Nonclassical carbocations,

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

SECTION 5.12. THE NORBORNYL CATION AND OTHER NONCLASSICAL CARBOCATIONS... 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations. ° If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown in the above examples, is a discrete reaction intermediate. If a carbon-carbon double or single bond participates in the departure of the leaving group to form a carbocation, it may be that a nonclassical carbocation is involved, but there is no necessary relation. In any particular case, either or both of these possibilities can be taking place. 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

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. 

In discussing nonclassical carbocations we must be careful to make the distinction between neighboring-group participation and the existence of nonclassical carbocations.95 If a nonclassical carbocation exists in any reaction, then an ion with electron delocalization, as shown... 

In the following pages we consider some of the evidence bearing on the questions of the participation of tt and a bonds and on the existence of nonclassical carbocations,96 though a thorough discussion is beyond the scope of this book.78... 

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. 

Core electron spectroscopy for chemical analysis (ESCA) is perhaps the most definitive technique applied to the differentiation between nonclassical carbocations from equilibrating classical species. The time scale of the measured ionization process is of the order of 10 16 s so that definite species are characterized, regardless of (much slower) intra- and intermolecular exchange reactions—for example, hydride shifts, Wagner-Meerwein rearrangements, proton exchange, and so on. 

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

The nonclassical carbocation problem and the inclusion of solvent effects... 

The presence of a transition metal is not necessarily required for hydrocarbon insertion. Alkyne incorporation has been reported for boracyclobutenes, as well as metallacyclobutene complexes of the transition elements. Boracyclobutene 51, a reactive intermediate prepared in situ (Section 2.12.9.2.1), inserts an additional equivalent of trimethylsilylacetylene into the B-C(sp2) bond to yield boracyclohexadiene 52 (Scheme 7). This isomerizes to the interesting bridged compound 53, an analogue of a nonclassical carbocation <1994AGE2306>. The related boracyclobutene 7 inserts the alkyne to yield a persistent boracyclohexadiene 54, but this product clearly arises from insertion into the boracyclobutene carbon-carbon bond rather than a boron-carbon bond <1994AGE1487>. 

The electron counting rules of Wade (S3), Williams (117), and Rudolph (118) can serve as a useful concept to explain structure and bonding in a variety of systems which at first glance are very different Zintl phases, boranes and carboranes, transition metal n complexes and carbonyl clusters, nonclassical carbocations, and also n complexes of main-group elements. According to... 

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