Benzhydryl carbenium ions

A very interesting paper80 reported studies of the reactions of several substituted benzhydryl carbenium ions, generated by laser flash photolysis, with halide ions in several solvents. This work provided the nucleophilicity N of chloride and bromide ions in several solvents. These data, along with the ionization rate constants and the solvolysis rate constants for the reactions of substituted benzyhdryl halides, was used to construct quantitative energy surfaces for the. S N 1 reactions of substituted benzhydryl halides in several solvents. 

The reaction of cyanide ion with substituted benzhydryl carbenium ions to form nitriles and isocyanides is controlled by the rates of reaction at carbon and nitrogen.124 In slow reactions, far from the diffusion limit, the attack is completely at the cyanide carbon. Very fast reactions, with little or no reaction barrier reacting at the diffusion-controlled limit, occur at both the N and the C of the cyanide ion. XN2 reactions occur almost exclusively at carbon regardless of the substrate or source of the cyanide ion. The HSAB principle cannot predict the products of these reactions. 

The reactivity sequence shown above corresponds well to Mayr s [18] model reactions of the electrophilic addition of benzhydryl carbenium ions to substituted alkenes. Table 2 lists the second-order rate constants for the addition of a diarylcarbenium ion to various alkenes and dienes [36]. One alkyl group offers little activation of the double bond a-olefins therefore form only oligomers with isomerized repeat units in low conversions under cationic polymerization conditions. One vinyl group activates the double bond slightly more than alkyl groups do. Table 2 also demon-... 

More reliable estimates of monomer reactivity are available from reactions of model compounds (Chapter 2). For example, the rate constants of addition of the same standard benzhydryl carbenium ion to various substituted styrenes correlate very well to Hammett s + = 4.9 [193]. Addition of various p-substituted benzhydryl cations to the same standard alkene yielded p(Acr = — 5.1 [193]. These results demonstrate that carbocationic polymerizations are extremely sensitive to even small changes in the monomer structure. They also demonstrate that the reactivity of carbenium ions scales nearly perfectly to the... 

Several substituted benzhydrols react with C of / -keto acids in 1,2-dichloroethane at 60 °C in the presence of an FeCl3 catalyst, giving the substitution product in what is thought to be an 5 1 reaction between the benzhydryl carbenium ion and a 0-keto acid. This is followed by a decarboxylation of the / -keto acid giving the final product, a (benzhydrylmethyl)alkyl- or aryl-ketone. The reaction also is successful when benzyl... 

Allyl halides heterolyze just as easily as benzyl halides because they also produce a resonance-stabilized carbenium ion. Even faster heterolyses are possible when the charge of the resulting carbenium ion can be delocalized by more than one unsaturated substituent and can thereby be stabilized especially well. This explains the remarkably high SN1 reactivities of the benzhydryl halides (via the benzhydryl cation) and especially of the triphenylmethyl halides (via the trityl cation) ... 

Fig. 11.13. Regioselectivity of the pinacol rearrangement of an asymmetric glycol. The more stable carbenium ion is formed under product-development control. Thus, the benzhydryl cation B is formed here, while the tertiary alkyl cation D is not formed.

If concentrations of carbenium ions are too low to be observed directly, they must be detected indirectly in kinetic studies of the racemiza-tion of optically active dormant species, ligand exchange and/or detailed studies of the effect of substituents, solvent and salts. Some of the most convincing and elegant work in this area was presented in Chapter 2 using primarily benzhydryl derivatives. As discussed in the next section, correlations between ionization rates and equilibrium constants, rates of solvolysis and rate constants of electrophilic addition can be interpolated and in some cases extrapolated to cationic polymerizations of alkenes to evaluate the reactivities of various active species and the dynamics of their isomerization. 

Ionization is exothermic and favored at lower temperatures in systems such as trityl, alkoxycarbenium, and benzhydryl derivatives which generate stabilized carbenium ions due to their electron-donating substituents. The exothermicity of ionization of benzhydryl species with BCl3 in CH2CI2 decreases from AH = -62 kJ/mol for di(p-anisyl) to -22 kJ/ mol for less stabilized p-tolyl and phenyl derivatives, and to approximately -8 kJ/mol for unsubstituted benzhydryl chloride [193]. These values, based on the solvolysis rates, extrapolate to AH = -12 kJ/mol for cumyl chloride (HCl adduct of a-methylstyrene) and AH = 0 kJ/mol for 1-phenyl-ethyl chloride (styrene adduct) and ( -butyl chloride (isobutene adduct) [240]. The reported entropies of ionization do not vary significantly and... 

Reactions of 2-trialkylsilyl- and trialkylstannyl-substituted furans with benzhydryl cations provided 2,5-disubstituted furans and ipso-substituted furans. Kinetic investigations of the reactions revealed that the monosubstituted product was produced from the protonolysis of the 2,5-disubstituted furylstannane, while the 2,5-disubstituted furan was derived from an electrophilic substitution of the mono-substituted furan. Introduction of a trialkylsilyl and a trialkylstannyl group to the 2-position of furan hardly affected the reactivity of this position towards carbenium ions ipso attack), while the 5-position is somewhat activated <01OL1629,01OL1633>. 

This example illustrates that the relative electrophilicities of two alkylating agents can be influenced by varying the amount of Lewis acid. Figure 5 shows that the reactivity differences become quite remarkable, when the difference of stabilization of the carbenium ions increases. The phenoxy substituted benzhydryl compound, for example, is 5400 times more reactive than the dimethoxy substituted compound, when an excess of BCI3 is used, whereas a reactivity ratio of 0.016 is observed with catalytic amounts of BCI3. 

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