摘要:

A general intramolecular Diels–Alder anionic oxy-Cope strategy for the synthesis of tricyclic skeletons and its application to the preparation of the gascardic acid precursor21is described.Insitugeneration of the appropriate cyclopentadiene9by isomerization (Et3N) afforded the [4+2] adduct directly or with EtAlCl2as catalyst. The requisite potassium salt of the 1,5 diene16rearranged with concomitant epimerization at 67 °C to the fused ring ketone18. Selective allylic oxidation and reduction provided21.Keywords: Diels–Alder, oxy-Cope, sesterterpene, synthesis, cycloaddition.

ISSN:0008-4042    

DOI:10.1139/v91-011

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

An argument is presented which suggests that hydrolysis of the vinyl ether group of 1-methoxycyclooctene may occur by reversible proton transfer from a catalyzing acid to the β-carbon atom of the substrate, instead of by the conventional reaction mechanism in which this proton transfer is rate determining and not reversible. Hydrolysis of this substrate is then examined by measuring rates of reaction in dilute aqueous solutions of strong mineral acids (perchloric and hydrochloric) as well as in buffer solutions of seven carboxylic acids, biphosphate ion, and 1,1,1,3,3,3-hexafluoro-2-propanol. General acid catalysis is observed and a Brønsted relation with the exponent α = 0.73 is constructed. That, plus the isotope effectskH/kD = 2.9 and 6.0 for catalysis by hydronium ion and acetic acid respectively, as well as the lack of deuterium incorporation into the substrate when the reaction is carried out in D2O with D2PO4−/DPO42−buffer at pD = 8, show that carbon protonation of the substrate is not reversible and that the conventional reaction mechanism is operative.Keywords: 1-methoxycyclooctene, vinyl ether hydrolysis, rate-determining proton transfer, Brønsted relation, solvent isotope effect.

ISSN:0008-4042    

DOI:10.1139/v91-012

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Porous membrane phase separators are used to study the adsorption of the cation tetrahexylammonium (Q+), of the anion bromothymol blue (HB−) and of the ion-pair formed between them (QHB) at the liquid–liquid interface in a rapidly stirred mixture of chloroform and aqueous buffer. Adsorption isotherms in all three cases follow the Langmuir equation. The anion HB−is much more strongly adsorbed than the ion-pair QHB. The porous membrane technique readily permits measurement of simultaneous adsorption of the two species HB−and QHB, and thereby allows a study of their competitive adsorption. When QHB is adsorbed in the presence of an excess of HB−both the saturated (monolayer) interfacial concentration of QHB and the logarithm of the adsorption equilibrium constant for QHB decrease linearly with an increase in interfacial concentration of HB−. This shows quantitatively that coadsorption of QHB and HB−involves a direct competition for space at the interface and also that the presence of adsorbed HB−changes the adsorbent properties of the interface. Analytical implications for solvent extraction are discussed.Keywords: interfacial adsorption, ion-pairs, liquid–liquid interface.

ISSN:0008-4042    

DOI:10.1139/v91-013

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

In the presence of a catalytic quantity of alkali metal mixtures of sulphur diimides RNSNR and R′NSNR′(R, R′ = Ph, 4-C6H4Me, 4-C6H4OMe, SiMe3, SPh) undergo a rapid scrambling of the R and R′ groups. When R and R′ are significantly different (e.g., R = Ph, R′ = SiMe3or SPh) the equilibrium is shifted to favour the unsymmetrical sulphur dimmide (R ≠ R′); the procedure thus represents an effective method for preparing such derivatives. A mechanism involving the centrosymmetric association and rearrangement of two sulphur diimide radical anions is suggested for the observed ligand scrambling. The synthesis and X-ray structure determination of the mixed diimide (4-NO2C6H4)SNSN(4-C6H4OMe) is reported. The crystals are monoclinic, space groupP21/n; the molecules stack as plates along theaaxis in a head-to-head fashion, producing an interplanar spacing between consecutive S2N2units of 3.421 Å.Keywords: sulphur diimide radical anions, skeletal rearrangements, (4-C6H4NO2)SNSN(4-C6H4OMe), X-ray structure.

ISSN:0008-4042    

DOI:10.1139/v91-014

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The reaction of metastable He(23S) and Ne(3P0,2) atoms with methy halides has been investigated by flowing afterglow technique. Emission spectra have been observed and formation rate constants of some fragments have been measured. The rotational and vibrational temperatures of CH(A2Δ) were also estimated.Keywords: flowing afterglow, methyl halides, metastable atoms.

ISSN:0008-4042    

DOI:10.1139/v91-015

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

α-Guaiacoxyacetoveratrone, a lignin model compound, has been studied using a combination of laser flash photolysis and steady state quantum yields. Triplet lifetimes range from 175 to 600 ns while the quantum yields of photodegradation vary from 0.06 to 0.6, depending on the solvent. Singlet processes (Φ ~ 0.1), previously unobserved for aromatic ketones, are implicated in the mechanism, and account for most or all of the degradation in aprotic solvents.Keywords: photodegradation, α-guaiacoxyacetoveratrone.

ISSN:0008-4042    

DOI:10.1139/v91-016

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The η-cyclopentadienyl (Cp) and η-allyl (C3H5) complexes [RhCp(CO)2], [RhCp(cod)] where cod = 1,5-cyclooctadiene, [Rh(η-C3H5)(CO)2], and [Rh(η-C3H5)3] have been shown to be useful precursors for the chemical vapour deposition (CVD) of rhodium films. The rhodium films contain carbon impurities but these can be greatly reduced if CVD is carried out in the presence of hydrogen. The films adhere well to a silicon substrate. The pyrolysis of [RhCp(CO)2] gives CO and [Rh2Cp2(CO)2(μ-CO)] and [Rh3Cp3(μ-CO)3] at intermediate stages. Pyrolysis of [Rh(η-C3H5)3] or [Rh(η-C3H5)(CO)2] gives 1,5-hexadiene as the only organic product, but similar pyrolysis in the presence of hydrogen gives much propene as well as 1,5-hexadiene.Keywords: rhodium, deposition, allyl, cyclopentadienyl.

ISSN:0008-4042    

DOI:10.1139/v91-017

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The activity coefficients of NaCl were estimated by measuring the EMFs of the cellat four ionic strengths,i.e., 0.5, 1.0, 2.0, and 3.0 mol/kg and at temperatures 25, 35, and 45 °C. The results were analyzed in terms of Harned's rule, the Pitzer and Rush–Johnson–Scatchard treatments. Osmotic coefficients and excess free energies of mixing were calculated at all ionic strengths and temperatures studied.Keywords: activity coefficients, sodium chloride, sodium acetate, Pitzer formalism, Scatchard equation.

ISSN:0008-4042    

DOI:10.1139/v91-018

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The organic reactions that take place during the thermal coking of bitumen are extremely complex and deep-seated, but obviously hydrogen atoms originally attached to carbons that form coke will have moved to a new carbon site (net addition of hydrogen, resulting in upgraded bitumen). Accompanying this process, one expects some general hydrogen shuttling between non-coking molecules. This paper describes an approach to determing the extent of this latter process by the addition of a small amount of perdeuterated pyrene to the bitumen prior to the coking operation. Theinsitupyrene functions as in indicator, allowing one to establish the extent and other mechanistic details of the "hydrogen donor" reactions that take place during coking. There is no indication that the pyrene substantially modifies the normal course of the reaction, hence the analogy to an indicator.Keywords: bitumen coking, pyrene, hydrogen exchange, donor solvent, free radicals.

ISSN:0008-4042    

DOI:10.1139/v91-019

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The crystal and molecular structures of Sn(O—t-Bu)4• and [Sn(O—i-Pr)4•HO—i-Pr]2have been determined by single-crystal X-ray diffraction. Sn(O—t-Bu)4crystallizes in the monoclinic crystal system with space groupC2/c, wherea = 17.382(6) Å,b = 8.742(2) Å,c = 15.518(5) Å, β = 116.44(1)°,Z = 4, andR = 2.5%. Sn(O—t-Bu)4is monomeric in the solid state, with a distorted tetrahedral tin coordination environment. [Sn(O—i-Pr)4•HO—i-Pr]2crystallizes in the monoclinic crystal system with space groupP21/n, wherea = 11.808(3) Å,b = 14.356(3) Å,c = 12.380(2) Å, β = 95.27(2)°,Z = 2, andR = 4.9%. [Sn(O—i-Pr)4•HO—i-Pr]2exhibits an edge-shared, bi-octahedral structure in the solid state that is distorted due to the presence of asymmetric hydrogen bonding between axially coordinated alcohol ligands and an isopropoxide ligand.13C NMR and IR spectroscopic data have been recorded for Sn(O—t-Bu)4and Sn(O—t-Bu-d9)4to establish criteria for unambiguous identification of solution structures of tin(IV) alkoxides. It is demonstrated that the two-bondcoupling constant is larger for terminal alkoxide ligands than for μ2-alkoxide bridges, and the ν(Sn—O) stretching frequency has been assigned. The dynamic solution behaviour of [Sn(O—i-Pr)4•HO—i-Pr]2has been studied using variable temperature1H and13C NMR spectroscopy. The data obtained are consistent with a process that involves rapid reversible dissociation of isopropanol at room temperature. Upon cooling, the equilibrium concentration of the species with coordinated alcohol increases, and the molecule undergoes rapid intramolecular proton transfer (AG≠ < 11.9 kcalmol−1). Upon further cooling, the13C NMR data are consistent with a solution structure analogous to that found in the solid state.Keywords: tin, alkoxide, NMR, dynamic, structure.

ISSN:0008-4042    

DOI:10.1139/v91-020

出版商:NRC Research Press    

年代:1991

数据来源: NRC