Dendritic BINOL ligands for asymmetric catalysis: effect of the linking positions and generations of the dendritic wedges on catalyst properties |
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Authors: | Guo-Hua LiuWei-Jun Tang Qing-Hua Fan |
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Institution: | a Laboratory of Chemical Biology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China |
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Abstract: | Three types of new chiral BINOL ligands (2, 3 and 4) bearing dendritic wedges have been synthesized through coupling reaction between 3-hydroxymethyl-2,2′-bis(methoxymethyl)-1,1′-binaphthol (7), 6,6′-dihydroxymethyl-2,2′-bis(methoxymethyl)-1,1′-binaphthol (12), 6-hydroxymethyl-2,2′-bis(methoxymethyl)-1,1′-binaphthol (15) and Fréchet-type polyether dendritic benzyl bromide, followed by deprotection of methoxymethyl groups by iPrOH/HCl, respectively. These new ligands obtained were assessed in enantioselective Lewis acid-catalyzed addition of diethylzinc to benzaldehyde. Compared to the enantioselectivity observed with dendrimer 1 bearing the dendritic wedges at 3,3′-positions of the binaphthyl backbone, higher enantioselectivity for all these ligands was observed. Difference in the effect of linking positions and generations on enantioselectivity and/or activity for all three kinds of dendritic ligand-derived catalysts was observed. Among these dendritic ligands, (R)-3/Ti(IV) catalyst with the dendritic wedges at 6,6′-positions of BINOL gave the highest enantioselectivity (up to 87% ee). |
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Keywords: | chiral dendrimer asymmetric catalysis alkylation BINOL |
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