Aggregation of the Lithium Enolate of p-Phenylisobutyrophenone in THF: The Unexpected Importance of Monomer

摘要

Several studies have shown that many metal enolates exist as aggregated species (dimers, tetramers, and higher units) in nonpolar or weakly polar solvents. However, the role of such aggregates in the reaction with electrophiles is still not clear. Understanding this role has synthetic significance because of mechanistic implications in reactivity and stereoselectivity. Jackman has reported that the methylation of the lithium salt of isobutyrophenone proceeds through aggregates, the only species he observed in 0.5 M solutions using NMR spectros-copy. Recently, we reported that the reactive intermediate in the methylation of the cesium enolate of 1-(4-biphenylyl)-2-methyl-1-propanone (p-phenylisobutyrophenone, PhIBP) in tetrahydrofuran (THF) is the free enolate ion. After that report was published, we discovered that the spectrum of the cesium enolate varies with concentration and deviates slightly from Beer's law. These changes indicate that different aggregates have slightly different spectra. Analyzing the data by singular value decomposition (SVD) changes the results sufficiently to show that the reactive species in alkylation is not the free ion but the monomeric ion pair. In this paper we present our preliminary results of the application of this approach to the lithium salt of PhIBP (LiPhIBP) in THF.