Electrophilic bromination of deuterated cyclohexenes in acetic acid containing varying [bromide]: Kinetics and product studies

Article abstract of DOI:10.1021/ja00057a018

The kinetics of bromine addition to various deuterated isotopomers of cyclohexene, namely the parent (3a), 3,3,6,6-tetradeuteriocyclohexene (3b), and cyclohexene-d₁₀ (3c), have been determined in HOAc (T = 25 °C) as a function of added [Br^-], and the products were determined. The observed kinetic isotope effect for bromination of 3a/3b is k_H/k_D ? with no added salt and indicates that deuteration of the allylic positions does not influence the kinetics. The kinetics of bromination of 3a/3c have been conducted at constant ionic strength (μ = 0.1 LiClO₄) as a function of [Br^-] to determine the kinetic isotope effect on the Br₂ and Br₃^- reactions: (k_Br2) 3a/3c = 1.08 (0.10) × 10³ M^-1 s^-1/2.15 (0.10) × 10³ M^-1 s^-1 = 0.5; (k_Br3-) 3a/3c = 3.09 (0.11) × 10² M^-1 s^-1 / 3.98 (0.11) × 10² M^-1 s^-1 = 0.78. The large inverse kinetic isotope effect (KIE) for k_Br2 3a/3c is consistent with a rate limiting step having considerable rehybridization of the olefinic carbons and may also indicate that the partitioning of the bromonium ion between reversal and product formation may be isotopically sensitive. The smaller inverse KIE for k_Br3- 3a/3c is consistent with a process wherein Br₂ nucleophilically captures an olefin/Br₂ charge-transfer complex or captures a reversibly formed bromonium/Br^- ion pair. The ratio of the dibromide/bromoacetate products is determined as a function of added Br^- and analyzed by a mechanism wherein the Br₂ reaction gives a bromonium ion/Br^- pair that collapses to give a 27:73 mixture of dibromide/bromoacetate, a Br₃^- reaction that gives only trans-dibromide, and an additional channel for production of dibromide resulting from external Br capture of a bromonium ion. Finally, the kinetics of bromine addition to 1,2-diphenylcyclohexene (4a), 3,3,6,6-tetradeuterio-1,2-diphenylcyclohexene (4b), 1,2-bis(p-chlorophenyl)cyclohexene (4c), and 1,2-bis(p-methoxyphenyl)cyclohexene (4d) have been determined and analyzed in terms of a mechanism involving an open β-bromocationic intermediate. In the case of 4a/4b a k_H/k_D of 1.4 in HOAc suggests that ion formation is reversible in that solvent, and the rate limiting step is Br promoted removal of an allylic H(D) from the intermediate. The k_H/k_D determined for 4a/4b in MeOH of 1.0 suggests that in this solvent the rate limiting step is ion formation and that all following steps are fast.