Stabilization conjugated carbocations

Shown in the margin are some dramatic rate effects that were measured under conditions that assist SnI pathways, with only a small fraction of an Sn2 pathway. Substitution of a hydrogen by a methyl on isopropyl chloride gave a 5.5 X lO increase in rate. Substitution of a methyl by a phenyl gives a 4.6 X 10 enhancement. As another series of examples, the relative rates of solvolysis of PhCHaCl, Ph2CHCl, and PhsCCl in mixtures of diethylether/ethanol are 1,1.75 X 10 and 2.5 X 10, respectively. Thus, the addition of phenyl rings dramatically stabilizes conjugated carbocations. 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

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. 

In general, the greater the resonance and hyper-conjugation the greater is the stability of carbocation. The stability also depends on the field strengths. The following examples illustrate this point. 

This concludes the discussion of the stabilities of carbocations with hydrocarbon-based structures and also of different methods for deriving equilibrium constants to express these stabilities. The remainder of the chapter will be concerned mainly with measurements of stabilities for oxygen-substituted and metal ion-coordinated carbocations. Consideration of carbocations as conjugate acids of carbenes and derivations of stabilities based on equilibria for the ionization of alkyl halides and azides will conclude the major part of the chapter and introduce a discussion of recent studies of reactivities. 

Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-stabilized allylic carbocation intermediate. 

Reactivity studies of conformationally fixed substrates has been an important technique for the study of the conjugative stabilization of carbocation centers afforded by cyclopropyl groups, and although Chapter 8 of this monograph is devoted to this topic, brief mention will be made of the subject here because of the direct relevance to the topic at hand. Examination of the long-lived ions themselves, principally by NMR, provides a further dimension to the analysis. 

The influence of cyclopropyl on the gas phase stability of carbocations as measured by ion cyclotron resonance is shown in Table 14, along with data for some reference compounds. The results are given as gas phase basicities, GB, and proton affinities, PA, defined as AG° and AH°, respectively, for dissociation of the protonated molecule, as in equation 11. In addition hydride affinities D(BH H ) for some cations defined as — AH° for equation 18 are included. For the gas phase basicities and proton affinities the products B are alkenes, amines, nitriles or carbonyl compounds, and thus for these values the stability of the cation is compared to a derivative where the substituent is conjugated with a carbon-carbon or carbon-oxygen double bond, or a nitrogen lone pair, whereas for hydride affinities the products are saturated. 

Carbocations are usually formed either by loss of a leaving group or by addition of an electrophile to a pi bond. Carbocation formation is very rare in nonpolar solvents that cannot stabilize the cation. Polar solvent molecules can cluster their lone pairs around the cationic center, forming an intermolecular donation of electron density that stabilizes the carbocation. Some carbocations are stable enough to be on the pA a chart the more stable ones are the weaker acids. Recall that when the pH equals the pATa. the conjugate acid and base concentrations are equal. A protonated amide with a pATa of -0.5 requires very acidic water before it is half-cation and half-neutral. A protonated ester with a pATa of -6.5 is much less stable. However, since most carbocations are not on the pAfa chart, we need another method to rank carbocation stability. 

Dienes are conjugated systems of two pi bonds. Above, the simplest diene, 13-butadiene, adds the electrophile to the end to produce a resonance stabilized allylic carbocation. As with alkenes, the more substituted the diene is, the more reactive. 

On the other hand, the organosilyl group is capable of stabilizing the carbocation (3 to silicon. This effect is believed to be attributable to the electron-donating property of the a-bond between silicon and a-carbon atoms ((a-p)7i conjugation) (Scheme IB). The reactions of allylsilanes or vinylsilanes with electrophiles are facilitated with this stabilization of intermediate carbocations (eq (10)). 

The p orbital of the positively charged carbon of the carbocation on the left is conjugated with the p orbitals on the carbons of the benzene ring. Therefore this carbocation is resonance stabilized. The carbocation on the right has no resonance stabilization because of the sp hybridized carbon that separates the p orbital of the positive carbon from the p orbital of the carbon of the benzene ring. 

From these studies, it can be concluded that it is not a simple matter to define the general substituent migratory aptitudes in chloiin and bacteriochlorin systems, because distant conjugated peripheral substituents can dramatically affect the stability of carbocation intermediates and hence the products. The facility of the rearrangement of various substituents depends not only on the intrinsic nature of the migratory group but also on the electronic and steric factors present elsewhere on the tetrapyrrolic systems. 

The first step of the mechanism is identical for both 1,2-addition and 1,4-addition, namely, the conjugated diene is protonated to give a resonance-stabilized, aUyfic carbocation and a bromide ion. However, the second step of the mechanism can occur via either of two competing pathways (shown in blue and red). Comparing these pathways, we see that 1,4-addition leads to a more stable product (lower in energy), while 1,2-addition occurs more rapidly (lower energy of activation). The 1,4-adduct is lower In energy because It exhibits a more substituted double bond ... 

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