Pentacoordinate carbocations,

The prototypical pentacoordinated carbocation, the parent nonclassical carbocation, CH, 1, has been extensively studied in the gas phase as well as by theoretical calculations.Among the higher coordinate carbocations, the hexacoordinated methylium dication, 2, and heptacoordinated trication, 3, were also shown to be energy minima by ab initio calculations. High-level ab initio calculations show that they have C2V and Csv structures, respectively.Their isoelectronic boron analogs were also studied. 

Other mechanisms with the involvement of an incipient methyl cation were also proposed.418,460,462 An attack of methyl cation released from extensively polarized methoxy groups on the carbon-hydrogen bond forms pentacoordinated carbocation intermediates that, in turn, yields ethyl methyl ether after the loss of a proton ... 

In superacids protonation of isobutane leads to pentacoordinate carbocations, the interconversion of which scrambles all hydrogens inclusive of the methine... 

Cram s original studies287 established, based on kinetic and stereochemical evidence, the bridged ion nature of (3-phenylethyl cations in solvolytic systems. Spectroscopic studies (particularly1H and 13C NMR)288-291 of a series of stable long-lived ions proved the symmetrically bridged structure and at the same time showed that these ions do not contain a pentacoordinate carbocation center (thus are not nonclassical ions ). They are spiro[2.5]octadienyl cations 111 (spirocyclopropylbenzenium ions)—in... 

Protonated alkanes (C H2 , 3 ) also play a significant role in alkane reactions. Saturated hydrocarbons can be protonated to alkonium ions, of which the methonium ion CH5+, 422 [Eq. (3.119)] is the parent, and formation of these pentacoordinate carbocations involves two-electron three-center (2e—ic) bonds. The dotted lines in the structure symbolize the bonding orbitals of the three-center bonds [Eq. (3.119)] their point of junction does not represent an additional atom. 

Superacid-catalyzed alkylation of adamantane with lower alkenes (ethene, propene, isomeric butenes) has been investigated by Olah et al.151 in triflic acid and triflic acid-B(0S02CF3)3. Only trace amounts of 1 -ferf-butyladamantane (37) were detected in alkylation with 1- and 2-butenes, whereas isobutylene gave consistently relatively good yield of 37. Since isomerization of isomeric 1-butyladamantane under identical conditions did not give even traces of 37, its formation can be accounted for by (r-alkylation, that is, through the insertion of the ferf-butyl cation into the C—H bond (Scheme 5.22). This reaction is similar to that between ferf-butyl cation and isobutane to form 2,2,3,3-tetramethylbutane discussed above (Scheme 5.21). In either case, the pentacoordinate carbocation intermedate, which may also lead to hydride transfer, does not attain a linear geometry, despite the unfavorable steric interactions. 

The nature of the spiro carbon atom is of particular importance in defining the carbocation nature of the ions. CMR spectroscopic studies clearly established the aliphatic tetrahedral nature of this carbon, thus ruling out a nonclassical pentacoordinated carbocation. 

On the other hand pentacoordinated carbocations play an equally important role in electrophilic reactions of o-donor saturated systems. 

The direct reduction of SbFs in the absence of hydrocarbon by molecular hydrogen necessitates, however, more forcing conditions (50 atm, high temperature), which suggests that the protolytic ionization of alkanes proceeds probably via solvation of the pentacoordinate carbocation by SbFs and concurrent ionization-reduction [Eq. (6.13)]. 

The diazomethane insertion into ketones and aldehydes to provide their homologs also involves migration of an alkyl group to an electron-deficient carbon center of a zwitterion through a pentacoordinate carbocation according to the mechanism depicted in Equation (6.29). 

The superacid-catalyzed alkylation of benzene with alkanes was also achieved. Alkyl cation formation for the required electrophilic attack again involves protolytic ionization of alkanes via pentacoordinate carbocations [Eq. (6.37)]. Sperenza and coworkers,on the other hand, have shown that phenyl cations generated in the gas phase readily insert into the C-H bonds of simple alkanes to provide the corresponding alkylated aromatics [Eq. (6.38)]. 

The nitronium ion is also capable of acting as an oxidizing agent effecting hydride abstraction from a variety of functionalized alkanes. The oxidation of diarylmethyl methyl ethers is best illustrated involving pentacoordinate carbocations [Eq. (6.44)]. 

The nitrosonium ion (NO ), the electrophilic species formed in nitrous acid media, is also a particularly effective hydride abstracting agent. Cumene reacts with NO+ to give various condensation products that involve intermediate formation of the cumyl cation. °The formation of the cumyl cation in a nonlinear hydride transfer reaction involves a pentacoordinate carbocation [Eq. (6.45)]. 

In contrast to NO, with superelectrophilic NOH and other hard electrophiles (H, I), (1 Ii, (. If . OH, F ), methane preferentially reacts similarly to its protonation to give CH5 or by hydride abstraction. These studies have reaffirmed the general pattern of electrophilic substitution of methane with strong electrophiles, which proceed via pentacoordinate carbocations involving 3c-2e C-H bonds, as previously suggested by Olah. 

Even lithiated pentacoordinate carbocations CH Li (n — 0-3) could be detected by flash vaporization mass spectroscopythe stability of which had been predicted by Schleyer and coworkers... 

Tricoordinate carbocations are customarily called carbonium ions for a clear discussion of terminology and a suggestion favoring the view that the term carbonium ion be reserved for pentacoordinate carbocations, with tricoordinate carbocations being referred to as carbenium ions, see G. A. Olah, J. Am. Chem. Soc. 94, 808 (1972). 

Olah s investigation of norbomyl cation provided him with an example illustrating a concept he had advanced in an independent line of research, namely, formation of pentacoordinate carbocation intermediates by attack of electrophiles on C-C and C-H bonds. Space does not permit discussion of the impressive experimental work associated with this concept, but it established that species analogous to the well-known CHs, so common in electron impact mass spectrometry, could be generated in solution. Attack of an electrophile on a C-H bond probably involves a triangular transition state, and results in hydride abstraction if a stable tricoordinate carbocation can be produced. Hydride transfer reactions between carbonium ions are quite common in media of low nucleophilicity ... 

Electrophilic attack on C-C bonds is less common, and most studies have been concerned with strained C-C bonds. These studies have established that cleavage of strained C-C bonds is a major reaction of such systems with electrophiles. The existence of the nonclassical pentacoordinate ion in SbFs-SOa requires that it be more stable than the classical ion. It is reasonable that the nonclassical pentacoordinate carbocation structure of the norbomyl cation could be more stable than the classical tricoordinate carbocation structure. The classical structure incorporates an electrophilic carbon at C(2) in a favorable geometric relationship for interaction with the strained C(l)-C(6) bond. 

The reaction was first described for pinacol (2,3-dimethyl-2,3-butanediol), a ditertiary 1,2-diol and catalyzed by sulfuric acid (73). Now it is known that the pinacol rearrangement is characteristic of all types of 1,2-diols, and most electrophilic catalysts are capable of promoting the process. Two possible mechanisms may account for the experimental observations. The stepwise mechanism involves the )8-hydroxy carbocationic intermediate 6 and the pentacoordinate carbocation 7 (Scheme 1). According to the concerted mechanism, the product carbonyl compoimd is formed through the hypercoordinate intermediate 8. 

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