摘要:

AbstractIn the reactions of 2,5‐di‐tert‐butyl‐1,4‐dimethoxybezene (1) with different oxidants, the radical cation 1+.is always detectable by EPR. However, the observed reactivity of 1+.depends greatly on the oxidation systems employed. In S2O 82−Cu2+HOAc and Ce4+HOAc systems (HOAc = acetic acid), 1+.appears to have long lifetimes and does not undergo fragmentation spontaneously. In contrast, in (RFCO2)2CF2CICFCI2(F113) systems, the readily formed 1+.is short‐lived, and large amounts of de‐tert‐butylation products have been isolated. Experimental results imply that the CC bond cleavage involved in de‐tert‐butylation could be a consequence of an attack by perfluoroacyloxy radical on 1+.in their original solvent cage. The fact that addition of methanol to the reaction mixture leads to the formation of a large amount oftBuOCH3(46%) and other evidence suggest that thetert‐butyl group leaves as a carbocation. On the basis of these results, we conclude that the reactions of 1 with (RFCO2)2are initiated by electron transfer and followed by a fast coupling of various radical species, namely, 1+.with RFCO2or with RFin the solvent cage, to form σ‐complexes which collapse or react with nu

ISSN:0894-3230    

DOI:10.1002/poc.610040102

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractX‐ray data on the crystal and molecular structure of 1,3‐dimethyl‐6,7‐ dimethoxy‐2‐benzopyrylium perchlorate show a good correlation with the geometry of the same cation calculated by the MINDO/3 and MNDO methods with total optimization of all geometrical parameters. The geometry of substituted pyrylium cations was determined by the MNDO and AM1 methods. On the basis of the data obtained, some effects of benzo [c] annelation are displayed for the 2‐benzopyrylium cation in comparison with the py

ISSN:0894-3230    

DOI:10.1002/poc.610040103

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe alkaline hydrolysis of ninepara‐substituted phenyl esters ofpara‐substituted benzoates was studied in the presence and absence of hexadecyltrimethylammonium bromide (CTAB) micelles. The second‐order rate constant for alkaline hydrolysis inCTABmicelles (K2m) was calculated using a pseudo‐phase ion‐exchange model. the substituent effect on the second‐order rate constant in the aqueous phase (k2w) and onk2mwas analysed using Hammett's σ ρ plots. The calculated ρ values in micelles were 0.8 unit. larger than those in water. The effective low dielectric constant in the micellar surface may be responsible for the general effect of increasing the sensitivity of reactions to the electronic effect of substituen

ISSN:0894-3230    

DOI:10.1002/poc.610040104

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe rate of intramolecular degradation of cephaclor, involving the attack of the amino group at C‐17 (N‐18) on the β‐lactam carbonyl, was increased up to 50‐fold by neutral (polyoxyethylene‐23 lauryl ether; Brij) and zwitterionic [3‐(N‐dodecyl‐N,N‐dimethylammonium) propane 1‐sulphonate; (SDP)] micelles. The rate ofOH−attack on cephaclor was increased 2–3‐fold by Brij andSDPmicelles. In the absence of micelles the rate of intramolecular degradation of cephaclor increased by up to 2‐fold by addition of organic solvents. Distance calculations, based on the crystal structure of the antibiotic, showed that the intramolecular degration can only proceed in a conformation involving acis‐amide bond (N‐14—C‐15). Micellar catalysis of the intramolecular degradation process was proposed to be due to the stabil

ISSN:0894-3230    

DOI:10.1002/poc.610040105

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractInteractions of theRauwolfiaalkaloids yohimbine, corynanthine, ajmalicine and reserpine with tetracyanoethylene were investigated by UV—visible spectroscopy. The results suggest the instantaneous formation of blue complexes whose thermodynamic and spectroscopic properties closely resemble those of 1 : 1 charge‐transfer complexes of tetracyanoethylene with indole derivatives. The complexes are stable when the piperidinic nitrogen atom of the alkaloids is protonated or methylated. In contrast, those of the free bases rapidly decompose to give the corresponding 3,4‐dehydro derivatives. The kinetics of these reactions have been studied and a mechanism is pro

ISSN:0894-3230    

DOI:10.1002/poc.610040106

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe intrinsic mechanism of circumambulatory rearrangements of 5‐bromo‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene, 5‐bromo‐1,2,3,4 5‐pentamethoxycarbonylcyclopentadien and 5‐bromo‐ 1,2,3,4,5‐pentaphenylcyclopentadiene due to sigmatropic shifts of bromine over the cyclopentadiene ring was proved, using the dynamic13C and1HNMRtechnique, to be governed by successive intramolecular 1,5‐sigmatropic shifts. Semi‐empiricalAM1andMINDO/3calculations of reactions paths performed for fluoro‐, chloro‐ and bromocyclopentadienes are in accord with the conclusion of a preference for a 1,5‐ over a 1,3‐shift reaction path of halogen migration over a cyclopentadiene system. Intramolecular 1,5‐sigmatropic shifts of chlorine in 5‐chloro‐5‐methyl‐1,2,3,4‐tetramethoxycarbonylcyclopentadiene with the free energy barrier of ΔG 298+= 26

ISSN:0894-3230    

DOI:10.1002/poc.610040107

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractSolvophobic and polarity parameters are used to explain theendo/exoselectivity of the reaction between cyclopentadiene and methyl acrylate. A good linear regression model withSpandE TNis obtained. The existence of an intrinsic correlation between theSpandE TNvalues, which makes the interpretation of the results difficult, is shown. A comparison of the results with other previously reported showed that the relative influence ofSpandET Non selectivity depends on the nature of the rea

ISSN:0894-3230    

DOI:10.1002/poc.610040108

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe hydrolytic decomposition of 2‐phenyl‐ and 2‐isopropyl‐3‐methyltetrahydro‐1,3‐oxazines to 3‐methylaminopropan‐1‐ol and appropriate aldehydes was studied by1H NMR and UV spectroscopy in acidic solutions. The time‐dependent spectra confirmed that the formation of the final products was preceded by an equilibration of three components, the two Schiff base intermediates and the starting material, of which the former produced a carbinolamine which in turn underwent heterolysis

ISSN:0894-3230    

DOI:10.1002/poc.610040109

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractThe concentration dependence of the1H and13C NMR chemical shifts in D2O and in CDCl3solutions were determined for (±)‐α‐phenylethylamine (I). Aqueous solutions of (−)‐Iand (+)‐I, 50% enantiomerically enriched in (−)‐ and (±)‐2, 2, 2,‐trifluoro‐1‐phenylethanol and the (+)‐ and (−)‐N‐formyl derivatives ofI, were also studied.1H nuclear Overhauser enhancements were used to check the conformations of the solutes at various concentrations and1HT1values were used to monitor the changes in molecular tumbling in solutions. The results are interpreted in terms of a spontaneous aggregation of solute molecules in water, with the possible determination of the critical micelle concentration. The time‐dependent splittings in the1H NMR spectra suggest further, more detailed, studies of the structures of the aggregates and the possibil

ISSN:0894-3230    

DOI:10.1002/poc.610040110

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY

摘要:

AbstractAqueous‐phase equilibrium acidity constants have been determined for urazole (1,2,4‐triazolidine‐3,5‐dione), several substituted urazoles and other selected imides. Aqueous phase pKavalues for urazole (5.8), 1‐methylurazole (5.3), 4‐methylurazole (5.7) and 1,4‐dimethylurazole (5.3) indicate that (in water) the methyl group acidifies the urazole moiety when bonded to N‐1 but has little effect on acidity when bonded to N‐4. Aqueous phase pKavalues for hydantoin (9.0) and 1‐methylhydantoin (9.1) suggest that a 1‐methyl substituent has little effect on the acidity of the imide proton present in hydantoin. These data, combined with aqueous phase pKavalues for 1,2‐dimethylurazole (7.5), a substituted urazole lacking amide protons, indicate that an amide proton in urazole (i.e. the proton bound to either N‐1 or N‐2 in urazole) isca1.5 pKaunits more acidic than the imide proton (i.e. the proton bound to N‐4). Changes in13C NMR chemical shifts for the carbonyl carbon atoms present in variously substituted D2O phase urazoles, hydantoins, succinimide and their conjugate bases also confirm that amide NH bonds found in urazole and 1‐methylurazole are weaker, in a heterolytic acid

ISSN:0894-3230    

DOI:10.1002/poc.610040111

出版商:John Wiley&Sons Ltd.    

年代:1991

数据来源: WILEY