Carboxonium Ions Research Articles

Comparison of the effect of methyl and phenyl substituents on the stabilities of some carboxonium ions. Stability decreases associated with increasing charge delocalization

Comparison of the effect of methyl and phenyl substituents on the stabilities of some carboxonium ions. Stability decreases associated with increasing charge delocalization

Acid catalysed hydrolysis of orthoesters: mechanism of hydrolysis of 2,4,10-trioxaadamantanes substituted in position 3

The rates of hydrolysis of orthoesters, 2,4,10-trioxaadamantanes, substituted in position 3 (R = H, CH3, C2H5, C6H5) have been determined at 25 and 40 °C in dioxane/water (60:40 by volume). Substitution of hydrogen by the groups CH3, C2H5, and C6H5 shows clearly a decrease in rate: 0.018 (CH3), 0.014 (C2H5), and 0.003 (C6H5). This decrease, in accord with substituent effects, plus the strongly negative entropies of activation observed, suggests that the rate-determining step is the addition of water to the carboxonium ion intermediate.

Kinetic α-deuterium isotope effects for enzymatic and nonenzymatic hydrolysis of nicotinamide-β-riboside

The kinetic α-secondary deuterium isotope effect, k H k D , for the pH-independent hydrolysis of nicotinamide riboside, yielding nicotinamide and ribose, in water at 25 ° is 1.14, establishing that this reaction proceeds with unimolecular substrate decomposition to yield a carboxonium ion, or related species, in the rate-determining step. Surprisingly, the corresponding isotope effect for the base-catalyzed decomposition of the same substrate is 1.12, a value indicating considerable sp 2 character at the Cl′ position in the transition state for this reaction. A similar result, k H k D = 1.15 , was obtained for base-catalyzed hydrolysis of NAD +. The kinetic alpha deuterium isotope effect for the pig brain NAD glycohydrolasecatalyzed hydrolysis of nicotinamide riboside is 1.08. This value suggests that CN bond cleavage to form an intermediate carboxonium ion, or structurally related species, is at least partially rate-determining. In contrast, the corresponding value for the hydrolysis of this substrate catalyzed by Escherichia coli nicotinamide ribonucleotide glycohydrolase is very near unity, a result consistent with several interpretations including a rate-determining enzyme isomerization reaction.

Ortho ester hydrolysis: rate-determining addition of water to a carboxonium ion intermediate

Hydrolysis of 2,4,10-trioxa-adamantane (I) and 3-methyl-2,4,10-trioxa-adamantane (II) suggests that the rate-determining step is the addition of wate to a carboxonium ion intermediate.

Hydride ion transfer between triphenylmethyl cation and tetrahydrofuran, 1,3-dioxolan, and 1,3-dioxepan

A polarographic study of the rates of hydride ion transfer from tetrahydrofuran, 1,3-dioxolan, and 1,3-dioxepan to the triphenylmethyl cation has provided data for comparing the effects of ring structure and size and of polarity of solvents on this reaction. The results are compared with those obtained by other authors with the same and with related compounds by spectroscopic methods. The reaction is insensitive to ionic strength and only slightly affected by polarity of solvent. The cyclic carboxonium ions which are the other reaction products, are too stable to be detected polarographically.

Über den thermischen abbau des poly(ε‐caprolacton)s

AbstractThe thermal and electron impact induced degradation reactions of poly(ε‐caprolactone) {poly[oxy(1‐oxohexamethylene)]} were investigated in a mass spectrometer. It can be shown that the favorated thermal degradation reaction is the cleavage of the ester bond and formation of ω‐hydroxyl and ketene endgroups (Eq. (iii)) and with a lower intensity the cleavage of the OCH2‐bond and formation of carboxyl‐and pentenyl endgroups (Eq. (vii)). After electron impact and fragmentation of the pyrolysis products to carboxonium ions an elimination of caprolactone via a “zip”‐mechanism can be observed (Eq. (v)).

Transformations of carboxonium compounds in carbohydrate and polyol chemistry

Abstract

Complexes of methylal with lewis acids

The results of an investigation of the interaction of the Lewis acids SbCl5 and SnCl4 with methylal in CH2C12 and CHC13 by the NMR method give evidence that the Lewis acids are present chiefly in the form of molecular complexes. Only in the further interaction of SbCl5 with CH3OCH2C1 in nitromethane can a signal corresponding to the formation of carboxonium ions be observed.

Die enzymatische umwandlung von 2-desoxy- D- lyxo-hexose (2-desoxy- D-galaktose) in 3,6-anhydro-2-desoxy- D- lyxo-hexose ( D-isogalaktal)

Incubation of 2-deoxy- D- lyxo-hexose with β-galactosidase, gave 2 in small (≈2%) yield a compound which was shown to be 3,6-anhydro-2-deoxy- D- lyxo-hexose (“ D-isogalactal” 2), identified by comparison of the reduced and acetylated product with an authentic sample of 2,3,6-tri- O-acetyl-1,4-anhydro-5-deoxy- D- arabino-hexitol. Enzymic synthesis of 2 is likely to start from the 2-deoxy- D- lyxo-hexopyranosyl cation ( 10) which undergoes β-elimination to give 2,3-dideoxy- D- threo-hex-2-enose ( 12, “ D-pseudogalactal”). Spontaneous intramolecular addition then yields 2. This pathway would support the hypothesis of a generally occurring formation of a free cyclic carboxonium ion in the action of β-galactosidase on sterically suitable aldohexopyranoses and aldohexopyranosides.

The preparation and properties of dicarbonyl-h5-cyclopentadienyliron h2-vinyl ether and h2-ketene acetal complexes

Abstract Metallation of >-haloacetals wth sodium dicarbonyl- h 5 -cyclopentadienyl- ferrate (Fp - ) provides a convenient point of departure for the synthesis of alhyde -iron complexes (FpCHRCHO) and of h 2 -vinyl alcohol and vinyl ether cations [Fp(CH 2 =CHOR)] + . These latter complexes are shown to be best described as distorted dihapto cation. Treatment of FpCOCH 2 OMe with strong acid leads to the ketene hemiacetal cation [Fp(CH 2 =C(OH)OMe)] + rather than to the expected ketene complex. This substance, as well as the acetal cation [Fp(CH 2 =C(OMe)OEt)] + prepared by alkylation of FpCH 2 COOMe, may possess the structure of an h 1 -metal complex incorporating a carboxonium ion. A correlation is shown to exist between the chemical shift of cyclopentadienyl protons and the average infrared carbonyl stretching frequency in a variety of Fp(olefin) + FpR complexes.

Polymerization of Bicyclic Acetals. II. Steric Course in the Cationic Polymerization of 6,8-Dioxabicyclo[3.2.1]octane

The polymerization of 6,8-dioxabicyclo[3.2.1]octane (DBO) was carried out in toluene, methylene chloride, and 1-nitropropane over the wide range of temperatures from −78 to 30°C. Boron trifluoride etherate was used as an initiator. The stereoregularity of the polymer obtained was evaluated from the relative intensities of the two NMR peaks appearing at τ 5.15 and 5.60, which were due to the equatorial and axial acetal protons of the tetrahydropyran ring, respectively. The EQ—H% of the polymer, defined as the percent of the equatorial acetal protons to the total acetal protons, decreases from nearly 100 to 70–80 depending on the solvents used with the rise in the polymerization temperature. On the basis of these results, it is concluded that the cationic polymerization of DBO proceeds at lower temperatures through an SN2 mechanism involving the reverse side attachment of the monomer to the acetal carbon atom of a propagating cyclic trialkyloxonium ion to afford a stereoregular polymer. At higher temperatures and in polar solvents, the participation of a car-boxonium ion derived from the cyclic trialkyloxonium ion in the propagation reaction and/or the competitive oxonium ion exchange reaction at the propagating chain end becomes significant, thus causing the loss of stereospecificity of the polymerization.

The type of reaction centres for polymerization of cyclic acetals

The conclusion that carboxonium ions are the reactive centres for polymerization of cyclic acetals hasbeen reached on the basis of our own and literature data. A modified mechanism of the chain propagation reaction, S N 1 is suggested and is based on a synchronous carboxonium ion formation as the main characteristic of the transition state.

Investigation of unimolecular decomposition and isomerisation reactions of some metastable carboxonium ions in the gas phase

The C3H5O+ cations generated from CH2CH·CH(OH)CH3, CH2CH·O·CH2·CH3, [graphic omitted] and CH3·[graphic omitted] have been shown to isomerise to the same structure or mixture of structures before fragmentation with a rate constant in the region 105–106 sec.–1. However, the C3H5O+ ion generated from CH3·CH2·CO2Me remains structurally distinct. Protonated carboxylic acid ions generated from the n-butyl esters of acrylic, crotonic, and propionic acids have been shown to remain distinct from isomeric ions before dehydration to the corresponding acylium ions in a metastable transition.

Acid-catalysed [2H6]ethanolyses of alkyl orthoacetates: evidence for keten dialkylacetals as intermediates

N.m.r. analyses of 2,6-dichlorobenzoic acid-catalysed [2H6]ethanolyses of 1,1,1-triethoxyethane, 1,1,1-trimethoxyethane, and 2-ethoxy-2-methyl-1,3-dioxolan indicate that in each case the orthoacetyl methyl protons are subject to deuterium exchange. This exchange, however, proceeds much more slowly than the accompanying alkoxy-exchange. For 1,1,1-triethoxyethane, deuterium incorporation (47 atoms % at equilibrium) was confirmed by hydrolysis and mass-spectral analysis of the resultant ethyl acetate. A reaction mechanism in which a keten dialkylacetal is formed by elimination of a proton from an intermediate carboxonium ion is suggested to account for the observations.

Stable carbonium ions. LVIII. Carbon-13 resonance investigation of protonated carboxylic acids (carboxonium ions) and oxocarbonium ions (acyl cations)

Stable carbonium ions. LVIII. Carbon-13 resonance investigation of protonated carboxylic acids (carboxonium ions) and oxocarbonium ions (acyl cations)

Mécanisme de l'hydrolyse acide des carbamates

AbstractN, N‐dialkyl carbamates decompose in strongly acidic media to carbon dioxide, olefin, alkyl halide and alcohol, the rate of reaction of the secondary esters closely following the acidity function. This fact, together with the variation in rate of hydrolysis of carbamates of cyclic alcohols with the ring size, shows that, unlike the solvolyses of the corresponding chloroformates which lead to carboxonium ions (\documentclass{article}\pagestyle{empty}\begin{document}$ {\rm R - O -}\mathop {\rm C}\limits^{{\rm (+)}} = {\rm O} $\end{document}), these reactions involve the intermediate formation of carbonium ions.

THE PREPARATION AND PROPERTIES OF α-D-GLUCOPYRANOSE 1,2-(ETHYL ORTHOACETATE) TRIACETATE

Crystalline ethyl and propyl 1,2-orthoacetates of α-D-glucopyranose triacetate were prepared. Conclusions are drawn regarding the chemical properties and configurations of these compounds which are based on the conformation of the 1,2-cyclic carboxonium ion believed to be formed when the substances are treated with acetic acid.

THE RELATIVE REACTIVITIES OF 1,2-trans-SUGAR ACETATES

The rates of exchange of the C1-acetoxy group of a variety of 1,2-trans-sugar acetates with acetate from stannic trichloride acetate labelled with carbon-14 were determined in chloroform containing stannic chloride. A correlation was found to exist between reactivity and configuration with the configuration at C3 playing a dominating role. The greater reactivity of the 2,3-trans-sugar compounds is attributed to steric inhibition to the formation of the resonance stabilized intermediate 1,2-cyclic carboxonium ion in the case of the 2,3-cis-compounds.