| Abstrakt: |
Hexabromocyclododecanes (HBCDs) are high production volume chemicals (>20000ty−1) used as flame retardants for plastics and textiles. Lately, we reported on the stereoselective isomerization of β-HBCDs. Herein we present insights into the mechanism and kinetics of (+)γ- to (+)α- and of (-)γ- to (-)α-HBCD isomerization. Only two of the six bromine atoms migrated, indicating that rearrangements of γ- to α-HBCDs are regio- and stereoselective as well. The apparent first-order isomerization rate constants increased from 0.0013 to 0.0031 to 0.0070min−1 at 120, 130, and 140°C, respectively, corresponding to half-lives of 540, 230, and 99min. Thus, a thermal treatment of materials containing γ-HBCDs at temperatures >100°C may induce the formation of α-HBCDs and, hence, may alter the diastereomeric ratio of a HBCD mixture. The inversion of vicinal dibromides in like-configurations (RR/SS) prevailed, whereas unlike-configurations (RS/SR) were not affected. An intramolecular, stereoselective migration of neighboring bromine atoms via a four-center transition state would explain the observed stereoisomer pattern and first-order kinetics. Despite the fact that vicinal dibromides in HBCDs prefer synclinal (gauche) conformations, antiperiplanar (staggered) conformations are assumed to facilitate concerted 1.2-shifts of both bromine atoms. A conformation analysis revealed that under kinetic control, only those bromine atoms in the more flexible part of the molecules are migrating, whereas those in the conserved triple-turn motive were not affected. Thus, this structural motive, common to all α-, β-, and γ-HBCDs, is more rigid and less reactive than the flexible part, containing the reacting dibromides in like-configurations. [Copyright &y& Elsevier] |
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