Experimental and AM1 calculational studies of the deprotonation of bicyclo[2.2.2]octane-2,5-dione and bicyclo[2.2.2]octane-2,6-dione: a study of homoconjugation, inductive, and steric effects on the rates and diastereoselectivities of α enolization

Abstract

The kinetics of NaOD-catalyzed HID exchange (enolization) at C3 u to the carbonyl group of bicyclo[2.2.2loctane-2,5-dione ( I ) and bicyclo[2.2.2]octane-2.6-dione (2) have been studied in 60:40 (v/v) dioxane-D,O at 25.0°C. The second-order rate constants for exchange are (9.7 + 1.5) x lo-' and (3.4 t 1.2)x lo-' L mol-' s-I for 1 and 2, respectively. Thus, 1, exchanges 76 times faster than bicyclo[2.2.2]octan-2-one (3) ( k = (1.27 + 0.02) x L mol-I s ' ) , but the 2,6-dione 2 unexpectedly is much less reactive (2.7 x lo-') than the monoketone. Unlike the large ex0 selectivity of 658 observed in the case of bicyclo[2.2.1]heptan-2-one, small and opposite selectivities, exo (1.2) for 1 and erzdo (2.1) for 2, are found for the isomeric [2.2.2] ketones. The results indicate that the incipient enolate of 1 is stabilized by a polar effect of the P carbonyl group at C5, not by homoconjugation. The source of the surprising low reactivity of 2 is unknown at this stage. The small diastereoselectivities, exo (1.2) for 1 and erzdo (2.1) for 2, correlate with relative energies of the diastereomeric pyramidal enolates calculated with AM I . L mol-' s-I; toutefois, la 2,6-dione (2) est beaucoup moins reactive (2,7 x 10-"ois moins reactif) que la monocCtone. Par opposition ?I la forte stlectivitt de 658 observCe dans 1e cas de la bicyclo[2.2. Ilheptan-'--one, on a observe de faibles sClectivitCs opposCes, exo (1.2) pour 1 et endo (2.1) pour 2, pour les cCtones [2.2.2] isomttres. Les rtsultats indiquent que I'Cnolate qui se forme dans le composC 1 est stabilist par un effet polaire du groupe carbonyle Pen C5 et non pas par homoconjugaison. La cause de cette faible rCactivitC surprenante du composC 2 est pour l'instant inconnue. Les faibles diastCrCostlectivitts, ex0 (1.2) pour 1 et endo (2,l) pour 2, I