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

Abstract

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 ca...