Indication of Pd-C or Cu-C Intermediates in Bimetallic Nanoclusters during Pd/Au-PVP- or Cu/Au-PVP-Catalyzed Oxidations of endo -4-Oxatricyclo[52.1.0^2,6]-8-decene and Tetrahydro-γ-carbolines

Catalytic oxidation of tricyclic endo-norbornene-fused tetrahydrofuran with the bimetallic nanocluster Cu/Au-PVP in the presence of H2O2 or t-BuOOH as the oxidant leads to C H bond oxidation adjacent to the ether function to give 4-oxa-tricyclo[5.2.1.0]-8,9-exoepoxydecane, however, oxidation with Pd/Au-PVP takes place at the C=C double bond to give the same epoxide and the oxidative threebond forming dimeric product, dodecahydro-1,4:6,9-dimethanodibenzofurano[2,3-b:7,8-b']bisoxolane. Formation of the latter product suggests the involvement of a reactive Pd C intermediate. Similarly, oxidative C C bond-forming reactions are observed in cycloaddition reactions of N2-Boc-1,2,3,4-tetrahydro-γ-carbolines and 2,3-dihydroxybenzoic acid with Cu/Au-PVP (2 5 mol%) and H2O2 at 25 °C, providing two-bond-forming [4+2] cycloadducts. Under similar reaction conditions, Pd/Au-PVP did not produce the corresponding cycloadduct, indicating a need for complexation between Cu and the carboxylic acid group of 2,3-dihydroxybenzoic acid and the allylic amine function of the γ-carbolines during the cyclization reaction. The reported intermolecular coupling reactions using Pd/Au-PVP or Cu/Au-PVP nanocluster catalysts under oxidative conditions at 25 °C are unprecedented.