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

Seiji Ogo, Keiji Uehara, Tsutomu Abura, Yoshihito Watanabe, Shunichi Fukuzumi

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Abstract

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*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 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 SO42- 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.

Original languageEnglish
Pages (from-to)16520-16527
Number of pages8
JournalJournal of the American Chemical Society
Volume126
Issue number50
DOIs
Publication statusPublished - Dec 22 2004
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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