Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides

Abstract

W(CNAr)₆ (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)₆ excited-state reduction potentials upon one-photon 445 nm excitation, with 10 mol % loading of the most powerful photoreductants W(CNDipp)₆ (CNDipp = 2,6-diisopropylphenylisocyanide) and W(CNDippPh^(OMe3))6 (CNDippPh^(OMe3) = 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)₆ 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 WII-diiodo [WI₂(CNAr)₅] complexes. Notably, we discovered that W(CNDipp)₆ can be regenerated by chemical reduction of WI2(CNDipp)5 with excess ligand present in solution.

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