title: Solid-state conformations and absolute configurations of (+) and (-) alpha-, beta-, and gamma-hexabromocyclododecanes (HBCDs).
abstract: Hexabromocyclododecanes (HBCDs) are high production volume chemicals used as flame retardants for plastics and textiles. They are currently produced in quantities exceeding 20,000 t/y. Despite this fact, the correct stereochemistry of most HBCDs is still not known. Six stereocenters are formed during bromination of cyclododecatrienes, resulting in mixtures of different stereoisomers. Considering all elements of symmetry, 16 different stereoisomers including six pairs of enantiomers as well as 4 meso forms are possible theoretically. Recently, we isolated 8 of the 16 possible stereoisomers from a technical HBCD mixture and assigned their relative configurations. Herein, we report on the isolation of 6 enantiomerically pure alpha-, beta-, and gamma-HBCDs, obtained from preparative chiral-phase liquid chromatography, and we present their absolute configurations, which were determined from X-ray diffraction analysis. The absolute configuration of (-) alpha-HBCD was found to be (1R,2R,5S,6R,9R,10S), while the one of (+) beta-HBCD is assigned to (1S,2S,5S,6R,9S,10R), whereas the one of (-) gamma-HBCD corresponds to (1S,2S,5S,6R,9R,10S). The given structural information allows the unambiguous identification of the six most important HBCD stereoisomers, which typically account for more than 95% of technical HBCDs. In addition, we compared the solid-state conformations of racemic and enantiomerically pure alpha-, beta-, and gamma-HBCDs. In all cases, vicinal dibromides adopted a synclinal (sc) conformation with torsion angles of 69+/-6 degrees. A unique structural motive was common to all examined HBCD solid-state conformations. This conserved structure was described as an extended triple turn consisting of an arrangement of three pairs of synclinal and two antiperiplanar torsion angles.
authors.affiliation: Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory of Solid State Chemistry and Catalysis, 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: Martin
authors.initials: M
authors.lastname: Kohler
authors.forename: Peter
authors.initials: P
authors.lastname: Schmid
authors.forename: Markus
authors.initials: M
authors.lastname: Zennegg
authors.forename: Max
authors.initials: M
authors.lastname: Wolfensberger
mesh_terms: D002853:Chromatography, Liquid; D018360:Crystallography, X-Ray; D005411:Flame Retardants; D006842:Hydrocarbons, Brominated / Q000737:chemistry*; D013058:Mass Spectrometry; D008958:Models, Molecular; D008968:Molecular Conformation; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: D005411:Flame Retardants; D006842:Hydrocarbons, Brominated; C089796:hexabromocyclododecane