Stoichiometric and Catalytic Oxidations with Hypervalent Organo-lambda3-iodanes

Isolation, characterization, and reaction of the activated iodosylbenzene monomer, hydroxy(phenyl)iodonium ion, as a complex with 18-crown-6 (18C6) are reported. The reaction of iodosylbenzene with HBF(4) in the presence of 18C6 afforded the hydroxy-lambda(3)-iodane complex PhI(OH)BF(4).18C6 as stable yellow prisms. X-ray structure analysis indicated that the close contacts between the iodine(III) and the three adjacent oxygen atoms of 18C6 will be responsible for the increased stability of the complex compared to the uncomplexed PhI(OH)BF(4). The aqua complex of the activated iodosylbenzene, PhI(OH)OTf.18C6.H(2)O, with a water molecule coordinated to iodine(III) was also prepared. These crown ether complexes are highly reactive and serve as versatile stoichiometric oxidants, especially in water. Thus, the complexes undergo oxidative transformations of a variety of functional groups such as olefins, alkynes, enones, silyl enol ethers, sulfides, and phenols under mild conditions. The latter part reports on the iodobenzene-catalyzed alpha-oxidation of ketones, in which diacyloxy(phenyl)-lambda(3)-iodanes generated in situ act as real oxidants of ketones and m-chloroperbenzoic acid (m-CPBA) serves as a terminal oxidant. The oxidation of a ketone with m-CPBA in acetic acid in the presence of a catalytic amount of iodobenzene, BF(3)-Et(2)O, and water at room temperature affords an alpha-acetoxy ketone in good yield. It is noted that the use of water and BF(3)-Et(2)O is crucial to the success of this alpha-acetoxylation.