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

Abstract

The first-order rate constants (k(Y)) at several temperatures in CDCl(3) were measured for thermal decompositions of YC(6)H(4)CH(2)CO(3)C(CH(3))(3) with Y being p-OCH(3), p-OPh, p-CH(3), p-Ph, p-H, p-Cl, m-Cl, and p-NO(2). The relative rates (k(Y)/k(H)) exhibit excellent rho(+)/sigma(+) Hammett correlations with rho(+) < 0, indicating a polar TS. Activation parameters (DeltaH()(Y) and DeltaS()(Y)) and their differential terms (DeltaDeltaH()(Y)(-)(H) and DeltaDeltaS()(Y)(-)(H)) were obtained from the Eyring plot. Differential activation terms (DeltaDeltaH()(Y)(-)(H) and DeltaDeltaS()(Y)(-)(H)) disclose an isokinetic relation with p-CH(3), p-Ph, p-H, p-Cl, and m-Cl (isokinetic temp, 230 K). However, p-OCH(3), and p-OPh show negative deviations, and a positive deviation occurs with p-NO(2). Plot of DeltaDeltaH()(Y)(-)(H) vs sigma(+) exhibits a good linear relation (r = 0.95) with a slope (alpha(1) = -3.34). A better linear correlation (r = 0.97) and steeper slope (alpha(2) = -5.22) were observed for TDeltaDeltaS()(Y)(-)(H) vs sigma(+). Negatively larger slope (alpha(2) = -5.22) may point to entropy control of rates. Differential activation parameters (DeltaDeltaH()(Y)(-)(H) and DeltaDeltaS()(Y)(-)(H)) reflect variations of activation process. Differential activation entropies (DeltaDeltaS()(Y)(-)(H)) are discussed in terms of contributions of translational and rotational entropies. Similar deviation behaviors of p-OCH(3), p-OPh, and p-NO(2) were again observed for the both plots. p-NO(2) can strongly destabilize the cationic site of the polar TS but serves an eminent spin delocalizer for the homolytic TS.

Induced Thermolysis of tert-Butyl Phenylperacetates by Thiophenol: Simultaneous Occurrence of Homolysis and Single Electron Transfer

“… a The figures in parentheses indicate mole concentrations (M) of thiophenol employed. b The values in parentheses are the rate constant ( k H ) reported by us with iodine trapping of the intermediate radicals of ref . The two figures show an excellent agreement. …”

“…Reactions at 80 °C of degassed and sealed Pyrex tubes containing TBPA (0.05 M), thiophenol (0.5−3.5 M), bibenzyl (0.01 M, internal standard), and potassium fluoride (acid scavenger: 50 mg) in CDCl 3 gave rise to formations of phenylacetic acid, tert -butyl alcohol, toluene, and phenyl disulfide. Scheme shows the two-bond homolysis of TBPA which has been thoroughly investigated by Bartlett et al and us . The initial transfer of an electron from thiophenol to TBPA results in formations of the radical anion 1 and the radical cation 2 , which eventually yield phenyl acetic acid and tert -butyl alcohol (Scheme ).…”

Mechanistic Insights into Fe Catalyzed α‐C−H Oxidations of Tertiary Amines

“…[29] Generally, many metal-oxo initiated CÀ H bond abstractions similar to cytochrome P 450 (radical rebound) mechanisms show similar negative values for the activation entropy. [27,30] In contrast, homolytic OÀ O bond cleavage of peroxyesters [31] or reactions proceeding via dissociation of substrate from the metal catalyst prior to C-H bond cleavage [32] are typically associated with more positive activation entropies, corresponding to a less ordered transition state structure.…”

Mechanistic Insights into Fe Catalyzed α-C-H Oxidations of Tertiary Amines

“…29 Generally, many metal-oxo initiated C-H bond abstractions similar to cytochrome P450 (radical rebound) mechanisms show similar negative values for the activation entropy. 27,30 In contrast, homolytic O-O bond cleavage of peroxyesters 31 or reactions proceeding via dissociation of substrate from the metal catalyst prior to C-H bond cleavage 32 are typically associated with more positive activation entropies, corresponding to a less ordered transition state structure.…”