Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes Containing Pyrrolide Ligands: Access to Catalysts with Sterically Demanding Alkoxides

Abstract

Neutral and cationic complexes of the general formulas Mo­(NR1)­(CHCMe2Ph)­(NC4H4)2(NHC) and [Mo­(NR1)­(CHCMe2Ph)­(NC4H4)­(NHC)]­[B­(ArF)4] (R1 = 2-tBu-C6H4, 2,6-iPr2-C6H3, 2,6-Me2-C6H3, tBu, Ad; B­(ArF)4 = tetrakis­(3,5-(CF3)2-C6H3)­borate; NHC = N-heterocyclic carbene) have been synthesized. Three NHC ligands with different electronic and steric properties, namely, 1,3-diisopropylimidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4-triazol-2-ylidene (TPT), and 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl) have been incorporated. The cationic complexes exhibit good activity in olefin metathesis at room temperature, provided no protic functional groups are present. Both neutral and cationic NHC (bis)­pyrrolide complexes serve as excellent precursors to molybdenum imido alkylidene NHC complexes with sterically demanding alkoxides. Here, introduction of the alkoxide ligands proceeds upon protonation of pyrrole with the respective alcohol. Application of the sterically demanding alcohols (HO-2,6-(2,4,6-Me3-C6H2)2-C6H3 1-H and 5,5′,6,6′,7,7′,8,8′-octahydro-3,3′-dibromo-2-silyloxy-1,1′-binapth-2′-ol 2-H) instead of the corresponding alkoxides circumvents previously described C–H activation reactions at the NHC. The derived sterically encumbered (chiral) complexes were accessible in good yields by simple washing with nonpolar solvents.

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

“…Catalysts containing the more basic alkyl‐substituted imido ligands ( 8 , 11 , 17 a ) displayed decreased reactivity compared to complexes with aromatic imido ligands. This was also observed earlier . To demonstrate that this is a reactivity issue, we mixed catalyst 11 with 10 equiv 10‐undecenoic acid and monitored the reaction by 1 H NMR (Figure S122).…”

“…Recently, we reported on oxo‐bridged dimers that formed in the reaction of a Mo‐imido alkylidene NHC complex containing a basic aryloxide with water . Moreover, it was shown that protonation of Mo( N ‐Ad)(CHCMe 2 Ph)(NHC)(pyrrolide)(aryloxide) selectively eliminates pyrrole . Also, other examples in which pyrrole was eliminated from metal imido alkylidene NHC bispyrrolide complexes or cationic metal imido alkylidene NHC monopyrrolide complexes were reported .…”

“…[16] Moreover, it was shown that protonation of Mo(N-Ad)(CHCMe 2 Ph)-(NHC)(pyrrolide)(aryloxide) selectively eliminates pyrrole. [21] Also, other examples in which pyrrole was eliminated from metal imido alkylidene NHC bispyrrolide complexes or cationic metal imido alkylidene NHC monopyrrolide complexes were reported. [15] All this evidence indicates that Xtype ligands are the first and preferred target for proton transfer.…”

“…This was also observed earlier. [12,15,21] To demonstrate that this is a reactivity issue, we mixed catalyst 11 with 10 equiv 10-undecenoic acid and monitored the reaction by 1 H NMR ( Figure S122). Even after 80 minutes, most of the catalyst was unchanged, clearly showing that proton transfer to either ligand was very slow.…”

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

“…[7] We also prepared silica-supported versions thereof [8] and established synthetic routes to anchor sterically demanding alkoxides to our catalysts, [7f,9] which are decisive for the control over stereochemistry. [7] We also prepared silica-supported versions thereof [8] and established synthetic routes to anchor sterically demanding alkoxides to our catalysts, [7f,9] which are decisive for the control over stereochemistry.…”