Dendrimers with inherently axially chiral units

Abstract

We have designed and successfully synthesised dendrimers with axially chiral units in the interior structure. Starting from chiral 2,2′-dimethoxy-1,1′-binaphthalene building blocks 2 and 4 and from the four-directional initiator cores 7 and 8 the dendritic homochiral and heterochiral oligomers 9–16 were prepared. Using the [ϕ]D and Δε values of monomers 2 and 4, we calculated [ϕ]D and Δε values for dendrons 11, 13, and dendrimers 9, 10, 15 and 16. Although the observed molar optical rotation [ϕ]D of the dendrimers agrees relatively well with the calculated values, the CD measurements of all the dendrimers in THF and CH2Cl2, except that of heterochiral dendrimer 16 in THF, were significantly different from the calculated values. The intensive hypochromism of the dendrimers (between 37–59% in THF) and the agreement between the calculated and observed Δε values of the dendrons (between 14 and 6% in THF) led to the assumption that the hypochromic effect is caused by intramolecular interactions. From the NMR measurements it was proved that in the homochiral dendrimer 15, the N–H groups of the amides can form intramolecular hydrogen bonds that in CHCl3, with the help of the axially chiral moieties, cause a different conformation of the molecule than in the diastereomeric dendrimer 16.