Oxidation of olefins by palladium(II). 18. Effect of reaction conditions, substrate structure and chiral ligand on the bimetallic palladium(II) catalyzed asymmetric chlorohydrin synthesis

The effect of electronic factors, solvent composition, identity of the chiral bidentate, and olefin structure on the yields and enantioselectivities of the asymmetric chlorohydrin synthesis were investigated. Electronic effects on the chlorohydrin reaction were tested by oxidation of phenyl allyl ether p-substituted by H, Cl, CH3O and CN. All species gave same similar yields and enantioselectivities indicating that electronic effects are not important. Varying the solvent composition of the THF–H2O mixtures indicated that the optimal solvent mixture contains more than 85% THF. Variation of added [Cl−] indicated that the added chloride had to be greater than 0.2 M for high yields and %eeʹs. Under ideal conditions the enantioselectivities of the chlorohydrins from the phenyl allyl ethers were more than 90%ee. Vinylacetic acid, methyl acrylate and trans-cinnamaldehyde were unreactive under the usual reaction conditions while 2-hydroxy-3-butene and allyl acetate give lower %eeʹs than did the phenyl allyl ethers. Styrene and α-methylstyrene gives comparable rates of reactions but the %eeʹs were lower with the latter. (2,6-Diisopropyl)phenyl allyl ether and 2-hydroxy-3-butene give high %eeʹs indicating that steric hindrance was not a major factor. All of the chiral bridging ligands tested gave satisfactory results except for DACH. A strange case was BZOX which did not give any induction at all. Structural studies showed the ligands are not large enough to bridge both Pd(II) in the bimetallic catalyst so one Pd(II) contained both ligand groups of the bidentate ligand and was thus unreactive. The other Pd(II) of the dimer was reactive but did not contain any chiral ligands to induce optical activity.