title: Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs): kinetics and mechanism of gamma- to alpha-HBCD isomerization.
abstract: Hexabromocyclododecanes (HBCDs) are high production volume chemicals (>20000 ty(-1)) used as flame retardants for plastics and textiles. Lately, we reported on the stereoselective isomerization of beta-HBCDs. Herein we present insights into the mechanism and kinetics of (+)gamma- to (+)alpha- and of (-)gamma- to (-)alpha-HBCD isomerization. Only two of the six bromine atoms migrated, indicating that rearrangements of gamma- to alpha-HBCDs are regio- and stereoselective as well. The apparent first-order isomerization rate constants increased from 0.0013 to 0.0031 to 0.0070 min(-1) at 120, 130, and 140 degrees C, respectively, corresponding to half-lives of 540, 230, and 99 min. Thus, a thermal treatment of materials containing gamma-HBCDs at temperatures >100 degrees C may induce the formation of alpha-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 alpha-, beta-, and gamma-HBCDs, is more rigid and less reactive than the flexible part, containing the reacting dibromides in like-configurations.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Analytical Chemistry, Uberlandstrasse 129, CH-8600 Dübendorf, Switzerland. norbert.heeb@empa.ch
authors.forename: Norbert V
authors.initials: NV
authors.lastname: Heeb
authors.forename: W Bernd
authors.initials: WB
authors.lastname: Schweizer
authors.forename: Peter
authors.initials: P
authors.lastname: Mattrel
authors.forename: Regula
authors.initials: R
authors.lastname: Haag
authors.forename: Andreas C
authors.initials: AC
authors.lastname: Gerecke
authors.forename: Peter
authors.initials: P
authors.lastname: Schmid
authors.forename: Markus
authors.initials: M
authors.lastname: Zennegg
authors.forename: Heinz
authors.initials: H
authors.lastname: Vonmont