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;Sal,Sal;S), 1b-(R;Ral,Ral;R), 1b-(S;Ral,Ral;S), 1c-(R;Ral;R), 1c-(R;Sal;R); X = BPh4, 2a-(R;Sal,Sal;R), 2c-(R;Ral;R)), which have been
fully characterized. The solid-state structure for complexes 1a-(S;Sal,Sal;S) and 1b-(R;Ral,Ral;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;Ral;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.