Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides.

W(CNAr) (CNAr = arylisocyanide) photoreductants catalyze base-promoted homolytic aromatic substitution (BHAS) of 1-(2-iodobenzyl)-pyrrole in deuterated benzene. Moderate to high efficiencies correlate with W(CNAr) excited-state reduction potentials upon one-photon 445 nm excitation, with 10 mol % loading of the most powerful photoreductants W(CNDipp) (CNDipp = 2,6-diisopropylphenylisocyanide) and W(CNDippPh) (CNDippPh = 4-(3,4,5-trimethoxyphenyl)-2,6-diisopropylphenylisocyanide) affording nearly complete conversion. Stern-Volmer quenching experiments indicated that catalysis is triggered by substrate reductive dehalogenation. Taking advantage of the large two-photon absorption (TPA) cross sections of W(CNAr) complexes, we found that photocatalysis can be driven with femtosecond-pulsed 810 nm excitation. For both one- and two-photon excitation, photocatalysis was terminated by the formation of seven-coordinate W-diiodo [WI(CNAr)] complexes. Notably, we discovered that W(CNDipp) can be regenerated by chemical reduction of WI(CNDipp) with excess ligand present in solution.