Reactivities of Five-Membered Heterocycles in Hydride Transfer Reactions.

2-[2'- And 3'-furyl and thienyl]-1,3-dimethylbenzimidazoline derivatives have been prepared in order to measure the rate of oxidation of these compounds by 1-benzyl-3-carbamoylpyridinium ion and by 1-benzyl-5-nitroisoquinolinium ion. The reactions were monitored spectrophotometrically in a solvent consisting of four parts of 2-propanol and one part of water at 25 degrees C. The reactivities of the five-membered heterocycles fail to correlate with the acid dissociation constants, pK(a), of the corresponding heteroaromatic carboxylic acids. The variations in k(2) appear to depend more heavily on resonance involving the heteroatoms while the pK(a) seems to depend mainly on the inductive effect of the heteroatoms. When rate constants for oxidation by the 1-benzyl-5-nitroisoquinolinium ion are plotted against rate constants for oxidation of the same imidazoline derivatives by 1-benzyl-3-carbamoylpyridinium ion, a linear plot with a slope of 0.96 +/- 0.06 is obtained. The slope of this plot can be estimated, using Marcus theory, by taking the rates of the two calculated alpha values. In this way 0.95 is obtained, in good agreement with the experimental value. In this calculation it was assumed that the reactivity of isoquinolinium is about the same as that of phenanthridinium ion. These results further extend the generality of the one-step mechanism of hydride transfer, without the intervention of high-energy intermediates.