Carbocation generation

Reactions of the 2-amino-4,5-substituted thiazole (52) in acetic acid with ethylene oxide has been reported to give the N-exocyclic disubstitution product (S3) (201) in a 40% yield (Scheme 38). The reactive species in this reaction is probably the carbocation generated in acetic acid by ethvlene oxide. 

Carbocations generated from alkanes using superacids react with carbon monoxide under mild conditions to form carboxyUc acid (188). In this process isomeric carboxyUc acids are produced as a mixture. However, when the reaction is mn with catalytic amounts of bromine (0.3 mmol eq) in HF-SbF solution, regio-selective carboxylation is obtained. / -Propane was converted almost exclusively to isobutyric acid under these conditions. 

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

The electrophile is a carbocation, generated by AICI3-assisted dissociation of an alkyl halide. 

The carbocations generated by non-Kolbe electrolysis can rearrange by alkyl, phenyl or oxygen migration. The migratory aptitudes of different alkyl groups have been studied in the rearrangement of a-hydroxy carboxylic acids (Eq. 34) [323]. 

No matter how produced, RN2 are usually too unstable to be isolable, reacting presumably by the SnI or Sn2 mechanism. Actually, the exact mechanisms are in doubt because the rate laws, stereochemistry, and products have proved difficult to interpret. If there are free carbocations, they should give the same ratio of substitution to elimination to rearrangements, and so on, as carbocations generated in other SnI reactions, but they often do not. Hot carbocations (unsolvated and/or chemically activated) that can hold their configuration have been postulated, as have ion pairs, in which OH (or OAc , etc., depending on how the diazonium ion is generated) is the coun-... 

The mechanism of these reactions is usually Sn2 with inversion taking place at a chiral RX, though there is strong evidence that an SET mechanism is involved in certain cases, ° especially where the nucleophile is an a-nitro carbanion and/or the substrate contains a nitro or cyano group. Tertiary alkyl groups can be introduced by an SnI mechanism if the ZCH2Z compound (not the enolate ion) is treated with a tertiary carbocation generated in situ from an alcohol or alkyl halide and BF3 or AlCla, or with a tertiary alkyl perchlorate. ... 

SCHEME 7.1 Carbocations generated from the 4-(2-methoxyisopropyl) styrene immer/TiCLt/isobutylene (IB) system. (Adapted from Puskas, J.E., Kwon, Y., Antony, P., and Bhowmick, A.K., J. Polym. Sci. Chem., 43, 1811, cover page, 2005. With permission.)... 

It was previously observed that with a catalytic amount of FeCls, benzylic alcohols were rapidly converted to dimeric ethers by eliminating water (Scheme 14). In the presence of an alkyne this ether is polarized by FeCls and generates an incipient benzylic carbocation. The nucleophilic attack of the alkyne moiety onto the resulting benzyl carbocation generated a stable alkenyl cation, which suffer the nucleophilic attack of water (generated in the process and/or from the hydrated... 

Sulfonium ylides can also be generated by in situ alkylation with diazo compounds. The alkylation can be carried out by reaction of a diazo compound with HBF4 and DBU.281 The reagents are added alternately in small portions and the reaction presumably proceeds by trapping of the carbocation generated by dediazonization and deprotonation. 

This conclusion is in accord with the results of MINDO/3 quantum-chemical calculations. Cation 7 is also prone to undergo a rather slow degenerate rearrangement by 1,2-bridge shift (AG is about 60 kJ/mol) (Scheme 8). This rearrangement was probed via 2H NMR spectroscopy, when the 9-CD3-analog of carbinol 5 was used as precursor for carbocation generation. At -50 °C cations 6-8 transform firstly into l,l,2,3,3a,4,8b-heptamethyl-l,3a,8b-trihydrobenzopentalenium ion (9) and then (partially) into 1,1,2,3,4,4,8b-heptamethyl-l,4,8b-trihydrobenzopentalenium ion (10) (Scheme 9). 

In addition to electron-deficient alkenes, under the catalysis of TiCLt, 1,2-allenylsi-lanes can react with aldehydes or N-acyliminium ion to afford five-membered vinylic silanes 71 and 72. Here the carbocations generated by a Lewis acid regiospecifically attack the C3 of the 1,2-allenylsilanes to produce a vinyl cation stabilized by hyper-... 

Ab initio MO calculations have been carried out for two carbocation-generating reactions the 6 nI reaction of protonated 1-phenylethanol (H2O leaving group) and the acid-catalysed hydration of styrene. Optimizations were done at the MP2/6-31G level. The 6 nI transition state lies half way between the reactant and the product with respect to the bond lengths, charge distribution, and secondary deuterium isotope effects. 

Fig. 4 Simplified carbocations generated as model for carbocations from the BaP and BeP skeletons.

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

Catalytic cracking proceeds through the formation of carbocations. The ease of formation depends upon the structural nature and stability of the carbocation generated. 

In the presence of the Lewis acid, the carbocation generated in situ can serve as an electrophile to replace, intramolecularly, the silyl substituent at the olefinic carbon153-155. The stereochemical requirements at the silyl substituent appear not to be important (equations 85-87). 

These methods are not always applicable or convenient. A more general method used by Richard and Jencks utilizes HPLC analysis of carbocation formation in alcohol-water mixtures.22 As shown in Scheme 2 for an a-aryl ethyl cation, formation of the ether product from reaction of the carbocation with the alcohol depends on the rate constant for carbocation formation kll and the partition ratio between product formation and the back reaction to form the alcohol kROiiAii2o- This ratio may be determined from the ratio of products formed from reaction of the carbocation generated from a suitable solvolytic precursor such as an alkyl halide. 

Laali et al. have characterized carbocations generated from substituted polycyclic aromatic compounds. The related cation 157 is a true aryl-methyl-type ion, whereas cations 158 have arenium ion character because the strongly electron-withdrawing a-CF3 group enhances charge delocalization into the pyrenyl and phenyl groups. 

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