Allylic alkylations catalyzed by palladium systems containing modular chiral dithioethers. A structural study of the allylic intermediates

Abstract

Palladium allylic systems containing modular chiral dithioether ligands were chosen as catalysts for a systematic study of homodonor ligands in allylic alkylation reactions. For this purpose, new type-DMPS (4-6) and -DEGUS (8, 9) ligands were synthesized. Dithioethers 4-6 afforded high activities and excellent selectivities in all Pd-catalyzed allylic reactions. Particularly the Pd/6 catalytic system provided b-VIII with an ee > 99% and a regioselectivity 1/b (VIII) = 1/1.6. A family of intermediate complexes containing several allyl groups (symmetrical, 13-16 and 20, and nonsymmetrical, 17-19, moieties) and dithioether ligands (4-6, 8, 9, 11, and 12) is described. The complexes were fully characterized both in solution and in the solid state. X-ray structures of four of these complexes (16-18 and 20) were determined. The diastereomers present in solution were studied by NMR spectroscopy, and in some cases it was possible to establish a relationship between the diastereomeric excesses and the selectivities found in the catalytic process.