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

• Published in: Tetrahedron letters 2011-01, Vol.52 (1), p.29-33
• Main Authors: Sazonov, P.K., Džambaski, Z., Shtern, M.M., Marković, R., Beletskaya, I.P.
• Format: Article
• Language: English
• Subjects: 2-(1-Bromoalkylidene)thiazolidin-4-ones; anions; bromides; chemical reactions; Halogenophilic reactions; hydrogen; Metal carbonyl anions; organobromine compounds; Proton transfer; sodium
• ISSN: 0040-4039; 1873-3581
• DOI: 10.1016/j.tetlet.2010.10.112

Bromophilic attack by the metal carbonyl anion, [Re(CO)5]− Na+; pKa of [Re(CO)5]H=21.1, on 2-(1-bromoalkylide-ne)thiazolidin-4-ones (Z=CN and COPh) is significantly faster than abstraction of an acidic lactam hydrogen (pKa ∼17–18). Bromophilic attack by the transition metal carbonyl anion, [Re(CO)5]Na (pKa=21.1), on 2-(1-bromoalkylidene)thiazolidin-4-ones is significantly faster than abstraction of an acidic lactam hydrogen (pKa ∼17–18), when the generated carbanion is stabilized by an α-CN or α-PhCO group. The bromophilic reaction of 2-(1-bromoalkylidene)thiazolidin-4-one, having an α-CN electron-withdrawing group, resulted in formation of a new metallacyclic anionic complex. With less reactive vinyl bromides, containing an α-CONHPh or α-CO2Et group, only deprotonation is observed. The role of the metal carbonyl anion is highlighted by a comparison with the 9-methylfluorenide carbanion (pKa of 9-methylfluorene is 22.3), which reacts exclusively via a deprotonation pathway.