Abstract
Aluminum complexes containing [RP(O)(2-O-3,5-tBu(2)C(6)H(2))(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-tBu(2)C(6)H(2))(2)](2-) [R = tBu (2a), Ph (2b)] analogs. Treating AlMe3 with one equiv of H-2[3a-b] in THF at 0 degrees C affords quantitatively [3a-b]AlMe, subsequent reactions of which with benzyl alcohol in THF at 25 degrees C generate {[3a-b] Al(mu(2)-OCH2Ph)}(2). The methyl [3a-b]AlMe and the benzyloxide {[3a-b] Al(mu(2)-OCH2Ph)}(2) are all active for catalytic ring-opening polymerization (ROP) of epsilon-caprolactone and rac-lactide (rac-LA). Controlled experiments reveal that {[3a] Al(mu(2)-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.