Dehydrogenative Coupling for Synthesis of Quinazolin-4(3H)-ones via Tandem Reaction using Ruthenium(II)-Phenyl-Azo-Naphthaldoxime: An Experimental and Theoretical Investigation.

The bidentate N, N, donor phenyl-azo-naphthaldoxime NpLH, 1 was used to synthesize the ruthenium(II) complex trans-[Ru(NpL)(CO)Cl(PPh)], 2. It has been characterized by SCXRD, electrochemical and spectral studies. Computational analysis indicates that the low-lying π*-LUMO of the complex has substantial azo-character of coordinated ligand. This property has been exploited to form an efficient electron transfer pre-catalyst to effectuate dehydrogenative functionalization of a large number of benzyl alcohols to quinazolin-4(3H)-ones via condensation with diverse o-amino benzamides as well as N-substituted benzamides under aerobic conditions (57 entries). A reaction mechanism has been projected via isolation of intermediates and certain control experiments. Furthermore, it has been substantiated by theoretical scrutiny using density functional theory (DFT) calculation. The catalytic cycle involves stepwise hydrogen atom transfer (HAT) from benzyl alcohols to the N atoms of the coordinated ligand with subsequent removal of the H-atoms from the N atoms to regenerate the active catalyst.