New Synthesis, Properties, and Oxidizing Ability of 1,3-Dimethyl-5,10-methanocycloundeca[4,5]furo[2,3-d]pyrimidin- 2,4(1,3H)-dionylium Tetrafluoroborate

Abstract

A novel synthesis of 1,3-dimethyl-5,10-methanocycloundeca[4,5]furo[2,3-d]pyrimidin-2,4(1,3H)-dionylium tetrafluoroborate (10(+).BF(4)(-)) was accomplished by the reaction of 3,8-methano[11]annulenone with dimethylbarbituric acid and following acidic cyclization, albeit in low yield. Remarkable structural characteristics were suggested on inspection of the spectral data and MO calculation, and it was clarified that the positive charge is largely localized at the C11. The pK(R+) value of cation 10(+) was determined spectrophotometrically to be 4.6, which is much smaller by 4.1 pH unit than that of 1,3-dimethyl-7,12-methanocycloundeca[4,5]furo[2,3-d]pyrimidin-2,4(1,3H)-dionylium tetrafluoroborate (pK(R+) = 8.7). This value is also smaller by 1.6 pH unit than that of the parent 1,6-methano[11]annulenylium ion (pK(R+) = 6.2). The feature is rationalized on the basis of the perturbation derived from the bond fixation of the parent cation. The electrochemical reduction of 10(+) exhibited less negative reduction potential at -0.39 (V vs Ag/AgNO(3)) upon cyclic voltammetry (CV). In a search for reactivity, reactions of 10(+) with some nucleophiles, hydride and diethylamine, were carried out to give mixtures of C11- and C13-adducts. In both reactions, the methano-bridge controls the nucleophilic attacks to the C13 to favor exo selectivity. The photoinduced autorecycling oxidation reactions of 10(+).BF(4)(-) toward some amines under aerobic conditions were carried out to give the corresponding imines (isolated by converting to the corresponding 2,4-dinitrophenylhydrazones) in 719-3286% yield (recycling number of 10(+).BF(4)(-): 7.2-32.9).