Benzhydryl cations alkylation

Equation 2.9 can also be interpreted as follows. Regardless of which nucleophile enters into an SN1 reaction with a given alkylating agent, the substitution product is produced with the same rate. Figure 2.12 illustrates this using as an example SN1 reactions of two different nucleophiles with P1l,CH—Cl. Both take place via the benzhydryl cation Ph2CH . In the first... 

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.

Rate constants and products have been reported for solvolysis of benzhydryl chloride and /7-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)-water and-ethanol, along with additional kinetic data for solvolysis of r-butyl and other alkyl halides in 97% TFE and 97% hexafluoropropan-2-ol. The results are discussed in terms of solvent ionizing power Y and nucleophilicity N, and contributions from other solvation effects are considered. Comparisons with other 3 nI reactions show that the solvolyses of benzhydryl chloride in TFE mixtures are unexpectedly fast an additional solvation effect influences solvolysis leading to delocalized cations. 

The O-alkylation of carboxylates is a useful alternative to the acid-catalyzed esterification of carboxylic acids with alcohols. Carboxylates are weak, hard nucleophiles which are alkylated quickly by carbocations and by highly reactive, carbocation-like electrophiles (e.g. trityl or some benzhydryl halides). Suitable procedures include treatment of carboxylic acids with alcohols under the conditions of the Mitsunobu reaction [122], or with diazoalkanes. With soft electrophiles, such as alkyl iodides, alkylation of carboxylic acid salts proceeds more slowly, but in polar aprotic solvents, such as DMF, or with non-coordinating cations acceptable rates can still be achieved. Alkylating agents with a high tendency to O-alkylate carboxylates include a-halo ketones [42], dimethyl sulfate [100,123], and benzyl halides (Scheme 6.31). 

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-... 

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