Highly Efficient Gold (I) Complex Catalyst for Hydration of Alkynes
Eiichiro Mizushima, Masato Tanaka,
Hydration of carbon-carbon unsaturated bonds is one of the environmentally benign routes to carbon-oxygen compounds. In this category, hydration of alkynes to give carbonyl compounds is well known reaction and has been extensively studied. Acid-catalysis is the most conventional method, while it is applicable only to electron rich acetylenes to give corresponding methylketones in satisfactory yields.[1,2] For the hydration of simple alkynes, another catalysts such as mercury(II) salts were required, which is inconsistent with the guiding principle of Green Chemistry. In this context, various kinds of transition metal complex catalysts such as ruthenium (II), ruthenium (III), rhodium, platinum, gold (III), and others have been attempted. However, catalyst efficiency of these catalyst systems is still insufficient. For example, in the hydration of 3-pentyn-1-ol catalyzed by cis-PtCl2(TPPTS)2 [TPPTS = P(m-C6H4SO3Na)3], initial turnover frequency (TOF) up to 550 /h has been claimed, while the overall TOF remains about 100 /h. Recently, it was reported that gold (I) species effectively catalyzes addition of methanol to simple acetylenes. In a patent application by the same authors, propargyl alcohol was claimed to be hydrated by the same catalyst system. We have found out that gold (I)-acid systems efficiently catalyze the hydration of simple acetylenes in methanol as a solvent. The efficiency was two orders of magnitude higher than reported transition metal catalyst systems.
1-Octyne (1 mmol), (Ph3P)AuCH3 (0.01 mmol, 1 mol %) and concentrated sulfuric acid (0.5 mmol, 50 mol %) were mixed in aqueous methanol (1.5 ml, methanol : H2O = 2 : 1 v/v) and the mixture was heated for 1 h at 70 °C to afford the Markovnikov hydration product, 2-octanone, in 95% yield without anti-Markovnikov hydration and possible methanol addition. In the absence of either the Au catalyst or sulfuric acid, the reaction did not proceed at all.
The reaction was significantly affected by the reaction media. Without using any solvent (otherwise the same conditions as above), the reaction did not furnish 2-octanone. The use of 2-propanol (71%), dioxane (56%), acetonitrile (53%) or THF (11%) gave lower yields as shown in parentheses, and the use of dichloromethane, DMF or toluene as solvent resulted in even lower yields. As a consequence, the use of methanol was the best choice for this reaction.
The addition of appropriate ligands significantly enhanced the catalytic performance, which enabled to minimize the quantity of the precious catalyst. When the reaction was carried out with 0.01 mol% catalyst (reaction conditions: (Ph3P)AuCH3 0.002 mmol, sulfuric acid 0.5 mmol, 1-octyne 20 mmol, water 1 ml, methanol 10 ml, 70 °C, 1 h), the yield of 2-octanone was 35% (TOF = 3500 /h). The yield was dramatically increased to 99% (TOF = 9900 /h) under atmospheric CO or to 90 % (TOF = 9000 /h) with the addition of P(OPh)3. The ligand effect is ambiguous at this time. It seemed to be associated with the stability of the catalyst. The reactions without ligand addition occasionally resulted in the catalyst deterioration, as visualized by metallic particle precipitation. However, in the presence of CO or phosphites, the particle formation was not evident any more by sight.
The other acid co-catalysts including CF3SO3H, CH3SO3H and H3PW12O40 also resulted in extremely high performance even in the absence of the coordinative additives. A reaction of 1-octyne using only 0.005 mol% of the gold catalyst, under otherwise identical conditions as before, formed 2-octanone in 70% yield (TOF = 14000 /h) under N2, and in 78% yield (TOF = 15600 /h) under CO.
This catalytic procedure could be applied to various alkynes. Both aliphatic and aromatic terminal alkynes including those bearing functionalities such as alkoxy, cyano, chloro, and olefinic groups were all hydrated under similar reaction to give corresponding methyl ketones. Ethynylcyclohexene and hexynonitrile reacted at the acetylenic groups remaining olefinic and cyano groups intact. Propargylic alcohols gave the mixture of methyl ketones and alpha,beta-unsaturated aldehydes which means anti-Markovnikov addition somewhat taking place. In the reactions of substituted phenylacetylenes, electronegative groups decreased the reactivity suggesting that the reaction mechanism is electrophilic.
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catalyst, Water, Ketone, Carbonyl Compounds, Alkynes, Addition, Gold Complex, Hydration