Metal Complexes Containing Enantiopure Bis(diamidophosphite) Ligands in Asymmetric Allylic Substitution and Hydroformylation Reactions

Enantiopure bis­(diamidophosphite) ligands with a heterocyclic terminal fragment derived from (R)- and (S)-N,N′-dimethyl-1,1′-binaphthyldiamine and bridging fragments derived from (S,S)-2,3-butanediol (a), (4R,5R)-4,5-di­(hydroxymethyl)-2,2-dimethyl-1,3-dioxolane (b), and (R)- and (S)-1,1′-bi-2-naphthol (c) were used to prepare the palladium complexes with general formula [Pd­(η3-2-CH3-C3H4)­(P-P)]­[X] (X = PF6, 1a-(S;S al ,S al ;S), 1b-(R;R al ,R al ;R), 1b-(S;R al ,R al ;S), 1c-(R;R al ;R), 1c-(R;S al ;R); X = BPh4, 2a-(R;S al ,S al ;R), 2c-(R;R al ;R)), which have been fully characterized. The solid-state structure for complexes 1a-(S;S al ,S al ;S) and 1b-(R;R al ,R al ;R) has been determined by X-ray diffraction. The catalytic performance of the palladium complexes has been evaluated in asymmetric allylic alkylation and amination reactions with the benchmark substrate. The influence of the nature and absolute configuration of both the terminal and bridging fragments of the bis­(diamidophosphite) ligands on the asymmetric induction is discussed. The best results in terms of enantioselectivity were obtained with 1c-(R;R al ;R), affording enantiomeric excesses up to 85% in both alkylation and amination reactions. A large match–mismatch effect between the absolute configurations of stereocenters of ligand c has been observed in the allylic amination process. Preliminary results in the rhodium-catalyzed asymmetric hydroformylation of styrene by using bis­(diamidophosphite) ligands a, b, and c disclosed in all cases low enantiomeric discrimination for the branched aldehyde. Both for the allylic alkylation and for the hydroformylation reaction, a related monodentate diamidophosphite d, derived from (R)-N,N′-dimethyl-1,1′-binaphthyldiamine and (S)-borneol, was also tested. Palladium complexes of this monodentate ligand showed fairly good enantioselectivity in allylic alkylation, but with very low rate, while the rhodium complex of d rendered better enantioselectivity (37% ee) than the bidentate ligands a–c in the hydroformylation of styrene.