Chiral chlorobismuthines stabilized by the intramolecular coordination of an N,N-dimethylamino group: X-ray structure analysis, asymmetric induction at the bismuth centre, and dynamic behaviour in solution

Abstract

Chiral chlorobismuthines 3 stabilized by intramolecular coordination of an N,N-dimethylamino group have been synthesized by the selective fluorodearylation of the bismuthines 1 with boron trifluoride–diethyl ether, followed by halogen exchange of the resulting fluorobismuthines 2 with brine. An X-ray crystallographic structure analysis of 3b revealed that the bismuth centre has a distorted pseudotrigonal bipyramidal structure through the formation of a hypervalent 3-centre 4-electron bond with the chlorine and nitrogen atoms at apical positions. ¹H NMR spectra of compounds 3 in CDCl₃ at room temperature reflected their fixed conformation in the solid state. Chiral chlorobismuthines 7a and 7b were obtained in a similar manner starting from optically pure (R)-N,N-dimethyl-1-phenylethylamine as a mixture of two diastereoisomers (77:23 and 78:22, respectively), the isomeric ratios of which were net influenced by the equatorial aryl groups present. Variable-temperature dynamic ¹H NMR spectra of the diastereoisomeric mixture of 7a provided important information about its behaviour in solution; in [²H₈]toluene dynamic but reversible dissociation of intramolecular Bi–N coordination was observed only at elevated temperatures, while in [²H₆]-DMSO it occurred quite easily below room temperature, leading to the inversion of configuration at the chiral bismuth centre.