Reactions of the dipyrido[1,2-a:2′, 1′ -c]pyrazinium dication in basic solution

Abstract

The reactions of the dipyrido[1,2-a:2′,1′-c]pyrazinium dication (1) in basic solutions have been investigated. In aqueous base, this cation is involved in two rapid pH-dependent equilibration reactions associated with pKa values of 9.9 ± 0.2 and 10.4 ± 0.2. Spectral studies indicate that pKa 9.9 is associated with pseudobase formation by the addition of hydroxide ion at C-2 or C-4 of one of the pyridinium rings. The pKa 10.4 is most probably attributable to the formation of a dipseudobase by addition of two hydroxide ions in the pyrazinium ring of 1. The kinetics of these pseudobase formation reactions have been investigated by stopped–flow spectrophotometry over the range pH 9–13. Similar methoxide addition reactions occur in basic maethanolic solutions.The dication 1 also undergoes a slow decomposition reaction in aqueous base to initially form the 1-(formylmethyl)-2,2′-dipyridyl cation (2). The rate of this reaction, which has been measured as a function of pH at 25 °C for pH 8–12, passes through a maximum in the vicinity of pH 10.2. The cation 2 undergoes a further decomposition reaction in these solutions to give an unidentified product.