Pinacol Coupling Under Ultrasound Irradiation

One of the most powerful methods for constructing carbon-carbon bond is the pinacol coupling reaction. 1,2-Diols obtained in the reaction were very useful synthons for a variety of organic syntheses, and were also used as intermediates for the construction of biologically important skeletons of natural products and asymmetric ligands for catalytic asymmetric reaction. One of the most powerful methods for the synthesis of pinacols is the reductive coupling of carbonyl compounds. Recent efforts have focused on the development of new reagents and reaction systems to improve the reactivity of the reagents and diastereoselectivity of the products.Ultrasound has increasingly been used in organic synthesis in the last three decades. Compared with the standard methods, this technique requires a special equipment, but is easily controlled. A number of organic reactions, especially metal-involved ones, can be carried out in higher yields, shorter reaction time, and milder conditions using ultrasound irradiation.Based on the reported pinacol coupling of aldehyde and ketone mediated by low valent titanium complex and metals in aqueous, we studied the reaction under ultrasound irradiation. Under ultrasound irradiation, some aromatic aldehydes and ketones can be reduced by low valent titanium complex affording the corresponding diols in high yield. Improved chemoselectivity has been observed in TiCl₄-Mg system compared with the traditional method. Under ultrasound irradiation, metal can be activated to active metal, which can reduce aromatic aldehydes and ketones to the corresponding pinacols. Water as the solvent is friendly to the environment.