Base-catalyzed enolization of 2,5-norbornanediones. Evidence for homoenolic assistance

Abstract

NICK HENRY WERSTIUK, ROLAND TAILLEFER, and SUJIT BANERJEE. Can. J. Chem. 56, 1148 (1978).The rate enhancements relative to the monoketones, for base-catalyzed ex0 and erldo hydrogen isotope exchange in 2,5-norbornanedione (2), 3,3-dimethyl-2,s-norbornanedione (3), and 1,3,3-trimethyl-2,s-norbornanedione (4) have been determined at 25 _+ 0.05"C in 60% dioxane-40% D 2 0 . After the increase in rate due to the inductive and strain effects of the second carbonyl group is taken into account, the homoenolic-assistance factors are extracted from the overall rate enhancements: for exo exchange in 2, 3, and 4, the factors are 13, 39, and 32; for erldo exchange in 2, 3, and 4, the factors are 26, 64, and 71, respectively.

Towards a complete account of the mechanism of hydrogen isotope exchange of diastereotopic protons of carbon acids. II. Acid- and base-catalyzed enolization of bicyclo[2.2.1]heptan-2-ones. Evidence for exchange by inversion

“…A research program was initiated at that time to detail the mechanistic intricacies of hydrogen isotope exchange (enolization) of bicyclic ketones and determine the effect of groups located remote (P or y) from an a enolizable site. A number of papers documenting our work have been published (9)(10)(11)(12)(13)(14)(15)(16). We noted, most importantly, from a study of the stereoselectivity of H -+ D and D -+ H exchange of l a and its 3,3-dideuteriated analog l b , that the k , , , , /k ,.,,, /,, (k,,,,/k , , , , ) ratio decreased from 660 for exchange of l a in deuteriated medium to 72 for exchange of l b in nondeuteriated medium.…”

“…Since it was not possible to determine the primary KIE for endo exchange of l a directly, l c and I d were prepared and the rates of exchange were determined by monitoring the rates of isomerization in deuteriated and nondeuteriated medium as described previously (13). The second-order rate constants for isomerization of l c and I d obtained by computer analysis of isomerization data are listed in Table 2.…”

“…It is interesting to note that on the basis of our proposal, using eqs. [9] and [14], given eq. [22], and assuming that SIEl 2 It is seen from the data in Table 1 (entries 2, 3, and 9) that (ky-D/kp-H)c,, = 1.24 k 0.19, in accord with our proposal, but not with the latter case, since LH is expected to be r 5.…”

“…A correction for dilution of the deuterium pool was not made in these studies because a previous study (13) showed that the corrected and uncorrected rate constants usually differed by less than 10%. A computer generated plot of the kinetic data for a typical run on l b is given in Fig.…”

AM1 calculational and experimental evidence that a β-nitroxyl group significantly enhances the thermodynamic and kinetic acidities of ketones

“…While a great deal of research work has been carried out to define the factors, e.g., orbital alignment, steric, torsional, and polar effects, that determine the rates of base-catalyzed a enolization of ketones (1)(2)(3)(4), there have been no reports detailing the effect of remote heteroatom-centered radicals on the kinetic and thermodynamic acidities of carbonyl compounds. Our longstanding interest in establishing how remote polar substituents enhance the rates of a and P(homoeno1ization) enolization of ketones and in using semiempirical calculations to predict and understand variations in the thermodynamic acidities of carbon and oxygen acids (5, 6 ) motivated us to undertake a calculational and kinetic study of the base-catalysed enolization of five ketones.…”