Aggregation and ion pair acidity in THF: lithium and cesium enolates of α-phenylcyclohexanone

Abstract

The lithium (1) and cesium (2) enolates of α-phenylcyclohexanone, PCH, have been studied in THF at 25°C. Both form monomer-dimer equilibria with K1,2 = 2.8 × 103 M-1 for 1 and 1.8 × 103 M-1 for 2. The ultraviolet-visible (UV-vis) spectra of the monomers and dimers were derived from the spectra of the enolates at a series of concentrations by singular value decomposition. Measurements of the ion pair pK values gave pK = 12.7 for 1 and 19.8 for 2. The monomers of 1 and 2 are alkylated by benzyl halides faster than are the dimers.Key words: carbon-acid ionization, enolate ions, ion pairs, ion pair acidity, aggregation, dimerization constants, alkylation.

Perspectives on Computational Organic Chemistry

“…With additional conjugation the electrostatic attraction of Li + to the enolate is reduced and ether coordination is enhanced, leading to lower dimerization constants. A typical example is that of phenyl conjugation: lithium 6-phenylcyclohexene-1-oxide is a tetramer in THF, whereas the conjugated lithium 2-phenylcyclohexene-1-oxide (LiPhCHX in Table 2) forms a dimer 46. Substituents close to the enolate center sterically inhibit aggregation.…”

“… (a) DMA = N,N-dimethylacetamide (b) Obtained by extrapolation (c) Ref 38 (d) Ref 39 (e) Ref 40 (f) Unpublished results (g) Ref 41 (h) Ref 42 (i) Ref 43 (j) Ref 44 (k) Ref 45 (l) Ref 46 (m) Ref 47 (n) Ref 48 (o) Ref 34, per hydrogen (p) For phenylacetylene (q) For aniline (r) For cyclohexyl phenyl ketone (s) For 1-indanone …”

Ion Pair First and Second Acidities of Some β-Diketones and Aggregation of Their Lithium and Cesium Enediolates in THF

“…41 For the cesium salts of enolates, the pK values relative to such indicators tend to be about 1 pK unit less than the DMSO pK a values. 23,25,28,69 In the present cases of the cesium salts of β-diketones, the pK of the cesium salt of the enol of 1, 10.9, is 3.3 units less than that of PhCOCH 2 COCH 3 in DMSO, 14.2. 70 The latter value is consistent with that of dibenzoylmethane in DMSO, 13.35, 71 which is expected to be a little lower because of the inductive effect of the second benzene ring.…”

“…20 Metal enolates of ketones, amides, esters, and other carbon derivatives are known to be frequently aggregated in ethereal solvents and in recent years this group has determined a number of such aggregation equilibrium constants and the relative ion pair acidities of the monomers. [21][22][23][24][25][26][27][28][29][30][31][32] Recently, the general approach was ap- plied to the lithium and cesium enediolates of an arylacetic acid, 33 which were found to form tight dimers. The knowledge of the actual reactive species involved in reactions of such systems is fundamental for mechanism hypotheses and particularly to understand reaction stereochemistry.…”

Role of Aggregates in Claisen Acylation Reactions of Imidazole, Pyrazole, and Thioesters with Lithium Enolates in THF1

“…We have recently reported the aggregation and alkylation kinetics of several lithium enolates in THF. The lithium enolates of p -phenylsulfonylisobutyrophenone, 1 , α-phenylcyclohexanone, 2 , and α- p -biphenylylcyclohexanone, 3 , form monomer−dimer mixtures in dilute solution, whereas that of p -phenylisobutyrophenone, 4 , is a monomer−tetramer mixture, but in all cases alkylation reactions are much faster with the monomers than with the aggregates. For example, for the reaction of 1 with a benzylic bromide k monomer is 3000 times greater than k dimer .…”