Carbocations equilibria

There are no reported studies of this rearrangement on the zeolite surface and we argued that it could give some clues to the alkyl-aluminumsilyl oxonium ion/carbocation equilibrium. In this work we show experimental and theoretical results on the rearrangement of the cyclopropylcarbinyl chloride over NaY zeolite, aiming at demonstrating the equilibrium between the carbocation and the alkyl-aluminumsilyl oxonium ion. 

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

The two dimers of (CH3)2C=CH2 are formed by the mechanism shown m Figure 6 16 In step 1 protonation of the double bond generates a small amount of tert butyl cation m equilibrium with the alkene The carbocation is an electrophile and attacks a second molecule of 2 methylpropene m step 2 forming a new carbon-carbon bond and generating a carbocation This new carbocation loses a proton m step 3 to form a mixture of 2 4 4 tnmethyl 1 pentene and 2 4 4 tnmethyl 2 pentene... 

It must be emphasized that we are not dealing with an equilibrium between two isomeric carbocations There is only one carbocation Its structure is not adequately represented by either of the individual resonance forms but is a hybrid having qualities of both of them The carbocation has more of the character of A than B because resonance struc ture A IS more stable than B Water attacks faster at the tertiary carbon because it bears a greater share of the positive charge... 

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

Figure 5.4 An energy diagram for the first step in the reaction of ethylene with HBr. The energy difference between reactants and transition state, AG, defines the reaction rate. The energy difference between reactants and carbocation product, AG°, defines the position of the equilibrium.

Most of these results have been obtained in methanol but some of them can be extrapolated to other solvents, if the following solvent effects are considered. Bromine bridging has been shown to be hardly solvent-dependent.2 Therefore, the selectivities related to this feature of bromination intermediates do not significantly depend on the solvent. When the intermediates are carbocations, the stereoselectivity can vary (ref. 23) widely with the solvent (ref. 24), insofar as the conformational equilibrium of these cations is solvent-dependent. Nevertheless, this equilibration can be locked in a nucleophilic solvent when it nucleophilically assists the formation of the intermediate. Therefore, as exemplified in methylstyrene bromination, a carbocation can react 100 % stereoselectivity. 

Unfortunately, it is not easy to measure acid strengths of very weak acids like the conjugate acids of simple unsubstituted carbanions. There is little doubt that these carbanions are very unstable in solution, and in contrast to the situation with carbocations, efforts to prepare solutions in which carbanions such as ethyl or isopropyl exist in a relatively free state have not yet been successful. Nor has it been possible to form these carbanions in the gas phase. Indeed, there is evidence that simple carbanions such as ethyl and isopropyl are unstable toward loss of an electron, which converts them to radicals. Nevertheless, there have been several approaches to the problem. Applequist and O Brien studied the position of equilibrium for the reaction... 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

The direction of rearrangement is usually toward the most stable carbocation (or radical), which is tertiary> secondary >primary, but rearrangements in the other direction have also been found,and often the product is a mixture corresponding to an equilibrium mixture of the possible carbocations. 

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... 

Alcohols are heterolysed into carbocations and water in the presence of the hydronium ion (6). From the equilibrium constant the free energy of heterolytic dissociation of the carbon-oxygen a bond, AGSe,(ROH + H30+), can be calculated. The AG°het(ROH + H3O+) value is related to the pi R+ of the carbocation by (26). 

The structure of nonclassical carbocations, such as norbomenyl 3, has been the subject of debate since the 1950s when Saul Winstein published his milestone studies on the solvolysis of tosylated norbomenyl compounds. It was proposed that the norbomenyl cation should be represented as the nonclassical structure 4+, with a 3-center, 2-electron cyclic system (3c-2e), rather than as the classical equilibrium... 

Rearrangement can also occur after the initial alkylation. The reaction of 2-chloro-2-methylbutane with benzene is an example of this behavior.35 With relatively mild Friedel-Crafts catalysts such as BF3 or FeCl3, the main product is 1. With A1C13, equilibration of 1 and 2 occurs and the equilibrium favors 2. The rearrangement is the result of product equilibration via reversibly formed carbocations. 

We consider the relatively high pKA values of 6-8 to be typical value for a cation-quinone methide equilibrium. The formation of a resonance-stabilized aromatic carbocation is one reason for these high pKA values. Another reason is the high energy of the quinone methide. The thermodynamic cycle shown in... 

Scheme 7.25 shows the role of quinone methide energy on the cation-quinone methide equilibrium. A high pKa value for this equilibrium is expected if the energy of the quinone methide approaches that of the carbocation. To construct this cycle, we used the Ka values that we determined for the protonated ketone (pKa — —0.9) and quinone methide (pKa = 6.6). This pKa difference requires that the keto form be more stable than the quinone methide by — 10.2kcal/mol. We obtained the calculated energy difference of lO.lkcal/mol from Hartree-Fock calculations using 6-31G and STO-3G basis sets, inset of Scheme 7.25. 

Boruah, R. C. Skibo, E. B. Determination of the pKa values for the mitomycin C redox couple by tritration, pH rate profile, and Nemst-Clark fits. Studies of methanol elimination, carbocation formation, and the carbocation/quinone methide equilibrium. J. Org. Chem. 1995, 60, 2232-2243. 

Richard, J. P. Amyes, T. L. Bei, L. Stubblefield, V. The effect of beta-fluorine substituents on the rate and equilibrium-constants for the reactions of alpha-substituted 4-methoxybenzyl carbocations and on the reactivity of a simple quinone methide. J. Am. Chem. Soc. 1990, 112, 9513-9519. 

The reaction can, however, be made preparative for (91) by a continuous distillation/siphoning process in a Soxhlet apparatus equilibrium is effected in hot propanone over solid Ba(OH)2 (as base catalyst), the equilibrium mixture [containing 2% (91)] is then siphoned off. This mixture is then distilled back on to the Ba(OH)2, but only propanone (b.p. 56°) will distil out, the 2% of 2-methyl-2-hydroxypentan-4-one ( diacetone alcohol , 91, b.p. 164°) being left behind. A second siphoning will add a further 2% equilibrium s worth of (91) to the first 2%, and more or less total conversion of (90) — (91) can thus ultimately be effected. These poor aldol reactions can, however, be accomplished very much more readily under acid catalysis. The acid promotes the formation of an ambient concentration of the enol form (93) of, for example, propanone (90), and this undergoes attack by the protonated form of a second molecule of carbonyl compound, a carbocation (94) ... 

In this context, it was suggested [53] that the reaction involved a tertiary carbocation intermediate that was in equilibrium with quasiphosphonium salt. The elimination of HC1 from the latter, leads to the formation of 2,5-dihydro-l,2-oxaphosphole derivatives. Macomber [54], however, has shown that bromination of optically pure... 

The determinations of absolute rate constants with values up to ks = 1010 s-1 for the reaction of carbocations with water and other nucleophilic solvents using either the direct method of laser flash photolysis1 or the indirect azide ion clock method.8 These values of ks (s ) have been combined with rate constants for carbocation formation in the microscopic reverse direction to give values of KR (m) for the equilibrium addition of water to a wide range of benzylic carbocations.9 13... 

Table 1 Rate and equilibrium constants for partitioning of substituted a-methyl carbocations R (R2)CCH3+ between nucleophilic addition of solvent (ks) and deprotonation (kp) (Scheme 7)°... 

Table 1 summarizes experimentally determined values of the following rate and equilibrium constants for the reactions of aliphatic and benzylic a-methyl carbocations (Scheme 7). 

Values of pA"R for the addition of water to carbocations to give the corresponding alcohols. The equilibrium constants KR (m) were determined as the ratio Hoh/ h> where fcHOH (s 1) is the first-order rate constant for reaction of the carbocation with water and H (m 1 s ) is the second-order rate constant for specific acid-catalyzed cleavage of the alcohol to give the carbocation.9,12 13... 

Values of Kadd for the addition of water (hydration) of alkenes to give the corresponding alcohols. These equilibrium constants were obtained directly by determining the relative concentrations of the alcohol and alkene at chemical equilibrium. The acidity constants pATaik for deprotonation of the carbocations by solvent are not reported in Table 1. However, these may be calculated from data in Table 1 using the relationship pA ik = pATR + logA dd (Scheme 7). 

The rate and equilibrium constants for the reactions of ring-substituted 1-phenylethyl carbocations (X-[6+]) in 50/50 (v/v) trifluoroethanol/water (Table 2 and Scheme 8),13 14 17 43, and for interconversion of ring-substituted 1-phenyl-... 

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