Carbocation concept, development

Interest within the physical organic community on the mechanism for the formation and reaction of ion-pair and ion-dipole intermediates of solvolysis peaked sometime in the 1970s and has declined in recent years. The concepts developed during the heyday of this work have stood the test of time, but these reactions have not been fuUy characterized, even for relatively simple systems. Richard and coworkers have prepared a short chapter that summarizes their recent determinations of absolute rate constants for the reactions of these weak association complexes in water. This work provides a quantitative basis for the formerly largely qualitative discussions of competing carbocation-nucleophile addition and rearrangement reactions of ion and dipole pairs. 

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. 

Development of the Carbocation Concept Early Kinetic and Stereochemical Studies... 

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-... 

History of physical organic chemistry is essentially the history of new ideas, philosophies, and concepts in organic chemistry. New instrumentations have played an essential role in the mechanistic study. Organic reaction theory and concept of structure-reactivity relationship were obtained through kinetic measurements, whose precision depended on the development of instrument. Development of NMR technique resulted in evolution of carbocation chemistry. Picosecond and femtosecond spectroscopy allowed us to elucidate kinetic behavior of unstable intermediates and even of transition states (TSs) of chemical reactions. 

Aldol reactions using a carbocation as an organocatalyst An organocatalytic aldol reaction based on a different concept was developed by the Chen group. The chiral triarylcarbenium ion 34 was used as a novel non-metallic Lewis acid catalyst in a Mukaiyama-type aldol reaction which led to enantiomerically enriched aldol products (Scheme 6.17) [67]. Although non-chiral trityl salt-mediated catalytic aldol reactions had previously been reported by Mukaiyama and co-workers [68], the construction of a suitable chiral carbenium ion remained a challenge. Optically active salts of type 34 were synthesized as Lewis acids based on a reactive carbe-... 

The concept of metabolic activation developed with AAF was then apphed to PAHs, aflatoxins, nitrosamines, nitrosoureas, hydrazines, urethane, and vinyl chloride. Several metabolic activation schemes are presented in Figure 6.2. In each case a highly reactive electrophilic carbocation is formed. We now know that the concept of metabolic activation applies to many genotoxic carcinogens and helps to explain... 

The energy required to proceed from reactants to products is AG, the free energy of activation, which is the energy at the transition state relative to the reactants. We develop the theoretical foundation for these ideas about reaction rates in Section 3.2. We first focus attention on the methods for evaluating the inherent thermodynamic stability of representative molecules. In Section 3.3, we consider general concepts that interrelate the thermodynamic and kinetic aspects of reactivity. In Section 3.4, we consider how substituents affect the stability of important intermediates, such as carbocations, carbanions, radicals, and carbonyl addition (tetrahedral) intermediates. In Section 3.5, we examine quantitative treatments of substituent effects. In the final sections of the chapter we consider catalysis and the effect of the solvent medium on reaction rates and mechanisms. 

Although the concept of the relative stabilities of carbocations had not been developed in Markovnikov s time, their relative stabilities are the underlying basis for his rule that is, the proton of H—X adds to the less substituted carbon of a double bond because this mode of addition produces the more stable carbocation intermediate. 

The crucial step in acid-catalyzed conversions of hydrocarbons is the formation of the intermediate trivalent or classical sp hybridized carbocation (car-benium ion). In the case of saturated hydrocarbons, this is interpreted by the interaction of the proton of the superacid and the bonding electron pair of the C—H a bond (a similar interaction between the proton and the C—C a bond would result in the cleavage of the carbon-carbon bond). This is based on the concept of a-basicity developed by Olah (65), which describes the ability of a bonds to share their bonded electrons with electrophiles. A hypervalent, pentacoordinate non-classical carbocation (carbonium ion) is formed, which possesses a three-center, two-electron (3c-2e) bond. This is transformed to the classical trivalent carbocation by the loss of hydrogen, that is, protolysis (protolytic cleavage) of the carbon-hydrogen bond occurs. The process is illustrated by the conversion of hexane to yield the 3-hexyl cation through the pentacoordinate carbonium ion (1) (eq. 45). 

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