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

Low-valent tungsten species generated from WCl and ,-bis(trimethylsilyl)-2,5-dimethyldihydropyrazine (-Me-DHP) promotes the catalytic formation of -phenyl-2,3,4,5-tetraarylpyrroles from diarylacetylenes and azobenzene (). An initial catalyst activation process is a three-electron reduction of WCl with -Me-DHP to afford transient 'WCl' species. Catalytically active bis(imido)tungsten(VI) species via successive one-electron reduction and N═N bond cleavage of was revealed by isolating W(═NPh)Cl(PMePh) from imidotungsten(V) trichloride and in the presence of PMePh. 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 and as well as -(2-bromophenyl)-2,3,4,5-tetraarylpyrrole by derivatizing their π-conjugated compounds , , and .