Intramolecular Hydroalkoxylation of Alkenes Catalyzed by Gold Nanoclusters under Aerobic Conditions

摘要

Gold nanoclusters (NCs) supported on metal oxides have paid much attention to develop aerobic oxidation catalysts since the landmark report by Haruta. On the other hand, few reports about the catalytic activity of non/weakly-supported homogeneous Au(0) NCs have appeared. We have recently succeeded in developing the preparative method of gold NCs stabilized by hydrophilic polymer, poly(N-vinyl-2-pyrrolidone) (Au:PVP NCs) with various range of cluster size (phi = 1.3-9.5 nm). We also found that smaller size (phi < 2.0 nm) Au(0):PVP NCs showed a high catalytic activity toward the aerobic oxidation of benzylic alcohols, homocoupling reactions of organoboron compounds, and generation of hydrogen peroxide. That key active species of the above reactions is believed to be the electron-deficient surface of Au(0) NC, generated by the adsorption of molecular oxygen as a superoxo-like form (A, in Scheme 1). It is indicated that thus O_2-adsorbed intermediate A behaves as a Lewis acid catalyst in spite of the formally zero-valent Au surface. We are interested in such a Lewis acidic character of Au(0) NC in aqueous media under aerobic conditions. Au(I) and/or Au(III) compounds have been applied to the Lewis acid catalyst especially to activate carbon-carbon multiple bonds. In this presentation, we wish to report that Au(0):PVP NC also works as a Lewis acid to catalyze the intramolecular hydroalkoxylation of unactivated alkenes under aerobic conditions.