Solvent-Free Synthesis of Substituted Benzimidazoles and Quinazolinones via Acceptorless Dehydrogenative Coupling Using Ferrocene-Hydrazone-Based Ru(II)-p-cymene Catalysts

A series of ferrocene-hydrazone based Ru(II)-p-cymene complexes were investigated for their catalytic activity towards acceptorless dehydrogenative coupling (ADC) reactions. Ru(II)-p-cymene isonicotinic Fc-hyrazone complex performed better in the solvent-free synthesis of 1,2-disubstituted benzimidazoles (up to 88%) and quinazolinones (up to 95%). The catalysis was extended to a wide range of benzylic alcohols. The synthesis of 1,2-disubstituted benzimidazoles was accomplished with electron-donating/-withdrawing o-phenylenediamines as well. A catalytic structure-activity relationship was established by varying the steric and electronic parameters on the substrates. All the catalytic experiments exhibited good to moderate yields, with hydrogen and water as the only by-products. The prepared complexes were well characterized using analytical and spectroscopic techniques. The molecular structures of the ligands and complex were resolved using single crystal X-ray crystallographic studies.