Page 886 - Advanced Organic Chemistry Part B - Reactions & Synthesis

862 + + .
C + C C C C C or C + C C C C C .
CHAPTER 10
Reactions Involving
Carbocations, Carbenes, Owing to the low barriers to bond formation, reactant conformation often plays a
and Radicals as Reactive
Intermediates decisive role in the outcome of these reactions. Carbocations, carbene, and radicals
frequently undergo very efficient intramolecular reactions that depend on the proximity
of the reaction centers. Conversely, because of the short lifetimes of the intermediates,
reactions through unfavorable conformations are unusual. Mechanistic analyses and
synthetic designs that involve carbocations, carbenes, and radicals must pay particularly
close attention to conformational factors.

10.1. Reactions and Rearrangement Involving Carbocation
Intermediates

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.

10.1.1. Carbon-Carbon Bond Formation Involving Carbocations

10.1.1.1. Intermolecular Alkylation by Carbocations. The formation of carbon-carbon
bonds by electrophilic attack on the system is a very important reaction in aromatic
chemistry, with both Friedel-Crafts alkylation and acylation following this pattern.
These reactions are discussed in Chapter 11. There also are useful reactions in which
carbon-carbon bond formation results from electrophilic attack by a carbocation on
an alkene. The reaction of a carbocation with an alkene to form a new carbon-carbon
bond is both kinetically accessible and thermodynamically favorable.

+ +
C + C C C C C

There are, however, serious problems that must be overcome in the application of this
reaction to synthesis. The product is a new carbocation that can react further. Repetitive
addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of
acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement.
A key requirement for adapting the reaction of carbocations with alkenes to the
synthesis of small molecules is control of the reactivity of the newly formed carbo-
cation intermediate. Synthetically useful carbocation-alkene reactions require a suitable
termination step. We have already encountered one successful strategy in the reaction
of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9).
In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is
formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also
favorable to the synthetic utility of carbocation-alkene reactions because the reactants
are more nucleophilic than the product alkenes.