Steric Control of Directional Isomerism in Dicopper(I) Helicates of Asymmetrically Substituted 2,2':6',2'':2'',6'''-Quaterpyridine Derivatives

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    Abstract: Derivatives of 2,2':6',2'':2'',6'''-quaterpyridine have been prepared which are asymmetrically substituted with alkyl groups in the 4- or 6-position and with various substituents in the 4-position. These ligands form dicopper- (I) double helicates which have been investigated by 1H and 13C NMR spectroscopic techniques. The formation of helical isomers is shown to depend on the intramolecular interactions between the constituent helicands of the double helicate; 4-methyl substituents undergo steric interactions with the 4-substituent of the partner helicand, leading to a modest selectivity, although bulky 4-substituents decrease selectivity. In the absence of 4-substituents, the smaller pitch permits steric interactions between like 4-substituents of the component helicands. In each case, formation of the head-to-head helicate isomer is preferred.