Probing the origins of asymmetric induction by 3-aminopyrrolidine lithium amides complexes: A Li-6/H-1/C-13 NMR study

The structure of two chiral N,N'-disubstituted-3-aminopyrrolidine lithium amides 3 and 4 in solution in THF-d(8) has been studied at low temperatures by high-field H-1, C-13, and Li-6 NMR spectroscopy. Despite their structural analogy, these two compounds adopt very different conformations in solution: while the amide 3 pyrrolidinic ring undergoes only minor changes with respect to its amino precursor 1, amide 4 presents a norbornyl-like bridged structure around Li-6(+). When excess (BuLi)-Li-6 is added to both amide solutions, 1:1 amide-(BuLi)-Li-6 complexes arise, and their structures appear, this time, very similar and organized around a parallepipedic N-Li-2-C core, the two lithium cations bridging the amide and the alkyl chain. The 4-(BuLi)-Li-6 complex appears very tight, with two distinct signals corresponding to each of the diastereotopic alpha-protons of butyllithium, by contrast with the looser 3-(BuLi)-Li-6 complex. From all these data, we propose a model to interpret the results obtained using these chiral amides in the asymmetric condensation of butyllithium with aromatic aldehydes.