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

Abstract

AbstractReaction between 2‐(1H‐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 (HOL1NH, 1a) or 2,4‐di‐tert‐butyl‐6‐(4,5‐dihydropyrrolo[1,2‐a]quinoxalin‐4‐yl)phenol (HOL2NH, 1b). Both 1a and 1b can be converted to 2‐(H‐pyrrolo[1,2‐a]quinoxalin‐4‐yl)phenol (HOL3N, 2a) and 2,4‐di‐tert‐butyl‐6‐(H‐pyrrolo[1,2‐a]quinoxalin‐4‐yl)phenol (HOL4N, 2b), respectively, by heating 1a and 1b in toluene. Treatment of 1b with an equivalent of AlEt3 afforded [Al(Et2)(OL2NH)] (3). Reaction of 1b with two equivalents of AlR3 (R = Me, Et) gave dinuclear aluminum complexes [(AlR2)2(OL2N)] (R = Me, 4a; R = Et, 4b). Refluxing the toluene solution of 4a and 4b, respectively, generated [Al(R2)(OL4N)] (R = Me, 5a; R = Et, 5b). Complexes 5a and 5b were also obtained either by refluxing a mixture of 1b and two equivalents of AlR3 (R = Me, Et) in toluene or by treatment of 2b with an equivalent of AlR3 (R = Me, Et). Reaction of 2a with an equivalent of AlMe3 afforded [Al(Me2)(OL3N)] (5c). Treatment of 1b with an equivalent of ZnEt2 at room temperature gave [Zn(Et)(OL2NH)] (6), while reaction of 1b with 0.5 equivalent of ZnEt2 at 40 °C afforded [Zn(OL2NH)2] (7). Reaction of 1b with two equivalents of ZnEt2 from room temperature to 60 °C yielded [Zn(Et)(OL4N)] (8). Compound 8 was also obtained either by reaction between 6 and an equivalent of ZnEt2 from room temperature to 60 °C or by treatment of 2b with an equivalent of ZnEt2 at room temperature. Reaction of 2b with 0.5 equivalent of ZnEt2 at room temperature gave [Zn(OL4N)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.