Reversibly photoswitchable catalysts for high regioselectivity and enantioselectivity in polymerization of allenes

Abstract

A pair of enantiomeric bidentate phosphine ligands L^R-azo and L^S-azo containing an azobenene moiety were prepared which could be directly coordinated with (π-allyl)NiOCOCF₃, generating two reversibly photoswitchable chiral Ni(II) catalysts for living/controlled polymerization of allene monomers. Ni(II)/L^R-azo and Ni(II)/L^S-azo exhibited different polymerization activities, regioselectivities, and enantioselectivities during the polymerization process due to their cis–trans photoisomerization of the azobenzene moiety on the catalyst under irradiation by light with different wavelengths. A high 2,3-regioselectivity (99.0%) was observed during the polymerization process of monomer 1 using Ni(II)/L^R-azo as catalyst under visible light (420 nm) irradiation. However, under dark conditions, the polymerization of 1 proceeded with lower regioselectivity (74.6%). Moreover, the polymerization rate ratio of D-2/L-2 was calculated to be ca. 55 under dark conditions, while the polymerization rate ratio of L-2/D-2 could be up to 21 under irradiation by visible light (420 nm) using Ni(II)/L^R-azo as initiator. High enantioselectivity was observed on polymerization of the racemates of L-2 and D-2, affording helical polyallenes with 60% ee_h value using Ni(II)/L^R-azo under dark conditions. Furthermore, the opposite enantioselectivity was observed during the polymerization of the racemates of L-2 and D-2 using Ni(II)/L^S-azo. Due to the cis- and trans-photoisomerization of the azobenene moiety, the Ni(II)/L^R-azo catalysts exhibited a reversible on–off switchable ability in the process of polymerization of the allene monomer of D-2 or L-2 under alternate irradiation by visible light (420 nm) and dark conditions.