Synthetic Cascade Sequences Initiated by Organoboranes

This project aims to combine radical reactions, initiated by organoboranes under mild conditions, with polar reactions to form industrially useful small molecules and polymers in one-pot processes. In Chapter 2, it is shown that commercially available organoborole, 2-propyl-1,3,2-benzodioxaborole (PBD), can be used to initiate radical reactions at room temperature, including the addition of organophosphorus hydride radical mediators to alkenes. Also, the polymerisation of maleimides has been achieved using PBD at room temperature. The hydroboration of alkenes with catecholborane can be used to form β-alkylcatecholborane radical initiators in situ, and these reagents have been used for the initiation of radical polymerisation reactions at room temperature. In Chapter 3, it is shown that the β-alkylcatecholboranes, formed in situ through the hydroboration of alkenes, react with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) to initiate conjugate addition-aminoxylation reactions with maleimides, to form alkoxyamines. It was observed that heating the alkoxyamines leads to the elimination of TEMPO-H, to re-form the maleimide C=C double bond. Thus, a new, mild, one-pot procedure for the synthesis of 3-substituted maleimides has been developed. Alternatively, the alkoxyamine can be heated with Zn in AcOH to afford an alcohol, which offers a new route to hydroxylated-succinimides. In Chapter 4, the newly developed method for the synthesis of 3-substituted maleimides was applied to the preparation of carbocyclic analogues of the natural product D-(+)-showdomycin. It was found that this procedure could be used directly with maleimide to produce carbocyclic analogues of D-(+)-showdomycin in reasonable overall yield, from cheap starting materials, and without the need for protecting groups (for the maleimide nitrogen or the hydroxyl group). In Chapter 5, the synthesis and reactivity of N-(2,3-epoxypropyl)maleimide (EPM) is explored, with respect to the use of this molecule as an additive in the polymer industry. It is shown that EPM can be prepared in an overall yield of 50%, at room temperature, from cheap starting materials maleic anhydride and N-allylamine. The reaction of EPM with primary amines is investigated, and it is shown that the amine reacts preferentially with the C=C double bond of the maleimide, rather than the epoxide ring. However, it is shown that the use of Lewis acid additives to the reaction can reverse the chemoselectivity, leading to preferential opening of the epoxide ring.