Organometallic Complexes of Group 5 Metals With Metal-Carbon Sigma and Multiple Bonds

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

“…Olefin metathesis (OM) is a crucial reaction to construct C–C double bonds utilized to produce commodity and fine chemicals. − The first reported well-defined complex that could promote catalytic OM was Tebbe’s reagent (Cp 2 TiCH 2 ClAlMe 2 ), which can slowly catalyze the exchange of methylene groups (degenerate metathesis) between two terminal olefins, as shown by Tebbe in 1979 . Subsequent research led to the development of highly efficient OM catalysts based on second- and third-row transition metals. − The recent progress, driven by green chemistry and economic factors, shows that the development of OM catalysts based on first-row metals, primarily vanadium, is achievable. − Thus, Nomura reported a series of highly active and selective V-based catalysts for ring-opening metathesis polymerization of cyclic olefins. − Some of those complexes have been utilized by Farrell in cross-metathesis (CM) but showed limited activity. , …”

“…This can be explained by the higher stabilization of a negative charge at the α-C atom by the aryl group in benzylidene than by the alkyl group in butylidene (alkylidene derived from 1-hexene). To overcome the bias in forming alkylidenes, we performed CIE between 13 C-labeled 4-phenyl-1-butene (9-13 C) and 1-hexene. However, the corresponding alkylidenes are not thermally stable.…”

[PNP]-Stabilized Niobium(IV) and Tantalum(IV) Complexes: Synthesis and Characterization of an Open-Shell Tantalum Alkylidene

“…In the 1970s, a cornerstone in organometallic chemistry was laid when Schrock and co-workers prepared the prototypical d 0 -configured tantalum and niobium alkylidenes ( t BuCH 2 ) 3 M=CH t Bu (M = Nb, Ta), which often serve as textbook examples for nucleophilic Schrock carbenes . Within a few years, closely related alkylidenes featuring chloride, cyclopentadienyl or alkoxide ligands, such as (thf) 2 Cl 3 M=CH t Bu, Cp 2 ClM=CH t Bu, or (Me 3 P)­(O t Bu)­Cl 2 M=CH t Bu (M = Nb, Ta) have been prepared and employed as well-defined catalysts for olefin metathesis reactions in homogeneous solution .…”

(Arylimido)niobium(V)–Alkylidenes as the Catalysts for Ring-Opening Metathesis Polymerization (ROMP) of Cyclic Olefins: Z-Specific ROMP of Cyclooctene by Nb(CHSiMe3)(NC6H5)[OC(CF3)3](PMe3)2

“…Certain high-oxidation-state metal–alkylidene complexes with early transition metals − have been known to play essential roles as catalysts in olefin metathesis reactions, − ,− which have been recognized as useful methods for the synthesis of functional polymers − as well as fine chemicals. − In particular, ring-opening metathesis polymerization (ROMP) of cyclic olefins is widely employed for the synthesis of advanced polymers. − ,, In contrast to successful examples of ROMP demonstrated by molybdenum­(VI)– and tungsten­(VI)–alkylidene catalysts, − , and later by vanadium­(V)–alkylidene catalysts, − reports of organo-niobium complex catalysts still have been limited until recently (Chart ). − After the first report by Mashima et al in 1997 (catalyst A ), there were two reports for the ROMP of norbornene (NBE) in 2016 (catalysts B and C ). , In particular, (imido)­niobium–alkylidenes, Nb­(CHSiMe 3 )­(NR)­[OC­(CF 3 ) 3 ]­(PMe 3 ) 2 ( C ), not only exhibited much higher catalytic activ...…”

“…52 It turned out that these niobium(V)− alkylidenes catalyzed the ROMP with high catalytic activities, and the activities by the fluorinated alkoxide analogues (1−6) were affected by the ligand set employed. The activity of Nb(CHSiMe 3 )(NAr)[OC(CF 3 ) 3 ](PMe 3 ) 2 (1−6) at 25 °C (initial NBE conentration 0.44 mmol/mL) increased in the order TOF = 11.6 s −1 (Ar = 2-MeC 6 H 4 , 2, run 3), 43 12.0 (2,6-Cl 2 C 6 H 3 , 3, run 5) 43 < 15.5 (2,6-Me 2 C 6 H 3 , 1, run 1), 41 16.5 (C 6 H 5 , 4, run 6) < 21.9 (2,6-F 2 C 6 H 3 , 5, run 7) < 56.7 (C 6 F 5 , 6, run 9). The C 6 F 5 analogue 6 showed the highest activity, suggesting that, as observed in the vanadium analogues, 32−36 the electron-withdrawing nature of the arylimido ligand plays a role.…”