On the novel function of the additive DBU. Catalytic stereoselective deprotonation by a mixed dimer of lithiated DBU and a chiral lithium amide †

摘要

The additive DBU is used to increase the selectivity and reactivity of e.g. chiral lithium amides in both catalysed andnnon-catalysed asymmetric syntheses. This has been attributed to the coordinating ability of DBU favoring morenreactive aggregates. However, we have found that LDA in THF deprotonates DBU to yield lithiated DBU (1) asnshown by multinuclear NMR studies. Furthermore, compound 1 is found to form a mixed dimer (5) with e.g. thennorephedrine-derived chiral lithium amide 2. Results of an investigation of the stereoselectivity of this novel reagentnin the epoxide deprotonation are also reported. Computational studies using PM3 and DFT show possible structuresnof 1 and 5 in line with the NMR results. In addition, the role of THF and DBU in the solvation of the aggregates hasnbeen investigated by computational modelling and favoured complexes in the equilibria between homo- andnheterocomplexes are also reported.