摘要:

The influence of preoxidizing the surface of polycrystalline copper electrodes on the efficiency of the controlled potential electrochemical reduction of nitrobenzene and azobenzene to aniline in basic aqueous methanol has been investigated. Electrodes electrochemically preoxidized to Cu(OH)2or to Cu2O gave high yields of aniline (90–100%) with high current efficiencies (88–99%) for the electrohydrogenation of nitrobenzene compared to electrodes preoxidized by air (7–29% of aniline, 64–74% current efficiency). For the electrohydrogenation of azobenzene, copper electrodes preoxidized to Cu2O were found to be more active (average yield of aniline of 28%, average current efficiency of 55%) than electrodes preoxidized to Cu(OH)2(3–11% of aniline, 42–53% current efficiency), and electrodes preoxidized by air were inactive (the electrohydrogenation stopped at hydrazobenzene). The improvement of activity induced by preoxidation of the copper surface is linked to the increase of the surface area.

ISSN:0008-4042    

DOI:10.1139/v94-301

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Numerous analogous dihydroxamates of rhodotorulic acid have been prepared, and their mode of complexation of iron(III) and the stability of these complexes studied as a function of pH, for comparison with ferrioxamine B and the complex of acetohydroxamic acid with iron(III). This study has shown that the stoichiometry of the complexes is 2:3 (at a pH of 7) and 1:1 (at a pH of 2), and that, in spite of a complete complexation, their electrochemical behaviour varies greatly with pH. The very strong complexation constant observed (log β110 = 21.3) at a pH of 1 with the ligandN,N′-bis(acetylhydroxamino-3′ propyl)-pentyloxy-4 pyridine-2,6-dicarboxamide confirms the strong complexation capacity of iron(III) by these complexes. The high relaxivity of the complex of iron(III) with theN,N′-bis(acetylhydroxyamino-3′ -propyl)-(amino-4″ -butyloxy)-4-pyridine-2,6-dicarboxamide suggests that this product could be a useful contrasting agent for nuclear magnetic imaging. [Journal translation]

ISSN:0008-4042    

DOI:10.1139/v94-302

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

A rigorous analytical solution to reverse pulse techniques for a plane electrode expanding with an arbitrary power of time is presented. Our results include the plane approximation for SMDE as well as the expanding plane electrode model for DME. In the case of SMDE, our results have been contrasted with those obtained by numerical solution of the problem, obtaining identical results. Finally, we have compared our treatment with previous solutions in literature.

ISSN:0008-4042    

DOI:10.1139/v94-303

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The general rigorous theory corresponding to double potential step chronoamperometry for a reversible charge transfer reaction in a spherical electrode whose area increases withtz, wherez ≥ 0, is presented. The static mercury drop electrode (z = 0) and the dropping mercury electrode (z = 2/3) are addressed specifically. The explicit equations for the current versus time response assuming unequal diffusion coefficients for the oxidized and reduced species are valid when the product is soluble in both the electrolytic solution and the electrode. Comparisons with previous general treatments in the literature are made. The theory is applied to the detection of amalgamation in RP and DNP techniques.Moreover, in this work we show the origin of errors appearing in some general treatments in the literature, these errors lead to physically meaningless results when amalgamation takes place.

ISSN:0008-4042    

DOI:10.1139/v94-304

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The photoprotonation of four dialkoxy-substituted benzenes in their excited singlet states has been studied. The parent systems4and5are regioselectively photoprotonated at the 2-position, with significant quantum efficiencies for deuterium incorporation at acidities greater than pH 2. The structurally related cyclophane derivatives6and7did not show any deuterium exchange over the same acidity range but fluorescence quenching by proton (in aqueous solution) and laser flash photolysis studies (in 1,1,1,3,3,3-hexafluoro-2-propanol) indicate that photoprotonation does take place with these compounds. Due to structural constraints imposed by the most stable conformation adopted by these molecules (6and7), the attacking proton (deuteron) is also the proton (deuteron) that is deprotonated from the cyclohexadienyl cation (2,6-dialkoxybenzenonium ion) intermediate, resulting in the absence of incorporation of deuterium.

ISSN:0008-4042    

DOI:10.1139/v94-305

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The regioselectivities ofp-toluenesulfonation reactions of dialkylstannylene acetals obtained from a number of carbohydrate-derived terminal 1,2-diols in the absence of added nucleophiles have been explored as functions of the carbohydrate structure, the nature of the alkyl group on tin, the solvent, and the reaction temperature. Virtually all dibutylstannylene acetals react preferentially on the primary oxygen atom, sometimes with excellent regioselectivity. Increasing the steric bulk of the alkyl substituents on tin increases the preference for reaction at the secondary oxygen atom, but at the expense of increased reaction times. Hexamethylenestannylene acetals react at about the same rate as dibutylstannylene acetals; in almost all cases, they react preferentially on the secondary oxygen atom, sometimes with excellent regioselectivity.

ISSN:0008-4042    

DOI:10.1139/v94-306

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The119Sn NMR spectra of dialkylstannylene acetals derived from a number of carbohydrate-based terminal 1,2-diols have been recorded. The spectra of chloroform-dsolutions at room temperature of most of these dialkylstannylene acetals were consistent with the presence of single symmetric dimers only although some did contain small proportions of higher oligomers. A general mechanistic scheme is presented that explains the regioselectivity ofp-toluenesulfonation reactions of dialkylstannylene acetals presented in the preceding paper. In the scheme, the reaction intermediates are the dialkylstannylene acetal dimers. Forp-toluenesulfonation reactions, equilibration between the three possible dimers is faster than reaction withp-toluenesulfonyl chloride. Reaction regioselectivity depends both on the position of the dimer equilibrium and on the rates of reaction of individual dicoordinate oxygen atoms in the dimers. The general scheme was also used to explain the regioselectivity obtained for the very fast oxidation reactions of dialkylstannylene acetals with bromine orN-bromosuccinimide. In these kinetically controlled reactions, regiochemistry is determined by the initial reaction of dicoordinate oxygen atoms in the most populated dimer(s) with the electrophilic oxidizing reagent.

ISSN:0008-4042    

DOI:10.1139/v94-307

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The total synthesis of marine sesquiterpenoid isoacanthodoral (3), in racemic form, has been accomplished, using the boron trifluoride catalyzed Diels–Alder reaction of dienone ester4and isoprene as the key step.

ISSN:0008-4042    

DOI:10.1139/v94-308

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

A tetramer of dimethyl acetylenedicarboxylate, tetramethyl 4-methoxy-5-[1,2,3-tris(methoxycarbonyl)-2-cyclopropen-1-yl]-7-oxabicyclo[2.2.1]hepta-2,5-diene-1,2,3,6-tetracarboxylate3, was recovered from old stocks of the monomer, and was also prepared thermally from the monomer by a variation of a previous method. NMR data and an X-ray crystal structure were determined for a red 1:1 adduct of this ester with triphenylphosphine. This red adduct, tetramethyl 4-methoxy-5-[1,2,3-tris(carbomethoxy)-3-triphenylphosphoranylidenepropen-1-yl]-7-oxabicyclo[2.2.1]hepta-2,5-diene-1,2,3,6-tetracarboxylate5, crystallized in the triclinic space groupP1 (No. 2) with two molecules in the unit cell (a = 12.315(2) Å,b = 12.321(2) Å,c = 14.652(2) Å, α = 110.60(1)°, β = 90.62 (1)°, and γ = 103.22(1)°). Refinement converged atR = 0.0694 (Rw = 0.0986) for 542 parameters using 4714 reflections withI > 2σ(I). Triphenylphosphine oxide did not react with tetramer3. Reaction of the tetramer of dimethyl acetylenedicarboxylate3with 1,2,5-triphenylphosphole gave an orange product with concomitant loss of a furan triester. NMR data confirmed that this was not a simple adduct, and examination of a crystal by X-ray established the structure as the dichloromethane complex of tetramethyl 1,9,10-triphenyl-1-phospha(V)tricyclo[5,2,1,05,10] deca-1,3,5,8-tetraene-2,3,4,6-tetracarboxylate10. This orange product crystallized in the monoclinic space groupP21/c(No. 14) with four molecules in the unit cell (a = 10.056(2) Å,b = 14.280(1) Å,c = 23.892(3) Å, α = 90.0°, β = 94.15(1)°, and γ = 90.0°). Refinement converged atR = 0.0683 (Rw = 0.0710) for 540 parameters using 3180 independent reflections withI > 3σ(I). The tetramer3did not react with 1,2,5-triphenylphosphole-1-oxide quickly, but after 9 months gave a white, probably polymeric, product.

ISSN:0008-4042    

DOI:10.1139/v94-309

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Dye solubility and stopped-flow kinetic studies were conducted in sodium dodecylsulfate/dodecyltrimethylammonium bromide and sodium dodecylsulfate/decyltrimethylammonium bromide micellar solutions with excess anionic surfactant. The enhanced rate in the presence of anionic micelles of the Ni2+(aq)/pyridine-2-azo-p-dimethylaniline (PADA) complexation reaction was used as a probe of the mixed micellar situation. PADA solubilities and the kinetic parameters derived through the Robinson model for micellar catalysis were consistent with a complete incorporation of the cationic surfactant into the sodium dodecylsulfate micelles.

ISSN:0008-4042    

DOI:10.1139/v94-310

出版商:NRC Research Press    

年代:1994

数据来源: NRC