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

Abstract

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-

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…135 Various cationic molybdenum imido alkylidene NHC complexes have also been investigated for the stereoselective olefin ring-opening cross metathesis. 136 The reactions could be accomplished with high E-or Z-selectivity depending on ligands steric hindrance around the Mo center. Importantly, in presence of most of these catalysts, no post-metathesis isomerization was observed, the products being formed under thermodynamic control.…”

Charge Distribution in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: A Combined X-ray, XAS, XES, DFT, Mössbauer, and Catalysis Approach

“…Molybdenum and tungsten imido/oxo alkylidene N -heterocyclic carbene (NHC) complexes have evolved as a new potent class of catalysts for olefin metathesis. , Particularly the high activities, expressed as turnover frequencies (TOFs), and their high productivities, expressed as turnover numbers (TONs), together with their exceptional high-temperature stability up to 160 °C and their pronounced functional group tolerance, render them highly attractive. − Their silica-immobilized versions have been reported to allow TONs of >1,200,000 and TOFs of >40 s –1 , which also makes them attractive for industrial use. , Complementarily, their permanent cationic charge renders them applicable to biphasic, continuous conditions . Also, fully thermally latent catalysts have been designed for reaction injection molding (RIM). , Finally, they can be used in ring-opening metathesis polymerization (ROMP) for the synthesis of highly tactic polymers − as well as in stereoselective ring-opening cross-metathesis (ROCM) and cross-metathesis (CM) …”

Recent Developments in Z‐Selective Olefin Metathesis Reactions by Molybdenum, Tungsten, Ruthenium, and Vanadium Catalysts

“…The same product 104 was obtained from RCM of diene 103 (R=Ph) in the presence of 1,4‐benzoquinone in toluene at 100 °C using Ru‐2 a (30 mol%) (Scheme 51). [106,107] …”