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

A pair of enantiomeric bidentate phosphine ligands LR-azo and LS-azo containing an azobenene moiety were prepared which could be directly coordinated with (& pi;-allyl)NiOCOCF3, generating two reversibly photoswitchable chiral Ni(ii) catalysts for living/controlled polymerization of allene monomers. Ni(ii)/LR-azo and Ni(ii)/LS-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)/LR-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)/LR-azo as initiator. High enantioselectivity was observed on polymerization of the racemates of l-2 and d-2, affording helical polyallenes with 60% eeh value using Ni(ii)/LR-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)/LS-azo. Due to the cis- and trans-photoisomerization of the azobenene moiety, the Ni(ii)/LR-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. Two enantiomers Ni(ii)/LR-azo and Ni(ii)/LS-azo could be used as reversible photo-switchable catalysts for the high regioselectivity and enantioselectivity polymerization of allene monomers.

Automated summary

Two enantiomeric bidentate phosphine ligands LR-azo and LS-azo containing azobenene moiety were prepared and directly coordinated with (& pi;-allyl)NiOCOCF3 to generate two reversibly photoswitchable chiral Ni(ii) catalysts for living/controlled polymerization of allene monomers. The cis-trans photoisomerization of the azobenzene moiety on the catalyst under irradiation with different wavelengths resulted in different polymerization activities, regioselectivities, and enantioselectivities for Ni(ii)/LR-azo and Ni(ii)/LS-azo.