Kinetic study of the effect of (OC)3Cr π-complexation upon the reactivities of tropone acetals and related tropylium cations

Abstract

The (OC)3Cr π-complexes (4) and (5) of acyclic and cyclic acetals of tropone undergo acidcatalysed heterolysis in water to give cationic complexes (2) and (6) that are stable in solutions of pH < ca. 5. These cations are thermodynamically more stable than the corresponding uncomplexed alkoxytropylium cations (8) and (13) but stabilisation is less than that the resulting from (OC)3Cr complexation of the tropylium cation. The spiro-acetals (5a–c) have very similar reactivities towards heterolysis and are somewhat less reactive than the acyclic acetals (4a–d). These heterolysis reactions, and their reverse, are exo-stereospecific. Reactions of the acyclic acetals with aqueous solutions of high pH give the tropone complex (9), the acetal heterolysis step being rate-limiting. The complexed alkoxytropylium cations (2) and (7) undergo base-catalysed conversion in water into the tropone complex; the intermediate (hydroxy) alkoxy compounds do not build up during these reactions whose rates are relatively insensitive to the identity of the alkoxy group. The reactivity of water relative to that of hydroxide ion towards these cations is much lower than predicted by Swain–Scott and Ritchie nucleophilicity scales. In aqueous alkali, the complexed (hydroxyalkoxy) tropylium cations (6) give apparently exclusively the corresponding spiro-acetals (5). These acetals and cations co-exist in aqueous solutions of pH 6–7 and their interconversions in these solutions are much faster than formation of the tropone complex. Conversion of the cations (6) into the acetals (5) is catalysed by borate but not by Tris buffer. Salts speed heterolysis of the acetals (4) and (5) and inhibit reactions of the cations (2), (6), and (7). Comparisons of reactivities of the (OC)3Cr-complexed diethyl, ethylene, and trimethylene acetals of tropone with those reported earlier for uncomplexed analogues show that complexation does not change the overall pattern of reactivity but suggest that the transition states for heterolysis of the complexed acetals are ‘earlier’(more acetal-like).