Carbocation Research Articles

George Andrew Olah (1927–2017)

George A. Olah, the recipient of the 1994 Nobel Prize in Chemistry for his groundbreaking work on superacids that led to isolation and observation of long lived carbocations, passed away on 8 March 2017 at his home in Beverly Hills, California.

Catalytic properties and reaction mechanism of the CrtO carotenoid ketolase from the cyanobacterium Synechocystis sp. PCC 6803

CrtW and CrtO are two distinct non-homologous β-carotene ketolases catalyzing the formation of echinenone and canthaxanthin. CrtO belongs to the CrtI family which comprises carotene desaturases and carotenoid oxidases. The CrtO protein from Synechocystis sp. PCC 6803 has been heterologously expressed, extracted and purified. Substrate specificity has been determined in vitro. The enzyme from Synechocystis is basically a mono ketolase. Nevertheless, small amounts of diketo canthaxanthin can be formed. The poor diketolation reaction could be explained by the low relative turnover numbers for the mono keto echinenone. Also other carotenoids with an unsubstituted β-ionone ring were utilized with low conversion rates by CrtO regardless of the substitutions at the other end of the molecule. The CrtO ketolase was independent of oxygen and utilized an oxidized quinone as co-factor. In common to CrtI-type desaturases, the first catalytic step involved hydride transfer to the quinone. The stabilization reaction of the resulting carbo cation was a reaction with OH− forming a hydroxy group. Finally, the keto group resulted from two subsequent hydroxylations at the same C-atom and water elimination. This reaction mechanism was confirmed by in vitro conversion of the postulated hydroxy intermediates and by their enrichment and identification as trace intermediates during ketolation.

Ultrafast dissociation pathways of diphenylmethyl chloride to generate reactive carbo cations

The primary processes in the formation of electrophilic precursor ions are studied on a microscopic scale by quantum chemical and quantum dynamical methods. For the competing reaction channels of heterolysis and homolysis in the photochemically induced dissociation of diphenylmethyl chloride, ab initio data for the ground and excited electronic states are evaluated in the gas phase. Here, the simultaneous formation of ionic and radicalic products can solely be attributed to the existence of conical intersections which are localized and characterized for the first time. The interaction of different electronic states also leads to the initial charge transfer from the phenyl π-system to the σ-bond of the leaving group which is essential for the fast dissociation and is explained by orbital analysis. Quantum dynamical calculations in one and two dimensions suggest that in gas phase the ion pair constitutes the main product despite its higher potential energy. Possible changes in polar solvents, where the ionic fragments are stabilized below the radical channel, are discussed.

Chromium Tricarbonyl‐Coordinated Carbocations

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access the actual ChemInform Abstract, please click on HTML or PDF.

Zwitterionic “Neutral” and “Anionic” Carbocation Analogues

AbstractFor Abstract see ChemInform Abstract in Full Text.

100 years of carbocations and their significance in chemistry.

100 years of carbocations and their significance in chemistry.

Polyfluorinated carbocations stabilized by oxygen and sulfur

Polyfluorinated carbocations stabilized by oxygen and sulfur

A New Rearrangement inIboga Alkaloids

The hydroxyindolenine 2, upon treatment with Ac2O in pyridine, rearranged into the bridged 16-spiro compound 3. Compound 3 can be considered to be a synthon of the carbocation at C(16) of ibogaine (7) since the solvolysis of 3 in EtOH/H2O gave a mixture of the 16-ethoxy- and 16-hydroxyibogaine derivatives 5 and 6, respectively.

Inhibitory effect of vitamin C on the mutagenicity and covalent DNA binding of the electrophilic and carcinogenic metabolite, 6-sulfooxymethylbenzo[a]pyrene.

6-Sulfooxymethylbenzo[a]pyrene has recently been shown to be a strong hepatocarcinogen in infant male B6C3F1 mice (Y.-J.Surh et al., Biochem. Biophys. Res. Commun., 172, 85-91, 1990) and appears to be an ultimate carcinogenic metabolite of 6-hydroxymethylbenzo[a]pyrene and possibly of benzo[a]pyrene. It produced high levels of aralkyl DNA adducts in the livers of B6C3F1 mice and also exhibited strong direct mutagenicity toward Salmonella typhimurium TA98 without metabolic activation. In the present study we found that ascorbic acid significantly reduced the bacterial mutagenicity and in vitro covalent DNA binding of 6-sulfooxymethylbenzo[a]pyrene. Ascorbic acid forms a mutagenically inactive covalent adduct with 6-sulfooxymethylbenzo[a]pyrene, which appears to account for its novel protective mechanism against this reactive sulfuric acid ester. It seems likely that the formation of this adduct involves aralkylation of an ascorbic acid anion by a presumed carbo cation derived from the electrophilic sulfuric acid ester.

Electronegatively substituted carbocations

Electronegatively substituted carbocations

ChemInform Abstract: Addition of Bromine Gas to Crystalline Dibenzobarrelene: An Enantioselective Carbocation Rearrangement in the Solid State.

Abstract Treatment of chiral crystals of 2,3:5,6-dibenzobicyclo[2.2.2]octa-2,5,7-triene (dibenzobarrelene, 1) with bromine vapor results in the formation of the rearranged product syn -8-bromo- endo -4-bromo2,3:6,7-dibenzobicyclo[3.2.1]octa-2,6-diene (2) in up to 8% enantiomeric excess.

ChemInform Abstract: Reactions of Carbocations with a Functionalized Hydrophobic Ammonium Ion.

AbstractThe kinetics of the reaction of the nonmicellizing hydroxyethyl ammonium salts (I) with the preformed carbocations (II) at high pH values are studied.

Synthesis of tetraisopropylethane and tetracyclopropylethane, and generation of the pentacyclopropylethyl carbocation

Synthesis of tetraisopropylethane and tetracyclopropylethane, and generation of the pentacyclopropylethyl carbocation

Macromers by carbocationic polymerization

Rapport de reactivite en copolymerisation a diverses conversions dans differents solvants et avec trois longueurs de chaine du macromere

Delocalization of charge by ferrocenyl and ruthenocenyl groups in ?-metallocenyl carbocations

The19F NMR chemical shifts of the ferrocenyl(p-fluorophenyl) and ruthenocenyl(p-fluorophenyl) carbocations testify to the greater effectiveness of the ruthenocenyl group over the ferrocenyl group in delocalizing the positive charge.

The efficiency of alkyl halide initiators in carbocationic polymerization

Etude theorique: application a la polymerisation du pentene-1 methyl-4 par le systeme halogenure d'alkyle-acide de Lewis

Structure of the cationoid intermediate formed during electrophilic addition to vinyl ethers

1. Use of the semiempirical MINDO/2 method has shown that the carbenium ion generated during the addition of a proton to the vinyloxy group has an acyclic structure. 2. An analysis of the electronic structure of a series of carbocations\(CH_3 \mathop C\limits^ + HOR\) where R is an alkyl or a phenyl radical, indicates that the oxonium form\(CH_3 CH = \mathop O\limits^ + R\) contributes substantially.

Déshydratation des diols-1,2 α,β-éthyléniques IV: rôle de la stéréomutation des carbocations allyliques α-hydroxylés sur l'orientation des réactions observées

The allylic carbocations[Formula: see text]formed during the dehydration of diols of type A[Formula: see text]are found to react by five different processes: three nucleophilic attacks (by solvent at the β or δ positions, or by the intramolecular hydroxyl group at δ), a degradation reaction between Cα, and Cβ and the classical pinacol rearrangement.Nucleophilic attack at the carbon δ is a very efficient reaction and sometimes (for monosubstituted substrates in β, γ or δ) gives quasi-exclusive products: a ketone for the reaction with H2O in δ or a 2,5-dihydrofuran product for the intramolecular reaction.In the case of γ and δ disubstituted carbocations, the selectivity is less and we show that it is possible to analyze the different steric effects commanding the stereomutation equilibrium of the allylic carbocation and so determining the orientation in the dehydration.

ChemInform Abstract: FLUORO‐CONTAINING CARBO CATIONS. XIII. GENERATION OF 2‐SUBSTITUTED 5‐METHYL‐ AND 5‐H‐1,1,3,4,6‐PENTAFLUOROBENZENONIUM IONS. EFFECT OF ORTHO‐ AND META‐SUBSTITUENTS ON THE RELATIVE STABILITY OF POLYFLUORINATED BENZENONIUM IONS

Chemischer InformationsdienstVolume 8, Issue 48 Preparative Organic Chemistry ChemInform Abstract: FLUORO-CONTAINING CARBO CATIONS. XIII. GENERATION OF 2-SUBSTITUTED 5-METHYL- AND 5-H-1,1,3,4,6-PENTAFLUOROBENZENONIUM IONS. EFFECT OF ORTHO- AND META-SUBSTITUENTS ON THE RELATIVE STABILITY OF POLYFLUORINATED BENZENONIUM IONS A. A. SHTARK, A. A. SHTARKSearch for more papers by this authorV. D. SHTEINGARTS, V. D. SHTEINGARTSSearch for more papers by this author A. A. SHTARK, A. A. SHTARKSearch for more papers by this authorV. D. SHTEINGARTS, V. D. SHTEINGARTSSearch for more papers by this author First published: November 29, 1977 https://doi.org/10.1002/chin.197748106Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume8, Issue48November 29, 1977 RelatedInformation

Preparation and reactions of a bis-homo(CH)5+-type carbocation

Preparation and reactions of a bis-homo(CH)5+-type carbocation