Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

Abstract

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.

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

“…In line with our concept, complexes containing the pentafluorophenoxide ligand were found to be the most active ones, as judged by the significantly higher TONs for allyl sulfide and the ether containing substrate S3 , as well as activity in the metathesis of methyl oleate. The productivity in the metathesis of pent‐4‐en‐1‐ol was similar or slightly lower than for the triflate complexes, this is in line with what has been observed for the analogous molybdenum complexes [7a] . Overall, tungsten complexes based on electron‐withdrawing imido ligands, that is, 2,6‐dichlorophenylimido and 2‐trifluoromethylphenylimido offered the highest productivities, likely due to the more electrophilic metal centers.…”

“…Having realized the influence of the nitrile ligand on the air stability of cationic tungsten imido alkylidene NHC complexes, we explored whether this concept could eventually be transferred to the analogous molybdenum compounds. First, we reevaluated the results of our previously conducted experiments on air stability of cationic molybdenum imido alkylidene NHC complexes [7a] . Interestingly, upon storage in air for 16 h, the nitrile‐free complexes all showed the uptake and coordination of water, accompanied by varying amounts of decomposition.…”

“…In the case of W‐08 , the addition of pivalonitrile was necessary to obtain crystalline products. The triflate anion was chosen because the related molybdenum complexes have proven highly reactive and tolerant towards alcohol‐containing substrates and, in some cases, air‐stable for several hours [7a] . The pentafluorophenoxide was chosen in view of an earlier observation that cationic tungsten imido alkylidene NHC complexes containing the pentafluorophenoxide ligand by far outperformed all other tungsten compounds in terms of productivity [10] .…”

“…Encouraged by the air stability of cationic molybdenum imido alkylidene NHC complexes, [7a] we set out to explore the stability of their tungsten analogs. For this purpose, a sample of the solid complex was exposed to air for up to two weeks.…”

Silica‐Supported Cationic Tungsten Imido Alkylidene Stabilized by an N‐Heterocyclic Carbene Ligand Boosts Activity and Selectivity in the Metathesis of α‐Olefins

“…Alkene metathesis is a robust and atom‐economic synthetic method for the creation of carbon‐carbon double bonds; it has thus found ample use in both academia and industry [1–3] . Over the last decades, substantial efforts have been put in developing efficient molecular catalysts, [4–15] but immobilized analogs have received increasing attention because of their benefits for process intensification [16,17] and their improved performance with the development of well‐defined supported systems, mostly prepared through surface organometallic chemistry (SOMC) [18–23] . SOMC, a molecular approach to generate well‐defined heterogeneous catalysts, in which the surface is exploited as a ligand to covalently anchor molecular complexes, has been particularly successfully employed for the preparation of silica‐supported Schrock‐type metathesis catalysts of the general formula [(≡SiO)M(E)(=CHR)(X)] (M=Mo or W, E=NR or O), that display high activity, selectivity and stability with performances often surpassing those of their homogeneous counterparts ( Scheme 1).…”