Facile and Regioselective Synthesis of Substituted 1H-Pyrazolo[3,4-b]quinolines from 2-Fluorobenzaldehydes and 1H-Pyrazol-5-amines

The present article concerns the scope and limitations of the regioselective condensation of 2-fluorobenzaldehydes with 1H-pyrazol-5-amines, leading to the synthesis of substituted 1H-pyrazolo[3,4-b]quinolines (PQ), in the presence of a base catalyst (DABCO and 2,4,6-trimethylpyridine). A method to obtain these nitrogen heterocycles with fluorine or trifluoromethyl substituents in different positions in the carbocyclic ring was developed as a part of a systematic research on the influence of fluorine-containing substituents on the parameters of PQ. Those compounds, characterized by high-fluorescence intensity, have been tested as emitters for the organic light-emitting diodes since 1997. The functionalization of PQ causes changes in various parameters, for example, HOMO and LUMO levels, which are important for the adjustment of fabricated organic light-emitting diodes. One of the easiest methods of PQ preparation, namely, the condensation of substituted anilines with 5-chloro-1H-pyrazole-4-carbaldehydes, is not regioselective. The method described in this study allows synthesizing of 1H-pyrazolo[3,4-b]quinolines with good yields and high selectivity - only the expected isomer is obtained. As various different 2-fluorobenzaldehydes are commercially available, and 1H-pyrazol-5-amines with different substituents are easy to prepare, the method could be a good alternative to the already known procedures. All possible mechanisms of the reaction were also thoroughly studied.