Cumyl chloride substitution reactions

For the reactivity parameters Y, n, a+ (but not a) andN+ the lack of curvature is not unexpected. This is because these parameters are defined with respect to the rate of some standard reaction (solvolysis of t-butyl chloride, substitution of methyl iodide, solvolysis of cumyl chlorides, combination reaction of nucleophiles with a standard electrophile). Therefore the resultant plot is of the type log k vs. log k, while the curvature shown in a typical Br nsted plot (Figure 5) results from a plot of log k vs. log K. This curvature is due to a gradual change from a reactant-like transition state, which is insensitive to a perturbation in the reactivity parameter, to a product-like transition state in which equilibrium perturbations are largely reflected in the transition state (and hence the rate). A log k — log k plot is not expected to show this effect and hence is not expected to show curvature. 

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (r-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized o-. Their procedure was to establish a conventional Hammett plot of log (.k/k°) against (t for 16 /wcra-substituted r-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

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. 

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. ArCMe2Cl82. This reaction follows an S l mechanism, with intermediate formation of the cation ArCMe2+. A —R para-substituent such as OMe may stabilize the... 

The ct" " substituent constant scale was developed based upon the solvolysis of substituted cumyl chlorides, where cumyl cations are formed in the rate-determining ionization. For the carbocation-forming reaction of Eq. 20, there is a good correlation of log versus the sum of the constants for the substituents directly on the carbon. Such a correlation clearly does not exist for the reaction where the... 

The Komblum reactions at the tertiary carbon atoms also belong to the dark SrnI substitutions. Let us compare the reactions of a-cumyl chloride and 4-nitro-a-cumyl chloride with the phenylthiolate ion (Komblum 1975) (Scheme 8-8). As seen, the substitution of the arylthio moiety for chlorine at the former position of the chlorine is observed only for the 4-nitroderivative. The optically active substrate gives the racemic substitution product upon reaction with the phenylthiolate ion (Scheme 8-9). 

The only available data concern some a-carbonium ion side-chain reactions (hydrolysis of <-cumyl chlorides and p-nitrobenzoates) they reveal that the substitution of nitrogen for a CH group, in furan, thiophene, or pyrrole, leads to a fairly uniform reduction in reactivity at the 2-position.211... 

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. 

In an analogous manner, through conjugation from para substituents can stabilize a positive ion. The para-methoxy substituent shows this type of stabilization for the cumyl (2-phenylprop-2-yl) cation 3. Rates of hydrolysis of substituted cumyl chlorides (reaction 3.10) in aqueous acetone were plotted against a. The meta-substituted points fell on the line substituents such as methoxy and methyl were faster than expected, and these values were used to derive cr+ values for para substituents of this type (Figure 3.5b). 

It was therefore felt that the rate and equilibria constants could be correlated better by the Hammett equation if two new types of reaction site that becomes electron-rich and an electron-withdrawing substituent, a- constant is used. The standard reactions for the evaluation of cr constants are the ionization of p-substituted phenols and p-substituted anilinium ions [75]. Likewise, when there is resonance between a reaction site that becomes electron-deficient and an electron-donating substituent, cj+ constant is used. The standard reaction for the evaluation of ct+ constants is the solvolysis of p-substituted tert-cumyl chlorides 178 using 90 % aqueous acetone [76]. Selected [Pg.193]

For such reactions, another set of substituent values, a, was developed [21]. The rates of solvolysis of meta- and para-substituted cumyl chlorides in 90% aqueous (aq) acetone (Eq. 4.52) were measured. The p for the reaction was found to be. 54 by plotting only the meta isomers against a. This was then used to obtain o values according to Equation 4.53 with... 

Such sign reversal in px at an isokinetic point is also observed in the reactions of anilines with cumyl (PhCMe2-) arenesulfonates63 (4-as) and chlorides (4-C1). Structurally, these systems correspond to R1 = R2 = Me in the general a-substituted benzylic compounds so that the reactivity and the reaction mechanism are expected to be related to those of other a-substituted benzylic series discussed above57 63. The second-order rate constants for the reactions of aniline are 33.9 x 10 3 M 1 s 1 for cumyl arenesulfonate... 

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