Intramolecular catalysis. Part 4. The intramolecular Cannizzaro reaction of biphenyl-2,2′-dicarbaldehyde, [α,α′-2H2]biphenyl-2,2′-dicarbaldehyde and 4,4′-or 5,5′- or 6,6′-disubstituted biphenyl-2,2′-dicarbaldehydes

Abstract

Biphenyl-2,2′-dicarbaldehyde, along with its α,α′-[2H2] isomer, and a series of 4,4′-, 5,5′- and 6,6′-disubstituted biphenyl-2,2′-dicarbaldehydes have been shown to undergo Cannizzaro reactions in 30%(v/v) dioxane-water containing base. The reaction has been shown to be intramolecular and of the third order; first order in substrate and second order in base. The rate coefficients have been measured at several temperatures and activation parameters evaluated. Studies of solvent isotope, solvent and salt effects have been made. The kinetic isotope effect, kH/kD, was found to be ca. 1.8. The effect of 4,4′- and 5,5′-disubstitution was assessed by use of the Hammett equation to give ρ at 30 °C to be ca. 4.6. Severe steric effects were noted for 6,6′-dimethyl substitution. The alkaline hydrolysis of the corresponding ε-lactones of 2′-hydroxymethylbiphenyl-2-carboxylic acids was studied under the same conditions. The intermediacy of the lactones in the intramolecular Cannizzaro reaction can be excluded. The evidence for the Cannizzaro reaction indicates a mechanistic pathway involving rate-determining hydride transfer from the dianion of a hydrated formyl group to the second formyl group. An estimate is made of effective concentration of the intramolecular hydride transfer.