We have a developed tunable tungsten-alkylidyne chromophores of the form W(CAr)L4X (Ar = aryl; L =neutral ligand; X= halide/pseudohalide) that possess excited-state potentials as low as -3.3 V (v. Cp2Fe0/+), enabling the efficient production of reactive organic radicals from substrates that are inaccessible to conventional photoredox chromophores such as [Ru(bpy)3]2+ or fac-Ir(ppy)3. We will describe the design principles that enable systematic control over the electronic absorption properties, redox potentials, and excited-state lifetimes of these chromophores. Excited-state quenching studies demonstrate that these chromophores are capable of generating organic radicals from difficult-to-reduce substituted benzenes (e.g., phenyl acetylene, styrene, benzaldehyde) via outer-sphere electron transfer. Similar photoredox sensitization of substituted phenyl and pyridyl halides (X = Cl, Br, I) produces radicals that, in the presence of base, undergo radical addition via C-C bond formation products with benzenes, furans, and pyrroles. Initial optimization studies indicate that these transformations can be accomplished with modest chromophore loadings (1-2 mol%) in high yields at reasonably short reaction times, even using sunlight. The scope and mechanisms of these transformations will be discussed.

Catalytic photoredox arene and heteroarene C-C bond-forming reactions driven without sacrificial reagents by tungsten-alkylidyne chromophores

255th National Meeting (2018)

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