Organic lithium compounds, especially lithium enolates, are a classic type of reagent used extensively in carbonyl functionalization. While it is well established that auxiliary ligands can alter the thermodynamics and kinetics of reactions involving lithium enolates, too little is known about these species’ solvation states to establish a similar correlation with their structures or their reactivity.
This complication has led Paul G. Williard and co-workers to investigate the influence of hexamethylphosphoramide (HMPA), a common additive used to “activate” lithium salts, on the aggregation behavior of lithium pinacolone enolate in both the solid and the solution states (DOI: 10.1021/jacs.5b01906). Combining crystal structure analysis and 1-D/2-D NMR titrations, the researchers examine the structures of a number of HMPA-solvated lithium enoloates and find that steric interaction within enolate aggregates dominates their solvation states.
“Crystal structures of HMPA-solvated lithium simple ketone enolates are reported for the first time,” the authors note in this study, which enables structural correlation with their aggregates in solution. More importantly, by identifying the key factor that controls enolate solvation, the results shed light on how to tune the reactivity of lithium enolates.
Xin Su, Ph.D.
Link: Spotlights on Recent JACS Publications, DOI: 10.1021/jacs.5b05796
