Group 3 Metal Initiators with an [OSSO]‐Type Bis(phenolate) Ligand for the Stereoselective Polymerization of Lactide Monomers

A series of 1,ω‐dithiaalkanediyl‐bridged bis(phenols) of the general type OSSO]H₂ with variable steric properties and various bridges were prepared. The stoichiometric reaction of the bis(phenols) 1,3‐dithiapropanediyl‐2,2′‐bis(4,6‐di‐tert‐butylphenol), 1,3‐dithiapropanediyl‐2,2′‐bis4,6‐di(2‐phenyl‐2‐propyl)phenol], rac‐2,3‐trans‐propanediyl‐1,4‐dithiabutanediyl‐2,2′‐bis4,6‐di(2‐phenyl‐2‐propyl)phenol], rac‐2,3‐trans‐butanediyl‐1,4‐dithiabutane diyl‐2,2′‐bis4,6‐di(2‐phenyl‐2‐propyl)phenol], rac‐2,3‐trans‐hexanediyl‐1,4‐dithiabutanediyl‐2,2′‐bis4,6‐di(2‐phenyl‐2‐propyl)phenol], 1,3‐dithiapropanediyl‐2,2′‐bis6‐(1‐methylcyclohexyl)‐4‐methylphenol] (C₁, R=1‐methylcyclohexyl), and 1,4‐dithiabutanediyl‐2,2′‐bis6‐(1‐methylcyclohexyl)‐4‐methylphenol] with rare‐earth metal silylamido precursors Ln{N(SiHMe₂)₂}₃(thf)_x] (Ln=Sc, x=1 or Ln=Y, x=2; thf=tetrahydrofuran) afforded the corresponding scandium and yttrium bis(phenolate) silylamido complexes Ln(OSSO){N(SiHMe₂)₂}(thf)] in moderate to good yields. The monomeric nature of these complexes was shown by an X‐ray diffraction study of one of the yttrium complexes. The complexes efficiently initiated the ring‐opening polymerization of rac‐ and meso‐lactide to give heterotactic‐biased poly(rac‐lactides) and highly syndiotactic poly(meso‐lactides). Variation of the ligand backbone and the steric properties of the ortho substituents affected the level of tacticity in the polylactides.