Determination of phosphite,hypophosphite, tellurium and mercury

A new catalytic oxidation procedure, involving the use of a cerium(IV) sulfate reagent in perchloric acid with a mixed silver(I)—manganese(II) perchlorate catalyst, has been developed for the determination of phosphite, hypophosphite, and tellurium. By this method 15—100 mg of phosphite, 5—35 mg of hypophosphite, and 10—105 mg of tellurium may be determined with standard deviations of ±0.17, ±0.32, and ±0.21% respectively. A direct titration procedure for mercury(I) is described using a ceric perchlorate solution as titrant with the mixed catalyst system. Samples from 65–510 mg may be analyzed with a standard deviation of ±0.36%.     

Syntheses of seven-membered rings: ruthenium-catalyzed intramolecular [5+2] cycloadditions

The Ru-catalyzed intramolecular [5+2] cycloaddition of cyclopropylenynes is investigated with respect to the regio- and diastereoselectivity as well as the functional group compatibility of the reaction. Evidence for the mechanism as occurring through a ruthenacyclopentene intermediate is elucidated from 1) the study of the diastereoselectivity of the cycloaddition; 2) the effect of variation of substituents on the regioselectivity of cyclopropyl bond cleavage in 1,2-trans- and 1,2-cis-disubstituted cyclopropanes and 3) examples that clearly do not involve ruthenacyclohexene as intermediates as products still incorporate the cyclopropyl moiety. The scope and limitations of the Ru-catalyzed cycloaddition are discussed and compared with the Rh-catalyzed reaction. The potential power of this methodology towards natural product total synthesis is demonstrated by the formation of several polycyclic systems with the chosen reaction conditions and readily available cyclopropylenyne substrates.     

Studies of the thermal degradation of acetaminophen using a conventional HPLC approach and electrospray ionization-mass spectrometry

The thermal degradation of acetaminophen is studied via conventional accelerated aging studies by initially thermally stressing the compound at temperatures between 160 degrees C and 190 degrees C and measuring the rate of decomposition by reversed-phase high-performance liquid chromatography. Rates of decomposition of the compound in the dry state and the activation energy for the process are determined and compared with previously published kinetic and thermodynamic data for the degradation of acetaminophen in solution. In addition, the thermal fragmentation of acetaminophen under electrospray ionization (ESI) conditions using an interface with a heated capillary inlet is studied and the apparent activation energy for this process also is characterized. A comparison of the data shows that acetaminophen is significantly more stable in the dry state than in solution. However, the gas-phase fragmentation of acetaminophen under ESI conditions occurs more readily than either dry- or solution-state degradation. Although the resulting electrospray fragmentation mimics the breakdown product that is formed when the compound undergoes either acid or base catalyzed hydrolysis in aqueous solutions, the mechanism that produces the fragment ion appears to involve a two-step process. Initially, the parent ion forms of the analyte are produced in the spray region of the interface followed by wall-catalyzed decomposition and re-ionization in the heated inlet capillary of the spectrometer.     

Kinetics and Mechanism of Monomolecular Heterolysis of Commercial Organohalogen Compounds: XXXVII.1 Effect of Nucleofuge and Solvent of the Relative Rates of Heterolysis of 1-Halo-1-methylcyclopentanes and 1-Halo-1-methylcyclohexanes. Correlation Analysis of Solvation Effects

The effect of solvent ionizing ability on heterolysis rate enhances in the series 1-chloro-1-methylcyclohexane < 1-bromo-1-methylcyclohexane 1-chloro-1-methylcyclopentane < 1-bromo-1-methyl- cyclopentane. The lower sensitivity of cyclohexyl substrates compared with cyclopentyl is determined by conformational effects. Bromides are more sensitive to solvent effects than chlorides because of the stronger polarizability of the C-Br bond.     

Application of neural network calibrations to an halide ISE array

The Neural Network (NN) technique was applied to the calibration of an ion selective electrode (ISE) array comprising a bromide selective electrode, two chloride ISEs and one thiocyanate ISE. The measured samples were synthetic mixture solutions of chlorides and bromides in concentration ranges such that interference occurs. The NN method allowed to perform the calibration without estimating the coefficients of the Nikolskii-Eisenman theoretical relation. Only the determination of bromide was detailed. The results obtained using this method were better than those obtained using linear multivariate calibration methods.     

Interfacial and intrafacial linkage isomerizations of rhenium complexes with aromatic molecules

The mechanisms for the interconversion of facial diastereomers of a variety of TpRe(CO)(L)(eta(2)-L(Ar)) complexes [L = (t)BuNC, pyridine (py), PMe(3), or 1-methylimidazole (MeIm); L(Ar) = benzene, anisole, naphthalene, 1-methylpyrrole, furan, or thiophene; Tp = hydridotris(pyrazolyl)borate] have been investigated by (1)H NMR spin saturation experiments. In addition, the rates and free energies of activation for these processes were calculated from spin saturation experiments and T(1) measurements. The operative mechanisms for interconversion of the pi diastereomers were found to be nondissociative, undergoing either an interfacial or intrafacial linkage isomerization. A comparison of the kinetic parameters for isomerization of related eta(2)-olefin complexes of the [TpRe(CO)(PMe(3))] and [CpRe(NO)(PPh(3))](+) fragments is also presented.     

Synthesis of 2,3,4,5-tetranitropyrrole

Dealkylation of 1-tert-butyl-2,3,4-trinitropyrrole in boiling trifluroacetic acid gave 2,3,4-trinitropyrrole, the structure of which was confirmed by X-ray crystallography. Treatment of this trinitropyrrole with nitric acid/oleum briefly at 60° gave 2,3,4,5-tetranitropyrrole.     

Synthesis and reactions of the transition metal substituted tin hydride HSn[Mo(CO)3C5H5]3

Reaction of HMo(CO)₃C₅H₅ and Sn(C₅H₅)₂ produces the tin hydride HSn[Mo(CO)₃C₅H₅]₃ (I). Reaction of I with CCl₄, CHCl₃, or CH₂Cl₂ gives ClSn[Mo(CO)₃C₅H₅]₃ (II). With hydrogen chloride the hydride I reacts to produce the dichloride Cl₂Sn[Mo(CO)₃C₅H₅]₂. The first step in this reaction is cleavage of the SnH bond to produce the chloride II. The hydride I reacts with acetic acid to produce the diacetate (CH₃COO)₂Sn[Mo(CO)₃C₅H₅]₂.     

Crotocaudin; a rearranged labdane type norditerpene from Croton caudatus geisel

The structure and stereochemistry of crotocaudin, a new norditerpene occurring as a minor constituent in Crotoncaudatus Geisel (Euphorbiaceae) have been established as ent-8S,10S-15,16-epoxy-19-norcleroda-4,11,13(16), 14-tetraene 18,6S: 20,12-diolactone 7 from the detailed studies of ¹H NMR spectra using lanthanide shift reagents, decoupling experiments and chemical reactions. The congener, teucvidin, ent-10S-15,16-epoxy-19-norcleroda-4,13(16), 14-triene-18,6S: 20,12R-diolactone 1 was obtained as a major component besides several triterpenoids viz taraxerone 2, taraxerol 3 and taraxeryl acetate 4. ¹³C NMR and ¹H NMR spectra and a few novel reactions of teucvidin and its conversion to teucvin, ent-(6R,12R)-15,16-epoxy-19-nor-9,4-friedolabda-4,13(16),14-triene-18,6:20, 12-diolactone 6 are also reported.     

The numbers of chiral and achiral alkanes and monosubstituted alkanes

Whereas the theory for the enumeration of the optical isomers of the lakyl radicals and the alkanes has long been understood, this is not the case for the corresponding archiral isomers. We present for the first time recurrence formulae for counting the number of archiral isomers of the alkyl radicals and the alkanes. For chiral and archiral alkanes and monosubstituted alkanes, numerical results up to C₁₄ are tabulated.After presenting the history of the problem and the necessary definitions, we proceed to derive functional equations on the various generating functions, which readily yield the more explicit recurrence formulae usefule for numerical calculations. In the process, we first re-derive Pólya's expression for planted steric trees using his classical enumeration theorem. This result is then extended to the enumeration of free steric trees using the now standard tree-counting method due to Otter and known as a dissimilarity characteristic equation.By definition, a steric tree is a quartic tree (all points having degree 1 or 4) in which the four neighbors of every carbon point are given a tetrahedral configuration. Building on the methods of the first two authors for counting chiral and archiral trees in the plane, we obtain the formula for counting achiral steric trees, thus setting a problem first enunciated by van't Hoff and Le Bel in 1874.