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* = eta(5)-C5Me5, 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 degreesC, 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 degreesC, 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 degreesC 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 2-4 were established by IR with C-13-labeled phenylacetylene (Ph(12)Cdrop(13)CH), electrospray ionization mass spectrometry (ESI-MS), and NMR studies including H-1, C-13, 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 (H2O)-O-18.