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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

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posted on 22.12.2004, 00:00 by Seiji Ogo, Keiji Uehara, Tsutomu Abura, Yoshihito Watanabe, Shunichi Fukuzumi
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 SO42- 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 24 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 24. The origins of the alkynyl, acyl, and ketonyl ligands of 24 were determined by isotopic labeling experiments with D2O and H218O.

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