Short communication: An alternative and effective catalyst for the silastannation of arylacetylenes with Me3SiSnBu3 at room temperature

The palladium‐catalyzed silastannation of acetylenes with tributyl(trimethylsilyl)stannane in the presence of triethylphosphite is reported for the first time. The reaction occurs at room temperature to give (Z)‐silyl(stannyl)ethenes in high yields. The protodemetallation of the resulting adducts with HCl–tetraethylammonium chloride is described first, which demonstrates that the reaction is governed only by the stability of a carbonium ion arising from the protonation to (Z)‐silyl(stannyl)ethenes rather than the hard and soft acid and base principle, i.e. the β‐cation stabilization effect (σ–π stabilization one) of a stannyl group in the carbonium ion is rather significant. Copyright © 2004 John Wiley & Sons, Ltd.     

Oxidation of benzyl alcohols by nitrous and nitric acids in strong sulfuric acid media

We have studied the oxidation of benzyl alcohols by nitrous and nitric acid in sulfuric acid media. The oxidation by nitrous acid is rapid and has an activation energy of 10.6 ± 0.8 kcal mol^?1. A Hammett plot of logk₂ vs. σ⁺ is linear with a ρ value of ?1.4. The oxidation by nitric acid in sulfuric acid media is autocatalytic. From the kinetic and product analyses, it is concluded that a common oxidant, the nitrosonium ion is involved when either nitrous or nitric acid is used. A mechanism is proposed which involves the abstraction of hydride from the alcohols as the rate determining step. It is demonstrated that the autoxidation of the alcohols is catalyzed by nitrous acid or nitric oxide.     

Carbonium ion rearrangement in the synthesis of γ-lactones from (E)-β-alkylcinnamic acids

A possible formation of a bridged carbonium ion intermediate in the lactonization of (E)-β-t-butylcinnamic acid is discussed on the basis of deuteriation and ¹³C NMR experiments.     

Mechanism of the etherification of 2-alkylamino-1-phenylethanol derivatives

Derivatives of 2-alkylamino-1-(4-hydroxyphenyl)-1-ethanol have been converted to the β-methylethers in good yield. Etherification of 2-alkylamino-1-(4-methoxyphenyl)-1-ethanol could not be accomplished. Based on this unreactivity, a mechanism is proposed whereby the etherification proceeds via a quinoidal intermediate and not via a carbonium ion intermediate. It is concluded that the acid catalyzed racemization of catecholamines may occur via a quinoidal intermediate.     

Alkylation mechanism of benzene with 1-dodecene catalyzed by Et3NHCl-AlCl3

The isotope exchange method was employed to investigate the catalytic mechanism of ionic liquid in alkylation of benzenes with olefins. It is proposed that alkylation was induced by the Lewis acid AlCl₃ which attracted π electrons of 1-dodecene to shift toward 1-carbon, thus forming a carbonium ion. The carbonium ion further reacted with benzenes to form a complex. Due to unstabilit of the complex, a deuterated ring proton was transferred into an electronegative 1-carbon of the side chain to substitute for the AlCl₃, accordingly 2-phenyldodecane was generated.     

One-Step Synthesis of 1,3-Bis(carboxymethyl) Adamantane

Addition of the carbonium ion to vinylidene chloride to give α-substituted acetic acid discovered by Bott^1,2 has been found of wide applicability. For example, the reaction of 1-bromo-, 1-hydroxy-, or 1-acetoxy-adamantane with vinylidene chloride in concentrated sulfuric acid gives a satisfactory yield of adamanty1-(1)-Acetic acid.^3,4     

Electrochemical oxidation of barbituric acids at low pH in the presence of chloride ion

Barbituric acid, 1-methylbarbituric acid and 1,3-dimethylbarbituric acid are electrochemically oxidized at the pyrolytic graphite electrode by way of a single voltammetric peak at pH 1 in the presence of chloride ion. At least four products are formed as a result of the reaction, the three major products, accounting for more than 80–90% of the oxidized barbituric acid, are the appropriately N-methylated 5,5′-dichlorohydurilic acids, 5,5-dichlorobarbituric acids and alloxans. The mechanism appears to proceed by an initial potential-controlling 1e/1H⁺ oxidation of the barbituric acids to give a barbituric acid radical. This can dimerize to hydurilic acid, which is then further electrochemically oxidized. However, this appears to be a minor route. The barbituric acid radical appears to be mainly further electrooxidized (1e) to a carbonium ion which further reacts with nucleophiles such as chloride ion to give 5-chlorobarbituric acid, or with water to give dialuric acid. Further electrochemical oxidation and chemical reactions of the latter species results in formation of the ultimate products.     

Studies on the preparation and exchange reactions of 5-deuterated uracils

The mechanism of base catalyzed proton exchange at the 5-position of uracil and its N-methylated derivatives has been studied. These reactions proceed by addition — elimination across the 5,6-double bond when the 1-nitrogen is substituted with a methyl group, or with anchimeric assistance of the N-1 anion if the 1-position is unsubstituted. The base catalyzed hydrolyses of 1,3-dimethyluracil and 3-methyluracil also appear to proceed through hydrated intermediates. A facile method for an acid catalyzed preparation of 5-deuterated uracils is described as well as a simple and accurate method for analysis of deuterium content.     

Spontaneous hydrolysis of ethyl formate: Isobaric activation parameters

Ethyl formate undergoes spontaneous autocatalytic hydrolysis via water catalyzed (neutral), as well as hydrogen‐ion catalyzed mechanisms. The activation parameters for the neutral reaction are ΔH‡ = 91 ± 8 kJ/mol and ΔS‡ = −48 ± 8 J K⁻¹/mol. This result is in contrast to the values reported in the hydrolyses of the more polar activated haloesters. The specific rate of neutral hydrolysis of ethyl acetate can also be predicted. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 67–71, 2000     

The amidoalkylation of aromatic hydrocarbons

The Nyberg procedure (the use of trifluoroacetic acid in chloroform) for the efficient amidoalkylation of aromatic hydrocarbons is limited to substrates more nucleophilic than benzene. The reaction involves protonation of the electrophile, cleavage to a carbonium ion and alkylation of the nucleophile by the carbonium ion. Either the cleavage step or the alkylation step may be rate-determining. The present work identifies some cases where a carbonium ion is formed but fails to alkylate the nucleophile (with benzene and nitro-substituted benzenes as nucleophiles) and other cases where the reaction conditions are not sufficient to permit cleavage of the protonated electrophile (the reactions of N-phthalimidomethylamides).     

Carbonium ion stabilization by (η5-cyclopentadienyl)dicarbonylnitrosylchromium; a 1H and 13C NMR study

the ¹H and ¹³C NMR spectra of a series of (η⁵-cyclopentadienyl)dicarbonylnitrosylchromium-substituted carbonium ions have been determined. These data suggest that the same mechanism is responsible for the stabilization of carbonium ions by (η⁵-cyclopentadienyl)dicarbonylnitrosylchromium as is operational in the analogous ferrocene and cymantrene complexes.     

The Synthesis and Acid-Catalyzed Hydrolysis of N-(4-Substitutedphenyl)-O-Benzenedisulfonimides

Abstract

The acid catalyzed hydrolyses of some cyclic disulfonimides, N-(4-substitutedphenyl)-o-benzenedisulfonimides (1a–d) have been studied in concentrated aqueous acidic solutions. Analysis of the data by the Excess Acidity Method, activation parameters, substituent, and solvent deuterium isotope effect are all indicate hydrolysis by an A-1 mechanism in the studied range.     

Complexes cationiques η-1′,2,3-[(methylene-2γ-butyrolactone)-yl-3]-(η-cyclopentadienyl)carbonylnitrosyl molybdene: Synthese et etudes des additions nucleophiles stereoselectives

The decomposition of hydroxyethyl-, ethoxyethyl- and phenoxyethyl(aquo)cobaloximes in sulfuric acid/water mixtures to produce ethylene has been studied spectrophotometrically and manometrically. Definitive kinetic evidence for formation of an intermediate which is common to all three starting complexes and accumulates at acidities greater than 7.3 M H₂SO₄ has been obtained. A complete rate law which accounts for all of the ionizations of starting materials and intermediate has been derived and fit to the rate data. The rate-determining step for product formation is decomposition of the intermediate at all acidities: the intermediate accumulates in strong acid because of a shift in the equilibrium for its formation due to the reduced activity of water in strongly acidic media. Activation parameters for the decomposition of the intermediate, which may be formulated as an ethylene-cobaloxime(III) π-complex or as a σ-bonded ethyl carbonium ion, have been obtained. ¹H and ¹³C NMR observations of the intermediate and its deuterated analog (from 1,1,2,2-tetradeuterio-2-hydroxyethyl(aquo)cobaloxime) have led to the conclusion that it is probably a σ-bonded ethyl carbonium ion which may be stabilized by σ-π hyperconjugation.     

Contiguous concerted participation in the rearrangement of anti-tricyclo[4.2.1.1.2,5]deca-3,7,-diene-9,10-diol,a highly hindered system.

The acid catalysed peclppangement of anti-tricyclo[4.1.1.^2,5]deca-3,7-diene-9, 10-diol ($\text{1a}$) to $\text{2a}$ is shown to proceed through concommitant electrophilic and nucleophilic attack on an alkene by an incipient carbonium ion and the rernaining alcohol function.     

Cubyl caged systems: a paradigm of tandem cyclobutylcyclopropylcarbinyl carbonium ion rearrangements

A novel and general carbonium ion rearrangement of homocubanone to tetracyclo(3.3.0.0^2,8.0^3,6)octyl system is reported.     

Active role of hydrogen bond and ambient water in Meyer–Schuster rearrangements in high‐temperature water

Meyer–Schuster rearrangements of 2‐phenyl‐3‐butyn‐2‐ol with H₃O⁺ and (H₂O)₆ model in high‐temperature water (HTW) have been investigated by the use of density functional theory calculations. In the substrate 2‐phenyl‐3‐butyn‐2‐ol catalyzed by H₃O⁺ and (H₂O)₆, the Meyer–Schuster rearrangements were predicted by the frontier molecular orbital theory. The results show that the rearrangement does not involve the carbonium ion intermediates, but the first transition state is carboniumion like. Dehydration and hydration may occur via the intermolecular proton relay along the hydrogen‐bond chains and the second step of reaction path is a total acid–base catalytic process. Based on the results, a model considered both HTW ambient and water molecules are proposed to represent mechanisms of other reactions in HTW. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Struktur und Mechanismus bei Fragmentierungsreaktionen 1. Teil. Die stereoisomeren 10-Chlor-decahydro-isochinoline. Fragmentierungsreaktionen, 23. Mitteilung

cis-10-Chloro-N-methyl-decahydro-isoquinoline ( 5 ) and its trans-isomer 6 undergo heterolytic fragmentation in 80% ethanol by different mechanisms. As predictable on stereo-chemical grounds the cis-isomer 5 reacts by the accelerated synchronous mechanism, the trans-isomer 6 , however, by the two-step carbonium ion mechanism. Synchronous fragmentation therefore dominates over the two-step process even when the latter would lead to a relatively stable tertiary carbonium ion. In both cases the more highly substituted and thermochemically more stable olefinic fragment 8 is formed.     

Intermediaires aryl carbonium en bromation. Nivellement de la contribution d'un cycle aromatique par les substituants donneurs de l'ion carbonium ArC+(R′)R″

The bromination of trans α-methyl stilbenes, Ar(Me)CHCHPh, is studied as a model of reactions with carbonium ion intermediates. Direct measurement of ρ, the sensitivity of bromination to the effect of substituents in an aromatic ring over a wide range of reactivity, is possible. Despite the linearity of the ρσ⁺ relationship, the real value of ρ, ?4·87, is only obtained by a Yukawa-Tsuno treatment which is justified by the absence of coplanarity in the aromatic ring and the carbonium ion. The effect of substituents in the ring decreases on going from the stilbenes, ρ = ?5·05, to the α-methyl stilbenes, ρ = ?4·87. Similar decreases are observed in solvolysis, protonation and electrophilic aromatic substitution. We conclude that the decrease in ρ, which occurs when the electron-donating ability of R′ and R″ in the cation ArC⁺(R′)R″ is increased, is a general feature of reactions with carbonium ion intermediates. This is attributed to two types of effect; the increase in the stability of the intermediate and “saturation” of charge stabilization by the substituents R′ and R″. The variation of ρ with structure of substituents is analysed.     

Heterocyclic N-glycosides. VIII. Anomerisation and rearrangement of 2′,3′-unsaturated N-glycosides

The acid catalyzed transformation of cither 1-(4 -O-acety1-2,3 -dideoxy-α- or β-L-glycero-pent-2 -cnosyl)-5,6-dichlorobenzotriazole gave the same mixture of unsaturuted glycosides. The main three components of this mixture were the above α and β anomers and a glycal-type compound which results from the allylic rearrangement of the 5,6-dichlorobenzotriuzole moiety to the C-3 position of the carbohydrate. Evidence is presented that these results can be rationalized as proceeding through an allylic carbonium ion. Preliminary experiments on transglycosidation reactions using 2′,3′-unsaturated N-glycosides are reported.     

Unimolecular reactions of isolated organic ions: Some isomers of [C7H16]+˙

The slow unimolecular reactions of six isomers of [C₇H₁₆]^+˙ are reported and discussed. These results are interpreted in terms of dissociation via complexes of incipient carbonium ions and the appropriate associated radical. In some cases, rearrangement of the incipient carbonium ion precedes or accompanies decomposition; such isomerization generally favours alkyl radical loss, relative to elimination of the corresponding alkane.