10-Dimethylamino derivatives of benzo[h]quinoline and benzo[h]quinazolines: fluorescent proton sponge analogues with opposed peri-NMe2/–N= groups. How to distinguish between proton sponges and pseudo-proton sponges

For the first time, 10-dimethylamino derivatives of benzo[h]quinoline 6 and benzo[h]quinazoline 7a–e as mixed analogues of archetypal 1,8-bis(dimethylamino)naphthalene (“proton sponge”) 1 and quino[7,8-h]quinoline 2a have been examined. Similar to 1 and 2, compounds 6 and 7 display rather high basicity, forming chelated monocations. At the same time, unexpected specifics of the protonated NMe2/—N═ systems consist of a strong shift of the NH proton to the 10-NMe2 group, contrary to the “aniline–pyridine” basicity rule. In case of 4H+, a rapid migration (in the NMR time scale) of the NH proton between two nitrogen atoms along the N–H···N hydrogen bond was registered at room temperature and frozen below −30 °C with the proton fixed on the NMe2 group. Two different approaches for classification of strong neutral nitrogen organic bases as proton sponges (kinetically inert compounds) or pseudo-proton sponges (kinetically active) are discussed. On this basis, benzoquinoline 6 was identified as staying closer to pseudo-proton sponges while 7a–e to proton sponges due to the presence in their molecules of bulky substituents in the pyrimidine ring. Other remarkable peculiarities of 6 and 7 are their yellow color and luminescence in the visible region distinguishing them from colorless 1 and 2a.

J. Org. Chem. 2016, 81, 5574–5587