Chirality in cyclotriphosphazenes with one stereogenic centre

Abstract

Mono-substitution reactions of gem symmetrically disubstituted cyclophosphazenes such as (1) and (2) provide a convenient way to investigate the chiral configurational properties of cyclophosphazene compounds with one stereogenic centre. Previous work by X-ray crystallography has shown that the mono-dibenzylamino derivative of (1), compound (3a), is chiral and exists as a racemate. In this work, compound (1) was reacted with piperidine, dimethylamine, pyrrolidine and phenol to give compounds (3b–3e), respectively, and compound (2) was reacted with dimethylamine to give (4). The structures and stereogenic properties of new compounds (3b), (3c) and (4) were determined by X-ray crystallography and, together with compound (3a), the chirality was confirmed by 31P NMR spectroscopy on addition of a chiral solvating agent (CSA), (S)-(+)-2,2,2-trifluoro-1-(9′-anthryl)ethanol. As compound (3d) did not give crystals suitable for X-ray crystallography and compound (3e) is an oil, their stereogenic properties were confirmed by 31P NMR spectroscopy on addition of CSA. The work confirms that cyclophosphazene derivatives containing one centre of chirality exist as racemates and shows that the chiral properties of such molecules may be determined by 31P NMR spectroscopy on addition of a chiral solvating agent in those cases where X-ray crystal structures are not available.