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

Abstract

Quinolino[7,8-h]quinoline is a superbasic compound, with a pK_aH in acetonitrile greater than that of 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 pK_aH 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 pK_aH values of quinolinoquinolines functionalised 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 that 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.