N,O‐chelate aluminum and zinc complexes: synthesis and catalysis in the ring‐opening polymerization of ε‐caprolactone

Reaction between 2‐(1 H ‐pyrrol‐1‐yl)benzenamine and 2‐hydroxybenzaldehyde or 3,5‐di‐ tert ‐butyl‐2‐hydroxybenzaldehyde afforded 2‐(4,5‐dihydropyrrolo[1,2‐ a ]quinoxalin‐4‐yl)phenol (HOL 1 NH, 1a) or 2,4‐di‐ tert ‐butyl‐6‐(4,5‐dihydropyrrolo[1,2‐ a ]quinoxalin‐4‐yl)phenol (HOL 2 NH, 1b). Both 1a and 1b can be converted to 2‐( H ‐pyrrolo[1,2‐ a ]quinoxalin‐4‐yl)phenol (HOL 3 N, 2a) and 2,4‐di‐ tert ‐butyl‐6‐( H ‐pyrrolo[1,2‐ a ]quinoxalin‐4‐yl)phenol (HOL 4 N, 2b), respectively, by heating 1a and 1b in toluene. Treatment of 1b with an equivalent of AlEt 3 afforded [Al(Et 2 )(OL 2 NH)] (3). Reaction of 1b with two equivalents of AlR 3 (R = Me, Et) gave dinuclear aluminum complexes [(AlR 2 ) 2 (OL 2 N)] (R = Me, 4a; R = Et, 4b). Refluxing the toluene solution of 4a and 4b, respectively, generated [Al(R 2 )(OL 4 N)] (R = Me, 5a; R = Et, 5b). Complexes 5a and 5b were also obtained either by refluxing a mixture of 1b and two equivalents of AlR 3 (R = Me, Et) in toluene or by treatment of 2b with an equivalent of AlR 3 (R = Me, Et). Reaction of 2a with an equivalent of AlMe 3 afforded [Al(Me 2 )(OL 3 N)] (5c). Treatment of 1b with an equivalent of ZnEt 2 at room temperature gave [Zn(Et)(OL 2 NH)] (6), while reaction of 1b with 0.5 equivalent of ZnEt 2 at 40 °C afforded [Zn(OL 2 NH) 2 ] (7). Reaction of 1b with two equivalents of ZnEt 2 from room temperature to 60 °C yielded [Zn(Et)(OL 4 N)] (8). Compound 8 was also obtained either by reaction between 6 and an equivalent of ZnEt 2 from room temperature to 60 °C or by treatment of 2b with an equivalent of ZnEt 2 at room temperature. Reaction of 2b with 0.5 equivalent of ZnEt 2 at room temperature gave [Zn(OL 4 N) 2 ] (9), which was also formed by heating the toluene solution of 6. All novel compounds were characterized by NMR spectroscopy and elemental analyses. The structures of complexes 3, 5c and 6 were additionally characterized by single‐crystal X‐ray diffraction techniques. The catalysis of complexes 3, 4a, 5a–c, 6 and 8 toward the ring‐opening polymerization of ε‐caprolactone was evaluated. Copyright © 2008 John Wiley & Sons, Ltd.