pH-Selective Synthesis and Structures of Alkynyl, Acyl, and Ketonyl Intermediates in Anti-Markovnikov and Markovnikov Hydrations of a Terminal Alkyne with a Water-Soluble Iridium Aqua Complex in Water

Chemoselective synthesis and isolation of alkynyl [Cp*IrIII(bpy)CCPh]+ (2, Cp* = η5-C5Me5, bpy = 2,2‘-bipyridine), acyl [Cp*IrIII(bpy)C(O)CH2Ph]+ (3), and ketonyl [Cp*IrIII(bpy)CH2C(O)Ph]+ (4) intermediates in anti-Markovnikov and Markovnikov hydration of phenylacetylene in water have been achieved by changing the pH of the solution of a water-soluble aqua complex [Cp*IrIII(bpy)(H2O)]2+ (1) used as the same starting complex. The alkynyl complex [2]2·SO4 was synthesized at pH 8 in the reaction of 1·SO4 with H2O at 25 °C, and was isolated as a yellow powder of 2·X (X = CF3SO3 or PF6) by exchanging the counteranion at pH 8. The acyl complex [3]2·SO4 was synthesized by changing the pH of the aqueous solution of [2]2·SO4 from 8 to 1 at 25 °C, and was isolated as a red powder of 3·PF6 by exchanging the counteranion at pH 1. The hydration of phenylacetylene with 1·SO4 at pH 4 at 25 °C gave a mixture of [2]2·SO4 and [4]2·SO4. After the counteranion was exchanged from SO4 2- to CF3SO3 -, the ketonyl complex 4·CF3SO3 was separated from the mixture of 2·CF3SO3 and 4·CF3SO3 because of the difference in solubility at pH 4 in water. The structures of 2−4 were established by IR with 13C-labeled phenylacetylene (Ph12C⋮13CH), electrospray ionization mass spectrometry (ESI-MS), and NMR studies including 1H, 13C, distortionless enhancement by polarization transfer (DEPT), and correlation spectroscopy (COSY) experiments. The structures of 2·PF6 and 3·PF6 were unequivocally determined by X-ray analysis. Protonation of 3 and 4 gave an aldehyde (phenylacetaldehyde) and a ketone (acetophenone), respectively. Mechanism of the pH-selective anti-Markovnikov vs Markovnikov hydration has been discussed based on the effect of pH on the formation of 2 − 4. The origins of the alkynyl, acyl, and ketonyl ligands of 2−4 were determined by isotopic labeling experiments with D2O and H2 18O.