Chemistry of transition metal carbene complexes: nucleophilic substitution reactions of cyanamide anion to Fischer carbene complexes

摘要: Rate constants for the reactions of cyanamide anion NC–NH− with Fischer carbene complexes type (CO)5MC(XR)C6H4Z M = Cr and W, XR SMe OMe, Z NMe2, Me, H, F, Cl CF3 in 50% MeCN–50% water (v/v) at 25 °C are reported. shows a much higher reactivity towards these than OH−, primary aliphatic amines (e.g. n-butylamine) or secondary alicyclic piperidine) but is slightly less reactive thiolate ions HOCH2CH2S−). The alkoxy were found to react faster thiomethyl derivatives, consistent previous findings alkoxide ion, CH(CN)2−, amine ion nucleophiles. Hammett ρ values 3.00 ± 0.08 (k1) 2.98 (k2) Cr–OMe–Z–NC–NH− 0.94 0.05 Cr-SMe-Z–NC–NH− reactions. reaction Cr-OMe-Z Cr-SMe-Z CH(CN)2− DABCO (1,4-diazabicyclo[2.2.2]octane) (in H2O comparable present hence value methoxy over corresponding derivatives best explained by considering substituent effects not only on transition state (TS) also reactant, previously observed pattern. Higher k1(W)/k1(Cr) ratios (thiomethyl)carbene as well ethoxycarbene related intrinsic rate constant which tungsten–carbene ones resulting an enhanced ratio. This can be electronegativity latter; result negative charge central metal atom more localized case W causing destabilization TS reactivity.