The First X-ray Structure Determination of an Optically Pure Bismuthane

Abstract

(4-Chlorophenyl){(S)-2-[(R)-1-(dimethylamino)ethyl]ferrocenyl}iodobismuthane (3b), the first example of an optically pure bismuthane, was synthesized with exclusive stereoselectivity using Bi−N intramolecular coordination and the planar chirality of ferrocene. The variable-temperature 1H NMR spectra measured in the range from −50 to +50 °C in CDCl3 revealed that no epimerization takes place at the bismuth center. An X-ray crystallographic study demonstrated that the bismuth atom is diastereoselectively introduced into the substituted cyclopentadienyl ring and that the 4-chlorophenyl group occupies the exo position toward the iron atom. The absolute configuration around the bismuth atom was determined to be R. The longer intramolecular Bi−N distance of 3b [2.71(1) Å] compared with those of chloro{2-[(R)-1-(dimethylamino)ethyl]phenyl}phenylbismuthane (1a,b) [2.55(2), 2.57(2) Å, respectively] indicates a weaker Bi−N coordination in the ferrocene system. The specific rotation of 3b, [α]D23 −546 (c 1.0, CHCl3), is negatively much larger than that of bis(4-chlorophenyl){(S)-2-[(R)-1-(dimethylamino)ethyl]ferrocenyl}bismuthane (5b), [α]D23 −330 (c 1.0, CHCl3), which is considered to be the result of generation of the optically active center at bismuth.