Aluminum Complexes Bearing N‑Protected 2‑Amino- or 2‑Imino-Functionalized Pyrrolyl Ligands: Synthesis, Structure, and Catalysis for Preparation of Pyrrolyl-End-Functionalized Polyesters

Reactivity
of N-protected 2-amino- or 2-imino-functionalized pyrroles
with aluminum alkyls was investigated, resulting in the isolation
of a series of aluminum alkyl complexes. Treatment of 2-imino-functionalized
pyrrole with AlMe3 produced only imino-coordinated aluminum
complex 1-Bn-2-(2,6-iPr2C6H3NCH)­C4H3NAlMe3 (1), while reactions of N-protected 2-amino-functionalized
pyrroles with aluminum alkyls produced the aluminum alkyl complexes
{[η1-μ-η1:η1-1-R1-2-(2,6-iPr2C6H3NCH2)­C4H2N]­AlR}2 (R1 = Bn, R = Me (2); R1 = Bn, R = Et (3); R1 = R
= Me (4); R1 = Me, R = Et (5)),
bearing 3-carbon bonded pyrrolyl ligands via C–H σ-bond
metathesis reaction. Further reactions of complexes 2–5 with a stoichiometric amount of isopropyl
alcohol (iPrOH) afforded the corresponding
aluminum alkoxide complexes [1-R1-2-(2,6-iPr2C6H3NCH2)­C4H3NAlR­(μ-OiPr)]2 (R1 = Bn, R = Me (6); R1 = Bn, R = Et (7); R1 = R = Me (8); R1 = Me, R = Et (9)) through selective
cleavage of the Al–C (Pyr) bonds. The solid-state structures
of the aluminum complexes 1–6 and 8 were confirmed by an X-ray diffraction study. These aluminum
alkyl complexes exhibited notable activity toward the ring-opening
polymerization of ε-caprolactone and l-lactide in the
absence of alcohol. The end group analysis of the ε-CL oligomer
gave strong support that the polymerization proceeded via a coordination–insertion
mechanism involving a unique Al–C (Pyr) bond initiation, providing
pyrrolyl-end-functionalized polyesters.