Enantioselective Formation of Double and Triple Helicates of Silver(I): The Role of Stacking Interactions

Abstract

Despite having ligands with almost identical conformations, the structures of the AgI complexes of 1 and 2 differ significantly: ligand 1 forms a double‐helical complex of P chirality, and 2 gives a triple‐helical complex (depicted on the right). The reason for the difference appears to be the existence of strong interligand stacking in the latter case.

Diastereoselectivity of Octahedral Cobalt(II) Pybox Complexes

“…Secondly, the phenyl groups can actually give an attractive interaction with the pyridine of the second ligand. This was clearly established in the trinuclear silver complex, [11] and, on the basis of the crystal structure, appears to be the case here. The lesser interligand repulsion of 1 compared to 2 and 3 can therefore explain the higher ligand field.…”

“…[17,18] The remarkable difference in diastereoselectivity may thus be explained in terms of the properties of the substituent. The phenyl group which stood apart from the others in helicate chemistry [11] also shows different behavior in simple octahedral complexes. It is reasonable to suppose that this effect might influence the enantioselectivity of catalytic reactions using these ligands, and, indeed, Evans and coworkers [10] did observe that the enantioselectivity was highest for the phenyl-substituted ligand when compared with benzyl, isopropyl and tert-butyl substituents.…”

“…Ligand A has the two phenyl rings oriented at roughly 90°to the oxazoline plane, in such a way as to lie almost parallel to the pyridine of ligand B [interplane angles: 3.7(7) and 3.0(7)°; interplane distances: 3.55(7) and 3.66(6) Å ] indicating a stacking interaction similar to that observed in the circular trinuclear helicate. [11] The phenyl groups of ligand A are offset with respect to the coordinated pyridine of ligand B, as expected according to the model of Hunter and Sanders. [13] This orientation of ligand A, however, brings the phenyl rings close to those of ligand B.…”

“…[10] We have recently used these ligands for the enantioselective synthesis of helical complexes. [11] During this work it was observed that the phenyl derivative 1 showed significantly different behavior from the other ligands 2 and 3, forming a cyclic trinuclear helicate rather than a linear dinuclear helicate. This difference could be attributed to intramolecular attractions between the phenyl substituents and the pyridine moiety.…”

Completely Stereospecific Self-Assembly of a Circular Helicate

“…In few cases circular helical assemblies have been reported. [5, 16±20] However, with the exception of a recently reported trinuclear species, [19] in all such cases racemates were obtained. Here we present an example of a completely stereospecific, self-assembled, circular helical structure.…”