Catalyst/Solvent Free Decarboxylation of α-Carboxylic Acids Grafted at N2 of 6-Arylpyridazin-3(2H)-ones and 4-Arylphthalazin-1(2H)-ones: Crystallography of N-Alkyl Derivatives and Intermediates

The straightforward decarboxylation of pyridazin-3(2H)-one-2-yl acetic acids and some of its alkyl homologues was achieved without the need for a catalyst and in solvent-free conditions by simple heating above the melting points. By this procedure N-alkylated pyridazin-3(2H)-ones 4a–e, tetrahydrophthalazin-1(2H)-one 5 and phthalazin-1(2H)-one 6 were obtained and the N-alkylated pyridazin-3(2H)-ones 4d,e, tetrahydrophthalazin-1(2H)-one 5 and phthalazin-1(2H)-one 6 were investigated by X-ray diffraction analysis and their supramolecular features were investigated by Hirshfeld analysis and CSD structure investigation, which revealed among other things that the 5,6,7,8-tetrahydrophthalazine framework had not been previously reported. We also investigated the ester precursors 2c,d. The reaction is feasible for a multi-gram scale approach. The compounds show weak hydrogen bonds of type C-H···O and C-H···Cl and in some cases π-stacking interactions. For some of the compounds we observed recurrent C-H···O patterns such as bifurcated H-bonds by switching the C=O acceptor.