Cyclic carbocation

A comparison of rate and equilibrium constants for partitioning of the cyclic carbocation [18+] with those for the l-(4-methylphenyl)ethyl carbocation Me-[6+] (Table 5) shows that placement of the cationic benzylic carbon in a five-membered ring results in the following complex changes in the reactivity of the carbocation towards deprotonation and nucleophilic addition of solvent (Scheme 15). 

Hydride transfer from alkenes was also proposed to occur during sulfuric acid-catalyzed alkylation modified with anthracene (77). Then the butene loses a hydride and forms a cyclic carbocation intermediate, yielding—on reaction with isobutene—trimethylpentyl cations. This conclusion was drawn from the observation of a sharp decrease in 2,2,3-TMP selectivity upon addition of anthracene to the acid. 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

These results were assumed to be ascribed to the neighboring group participation via the intermediary cyclic carbocation (4,) as shown in equation 4, as well as the high nucleophilicity of DTC and acceleration by use of the dipolar aprotic (DA) solvent. The formation and reactivity (stability) of such carbocation were examined at length with the corresponding... 

When the same substituents are at each end of the double or triple bond, it is called symmetrical. Unsymmetrical means different substituents are at each end of the double or triple bond. Electrophilic addition of unsymmetrical reagents to unsymmetrical double or triple bonds follows Markovnikov s rule. According to Markovnikov s rule, addition of unsymmetrical reagents, e.g. HX, H2O or ROH, to an unsymmetrical alkene proceeds in a way that the hydrogen atom adds to the carbon that already has the most hydrogen atoms. The reaction is not stereoselective since it proceeds via a planar carbocation intermediate. However, when reaction proceeds via a cyclic carbocation intermediate, it produces regiospecific and stereospecific product (see below). A regioselective reaction is a reaction that can potentially yield two or more constitutional isomers, but actually produces only one isomer. A reaction in which one stereoisomer is formed predominantly is called a stereoselective reaction. 

Pentafluorophenyl has an excellent ability to enchance the stability of allylic linear and cyclic carbocations. Species 26 and 27 were observed as long-lived species in the reaction of SbF5 with corresponding olefins [69] ... 

Very similar geometries are observed for Br2 and Cl2. The three-membered ring structure is clearly visible and the departure of the X anion is already in progress. Point charges are developing and the Br-Br and Cl-Cl bonds are lengthened from their equilibrium distances by 37 and 43%, respectively.131 A subsequent SN2 attack of the displaced X on the cyclic carbocation will give the experimentally observed trans-addition product 132... 

Isbell was also ahead of his time in grasping the concept of reaction mechanisms. In collaboration with the late Ward Pigman, he proposed two important mechanisms one for the anomerization of aldopyranoses by ring opening (see Fig. 3), and the other for the anomerization of their esters via a cyclic carbocation (see Fig. 4). 

In the case of d- or d,Mimonene polymerization, the chain bears, on the average, one unsaturation per two monomer units and this indicates that both endo- and exocyclic double bonds of the monomer are involved. The tertiary cyclic carbocation could propagate via cycloaddition and lead to bicyclic saturated units in the chain (Fig. 15). 

Squalene oxide is protonated on oxygen, and the epoxide ring is opened by nucleophilic attack of the double bond six carbons away, to yield a six-membered, cyclic carbocation intermediate. This step is similar to the acid-catalyzed epoxide openings we saw in Section 18.8. 

Draw the structures of the intermediate bromonium and cyclic carbocation, and propose mechanisms for all three steps. 

As discussed in Section 7.4, conformational control in deamination of open-chain amines is difficult to evaluate because the activation energies of conformational changes are often smaller than those of the steps in deamination reactions. Alicyclic amines are more suitable for such mechanistic investigations. In addition, the con-formers of such amines can be locked if they contain bulky substituents (tert-hvXyX) or if the amines are based on bi- and polycyclic hydrocarbons (decalinamines, cholestaneamines, norbornylamines, etc.). We shall therefore concentrate first on the deamination of the epimeric 4-( er butyl)cyclohexylamines. Then, we will discuss the structural problems of cyclic carbocations formed in deamination of norbor-nylamine, cyclopropylmethylamine, and cyclobutylamine, i. e., compounds that are at the center of interest in the debate on classical versus nonclassical carbocations. 

Most attempted conversions of (25) to (26) by conventional acid catalysis (e.g. HCOOH in pentane, AICI3 in ether or BF3 Et20 in CH2CI2) lead predominantly to mixtures of the alkenes rather than the desired (26) (<30%) [42]. Under these conditions, deprotonation of the cyclic carbocation species takes precedence over return of the OH-group or - if formed - (26) is unstable under the reaction conditions. A rationalization of the successful conversion by EGA catalysis in acetone is the trapping of the cy-clized carbocation by acetone giving (27), which is hydrolyzed to (26) (52%, 0% de). Scheme 16. Other solvents gave none of... 

The biosynthesis of kaurane diterpenoids shown in Scheme 102.6a starts from the cyclic carbocation formed by cyclization of copalyl PP. 

Some special cyclic carbocations undergo ring expansion with an alkyl shift (as shown below). 

The reactive nature of carbocationic THP intermediates in Prins cyclizations is not limited to intermolecular nucleophilic trapping. The cyclic carbocation 133... 

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