Chiral (1,2)-Diphenylethylene-Salen Complexes of Triel Metals: Coordination Patterns and Mechanistic Considerations in the Isoselective ROP of Lactide

The synthesis of enantiomerically pure aluminium, gallium and indium complexes supported by chiral (R,R)‐(HHONNOHH) (1), (R,R)‐(MeHONNOHMe) (2), (R,R)‐(tButBuONNOtButBu) (3), (R,R)‐(MeNO2ONNOMeNO2) (4), (R,R)‐(HOMeONNOHOMe) (5) and (R,R)‐(ClClONNOClCl) (6) (1,2)‐diphenylethylene‐salen ligands is described. Several of these complexes have been crystallographically authenticated, which highlights a diversity of coordination patterns. Whereas all Ga complexes form [Ga2(CH2SiMe3)4(ONNO)] bimetallic species (ONNO=1–3), aluminium [AlR(ONNO)] (R=Me, CH2SiMe3) and indium [In(CH2SiMe3)(ONNO)] derivatives are monometallic for ONNO=1, 2 and 4–6, and only form the bimetallic complexes [Al2R4(ONNO)] and [In2(CH2SiMe3)4(ONNO)] for the most sterically crowded ligand 3. The [AlMe(ONNO)] complexes react with iPrOH to give [AlOiPr(ONNO)] complexes that are robust towards further iPrOH. The [In(CH2SiMe3)(ONNO)] congeners are inert towards excess alcohol, whereas the Ga compounds decompose easily. All these alkyl complexes, as well as the [AlOiPr(ONNO)] derivatives, catalyse the ring‐opening polymerisation (ROP) of racemic lactide (rac‐LA). The [AlMe(ONNO)] complexes require additional alcohol to afford controlled reactions, but [AlOiPr(ONNO)] complexes are single‐component catalysts for the isoselective ROP of rac‐LA, with values of Pm in the range 0.80–0.90. Experimental evidence unexpectedly shows that chain‐end control leads to the isoselectivity of these aluminium catalysts; also, the more crowded the coordination sphere, the higher the isoselectivity. The bimetallic Ga complexes do not afford controlled reactions, but the binary [In(ONNO)(CH2SiMe3)/(PhCH2OH)] systems competently mediate non‐stereoselective ROP; evidence is given that an activated monomer mechanism is at work. Kinetic studies show that catalytic activity decreases when electronic density and steric congestion at the metal atom increase. Class act: A new range of monometallic chiral salen–aluminium complexes based on a less‐classical chiral backbone has been developed. These constitute efficient precatalysts for the isoselective ring‐opening polymerisation of racemic lactide. According to chain‐end control, they afford poly(lactide) (PLA) featuring high isotacticity (Pm≈0.80–0.90) in the upper range reported to date (see figure).