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 K 1,2 = D/M2 = 2.89 × 104 M-1 and K 1,4 = 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 the concentration-dependent extinction coefficient and absorption band shape 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 °C indicate greater aggregation at the lower temperature. Alkylation reactions with MeOTs and BnCl again occur predominantly with the monomer. The reaction products at room temperature are those of C-alkylation with BnCl and equal amounts of C- and O-alkylation with MeOTs.

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran1

“…This explanation finds support in the relative pK values; CsPhIBP has a pK 9.3 units higher than that of CsBPCH compared to a difference of only 3.5 units for the lithium enolates. 7,9 Moreover, in line with the relative basicities of the cesium enolates, CsPhIBP monomer is more reactive (by about an order of magnitude) toward methyl tosylate and p-tert-butylbenzyl chloride than is CsBPCH, the opposite reactivity order of the lithium enolates.…”

“…The two cations close to the enolate oxygen in the dimer electrostatically oppose this transition dipole more than the single cation in the monomer, which results in a higher energy transition. 7,8 We have learned that the reliability of the SVD results needs to be tested. 7,8 The approach used for the present results is to compare the self-consistency of the extrapolated values for λ max of monomer and dimer with those of the direct SVD.…”

“…7,8 We have learned that the reliability of the SVD results needs to be tested. 7,8 The approach used for the present results is to compare the self-consistency of the extrapolated values for λ max of monomer and dimer with those of the direct SVD. The spectra shown in Figure 2 were used to determine the composition of each spectrum used in the SVD analysis in terms of the monomer and dimer concentration; these numbers are included in Table S2 (Supporting Information), and the λ max for each such spectrum was plotted against the monomer concentration.…”

“…[4][5][6] Recent work in our laboratory has used two complementary methods for determining the stoichiometry of such aggregates and the equilibrium constants among them. 7,8 One method exploits the different UV-vis spectra of the aggregates. Singular value decomposition (SVD) analysis of a series of spectra covering a range of concentrations is used to deduce the components and their spectra.…”

“…Aggregation stoichiometry and equilibrium constants are then obtained from the observed changes in the proton transfer equilibrium with concentration. We reported recently that the cesium enolate of p-phenylisobutyrophenone (CsPhIBP) exists as monomers, dimers, and tetramers in THF solution, 7 and that the corresponding lithium enolate (LiPhIBP) is a monomer-tetramer mixture. 8 In both cases, alkylation reactions involve dominantly the monomer.…”