3H-1,2,3,4,6-penta-azaindenes (“8-azapurines”). Part II. Rates and equilibria for reversible water addition

The cations of 8-azapurine, 2-methyl-, and 2-amino-8-azapurine have been shown from rapid-reaction studies and nuclear magnetic resonance spectra to undergo reversible addition of water across the 1,6-double bonds to form the corresponding 1,6-dihydro-6-hydroxy-8-azapurine (6,7-dihydro-7-hydroxy-3H-1,2,3,4,6-penta-azaindene) cations. The neutral molecules are essentially anhydrous. Acid dissociation constants, equilibrium constants, and nuclear magnetic resonance spectra of some of the covalently hydrated and the anhydrous species are recorded, together with first-order rate constants at 20° for the addition and removal of water. Whereas the stable anion of 2-hydroxy-8-azapurine is anhydrous, its netural molecule exists mainly as the covalently hydrated species. It is concluded that changes in n.m.r. spectra can provide a convenient method for demonstrating covalent hydration in heterocyclic molecules.