Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs): Kinetics and mechanism of β-HBCD racemization

Abstract

Hexabromocyclododecanes (HBCDs) are high production volume chemicals currently produced in quantities exceeding 20 000 t y −1. They are used as flame retardants for plastics and textiles. HBCDs are thermally labile compounds, rapidly decomposing at temperatures above 250 °C to form bromine radicals, which scavenge other radicals formed during pyrolysis. But certain HBCD stereoisomers must reach the environment without decomposition, because their levels in soils, sediments, and biota are increasing worldwide. The fate of individual HBCD stereoisomers during production, product use, disposal, and transformation in the environment remains unclear. Herein we report on the thermally induced, highly selective isomerization of (+) and (−)β-HBCD. Regio- and stereoselective migration of only two of the six bromine atoms resulted in the racemization of both β-HBCDs. First order rate constants ( k rac) increased from 0.005, 0.011, 0.021, to 0.055 min −1 at 130, 140, 150, and 160 °C, corresponding to half life times τ 1/2 of 143, 63, 29, and 14 min, respectively. From the deduced kinetic model, we conclude that any thermal treatment of enantiomerically enriched β-HBCDs in the range of 100–160 °C will result in a loss of most optical activity within few hours. The simultaneous inversion of two asymmetric centers occurred with perfect stereocontrol. Selectively, vicinal dibromides with the RR- and the SS-configurations migrated at these temperatures. An intramolecular reaction mechanism with a four-center transition state is postulated, based on the obtained stereoisomer pattern and the observed reaction kinetics. Crystal structure analysis revealed that all vicinal dibromides in β-HBCDs prefer synclinal (gauche) conformations. However, an antiperiplanar (staggered) conformation is assumed to facilitate the concerted 1.2-shifts of both bromine atoms, resulting in an inversion of both neighboring carbon atoms. First experiments with other HBCD stereoisomers suggest that the presented isomerization mechanism is of relevance for those stereoisomers as well.