INVESTIGATION OF THE EARLY STAGES OF ELECTROPHILIC ADDITION OF BR2 TO OLEFINS - KINETIC EVIDENCE FOR A REVERSIBLY FORMED BROMONIUM ION IN THE REACTION OF BR2 WITH TETRAISOBUTYLETHYLENE

The early stages of electrophilic bromination of a hindered olefin, tetraisobutylethylene (TIBE, 2), have been investigated with an aim to determining the formation constant (kf) for the 1:1 charge-transfer complex and information about the reversible formation of the corresponding bromonium ion. In dichloroethane and acetic acid at 25 °C, the respective Kf values are 9.71 (0.19) and 1.72 (0.2) M−1, while in methanol, the value can be no larger than ~2.5 M−1. On the basis of the values of Kf in dichloroethane at 6, 25, and 50 °C, the thermodynamic parameters for CTC formation from TIBE + Br2 are ΔH =-4.07 (0.35) kcal/mol and °S =-9.20 (0.11)eu. TIBE reacts with bromine in acetic acid to form a double bond rearranged allylic bromide. The reaction is not consistent with a free radical process and is more consistent with one proceeding via an ionic bromonium ion intermediate. The kinetics of bromination in acetic acid at 25 °C of TIBE (2-H8) and d8-TIBE (2-D8), in which theeight allylic positions are deuterated, reveal a substantial kinetic isotope effect (KIE) of2.3 (0.1). This value is too large for any imaginable β-secondary KIE on the bromonium ion formation and is most consistent with a primary effect in which a CH or CD (CL) bondisremoved in a rate limiting or partially rate limiting step. This step is suggested to bedeprotonation of the bromonium ion which leads to formation of double-bond rearranged allylic bromide. The KIE data provide the first kinetic isotope evidence for the reversible formation of a bromonium ion. © 1990, American Chemical Society. All rights reserved.