Synthetic Access to Ring-Expanded N-Heterocyclic Carbene (RE-NHC) Copper Complexes and Their Performance in Click Chemistry

Abstract

The facile syntheses of ring-expanded N-heterocyclic carbene (RE-NHC) copper­(I) halide complexes are reported. The method makes use of a weak inorganic base in a green solvent. The reaction times can be greatly reduced by use of this weak-base route under microwave irradiation. The easy access to these complexes permits an evaluation of the catalytic activity and reaction profiling of [Cu­(RE-NHC)­X] complexes in the Huisgen 1,3-cycloaddition reaction.

Ambiphilicity of ring-expanded N-heterocyclic carbenes

“…In marked contrast, using CAAC-5 the oxidative addition product 1a was quantitatively obtained within minutes. Under the same conditions a rapid and clean reaction was also observed with NHC-6 13 a NHC-7 15 and NHC-8 13 c giving adducts 1b–d as shown by characteristic 1 H NMR signals at 6.04, 5.87 and 5.64 ppm, and 13 C NMR signals at 72.3, 73.9 and 77.1 ppm, respectively. The structure of adduct 1c (from NHC-7) was confirmed by X-ray crystallography.…”

“…Compared to NHC-5s, RE-NHCs display a larger N–C–N bond angle (∠ carb ) which imposes greater steric constraint when used as a ligand for transition metals, a feature used to enhance catalytic activity. 15 Arguably less emphasized, is the larger carbene bond angle, which increases the p-character of the lone pair, and thus the energy level of the HOMO. 16 Comparatively, the LUMO is less affected since ring expansion does not significantly disrupt the planarization of the α-amino fragments, which leaves the mesomeric stabilization of the p π orbital by the nitrogen lone pairs nearly identical.…”

Synthesis, DFT calculations, and investigation of catalytic and biological activities of back‐bond functionalized re‐NHC‐Cu complexes

“…This feature was also observed for other copper complexes of similar structure. 50 The frontier molecular orbitals (FMO) were also calculated at the same level of theory, and the obtained results are shown in Figure 2. As is well known, the FMO, that is, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), mainly determines the electronic properties and reactivity of molecules.…”

“…[29][30][31] Re-NHCs, in particular, have distinct electronic and steric characteristics that lead to highly stable M-NHC bonds, which help to improve catalytic efficiency in click reactions. [32][33][34][35] With light on these info, we aimed to synthesize back-bond functionalized re-NHC-Cu complexes with different azol ring size and different substituents, which can be functional in different applications such as catalysis and medical applications.…”

Dial-a-base mechanochemical synthesis of N-heterocyclic carbene copper complexes

“…15,16 This strategy is attractive as it is tolerant to air, and exploits inexpensive bases such as potassium carbonate. Alongside a single report of a microwave assisted weak base method, 17 access to RE-NHC systems relies on the strong base approach. For example, synthesis of (6-Dipp)CuCl generally occurs via the deprotonation of [6-Dipp]HCl with NaHMDS followed by addition of CuCl.…”