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 chira l bidentate oxazoline ligands (from pyridine (a-d), quinoline (e-g), and pi perazine (h)), containing different kinds of allyl groups (1-5), were easil y obtained, and their structures in solution were elucidated by NMR spectro scopy. Complex 1d was also characterized by X-ray diffraction. The activity of these complexes in palladium-catalyzed allylic alkylation for acyclic a nd cyclic substrates was tested. For catalytic systems with ligands a-g, th e activity was very high, although the enantioselectivity was low for pyrid ine derivatives and moderate for quinolino-oxazoline ligands, showing the b eneficial effect of chelate size in the latter case. In contrast, the palla dium tetraaza catalytic system (Pd/h) exhibited a low activity and a low as ymmetric induction. Allylic complexes 1a, 1d, and 1e were also active in th e Pd-catalyzed allylic amination, but the activity of these systems was low er than for the alkylations. Under in situ catalytic conditions, the coordi nation competition of the benzylamine with the bidentate ligand led to diff erent results than using previously isolated precursors.