摘要:

An improved route for the synthesis of cyclodextrin (CD)-based enzyme models having catalytic groups on the secondary face is presented. An epoxide derived from heptakis(6-O-tert-butyldimethylsilyl)-β-CD was prepared via intermediates that can be purified by conventional flash chromatography on apreparativescale. Reaction of β-cyclodextrin with;tert-butyldimethylsilyl chloride in pyridine gave heptakis(6-O-tert-butyldimethylsilyl)-β-CD (1). Treatment of1withN-tosylimidazole and NaOMe in chloroform gave mono(2-O-tosyl) heptakis(6-O-tert-butyldimethylsilyl)-β-CD (2) in 22% yield. The latter was converted smoothly to mono(2A, 3A-anhydro) heptakis(6-O-tert-butyldimethylsilyl)-β-CD (3), in which one glucose subunit has been converted to a manno-epoxide, by treatment with KOEt in refluxing ethanol (87% yield). Compounds1,2, and3were characterized by a variety of one- and two-dimensional NMR techniques. These results open the door to attachment of catalytic groups to the cyclodextrin in a well-defined manner. Nucleophilic attack on the epoxide, followed by removal of the silyl groups, can be used to prepare a wide variety of enzyme model systems.Keywords: functionalized cyclodextrins, silylation, 2D NMR, enzyme models.

ISSN:0008-4042    

DOI:10.1139/v91-021

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Phenylcyclopropylketene (4),tert-butylcyclopropylketene (5), and dicyclopropylketene (6) were formed by dehydrochlorination of the corresponding acyl chlorides by Et3N in THF, and are the first cyclopropylketenes to be isolated and purified. Reaction of4withn-BuLi and capture of the intermediate enolates with Me3SiCl gave the stereoisomeric silyl enol ethersc-PrCPh=C(OSiMe3)-n-Bu with a 79:21 preference for formation of theZisomer resulting from nucleophilic attacksynto cyclopropyl, whereas the corresponding reaction oft-BuLi gave a 9:91 preference for attackantito cyclopropyl. Some isopropyl-, cyclopentyl-, and cyclohexylketenes gave comparable results. Analyses of the relative sizes of the ketene substituents in the ground state by steric parameters, and of the product stabilities by molecular mechanics, both fail to predict the observed similarities in the results with differentsecondaryalkyl groups. The hydration reactivities of4and6show that, in neutral H2O/CH3CN,c-PrCPh=C=O is more reactive thani-PrCPh=C=O, a result ascribed as mainly due to the smaller size of cyclopropyl.c-Pr2C=C=O has the same reactivity in neutral water as Et2C=C=O, but is 22 times less reactive with acid, a result attributed to the inability of the β-cyclopropyl groups to directly stabilize the cationic transition state for protonation.Keywords: cyclopropylketenes, ketenes, nucleophilic addition, hydration kinetics.

ISSN:0008-4042    

DOI:10.1139/v91-022

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The photolyses of electron donor–acceptor (EDA) complexes formed between bothcis-stilbene (cS) andtrans-stilbene (tS) as donors and mefhylviologen (MV+2) as acceptor have been carried out in homogeneous ethanolic and in micellar sodium dodecyl sulfate (SDS) solutions, in the presence of oxygen. The stilbene loss quantum yields (Φ−S) have been determined in both media. Quantum yields were observed to be dependent upon methylviologen concentration and the acidity of media. A reaction scheme is proposed, which accounts for experimental results and can be related to the photochemistry of MV+2and its cation radical MV•+.Keywords: EDA complexes, micelles, methyl viologen, stilbene.

ISSN:0008-4042    

DOI:10.1139/v91-023

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

We have investigated polarographically the dependence of the dioxygen yield on the concentration of 2,4-dinitrophenol in photooxidation of water in an octane/water system containing hydrated oligomer of chlorophyll, a water-soluble electron acceptor ferricyanide and a buffer. A linear correlation between the rate of dioxygen evolution and the photopotential at the interface between the two liquid phases is observed. Investigation of the dependences of photopotential and rate of dioxygen evolution on the dielectric permittivity of the nonaqueous phase has shown that with increasing dielectric permittivity, one can observe a decrease in both the reaction rate and the potential difference. The lifetime of chlorophyll in the course of water photooxidation reaction has been measured.Keywords: chlorophyll, oxygen, water photooxidation, oil/water interface, artificial photosynthesis.

ISSN:0008-4042    

DOI:10.1139/v91-024

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

In models of the kinetics of chemical reactions in solution the solvent is commonly assumed to be a uniform continuum. An example is the Smoluchowski–Debye–Stokes–Einstein equation for the rate constantk2of a bimolecular reaction between charged or polar species:where κ is the probability that a reactant encounter pair will react,Ris the gas constant,Tis the temperature,fis a factor that reflects the effect of electrostatic interaction between the reactants on their probability of attaining the closeness of approachrrat which reaction occurs, η is the solvent viscosity, andrdis the effective radius of the reactant entities for mutual diffusion. The equation is useful in evaluating effects of bulk fluid properties on reaction rates. Residual effects are attributed to more specific solvent behaviour.Rate constantsk2, activation energiesE2, and pre-exponential factorsA2of reactions of solvated electronswithandions vary with the composition of 1-propanol/water and 2-propanol/water mixed solvents. Plots ofk2η/fTagainst solvent composition are nonlinear and change with the solvent pair and with reactant pair. Measured molar conductivitiesandindicate that the values ofrdfor the mutual diffusion of the cations and anions have a minimum near 90 mol% water, and that the values in pure propanol-1 or −2 (150–190 pm) are larger than those in pure water solvent (26 pm for70 pm for the metal ions). The liquid structure influences both the rate of diffusion and the probability of reaction of a reactant encounter pair.Keywords: alcohol/water mixed solvents, positive ions, reaction kinetics, solvated electron, solvent effects.

ISSN:0008-4042    

DOI:10.1139/v91-025

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The reaction of Cp2TiPh2with 2,2-dimethylpropane-1,3-diol proceeds with the evolution of benzene affording the bimetallic titanocene alkoxide species Cp2Ti(Ph)(μ-OCH2C(CH3)2CH2O)(Ph)TiCp2,1. Complex1crystallizes in the monclinic space groupCcwitha = 25.448(7) Å,b = 8.483(s) Å,c = 14.512(3) Å, β = 101.52(2)°,Z = 4, andV = 3070(1) Å3. The structure was refined employing 1471 data withFo > 3σFoand 138 variables toRvalues ofR = 0.0588 andRw = 0.0828. the dimeric nature of1is confirmed. The coordination spheres of the two Ti centres are pseudo-tetrahedral. The Ti—C σ bonds were found to be 2.21(1) and 2.25(1) Å. Although the phenyl substituents are directed towards each other, steric effects appear to be overcome by the approximately perpendicular orientation of the phenyl rings (in fact the angle between the planes is 81.9°). The Ti … Ti separation in1is 6.725 Å. The implications of the preparation of1regarding the mechanism of formation of early metal macrocyclic compounds are discussed. In addition, the ramifications for the synthesis of dissymmetric early metal macrocycles are considered.Keywords: titanium alkoxides, alkoxide bridged, structure.

ISSN:0008-4042    

DOI:10.1139/v91-026

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Reaction of substituted phenoxides with TiCl4affords the species Ti(OAr′)4(Ar′ = C6H4(2-t-Bu),1; Ar′ = C6H(2,3,5,6-Me)4,2). The compound Ti(OC6H4(2-t-Bu))4,1, crystallizes in the tetragonal space group, witha = 15.203(4) Å,c = 8.026(3) Å,Z = 2, andV = 1855(2) Å3. The compound Ti(OC6H(2,3,5,6-Me)4)4,2, crystallizes in the orthorhombic space groupPbcn, witha = 16.539(7) Å,b = 16.136(6) Å,c = 27.716(12) Å,Z = 8, andV = 7397(9) Å3. The geometry of the Ti coordination sphere in these complexes is best described as pseudo-tetrahedral. In the case of1strict crystallographicsymmetry is imposed. The complex2exhibits reversible cyclic voltammetric behaviour consistent with a one electron reduction to the Ti(III) analogue. Chemical reduction of2employing sodium amalgam affords the quantitative formation of (C6H(2,3,5,6-Me)4O)2Ti(μ-OC6H(2,3,5,6-Me)4)2Na(THF)2,3. The reaction of3with [(COD)Rh(μ-Cl)]2does not afford the Ti(III)/Rh(I) early–late heterobimetallic (ELHB) complex (C6H(2,3,5,6-Me)4O)2Ti(μ-OC6H(2,3,5,6-Me)4)2Rh(COD). The nature of all products is not known; however, redox chemistry, in which electron transfer from Ti(III) to Rh(I) occurs is evidenced by the generation of2and Rh(0). In addition, ligand transfer reactions giving uncharacterized Rh-alkoxides are suggested by the spectral data. The implications and ramifications for the synthesis of alkoxide bridged ELHB models of bimetallic heterogeneous catalyst systems are discussed.Keywords: titanium phenoxides, redox chemistry, structures.

ISSN:0008-4042    

DOI:10.1139/v91-027

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The plane-nonsymmetrical diene spiro(bicyclo[2.2.1]heptane-2,1′-[2,4]cyclopentadiene) (2) was synthesized, and the π-facial stereoselectivity of its Diels–Alder cycloadditions with three cyclic dienophiles was examined. Adduct ratios were very similar, and the selectivity is surmised to be mainly the result of steric interactions at the transition state.Keywords: facial stereoselectivity, Diels–Alder, cycloaddition.

ISSN:0008-4042    

DOI:10.1139/v91-028

出版商:NRC Research Press    

年代:1991

数据来源: NRC