Palladium allylic complexes with enantiopure bis(diamidophosphite) ligands bearing a cyclohexane-1,2-diamine skeleton as catalysts in the allylic substitution reaction

Abstract

A series of cationic allyl palladium complexes [Pd(η3-CH3-C3H5)(P-P)]X (X = PF6, 2a-c, 2e; and X = BPh4, 3a, 3b, 3d, 3e) and [Pd(η3-1,3-Ph2-C3H3)(P-P)]X (X = PF6, 6b; and X = BPh4, 7a) have been prepared. The bis(diamidophosphite) ligands (P-P) contain a diazaphospholidine terminal fragment derived from (R,R)- and (S,S)-N,N’-dibenzyl- and (R,R)-N,N’-dimethyl-cyclohexane-1,2-diamines and dialcoxy bridging fragment derived from (R,R)- and (S,S)-butanediol, (R,R)-cyclohexanediol, (4R,5R)- and (4S,5S)-4,5-di(hydroxymethyl)-2,2-dimethyl-1,3-dioxolane and (R)- and (S)-binaphthol. Complexes [Pd(η3-CH3-C3H5l)P2]X (X = PF6, 4f, 4g; and X = BPh4, 5f), where P are monodentate diamidophosphite ligands with diazaphospholidine heterocyclic backbone obtained from (R,R)- and (S,S)-N,N’-dibenzylcyclohexane-1,2-diamine and alcoxy groups coming from (R)-phenyl-ethanol and (S)-borneol have been also prepared. Neutral palladium complexes [PdCl2(P-P)] (1a, 1c) were synthesized to prove the C2 symmetry of the P-P ligand. The new compounds were fully characterized in solution by NMR spectroscopy. The X-ray crystal structure determination for 2e-(R,R,Ral,Ral;R,R) and 1a-(S,S;Sal,Sal;S,S) has been achieved.The new allyl-palladium complexes were applied in the asymmetric allylic substitution reaction of the benchmark substrate rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate and benzylamine as nucleophiles in order to test their catalytic potential. The best results were obtained with the 3a-(R,R;Ral,Ral;R,R) precursor (up to 84% ee) while complexes with the e ligand derived from the (R,R)-N,N’-dimethylcyclohexane-1,2-diamine terminal fragment resulted inactive in the process. The influence of the nature and the absolute configuration of both the bridging and the terminal fragments of the bis(diamidophosphite) ligand on the asymmetric induction is discussed. A preliminary study of the anion effect (PF6− vs. BPh4-) on the activity and the enantioselectivity of the Pd-catalysed allylic substitution has also been performed.