Stereoselective Olefin Ring‐Opening Cross Metathesis Catalyzed by Molybdenum Imido Alkylidene N ‐Heterocyclic Carbene Complexes

Effect of liquid confinement on regioselectivity in the hydrosilylation of alkynes with cationic Rh(I) N-heterocyclic carbene catalysts

Polymeric mesoporous monoliths were prepared via ring-opening metathesis polymerization (ROMP) from norbornene (NBE), 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene (DMN-H6), tris(norborn-2-enylmethylenoxy)methylsilane and the 1st-generation Grubbs catalyst [RuCl2(PCy3)2(CHC6H5)] in the presence of 2-propanol and toluene and surface grafted with 1-(2-((norborn-5-ene-2-carbonyl)oxy)ethyl)-3-ethyl-1H-imidazol-3-ium tetrafluoroborate. Subsequently, a supported ionic-liquid-phase (SILP) system was created by immobilizing the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] with the cationic catalyst [Rh((1-pyrid-1-yl)-3-mesitylimidazol-2-ylidene)(COD)+BF4-] (Rh-1; COD = 1,4-cyclooctadiene) dissolved therein. The regio- and stereoselectivity of Rh-1 dissolved in the IL and supported on the mesoporous monolith, referred to as Rh@SILPROMP, in the hydrosilylation of 1-alkynes with HSiMe2Ph was studied and compared to that of the homogeneous catalyst Rh-1 under biphasic conditions using methyl tert-butyl ether (MTBE) as a second organic phase. Different amounts of IL were used, which allowed for the creation of SILPs with different layer thicknesses. Rh@SILPROMP provided by far better β-(Z) selectivity for both aromatic and aliphatic 1-alkynes in comparison to Rh-1 used under biphasic conditions. The highest β-(Z) selectivity was obtained with the thinnest IL layer. No leaching of the IL or rhodium from the SILP system into the organic phase was observed, resulting in virtually metal-free hydrosilylation products. The data obtained with Rh@SILPROMP were also compared with those from previous studies with Rh-1 in the same IL supported on polyurethane-derived mesoporous monolithic supports (Rh@SILPPUR) and on mesoporous SBA-15 (Rh@SILPSBA-15). For the first time, the use of a liquid confinement created by both a SILP and the support itself to tune the transition state of an organometallic catalyst by non-covalent interactions and thus stereo- and regioselectivity is outlined.

Late-stage Modification of Cage Diones by Tandem Metathesis

Herein, we report the application of tandem ring-opening cross-metathesis (ROCM) and tandem ring-opening cross-metathesis followed by cross-metathesis (ROCM/CM) in highly strained diastereomeric heptacyclic cage diones (HCCD's) catalyzed by ruthenium catalysts. These caged diastereomeric compounds have a high degree of ring strain as well as steric congestion. Therefore, the present work related to ROCM is unique and intricate as compared to simple norbornene derivatives. It is useful to prepare new hexacyclic cage derivatives suitable for high-energy density materials. The synthetic strategy of starting cage compounds features the Diels-Alder reaction, and [2+2] photocycloaddition as key steps.

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

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.