Carbocation reactions

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

Figure 5.7 An energy diagram for the overall reaction of ethylene with HBr. Two separate steps are involved, each with its own transition state. The energy minimum between the two steps represents the carbocation reaction intermediate.

In this section, the emphasis is on carbocation reactions that modify the carbon skeleton, including carbon-carbon bond formation, rearrangements, and fragmentation reactions. The fundamental structural and reactivity characteristics of carbocations toward nucleophilic substitution were explored in Chapter 4 of Part A. 

Chapter 10 considers the role of reactive intermediates—carbocations, carbenes, and radicals—in synthesis. The carbocation reactions covered include the carbonyl-ene reaction, polyolefin cyclization, and carbocation rearrangements. In the carbene section, addition (cyclopropanation) and insertion reactions are emphasized. Recent development of catalysts that provide both selectivity and enantioselectivity are discussed, and both intermolecular and intramolecular (cyclization) addition reactions of radicals are dealt with. The use of atom transfer steps and tandem sequences in synthesis is also illustrated. 

Disubstituted Alkenes. Simple 1,2-disubstituted alkenes such as 2-octene or cyclohexene, which produce only secondary aliphatic carbocation reaction intermediates, do not undergo reduction upon treatment with a Brpnsted acid and an organosilicon hydride. Even when extreme conditions are employed, only traces of reduction products are detected.192 203 207-210,214 An exception is the report that 4-methyl-2-pentene forms 2-methylpentane in 70% yield when heated to 50° for 20 hours with a mixture of Et3SiH/TFA containing a catalytic amount of sulfuric acid. It is believed that 4-methyl-2-pentene is isomerized to 2-methyl-2-pentene prior to reduction.203... 

In the case of primary carbocation, reaction with nitrile oxide gives a mixture of two regioisomeric oximes (Scheme 1.41). Probably, this is a result of the attack of the nitrile oxide - BF3 complex on neutral 3-chloromethylanisole. 

Although the geometric relationship suggested by Shiner and by Sunko and their co-workers clearly demonstrates that hyperconjugation is the major contributor to the secondary /3-deuterium KIE in carbocation reactions, Williams (1985) has suggested that there is a significant inductive component to these KIEs. Williams used ab initio MO methods to calculate the geometries of the substrates and the isopropyl carbocation formed in a gas-phase heterolysis (30) of series of isopropyl derivatives at the RHF/4-31G level. 

Azide ion is a modest leaving group in An + Dn nucleophilic substitution reactions, and at the same time a potent nucleophile for addition to the carbocation reaction intermediate. Consequently, ring-substituted benzaldehyde g m-diazides (X-2-N3) undergo solvolysis in water in reactions that are subject to strong common-ion inhibition by added azide ion from reversible trapping of an o -azido carbocation intermediate (X-2 ) by diffusion controlled addition of azide anion (Scheme... 

Figure 2.3 may be used to illustrate the following progression in pathways for nucleophilic substimtion at X-l-Y and X-2-Y with changes in the lifetime of the respective carbocation reaction intermediates. 

Stepwise solvolysis of chiral substrates through a planar achiral carbocation reaction intermediate normally results in the formation of racemic products. However, the solvolysis of chiral tertiary derivatives 6-Y proceeds with either... 

The stepwise formulation explains why boron becomes attached to the less-substituted carbon, but does not account for the fact that the reactions show no other characteristics of carbocation reactions. This could be because of an expected, extraordinarily fast rate of hydride-ion transfer to the carbocation. A more serious objection to the stepwise mechanism is that alkynes react more rapidly than alkenes, something which normally is not observed for stepwise electrophilic additions (cf. Section 10-5). 

With propanamine, loss of nitrogen from the diazonium ion gives the very poorly stabilized propyl cation, which then undergoes a variety of reactions that are consistent with the carbocation reactions discussed previously (see Sections 8-9B and 15-5E) ... 

Richard has also shown that intrinsic barriers for carbocation reactions depend not only on the extent of charge delocalization but to what atoms the charge is delocalized. In a case where values of pifR for calculation of A were not available, comparisons of rate constants for attack of water kH2o with equilibrium constants for nucleophilic reaction with azide ion pKAz for 65-67 showed qualitatively that delocalization to an oxygen atom leads to a lower barrier than to an azido group which is in turn lower than to a methoxyphenyl substituent.226... 

In addition to the work on carbocation reactions already described Mayr,28 together with Richard and McClelland, has been concerned with problems raised by the lack of dependence of selectivity on reactivity apparent both from the normal constancy and specific variations in values of the slope parameters, sN (and, vh ).2x We conclude this chapter, therefore, with a brief discussion of... 

A particular difficulty arises for the comparison of hard and soft nucleophiles. This difficulty indeed is amplified if one goes beyond carbocation reactions to consider softer or harder electrophilic centers, such as transition metals or protons. Interpreting differences between reacting atoms presents an ultimate challenge for attempts to understand reactivity. Richard has gone a considerable way toward offering a rational analysis of the principal factors to be considered in such an endeavor. However, this is one issue likely to attract attention in the next one hundred years of carbocation chemistry and in the wider field of electrophile nucleophile combination reactions. 

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