Intramolecular catalysis. Part 6. The intramolecular Cannizzaro reaction and hydration of phenanthrene-4,5-dicarbaldehyde and [α,α′-2H2]phenanthrene-4,5-dicarbaldehyde

Phenanthrene-4,5-dicarbaldehyde has been shown to form a bridged (cyclic) hydrate in aqueous solution, as a cis–trans mixture. The rate coefficients for the uncatalysed hydration has been measured at two temperatures and the activation parameters evaluated. The dialdehyde, as well as the α,α′-dideuterio isomer, has been shown to undergo a Cannizzaro reaction in 70%(v/v) dioxane-water. The reaction has been shown to be intramolecular and of the second order, i.e. first order in the monoanion of the hydrate and in base. The rate coefficients have been measured at several temperatures and activation parameters evaluated. The kinetic isotope effect, kH/kD, was found to be ca. 1.7 and the kinetic solvent isotope effect, kH2O/kD2O, ca. 0.59. The alkaline hydrolysis of the corresponding lactone of 5-(hydroxymethyl)phenanthrene-4-carboxylic acid was studied under the same conditions. As an intermediate in the intramolecular Cannizzaro reaction, the lactone can be excluded as it cannot be detected directly under conditions where it is relatively long-lived. The evidence for the Cannizzaro reaction indicates a mechanistic pathway involving rate-determining hydride transfer from the chain tautomer of the dianion of the hydrate.