Synthesis and Basicity Studies of Quinolino[7,8‑h]quinoline Derivatives

Quinolino­[7,8-h]­quinoline is a superbasic compound, with a pKaH in acetonitrile greater than that of 1,8-bis­(dimethylaminonaphthalene) (DMAN), although its synthesis and the synthesis of its derivatives can be problematic. The use of halogen derivatives 4,9-dichloroquinolino­[7,8-h]­quinoline (16) and 4,9-dibromoquinolino­[7,8-h]­quinoline (17) as precursors has granted the formation of a range of substituted quinolinoquinolines. The basicity and other properties of quinolinoquinolines can be modified by the inclusion of suitable functionalities. The experimentally obtained pKaH values of quinolino­[7,8-h]­quinoline derivatives show that N4,N4,N9,N9-tetraethylquinolino­[7,8-h]­quinoline-4,9-diamine (26) is more superbasic than quinolino­[7,8-h]­quinoline. Computationally derived pKaH values of quinolinoquinolines functionalized with dimethylamino (NMe2), 1,1,3,3-tetramethylguanidino (NC­(NMe2)2) or N,N,N′,N′,N″,N″-hexamethylphosphorimidic triamido (NP­(NMe2)3) groups are significantly greater than those of quinolino­[7,8-h]­quinoline. Overall, electron-donating functionalities are observed to increase the basicity of the quinolinoquinoline moiety, while the substitution of electron-withdrawing groups lowers the basicity.