Physical Organic Chemistry of Transition Metal Complexes. 31. Nucleophilic Attachment of the Malononitrile Anion to Fischer Carbene Complexes

Abstract

Rate constants for the attachment of CH(CN)2- to Fischer carbene complexes of the type (CO)5MC(XR)C6H4Z with M = Cr, W and XR = OMe, OEt, SMe in 50% MeCN−50% water (v/v) at 25 °C are reported. The malononitrile anion shows a much higher reactivity toward these carbene complexes than OH-, primary aliphatic amines (e.g. n-butylamine), or secondary alicyclic amines (e.g. piperidine) but is slightly less reactive than thiolate ions (e.g. HOCH2CH2S-). The alkoxycarbene complexes react more quickly than the thiomethyl derivatives, consistent with previous findings for alkoxide ion, OH-, amine, and thiolate ion nucleophiles. Hammett ρ values are 0.69 for the reaction with (CO)5CrC(SMe)C6H4Z and 2.88 for the reaction with (CO)5CrC(OMe)C6H4Z. The much larger ρ value for the methoxycarbene complexes fits a pattern observed previously with other nucleophiles and can be explained by the π-donor effect of the methoxy group, which is much stronger than that of the thiomethyl group.