Complexes of (η⁵-Cp*)Ir(<scp>iii</scp>) with 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone: catalyst for oxidation and 1,2-substituted benzimidazole synthesis

Abstract

The treatment of 1-benzyl-3-phenylthio/selenomethyl-1,3-dihydrobenzoimidazole-2-thione/selenone [L1-L4] with [(η⁵-Cp*)IrCl(μ-Cl)]₂ at 25 °C followed by NH₄PF₆ results in [(η⁵-Cp*)Ir(L)Cl][PF₆] (1-4 for L = L1 to L4), authenticated with high-resolution mass spectrometry (HR-MS) and multi-nuclei nuclear magnetic resonance (NMR) imaging (¹H, ¹³C{¹H} and ⁷⁷Se{¹H}). The structures of 1-4, established with single-crystal X-ray diffraction, reveal a "piano-stool" geometry around the Ir. The Ir-thio/selenoether (Ir-S/Ir-Se) bond distances (Å) are 2.347(18)-2.355(4)/2.4663(12)-2.4663(13) and Ir-thione/selenone (Ir-S/Ir-Se) distances are 2.4146(19)-2.417(2)/2.5141(16)-2.5159(12). The reaction of 1,2-phenylenediamine with benzylic alcohols and furfuryl alcohol under mild and ambient conditions, catalyzed efficiently with complexes 1-4, generates bisimine in situ. Cyclization and rearrangement via 1,3-hydride shift triggered by its electrophilic activation with Ir(iii) species finally results in 1,2-disubstituted benzimidazole. The yield of the heterocycles in this one-pot synthesis is excellent to good. The aldehydes generated in situ by aerial oxidation of alcohols in the presence of 1-4 as catalysts are precursors to the bisimine as the protocols of this heterocycle synthesis carried out in the absence of 1,2-phenylenediamine give them in excellent-to-good yield. The oxidation of alcohols by hydrogen transfer to acetone was catalyzed efficiently with complexes 1-4 and resulted in aldehyde/ketone in excellent-to-good yield. Each catalytic process is marginally more efficient with 1 than its counterparts.