Carbocations initiating

Carbocations initially formed upon addition of an electrophile to an alkene may be able to undergo skeletal rearrangement depending on whether or not a more stable cation exists and, if it does exist, whether or not it can be reached via a low-energy pathway. Consider addition of HBr to 3-methyl-1-butene, the product of which is 2-methyl-2-butyl bromide. 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

When the carbocation initially formed in the addition of HX to an alkene can rearrange to a more stable one => rearrangements invariably occur. 

The intermediate carbocation initially appears secondary, but it is also benzylic and, therefore, resonance stabilized by the aromatic ring. 

The initiation with some carbocations, especially trityl, does not involve direct addition to monomer. The carbocation abstracts a hydride ion from the a-carbon of monomer and the newly formed carbocation initiates polymerization [Afsar-Taromi et al., 1978 Kuntz, 1967]. This hydride ion abstraction is so facile with 1,3-dioxolane that it is used to preform stable l,3-dioxolan-2-ylium salts (XII) that can be used subsequently as initiators [Jedlinski et al., 1985],... 

The polymeric carbocation initiates polymerization of isobutylene, styrene, tetrahydrofuran, and other monomers [Cai and Yan, 1987 Cameron and Sarmouk, 1990 Kennedy and Ivan, 1992 Martinez et al., 2001 Pi and Kennedy, 2001]. 

The isobutyl carbocation, initially formed when l-chloro-2-methylpropane and AICI3 react, rearranges via a hydride shift to give the more stable tert-butyl carbocation, which can then alkylate benzene to form rert-butylbenzene. 

Propagation according to scheme (168a) is typical of carbocation initiators both modes were observed with acylium (RC+=0) initiators. During chain growth from y -propiolactone, the concentracion of acylium ions decreases until oxonium ions become the active centres. In e-caprolactone polymerization, both types of centre continue to operate. 

Keating and Skell" and later Laurent and Thomalla" have found that a similar mixture of products was obtained in the anodic oxidation of either cyclobutanecarboxylic acid, allylacetic or cyclopropaneacetic acid. They accounted for these results by a rapid equilibria among the carbocations initially formed. A similar observation is that in the anodic oxidation of the corresponding iodo reactants in acetonitrile N-(cyclopropylmethyl)acetamide was found in each product mixture". It is noteworthy that the same identity in products also holds for the chemical reactions of solvolysis and... 

Formation of ring junctions with substrates containing an (Ej-alkene in a 5,6-relationship to the carbocation initiator leads to the thermodynamically more stable transfused rings, as exemplified by the cyclization of diene A to the trans-bicyclic product B. Interestingly, the same product B was obtained when the (Z)-isomer C was subjected to the same cyclization conditions. It appears that both dienes A and C cyclize initially to a common monocyclic intermediate E. Cyclization of E and trapping of the incipient cation by formic acid proceeds in a rrans-manner to give the bicyclic formate ester F, which is hydrolyzed under the reaction conditions to the alcohol B. ... 

The 1° carbocation initially formed is very unstable some chemists believe that rearrangement occurs at the same time as the leaving group leaves. At most, the 1° carbocation has a very short lifetime. 

With some cationic initiators, such as the triphenyl methyl (trityl) salts of the general formula PhsC " X , where X is a stable anion such as CIO4, SbClg, and PFg, the initiation does not involve direct addition to monomer. The carbocation abstracts a hydride (H ) ion from the a-carbon of the monomer and the resulting new carbocation initiates polymerization. Thus the reaction of trityl hexafluo-rophosphate and tetrahydrofuran has been shown to proceed as follows (Cowie, 1991) ... 

Acid-butanol assay uses an acid-catalyzed oxidative depolymerization of coti-densed tannins to yield red anthocyanidins. Proanthocyanidins depolymerize when treated in hot mineral acid to produce colored anthocyanidins with an absorbance maximum around 550 nm [88,89]. The anthocyanidins result from the autoxidatimi of the carbocations initially formed by cleavage of the interflavonoid bond. Irmi sulfate can be added to the reagent to accelerate the autoxidation reaction that increases color intensity and consistency of anthocyanidin yield [90]. 

If the carbocation initially formed can rearrange to generate a more stable carbocation, then a rearrangement will occur. 

Why does the carbocation initially formed in the final reaction of Figure 10.28 not rearrange to give a tertiary cation ... 

We have two effects to consider—inductive and resonance. Halogens are inductive withdraw-ers of electron density, but they also have lone pairs that can assist in stabilizing a carbocation. Initially, you may feel uncomfortable with the idea that halogens can be resonance donors, but it should not be so surprising, as we have seen it before when we considered the addition of bromine... 

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