摘要:

The hitherto elusive mono-O-protonated deltic acid C3O3H3+was prepared by protolysis of di-tert-butoxy deltate in FSO3H-SO2ClF and in FSO3H:SbF5(Magic Acid; 1:1 molar solution) in SO2ClF as solvent at -78°C and was characterized by1H and13C NMR spectroscopy. The structure and NMR chemical shifts were also calculated by the ab initio/IGLO method. No NMR evidence was found for persistent di-O-protonated deltic acid under these conditions, although a limited equilibrium with the mono-O-protonated species can be involved. Di-, tri-, and tetra-O-protonated deltic acids were also studied by ab initio/IGLO method.Key words: protonated deltic acid, aromaticity, superacids, NMR spectroscopy, ab initio and IGLO calculations

ISSN:0008-4042    

DOI:10.1139/v98-166

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

We have studied the equilibration shown in eq. [3] of 4,4prime-dimethoxytrityl alcohol in aqueous perchloric and nitric acids containing low proportions of acetonitrile using stopped-flow kinetics techniques. The rate constants for the overall progress to equilibrium,kobs, have been resolved into forward and reverse components using the equilibrium UV absorbance and a value for the molar absorptivity of the 4,4prime-dimethoxytrityl carbenium ion determined in concentrated aqueous perchloric acid. The forward reaction (rate constantkf) is first order in both the alcohol and the acid concentrations; the reverse reaction (rate constantkr) is pseudo first order with respect to the carbocation. At constant hydronium ion concentration, the forward rate constant increases linearly with the concentration of electrolyte, whereas the reverse rate constant decreases. These effects depend upon the nature of the anion, but not the cation, and are not ionic strength effects. At constant anion concentrations,kfin both acids, andkrin perchloric acid, are independent of hydronium ion concentration; however,krdecreases with increasing hydronium ion concentration at constant nitrate concentration. At nonconstant ionic strength, changes inkfandkrobserved in increasing concentrations of perchloric acid are attributable wholly to changes in perchlorate concentration. A mechanism is proposed which involves pre-equilibrium protonation of the alcohol, heterolysis of the protonated alcohol to give a 4,4prime-dimethoxytrityl carbenium ion - water ion-molecule pair, then conversion of this into a dissociated carbenium ion in equilibrium with ion pairs. To account for the strong effects of perchlorate and nitrate upon the forward rate constants, it is proposed that these anions provide additional reaction channels from the ion-molecule pair. However, we find no evidence of acid catalysis in the reaction of the ion-molecule pair (in contrast to our finding for the reaction of the corresponding ion-molecule pair formed from dimethoxytritylamine in acidic media). Some of the elementary rate and equilibrium constants of the proposed mechanism have been evaluated.Key words: trityl, carbenium ion, stopped-flow, ion pair, ion-molecule pair.

ISSN:0008-4042    

DOI:10.1139/v99-003

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The reactions of the singlet methylene (1a) and dimethylcarbene (1b), with their diazirine precursors, diazirine (2a), and dimethyldiazirine (2b), have been studied theoretically using ab initio and density functional theory. The reaction has no activation barriers for the parent system (1a+2a) and proceeds via a reactive complex and a transition state with a small negative enthalpy of activation &Dgr;Hnot =298= -1.1 kcal mol-1, &Dgr;Snot =298= -34.4 cal mol-1K-1, &Dgr;G°298= 9.2 kcal mol-1) for the dimethyl derivatives (1b+2b). The formation ofN-methylene diazirinium ylides (3a,b) is exothermic by 64-80 kcal mol-1. The isomer, 1,3-diazabicyclo[1.1.0]butane (4a), is more stable (5-12 kcal mol-1) than isomer3a, but can neither be formed by direct thermal reaction of1awith2anor undergo the direct rearrangement into formaldazine (5a). The rearrangement of ylides3a,binto azines5a,bproceeds by conrotatory C3-N1ring opening. The predicted activation barrier of ca. 15 kcal mol-1for the ring opening in ylide3bis in excellent agreement with experimental data. The formation of pyridinium ylides from carbenes and pyridine is also studied.Key words: diazirinium ylide, ab initio MO (molecular orbital) theory, density functional theory, pyridinium ylide, CIS (singles configuration interaction) transition energies

ISSN:0008-4042    

DOI:10.1139/v99-012

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

TheS-nitrosothiols 2-acetamido-2-deoxy-S-nitroso-1-thio-&bgr;-D-glucopyranose 3,4,6-triacetate (GPSNO) andS-nitroso-N-carbamyl-D,L-penicillamine (SNCP) were synthesized byS-nitrosation of the corresponding thiols, isolated, and fully characterized. The nitrosothiol (TGSNO) from 1-thioglycerol was obtained as a red gelatinous liquid, which decomposed rapidly at room temperature and so was not characterized. The kinetics of decomposition of GPSNO showed that there is a surprisingly large thermal pathway overlaid with a Cu2+/RS-catalyzed reaction. The results strongly suggest that the product disulfide complexes Cu2+(for which there is some spectral evidence), leading to incomplete conversion by that route. Ascorbate also acts as a Cu2+reductant. AnotherS-nitroso sugar,S-nitroso-1-thio-&bgr;-D-glucose (SNTG), behaved very similarly from solutions generated and used in situ. The decomposition of TGSNO shows induction periods suggesting that slow initial generation of Cu+(the true catalyst) is taking place. There appears to be also a significant alternative pathway (analogous to that found for GPSNO), where the rate appears to be independent of [Cu2+], but very unusually this pathway is effectively halted by addition of EDTA either at the start of the reaction or at a later time. Reaction schemes are put forward to account for these unusual reaction characteristics.Key words:S-nitrosothiols, nitric oxide, ascorbate, copper catalysis.

ISSN:0008-4042    

DOI:10.1139/v99-026

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

An equal mixture of monodeuterated isotopomers of maleylacetone, viz., 4-hydroxy-6-oxo-2,4-heptadienoic-2-d1acid (1) and 4-hydroxy-6-oxo-2,4-heptadienoic-3-d1acid (2) was synthesized and subjected to catalyzedcis-transisomerization by the maleylacetoacetatecis-transisomerase-glutathione system. These studies were designed to probe the detailed mechanism of enzyme-catalyzedcis-transisomerization about the C-2—C-3 bond of the substrate. In the competitive reaction,2accumulates with respect to1at high extent of reaction.1H NMR analysis indicates that1reacts 15% faster than2in line with a nucleophilic addition mechanism at C-2 rather than a conjugate addition mechanism at C-2 and C-3, to convert the C-2—C-3 double bond to a single bond to allow facile internal rotation.Key words: enzyme-catalyzedcis-transisomerization, secondary &agr;-deuterium isotope effect, maleylacetoacetatecis-transisomerase,cis-transisomerization mechanis

ISSN:0008-4042    

DOI:10.1139/v98-185

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Reactions of methyl(vinyl)iodonium ion and the &bgr;-substituted derivatives as well as divinyliodonium ion with chloride ion were examined theoretically by ab initio MO (MP2) calculations at the double-zeta (DZ) + d level. Interaction of the iodonium ion1with Cl-leads to chloro-&lgr;3-iodane2. Transition states for the SN2, ligand-coupling substitution (LC), and &bgr;-elimination (&bgr;E) are found for reactions at the vinyl group. The barrier to LC is usually the lowest in the gas phase, but relative barriers to SN2 and to &bgr;E change with the substituents. Effects of solvent were evaluated by a dielectric continuum model and found to be large on SN2 but small on LC. The experimental observations in solution that the SN2 is the most facile for the &bgr;-methyl-substituted1while it is retarded by the &bgr;-tert-butyl and &bgr;-chloro substitutions are reproduced by the calculations that take medium effects into account.Key words: vinyliodonium salt, polyvalent iodine, vinylic SN2 reaction, ligand coupling, ab initio MO calculation.

ISSN:0008-4042    

DOI:10.1139/v99-024

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The reaction of thiomethoxybenzylidene Meldrum's acid (5-SMe) with thiolate and alkoxide ion nucleophiles is shown to proceed by the two-step addition-elimination SNV mechanism in which the tetrahedral intermediate accumulates to detectable levels. For the reactions with thiolate ions, rate constants for nucleophilic addition (k1RX), its reverse (k-1RX), and for conversion of the intermediate to products (k2RX) were determined. For the reactions with alkoxide ions, onlyk1RXandk-1RXcould be obtained; the intermediate in these reactions did not yield the expected substitution products, and hence nok2RXvalues could be determined. The reactions with OH-and water are believed to follow the same mechanism, but the respective intermediates remain at steady-state levels, and onlyk1OHandk1H²Ofor nucleophilic attack on5-SMewere measurable. New insights regarding structure-reactivity behavior in SNV reactions are gained from comparisons of rate and equilibrium constants in the reactions of5-SMewith the corresponding parameters in the reactions of methoxybenzylidene Meldrum's acid (5-OMe) and benzylidene Meldrum's acid (5-H). In particular, the relative importance of steric and pi-donor effects of the MeS vs. MeO group in5-SMeand5-OMe, respectively, and their role in affecting the intrinsic rate constants for nucleophilic addition, has been clarified by these comparisons. Our results also add support to a previous suggestion that soft-soft type interactions tend to increase intrinsic rate constants for thiolate ion addition to vinylic substrates, especially5-SMewith the soft MeS group.Key words: nucleophilic vinylic substitution, intrinsic rate constants, transition state imbalances, steric/pi-donor/anomeric effects

ISSN:0008-4042    

DOI:10.1139/v99-009

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions,2–>1and3–>1, were measured. The rates of the reverse reactions,1–>2and1–>3, were determined from the rates of acid-catalyzed hydron exchange at theortho-andpara-positions of1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2tand -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of2and3through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, &tgr;(2) = 260 &mgr;s and &tgr;(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profile

ISSN:0008-4042    

DOI:10.1139/v99-048

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Protonation of ferrocene has been suggested to take place on carbon (exo-protonation) or iron (endo-protonation). However, experiments have not been conclusive because of interfering exchange reactions. Now low-temperature protonation of ferrocene and [2H10]-ferrocene in superacid and direct observation of the carbocation by1H NMR at low temperature shows only primary protonation and that it exclusively takes place in anendo-fashion. Studies by DFT calculations using B3LYP hybrid functional indicate the presence of an intramolecular nonlinear C—H—Fe bond and that the proton might be delocalized between carbon and iron. Potential energy barriers for degenerate rearrangements of the hydride bridged carbocation are low, suggesting that the proton might be delocalized between all 10 carbons and iron. The NMR results are consistent with such an interpretation.Key words: regioselective, protonation, superacid, ferrocenonium io

ISSN:0008-4042    

DOI:10.1139/v99-053

出版商:NRC Research Press    

年代:1999

数据来源: NRC

摘要:

Equilibrium constants for the ionization of a variety of phenyl-substituted 2-tetralones (pKaK), for the ionization of their enols (pKaE), and for keto-enol tautomerization (pKE) were determined. Hammett plots of pKaKand pKaEvs. &sgr;-are linear with slopes (-&rgr;) of -1.66 ± 0.06 and -0.90 ± 0.03, respectively, except for deviations of the points corresponding to 6-nitro-2-tetralone (1b) and its enol. We have previously attributed the negative deviation of1bfrom the correlation for the acidities of the ketones obtained with the more limited set of data to the lack of a free electron pair on C-1 of the free tetralone (Nevy et al.). The negative deviation of the point for1bfrom the correlation for the acidities of the enols suggests that charge transfer from the hydroxyl group of the enol to the nitro group is less important than it is for phenols. This study represents the first systematic study of electronic effects on equilibria among ketone, enol, and enolate in aqueous solution.Key words: enol, acidity, equilibrium, substituent, conjugation.

ISSN:0008-4042    

DOI:10.1139/v99-006

出版商:NRC Research Press    

年代:1999

数据来源: NRC