Secondary α-Deuterium Kinetic Isotope Effects Signifying a Polar Transition State in the Thermolysis of Ring-Substituted tert-Butyl Phenylperacetates

Abstract

Several ring-substituted tert-butyl phenylperacetates (YC6H4CH2CO3Bu t ) and their deuterated versions (YC6H4CD2CO3Bu t ) were prepared (Y: p-OCH3, p-CH3, p-H, and p-NO2). Thermolyses at 80 °C in CDCl3 showed excellent first-order kinetics. The rates have been measured as k YH × 104 and k YD × 104 s-1: 11.9 and 9.20 (p-OCH3), 2.64 and 2.22 (p-CH3), 1.06 and 0.93 (p-H), 0.164 and 0.156 (p-NO2). Hammett correlations were derived to yield ρYH + = −1.17 and ρYD + = −1.12. However, better Hammett plots were obtained with three points (p-OCH3, p-CH3, and p-H) showing ρYH + = −1.35 and ρYD + = −1.28. SDKIE was calculated as 1.293 (p-OCH3), 1.189 (p-CH3), 1.140 (p-H), and 1.051 (p-NO2), showing substantial substituent effects. Values of k YH/k YD for p-NO2 showed little temperature dependence. Hammett correlations and SDKIE were derived from the same kinetic entity that is the bond cleavage.

Photobrominations of Substituted Cumenes by N-Bromosuccinimide: Charge Delocalizations, Inductive Effects, and Spin Dispersions Triggered by Substituents

“…Accordingly, values of ΔΔ H ≠ Y - H became trivial showing almost invariable fluctuations of ΔΔ H ≠ Y - H against σ + (Figure ). Similar values of ΔΔ H ≠ Y - H close to zero were also observed with fragmentation reactions of benzyl methyl substituted benzyl carbinyloxy radicals 7a 5 Plot of ΔΔ H ⧧ Y-H vs σ + .…”

“…The negative entities repeatedly suggest that electron-donating substituents (e.g., p -OCH 3 ) promote bond breakings to loosen TS structures. Quite similar loosening of TS has been also detected by secondary α-deuterium kinetic isotope effects occurring in thermolysis of tert -butyl phenylperacetates 7e…”

“…Substituents control the rates of radical reactions through charge delocalizations (σ + ), inductive effects (σ), or spin dispersions (σ • ). The polar effects of radical reactions − are due mainly to the charge delocalizations and involve entropy control of rates. 6c, Furthermore, dominance of entropy could maintain the relative rates ( k Y / k H ) either constant or higher with increasing temperature (a violation of reactivity/selectivity principle). The inductive effects 7c,, have been operated only with TS bearing a carbanionic moiety where localization of negative charge occurs on the benzylic carbon atom.…”

“…The polar effects of radical reactions − are due mainly to the charge delocalizations and involve entropy control of rates. 6c, Furthermore, dominance of entropy could maintain the relative rates ( k Y / k H ) either constant or higher with increasing temperature (a violation of reactivity/selectivity principle). The inductive effects 7c,, have been operated only with TS bearing a carbanionic moiety where localization of negative charge occurs on the benzylic carbon atom. Substituents could also provoke spin delocalizations − to influence the rates of radical reactions.…”

Thermolysis of tert-Butyl Phenylperacetates: Delicate Control of the Rates through Contributions from Translational and Rotational Entropy

“…Thermal decompositions of tert -butyl phenylperacetates − occur through simultaneous two-bond scissions to yield a carbon dioxide, benzyl and tert -butoxy radical. Both activation enthalpy (Δ H ⧧ ) and activation entropy (Δ S ⧧ ) gradually decrease 1 when R in RCO 3 C(CH 3 ) 3 is varied in the order of C 6 H 5 (Δ H ⧧ = 33.5 kcal mol -1 , Δ S ⧧ = 7.8 eu), C 6 H 5 CH 2 (Δ H ⧧ = 28.7 kcal mol -1 , Δ S ⧧ = 3.9 eu), (C 6 H 5 ) 2 CH (Δ H ⧧ = 24.3 kcal mol -1 , Δ S ⧧ = −1.0 eu), C 6 H 5 −CHCH−CH 2 (Δ H ⧧ = 23.5 kcal mol -1 , Δ S ⧧ = −5.9 eu) .…”