Carbocations five-coordinate

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

Some of the most important reaction intermediates in organic chemistry are the carbocations. Neglecting some heteroatom-stabilized cations, most carbocations are divided into two groups trivalent carbenium ions and five-coordinate or higher coordinate carbonium ions. The parent carbenium ion is CHJ, and the parent carbonium ion is CHJ. Carbonium ions have been proposed as reactive intermediates in superacid-catalyzed reactions however, they have never been directly observed in condensed media. In contrast, a variety of carbenium ions have already been prepared in superacidic media and been characterized by various physical methods, mainly 13C NMR spectroscopy (5). 

Protolytic reactions of saturated hydrocarbons in superacid media21 were interpreted by Olah as proceeding through the protonation (protolysis) of the covalent C—H and C—C single bonds. The reactivity is due to the electron donor ability of the <7 bonds via two-electron, three-center bond formation. Protolysis of C—H bonds leads via five-coordinate carbocations with subsequent cleavage of H2 to trivalent ions, which then themselves can further react in a similar fashion ... 

There is clear differentiation of the alkylation of alkenes (jt-donor nucleophiles) and alkanes (a donors). The former follows Markovnikov addition, giving a trivalent carbocation and derived branched products. The latter proceeds through a five-coordinate carbocation without involvement of trivalent carbenium ions and thus without necessary branching. 

The hydrogen-bridged carbocation (B) contains a two-coordinate hydrogen atom. Hypercoordination— which includes two-coordination for hydrogen and at least five-coordination for carbon—is generally... 

A carbocation, real or hypothetical, that contains at least one five-coordinate carbon atom. 

Cations are kinetic chain carriers in cationic polymerizations. Such cations may be, for example, carbocations or oxonium ions. All electrophilic carbon atoms may be described as carbocations. These can be classified as carbenium ions (trivalent carbocations) and as carbonium ions (carbocations with coordination numbers of four or five). Carbenium ions such as, for example, R3C are classical carbocations. Carbonium ions such as, for example, R5C or R5C2 are nonclassical ions. 

More recently it was realized that hydrocarbon ions (carbocations) also encompass five (or higher) coordinate carbonium ions for which CH5+ is parent.94 Alkanes having only saturated C—H and C—C bonds were found to be protonated by very strong acids, specifically, superacids, which are billions of times stronger than concentrated sulfuric acid.21... 

This chapter begins with a short historic retrospect about the development of the carbocation concepts and covers the techniques used for their generation, observation, and characterization under superacidic long-lived conditions. This is followed by an extensive coverage of the multitude of trivalent (classical) and equilibrating (degenerate) and higher (five or six) coordinate (nonclassical) carbocations. 

EQUILIBRATING (DEGENERATE) AND HIGHER (FIVE OR SIX) COORDINATE (NONCLASSICAL) CARBOCATIONS... 

A carbocation, real or hypothetical, whose structure cannot adequately be described by two-electron two-center bonds only. (The structure may involve carbon atoms with a coordination number greater than five.)... 

The second class of carbocations contains one or more hypercoordinate carbon atoms. These hypercarbons are coordinated to five or more atoms within reasonable bonding distance. Hypercoordinate or nonclassical carbocations cannot be described by using 2c-2e single bonds alone, but necessitate the involvement of three- (or multi-) center, two-electron bonds, e.g., 3c-2e bonds. Each hypercarbon in the cation is always associated with eight electrons, although the ion, overall, is electron deficient. The methonium ion (Cd I5 ) may be considered the parent of the hypercoordinate carbocations (Fig. 5.1). 

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