AGGREGATION AND REACTIVITY OF THE CESIUM ENOLATE OF P-PHENYLISOBUTYROPHENONE IN TETRAHYDROFURAN

Reinvestigation of the cesium enolate (CsPhIBP) of 1-biphenylyl-2-meth ylpropanone (p-phenylisobutyrophenone, PhIBP) in tetrahydrofuran (THF) solution at 25.0 degrees C shows that its UV-visible spectrum changes with concentration with lambda(max) moving to longer wavelengths in m ore dilute solution. Analysis of the spectral data over a wide concent ration range by singular value determination (SVD) combined with the p K measurements indicates a mixture of monomer (M), dimer (D), and tetr amer (T) with equilibrium constants of K-1,K-2 = D/M-2 = 2.89 x 10(4) M-1 and K-1,K-4 = T/M-4 = 7.78 x 10(12) 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 f ind that the cesium ion pair is more highly aggregated and much more b asic than the lithium ion pair. The average aggregation number of CsPh IBP at 10(-3) M is 3.2, substantially greater than the value of 2.2 re ported previously; the revision arises from our taking into account th e concentration-dependent extinction coefficient and absorption band s hape of CsPhIBP. The revised value also requires that a correction be applied to our previously reported kinetics of the reaction of CsPhIBP with methyl tosylate (MeOTs); the data indicate that the CsPhIBP ion pair monomer reacts, instead of the free enolate ion that we reported previously. Similarly, alkylation by p-tert-butylbenzyl chloride (BnCl ) occurs dominantly via the monomer. More limited studies at -20 degre es C indicate greater aggregation at the lower temperature. Alkylation reactions with MeOTs and BnCl again occur predominantly with the mono mer. The reaction products at room temperature are those of C-alkylati on with BnCl and equal amounts of C- and O-alkylation with MeOTs.