Isolation and crystal structures of both enol and keto tautomer intermediates in a hydration of an alkyne-carboxylic acid ester catalyzed by iridium complexes in water

Hydration of tetrolic acid ethyl ester as an alkyne-carboxylic acid ester catalyzed by an Ir-aqua complex [Ir^IIICp*(bpy)(OH ₂)]²⁺ (1, Cp* = η⁵-C ₅Me₅, bpy = 2,2′-bipyridine) in water provides ethyl acetoacetate as a β-keto acid ester. We report the successful isolation of both an Ir-enol tautomer intermediate [Ir^IIICp*(bpy){CH ₃C(OH)=CC(O)OC₂H₅}]⁺ (2) and an Ir-keto tautomer intermediate [Ir^IIICp*(bpy){CH ₃C(O)-CHC(O)OC₂H₅}]⁺ (3) in the catalytic hydration by optimizing the conditions of the isolation, such as pH of the solution, reaction time, and selection of counteranions. The structures of the enol and keto complexes with characteristic Ir-(sp²carbon) bond and Ir-(sp³ carbon) bond, respectively, were unequivocally determined by X-ray analysis, IR, electrospray ionization mass spectrometry (ESI-MS), and NMR studies including ¹H, ¹³C, distortionless enhancement by polarization transfer (DEPT) and correlation spectroscopy (COSY) experiments. It was confirmed that the hydration of tetrolic acid ethyl ester catalyzed by 2 or 3 as initial catalysts provides ethyl acetoacetate. Mechanism of the catalytic hydration of tetrolic acid ethyl ester as an alkyne-carboxylic acid ester is discussed based on isotopic labeling experiments with the Ir-enol and Ir-keto tautomers.