Constitution, configuration, and the optical activity of chiral dendrimers

Date of Completion

January 1998

Keywords

Chemistry, Organic

Degree

Ph.D.

Abstract

The efficient preparation of four chiral AB2 monomers suitable for the construction of dendrimers is presented. Chirality is introduced in the form of a 1,2-diol unit, protected as an acetonide derivative, using the osmium catalyzed asymmetric dihydroxylation. reaction (AD) (>97% ee in all cases). Using these cinnamate and stilbene based monomer subunits, dendrons and dendrimers have been prepared up to the third generation using a convergent dendrimer synthesis. Investigation of the chiroptical properties of the dendrons and dendrimers prepared revealed a significant change in molar rotation per chiral subunit ([Φ]D/n) upon increasing generation for the cinnamate based series, a possible indication of chiral conformational order in this system. However, preparation and investigation of the chiroptical properties of low molecular weight model compounds, prepared to simulate different regions of the dendrimer structure, revealed that any observed chiroptical anomalies were a result of slight constitutional changes within the dendritic architecture and not chiral conformational order. ^ The deprotection of the chiral 1,2-diol units for dendrimers up to the first generation is presented. We found that the solubility of these more polar polyhydroxylated chiral dendrimers was low in non-polar organic solvents. Consequently, dendrimer peripheries were modified with long chained alkyl groups, resulting in dendrimers which were more compatible with non-polar media (e.g. n-hexane). Dendrimers; like these, containing chiral polar interiors (after deprotection) and non-polar peripheries, may potentially serve as chiral inverse unimolecular micelles. ^ The novel construction of photolabile dendrimers containing o-nitrobenzyl ether linkages is presented. A variety of dendrimers were prepared with photolabile linkers positioned at the core, interior, or periphery of the dendrimer. Irradiation with ca. 350 nm light effectively cleaves the o-nitrobenzyl groups, as evidenced by UV, 1H NMR, and chiroptical data. Photolabile dendrimers offer great potential as hosts for the selective encapsulation and release of guests through the implementation of light energy. ^

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