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

Low-valent tungsten species generated from WCl₆ and N,N′-bis(trimethylsilyl)-2,5-dimethyldihydropyrazine (Si-Me₂-DHP) promotes the catalytic formation of N-phenyl-2,3,4,5-tetraarylpyrroles 3aa-ka from diarylacetylenes 1a-k and azobenzene (2a). An initial catalyst activation process is a three-electron reduction of WCl₆ with Si-Me₂-DHP to afford transient ‘WCl₃’ species. Catalytically active bis(imido)tungsten(VI) species via successive one-electron reduction and N═N bond cleavage of 2a was revealed by isolating W(═NPh)₂Cl₂(PMe₂Ph)₂ from imidotungsten(V) trichloride and 2a in the presence of PMe₂Ph. The superior catalytic activity of the tungsten catalyst was clarified by a density functional theory study: activation energies for the key three steps, [2 + 2]-cycloaddition of W═NPh and diarylacetylene to form (iminoalkylidene)tungsten species, enyne metathesis with second diarylacetylene, and C-N bond formation, are reasonable values for the catalytic reaction at 180 °C. In addition, this tungsten catalyst overcame two distinct deactivation processes: α-enediamido formation and aggregation of the low-valent species, both of which were observed for previously developed vanadium and titanium catalysts. We also demonstrated the synthetic utility of pentaarylpyrroles 3aa and 3ba as well as N-(2-bromophenyl)-2,3,4,5-tetraarylpyrrole 3ab by derivatizing their π-conjugated compounds 9aa, 10ba, and 11ab.