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On the mechanism of imine elimination from Fischer tungsten carbene complexes

2016; Beilstein Institute for the Advancement of Chemical Sciences; Volume: 12; Linguagem: Inglês

10.3762/bjoc.12.125

2195-951X

Philipp Veit, Christoph Förster, Katja Heinze,

N-Heterocyclic Carbenes in Organic and Inorganic Chemistry

(Aminoferrocenyl)(ferrocenyl)carbene(pentacarbonyl)tungsten(0) (CO) 5 W=C(NHFc)Fc ( W(CO) 5 ( E -2) ) is synthesized by nucleophilic substitution of the ethoxy group of (CO) 5 W=C(OEt)Fc ( M(CO) 5 (1 Et ) ) by ferrocenyl amide Fc-NH – (Fc = ferrocenyl). W(CO) 5 ( E -2) thermally and photochemically eliminates bulky E -1,2-diferrocenylimine ( E -3 ) via a formal 1,2-H shift from the N to the carbene C atom. Kinetic and mechanistic studies to the formation of imine E -3 are performed by NMR, IR and UV–vis spectroscopy and liquid injection field desorption ionization (LIFDI) mass spectrometry as well as by trapping experiments for low-coordinate tungsten complexes with triphenylphosphane. W(CO) 5 ( E -2) decays thermally in a first-order rate-law with a Gibbs free energy of activation of Δ G ‡ 298K = 112 kJ mol −1 . Three proposed mechanistic pathways are taken into account and supported by detailed (time-dependent) densitiy functional theory [(TD)-DFT] calculations. The preferred pathway is initiated by an irreversible CO dissociation, followed by an oxidative addition/pseudorotation/reductive elimination pathway with short-lived, elusive seven-coordinate hydrido tungsten(II) intermediates cis (N,H)-W(CO) 4 (H)( Z -15) and cis (C,H)-W(CO) 4 (H)( Z -15) .