Unraveling the Competition of Two C–H and Two M–C Bonds in Guiding the Mechanism of Rhodium(III)-Catalyzed C–H Activation–Annulation

Abstract

Imidazolium motif-containing molecules are known to be of significant interest in diverse research areas, but there is a lack of functionalization protocols of these molecules. In a programme to overcome this challenge, recently we developed a unique dual-role of latent imidazolium C-H bond which not only generated a metal-C NHC bond upon activation but also directed further aryl/heteroaryl C-H activation to furnish a library of potentially valuable products. Mechanistic investigation of any newly-discovered catalytic reaction is at the heart of its future development for potential application in diverse fields. Motivated by this philosophy, the presented work delineates the key mechanistic insights of this annulation reaction which unravel the crucial competition of two C-H bonds (imidazolium and aryl C-H) and two M-C bonds (M-C NHC and M-C aryl ) in establishing the rate-limiting step and the alkyne-insertion regioselectivity in the reaction. Through careful isolation and X-ray structural characterization of the key seven-membered inserted intermediate along with DFT rationale, the exclusive regioselectivity of the alkyne-insertion to the M-C NHC bond was established. Kinetics studies were used to evaluate the rate-determining step of the reaction which was found to be the initial nondirected imidazolium C-H activation step. These mechanistic insights seem to be useful in understanding similar C-H activation processes in general which are topical in the area of catalysis.

Small “Yaw” Angles, Large “Bite” Angles and an Electron‐Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride‐Transfer Activity

“…Hydrogen atoms and solvents were not shown for clarity. Crystal structures of 1 and 6 were reported earlier …”

“…Complexes 6 – 10 were designed to represent six‐membered iridacyclic congeners of complexes 1 – 5 , respectively. All the complexes 1 – 10 were characterized fully by 1 H and 13 C{ 1 H} NMR spectroscopy, electron‐spray ionization mass spectrometry (ESI‐MS), and single‐crystal X‐ray diffraction analyses (Figure b, for details, see Supporting Information). It was interesting to note that the five‐membered cyclometalated complexes 1 – 5 exhibited bite angle values in the range of 76–77°, which is correlated to piano‐stool geometry.…”

“…Complexes 1-5 consisted of fivemembered iridacycles with electronic perturbation at the Nsubstituent (1 vs. 2)o ra tt he cyclometalated aryl ring (1 vs. 3, 4, 5). Complexes 6-10 were designed to represent six-membered iridacyclic congeners of complexes 1-5,r espectively.A ll the complexes 1-10 were characterizedf ully by 1 Ha nd 13 C{ 1 H} NMR spectroscopy,e lectron-spray ionization mass spectrometry (ESI-MS), and single-crystalX -ray diffraction analyses [10] (Figure 2b,f or details, see Supporting Information). It was interesting to note that the five-memberedc yclometalated complexes 1-5 exhibitedb ite angle values in the range of 76-778,w hich is correlated to piano-stool geometry.T he bite angle of the sixmembered congeners 6-10 wasf ound to be larger (ca.…”

“…[7] The 13 C{ 1 H} NMR chemical shift of the metal-boundC NHC resonance was about1 64 AE 2ppm in complexes 1-5 and 156 AE 2ppm in complexes 6-10 (Table 1), suggestingam ore electron-rich C NHC in the six-membered rings than in the five-membered conge- Hydrogen atoms and solvents were not shown for clarity.Crystal structures of 1 and 6 were reported earlier. [10] ners. This difference may be ascribed to the variable electronic effect of the NHC ligands in the two types of complexes,d ue to the presence (in the case of complexes 1-5)o ra bsence (in complexes 6-10)o fe lectronic delocalization of the nitrogen lone pair with the pendant aryl ring, Ac omparison of the electronically originated 13 C{ 1 H} NMR chemicals hifto fC NHC with the geometrically originated yaw and bite angle indicated similar and opposite trends, respectively,o nm ovingf rom five-to six-membered cyclometalated complexes ( Figure 3).…”

C–H activation-annulation on the N-heterocyclic carbene platform

“…Upon successful exhibition of directing group behaviour of NHC to activate aromatic 17,19,21,24 and non-aromatic sp 2 C-H bonds at Rh-NHC platform, 27 we extrapolated our investigations to difficult-to-functionalise chelating organic molecules having strongly coordinating heteroatoms in suitable metalchelating positions. Thus, in early 2016, our group reported an unprecedented 'rollover' C-H activation-annulation of such chelating molecules based on 2-pyridylimidazolium halide motifs as NHC backbone.…”