Synthesis of Tungsten(VI) Imido Alkylidene Bispyrrolide Complexes via the Isocyanate Route

Abstract

Scheme 1. Established Synthetic Route to Tungsten MAP-Type Catalysts via Tungsten(VI) Bisimido Dichloro DME Complexes Communication pubs.acs.org/Organometallics

Cationic Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes That Contain Bulky Ligands

“…Additionally, the small NHC ligands 1,3-dimethylimidazol-2-ylidene and 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene were employed as the respective silver­(I) iodide adducts, in the hope of minimizing steric interactions of the NHC with other ligands. As has already been outlined, W imido alkylidene NHC complexes could not be prepared from highly electrophilic bis-triflate precursors but rather from the corresponding bis-halide complexes. , Investigations for the tert -butylimido alkylidene catalysts therefore started with the dipyridine complexes 1a , b . It was not surprising to find that the tungsten–pyridine bonds are extraordinarily strong.…”

“…As has already been outlined, W imido alkylidene NHC complexes could not be prepared from highly electrophilic bis-triflate precursors 1 but rather from the corresponding bis-halide complexes. 3,14 Investigations for the tert-butylimido alkylidene catalysts therefore started with the dipyridine complexes 1a,b. It was not surprising to find that the tungsten−pyridine bonds are extraordinarily strong.…”

Cationic Group VI Metal Imido Alkylidene N‐Heterocyclic Carbene Nitrile Complexes: Bench‐Stable, Functional‐Group‐Tolerant Olefin Metathesis Catalysts

“…However, the tungsten 2,6‐dichlorophenylimido and 3,5‐dimethylphenylimido alkylidene dichloride complexes were prepared following our recently published alternative synthetic procedure, [13] in which the imido ligand is introduced by reaction WOCl 4 with an isocyanate (Scheme 6 b). [14]…”

“…The use of excess KBr as well as finely powdering the reagent were found to speed up the reaction significantly.H owever, on al arger scale of several grams,r eaction times of up to 4days were sometimes necessary to achieve full conversion due to the heterogeneous nature of the reaction. However,no side-products were formed and the dibromide complex could be stored at À35 8 8Cu nder aN 2 -atmosphere for months.D ue to the apparent equivalency of the DME protons in 1 H-and 13 C-NMR, an octahedral structure with the bromide ligands trans to each other, similar to the corresponding bistriflate complexes,c an be assumed. Reaction of the dibromide complex with 1,3-dimesitylimidazol-2-ylidene (IMes)i nb enzene led to displacement of DME by the NHC,y ielding the tungsten imido alkylidene NHC dibromide complex W-02.…”

“…However,t he tungsten 2,6-dichlorophenylimido and 3,5dimethylphenylimido alkylidene dichloride complexes were prepared following our recently published alternative synthetic procedure, [13] in which the imido ligand is introduced by reaction WOCl 4 with an isocyanate (Scheme 6b). [14] Thet ungsten 2-tert-butylphenylimido alkylidene dichloride complex W-19 was obtained by protonation of the corresponding bispyrrolide complex with HCl in diethyl ether in the presence of DME (Scheme 6c).…”