Probing the Origins of Asymmetric Induction by 3-Aminopyrrolidine Lithium Amides Complexes: A ⁶Li/¹H/¹³C NMR Study

Abstract

The structure of two chiral N,N‘-disubstituted-3-aminopyrrolidine lithium amides 3 and 4 in solution in THF-d8 has been studied at low temperatures by high-field 1H, 13C, and 6Li 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 6Li+. When excess Bu6Li is added to both amide solutions, 1:1 amide−Bu6Li complexes arise, and their structures appear, this time, very similar and organized around a parallepipedic N−Li2−C core, the two lithium cations bridging the amide and the alkyl chain. The 4−Bu6Li complex appears very tight, with two distinct signals corresponding to each of the diastereotopic α-protons of butyllithium, by contrast with the looser 3−Bu6Li 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.