Unusual formation of 5-halo-2,2'-bipyridyls by treatment of tris(2-pyridyl)phosphine derivatives with halogens

Article abstract of DOI:10.1016/0040-4039(95)00719-S

Reaction of tris(2-pyridyl)phosphine with chlorine in dichloromethane or acetonitrile gave chlorotris(2-pyridyl)phosphonium chloride which afforded a coupling product, 2,2'-bipyridyl, by treating with dilute HCl. Treatment of tris(2-pyridyl)phosphine or tris(2-pyridyl)phosphine oxide with chlorine or bromine in methanol gave unusual 5-halo-2,2'-bipyridyl as a major coupling product. This is undoubtedly an electrophilic substitution on a pyridyl ring in the pentacovalent phosphorane.