Magnesium Pyrazolyl-Indolyl Complexes as Catalysts for Ring-Opening Polymerization of L-Lactide

A series indole-based ligand precursors, Pz(R)IndH (R = H, Pz(H)IndH; R = Me, Pz(Me)IndH; R = t-Bu, Pz(tBu)IndH; and R = Ph, Pz(Ph)IndH), have been synthesized via copper-catalyzed N-arylation (for Pz(H)IndH) or the Bartoli indole synthesis (for Pz(Me)IndH, Pz(tBu)IndH and Pz(Ph)IndH) reactions with moderate to high yield. Reactions of these ligand precursors with 0.7 equivalent of (MgBu2)-Bu-n in THF (for 1) or hexane (for 2-4) afforded the bis-indolyl magnesium complexes 1-4, respectively. All the ligand precursors and related magnesium complexes have been characterized by NMR spectroscopy and elemental analyses. The molecular structure is reported for compound 1. These novel magnesium complexes demonstrate efficient catalytic activities for the ring-opening polymerization of L-lactide in the presence of alcohol.