Reversible N‐Heterocyclic Carbene‐Induced α‐H Abstraction in Tungsten(VI) Imido Dialkyl Dialkoxide Complexes

Abstract

The first reversible N‐ heterocyclic carbene (NHC) induced α‐H abstraction in tungsten(VI) imido‐dialkyl dialkoxide complexes is reported. Treatment of W( N Ar)(CH 2 Ph) 2 (O t Bu) 2 (Ar=2,6‐dichlorophenyl, 2,6‐dimethylphenyl, 2,6‐diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W( N Ar)(CHPh)(NHC)(CH 2 Ph)(O t Bu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert‐ butoxide ligand as t BuOH in the course of the reaction was observed. The formed alkylidene complexes and t BuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by 1 H NMR spectroscopy. A correlation between the steric and electronic properties of the NHC and the reaction rates was observed. Kinetics of a deuterium‐labeled complex in comparison to its non‐deuterated counterpart revealed the presence of a strong primary kinetic isotope effect (KIE) of 4.2, indicating that α‐H abstraction is the rate‐determining step (RDS) of the reaction.

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

“…20 To prove this assumption, we reacted W(N-2,6-Cl 2 − C 6 H 3 )(OtBu) 2 Cl 2 (THF) with 3.0 equiv of the sterically less demanding benzylmagnesium chloride. Although we recently showed that also dibenzyl complexes of the type W(NAr)-(CH 2 Ph) 2 (OtBu) 2 (Ar = 2,6-Me 2 -C 6 H 3 , 2,6-i Pr 2 -C 6 H 3 , 2,6-Cl 2 -C 6 H 3, ) can be synthesized, 31 in fact in this case, the corresponding tribenzyl complex W(N-2,6-Cl 2 -C 6 H 3 )-(CH 2 Ph) 3 (OtBu) (see the Supporting Information for its single crystal X-ray structure) was isolated in 42% yield. Here, no substitution of the second alkoxide occurred, which can again be attributed to the lower steric bulk of the benzyl groups compared to the neophyl groups in, e.g., W-4b.…”

Tungsten Oxo and Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes for the Visible-Light-Induced Ring-Opening Metathesis Polymerization of Dicyclopentadiene

“…Our group has a long-standing track record in the synthesis of group 6 olefin metathesis catalysts bearing an NHC ligand. − In previous studies, we found that five-coordinate NHC complexes usually possess high activity and functional group tolerance; the activation of those complexes occurs via a cationic species. , Indeed, later we succeeded in isolating cationic group 6 complexes that were outstandingly active (TON of up to 1.2 million for silica-supported cationic catalysts). ,, However, we noticed that certain representatives of the five-coordinate, neutral complexes were only active at elevated temperatures and some of those complexes were even completely inactive, which is especially true for tungsten-based catalysts. , …”

Stereoselective Ring‐Opening Metathesis Polymerization with Tungsten Sulfido Alkylidene N‐Heterocyclic Carbene Complexes

“…In many cases, cis-isotactic (cis-it), trans-isotactic (trans-it), cis-syndiotactic (cisst), or trans-syndiotactic (trans-st) structures are formed virtually selectively. [7,8] More recently, the new class of cationic DOI: 10.1002/macp.202200350 molybdenum and tungsten alkylidene NHC catalysts bearing imido, oxo and sulfido ligands, [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] respectively, has been developed. Compared to Schrock-type catalysts, this class of catalysts is characterized by a generally increased tolerance toward oxygen, moisture and functional groups [14,15,29] as well as by higher activities and productivities with turnover numbers exceeding 1.2 million when supported on silica.…”