Easily recoverable BINOL ligand with ionic tag for asymmetric catalysis

A new chiral task specific ionic species containing chiral (S)-binaphthyl unit was synthesized and used as an auxiliary in the Ti-promoted addition of diethylzinc to benzaldehyde. The utilization of the ionic ligand containing the (S)-BINOL substructure led to enantioselectivities similar to those of the related non-ionic counterpart. The ionic substructure allowed an appropriate tuning of the solubility of the BINOL ligands in order to ensure reaction in a homogeneous medium and recovery of the chiral auxiliary. The reaction was performed in CH2Cl2 while the chiral ionic auxiliary was re-used in three catalytic cycles without a loss in activity or enantioselectivity.

Figure optionsDownload as PowerPoint slide

(S)-6-Bromo-2,2′-dihexyloxy-1,1′-binaphthylC32H37BrO2[α]D25=-67.1 (c 1.46, CH2Cl2)Absolute configuration: (S)Source of chirality: (S)-2,2′-dihydroxy-1,1′-binaphthyl

(S)-2,2′-Dihexyloxy-1,1′-binaphthyl-6-carbaldehydeC33H38O2[α]D25=-54.9 (c 2.06, CH2Cl2)Absolute configuration: (S)Source of chirality: (S)-2,2′-dihydroxy-1,1′-binaphthyl

(S)-3-Butyl-1-{3-[(2,2′-dihexyloxy-[1,1′]binaphthalenyl-6-ylmethyl)-amino]propyl}-3H-imidazol-1-ium bis-(trifluoromethanesulfonyl)imideC45H58F6N4O6S2[α]D25=-35.5 (c 1.04, CH2Cl2)Absolute configuration: (S)Source of chirality: (S)-2,2′-dihydroxy-1,1′-binaphthyl

(S)-3-Butyl-1-{3-[(2,2′-dihydroxy-[1,1′]binaphthalenyl-6-ylmethyl)-amino]propyl}-3H-imidazol-1-ium bis-(trifluoromethanesulfonyl)imideC33H34F6N4O6S2[α]D25=-40.3 (c 1.54, DMSO)Absolute configuration: (S)Source of chirality: (S)-2,2′-dihydroxy-1,1′-binaphthyl