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For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing acyclic diene metathesis (ADMET) oligomerization. With cationic molybdenum imido ...

the stereospecific ring-opening metathesis polymerization (romp) of endo,exo-2,3-dimethoxymethylnorborn-5-ene (dmmnbe) was accomplished using cationic tungsten imido alkylidene n-heterocyclic carbene (nhc) complexes as initiators. these even outperform a cationic molybdenum imido alkylidene nhc reference initiator and allow for high trans-isospecifity of up to 95%. furthermore, tuning the steric demand of the ligands of the metal complexes allowed for a change in stereoselectivity. thus, for the first time, cis-syndiotactic polymers were synthesized by the action of non-chelating cationic group 6 metal alkylidene nhc complexes. both selectivities were not limited to dmmnbe, but also found for other monomers, namely endo,exo-2,3-dicarbomethoxynorborn-5-ene (dcmnbe), methyl-n-(s)-(−)-α-methylbenzyl-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate and 2,3-bis[(menthyloxy)carbonyl]norbornadiene.

The charge delocalization between the N-heterocyclic carbene (NHC) and the metal in cationic molybdenum imido alkylidene NHC mono(nonafluoro-tert-butoxide) complexes has been studied for different ...

Despite their excellent selectivities and activities, Mo‐and W‐based catalysts for olefin metathesis have not gained the same widespread use as Ru‐based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air‐stable cationic‐at‐metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio‐) esters, (thio‐) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron‐withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes., The presence of a nitrile ligand renders cationic group VI metal imido alkylidene NHC complexes air‐stable, functional‐group‐tolerant, and olefin metathesis‐active. No pre‐activation is required and no loss of activity, productivity, or selectivity in olefin metathesis is observed due to the presence of a nitrile ligand.

The first reversible N‐heterocyclic carbene (NHC) induced α‐H abstraction in tungsten(VI) imido‐dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH2Ph)2(OtBu)2 (Ar=2,6‐dichlorophenyl, 2,6‐dimethylphenyl, 2,6‐diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W(NAr)(CHPh)(NHC)(CH2Ph)(OtBu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert‐butoxide ligand as tBuOH in the course of the reaction was observed. The formed alkylidene complexes and tBuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by 1H 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., Time to say good bye: The N‐heterocyclic carbene‐induced α‐H elimination in tungsten(VI) imido dialkyl dialkoxide complexes offers access to tungsten(VI) imido alkylidene monoalkyl monoalkoxide complexes in excellent yields of up to 96 %.

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts., Sometimes less is more: reduction of electron density in cationic Mo imido alkylidene N‐heterocyclic carbene complexes substantially enhances tolerance to hydroxyl groups in olefin metathesis reactions including acyclic diene metathesis polymerization, cross‐metathesis and ring‐opening cross metathesis.

All primary data files for the publication can be found here. Procedures, reaction conditions and used analytical equipment is discussed in detail in the experimental section or the supporting information of the paper. Nuclear magnetic resonance (NMR) Spectra are named according to the numbering in the publication. ¹H-NMR, ¹⁹F-NMR and ¹³C-NMR spectra were recorded using a Bruker Avance III 400 (400MHzfor 1H, 101MHz for 13C) spectrometer. NMR files can be opened with software such as MestReNova or TopSpin. Size exclusion chromatography (SEC) and Matrix Assisted Laser Desorption Ionization - Time of Flight Mass Spectrometry (MALDI-TOF-MS) data are provided as text-files and can be plotted with software such as MS Excel.

All primary data files that were contributed to the publication by the Buchmeiser Research Group can be found here. Data obtained by the Coperet Research Group is not included. Procedures, recation conditions and used analytical equipment is discussed in detail in the experimental section or the supporting information of the paper. NMR Spectra are named according to the numbering in the publication. 1H-NMR, 19F-NMR and 13C-NMR spectra were recorded using a Bruker Avance III 400 (400MHzfor 1H, 101MHz for 13C) spectrometer.

Both solvent-free and acetonitrile-containing cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NR')(CHCMe2 R)(NHC)(X)+ A- ] (R' = 2,6-Cl2 -C6 H3 , tBu, 2-CF3 -C6 H4 , 2-tBu-C6 H4 , 2,6-iPr2 -C6 H3 , 2,6-Me2 -C6 H3 ; R = Me, Ph; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-di-iPr-imidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4-triazol-2-ylidene); X = CF3 SO3 , C6 F5 O, OCH(CF3 )2 , OC(CF3 )3 , pyrrolide, C6 F5 COO, 2,6-(CF3 )2 -C6 H3 COO; A- = B(ArF )4- , Al(OC(CF3 )3 )4- ), have been investigated for their propensity to cyclopolymerize 4,4-disubstituted 1,6-heptadiynes. All metal complexes contain a stereogenic (chiral) metal center, which accounts for the high reactivity and high regioselectivity of insertion (>99%) that are observed for all metal complexes, leading to highly conjugated, α-insertion-derived polyenes that are based on a highly regular polymer backbone and that show absorption maxima close to 600 nm. With the chiral monomer 4-(ethoxycarbonyl)-4-(1S,2R,5S)-(-)-menthoxycarbonyl-1,6-heptadiyne, high syndiospecifity (>99% syndiotactic) is observed. A mechanism explaining the high regio- and stereoselectivity is presented. Thus, α-addition of the monomers proceeds chain-end-controlled trans to the NHC and is preferred over β-addition through intramolecular Mo-O chelation. Insertion of the monomers entails double inversion at the stereogenic metal center in the course of one complete monomer insertion.

The concept of metal imido and metal oxo alkylidene N-heterocyclic carbene (NHC) catalysts for olefin metathesis is outlined. This includes the synthetic concepts for their synthesis, their reactivity, the role of the metal, the origin of functional group tolerance as well as relevant aspects of regio- and stereoselectivity. Attention is also devoted to the role of the NHC in the generation and stabilization of cationic metal complexes. Indeed, the novel concept allows for the preparation of neutral, pentacoordinate pre-catalysts that serve as excellent progenitors to the corresponding tetracoordinate cationic systems. The activation process entails the release of an anionic ligand and can be regulated by the careful choice of ligands. This allows for the design of precursor catalysts that are fully thermally latent in the presence of monomer, which can be used in single-component, dicyclopentadiene (DCPD) based resin systems for reaction injection molding (RIM) or resin transfer molding (RTM). Mechanistic aspects, i.e. an associative vs. a dissociative mechanism, and their importance for this particular feature are outlined, too. Also, concepts for preparing all-cis or all-trans-tactic polymers and for accomplishing E- and Z-selective olefin metathesis reactions are summarized. In addition, the immobilization of this new type of olefin metathesis catalysts on silica, the reactivity of the resulting supported catalysts, both in olefin or alkyne metathesis, is described. In addition, aspects of biphasic catalysis, for which cationic catalysts can be used, are addressed. Finally, the extension of this concept to neutral metal alkylidyne NHC catalysts for alkyne metathesis, their state of the art and remaining challenges are outlined.