Sintese en modellering van alisikliese dendrimere
Boshoff, Anina Elizabeth
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Research in the field of dendrimer chemistry has escalated in the past two decades. Dendrimers are macromolecules closely related to polymers but with a much smaller mass distribution. Because of this property, they possess unique three dimensional characteristics. Dendrimers are increasingly used in homogeneous catalysis due to the ease of separation from the reaction mixture with the use of nanofiltration membranes. Dendritic catalysts are limited to lower generation dendrimers, which unfortunately aren't large and robust enough to be constrained by nanofiltralion membranes. The reason is that higher generation dendrimers sterically hinder access to the active point in the catalyst. Incorporation of rigid. three dimensional alicydic compounds in dendrimers holds a possible solution to this problem. Two objectives were set: 1. Suitable alicyclic compounds were to be syntethised and incorporated into the core of a dendrimer. 2. Suitable alicyclic compounds were to be syntethised and used as periphery molecules for a dendrimer. • Alicyclic compounds 16, 23 and 28 were identified as suitable core molecules for alicyclic dendrimers. Compound 8 was identified as a suitable dendron to be coupled lo an alicyclic compound. • Compound 16 is synthesised through reduction of the p-benzoquinone-cyclopentadiene- adduct 3 by sodiumborohydride in a ceriumtrichloride heptahydrate solution in methanol before irradiation by ultraviolet light. • Coupling alicyclic compound 16 with dendron 8, was done in alkaline dimethyl sulfoxide. Only one hydrogen was substituted with a dendron. Removal of the excess compound 8 proved to be problematic and very little of the pure product could be obtained. • Compound 23 was synthesised by reducing compound 22, which was synthesised as follows. • Coupling alicyclic compound 23 with dendron 8, was done in alkaline dimethyl sulfoxide. The raw product was a yellow oil and couldn't be separated from the excess compound 8. No analytical data could therefore be obtained. The only conclusion that could be made as to the nature of the product was based on semi-empirical calculations performed with Accelrys MS Modeling, which indicated that only one hydrogen would most likely have been substituted by compound 8 and probably at the oxygen indicated below. • The third alicyclic compound synthesised was 28, for which the starting compound is synthesised as follows. • Clemmensen reduction yields a mixture of products of which 28 is the main product. 27 could be separated by steam distillation, after which 28 could be retrieved from the remaining water. • Coupling alicyclic compound 28 with dendron 8, was done in alkaline dimethyl sulfoxide. Compound 32 could be separated from the excess dendron 8 as a colourless crystalline product by flash chromatography using petroleum ether:ethyl acetate:methanol (20:20:1) as eluent. Substitution again only took place at one oxygen. • A possible explanation for the single substitution of dendron 8 on the alicyclic compounds, could be gained from molecular modelling data performed with Accelrys MS Modeling. The moment one aromatic substituent is present in an alicyclic structure, all the HOMO electrons move from the remaining hydroxyl group to the aromatic substituent, leaving no electrons available for nucleophilic attack on another molecule of 8. See example below. • Compound 38 was identified and synthesised as a suitable alicyclic structure for incorporation in the periphery of a dendrimer. • Compound 41 was used as the aromatic core molecule. Both benzylic bromide positions was substituted by molecules of 38, thus obtaining an alicyclic dendrimer 42 with two alicyclic compounds on the periphery. • Compounds 32 and 42 were successfully synthesised as alicyclic dendrimers.