Engendering reactivity at group 5-heteroatom multiple bondsviaπ-loading

Abstract

The proof is in the π: when more than one π-donor ligand is bound to a group 5 metal center, imido-based reactivity increases.

Evaluation of Tungsten Catalysis among Early Transition Metals for N-Aryl-2,3,4,5-tetraarylpyrrole Synthesis: Modular Access to N-Doped π-Conjugated Material Precursors

“…One-electron reduction of B by the solvent or substrates at the high reaction temperature produces imidotungsten­(IV) C , and the cleavage of the NN bond of 2a by C generates bis­(imido)­tungsten­(VI) species D . [2 + 2]-Cycloaddition with a diarylacetylene 1 , giving imidoalkylidene species E , and successive enyne metathesis of E with the second alkyne results in the formation of metallacycle species F . , Finally, reductive electrocyclization of F yields the corresponding pyrrole-coordinated tungsten species G , and then product 3 is released with regeneration of imidotungsten­(IV) C . Details of the structures of E and F as well as the C–N bond-forming step are discussed in the DFT study below.…”

“…The reaction of bis­(imido)vanadium complex 6 and diarylacetylene 1a in the presence of CuCl as a phosphine scavenger gave α-diimine derivative 7a in 67% yield along with pyrrole 3aa in 26% yield (Scheme a), which was consistent with the observation of 7a in Table , entry 11. Formation of an α-diimine through consumption of both imido ligands on V can be seen as a catalyst deactivation pathway as the V-catalyzed reactions rely on π-loading of the bis-imido moiety for productive [2 + 2] cycloaddition reactivity. , In contrast, the use of 5-decyne ( 1l ) for the stoichiometric reaction with 6 produced the corresponding tetra-alkylated pyrrole 3la in a selective manner without the α-diimine 7l , suggesting that the side reaction to form α-diimine is accessible only in the case of diarylacetylenes.…”

Zr vs. Ti molecular and silica-supported imido complexes in stoichiometric and catalytic oxo/imido heterometathesis

“…Early transition metal imido complexes have been extensively studied due to a diverse spectrum of their reactivity [1][2][3][4][5][6][7][8][9][10][11][12][13] and participation in numerous catalytic reactions such as alkyne hydroamination, 14,15 carboamination 16,17 and iminoamination, 18 imine 6 and carbodiimide 19 metathesis, and guanylation of amines, 20 to name but a few. A new wave of interest in these highly reactive species has recently been associated with various redox nitrene-transfers, 7,[21][22][23] oxidative amination and coupling reactions [24][25][26] including synthesis of pyrroles [27][28][29] , and other nitrogen heterocycles. 15,18,30,31 Among others, imido-transfer reactions of imido complexes with organic and inorganic "oxo" compounds (i.e.…”

Observing Similarities and Differences in the Properties of Isostructural Niobium(V)/Tantalum(V) Coordination Compounds with Strong Pi-Donor Ligands

“…14 Fostvedt, Arnold, Bergman, and coworkers recently posited that, in the case of bis-imido group 5 complexes, pi-bonding ligands compete with metal based dorbitals, increasing the energy of the frontier orbitals at the imido ligands, to incrase reactivity. 15 While the effect has been well-documented, few reports have appeared to explore the possibility of pi-loading for divergent Nb/Ta properties and reactivity. Additionally, such heavily pi-loaded group 5 complexes are relatively transient and unstable in air and water.…”

“…And Helgert, Webster, Hollis, and co-workers recently described a reactive tantalum bis -imido species capable of group transfer catalysis . Fostvedt, Arnold, Bergman, and co-workers recently posited that, in the case of bis -imido group 5 complexes, pi-bonding ligands compete with metal based d-orbitals, increasing the energy of the frontier orbitals at the imido ligands, to incrase reactivity . While the effect has been well-documented, few reports have appeared to explore the possibility of pi-loading for divergent Nb/Ta properties and reactivity.…”