Copper-catalyzed diboration of ketones: facile synthesis of tertiary a-Hydroxyboronate esters

The versatility of the C-B bond in organic synthesis has led to extensive development of new methods for the selective incorporation of boron into organic substrates. The incorporation of boron substituents alpha to heteroatoms provides direct entry into substrates analogous to those formed and utilized in the Matteson homologation reaction. Our group has explored the diboration of various ketones using the (ICy)CuOt-Bu catalyst developed by Sadighi and co-workers. We found that (ICy)CuOt-Bu could be generated in situ using 3 mol % (ICy)CuCl and 5 mol% NaOt-Bu (Scheme 1). This catalyst system was shown to cleanly afford the diboration of various ketones in toluene at 50 degreesC over 2--22 hours. Treatment of the resulting products with SiO2 provides the corresponding α-hydroxyboronate esters. Products derived from aryl ketones are prone to decomposition during isolation via silica gel chromatography. Protection of alcohols has been achieved in a two step one flask procedure using TBSOTf. Matteson homologation products have been obtained in low yields by homologation of TBSprotected alcohols.