New chiral auxiliaries for dynamic kinetic resolution: From theory to experiment

New efficient chiral auxiliaries for dynamic kinetic resolution (DKR) of bromides into amines are proposed, based on a theoretical rationalisation of known literature results. One example was synthesized and tested, affording diastereoselectivities up to 100%. Several results of DKR reactions are known, based on oxazolidinone or imidazolidinone units as chiral auxiliaries. Nevertheless, their behaviour was not fully understood until a recent paper that we published. We now used our proposed mechanism to rationalize the behaviour of other similar chiral auxiliaries and to propose small structure changes in imidazolidinone rings which could largely improve their performance. We could show that the good performance of these molecules as chiral auxiliaries for DKR reactions where bromine is the leaving group and a primary or secondary amine is the nucleophile is due, in a first step, to the formation of a hydrogen bond between the amine and the ring carbonyl oxygen and) in a second step, to the strong electrostatic interaction between the leaving bromide and the carbonyl oxygen in the C-3 substituent. Considering the behaviour of this substituent which rotates to minimize the electrostatic repulsion with the bromide when reaching the transition state, we proposed the introduction of a second substituent in the C-4 position of the imidazolidinone ring, which prevents such rotation, thus increasing the energy difference between the transition states of the two distereoisomers. With such an auxiliary we were able to increase the best de known in literature (88%), when benzylamine is used as nucleophile, to 99, or even 100%, when iodide replaces the bromide in the substrate.