Thiazoline Derived Orthometalated Palladium Pincer Complexes for Quinoline Synthesis via Acceptorless Dehydrogenation and Subsequent C( _{sp ²} )–C( _{sp ²} ) Coupling Reactions: Experimental and Computationa

Herein, we present the synthesis of a thiazoline-derived N∩NH∩CH pincer ligand and a series of orthometalated N∩N∩C palladium(II) complexes as catalysts in solvent-free quinoline synthesis via the acceptorless dehydrogenation (ADH) process and subsequent C(sp2)–C(sp2) bond-forming reactions. The N∩NH∩CH (2) pincer ligand was produced by the equimolar reaction of thiazoline amine (N∩NH2, 1) and benzoyl chloride, which upon further treatment with Pd(OAc)2 in acetonitrile, afforded the orthometalated palladium(II) complex [(κ3–N∩N∩C)Pd(NCCH3)] (3). A series of heteroleptic palladium(II) complexes of [(κ3–N∩N∩C)Pd(L)] 4–12 was explored by reacting complex 3 with various substituted pyridine derivatives. Proposed structures and chemical compositions were confirmed using 1H, 13C{1H} NMR, and HR–MS spectroscopic techniques. Additionally, the molecular structures of complexes 3, 4, 6, 10, and 12 were established through single-crystal X-ray diffraction studies. The catalytic activity of palladium(II) complexes 3–12 was evaluated for the synthesis of quinoline derivatives from 2-aminobenzyl alcohol and substituted acetophenones via ADH reactions. The further utility of the complexes in C(sp2)–C(sp2) coupling reactions was also explored using 2-(4-bromophenyl)quinoline with phenylboronic acid derivatives. A tenable reaction mechanism has been suggested through several control experiments and spectroscopic investigations. DFT calculations have been employed to prove the thermodynamics associated with this conversion.