Carbocation higher-coordinate

On the basis of my extensive study of stable, persistent carbocations, reported in more than 300 publications, I was able to develop the general concept of carbocations referred to in Chapter 9. Accordingly, in higher-coordinate (hypercoordinate) carbonium ions, of which pro-... 

The remarkable stability of the gold complexes is due to significant metal-metal bonding. However, their isolation and structural study are remarkable and greatly contributed to our knowledge of higher-coordinate carbocations. 

Carbocations are central to hydrocarbon chemistry (/). Much of this chemistry is based on acid catalysis, which leads to generation of positive ions of carbon. The resulting intermediates are classified as carbenium and carbonium ions, as proposed by Olah (2-4). Carbonium ions are the penta- or higher coordinate carbocations that maintain 8 valence electrons via 2-electron/3-center bonding, quite different from carbenium ions that possess only 6 valence electrons. Figure 1 shows a systematic classification of 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. 

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). 

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... 

An empirical criterion based on additivity of 13C NMR chemical shifts for distinguishing classical trivalent and higher coordinate carbocations has been developed by... 

Schleyer, Olah, and co-workers.55 In this method, the sums of all 13C chemical shifts of carbocations with their respective hydrocarbon precursors are compared. A trivalent carbocation has a sum of chemical shifts of at least 350 ppm higher than the sum for the corresponding neutral hydrocarbon. This difference can be rationalized by partly attributing it to the hybridization change to sp2 and to the deshielding influence of an unit positive charge in the trivalent carbocation. Since higher coordinate carbocations (nonclassical ions) have penta- and hexa-coordinate centers, the sum of their chemical shifts relative to their neutral hydrocarbons is much smaller, often by less than 200 ppm. 

The involvement of intermediates and transition states containing higher coordinate carbon has been originally proposed by Olah in both electrophilic reactions involving electron-deficient systems such as carbocations, heterocations, carbenes, nitrenes, silylenes, coordinatively unsaturated metal compounds, and so on, as well as in nucleophilic Sn2 reactions by Ingold. Whereas in electrophilic reactions, the pentacoordinate carbocationic centers are associated with eight electrons and thus can be intermediates (albeit... 

Cycloalkanes also undergo related acid-catalyzed rearrangements. Higher coordinate carbocations again play a key role in the mechanism as shown in the cyclohexane-methylcyclopentane rearrangement. ... 

Olah GA. Higher coordinate (hypercarbon containing) carbocations and their role in electrophilic reactions of hydrocarbons. Pure Appl Chem 1981 53 201. 

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... 

In general, acetylenes are more reactive than olefins in a coordination reaction since the former have stronger coordinating ability, while vice versa in an cationic reaction because of the higher stability of carbocations formed from olefins. In the copolymerization of phenylacetylene with styrene by WC16—Ph4Sn, essentially only phenylacetylene polymerizes 86). This result supports the idea that the present polymerization is a sort of coordination reaction. 

Unlike mononuclear carbonylates, reaction of carbocation reagents with anionic polynuclear carbonylates often results in coordination at the bridging carbonyl oxygen rather than at the metal center. This type of interaction was first observed for the reaction of [HFe3(CO)II] with CH3S03F (88) [Eq. (12)], and a higher yield route is indicated in Eqs. (13) and (14). The X-ray crystal structure of this molecule is shown in Fig. 13. 

---