Reactions of transition metal carbonyl anions with 2-(1-bromoalkylidene)thiazolidin-4-ones: halogenophilic attack or deprotonation

Abstract

Bromophilic attack by the transition metal carbonyl anion, [Re(CO)(5)]Na (pK(a) = 21.1), on 2-(1-bromoalkylidene)thiazolidin-4-ones is significantly faster than abstraction of an acidic lactam hydrogen (pK(a) similar to 17-18), when the generated carbanion is stabilized by an alpha-CN or alpha-PhCO group. The bromophilic reaction of 2-(1-bromoalkylidene)thiazolidin-4-one, having an alpha-CN electron-withdrawing group, resulted in formation of a new metallacyclic anionic complex. With less reactive vinyl bromides, containing an alpha-CONHPh or alpha-CO(2)Et group, only deprotonation is observed. The role of the metal carbonyl anion is highlighted by a comparison with the 9-methylfluorenide carbanion (pK(a) of 9-methylfluorene is 22.3), which reacts exclusively via a deprotonation pathway.