PROTONATION OF 2,4-DIAMINOPYRIMIDINES .I. DISSOCIATION CONSTANTS AND SUBSTITUENT EFFECTS

The basic dissociation constants of a series of approximately 70 2,4-diaminopyrimidines and condensed pyrimidine derivatives have been obtained. The major effect of 5 substitution is inductive, but there is a greater resonance component than can be accounted for by correlation with Hammett σm constants. Maximum correlation is achieved with the equation log (K/Ko) = p[0.72σI + 0.28σR], in which σR− has been substituted for σR with the - R substituents. The effect of 6 substitution, on the other hand, is almost completely inductive. The equation log (K/Ko) = p[0.96σI + 0.04σR] best satisfies the data in this case. Similar relationships have been found with 4-amino-6-substituted pyrimidines. In some cases hydrogen bonding renders such correlations imprecise. Dissociation constants of 4-substituted pyrimidines can be correlated with σp constants, but 2- substituted derivatives appear to have a considerably greater inductive component. The shifts in ultraviolet maxima of 2,4-diamino-6-substituted, but not 5-substituted, pyrimidines have been found to have a dependence on the + R or - R character of the substituents. Ion pair formation between certain diaminopyrimidines and divalent ions in aqueoussolution has been postulated on the basis of uv studies. © 1969, American Chemical Society. All rights reserved.