Carbonium ions reviews

The study of the carbonylation of carbonium ions, as summarized in this Review, has afforded valuable information on a number of problems. 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

Among the various intermediate species that may participate in a reaction sequence are stable molecules, ions, free atoms, free radicals, car-banions, carbonium ions, molecular and ionic complexes, and tautomeric or excited forms of stable molecules. If the intermediate is, indeed, a stable substance, then its presence can be detected by any of the standard techniques of chemical analysis, provided that the intermediate can be isolated (i.e., prevented from participation in the processes that would normally follow its formation). If isolation is impossible, then the techniques available for the study of stable intermediates are the same as those for the study of highly reactive species. For a detailed discussion of appropriate experimental techniques, consult the references listed in Section 3.2.2 or the review by Wayne (1). 

It needs to be said at the outset that my attempts at clarification have not been made easier by the discovery [4] of the pseudocationic polymerizations early in 1964. Since exploration and revaluation of these reactions are still only in their early stages, there are inevitably many loose ends and open questions and probably also some inconsistencies in the present work. Some aspects of pseudocationic polymerization have been reviewed [5-7]. It should be noted that this discovery makes many of the theoretical discussions in Reference 1 of purely historical interest. Since the publication of Reference 1 several reviews on, and relevant to, cationic polymerization [8] and on carbonium ions [9] have appeared. 

Five years ago a brief review focused on the applications of nuclear magnetic resonance (nmr) as a method for determining charge density in carbonium ions and pointed out some of the precautions required (Fraenkel and Famum, 1968). Since then, proton nmr (pnmr), which was emphasized in that review, has continued to attract primary attention as a probe into the structure and charge density of organic cations and anions (Olah and Schleyer, 1968,1970, 1972, 1973 Oth ef al., 1972 Takahashi et a/., 1973 van... 

Infra-red studies [31] and molecular orbital calculations [33,34] have led to the description of the guanidinium ion as a tri-amino carbonium ion with the TT-electron charge distribution shown (IX) Most of the positive charge is located in the vicinity of the central carbon atom. The relevance of this description to the pharmacological properties of guanidinium ions will be discussed later. For typographical convenience, guanidines will be formulated in this review in the unprotonated form. 

Although no mechanistic studies of this hydrolysis have been reported, the data available suggest that the mechanism is similar to that proposed for the hydrolysis of a-D-glucopyranosyl phosphate in weakly acidic media,332 namely protonation of the glycosyl pyrophosphate derivative and slow heterolysis of the resulting monoanion (79) to produce a cyclic carbonium ion (80), a species considered to be an intermediate in the hydrolysis of glycosides (for a review, see Ref. 333). 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

For reviews of organic compounds protonated at O, N, or S, sec Olah White O Brien Chem. Rev. 1970. 70. 561-591 Olah White O Brien, in Olah Schleycr Carbonium Ions, vol. 4 Wiley New York, 1973, pp. 1697-1781. 

For a review, sec Fort, in Olah Schleyer Carbonium Ions, vol. 4 Wiley New York, 1973. pp. 1783-1835. 

For monographs, see Olah Schleyer Carbonium Ions, vol. 3 Wiley New York, 1972 Bartlett Nonclassical Ions W.A. Benjamin New York, 1965. For reviews, see Barkhash Top. Curr. Chem. 1984, 1161117, 1-265 Kirmse Top. Carr. Chem. 1979, 80, 125-311, pp. 196-288 McManus Pittman, in McManus Organic Reactive Intermediates Academic Press New York, 1973, pp. 302-321 Bethell Gold Carbonium Ions Academic Press New York. 1967 pp. 222-282. For a related review, see Prakash Iyer Rev. Chem. Interned. 1988, 9, 65-116. 

For reviews of arenium ions formed by addition of a proton to an aromatic ring, sec Brouwer Mackor MacLcan. in Olah Schleyer Carbonium Ions. vol. 2 Wiley New York. 1970, pp. 837-897 Pcrkampus Adv. Phvs. Org. Chem. 1966, 4. 195-304. 

For reviews of electrophilic addition to alkynes, including much evidence, see Rappoport React. Interned. (Plenum) 1983, 3, 427-615, pp. 428-440 Stang Rappoport Hanack Subramanian Vinyl Cations, Academic Press New York. 1979. pp. 24-151 Stang Prog. Phys. Org. Chem. 1973, 10. 205-325 Modena Toncllato Adv. Phys. Org. Chem. 1971, 9, 185-280. pp. 187-231 Richey Richey, in Olah Schleycr Carbonium Ions, vol. 2 Wiley New York, 1970. pp. 906-922. 

Studies reported on the hydrocarbon monomers show that there are three main areas of ionicities which produce different initiation, termination and termination reactions. The strong cationic systems involve the transfer or elimination of protons or carbonium ions. This has been well reviewed by Kennedy and Langer (1). At the other extreme, strong anionic systems react by hydride transfer. For the olefinic monomers, this region extends to include alkyl aluminum which undergo easy exchange to produce dimers (72). 

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