Resolution of Racemic 1,2-Dibromohexafluoropropane through Halogen-Bonded Supramolecular Helices

Abstract

Halogen bonds, attractive intermolecular interactions between perfluoroalkyl bromides and bromide ions, are present in cocrystals of (-)-sparteinium hydrobromide (1) and (S)-1,2-dibromohexafluoropropane (2; shown schematically), and result in enantiopure and infinite supramolecular helices. The perfluorocarbon-hydrocarbon self-assembly allows the resolution of racemic 2.

Halogen Bonding: A Paradigm in Supramolecular Chemistry

“…[19] An extented CÀX (X Br, I) bond length was observed in the co-crystals with respect to the pure halo PFC, so generally the stronger the halogen bonding, the greater the lengthening turns out to be, consistent with an electron donation from nitrogen to iodine. [20] A covalent bond can be characterised by its length and dissociation energy, or alternatively by the selective reactions; the selectivity then is merely a result of the peculiarities of the bond and its surrounding. Similarly, a non-covalent interaction can be characterised by its geometric and energetic properties or by the selective supramolecular synthesis.…”

“…For instance, preliminary experiments show how 2,3-dibromohexafluoropropane forms infinite networks which are solid at room temperature when bromide anions work as electron donor motifs and the self-assembly process is effective and selective enough to allow the resolution of the racemic fluorocarbon into single enantiomers when the hydrobromide of an enantiopure amine is used as donor species. [20] Selectivity amongst different heteroatoms as electron donors: While different heteroatoms with free lone pairs (e.g. O, S, N, P) can work as electron donors to PFC as well as HC halides, up till now heteroatom NMR spectroscopy has been used to detect the direct involvement of the heteroatom in the halogen bonding only for nitrogen substituted electron donors.…”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…The large amount of structures produced allows many interesting contributions based on the analysis of structural databases [9][10][11][12][13]. And, indeed, most of the applications of XB are in the materials science: porous systems [14][15][16], liquid crystals [17], light-emitting materials [18,19] and magnetic materials [20,21] are only some applicative fields fruitfully explored with XB-based materials.…”

Combining halogen bonding and chirality in a two-dimensional organic metal (EDT-TTF-I2)2(D-camphorsulfonate)·H2O

“…15 Based on its high strength and directionality, it appears now as a very efficient tool in crystal engineering, 16 also used for example for the resolution of racemic organic halides. 17 Note also that the oxidation of such halogeno TTF derivatives into radical cations actually enhances their halogen bond donor ability. 18 We describe here a very first example along these lines, where a partially oxidised iodinated tetrathiafulvalene, EDT-TTF-I 2 ,…”