Solvent Effects on Anchimerically Assisted Solvolyses. II. Solvent Effects in Solvolyses of threo-2-Aryl-1-methylpropyl p-Toluenesulfonates

Abstract

The solvent effects on the solvolysis of threo-2-aryl-1-methylpropyl p-toluenesulfonates were analyzed with the aid of the extended Winstein–Grunwald Equation. The aryl-assisted kΔ solvolysis of 2-(p-methoxyphenyl)-1-methylpropyl and 2-methyl-2-phenylpropyl p-toluenesulfonates failed to give a single linear correlation with the YOTs parameter. The dispersion against YOTs cannot be due to nucleophilic solvent assistance. The m values from the plots of 2-(p-methoxyphenyl)-1-methylpropyl and 2-methyl-2-phenylpropyl p-toluenesulfonates against YOTs were significantly lower than m = 1.0 for the limiting kc solvolysis. Significantly improved correlations (R > 0.99; SD < 0.14) were obtained for the p-methoxy, 3-chloro-4-methoxy, and p-methyl derivatives with the YΔ scale based on the solvolysis of 2-(p-methoxyphenyl)-2-methylpropyl p-toluenesulfonate, instead of with YOTs. No improvement by the YΔ treatment was obtained for the unsubstituted compound (R = 0.97; SD = 0.22). A new similarity comparison analysis in terms of YOTs and YΔ, i.e., log (k/k80E) = mcYOTs + mΔYΔ, was applied successfully to the present kΔ solvolyses. The degree of YΔ character relative to the YOTs one, scaled by mΔ/(mc + mΔ), varies for 1-methyl-2-phenylpropyl tosylates in the range from 1.0 to 0.00 in the consistent manner expected if the dispersion in the YOTs plot becomes less pronounced for solvolyses of less significant anchimeric assistance. There is a continuous spectrum of behavior between the extreme pure kc mechanism and the extreme pure kΔ one.

Detailed analysis for the solvolysis of isopropenyl chloroformate

“…Hence a series of Y X scales [37–43] are now available. Whenever the reaction center is adjacent to a π-system [44,45], or in α-haloalkyl aryl compounds that proceed via anchimeric assistance ( k Δ )[46], Kevill and D’Souza recommended [2,47–50] the addition of an additional aromatic ring parameter ( hI ) term to equation 1 give equation 2. In equation 2, h represents the sensitivity of solvolyses to changes in the aromatic ring parameter I .…”

Use of the Simple and Extended Grunwald-Winstein Equations in the Correlation of the Rates of Solvolysis of Highly Hindered Tertiary Alkyl Derivatives

“…The equation including the hI term was very usefully applied [2, 29] to reactions involving a 1,2-aryl shift, where it can be shown to be equivalent to the original treatment [30, 31, 32]. It has the advantage that it would not require the development of new solvent ionizing power scales, requiring only the appropriate Y X scale, if the study was extended from a p -toluenesulfonate leaving group to other leaving groups.…”

Correlation of the rates of solvolysis of arylmethylp-toluenesulfonates: application of the aromatic ring parameter and a discussion of similarity models

“…It has been found, however, that these scales do not adequately correct for the perturbation involved when multiple aromatic rings are capable of entering into conjugation with the carbocationic center and the solvolysis of 2,2-dimethyl-1-(β-naphthyl)propyl tosylate was proposed as a similarity model (23). As a variant, multiple correlation analysis against a linear combination of both Y ∆ and Y OTs terms has been proposed, primarily for ionizations proceeding with 1,2-aryl group migration (24) but also for the solvolyses of benzylic systems (25).…”