Hydration of phenylacetylene

A more detailed study of the hydration of phenylacetylene, 9a, and three substituted phenylacetylenes, p-methoxy 10, p-methyl 11, and p-chlorophenyl-acetylene 12, in aqueous sulfuric acid containing 5% ethanol has been carried out by Noyce and co-workers (19,20). The hydration obeys general acid catalysis and gives a linear Hq dependence. The slopes for the logarithm of the observed rate constants versus Ho and the activation parameters for the hydration of these phenylacetylenes are summarized in Table II. 

Acidity Dependence and Activation Parameters for Hydration of Phenylacetylenes ... 

Solvent Deuterium Isotope Effects for the Hydration of Phenylacetylenes at 25° ... 

The experiments of Bott (17) and Noyce (19-21) show that a vinyl cation best represents the intermediate in the hydration of phenylacetylenes. In particular, the large solvent Isotope effects observed indicate a rate-limiting protonation and formation of a vinyl cation, for these values are not in agreement with solvent isotope effects observed for compounds which react by other possible mechanisms, such as one involving equilibrium formation of the vinyl cation followed by the slow attack by water. 

Figure 1. Energy-reaction coordinate diagram for the acid-catalyzed hydration of phenylacetylene. The ordinate is not to scale (20).

Sanz, S., Jones, L.A., Mohr, F. and Laguna, M. (2007) Homogenous Catalysis with Gold Efficient Hydration of Phenylacetylene in Aqueous Media. OrganometaUics, 26(4), 952-957. 

Thus for these reactions m is necessarily less than unity, a result that has now been widely observed in practice,117,118,120,161,180,181 and thus the m1 value offers a clear distinction between the A1 and A-SE2 mechanisms, which is not the case with the H0 correlations discussed above. A number of different excess acidity plots according to equation (56), covering a wide reactivity range, are shown in Fig. 9. These are for the hydration of oc-methylstyrene,120 equation (58), and the mechanistically similar hydration of phenylacetylene 118 for the isomerization of m-stilbene 120 and for the detritiation of tritiated benzene, equation (28) above."7 As can be seen, all four plots are good straight lines the references cited may be consulted for the details. The slopes look steep, but m values for carbon protonation approximate 1.8,36 and the nfi values are all calculated to be... 

Two tetrahalogen-gold(III) compounds (AuCf, AuBrCfi ) in ionic liquids were studied by Raubenheimer et al. for the hydration of phenylacetylene. Although the activity was lower than that reported in previous studies, this strategy described the re-use of the catalyst achieved by recycling the ionic liquid phase [95]. 

Since photoexcitation greatly enhances the reactivity of acetylenes, formation of the enol intermediate becomes faster than its rearrangement to ketone. As a result, the intermediate acetophenone enol in the hydration of phenylacetylene could be directly observed 42... 

Heteropolyacids are much more active than H2S04 and HC104 for hydration of phenylacetylene [Eq. (12)] (173). Also in this case, the rate of reaction in the presence of heteropolyacids shows an approximately second-order dependence on the catalyst concentration. This observation suggests that this reaction proceeds by a mechanism similar to that of Scheme 3 ... 

A variety of gold(I) complexes containing the water-soluble phosphine ligands TPPMS, TPPDS, and TPPTS were found active catalysts for the hydration of phenylacetylene to acetophenone in aqueous media. The isolated [Au(C=C Bu)(TPPTS)] showed a very high activity (1000 h ) at refiux temperature in methanol/water 5 1 in the presence of 10 mol% H2SO4 as cocatalyst (224). Similarly, various water-insoluble [AuBralNHC)] (225) and water-soluble [AuCl(NHC)] complexes (226) were effective in hydration of various terminal alkynes to the corresponding 2-oxo-derivatives. 

Mohr and Laguna reported in 2007 the ability of water-soluble Au-phosphine complexes XVII-XVIII to promote hydration of phenylacetylene in water (Scheme 16.29). Using 0.1 mol% of the gold complex, and 10 mol% of H2SO4 as additive, they obtained moderate conversions to the hydrated product. Although the obtained conversions were lower than the ones... 

Catalytic hydration of phenylacetylene has been accomplished in a biphasic mixture of ionic liquids and toluene using [BMTz] [AuCl3Br] 15 as a catalyst (Scheme 6) [114], Several imidazolium derived ionic liquids, as weU as 15, can be... 

Scheme 6 Gold-catalyzed hydration of phenylacetylene in an ionic liquid...

The active organometallic gold(III) catalysts in the hydration of phenylacetylene proved to be also efficient catalysts for the addition of MeOH giving the enol ether 17 and the acetal 18 (Table 3). 

Water-soluble phosphine ligands TPPMS, TPPDS, and TPPTS (mono-, di-, and tri-sulfonated triphenylphosphine, respectively) were tested as ligands for the hydration of aUcynes in aqueous media [116]. Complexes 19 and 20 (Fig. 1) gave the highest turnover frequencies ever reported (1,000 and 1,060 h , respectively) for the hydration of phenylacetylene under optimum conditions (0.1 mol% catalyst loading, 10 mol% H2SO4, reflux, and MeOH/H20). 

---