Silver- and Acid-Free Catalysis by Polyoxometalate-Assisted Phosphanegold(I) Species for Hydration of Diphenylacetylene

• Published in: Organometallics Vol. 35; no. 11; pp. 1658 - 1666
• Main Authors: Arai, Hidekazu, Yoshida, Takuya, Nagashima, Eri, Hatayama, Akifumi, Horie, Shogo, Matsunaga, Satoshi, Nomiya, Kenji
• Format: Journal Article
• Language: English
• Published: American Chemical Society 13.06.2016
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.6b00114

A DMSO-soluble intercluster compound consisting of a tetra­{phosphanegold­(I)}­oxonium cation and an α-Keggin polyoxometalate (POM) anion, [{Au­(PPh3)}4(μ4-O)]3­[α-PW12O40]2 (1), was found to be an effective precatalyst for the silver- and acid-free catalysis of diphenylacetylene hydration (0.67 mol % catalyst; conversions 36.1%, 55.2%, and 93.7% after 4, 6, and 24 h reactions, respectively). The reaction proceeded in the suspended system in 6 mL of 1,4-dioxane/water (4:1) at 80 °C because of the low solubility of 1. Similar POM-based phosphanegold­(I) compounds [{{Au­(PPh3)}4(μ4-O)}­{{Au­(PPh3)}3(μ3-O)}]­[α-PW12O40]·EtOH (5), which is composed of a heptakis­{triphenylphosphanegold­(I)}­dioxonium cation and an α-Keggin POM anion, and [Au­(CH3CN)­(PPh3)]3­[α-PMo12O40] (6), which consists of an acid-free monomeric phosphanegold­(I) acetonitrile cation and an α-Keggin molybdo-POM anion, also exhibited acid-free catalysis for the hydration of diphenylacetylene. An induction period was observed in the catalysis by 5. On the other hand, their component species, or phosphanegold­(I) species without the POM anion, such as [{Au­(PPh3)}4(μ4-O)]­(BF4)2 (2) and [{Au­(PPh3)}3(μ3-O)]­BF4 (3), and the monomeric phosphanegold­(I) complex [Au­(RS-pyrrld)­(PPh3)] (4) (RS-Hpyrrld = RS-2-pyrrolidone-5-carboxylic acid), the last of which has been used as a precursor for the preparation of 1, 5, and 6, showed poor activities in this reaction (0.67 mol % catalysts; conversions 1.8%, 1.7%, and 0.5% after 24 h reactions, respectively). However, upon adding the free-acid form of Keggin POM, i.e., H3[α-PW12O40]·7H2O (H-POM: 0.67 mol %), 2–4 exhibited remarkably enhanced activities (conversion 97.6% each after 24 h reactions). In contrast, the activities were not enhanced after adding either the sodium salt of the Keggin POM, Na3[α-PW12O40]·8H2O (Na-POM; 0.67 mol %), or a Brønsted acid 10% HBF4 aqueous solution (0.67 mol %). Both H-POM and Na-POM themselves exhibited no activity. Catalysis by the phosphanegold­(I) species for diphenylacetylene hydration was influenced significantly under the free-acid form or sodium salt of the Keggin POM. Acid-free catalytic hydration by 1 of other alkynes, such as phenylacetylene and 1-phenyl-1-butyne, was also examined.