Chiral 27-membered [3 + 3] Schiff-base macrocycles and their reactivity with first-row transition metal ions

Abstract

27-Membered Schiff-base macrocycles 1 and 2 are prepared via the [3 + 3] template condensation of 2,6-diformyl-4-substituted phenol (R = Cl in 1 and R = CH 3 in 2) with enantiomeric [(1 R,2 R) or (1 S,2 S)] and racemic trans-cyclohexane-1,2-diamines. Among them two chiral [3 + 3] macrocyclic single-crystals ( S, S)- 1 and ( R, R)- 2 in their backbones have been structurally characterized. Both of them have triangular holes (∼42.36 Å 2) in which one phenyl ring of the neighboring molecule is inserted as a guest by π–π stacking interactions with the centroid–centroid separation of 3.599 Å. The treatment of aforementioned macrocycles with certain first-row transition metal salts [Cu(II), Zn(II), Fe(III)] with different counterions leads to the cleavage of Schiff-base C N double bonds and degradation of the macrocycles due to the size matching effect of macrocycles as well as the restriction of coordination geometry of the central metals and the counterions used. As a result, [2 + 2], [2 + 1] and [1 + 2] types of complexes (( R, R)- 3, 4, 5 and 6) are yielded. In the case of 6, a new type of dimethyl acetal complex is produced in methanol where FeCl 3 serves as a reactive Lewis acid catalyst. The intrinsic chiral differences of the macrocycles and their transition metal complexes are demonstrated by circular dichroism (CD) spectra.