Characterization of a chiral enolate aggregate and observation of 6Li-1H scalar coupling

A chiral enolate aggregate 1 containing a lithium enolate and a chiral lithium amide was systematically investigated by various NMR techniques. H-1 and C-13 DOSY at 25 and -78 degrees C provide its solution structure, aggregation number, and formula weight. Multiple 2D Li-6 NMR techniques, such as Li-6-Li-6 EXSY, Li-6-H-1 HOESY, were utilized to investigate its stereochemical structure. The configuration of the enolate in complex 1 was confirmed by Li-6-H-1 HOESY and trapping with TMS-Cl. A unique Li-6-H-1 coupling through the Li-N-C-H network was observed. This scalar coupling was corroborated by Li-6-H-1 HMQC, deuterium labeling experiments, and selective H-1 decoupling Li-6 NMR. The stereostructure of 1 provides a model for the origin of enantioselectivity of chiral lithium amide-induced enolate addition reactions.