Substituent effects on the solvolysis of 1,1-diphenyl-2,2,2-trifluoroethyl tosylates. Part III. Effects of electron-donating substituents in the fixed aryl moiety

Abstract

The solvolysis rates of 1-aryl-1-(p-methoxyphenyl)-2,2,2-trifluoroethyl and 1-aryl-1-(p-phenoxyphenyl)-2,2,2-trifluoroethyl bromides and chlorides were conductimetrically measured at 25.0°C in 80% aqueous ethanol. The solvolysis rates of 1-(p-methylphenyl)-fixed series were also set up to analyze the substituent effect. The substituent effects on these Y-series of solvolyses were analyzed on the basis of the Yukawa-Tsuno equation. The pmethoxyphenyl-fixed series showed a linear correlation with r = À1.7 and r = 1.0 for the substituent range less reactive than p-Me including all meta substituents, and a discrete correlation with r = À3.9 and r = 1.26 for the strong p-donor class substituents more reactive than p-PhO. A similar bilinear correlation was obtained for the Y = p-PhO series. The partial correlations for the strong p-donor class substituents in both Y-series are nearly the same as the correlation r = À4.2 and r = 1.2 for symmetrically disubstituted (X = Y) series. These bilinear (or non-linear) correlations were explicable in terms of changes in the coplanarity of the two benzene rings depending upon the resonance capabilities of X and Y substituents. For strong p-donor class X substituents as strongly p-electron donating as the Y substituent, both aryl rings are equivalently twisted as in the symmetrical X = Y series, and for less electrondonating X substituents, the fixed p-methoxyphenyl (or p-phenoxyphenyl) ring remains in a coplanar conformation and X-phenyl is twisted more out of coplanarity. In the fixed Y = p-Me series, a significantly non-linear correlation was found, comprising three distinct correlations characteristic of three conformers, a conformer with the X-aryl coplanar and Y-aryl twisted, a symmetrical conformer with X being a weak or moderate p-electron donor and both aryls being equivalently twisted, and a conformer with Y-aryl coplanar and X-aryl twisted. The relative stabilities of these conformers were estimated by the ab initio MO optimization of the mono-p-methoxycarbenium ion, confirming the conclusions above.

Photochemical Generation of Highly Destabilized Vinyl Cations: The Effects of α- and β-Trifluoromethyl versus α- and β-Methyl Substituents

“…Substituent effects on carbocations have received tremendous attention over the years . Recent interest has focused on alkyl cations bearing electron-withdrawing substituents, including CN, CF 3 , and C 2 F 5 , mainly at the α-position . For these substituents next to their α-effects also their β-effects have been studied, leading to measures of relative (de)stabilization by α- and β-substituents .…”

“…In this study we report the photogeneration of the vinyl cations 3 + and 4 + substituted at the α- and β-position, respectively, by a trifluoromethyl group and thus the production of highly destabilized vinyl cations (see Figure ). By virtue of these species being vinyl cations and the presence of a CF 3 group these species are doubly destabilized carbocations. 2b, For comparison also the corresponding α- and β-methyl-substituted vinyl cations 1 + and 2 + are photogenerated, which allows the assessment of the effects of the CF 3 substituent compared to the CH 3 group at both the α- and β-position on the (in)stability and reactivity of the vinyl cations. To generate these species the halostyrene precursors 1 − 4 have been synthesized and their photochemistry in methanol has been studied.…”