Vanadium Imido NHC Complexes for Ring-Closing Olefin Metathesis Reactions

Abstract

Vanadium bis-phosphine imido and oxo chloride alkylidenes have been extensively applied in the ring-closing metathesis of various acyclic olefins. However, their reactions involving ethylene have shown limited productivity due to rapid decomposition. The primary degradation pathway involving V bis-phosphine imido complexes is β-H elimination at an unsubstituted metallacyclobutane. In contrast, β-H elimination is disfavored for V oxo species, but bimolecular decomposition precludes its high productivity. Herein, we present the synthesis of V imido NHC complexes that are the most productive V catalysts toward various terminal olefins in ring-closing metathesis reactions. Experimental and computational studies suggest that β-H elimination and bimolecular decomposition are disfavored for V imido NHC complexes.

Singlet Spin State Drives [V]-Carbene To Catalyze Olefin Metathesis: A Computational Analysis

“…More recently, in a different [V]-carbene class, one of the ketamides was replaced by an aryloxy ligand V V (CHSiMe 3 )­(NAr)­(O-2,6- i Pr 2 C 6 H 3 )­(PMe 3 ) (Scheme c), showing the highest catalytic activity so far toward the ROMP of norbornene and other strained olefins. , This latter case triggered a family of complexes with enhanced catalytic activity, including a type of complex with a fluorinated alkoxy ligand V V (CHSiMe 3 )­(NR)­[OC­(CF 3 ) 3 ]­(PMe 3 ) 2 (Scheme d) that catalyzes cis -specific ROMP and is thermally robust. , Lately, vanadium carbenes with both imido (Scheme e) and oxo (Scheme f) ligands have been used to catalyze ring-closing metathesis, − and their computational studies have shown that β-hydride elimination plays a subordinate role in deactivating the reaction. Both Schrock carbene catalysts and this family of [V]-carbene above described are high-oxidation early transition metal complexes with similarities, such as bearing ketamide and an alkoxide ligand, as well as the metal center in its highest oxidation center.…”

Synthesis and Activity of Vanadium Oxo NHC Alkylidenes. Remarkable Preference for Degenerate Metathesis and Application for Carbon Isotope Exchange

“…The main degradation pathway for catalysts 5 is the exchange of NHC to phosphine that forms during the initiation step, leading to a bis phosphine complex analogous to 3, which is unstable toward ethylene. 21 The next logical step for the catalyst optimization is the synthesis of V oxo NHC alkylidenes to preserve remarkable stability toward β-H elimination of V oxo complexes and disfavor bimolecular decomposition by introducing a large NHC ligand. In addition, we hypothesized that the shift to phosphine-free V oxo alkylidenes would improve catalyst performance, since phosphines can participate in side reactions and deactivation pathways, such as reduction of highoxidation-state V complexes 22 and reaction with alkylidenes.…”

“…Recently, our group has developed V catalysts for ring-closing metathesis (RCM) of internal and terminal olefins and reported the highest V-based OM productivity involving terminal dienes. − Representative V alkylidene complexes 3 – 5 and their productivities (turnover numbers, TONs) in RCM involving the model substrate 1 are shown in Scheme . The dissociation of one neutral ligand is required to access four-coordinate 14-electron active catalysts 3a – 5a during the reaction.…”

“…Exchange of phosphine in imido complex 3 to an N-heterocyclic carbene (NHC) ligand increases the TON from 6 to 170 in the reaction with 1 by suppressing both β-H elimination from MCB and bimolecular decomposition. The main degradation pathway for catalysts 5 is the exchange of NHC to phosphine that forms during the initiation step, leading to a bis phosphine complex analogous to 3 , which is unstable toward ethylene …”

Effect of Phosphine, B(C₆F₅)₃ in Ring Opening Metathesis Polymerization (ROMP) of Cyclic Olefins by (Arylimido)Vanadium(V)–Alkylidene Catalysts and the Chain Transfer ROMP of Cycloheptene

“…1 H NMR monitoring of the reaction of 4 with B(C6F5)3 by revealed that the sole reaction prod-uct observed was Me3SiHC＝CHSiMe3 (cis/trans mixture), additionally identified by GC-MS spectrometry. These results clearly suggest a decomposition pathway via dimeric alkylidene coupling 25),26),29), 30) . In addition, these results also contrast with the formation of another alkylidene species after addition of B(C6F5)3 into a…”