Mass spectra of the coordination compounds of nickel with the tetradentate Schiff bases of S-substituted thiosemicarbazides with 2,4-pentanedione and 3-oxo-2,4-pentanedione

The mass spectra of the coordination compounds of nickel(II) with the bis-S-substituted thiosemicarbazones of 2,4-pentanedione (R₂H₂L) with the general formula [NiR₂HL]X and of 3-oxo-2,4-pentanedione (H₂R₂LO) with the general formula NiR₂LO were studied. A distinguishing feature of the mass spectra of [NiR₂HL]X is the absence of molecular ion peaks. The initial point of fragmentation is the [M - HX] ions. The strongest lines in spectra of NiR₂LO are those due to the elimination of R and R-H. At the first stage the fragmentation of all the investigated compounds takes place mainly through only one of the thiosemicarbazide residues. At the second stage the 2,4-pentanedione (3-oxo-2,4-pentanedione) residue is eliminated, after which the remaining thiosemicarbazide part dissociates. A significant part of the fragmentation paths was confirmed by the peaks of the metastable ions and also by the mass spectra of the deuterated analogs.Institute of Chemistry, Academy of Sciences of the Moldavian SSR, Kishinev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 2, 205–211, March–April, 1991. Original article submitted August 19, 1986.     

How does product isotope effect prove the operation of a two-state "rebound" mechanism in C-H hydroxylation by cytochrome P450?

C-H hydroxylation is a fundamental process. In Nature it is catalyzed by the enzyme cytochrome P450, in a still-debated mechanism that poses a major intellectual challenge for both experiment and theory; currently, the opinions keep swaying between the original single-state rebound mechanism, a two-oxidant mechanism (where ferric peroxide participates as a second oxidant, in addition to the primary active species, the high-valent iron-oxo species), and two-state reactivity (TSR) mechanism (where two spin states are involved). Recent product isotope effect (PIE) measurements for the trans-2-phenyl-methyl cyclopropane probe (1), led Newcomb and co-workers (Newcomb, M.; Aebisher, D.; Shen, R.; Esala, R.; Chandrasena, P.; Hollenberg, P.; Coon, M. J. J. Am. Chem. Soc. 2003, 125, 6064-6065) to rule out TSR in favor of the two-oxidant scenario, since the direction of the PIE was at odds with the one predicted from calculations on methane hydroxylation. The present report describes a density functional theoretical study of C-H hydroxylation of the Newcomb probe, 1, leading to rearranged (3) and unrearranged (2) products. Our study shows that the reaction occurs via TSR in which the high-spin pathway gives dominant rearranged products, whereas the low-spin pathway favors unrearranged products. The calculated PIE(2/3) values based on TSR are found to be in excellent agreement with the experimental data of Newcomb and co-workers. This match between experiment and theory makes a strong case that the reaction occurs via TSR mechanism.     

Cycloaddition reactions of 1.3-benzothiazines v synthesis of new tetracyclic ring systems

New tetracyclic derivatives (3a,$\text{b}$, 5, 8) similar to protoberberines, but containing a lactam structural element and other hetero atoms besides the bridge-head nitrogen in rings B and C, were prepared by cycloaddition of 6,7-dimethoxy-2H-1,3-benzothiazine (1a) with $\text{o}$-substituted aromatic carboxylic acid derivatives (2a,$\text{b}$, 4) and from 4- methyl-6,7-dimethoxy-2$\text{H}$-1,3-benzothiazine (1b) with 3.5-dinitrobenzoyl chloride.     

Inorganic reaction mechanisms. vol. 4 (A chemical society specialist report) : A. McAuley,Senior reporter,The Chemcial Society,London, 1976, xviii + 398 pages, £ 23.00, $63.25.

The palladium(I) and platinum(I) complexes [(CH₃NC)₆M₂]²⁺ undergo substitution reactions with isocyanides, phosphines and halide or pseudohalide ions. With triphenylphosphine, axial substitution is preferred. The product [(CH₃NC)₅(Ph₃P)Pd₂]²⁺ is fluxional.     

Capillary electrophoresis separation of vinpocetine and related compounds: prediction of electrophoretic mobilities in partly aqueous media

Offord's equation, a relationship between electrophoretic mobility and charge, size and shape of peptides, has been extended to quantitate the electrophoretic mobility of vinca alkaloids. Partly aqueous protonation constants and the derived theoretical mobilities have been proven to be able to predict experimental electrophoretic mobilities. In practice, seven vincamine derivatives of very low water-solubility were separated by capillary electrophoresis. Buffer total concentration, apparent pH and methanol content, the three most important parameters of the running buffer, were used in triangular resolution mapping to characterize separation. Even though electrophoresis is well known to slow down in partly aqueous media, under our optimized circumstances a baseline separation was achieved within 8 min in each case.     

Missing-link macrocycles: hybrid heterocalixarene analogues formed from several different building blocks

The synthesis and structural characterization of hybrid heterocalix[4]arene analogues containing pyrrole, benzene, methoxy-substituted benzene, and pyridine subunits is described. Macrocycles 1 and 2, examples of calix[2]benzene[2]pyrrole and calix[1]benzene[3]pyrrole systems, respectively, are synthesized by the condensation of pyrrole and an appropriate phenylbis(carbinol). Macrocycles 3 and 7, examples of calix[2]benzene[1]pyridine[1]pyrrole and calix[1]pyridine[3]pyrrole, respectively, are synthesized by the use of a carbene-based pyrrole-to-pyridine ring-expansion procedure. Single-crystal X-ray analysis reveals that compounds 1a, 1b, and 2b adopt 1,3-alternate conformations in the solid state, whereas compounds 3 and 7 display structures that are best described as "flattened partial cones" in terms of their conformation. (Series a refers to pure benzene-derived systems, whereas series b indicates macrocycles containing 5-methoxyphenyl subunits). In the solid state, the methoxy-functionalized macrocycles 1b and 2b, and the chloropyridine-containing macrocycle 7 exist as dimers. In the case of 1a and 7, these compounds interact with neutral solvent in the solid state. The conformations of the macrocycles in solution were explored by temperature-dependent proton NMR and NOESY spectral analysis. At 188 K, macrocycles 1a and 2a adopt flattened 1,3-alternate conformations, whereas macrocycles 3 and 7 exist in the form of flattened partial-cone conformations. Standard proton NMR titration analyses were carried out in the case of macrocycles 1a and 2a, and reveal that at least the second of these systems is capable of binding fluoride and chloride anions in CD(2)Cl(2) solution at room temperature (K(a)=571 and 17M(-1) in the case of 2a and F(-) and Cl(-), respectively).     

On Triazoles. XIII. The reaction of 5-benzalimino-1,2,4-triazoles with substituted acetyl chlorides

The reaction of Schiff bases prepared from 1- and 2-substituted-5-amino-1,2,4-triazoles with phenoxyacetyl chlorides in the presence of triethylamine and a mixture of phosphorus oxychloride and dichloroacetic acid in dimethylformamide to yield β-lactam 4 , a dihydro-1,2,4-triazolo[4,3-a]pyrimidine-5(1H)-one 5, a 1,2,4-triazolo[1,5-a]pyrimidin-5(3H)-one 9 and the corresponding 1,2,4-triazolo[4,3-a]pyrimidine-5(1H)-one 10 derivatives was studied.