キラリティースイッチングを特徴とするらせん高分子配位子を用いた触媒的不斉合成

概要

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Polyquinoxaline-based helically chiral phosphine ligands PQXphos, bearing chiral side chains, were synthesized via living aromatizing copolymerization of o-diisocyanobenzene monomers. The chiral reaction environment of PQXphos is created on the basis of its single-handed helical structure, leading to high enantioselectivities in palladium-catalyzed asymmetric hydrosilylation of styrenes (up to 98% ee), Suzuki-Miyaura coupling (up to 98% ee), and silaborative C-C cleavage of meso-methylenecyclopropanes (up to 97% ee). While most common organic solvents induce right-handed helix to (R)-PQXphos bearing (R)-2-butoxymethyl side chains, 1,1,2-trichloroethane and some solvents induce left-handed helix to it. The inverted, left-handed (R)-PQXphos also showed high enantioselectivities for the production of enantiomers of the products obtained with right-handed (R)-PQXphos. PQXphos formed insoluble polymer complex through coordination to palladium. The insoluble palladium complex of PQXphos was reused 8 times, while keeping catalyst activity and enantioselectivity. In the silaborative desymmetrization of methylenecyclopropanes, PQXphos showed not only high enantioselectivity, but also high catalyst activity in comparison with the corresponding low-molecular-weight ligand.