Photocatalytic applications in organic synthesis
Linklater, Paul Thomas
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As the search for new pharmaceuticals continues our understanding of their operation grows. A result of this is that design of new drugs is more often being limited only by the ability of synthesis techniques to produce them. The development of new synthesis techniques is therefore always of great interest. Among these techniques is semiconductor photocatalysis, most well known for its use in the clean-up of contaminated water; however it is capable of a wide range of reactions, from oxidation to cyclisation. This study examined some of the factors and influences involved in the applications of the technique for the oxidation of alcohols to ketones. For the photocatalysis the steroid 6β-hydroxy-3α,5-cyclo-5α-androstan-17-one was synthesised, a compound with suspected antiprogesterone properties. A feature of the compound is the presence of a cyclopropane group within the A ring. It was synthesised easily through normal techniques; the presence of the cyclopropane group however makes the safe oxidation of the compound a difficult task. The photocatalytic oxidation studies started with cyclohexanol as the base compound; this is the target area of the steroid without the influences of the other rings. Derivatives of cyclohexanol were used to study the effect of neighbouring groups on the reaction rate. The compounds used were 2-methylcyclohexanol, menthol, 2- chlorocyclohexanol and 1,2,3,4-tetrahydronaphthol; these being chosen to give a mixture of compounds with electron donating and electron withdrawing groups. It was found that the presence of electron donating groups gave a slower reaction rate while electron withdrawing groups gave a faster reaction rate. The reaction rate of 1.066x10-8 mol s-1 for the cyclohexanol dropped to 1.023x10-8 mol s-1 with the addition of the methyl group in 2-methylcyclohexanol. However the chloro group raised the rate to 1.689x10-8 mol s-1. The rate increased further due to the presence of the benzene ring in 1,2,3,4-tetrahydronaphthol. These results showed the influence of induction on the compound, the groups affecting the electron density around the hydroxyl group. It was also found that the catalyst used had a large impact on the reaction rate; the anatase to rutile ratio had an effect but the major influence was the surface area of the catalyst. In cyclohexanol conversions of 41% were achieved with UV100 and 38% with PC500. The P25 gave a conversion of 43% but the Aldrich anatase just 25%. While the steroid could not eventually be oxidised photocatalytically a greater understanding was gained and a number of areas for further study found. The study did succeed in examining the influence of vessel absorption on the UV light reaching the sample, it also synthesised a steroid containing a cyclopropane group. The study also examined the effect of electron donating and withdrawing groups on the photocatalytic oxidation of alcohols. And finally it examined the effect of the catalyst used on reaction rate.