Carbenium ions classes

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Reaction of glycosylmethylamines with aryldiazonium salts gives a class of compounds which, by acid catalysis or unknown factors of enzymic catalysis, generate glycosylmethyldiazonium ions. These, in turn, lose nitrogen, to yield highly electrophilic carbenium ions, as illustrated for the ) -d-galactosyl derivative 38 (see Scheme 8). 

In addition to the species Pn+ and Pn+ M, one must consider the complexes formed by the carbenium ions with other n- or n-donors in the system, in particular the polymers formed from monomers containing aromatic groups or hetero-atoms. This means that the polymers formed from non-aromatic hydrocarbons, e.g., isobutene, form a distinct class of noncomplexing polymers we will call these the Class A polymers. It is likely that the internal double-bonds in, for example, poly-(cyclopentadiene) are such poor complexors for steric reasons, that polymers containing them can be placed into the same class. 

Example The mass spectra of both acetone and butanone show typical acyiium ion peaks at m/z 43, whereas the signals in the spectra of isopropyl ethyl thioether (Fig. 6.9), of 1-bromo-octane, (Fig. 6.10), and of isomeric decanes (Fig. 6.18) may serve as examples for carbenium ion signals. The superimposition of both classes of ions causes signals representing an average pattern. The properties of larger carbenium ions are discussed in the section on alkanes (Chaps. 6.6.1 and 6.6.3). 

Example The El mass spectmm of -decane is typical for this class of hydrocarbons (Fig. 6.18a). Branching of the aliphatic chain supports cleavage of the bonds adjacent to the branching point, because then secondary or tertiary carbenium ions and/or alkyl radicals are obtained (Fig. 6.18b,c). This allows for the identification of isomers to a certain degree. Unfortunately, hydrocarbon molecular ions may undergo skeletal rearrangements prior to dissociations, thereby obscuring structural information. 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

As a class, nitrenium ions are rather poorly characterized relative to similar reactive intermediates such as carbenes and carbenium ions. This simation alone is sufficient to motivate many fundamental studies into their structures and behavior. There are also several practical considerations that motivate their study. The following is intended as a brief overview of these latter areas. 

The allyl cation (9) is the simplest member of the class of resonance-stabilized cations that includes the alkyl-substituted cyclopentenyl cations. But one could also say that the carbenium ion (CH3) is the simplest member of a class of cations that includes the trityl cation. In each case, 10 or so orders of magnitude of acidity separate the primitive member from its more elaborate derivatives. 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Several classes of compounds initiate cationic polymerizations of alkenes, including protonic acids, Lewis acids (usually in combination with a cation or proton source), stable carbenium ions, oxidizing reagents, and other strong electrophiles. This section attempts to explain the mechanism of initiation with quantitative information when available physical means of initiation (electric current, y-rays, field ionization and emission, nuclear chemical initiation) will not be discussed. 

Particularly relevant to the present crmtext is the fact that the olefinic double bond is considered as a soft base in Pearson s theory, while many Lewis acids used in cationic polymerisation (BF3, BCI3, AICI3, etc.) are classed as hard acids. Obviously, n-acceptors like chloranil or tetracyanoethylene are considered as soft acids. Thus, the interactions between Lewis acids and olefins must be considered as very weak in the context of the HSAB theory. This prediction is well substantiated by the tenuous character of the complexes observed in experimental studies (see Chap. IV). On the other hand, carbenium ions are usually placed at the borderline between hard and soft acids and are definitely softer than the Lewis acids mentioned above. Consequently, their interactions with olefins must be rather strong, which suggests that that propagation in cationic polymerisations promoted by Lewis acids should be faster than initiation. 

Olah s definition of two main classes of carbocations is most appropriate. A carbenium ion is a classical entity which contains an electron-deficient trivalent carbon atom possessing sp hybridisation and six electrons in the valence shell, e.g., CH3, (CH3)3C, CgH7 (benzenium ion), etc. The three atoms bound to such as carbon atom tend to be... 

In principle, any functionality capable of producing a carbenium ion under strongly acidic conditions will be able to participate in a Ritter-type reaction. Such classes of compounds include alcohols, aldehydes, alkanes, alkenes, alkyl halides, carboxylic acids, dienes, epoxides, esters, ethers, glycols, ketones, IV-methylolamides and oximes. Consequently, an enormous number of examples is reported and only a representative selection can be presented here. A comprehensive listing of examples reported up to 1966 is provided in the review by Krimen and Cota. ... 

Padmanabhan, S., Nicholas, K. M. Carbon-13 NMR study of (propargyl)dicobalt hexacarbonyl cations a structurally unique class of metal-stabilized carbenium ions. J. Organomet. Chem. 1984, 268, C23-C27. 

Recently, Chen has synthesized and resolved chiral suberyl carbenium ions and utilized these as catalysts for enantioselective Mukaiyama aldol addition reactions (Eq. (8.22)) [34]. Thus the reaction of the ethyl acetate-derived silyl ketene acetal with benzaldehyde in the presence of 10-20 mol% of catalyst afforded the corresponding adduct in 50% ee. The enantioselectivity of the process proved sensitive to the nature of the cation, consistent with observations previously highlighted by Denmark in related studies [35]. Although at the current level of development the selectivities are modest, the study documents a novel class of metal-free Lewis acidic agents. 

The two classes of carbocations are not mutually exclusive but represent the limits of a spectrum of carbocations with various types and degrees of charge delocalization. The interaction of neighboring groups with the vacant p-orbital of a carbenium ion center can contribute to the ions stability. This may involve the interaction of unshared electron pairs (n-donors), bent... 

In less than 10 years, organosilanes have emerged as an unusually versatile class of nucleophiles for terminating Mannich-type cyclizations. Their utility derives from the ability of the silicon substituent to control the regioselectivity of bond formation and dictate the postcyclization destiny of the carbenium ion intermediate produced upon cyclization. The characteristics of silicon that are responsible for this exceptional control have been discussed in several recent reviews. ... 

Arenium ions represent a large class of organic cations—carbenium ions that have been intensively studied in recent years. Thus, benzenium ions can be regarded as derivatives of cyclohexadienyl cations (1) belonging to the group of cyclic enyl cations with the open rc-electron system including also the derivatives of cychj-pentenyl (2), cyclobutenyl (3), cycloheptadienyl (4) and other analogous cations... 

Protonated saturated hydrocarbons contain a carbon atom which is formally 5-coordinate. There are insufficient electrons to describe such a carbocation by a classical structure and it is necessary to invoke a 3-centre 2-electron bond. This family of non-classical alkonium ions are generally known as carbonium ions. Removal of H2 from an alkonium ion leaves a carbocation containing a 3-coordinate carbon. These trivalent alkyl cations are known as carbenium ions. The interrelation between the two classes of carbocations is shown for the parent ions in equation 2. 

Nitrogen-containing compounds form an important class of N-bases due to their role in life processes. This is one of the reasons for our interest in reactions of free carbenium ions with amines [1-3]. In the course of these studies we found that, in contrast to the other N-bases, the interaction of carbocations with amines occurs via two competing channels, i.e. the formation of the condensation complex as a result of the overlap of the vacant p-orbital of the cation with the lone pair orbital of nitrogen and the proton transfer from the carbenium cation to the amine. The latter channel is very effective, due to the high proton affinities of amines. 

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