Diarylmethyl Cations Research Articles

Crystallographic and spectroscopic studies on persistent triarylpropargyl cations.

By acid treatment of precursor alcohols, mesitylethynyl-substituted diarylmethyl cations were isolated as stable solids, X-ray structural analyses of which revealed a planar geometry. Furthermore, the ion pairs including these triarylpropargyl cations form charge-segregated assemblies in the crystal, and effective intermolecular interaction induces a red-shift of absorption in the crystal.

Brønsted Acid Catalyzed α‐Alkylation of Aldehydes with Diaryl Methyl Alcohols

The catalytic a-alkylation of carbonyl compounds is a common approach in organic synthesis. In recent years, many efforts have been directed towards the activation of ketones and aldehydes by means of enamine catalysis to react with a broad range of electrophiles. In 2004, List reported an elegant intramolecular alkylation of aldehydes with alkyl halides using proline-based amine catalysts. While the long-sought intermolecular versions of such enamine-catalytic SN2-type a-alkylation of aldehydes [3] remain challenging, research by the groups of Melchiorre, Cozzi, Jacobsen and others have pioneered the amine-mediated SN1type intermolecular reactions between aldehydes and arylsulfonyl indoles, diaryl alcohols, or halides. In the approach used by Cozzi and others, amine catalysts were used to activate the aldehydes via enamine intermediates, and Bronsted acids were used to mediate the formation of diaryl methyl cations from the corresponding diaryl methanols. Despite the success of this amine/acid co-catalysis approach for diaryl methyl alcohols (e.g., 1a) that generate highly stabilized carbocations, the scope of the reactions still remains limited: diaryl methyl alcohols (such as 1b–f) that lead to “less stabilized” diaryl methyl carbocation electrophiles are ineffective substrates in these alkylation reactions. Instead of using enamine catalysis for aldehyde activation, here we disclose the use of Bronsted acids alone to catalyze these SN1 type aldehyde alkylations. The acid catalyst is believed to facilitate the formation of alcohol-derived carbocations and to accelerate the enolization of aldehydes. A much broader scope of substrates is thereby realized: diaryl methyl alcohols that lead to less stabilized carbocations can be used; aldehydes with a,a-disubstituents react efficiently as well to generate products containing quaternary carbon centers. Our work began by using diaryl methanol 1b as a model substrate to develop an acid-catalysis approach. According to the Mayr reactivity scales, such substrates lead to lessstabilized carbocations (e.g., compared to that from 1a) and were ineffective using the enamine/acid co-catalytic strategies. The results of our initial studies are summarized in Table 1. Weak acids, such as acetic acid, could not mediate

Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites — Deprotonation and oxidation reactions

The oxidation of diarylmethanes is a multistep process involving initial formation of a radical cation, deprotonation of the radical cation to the radical, and oxidation of the radical to the carbocation. The dynamics and efficiency of the last two steps in this process, namely deprotonation and oxidation, in acidic zeolites and non-acid zeolites are examined in the present work as a function of the acidity of the diarylmethane radical cations and the oxidation potential of the diarylmethyl radicals. Our results indicate that rate constants for deprotonation strongly depend on the acidity of the radical cations, but not on the composition of the zeolites. In addition, oxidation of the radicals to the diarylmethyl cations is strongly dependent on both the oxidation potential of the radicals and the oxidizing ability of the zeolite. This dependence allows oxidation potentials of the zeolites to be estimated.Key words: radical cations, carbocations, zeolites, laser flash photolysis.

Cover Picture: Structure–Nucleophilicity Relationships for Enamines (Chem. Eur. J. 10/2003)

AbstractThe cover picture shows the exponential decay of the UV/Vis absorption of a diarylmethyl cation in presence of 46 equivalents of 1‐(methylphenylamino)cyclohexene. Using diarylmethyl cations of different electrophilicity E allows us to perform a comprehensive comparison of enamine nucleophilicity (N) on the basis of the depicted correlation equation. Enamines, commonly characterized as strong nucleophiles, show dramatically different reactivities. For a typical s value of 0.85, the nucleophilicity range from N = 16 to N = 4 corresponds to relative reactions times of one minute versus 20 000 years. A detailed report on the relationships between structure and nucleophilicities of enamines is given by H. Mayr et al. on p. 2209 ff.

Reaction of the tamoxifen cation and the bis-(4-methoxyphenyl)methyl cation in aqueous solutions containing 2'-deoxyguanosine

The competition between 2'-deoxyguanosine (dG) and water has been quantitatively evaluated for the allylic carbocation derived from tamoxifen and for the stabilized diarylmethyl cation (bis-(4-methoxyphenyl)methyl). Both systems were examined by the competition kinetics method, in which the products were quantitatively analyzed after the SN1 solvolysis of the corresponding acetate esters in aqueous solutions containing the nucleoside. The principal product of the reaction of both cations with dG is the adduct at the NH2 group, a characteristic of delocalized carbocations. The tamoxifen cation was also examined by laser flash photolysis, with absolute rate constants for the reaction with dG and water being obtained and converted into rate constant ratios. The principal result of this study is that there is a three orders of magnitude difference in the reactivity of these cations towards the neutral form of dG and its conjugate base. Under acidic conditions where the reaction occurs with neutral dG, the guanine–water selectivity is low. Even at relatively high concentrations of dG, the majority of the product is alcohol derived from the water reaction. At pH 10 to 11, in contrast, dG is present as the anion and this is highly competitive. Yields of adduct as high as 90% can be attained. A consequence of the large difference in reactivities is that at neutral pH the majority of the reaction of the cation with dG is actually occurring via the small amount of conjugate base present. A further feature of the results is that the NH2 adduct is the predominant stable product from the anion. To explain the high rate constant for the reaction forming this product, a mechanism is proposed whereby one of the protons of the NH2 group is transferred to N1 as the N2-cation bond is forming.Key words: guanine, DNA adduct, carbocation, tamoxifen.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

Transition-state structures for the carbocation–nucleophile combination reactions of (4-substituted-4'- methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile–water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50–65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50–65% bond formation in the transition state independent of reactivity, have previously been made in correlations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon—halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ~25 and ~40% bond formation in the transition states for the reactions with bromide and chloride, respectively.Key words: carbocation, isotope effect, transition state, halide.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

Hydroxyalkylation of benzene derivatives by benzaldehyde in the presence of acid zeolites

Condensation of benzene, anisole and N, N-dimethylaniline with benzaldehyde has been carried out in the presence of a USY zeolite (Si/Al 17.5). A combined study of the product distribution present in the liquid phase (di- and triarylmethanes together with N-benzylated byproducts in the case of dimethylaniline) as well as the organic material entrapped inside the zeolite micropores (triphenylmethane or trityl ions) has established that there is a direct relationship between the reactivity of the arene and the conversion of benzaldehyde, percentage of diarylmethanes in the liquid phase and population of trityl cations. A hydride transfer from triarylmethanes to diarylmethyl cation intermediates has been proposed as a key step for the simultaneous presence of trityl cations adsorbed in the solid and diarylmethanes in the liquid phase.

Homolytic vs Heterolytic C−H Bond Cleavage in Alkylaromatic Radical Cations. Formation of Diarylmethyl Cation in the Photoinduced Electron Transfer Reaction of Bis(4-methoxyphenyl)methane Sensitized by Chloranil

Homolytic vs Heterolytic C−H Bond Cleavage in Alkylaromatic Radical Cations. Formation of Diarylmethyl Cation in the Photoinduced Electron Transfer Reaction of Bis(4-methoxyphenyl)methane Sensitized by Chloranil

Biphotonic generation of carbenes and carbocations by laser flash photolysis

In protic solvents, trans-2,3-diphenylaziridinylimines of diaryl ketones are (flash-)photolyzed in two consecutive steps, via diaryldiazomethanes and diarylcarbenes, to give eventually diarylmethyl cations. The same intermediates arise from diazo ketones by way of Wolff rearrangement and ketene photolysis. The method has been used to demonstrate the protonation of fluorenylidene by hexafluoroisopropanol.

Comparison of the Nucleophilicities of Alkynes and Alkenes. Quantitative Determination of the Nucleophilicities of Alkynes toward Carbenium Ions

AbstractRelative reactivities of alkenes and alkynes toward diarylmethyl cations have been determined by direct rate measurements and by competition experiments in dichloromethane. At −70°C alkynes are found to be one to two orders of magnitude less reactive than analogously substituted alkenes (e.g. phenylacetylene/styrene), but the reactivity difference reduces strongly as the temperature is raised. The stereochemistry of the vinyl chlorides produced by addition of benzhydryl chlorides to alkynes is characterized.

Reactivities of diarylmethyl and triarylmethyl cations with primary amines in aqueous acetonitrile solutions. The importance of amine hydration

By use of the technique of laser flash photolysis, rate constants k(RNH 2 ) have been directly measured for the reactions of primary amines RCH 2 NH 2 (R=CH 3 CH 2 , CH 3 OCH 2 , NCCH 2 , CF 3 ) with diarylmethyl cations (D + ) in acetonitrile/water solutions

ChemInform Abstract: Reactivities of Triarylmethyl and Diarylmethyl Cations with Azide Ion Investigated by Laser Flash Photolysis. Diffusion‐Controlled Reactions

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Reactivities of triarylmethyl and diarylmethyl cations with azide ion investigated by laser flash photolysis. Diffusion-controlled reactions

By use of the technique of laser flash photolysis, rate constants k AZ and k S have been directly measured for the reactions at 20 o C in acetonitrile-water (AN-W) solutions of varying composition of 18 triarylmethyl and 10 diarylmethyl cations with azide and solvent. The cations have k S that depend on substituent and vary from ∼10 1 to ∼10 7 s −1 . For the more stable ions k AZ also varies, increasing with decreased electron donation and also increasing by as much as 10 3 with increasing acetonitrile content. For less stable cations, however, the rate constant becomes independent of substituent

ChemInform Abstract: Flash Photolysis Generation and Reactivities of Triarylmethyl and Diarylmethyl Cations in Aqueous Solutions.

ChemInformVolume 20, Issue 36 Preparative Organic Chemistry ChemInform Abstract: Flash Photolysis Generation and Reactivities of Triarylmethyl and Diarylmethyl Cations in Aqueous Solutions. R. A. MCCLELLAND, R. A. MCCLELLAND Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this authorV. M. KANAGASABAPATHY, V. M. KANAGASABAPATHY Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this authorN. S. BANAIT, N. S. BANAIT Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this authorS. STEENKEN, S. STEENKEN Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this author R. A. MCCLELLAND, R. A. MCCLELLAND Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this authorV. M. KANAGASABAPATHY, V. M. KANAGASABAPATHY Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this authorN. S. BANAIT, N. S. BANAIT Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this authorS. STEENKEN, S. STEENKEN Dep. Chem., Univ. Toronto, Toronto, Ont. M5S 1A1, Can.Search for more papers by this author First published: September 5, 1989 https://doi.org/10.1002/chin.198936118Read 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 onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume20, Issue36September 5, 1989 RelatedInformation

Flash-photolysis generation and reactivities of triarylmethyl and diarylmethyl cations in aqueous solutions

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFlash-photolysis generation and reactivities of triarylmethyl and diarylmethyl cations in aqueous solutionsRobert A. McClelland, V. M. Kanagasabapathy, Narinder S. Banait, and Steen SteenkenCite this: J. Am. Chem. Soc. 1989, 111, 11, 3966–3972Publication Date (Print):May 1, 1989Publication History Published online1 May 2002Published inissue 1 May 1989https://doi.org/10.1021/ja00193a031RIGHTS & PERMISSIONSArticle Views1712Altmetric-Citations110LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (820 KB) Get e-Alerts Get e-Alerts

Electrophilic attack at allylsilanes: a quantitative determination of the β-silyl effect

The relative reactivities of allylsilanes and alkenes towards diarylmethyl cations have been determined by competition experiments. Introduction of a β-trimethylsilyl group increases the reactivity of propene towards the diphenylmethyl cation by a factor of 30700.

Reactions of peralkyl organotin and organomercury compounds with di- and triarylmethyl salts

1. In the reactions of peralkyl organotin compounds containing a primary β-H atom with diarylmethyl salts, β-elimination and substitution occur simultaneously. 2. The proportion of the β-elimination product in the reaction mixture increases as the stability of the diarylmethyl cation increases. 3. The reactions of dipropylmercury with diarylmethyl salts, like those of peralkyl organotin compounds with triarylmethyl salts, result in exclusive β-elimination, irrespective of the stability of the corresponding cation.

Arylallyl cations and their stereospecific cyclization to bicyclic trienylic cations

A series of arylallyl alcohols, where aryl = 2,6-dimethylphenyl, mesityl, duryl, or pentamethylphenyl, was prepared. On addition to strong acid, these generate arylallyl cations. In some cases these allyl cations are observable and were characterized as insitu solution species by nmr spectroscopy. In other cases, an immediate cyclization takes place, leading stereospecifically to a series of bicyclic trienyl cations, which were also thoroughly characterized. The stereochemistry of this cyclization reaction, which was initially expected to follow the "rules" for electrocyclic ring closures, was probed in some detail. It is tentatively concluded that the stereochemistry in the products is not that expected from a conrotatory ring closure. Two diarylmethyl cations were also prepared. However, these did not cyclize.

Reaction of pyrimidines with diarylmethyl cations

Reaction of pyrimidines with diarylmethyl cations