Stereochemistry and mechanism of catalytic hydrogenation of substituted methylenecyclohexanes

Substituted alkylmethylenecyclohexanes were hydrogenated over several transition metal catalysts. The ratios of the epimeric products were almost unity over freshly prepared Raney Ni but the axial Me counterparts were favoured over aged catalyst. The axial Me products were also preferred on Pt or Rh catalysts. Pd catalysed hydrogenation gave predominantly the equatorial Me isomers at high catalyst ratio, while the axial Me counterparts were favoured at the early stage of the reaction at a catalyst-substrate ratio of 1:20. The change of the ratio of epimeric products depending on the amount of catalyst is due to the rapid migration of exo-cyclic double bond to the inside of the cyclohexane ring at high catalyst ratio. The mechanism of hydrogenation is discussed in terms of the modified Horiuti-Polanyi mechanism.     

Stereochemistry and mechanism of the hydrogenation of substituted cyclopentenes

1,2- and 2,3-disubstituted cyclopentenes were hydrogenated with several transition metal catalysts. The hydrogenation of 1,2-disubstituted cyclopentenes gave preferably the cis products with Raney Ni, whereas 2,3-disubstituted cyclopentenes gave preferably the trans isomers. The trans products were favoured in the hydrogenation of 1,2- and 2,3-dialkylcyclopentenes with Pt or Pd catalysts. A detailed product analysis of the hydrogenation of 1,2-dialkylcyclopentenes indicates that the double bond migration occurred to form 2,3-dialkylcyclopentenes in advance of the hydrogenation. In contrast, 1-methyl-2-phenylcyclopentene gave predominantly the cis product irrespective of the kind of catalyst. These results suggest the formation of a π-benzylic species during the course of the reaction. The mechanism is discussed in terms of the modified Horiuti-Polanyi mechanism.     

New catalytic system for the hydrogenation of ketones

Alcohols are produced in high yield by hydrogenation of several aliphatic and aromatic ketones with rhodium complexes as catalysts in the presence of strong alkali.     

The stoichiometric hydrogenation of substituted phenyl alkenes by hydridocobalt tetracarbonyl

The relative rates of hydrogenation of a series of styrenes, phenylpropenes, 1,1-diphenylethylenes, and 1,1-diphenylpropenes were measured. Compared to 1,1-diphenylethylene (k₂ = 2.42 X 10^-2 I mol^-1 sec^-1), 1,1-diphenylpropene and styrene have relative rates of 0.0045 and 0.011 respectively. The effect of 4-chloro and 4-methoxy substituents on both styrene and diphenylethylene is slightly rate enhancing. An unusual kinetic dependence occurs with mixtures of alkenes.     

Analytical reactions of substituted pyrimidines

Analytical aspects of the chemistry of substituted pyrimidines are reviewed.     

Palladium-catalyzed synthesis of substituted cyclopentadienes and indenes from allylic esters

Various mono- and di-substituted cyclopentadienes have been prepared by palladium-catalyzed alkylation of allylic esters with cyclopentadienide and t-butyl-cyclopentadienide anions. The same procedure has been applied to the preparation of substituted indenes.     

Electron-transfer reactions and conformational changes associated with the reduction of substituted bianthrones

The electrochemical reduction of several substituted bianthrones is similar to that of the parent compound. 3,3′-Dimethylbianthrone (II), 3,3′-di-n-heptylbianthrone (III). 3,3′-dimethoxybianthrone (IV) and 1,1′-dimethylbianthrone -(V) were studied in dimethylformamide using cyclic voltammetry and transmission mode spectroelectrochemistry. For each compound the low temperature A form is reduced in a two-electron irreversible reaction to a twisted dianion, B^2?. Upon oxidation, B^2? forms first B, then B, whose spectral properties are identical to those of the high-temperature thermochromic form of the bianthrones. Rate constants for the B-A reaction were determined for each compound. The reduction of 2,3,2′,3′-dibenzo-7,7′-dimethylbianthrone (VI) showed somewhat different features which were tentatively interpreted in terms of redox catalysis.     

Stereochemistry of reactions in the 1,2-dimethylsilacyclopentane ring systems. I. Stereospecific transformations

Preparations, separations of geometric isomers, and structural assignments based on nmr and on chemical evidence are described for a number of 1-substituted 1,2-dimethylsilacyclopentanes. A number of stereospecific reactions have been observed, and the stereochemistry is in all cases the same as that observed for acyclic silanes. A discussion of the role of ring strain in determining stereochemical outcome and reaction rates is presented.     

Preparation and properties of substituted iron tungstates

Polycrystalline samples of members of the systems Fe_2?xCr_xWO₆ and Fe_1?xMn_xWO₄ were prepared and single crystals of Fe_1?xMn_xWO₄ were grown by chemical vapor transport. Their crystallographic parameters and electrical properties were characterized. Fe₂WO₆ crystallizes with the tri-α-PbO₂ structure and is an n-type semiconductor. For 0.3 ≤ x ≤ 2, the system Fe_2?xCr_xWO₆ crystallizes with the inverse trirutile structure and is nonconducting due to blocking of iron(II)-iron(III) conduction paths by chromium(III). For 0 ≤ x ≤ 1, the system Fe_1?xMn_xWO₄ crystallizes with the wolframite structure and shows p-type semiconducting behavior. The nature of the variation of resistivity with x of Fe_1?xMn_xWO₄ suggests that interchain electron transfer may occur in this structure.     

Convenient synthesis of (triphenyl)germyl and (triphenydstannyl substituted allenes

Allenyl-germanes and -stannanes, Ph₃MC(R)CCR′R″ (M = Ge, Sn) can be obtained, generally in excellent yield, through alkylcopper(I)-induced 1,3-substitution of the propargylic chlorides Ph₃MCCCR′R″Cl. In the tin series, however transmetallation is the main process when MeCu, H₂CCHCu or PhCu are used. The allenyl compounds in which R is (trimethylsilyl)ethynyl or 4,4-dimethyl-1,2-pentadienyl can be obtained by using the organozinc compounds instead of the copper(I) compound and using tetrakis(triphenylphosphine)palladium as catalyst.     

Solid—liquid phase diagrams of mixtures containing substituted phenols and amine mixtures

The thermal decomposition of CaCO₃ was studied under high vacuum by means of both TG and the more recently developed constant decomposition rate thermal analysis (CRTA) which allows the monitoring of both reaction rate and the residual pressure over the sample. The reliability of the kinetic results seems to be much higher with the latter technique which actually allows the reduction of the reaction rate and therefore the heat and mass transfer effects over a broad range of sample size. For instance, it was necessary, by conventional TG started under a vacuum of 2 10^?6 torr with a heating rate of 0.5 K min^?1, to lower the amount of sample to 2 mg in order to obtain the same activation energy as that calculated from CRTA with various samples weighing up to 50 mg. The TG experimental conditions quoted above (and which are upper limits of mass and heating rate) are beyond the limit of sensitivity of most available conventional TG equipment.     

Structural characterization of polyolefins by pyrolysis—hydrogenation glass capillary gas chromatography

In order to obtain quantitatively specific and high-resolution pyrograms of high polymers, fundamental splitting conditions for pyrolysis—gas chromatography were studied using a vertical micro-furnace type of pyrolyzer attached to a glass capiliary separation column. High-resolution pyrograms were measured for polyolefins such as polyethylenes (PE), polypropylenes (PP) and various kinds of ethylen—propylene copolymers [P(E-co-P)] using a pyrolysis—hydrogenation device. Microstructures such as short branchings for PE, stereoregularities and chemical inversions for PP and sequence distributions for P(E-co-P) are discussed on the basis of the pyrograms.     

The preparation and some catalytic activity of polymer-supported η5-cyclopentadienyl-rhoduim and -cobalt dicarbonyls

Polymer-supported η⁵-cyclopentadienylrhoduim dicarbonyl and η⁵-cyclopentadienylcobalt dicarbonyl have been prepared with 20% divinylbenzene-polystyrene copolymer, macroporous beads. The beads have been tested for a variety of types of catalytic activity. The cobalt-containing beads have not proven to be catalysts. The rhodium beads were effective in hydrogenation of olefins, aldehydes and ketones, isomerization of olefins, disproportion of 1,4,-cyclohexadiene and cyclohexene, cyclotrimerization of ethyl propiolate, and hydroformylation of 1-pentene and 1-hexene. Decomposition of the rhodium catalysts occurs except in hydroformylation, although only slight loss of the carbonyl groups and catalytic activity was observed in cyclotrimerization.     

Oxidation of ferrocene and some substituted ferrocenes in the presence of horseradish peroxidase

The enzyme catalysed oxidation of ferrocene and some substituted ferrocenes to the corresponding ferricinium ions by hydrogen peroxide in the presence of native or immobilized horseradish peroxidase has been studied. Initial and maximum rates of oxidation have been determined. It was found that the oxidation was independent of the hydrogen peroxide concentration. The oxidation of ferrocene was effected also by horseradish peroxidase in a coupled system with glucose oxidase in the absence of any added hydrogen peroxide.     

On the mechanism of hydrogen evolution at GaAs electrodes

The hydrogen evolution reaction at n- and p-GaAs electrodes has been reinvestigated. As in the case of metal electrodes, hydrogen evolution can occur in two ways: at ?0.5 V (SCE) hydronium ions are reduced, at ?1.25 V (SCE) reduction of water molecules takes place. It is confirmed that in both cases conduction band electrons are responsible for the two reduction steps, forming adsorbed hydrogen atoms in the first and hydrogen molecules in the second step. Hole injection can occur only to a negligible extent, although it appears energetically feasible.     

Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : III. Proton data at 100 MHz and the derived Hammett σ-constant of the trimethyltin group

Proton NMR data at 100 MHz are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, XC₆H₄Sn(CH₃)₃, where X = para-N(CH₃)₂, para-OCH₃, para-OC₂H₅, para-CH₃, meta-CH₃, -H, para-F, meta-OCH₃, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH₃)₃. Correlation coefficients with Hammett σ-constants of greater than 0.95 are obtained with the methyltin proton chemical shifts and coupling constants to carbon [¹J(¹³C¹H)] and tin [²J(SnC¹H)]. Solvent effects and other extraneous factors invalidate comparisons of ? values in terms of the relative attenuation of the transmission of substituent effects through homologous carbon, silicon, germanium and tin systems, but coupling constant data reflect a diminution of ca. one tenthfold per bond in the order ?[C(1)Sn] > ? [SnC] > ? [CH]. Satisfactory correlations (r > 0.95) are obtained in this series of closely-related compounds among the conventionally recorded two-bond, ²J(SnC¹H) and the constituent, one-bond ¹J (Sn¹³C) and J(¹³C¹H) coupling constants, but the correlation coefficient for the comparison between the two one-bond couplings, ¹J(Sn¹³C) and ¹J(¹³C¹H) is lower (r = 0.872). Changes in the couplings at the methyltin carbon bond tin-119 atoms are interpreted in terms of isovalent hybridization; a model based upon effective nuclear charges is tested with respect to both NMR coupling constants and ¹¹⁹Sn Mössbauer Isomer shifts at tin and is invalidated. Proton and carbon-13 NMR, chemical shift and coupling constant data are used to derive a Hammett σ-constant for the para-trimethyltin group of ?0.14, and the significance of this value is discussed.     

Solid state studies on substituted CuCr2O4 spinel

Changes in crystallographic, electrical, and thermal properties of CuCr₂O₄ spinel were investigated by replacing Cu with Mg, i.e., Cu_1?xMg_xCr₂O₄, and Cr with Al, i.e., CuCr_2?xAl_xO₄. The tetragonal distortion in CuCr₂O₄ disappeared with 60% replacement of Cu by Mg (x = 0.6) or 50% replacement of Cr by Al (x = 1.0). The temperature variation of electrical resistivity for all the tetragonal samples was similar to that of CuCr₂O₄. The first order, diffusionless phase transition was manifest in the hysteresis loops of log ? vs $\text{1}\text{T}$ plots. The resistivity and activation energy for conduction changed sharply near the phase transition composition. With the replacement of Cr by Al, the conduction in CuCr₂O₄ was found to change from p type to n type. The low thermal stability of the spinel was found to be due to a high concentration of tetrahedral Cu²⁺ ions (>80%) and compressed tetragonal distortion which strains the spinel lattice. This strain is removed by replacing either Cu with Mg or Cr with Al, whereby the spinel becomes stable.