Metal-ion-controlled transamination in the synthesis of macrocyclic Schiff-base ligands. Part 1. Reactions of 2,6-diacetylpyridine and dicarbonyl compounds with 3,6-dioxaoctane-1,8-diamine
Possible routes to the metal-ion-controlled synthesis of [2 + 2] macrocyclic ligands derived from the Schiff-base condensation of two molecules of 2,6-diacetylpyridine (or related dicarbonyl) with two molecules of 3,6-dioxaoctane-1,8-diamine are considered. It is proposed that in the presence of Ba2+ ion, macrocycle formation proceeds via the open-chain intermediate derived from one molecule of dicarbonyl and two molecules of diprimary amine. The intermediates, isolated as their barium(II) complexes, are shown to undergo ring closure in anhydrous MeOH on treatment with a further mol of dicarbonyl to give complexes of the 30-membered macrocyclic ligands, including an ‘asymmetric’ ligand containing both pyridyl and furyl moieties. In one case, reversible ring closure occurs in the absence of added dicarbonyl and a transamination mechanism involving a sequence of nucleophilic additions (followed by deamination) of NH2 to co-ordinated CN groups is proposed. Further evidence for transamination as a route to macrocycle synthesis is provided by the ring closure, with elimination of four molecules of e.g. n-propylamine, observed on treatment of some six-co-ordinate copper(II) or zinc(II) bis complexes of pyridine-2,6-bis(N-propylcarbaldimine)(or related open-chain ligand) with 3,6-dioxaoctane-1,8-diamine. An analogous transamination mechanism is proposed for the conversion of a 30-membered macrocyclic ligand to an 18-membered macrocycle co-ordinated to PbII, and for the conversion of a magnesium(II) complex of the same 30-membered ring to a magnesium(II) complex of a 15-membered ring.