Quino quinoline, proton sponge

Compounds such as 2, 4, and 5 are known as proton sponges. Another type of proton sponge is quino[7,8-A]quinoline (6). Protonation of this compound also gives a stable monoprotonated ion similar to 3, but the steric hindrance found in 2, 4, and 5 is absent. Therefore 6 is a much stronger base than quinoline (7) (p/fg values of the conjugate acids are 12.8 for 6 and 4.9 for 7), but proton transfers are not abnormally slow. 

Quino[7,8-/i]quinoline (50) and benzo[l, 2-h 4,3-/r Jdiquinoline (52) belong to the so-called proton sponges and possess abnormally high basicities, pKa= 12.8 and 10.3, respectively (89AG(E)84, 89AG(E)86). This is mostly due to strong destabilization of both bases because of electrostatic repulsion... 

Incorporation of a dialkylamino group of proton sponge into an aromatic ring such as in quino[7.8-h]quinoline (18) results in an increase in basicity [18,19]. In contrast to... 

Zirnstein, M.A. and Staab, H.A. (1987) Quino[7,8-h]quinoline, a new type of proton sponge . Angewandte Chemie (International Edition in English), 26, 460-462. 

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