A Dinuclear Palladium Catalyst for α-Hydroxylation of Carbonyls with O₂

Abstract

A chemo-and regioselective α-hydroxylation reaction of carbonyl compounds with molecular oxygen as oxidant is reported. The hydroxylation reaction is catalyzed by a dinuclear Pd(II) complex, which functions as an oxygen transfer catalyst, reminiscent of an oxygenase. The development of this oxidation reaction was inspired by discovery and mechanism evaluation of previously unknown Pd(III)-Pd(III) complexes.Bimetallic catalysis, as we defined it in our previous work, is catalysis with synergistic redox cooperation between two metal centers.1 Several dinuclear complexes are known to catalyze redox transformations in organic chemistry and Nature,2 but the design of new dinuclear complexes for bimetallic redox catalysis is rare.3 While in monometallic redox transformations only one metal participates in redox chemistry, two metals share the redox work in bimetallic catalysis and can therefore lower activation barriers compared to mononuclear catalysts.1 In 2009, we reported the first reactions from organometallic Pd(III) compounds and implicated dinuclear Pd(III) complexes with metal-metal bonds in bimetallic directed oxidation catalysis.4 Subsequently, we sought to evaluate dinuclear Pd catalysts with potential for metal-metal redox cooperation for other challenging redox transformations. Here we report a chemo-and regioselective α-hydroxylation reaction of carbonyl compounds with molecular oxygen or air as oxidant, catalyzed by the dinuclear Pd(II) complex 1 (eq 1). into a substrate and are less common in organic synthesis.6 Here we report the first dinuclear Pd(II) oxygen transfer catalyst, which functions as a dioxygenase. We identified the dipalladium paddlewheel complex 1, originally developed by Cotton,7 which features four bridging hexahydro-2H-pyrimido[1,2-a]pyrimidine ligands (hppH, 2), and its use for chemo-and regioselective α-hydroxylation of carbonyl compounds to form tertiary alcohols with O 2 as the only oxidant (eq 1).Oxidation of α-methyl-β-tetralone (3) under one atmosphere of O 2 in the presence of 5 mol % 1 produced α-hydroxy-α-methyl-β-tetralone (4) in 77% isolated yield in the absence of any other reagent or additive except solvent. α-Hydroxylation reactions of carbonyl compounds are typically performed by oxidation of the corresponding enolates or silyl enol ethers with an oxygen transfer reagent, such as m-CPBA, dimethyldioxirane (DMDO), or Nsulfonyl oxaziridines.8 During deprotonation, silyl enol ether formation, and oxidation, a stoichiometric amount of waste is generated in each operation. Other metal-mediated α-hydroxylation reactions of carbonyls with dioxygen typically afford hydroperoxides and require subsequent reduction, or have significantly smaller demonstrated substrate scope.9Hydroxylation proceeded smoothly between 0 °C and 6 °C in THF or toluene (for results in toluene and benzene, see Supporting Information). When the reaction was performed under one atmosphere of air instead of O 2 , isolated yields of hydroxylated products were 20-30% lower. The reaction could be ap...

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