The Mechanism of NO Bond Cleavage in Rhodium-Catalyzed CH Bond Functionalization of Quinoline N-oxides with Alkynes: A Computational Study.

Metal-catalyzed CH activation not only offers important strategies to construct new bonds, it also allows the merge of important research areas. When quinoline N-oxide is used as an arene source in CH activation studies, the NO bond can act as a directing group as well as an O-atom donor. The newly reported density functional theory method, M11L, has been used to elucidate the mechanistic details of the coupling between quinoline NO bond and alkynes, which results in CH activation and O-atom transfer. The computational results indicated that the most favorable pathway involves an electrophilic deprotonation, an insertion of an acetylene group into a RhC bond, a reductive elimination to form an oxazinoquinolinium-coordinated RhI intermediate, an oxidative addition to break the NO bond, and a protonation reaction to regenerate the active catalyst. The regioselectivity of the reaction has also been studied by using prop-1-yn-1-ylbenzene as a model unsymmetrical substrate. Theoretical calculations suggested that 1-phenyl-2-quinolinylpropanone would be the major product because of better conjugation between the phenyl group and enolate moiety in the corresponding transition state of the regioselectivity-determining step. These calculated data are consistent with the experimental observations. [ABSTRACT FROM AUTHOR]