Thermodynamic conjugated carbocations

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

As thermodynamic stability indexes for the hydrocarbon ions, pA R+ and pA a values [(4) and (5)] have been widely applied for the carbocation and carbanion, respectively, in solution. Here K + stands for the equilibrium constant for the reaction (6) of a carbocation and a water molecule stands for the equilibrium constant for the reaction (7) of a hydrocarbon with a water molecule to give the conjugate carbanion. The equilibrium constants are given by (8) and (9) for dilute aqueous solutions. Obviously, the reference system for the pKn+ scale is the corresponding alcohol, and... 

Presumably, isoxazoline (175) is thermodynamically more favorable than overcrowded aziridine (176) due to jt,jt conjugation. Elimination of the bromide anion from intermediate (175) is also hindered due to instability of the carbocation that formed. 

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). 

The neutral 1,4- and 1,2-quinone methides react as Michael acceptors. However, the reactivity of these quinone methides is substantially different from that of simple Michael acceptors. The 1,6-addition of protonated nucleophiles NuH to simple Michael acceptors results in a small decrease in the stabilization of product by the two conjugated 7T-orbitals, compared to the more extended three conjugated 7T-orbitals of reactant. However, the favorable ketonization of the initial enol product (Scheme 1) confers a substantial thermodynamic driving force to nucleophile addition. By comparison, the 1,6-addition of NuH to a 1,4-quinone methide results in a large increase in the -stabilization energy due to the formation of a fully aromatic ring (Scheme 2A). This aromatic stabilization is present to a smaller extent at the reactant quinone methide, where it is represented as the contributing zwitterionic valence bond structure for the 4-0 -substituted benzyl carbocation (Scheme 1). The ketonization of the product phenol (Scheme 2B) is unfavorable by ca. 19 kcal/mol.1,2... 

The first formed product now cyclizes to form the second six-membered ring. This recreates a carbocation at the tertiary centre like the one that set off the fragmentation as the more nucleophilic end of the isolated alkene attacks the end of the conjugate electrophile. This is a thermodynamically controlled reaction with the new stereogenic centre choosing an equatorial substituent. 

SOLUTION TO 11a First we need to determine which of the terminal sp carbons of the conjugated system is going to be the C-1 carbon. The proton will be more apt to add to the indicated sp carbon because the carbocation that is formed shares its positive charge with a tertiary allylic and a secondary allylic carbon. If the proton were to add to the sp carbon at the other end of the conjugated system, the carbocation that would be formed would be less stable because its positive charge would be shared by a primary allylic and a secondary allylic carbon. Therefore, 3-chloro-3-methylcyclohexene is the 1,2-addition product and 3-chloro-l-methylcyclohexene is the 1,4-addition product. 3-Chloro-3-methylcyclohexene is the kinetic product because of the chloride ion s proximity to C-2, and 3-chloro-l-methylcyclohexene is the thermodynamic product because its more highly substituted double bond makes it more stable. 

Mechanisms under acidic conditions usually start by a protonation step. In this case, protonation of the conjugated double bond would lead to carbocation 6 in equilibrium with the more stable tautomer 7. Cyclization onto the aromatic ring would lead to conjugated cation 8, which after aromatization yields the final product 3. The experimental fact is that the compound having the trans arrangement between the substituents on Cl and C2 positions is exclusively obtained. That is, the formation of the thermodynamically more stable product seems to be favored (Scheme 14.3). 

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