2D- and VT-¹³C-NMR. Substitution of CO by X^? in 1 takes place at a Rh-atom giving [Ir₂Rh₂(CO)₈(μ₂-CO)₃X]^? (X = Br, I). Substitution by bidentate ligands gives [Ir₂Rh₂(CO)₇(μ₂-CO)₃(η⁴-L)] (L = norbornadiene, cycloocta-1,5-diene) where the ligand L is chelating a Rh-atom of the basal IrRh₂ face. Carbonyl substitution by tridentate ligands gives [Ir₂Rh₂(CO)₆(μ₂-CO)₃(μ₃-L)] (L = 1,3,5-trithiane, tripod) with L capping the triangular basal face of the metal tetrahedron. Carbonyl scrambling is also observed in these substituted derivatives of 1 and is mainly due to the rotation of three terminal CO's about a local C₃ axis on the apical Ir-atom.     

Artificial neural network applied to solid state thermal decomposition

A multi-layer neural network is constructed to describe the thermal decomposition of rhodium acetate. Critical analysis of the residual, trained, interpolated and extrapolated errors, with the number of neurons, indicates the efficiency of the present approach. It was possible, within this framework, to improve the A _n model, with a better correlation between the results. A new value of the activation energy, E _a, and frequency factor, Z, are calculated for the decomposition process. Since the neural network is more precise than a particular model, the calculated values for these quantities are believed to be more precise. The computed values are E _a=194.0 kJ mol^-1 and Z=5.23·1016 s^-1. The neural network eliminates the step to decide, among the available models, the one that best fit the data. An agreement up to four significant figures can be achieved even for data not used in the training process, both in the interpolated and extrapolated regions. This method suggests, therefore, an important alternative tool for the experimentalists. The present approach can also be adapted to other systems and to data in two dimensions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thioorthoesters in the activated Pictet-Spengler cyclization. Synthesis of 1-thiosubstituted tetrahydroisoquinolines and carboncarbon bond formation via sulfonyl iminium ions generated from N,S-sulfonyl acetals

The elaboration of 1-alkylthio- and 1-arylthio-tetrahydroisoquinolines by means of the activated Pictet-Spengler reaction of N-sulfonyl-β-phenethylamines with thioorthoesters as electrophiles, and their use as sulfonyl iminium ion precursors for carboncarbon bond formation, leading to 1-substituted tetrahydroisoquinoline derivatives, is reported.     

TheK-shell conversion coefficient of the 135.5 keVM4 transition in193mPt decay

TheK conversion coefficient of the 135.5 keVM4 transition in^193m pt has been determined by the XPG method. The result is _K =135.2±10.5 and the value exhibits a deviation 3.4% below the theoretical prediction of Hager and Seltzer. Similar deviations have been systematically observed in precision values ofE3 andM4 conversion coefficients reported in the literature.     

Vibrational spectra of tris(dmit) complexes of main group metals: infrared, Raman and ab initio calculations

Infrared Fourier Transform investigation of several metal tris-complexes of 1,3-dithiole-2-thione-4,5-dithiolate (dmit) ligand have been recorded within a theoretical-experimental investigation of the vibrational molecular spectra of crystalline [NEt4]2[Sn(dmit)3] and [NEt4][Sb(dmit)3] compounds. For the [Sn(dmit)3]-2 anion we recorded as well the Raman Fourier Transform spectra. Ab initio calculations have been carried with several ECPs, basis sets and methodologies (RHF and DFT) in order to assess family and methodological errors precisely. Geometry optimization and subsequent hessian calculation lead to the vibration frequencies reported. These calculated frequencies and intensities assisted the fundamental, overtones and combinations bands assignments. Remarkable agreement has been found between the experimental geometries and frequencies to those here calculated. Besides the bands traditionally studied for the dmit compounds, as CC and CS stretchings, also the region below 500 cm(-1) was evaluated, allowing to characterize several modes involving angular deformation of the dmit as the MS stretching of these octahedral distorted systems.