Enantioselective rhodium-catalysed nucleophilic allylation of cyclic imines
Hepburn, Hamish Bruce
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A highly enantioselective and diastereoselective rhodium-catalysed addition of potassium allyltrifluoroborates to cyclic imines is described within. By utilising rhodium-chiral diene complexes, a wide range of cyclic imines were successfully allylated in high yields and enantioselectivities. Using a variety of more highly substituted allyl reagents, additional stereocentres and further molecular complexity was achieved with good yields, enantioselectivities and diastereoselectivities. Investigations involving isomeric allyl species and deuterated allyl species provided results that gave mechanistic insight, leading to the proposal of a plausible mechanistic pathway and suggested the formation of interconverting allylrhodium intermediates. Furthermore, during these investigations, a highly interesting isomerisation of the allylrhodium intermediate was discovered. Such isomerisation led to the in situ formation of the more complex allylrhodium intermediates which led to complex products upon allylation with cyclic imines that would be difficult to synthesis via other methods. This isomerisation was found to occur for a range of cyclic imines and disubstituted allyltrifluoroborates, proceeding in good yields and diastereomeric ratios. Deuterium studies indicate it is probable that this isomerisation proceeds via a 1,4 rhodium migration and a plausible mechanism is proposed explaining both the connectivity of the products and the relative stereochemistry.