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*Ir ^III(bpy)CCPh]⁺ (2, Cp* = η⁵-C ₅Me₅, bpy = 2,2′-bipyridine), acyl [Cp*Ir ^III(bPy)C(O)CH₂Ph]⁺ (3), and ketonyl [Cp*Ir^III(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*Ir^III(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 PF ₆) 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 [4] ₂·SO₄ and [4]₂·SO₄. After the counteranion was exchanged from SO₄^2- to CF ₃SO₃^-, the ketonyl complex 4·CF ₃SO₃ was separated from the mixture of 2·CF ₃SO₃ 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 (Ph ¹²C≡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 H ₂¹⁸O.