DFT study of pericyclic and pseudopericyclic thermal cheletropic decarbonylations. Evaluation of magnetic properties

A comprehensive B3LYP/6-31G** study of various thermal cheletropic decarbonylations was conducted. The complete pathway for each reaction was determined, and changes in magnetic susceptibility and its anisotropy were monitored with a view to estimating the aromatization associated to each process. This information, together with the energy and structural results, allowed us to clarify cases that are not clearly pseudopericyclic or pericyclic from previous work. Also, our results reveal that pericyclic reactions involve no disconnection in the cyclic array of overlapping orbitals. Therefore, a pseudopericyclic reaction involves at least one such disconnection. In any case, pseudopericyclic reactions involving two disconnections exhibit much clearer features, which facilitates their classification.     

Choice of coordination number in d10 complexes of group 11 metals

The distribution of di-, tri-, and tetracoordination among the d(10) ions of the group 11 metals is theoretically analyzed by means of density functional calculations on more than 150 model complexes of general formula [MX(m)L(n)](1-m) (where M = Cu, Ag, or Au; L = NH(3) or PH(3); X = Cl, Br, or I; m + n = 2-4). The energy of a ligand association reaction has been found to be practically determined by two contributions: the interaction energy and the energy of deformation of the metal coordination sphere. The larger deformation energy of gold complexes compared to copper and silver ones explains the predominance of dicoordination among Au(I) complexes, in comparison with Cu(I) and Ag(I), for which dicoordination is far less common than tri- and tetracoordination. Other experimental trends can be explained by looking at the fine details of these two energetic components.     

Acid-catalyzed Grob fragmentation reactions of acetonides derived from terpenes

[reaction: see text] Acetonides derived from different terpenes undergo Grob fragmentation by treatment with a catalytic amount of acid, triflic acid, or boron trifluoride, giving aldehydes containing a cyclopropane or cyclobutane ring with good yields and complete diastereoselectivity. The structure and the stereochemistry of the starting acetonide have a crucial influence on the reaction course.     

Selective spectrophotometric kinetic determination of copper with 2-methyl-1, 3-cyclohexanedione bis(4-phenyl-3-thiosemicarbazone)

A non-catalytic kinetic method is described for the determination of copper(II), based on the slow rate of complexation between the metal ion and 2-methyl-1, 3-cyclohexanedione bis(4-phenyl-3-thiosemicarbazone), which is due to the slow syn/anti equilibration of the reagent. The reaction of pH <2 is followed by measuring the rate of change of the absorbance at 480 nm. The calibration graph is linear over the copper range 2–10 μg ml^?1, with relative standard deviations of 1.20% (tangent method), 1.85% (fixed-time method) and 1.75% (fixed-absorbance method). The only serious interferences are from Pd, Sb, Mo, Hg and periodate.     

19F,31P,1H NMR and X-ray crystallographic studies of (pentafluorophenyl)3−n (4-pyrazol-1-yl-2,3,5,6-tetrafluorophenyl) n=1,2,3 Phosphines

New phosphines1–3 have been synthetized by reaction of pyrazolate anion with tris(pentafluorophenyl)phosphine and characterized by¹H,³¹P, and¹⁹F NMR studies.¹⁹F NMR spectral data contribute to the evidence for apara-substitution of tetrafluorophenyl rings. The crystal structure of tris(4-pyrazol-1-yl-2,3,5,6-tetrafluorophenyl)phosphine 1 has been determined, proving that the assignment based on spectroscopic data was correct: C₂₇H₉F₁₂N₆P,M _r = 676.37, monoclinic, space group P2_l/c,a=10.754(2) å,b=10.316(2) å,c = 23.598(5) å,=95.36(3),V=2607(1), å³,Z=4,R ₁=0.042, andwR ₂=0.122.     

Ligand effects and dimer formation in dicoordinated copper(I) complexes

The dimers of dicoordinated Cu(I) complexes of the type [CuXL], where X^? = Cl^?, Br^?, I^?, or CN^? and L is a neutral ligand such as NH₃, PH₃, or CNCH₃, have been studied by means of ab initio and density functional calculations. The performance of density functional calculations using the hybrid B3LYP method has been compared with MP2 results. Ligand and conformational effects are analyzed for two types of dimers, notably those in which the monomers retain their linear conformation and associate through Cu–Cu contacts and those in which the X ligands act as bridges resulting in tricoordination of each Cu atom and a rhombic Cu₂X₂ core. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Intracellular oxidation of dipeptides. Base-promoted elimination from N-halodipeptides to 2-[N-alkyl-N-(2-N-alkylimino-2-alkyl-ethanoyl)amino]-2,2-dialkylethanoic acids

One of the possible ways of intracellular oxidation of peptides is via the formation of the corresponding (N-X)-dipeptides, that then undergo base-promoted elimination to yield intermediate 2-[N-alkyl-N-(2-N-alkylimino-2-alkylethanoyl)amino]-2,2-dialkylethanoic acids, which subsequently hydrolyze. Such an elimination process is general-base catalyzed, with Br?nsted beta values ranging from 0.26 to 0.31, which suggests an essentially constant degree of proton transfer at the TS. For (N-X)-dipeptides, the ratio k(N-Br)/k(N-Cl) ranges from 2.5 to 15, suggesting a structural dependence of the degree of N-X bond breaking at the TS. The values of beta and k(N-Br)/k(N-Cl) support a concerted asynchronous A(xh)D(H)D(N) mechanism, its TS changing from reactant-like to slightly nitrenium-like depending on the structure of the starting dipeptide. As a consequence of the antiperiplanarity requirements of the reaction, the steric interaction between the leaving group and the substituent on the C bearing the H to be eliminated controls the reaction rate. Such steric interaction is rather important, as indicated by the steric crossed-interaction coefficient (p(ssy') = 0.33). Semiempirical calculations show that bulky substituents in the vicinity of the reaction center imply additional energy requirements for the system to achieve the antiperiplanarity needed at the TS for the reaction to proceed. From the observations reported it follows that (N-X)-dipeptides lose their oxidizing power more readily than analogous (N-X)-amino acids or (N-X)-amines, opening a possible pathway to lessen intracellular halogen-based oxidative stress.     

Micellar electrokinetic capillary chromatographic method for simultaneous determination of drugs used to treat advanced breast cancer

Summary A micellar electrokinetic capillary chromatography (MEKC) method has been developed for quantification of four drugs-tamoxifen, anastrozole, letrozole, and methotrexate—used to treat advanced breast cancer. Separation was performed at 25°C and 25kV, with 20mm borate buffer (pH 9.2) containing 40mm sodium dodecylsulfate as electrolyte solution. Under these conditions analyses were performed in 12 min. The linearity of the response was investigated for the concentration range 2.0–20.0 mg L⁻¹. The intra-day residual standard deviation (n=4 graphs) between the slopes of the calibration graphs was acceptable for the four drugs studied. Detection limits (signal-to-noise ratio=3) were below 1 mg L⁻¹ for all the compounds. The simplicity, precision, and sensitivity of MEKC proved suitable for quality control of pharmaceutical preparations used to treat advanced breast cancer. Six different pharmaceutical preparations, each containing one of the above-mentioned drugs, were successfully analyzed.     

Separation and determination of aminohalogenbenzophenones by high-performance liquid chromatography with electrochemical detection.

A high-performance liquid chromatographic method with electrochemical detection has been developed for the determination of three aminohalogenbenzophenones: 2-amino-2',5-dichlorobenzophenone, 2-amino-5-chlorobenzophenone and 2-amino-5-bromo-2'-fluorobenzophenone, metabolites of benzodiazepinooxazoles and other psychotropic drugs. A mobile phase of methanol-water (65:35), containing 5 mM KH2PO4 appeared to be the optimal when a 4-microns, 60-A Nova-Pak C18 column and a flow-rate of 0.75 ml/min (130 bar) were used. The temperature was optimized at 30 degrees C. The amperometric detector, equipped with glassy carbon electrode, was operated at 1.3 V versus Ag/AgCl in the DC mode. The method was applied to the determination of these compounds at two concentration levels: ppm and ppb (ng/cm3) using 2-amino-5-chlorobenzophenone as internal standard. The limit of determination was 750 pg/ml of biological fluid for each compound, and recoveries greater than 97% were obtained for spiked samples of urine and serum, using C18 Sep-Pak cartridges in the sample clean-up procedure.     

Axial Bonding Capabilities of Square Planar d(8)-ML(4) Complexes. Theoretical Study and Structural Correlations

A qualitative molecular orbital study and a structural analysis of the bonding capabilities of the metal atoms in square planar ML(4) complexes of d(8) ions are presented. In addition to analyzing the donor-acceptor properties of the metal atom in such complexes, the following aspects are also studied: (a) the effect of axial groups (bases or acids) on the donor-acceptor properties of the metal atom; (b) the effect of the axial groups on the deviation of the ML(4) ensemble from planarity; (c) the effect of an axial group on the bond between the metal atom and another group in trans; and (d) the implications on chemical reactivity.