Aggregation and Reactivity of the Cesium Enolate of p-Phenylisobutyrophenone in Tetrahydrofuran1

Abstract

Reinvestigation of the cesium enolate (CsPhIBP) of 1-biphenylyl-2-methylpropanone (p-phenylisobutyrophenone, PhIBP) in tetrahydrofuran (THF) solution at 25.0 °C shows that its UV−visible spectrum changes with concentration with λmax moving to longer wavelengths in more dilute solution. Analysis of the spectral data over a wide concentration range by singular value determination (SVD) combined with the pK measurements indicates a mixture of monomer (M), dimer (D), and tetramer (T) with equilibrium constants of K1,2 = D/M2 = 2.89 × 104 M-1 and K1,4 = T/M4 = 7.78 × 1012 M-3. The pK of the monomer is 25.08. The data are also compared with the corresponding data obtained for the lithium ion pair system reported in the following paper. We find that the cesium ion pair is more highly aggregated and much more basic than the lithium ion pair. The average aggregation number of CsPhIBP at 10-3 M is 3.2, substantially greater than the value of 2.2 reported previously; the revision arises from our taking into account ...