Structural Studies of the Template Effect: Formation of Macrocyclic Ligands Incorporating Heavy Donor Atoms

Abstract

X-ray structural studies of both products and reactants involved in the template formation of the macrocycle 'cyclars' (5,14-dimethyl-5,6- 7,9,14,15,16,18-octahydrodibenzo[e,l]- dithiadiarsacyclotetradecin ) have been performed, and the results used to define several aspects of possible reaction mechanisms. The structure of the racemic form of cyclars shows that this free arsenic/sulfur donor ligand is not 'preorganised' for square-planar coordination, though metal ion/donor atom distances in its palladium(II) complex (Pd-As 2.388 Ǻ, Pd-S 2.314 Ǻ) do appear normal. In contrast, the structure of the palladium(II) complex of the arsenic meso ligand, which is not a direct product of the template reaction, shows relatively short palladium/donor atom distances (Pd-As 2.361 Ǻ, Pd-S 2.278 Ǻ), suggesting that template formation may be inhibited because the 'hole size' of this ligand diastereomer is too small for palladium(II). Further, the structure of a bis (bidentate) ligand precursor complex which could, in principle, generate the meso macrocycle shows that, in the solid at least, the reactant groups are not favourably juxtaposed for cyclization. The structure of the platinum(II) complex of a sulfenate derivative of the same bidentate ligand suggests that electrophili c attack at coordinated thiolate, the crucial step in the macrocycle formation, is completely unselective for an unconstrained electrophile.