Synthesis and chiroptical properties of layer-block dendrimers

A series of optically active, tartaric acid-based, monodisperse dendrimers 1 4 was prepared. The central core and branching junctures of these dendritic compounds were made up from phloroglucinol units and the surface sectors used were 4-tert-butylphenoxy moieties. The optically active elements, which were derivatives of either (D)or (L)-tartaric acid, served as the chiral linker branches of these dendrimers. For this series of structurally flexible, low generation optically active dendrimers, the observed molar optical rotation was proportional to the number of chiral units residing within the dendritic matrix, with the chiroptical property of the (D)-tartrate unit cancelling that of the (L)-tartrate unit on a one to one mole basis. Circular dichroism studies, however, revealed that this cancellation effect was more effective when both the (D)and (L)-chirons were situated within the same layer. INTRODUCTION Recent interest in the preparation of optically active dendrimers stemmed from their potential use as chiral hosts for enantiomeric resolutions and as chiral catalysts for asymmetric synthesis (ref. 1). In order to understand the structure-property relationship of a chiral dendrimer, it is necessary to establish the intriguing relationship between the overall chiroptical properties of the dendrimer and the stereospatial arrangement of its constituent chiral elements. Homochiral, optically active, structurally defined dendrimers are idea models for this type of study since the repeating chiral elements are expected to distribute in an orderly manner inside a dendritic matrix. By selecting appropriate chiral elements and placing them in pre-defined positions by either convergent or divergent synthetic methodology, the influence of the three dimensional disposition of these chiral elements on the overall macroscopic chiroptical properties of optically active dendrimers can then be evaluated. Today, the use of optically active, naturally occurring biological molecules such as carbohydrates or amino acids as monomeric units for the construction of chiral bio-active dendrimers has been well exemplified (ref. 2). On the other hand, relatively few studies had been conducted on the preparation of abiotic chiral dendrimers and their structural-chiroptical properties. Some time ago, Newkome reported the synthesis of a series of tryptophane terminated dendritic macromolecules (ref. 3) and showed that the molecular ellipicity of these molecules was proportional to the number of chiral surface groups. Recently, Seebach reported the preparation of optically active dendrimers using (R)-3-hydroxybutanoic acid derivatives as the chiral branching junctures (ref. 4). In this particular study, it was discovered that attachment of achiral branches to a chiral centre piece led to a dilution of the overall molar optical rotation of the resulting dendrimers (ref. 4c). On the other hand, Meijer reported that sterically congested dendrimers with multiple optically active units located on the surface sector displayed diminishing optical rotation property (ref. 5) . This paper summarizes our independent work on the syntheses and chiroptical property investigations on a series of optically active homo-dendrimers 1 2 (ref. 6 ) and layer-block co-dendrimers 3 4 (ref. 7), using optically pure tartrate derivatives as the chiral branches. SYNTHESIS Our chiral dendrimers 1 4 utilize phloroglucinols as the branching junctures and 4-tert-butylphenoxjr moieties as the end groups (Scheme 1). The optically active linkers are made up of either (D)or (L)tartrate derived chiral units, because they are inexpensive and readily available in both enantiomeric pure form. The chiral linkers of first two homochiral dendrimers 1 and 2, comprise of only @)-tartrate derived units. On the other hand, the layer-block co-dendrimers 3 and 4, consist of both (D)and @)-tartrate derived functionalites as the chiral linkers.