Synthesis of Thiophenyl Schiff Bases via Buchwald-Hartwig Coupling of Piperazines to Quinoline Motif

Buchwald-Hartwig coupling reaction was employed to furnish new class of substituted piperazine-fused thio-quinolinyl Schiff bases. Synthesis of thiocarbonyl quinoline moiety was attempted using Lawesson’s reagent followed by esterification achieved using ethyl bromoacetate in refluxing acetone. Substantial C-N coupling of various piperazine or piperidine bases to the C-6 position of quinoline ring system was accomplished using X-Phos as optimal ligand for the Pd(0)-catalyzed Buchwald-Hartwig amination. After the successful coupling in satisfactory yields, the ester derivatives were hydrazinolyzed using 99% hydrazine hydrate and the resulted analogs were reacted with thiophene-2-carbaldehye in refluxing ethanol using catalytic amount of acetic acid to access the intended class of Schiff bases. New products would be of significant interest for the biological applications verifying that Schiff bases are valuable biologically active components. The proposed structures of the new products were confirmed with the aid of spectroscopy techniques (FT-IR and 1H NMR) and elemental (CHN) analysis.