Synthesis, properties, and oxidizing function of 6-substituted 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrole-8(7H),10(9H)-dionylium tetrafluoroborates

Uracil-annulated heteroazulenes, 6-substituted 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrole-8(7H),10(9H)-dionylium tetrafluoroborates 7a,b.BF4-, which are the isoelectronic compounds of 5-dezazaflavin, were synthesized. X-Ray crystal analysis and MO calculations were carried out to clarify the structural characteristics of 7a,b.BF4-. The stability of cations 7a,b is expressed by the pK(R+), values which were determined spectrophotometrically to be 10.9 and 11.2, respectively. The electrochemical reduction of 7a,b exhibited high reduction potentials at -0.84 and -0.87 (V vs Ag/AgNO3) upon cyclic voltammetry (CV). A good linear correlation between the pK(R+) values and reduction potentials (El(red)) of 7a,b.BF4- and reference compounds 4.BF4- and 5.BF4- was obtained. In a search of the reactivity, reactions of 7a,b.BF4- with some nucleophiles, hydride and diethylamine, were carried out to clarify that the introduction of nucleophiles to give regio-isomers is dependent on the nucleophile. The photo-induced oxidation reactions of 7a,b.BF4- toward some alcohols under aerobic conditions were carried out to give the corresponding carbonyl compounds in more than 100% yield [based on compounds 7a,b.BF4-], suggesting the oxidizing function of 7a,b.BF4- toward alcohols in the autorecycling process. (C) 2003 Published by Elsevier Ltd.