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

Abstract

The cationic tetra-coordinated 16 electron complex [Ir(trop 2 dach)] + OTf − ( 1 ) where (OTf − = CF 3 SO 3 − ) and the neutral amine amido complex [Ir(trop 2 dach-1H)] ( 2 ) were isolated and structurally characterized. The NH function in 1 is easily deprotonated (p K a DMSO = 10.5) to yield the amino amido complex [Ir(trop 2 dach-1H)] ( 2 ), which is deprotonated at p K a DMSO = 19.6 to the anionic di(amido) iridate [Ir(trop 2 dach-2H)] − ( 3 ); [( R , R )-top 2 dach stands for the tetrachelating diamino diolefin ligand ( R , R )- N , N ′-bis(5 H -dibenzo[ a , d ]cyclohepten-5-yl)-1,2-diaminocyclohexane; ( R , R )-top 2 dach-1H and ( R , R )-top 2 dach-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(trop 2 dach-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, RCH 2 OH + BQ → RCH O + H 2 BQ ( H 2 BQ = 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.