Oxidation of olefins by palladium(II). 16. A new palladium(II)-catalyzed asymmetric chlorohydrin synthesis

Previous studies showed that palladium(II) catalysts containing a pyridine ligand give mainly ethylene chlorohydrin as a product of the oxidation of ethene in the presence of CuCl2 at [Cl-] = 0.2 M, Under the same conditions PdCl42- produced only ethanal. The replacement of pyridine by a chiral monoamine gave a catalyst which oxidized propane and methyl vinyl ketone to optically active chlorohydrins. However, the optical yields were low, with ec's of only 8-12%. The use of chelating diphosphines would be expected to greatly improve optical yields. However, the neutral catalysts containing diphosphine ligands were insoluble in the mixed H2O-THF (4/1) reaction media. The chiral ligands (2S,4S)-bis(diphenylphosphino)butane ((S,S)-BDPP), (2S,3S)-bis(diphenylphosphino)butane ((S,S)Chiraphos), and (R)-(+)-2,2'-bis(di-p-tolylphosphino)-1,1'-binaphthyl, ((R)-Tol-BINAP) were sulfonated to give water-soluble catalysts. These catalysts gave poor to modest ee's (28-76%) in the oxidation of propene, methyl vinyl ketone, and allyl phenyl ether. In II2O-THF mixed: solvents richer in TI-IF (1/2 H2O/THF), unsulfonated (R)-Tol-BINAP is soluble to some extent. Oxidation of the above olefinic substrates with this system gave the corresponding chlorohydrins with slightly higher enantioselectivities than with the sulfonated catalysts, The absolute configuration of the chlorohydrin from propane with the (R)-Tol-BINAP catalyst was R. The oxidation of 2,3-dihydrofuran gave chlorohydrins with the E conformation. These two stereochemical results are consistent with anti addition to the most stable pi-complex followed by a Pd(II)-carbon bond-breaking step which places a chloride from the coordination sphere of Pcd(II) on the carbon which is leaving.