REACTIONS OF PERHALOACETONES WITH DIHYDROPYRIDINES AND OTHER ELECTRON-DONORS

The kinetics of the reduction of hexachloroacetone by 3-substituted 1-benzyl-1,4-dihydropyridines is 1st order in each reactant. The rate of reduction is sensitive to the electron-withdrawing power of the 3 substituent. The efficient, nonenzymic reductions of thiobenzophenones and halo ketones by 1-substituted 1,4-dihydronicotinamides are approximations to the biological reductions of simple carbonyl groups by the coenzyme, NADH. Attachment of an indole moiety at either the 3 or the 1 position of the dihydropyridine ring resulted at most in a small decrease in the rate. Activation energies for reduction by the 3-carbamoyl- and 3-cyanodihydropyridines are low (5-7 kcal mol-1) and the entropies of activation are very negative (-46, -47 eu [entropy units]). Reduction by the 3-carbamoyl derivative proceeds 33 times more rapidly in acetonitrile than in benzene. The isotope effect (kH/kD) in the product-forming step in reactions of hexachloroacetone, pentachloroacetone and sym-tetrachloroacetone with 1-benzyl-3-carbamoyl-1,4-dihydropyridine-4-d is essentially invariant with the nature of the halo ketone. Changes in the UV-visible spectra are observed with dihydropyridines and halo ketones are mixed, suggesting the possible intervention of intermediate complexes in the reduction. Although ESR studies indicated the lack of detectable radicals in these reactions, 1-electron transfer occurs from N,N,N'',N''-tetramethyl-p-phenylenediamine to hexafluoroacetone to yield the cation radical of the amine. Pentachloroacetone is the product from hexachloroacetone and the diamine. 1,4,4-Trimethyl-1,4-dihydropyridine in acetonitrile gives highly colored solutions when mixed either with hexachloroacetone or chloranil. It was not possible to identify products from these reactions.