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.

The influence of the structure of cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalysts, i. e. of [Mo(N-2-tert-butyl-C 6 H 4 ) (CHCMe 2 Ph)(NHC)X + B(Ar F ) 4 À ] (NHC = 1,3-di(2-Pr) imidazol-2-ylidene (iPr), 1,3-dimesitylimidazol-2-ylidene (IMes); X = pyrrolide, OCH(CF 3 ) 2 , B(Ar F ) 4 À = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) and of [Mo(N-3,5-Me 2 -C 6 H 3 )(CHCMe 2 Ph)(NHC)(CH 3 CN)X + B(Ar F ) 4 À ] (NHC = 1,3-dimesitylimidazol-2-ylidene, 1,3-dimesitylimidazolin-2-ylidene (IMesH 2 ); X = CF 3 SO 3 , OCPh(CF 3 ) 2 ) on E/Z-selectivity in the ring-opening cross-metathesis (ROCM) of endo, endo-2,3dicarbomethoxynorborn-5-ene (endo, endo-DCMNBE), exo, exo-2,3-dicarbomethoxynorborn-5-ene (exo, exo-DCMNBE), endo, exo-2,3-dicarbomethoxynorborn-5-ene ((+) DCMNBE) and 2,3-exo,exo-bis(acetoxymethyl)-7-oxabicyclo[2.2.l]hept-5-ene (7-oxa-NBE) with 1-pentene, styrene, allyltrimethylsilane, allyl benzyl ether, allyl phenyl ether and allyl ethyl ether has been studied. With the exception of the ROCM reaction of endo, endo-DCMNBE with styrene, all other ROCM reactions of endo, endo-DCMNBE proceeded under thermodynamic control without any post-metathesis isomerization reactions with full retention of the configuration of the newly formed 1,2-disubstituted double bond as confirmed by kinetic studies. Similar accounts for selected homometathesis reactions. Catalyst structure-selectivity correlations based on the buried volume, V bur , of the N-imido ligand are presented.ROCM, [2] this has been accomplished by the use of chiral catalysts having a stereogenic metal center, while in the case of ruthenium alkylidene-, [3] i. e. Grubbs-and Grubbs-Hoveyda-type catalyst-triggered ROCM, this was realized by the use of chiral Nheterocyclic carbene (NHC) ligands [3] or, again, with the aid of a stereogenic metal center. [4] In several reports [3a,5] the use of 7-oxa-or 7-azanorbornenes compounds has been described, to make use of the heteroatom to increase stereoselectivity. There are also reports, which exclusively use aryl-substituted alkenes i. e. styryl-alkenes or enol ethers or enoates as a cross-

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.

We report the synthesis of O- and N-chelated hexacoordinated molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and pentacoordinated molybdenum imido alkylidene NHC monotriflate monoalkoxide complexes and their use as thermally latent and in some cases air-stable precatalysts in the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Introduction of electron-withdrawing and electron-donating groups at the O-chelated alkylidene ligand allowed for the tuning of both the onset temperature of polymerization (T onset) and the temperature of the exotherm maximum (T exo,max). In addition, N-chelated complexes were synthesized using easily available 2-vinylpyridine or 2-vinyl-N,N-dimethylaniline to yield five-membered and, for the first time, four-membered molybdenum imido alkylidene NHC chelates. With these precatalysts, T onset and T exo,max could be varied between 52 and 142 °C and between 99 and 174 °C, respectively.

A series of thermotropic liquid crystalline polyesters derived from bis-(4-hydroxybenzoyloxy)-2methyl-1,4-benzene (BHBOMB) and aliphatic dicarboxylic acid chlorides were investigated. All these polyesters were synthesized by interfacial polycondensation method and characterized by differential scanning calorimetry and wide-angle X-ray diffractometer. These polyesters consist of BHBOMB as a mesogenic diol and aliphatic diacid chlorides were used as flexible spacers. The length of oligomethylene units in polymer was varied from the trimethylene to the dodecamethylene groups. The transition temperatures and thermodynamic properties were studied for all these polymers. All these polyesters were soluble in chlorinated solvents such as chloroform, dichloromethane, dichloroethane, etc. More importantly, all these polyesters exhibited very large mesophase stability.