published_or_final_version / abstract / Ecology and Biodiversity / Master / Master of Philosophy

published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

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The initial aim of this research project was to oxidise Cr C4 alcohols to their respective aldehydes. However, isolation of these aldehydes was impossible due to their low boiling points. Another problem was the proximity of the boiling points of these aldehydes with the boiling points of solvents used in reactions. As a result, the alcohols investigated in this project were changed to C4-C6 alcohols. Different oxidising reagents are known that oxidise oxidise primary alcohols to aldehydes. Of these, chromium (VI) has been used the most. In this project, Chromium (VI) supported on different inert: inorganic supports was chosen as the oxidising reagent. The first task was therefore to synthesise these supported oxidants. The synthesis of some of these oxidants was lengthy. This is advantage the preparation of supported oxidants. Unsupported oxidants like PCC and chromium trioxide were only used as a basis of comparison to emphasise the usefulness of supported oxidants. Pyridinium chlorochromate was used to oxidise primary alcohols to aldehydes in moderate yields. The reaction was found to be moisture sensitive. The yield and the duration of the reaction was found to be better with the use of molecular sieves. Silica gel-supported pyridinium chromate was also used to oxidise alcohols to aldehydes in good yields. Poly[vinyl(pyridinium chlorochromate)] was used to oxidise I-hexanol and I-pentanol to their respective aldehydes also in good yields. The duration of these reactions ranged from 30 min. to 3 days. During the oxidation of alcohols using silica gel-supported chromium trioxide and diethyI ether/chromium trioxide it was found that the reaction conditions can be changed so that either aldehydes or symmetrical esters were the products. This was achieved by varying the duration of the reaction. Silica gel-supported chromium trioxide resulted in both the aldehydes and esters in good yields while the diethyl iii Aluminium-supported chromium trioxide, kieselguhr-supported chromic acid, alumium silicate-supported chromic acid and alumina-supported pyridinium chlorochromate were used to oxidise primary alcohols to symmetrical and unsymmetrical esters in good to excellent yields. The duration of these reactions varied from 1 hour to 48 hours depending on the conditions employed and the chain length of the reagents. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.

by Alexandra M. Leung. / Thesis (M. Phil.)--Chinese University of Hong Kong, 1981. / Bibliography: leaves 208-224.

The application of MIDA boronates (MIDA = N-methyliminodiacetic acid) in Suzuki- Miyaura reactions has increased over the last years. This is mainly because in many cases, the replacement of the boronic acid for the respective MIDA boronate has a positive result in the reaction yield. The key feature that makes MIDA boronates efficient coupling partners is that they can undergo a slow hydrolysis reaction under Suzuki-Miyaura conditions, which generates the boronic acid in situ. The control of the hydrolysis rate is crucial to keep a low concentration of the boronic acid to avoid some of the side reactions frequently observed. The kinetics of the hydrolysis reaction from a range of alkyl, aryl, and heteroaryl MIDA boronates have been determined under different reactions conditions. In addition, competition experiments and computational calculations have resulted in the proposal of three distinct mechanisms for the hydrolysis of MIDA boronates: ‘base-promoted’, ‘water-promoted’, and ‘acid-promoted’. The base and acid mediated processes occur at faster rates than the neutral pathway and involves a rate-limiting addition at the MIDA carbonyl carbon by hydroxide or water, respectively. Whilst the 'neutral' hydrolysis requires neither base nor acid and involves ratelimiting B-N bond cleavage by a water cluster. Under certain conditions the neutral mechanism can operate in parallel with the base or with the acid mediated mechanism; relative rates are easily quantified by 18O incorporation in the MIDA, after this is released from the hydrolysis reaction. This insight is expected to assist informed application and optimisation of MIDA boronates in synthesis as well as the design of new MIDA boronate derivatives.