Iridium aminyl radical complexes as catalysts for the catalytic dehydrogenation of primary hydroxyl functions in natural products

The cationic tetra-coordinated 16 electron complex [Ir(trop₂dach)]⁺OTf⁻ (1) where (OTf⁻ = CF₃SO₃⁻) and the neutral amine amido complex [Ir(trop₂dach-1H)] (2) were isolated and structurally characterized. The NH function in 1 is easily deprotonated (pK_a^DMSO = 10.5) to yield the amino amido complex [Ir(trop₂dach-1H)] (2), which is deprotonated at pK_a^DMSO = 19.6 to the anionic di(amido) iridate [Ir(trop₂dach-2H)]⁻ (3); [(R,R)-top₂dach stands for the tetrachelating diamino diolefin ligand (R,R)-N,N′-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane; (R,R)-top₂dach-1H and (R,R)-top₂dach-2H indicate the mono and double deprotonated form]. Complex 3 is easily oxidized by 1,4-benzoquinone (BQ) to the neutral iridium aminyl radical complex [Ir(trop₂dach-2H)] (4). In combination with BQ as hydrogen acceptor and catalytic amounts of base, 4 serves as catalyst in the highly efficient dehydrogenation of functionalized primary alcohols to the corresponding aldehydes, RCH₂OH + BQ → RCHO + H₂BQ (H₂BQ = catechol). Alcohols like geraniol and retinol are rapidly converted to geranial and retinal, while the conversion of sterically hindered alcohols like lavandulol is slower and the primary product, lavandulal, isomerizes to isolavandulal in a classical base-catalyzed reaction.