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

Reaction 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 ((HOLNH)-N-1, 1a) or 2,4-di-tert-butyl-6-(4,5-dihydropyrrolo[1,2-a]quinoxalin-4-yl) phenol ((HOLNH)-N-2, 1b). Both 1a and 1b can be converted to 2-(H-pyrrolo[1,2-a]quinoxalin-4-yl)phenol ((HOLN)-N-3, 2a) and 2,4-di-tert-butyl-6-(H-pyrrolo[1,2-a]quinoxalin-4-yl)phenol ((HOLN)-N-4, 2b), respectively, by heating 1a and 1b in toluene. Treatment of 1b with an equivalent of AlEt3 afforded [Al(Et-2)((OLNH)-N-2)] (3). Reaction of 1b with two equivalents of AlR3 (R = Me, Et) gave dinuclear aluminum complexes [(AlR2)(2)((OLN)-N-2)] (R = Me , 4a; R = Et, 4b). Refluxing the toluene solution of 4a and 4b, respectively, generated [Al(R-2)((OLN)-N-4)] (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(Me-2)((OLN)-N-3)] (5c). Treatment of 1b with an equivalent of ZnEt2 at room temperature gave [Zn(Et)((OLNH)-N-2)] (6), while reaction of 1b with 0.5 equivalent of ZnEt2 at 40 degrees C afforded [Zn((OLNH)-N-2)(2)](7). Reaction of 1b with two equivalents of ZnEt2 from room temperature to 60 degrees C yielded [Zn(Et)((OLN)-N-4)] (8). Compound 8 was also obtained either by reaction between 6 and an equivalent of ZnEt2 from room temperatureto 60 degrees 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((OLN)-N-4)(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 e-caprolactone was evaluated. Copyright (C) 2008 John Wiley & Sons, Ltd.