(674e) Phenyl-Alkyne Hydration and Phenyl-Methyl-Ether Deprotection in High-Temperature Water Catalyzed by Water-Tolerant Lewis Acids

We present results from kinetic investigations of two different reactive systems: hydration of 1-phenyl-1-propyne to form propiophenone, and hydrolysis of anisole to form phenol. Alkyne hydration is a convenient route toward synthetically useful ketones. However, the traditional synthesis, catalyzed by mercuric salts, requires pronounced safety precautions. Anisole hydrolysis is a model instance of deprotection. In multistep synthesis, one must often protect a labile hydroxy group by masking it as an ether. Such protecting groups must be removed in a later synthetic step. Our work presents promising and potentially useful alternatives to traditional methods for both of these transformations.

Our work investigates the ease and robustness of these transformations in high-temperature water catalyzed by water-tolerant Lewis acids without the use of any organic solvent. Our survey of water-tolerant Lewis acids includes In(OTf)₃, Sc(OTf)₃, Yb(OTf)₃, and InCl₃, and compares reactivity with that of mineral acids HCl and H₂SO₄. We further investigate the kinetics within the temperature range of 150 to 300°C, determine the order of reaction in substrate and catalyst, and compare activation energy and frequency factor with other well studied preparations. Our kinetic studies conclude with mechanistic pathways that are consistent with experimental results.