Palladium Complexes with Chiral Oxazoline Ligands. Effect of Chelate Size on Catalytic Allylic Substitutions

Mono- and bimetallic allylic palladium complexes (1L−5L) with L being chiral bidentate oxazoline ligands (from pyridine (a−d), quinoline (e − g), and piperazine (h)), containing different kinds of allyl groups (1−5), were easily obtained, and their structures in solution were elucidated by NMR spectroscopy. Complex 1d was also characterized by X-ray diffraction. The activity of these complexes in palladium-catalyzed allylic alkylation for acyclic and cyclic substrates was tested. For catalytic systems with ligands a−g, the activity was very high, although the enantioselectivity was low for pyridine derivatives and moderate for quinolino-oxazoline ligands, showing the beneficial effect of chelate size in the latter case. In contrast, the palladium tetraaza catalytic system (Pd/h) exhibited a low activity and a low asymmetric induction. Allylic complexes 1a, 1d, and 1e were also active in the Pd-catalyzed allylic amination, but the activity of these systems was lower than for the alkylations. Under in situ catalytic conditions, the coordination competition of the benzylamine with the bidentate ligand led to different results than using previously isolated precursors.