Oxidation of Olefins by Palladium(II). 15.1 Oxidation of (R)-(−)-(Z)- and (R)-(+)-(E)-3-Penten-2-ol Using Several Nucleophiles To Give Chiral β-Substituted Ketones. A Method of Finding Modes of Palladation of Olefins Using Chirality Transfer

Abstract

The oxidation of (R)-(−)-(Z)- and (R)-(+)-(E)-3-penten-2-ol with Pd(II) using hydroxyl, methoxyl, acetate, and phenyl as nucleophiles produced β-substituted ketones. The ketone products are optically active, indicating the hydroxyl group was preferentially directing the Pd(II) to one of the two faces of the double bond in forming the initial π-complex. The absolute configuration of the product depends on the absolute configuration of the starting alcohol, the π-complex face to which the nucleophile adds, and the stereochemistry of addition. Because of steric interactions between the two terminal methyl groups, one π-complex is more stable than the other. Phenylpalladium, a reagent which is known to add syn, gave (R)-4-phenyl-2-pentanone, a result that is consistent only with syn addition to the most stable π-complex. With this information, determination of the stereochemistries of addition of the other nucleophiles is possible. In aqueous solution and methanol at low [Cl-], the 4-hydroxy, and 4-methoxy-2-pentanone products had the R configuration, indicating syn addition. These results are consistent with recent stereochemical and kinetic studies. A surprise was the finding of the R configuration, indicating syn addition, for the 4-acetoxy-2-pentanone product in acetic acid solvent. Previous kinetic studies were interpreted in terms of anti addition. Another unexpected result was the reversal of addition to anti at high [Cl-] in acetic acid. As expected, the Z-isomer gave higher degrees of chirality transfer than the E-isomer.