Cation arxl concentration dependence of anionic rearrangements involving hydride shifts

Abstract

The alkali metal salts of the polycyclic hydroxy-ketones 7 and 8 rearrange by intramolecular transfer of hydride between alkoxide and carbonyl carbons. Rates in DMSO solution, determined by dynamic 1H NMR methods, show strong concentration dependence, increasing with dilution of the alkoxides. Rates are also strongly cation dependent, increasing in the order Li<Na<K. Intramolecular 0·0 distances in these rigid polycylic structures are too large to permit their co-ordination to a common alkali metal cation, as occurs in the intermolecular Meerwein-Ponndorf-Verley-Oppenauer redox couple, and their behaviour is consistent with a mechanism involving dissociation of the salts prior to rearrangement. Analogies are drawn with other reactions involving formal collapse of alkoxide to carbonyl with expulsion of very strongly basic leaving groups. For the sodium and potassium salts, excess added [2.2.2] crystand eliminates concentration dependence, and suppresses, but does not entirely eliminate cation dependence. Under these conditions, the salts of 8 are about 10 times more reactive than those of 7 .