Carbocationic chemistry

The dissociation constant KD, which is often derived from spectral properties in carbanion chemistry, therefore includes a covalent term that corresponds to Kx in carbocationic chemistry. As one would not expect equal reactivity of benzhydryl chloride and benzhydryl cations, one also should not expect equal reactivity for benzhydryl lithium and benzhydryl anions. As one realizes that the terms contact ion-pair and dissociation have a different meaning in carbocation and carbanion chemistry, the apparent discrepancies quoted above, will disappear. 

The conventional acid-induced transannular cychzation of 1,10-epoxy-costunolide, via carbocationic chemistry, results in a eudesmanohde mix-... 

The transformation of 1,4-dien-3-ones such as 295 (Scheme 2.111) into cyclopentenone 296 represents an example of the preparative utilization of carbocationic chemistry for cyclopentenones. This reaction, known as Nazarov s cyclization, was discovered almost half a century ago. While the initial... 

A sizable number of reports describe carbocationic chemistry initiated by transition metal catalysts. This includes a variety of cyclization and ring-opening transformations. For example, a Rh2(II)-catalyzed synthetic method has been developed as a means of preparing 2,3-disubstituted indoles from styryl azides (Scheme 30)7 The chemistry... 

In the dehydrocyclization of alkanes it is clear that ring closure can take place both in a metal-catalyzed reaction and as a carbocationic process. The interpretation of the reforming process proposed by Heinemann and coworkers,123 therefore, is not a complete picture of the chemistry taking place. The scheme they presented (Fig. 2.1) attributes cyclization activity solely to acidic sites. The ample evidence available since requires that metal-catalyzed C5 and C6 ring-closure possibilities be included in a comprehensive interpretation. Additionally, the metal component plays and important role in carbocationic reactions in that it generates carbocations through the formation of alkenes. 

A controversial issue of heteroatom-stabilized cations is the relative stabilization of carbocationic centers adjacent to oxygen and sulfur.541 In solution studies, a-O-substituted carbocations were found to be stabilized more than a-iS -substituted carbocations.677 Gas-phase studies reached an opposite conclusion,678 679 whereas subsequent theoretical studies (high-level ab initio methods) supported the findings of solution chemistry. Recent results, namely, basicities of various vinylic compounds (365-370) measured in the gas phase also support this conclusion.680 Although monoheteroatom-substituted compounds 365 and 366 were found to have similar proton affinities, an additional a-methyl group increased the stability of the carbenium ion derived from 367 more than that of the sulfur counterpart 368. Even larger differences were found between proton affinities of the bis-heteroatom-substituted compounds 369 and 370. 

Collado and co-workers made detailed studies of the chemistry of the sesquiter-penoid caryophyllene and its hydroxylated products including rearrangements in-duced by superacids. They have recently reported novel rearrangements of the sesquiterpenoid panasinsane derivatives 213 to provide three products and interpreted the transformations by the involvement of the common carbocationic intermediate 214 [Eq. (5.304)]. 

Superelectrophilic onium dications have been the subject of extensive studies and their chemistry is discussed in chapters 4-7. Other multiply charged carbocationic species are shown in Table 2. These include Hogeveen s bridging, nonclassical dication (14)26 the pagodane dication (15)27 Schleyer s l,3-dehydro-5,7-adamantane dication (16)28 the bis(fluroenyl) dication (18)29 dications (17 and 19) 19a trications (20-21)19a,3° and tetracations (22-23).31 Despite the highly electrophilic character of these carbocations, they have been characterized as persistent ions in superacids. 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

The situation is quite different for carbanionic systems, for which large reactivity differences between free ions and ion pairs are well established [138]. What makes the difference Scheme 35 outlines that the term ion pairing has a different meaning in carbocationic and carbanionic chemistry [139],... 

In this section we will review some details concerning the chemistry of these systems with a special emphasis on the structure of the active species, kinetics, as well as on the molecular weights and the evolution of molecular weight distributions with conversion. In the last Section VII, we will discuss the mechanistic features of new controlled carbocationic polymerizations. 

Reaction of (284) with an aldehyde, ketone, or enol ether in the presence of acid results in an electrophilic substitution that produces a -ferrocenylalkyl carbocations that may be trapped by nucleophiles (azides, amines, thiols). This chemistry may be used to prepare enantiomerically pure ferrocene derivatives in a maimer that avoids resolution procedures (Scheme 86)." For example, the enol ether from (-)-menthone affords a kinetic carbocation (302) that may be trapped or allowed to rearrange to the more thermodynamically stable cation (303) and then trapped, thus offering a means of controlling the configuration of the stereocenter adjacent to the ferrocene unit. Use of an enantiomerically pure aldehyde derived from Q -pinene (304) affords a 1 1 carbocationic mixture that similarly isomerizes to a single cation. 

If other elements of chirality are present in the molecule, the descriptors should be arranged in the order central > carbocationic > axial > planar > torsional (note that Sokolov [6] uses a different order for the descriptors). The effects caused by such other types of chirality are discussed elsewhere in some detail [41], e.g., S-cis/S-trans-isomerism in ferrocenyl ketones and alkenes, as well as the chemistry of biferrocenyl (containing a direct bond between two ferrocenes). 

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