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Abstract:
In the last few decades aluminum hydride (AlH3) and complex aluminum hydrides (M(AlH4)n, where M is a metal and n = 1,2) have received special attention for their use as hydrogen storage materials, but they have also attracted particular interest for their use as reducing agents in organic chemistry.
For decades, LiAlH4 as well as NaBH4 have been widely used to accomplish many challenging reactions, e.g. reduction of carboxylic acids (RCOOH) to the corresponding alcohol (RCH2OH). Traditionally, organic chemists have focused on developing new materials using transition metals bonded to organic ligands in order to achieve selective transformations. However, these specific metal-organic complexes, which are designed for each kind of reaction, require several synthesis steps.
Due to the interest in novel materials for hydrogen storage, a wide series of novel complex aluminium hydrides, known as alanates, were synthesized over the last years. Since all alanates have a similar structure to the commonly used LiAlH4, it was assumed that their reactivity was identical and, therefore, their use as potential reducing agents has not yet explored. However, in this thesis various alanates are shown to behave quite differently under certain reactions conditions.
The work of this thesis is divided into two parts: in the first part, several alanates were synthesised and characterised by XRD; in the second part, their reactivity was tested with various substrates.
Instead of using traditional methods, the alanates were synthesized by mechanochemistry. Planetary ball milling is a technique where materials are mixed with balls at high rotational speed, and reactions are catalysed by the effect of the impacts between the balls. Thereby, direct synthesis of group I (alkali metals) and group II (alkaline-earth metals) alanates was achieved. Additionally, ball milling was also used for the synthesis of other complex aluminium hydrides known as hexaalanates (Li3AlH6 and Na3AlH6).
For testing their reactivity, compounds containing functional groups that are normally difficult to reduce such as amides, nitriles, alkynes and α,β-unsaturated compounds were investigated. The experiments were carried out heterogeneously, using a suspension of the alanate in organic solvent containing the substrate. This could be achieved because alanates possess limited solubility in organic solvents, being those from group I and II only soluble in some ethers and hexaalanates completely insoluble. Accordingly, the system differs from the typical organic procedures, where the reaction occurs in a single phase (i.e. homogeneously).
Promising results were obtained for the reaction of these complex hydrides with α,β-unsaturated compounds, where the C=O group was selectively reduced to the corresponding alcohol without reduction of the C=C double bond. This selective reaction is similar to the Luche reduction, where the carbonyl group is reduced with sodium borohydride NaBH4 in a methanol solution in the presence of CeCl3. Unlike the Luche reduction, which requires many different components, our approach involved a simple one-step synthesis, proving that complex aluminium hydrides show promise as selective reducing agents.
Finally, reduction of some α,β unsaturated compounds with complex hydrides was also achieved mechanically, with similar conversions and selectivities to those reported in solution. Interestingly, even with ball milling the Lewis catalyst, CeCl3, is required to selectively yield the corresponding allylic alcohol. Additionally, methanol, the solvent used for reducing α,β-unsaturated carbonyl compounds to allylic alcohols, is also required in order to carry out this reaction by ball milling, indicating that it is directly involved in the reaction mechanism, perhaps as a proton donor or co-catalyst.
