KINETISCHE UND MECHANISTISCHE UNTERSUCHUNGEN VON UEBERGANGSMETALLKOMPLEX-REAKTIONEN. VI. KINETIK UND MECHANISMUS DER EINSCHIEBUNG VON DIMETHYLCYANAMID IN DIE METALL-CARBENKOHLENSTOFF-BINDUNG VON PENTACARBONYL(ARYLPHENYLCARBEN)WOLFRAM

Article abstract of DOI:10.1016/S0022-328X(00)84684-7

Pentacarbonyl(arylphenylcarbene)tungsten complexes, (CO)5W (Ia, R=OCH3; Ib, R=CH3; Ic, R=H; Id, R=Br; Ie, R=CF3) react with dimethylcyanamide (II) via insertion of the C<*>N group into the metal-carbene bond.The formation of pentacarbonyltungsten(0) (IIIa-IIIe) follows a second-order rate law: d/dt=k.Replacement of R=H by electron-withdrawing substituents (Br, CF3) results in an increase, by electron-donating groups (CH3, OCH3) in a decrease of the reaction rate.The rate constants correlate well with Hammett's ?-constants.The activation enthalpies ΔH^<*> are low (37.3-41.6 kJ mol^-1), the activation entropies ΔS^<*> strongly negative (-119 to -133 J mol^-1 K^-1).The results are discussed on the basis of an associative stepwise mechanism with a nucleophilic attack of the C<*>N group of II at the carbene carbon in the first reaction step.