Enhanced Reactivity of Aluminum Complexes Containing P-Bridged Biphenolate Ligands in Ring-Opening Polymerization Catalysis

摘要

Aluminum complexes containing [RP(O)(2-O-3,5-tBu2C6H2)2]²⁻ [R = tBu (>3a), Ph (>3b)] have been synthesized, structurally characterized, and their reactivity studied in comparison with those of their [RP(2-O-3,5-tBu2C6H2)2]²⁻ [R = tBu (>2a), Ph (>2b)] analogs. Treating AlMe3 with one equiv of H2[>3a-b] in THF at 0°C affords quantitatively [>3a-b]AlMe, subsequent reactions of which with benzyl alcohol in THF at 25°C generate {[>3a-b]Al(μ2-OCH2Ph)}2. The methyl [>3a-b]AlMe and the benzyloxide {[>3a-b]Al(μ2-OCH2Ph)}2 are all active for catalytic ring-opening polymerization (ROP) of ε-caprolactone and rac-lactide (rac-LA). Controlled experiments reveal that {[>3a]Al(μ₂-OCH₂Ph)}₂ is competent in living polymerization. Kinetic studies indicate that [>3a]AlMe, in the presence of benzyl alcohol, catalyzes ROP of rac-LA at a rate faster than [>3b]AlMe and [>2a]AlMe(THF) by a factor of 1.8 and 23.6, respectively, highlighting the profound reactivity enhancement in ROP catalysis by varying the P-substituents of these biphenolate complexes of aluminum.