Carbon–Carbon Bond Formation between N-Heterocyclic Carbene Ligand on Ruthenium Carbene Catalysts and 1,4-Naphthoquinone via Intramolecular Carbon(sp³)–Hydrogen Bond Activation

Abstract

Reactions of ruthenium carbene catalysts, including their metathesis, nonmetathesis, and decomposition reactions, have been extensively studied because of their importance in organic synthesis. Here, we developed a novel cross-coupling reaction between 1,4-naphthoquinone and NHC ligands on ruthenium benzylidene complexes via intramolecular C­(sp3)–H bond activation. Also, we discovered a novel tricyclic blue pigment, 2-(1-mesityl-5,7-dimethyl-2,3-dihydro-1H-imidazo­[1,2-a]­indol-9-yl)­naphthalene-1,4-dione, as a product of the reaction.

Ion mobility mass spectrometry uncovers regioselectivity in the carboxylate-assisted C–H activation of palladium N-heterocyclic carbene complexes

“…31 Cyclometallated NHC complexes are also intermediates en route to annulated NHC compounds. 32,33 The ability of promoting these annulation reactions in a regioselective fashion would imply directing the formation of regiospecific cyclometallated NHC complexes with implications also for regiodivergent catalysis. Therefore, a detailed understanding of the intimate mechanism of NHC cyclometallation is highly desirable to understand both reactivity and regioselectivity issues.…”

Introducing Ion Mobility Mass Spectrometry to Identify Site-Selective C–H Bond Activation in N-Heterocyclic Carbene Metal Complexes

“…The mechanistic picture of the above-mentioned transformations is often triggered by an intramolecular C–H activation in the N-bound groups of the NHC. Such C–H bond activation is frequently found to be a reversible process and produces highly reactive and/or short-lived cyclometalated intermediates. − Although having a comprehensive mechanistic proposal is crucial to success, the identification of the site-selective C–H bond activation in these NHC complexes may become challenging using NMR or X-ray diffraction methods. In this context, soft ionization mass spectrometry (MS) techniques, such as electrospray ionization (ESI), and its tandem version are commonly used to uncover speciation and mechanistic insights in catalysis. − However, MS detection itself does not provide structural and conformational information about the isomers resulting from competitive C–H activation unless selective deuteration is carried out (a task difficult to achieve) .…”