Substituent effects cumyl chloride solvolysis

Any Sn1-Sn2 mechanistic complication should be absent in the solvolysis of a-t-butylbenzyl tosylates [15], which have a neopentyl-type structure (Tsuji et al., 1990). Indeed, the substituent effect in the solvolysis is accurately described by (2) with an r value of 1.09 which differs from the value r = 1.0 for the a-cumyl chlorides solvolysis. Based on the linearity of the correlation of the substituent effects on the solvolyses of [14] and [15] in 80% aqueous acetone, an Sn1-Sn2 mechanistic duality is also unlikely to be the cause of the exalted r value observed in the solvolysis of [14]. 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

The rates of solvolysis of substituted cumyl chlorides (equation 3) are used to define and (Tp parameters, and this reaction has been studied in several different laboratories These constants reflect the enhanced stabilization when direct conjugation of the p-substituent with the cationic center is possible and clearly demonstrate the importance of this effect for cyclopropyl. 

These parameters were intended to apply to XGY systems in which the skeletal group is phenylene. Hammett found it necessary to define an additional set of parameters, cTp , in order to account for substituent effects in 4-substituted benzene systems with an active site that has a lone pair on the atom adjacent to the benzene ring. The reference set was the ionization constants of 4-substituted phenols in water at 25 °C. Brown and his coworkers later defined another set of constants, o p, to account for substituent effects in benzene derivatives with electronically deficient active sites. The reference set was the rate constants for the solvolysis of 4-substituted cumyl chlorides in 90% aqueous acetone at 25 °C. Finally, Wepster and coworkersand Taft both independently proposed constants intended to represent substituent effects in benzene derivatives with minimal delocalized effect. Using the Taft notation these constants are written as cr . The reference systems had a methylene group inserted between the benzene ring and the active... 

Meanwhile, other solvolytic data from the work of Brown and Peters has been interpreted as evidence for the lack of ability of norbornyl groups to stabilize electron deficiency. The rates of solvolysis of a series of p-alkyl-t-cumyl chlorides in 90 % aqueous acetone at 25 °C have been measured, and rate constants and Hammett-Taft a values compared. These results show a small differential electronic effect for the norbornyl substituents which is larger from the exo-direction than from the endo-direction, but also that these groups exhibit only a moderate ability to stabilize positive charge. Studies have been extended to the solvolysis of l-(p-cyclopropyi-phenyl)- and l-(p-isopropylphenyl)-l-arylethyl chlorides, and previous conclusions concerning the lack of a-participation in 2-norbornyl systems have been restated. ... 

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