Studies towards controlling selectivity in Minisci reactions
The addition of free radicals to electron-deficient heteroarenes, commonly referred to as Minisci-type reactions, comprises an important class of transformations for the direct functionalisation of heteroaromatic compounds. Recent advances in radical generation methods, particularly due to developments in photoredox catalysis, have allowed for milder reaction conditions and a multitude of radical precursors. However, controlling regioselectivity in Minisci reactions remains challenging, when there are multiple accessible positions on the heteroarene. Furthermore, when prochiral radicals are used, a new stereocentres arises in the product. This thesis focuses on studies towards controlling these two aspects of selectivity in Minisci-type additions to heteroarenes.
In the first part of the thesis, the effects of solvent and achiral Brønsted acid catalysis on regioselectivity in the Minisci-type addition of N-acyl α-amino radicals to quinolines was investigated. Two sets of conditions, that favour either the C2- or C4-position, were identified and a scope of compatible substrates is presented.
The second part of the thesis focuses on probing the identity of the enantiodetermining step in the enantioselective Minisci reaction previously reported by our group. A positive non-linear effect between catalyst and product enantiomeric excesses was initially observed but further experiments at lower catalyst loading suggested a linear relationship. Precipitation studies suggested selective formation of a heterochiral aggregate to be responsible for the non-linear effect at the higher catalyst loading. These observations led us to conclude that is it unlikely that two molecules of catalyst are involved in the enantiodetermining step of the reaction, as had previously been hypothesised. Computational studies carried out by a collaborator deduced that the mechanism proceeds via an unexpected mode of intramolecular enantiodetermining deprotonation enacted by the amide group, with the aid of a single associated chiral phosphate.
The final part of the thesis details work aimed at enabling the enantioselective addition of secondary amine-derived α-amino radicals to heteroarenes, specifically heterocyclic secondary amines. These important types of radicals had given poor selectivity in our previous protocol which was hypothesised to be due to the lack of a hydrogen bond donor on the radical to permit interaction with the catalyst. By incorporating a urea group on the secondary amine nitrogen, we have shown that the enantioselective Minisci-type addition of cyclic ureas can be achieved with high enantioenrichment in the product, but presently in low to moderate yields. We also describe initial studies towards an alternative strategy of a diastereoselective Minisci reaction using a chiral auxiliary on the amide.