Preparation of Rhodium(III) Di-NHC Chelate Complexes Featuring Two Different NHC Donors via a Mild NaOAc-Assisted C–H Activation

Abstract

Two molecules of 4,5-dichloroimidazole react with dibromomethane to give the diimidazole 1. Reaction of 1 with iodomethane or 2-fluorobenzyl bromide yields the monoalkylated imidazolium/imidazole salts [2]I and [3]­Br, respectively. Salt [2]I reacts with Ag2O followed by transmetalation to [Rh­(Cp*)­(Cl)2]2 to give the RhIII-NHC complex [4], bearing an NHC ligand with a pendant imidazole group. The pendant imidazole can be deprotonated using NaOAc to yield complex [5], bearing a doubly C-metalated C∧C chelate ligand. Reaction of [2]I or [3]Br with NaOAc and [Rh­(Cp*)­(Cl)2]2 yields the C∧C chelate complexes [7] and [9], respectively, in a one-pot reaction. The imine ring nitrogen atom in complexes [7] and [9] can be protonated using HBF4·Et2O to give complexes [8]­BF4 and [11]­BF4, each bearing a C­(NHC)∧C­(pNHC) chelate ligand (pNHC = protic NH,NR-NHC ligand). Alkylation of the imine ring nitrogen atom in [9] yields complex [10]­BF4, bearing a unique unsymmetrical (NHC)∧C­(NHC′) chelate ligand.

Iridium(III) Complexes Bearing Chelating Bis‐NHC Ligands and Their Application in the Catalytic Reduction of Imines

“…Apparently, both the imidazolium and imidazole groups of 2 I and 3 Br can be deprotonated by NaOAc. Similar behavior has been observed during the preparation of rhodium(III) NHC complexes from imidazolium/imidazole NHC precursors . The presence of NaOAc is essential for the reaction to proceed, although its precise role apart from acting as a base has not been determined at this time.…”

“…The 2‐fluorobenzyl group in 3 Br was introduced because the resulting compound is insoluble in acetonitrile and can be easily isolated from this solvent by filtration. A detailed description of the synthesis of 2 I and 3 Br has been reported …”

“…4,5‐Dichloroimidazole, iodomethane, 2‐fluorobenzyl bromide, N ‐benzylideneaniline, N ‐benzylidene‐ N ‐(diphenylmethyl)amine and N ‐benzylidene‐ tert ‐butylamine were used as received from commercial sources. [{Cp*IrCl(µ‐Cl)} 2 ] and the ligand precursors 2 I and 3 Br were prepared as described previously. Satisfactory microanalytical data for 4 and 5 could not be obtained due to the sensitivity of the compounds or the fluorine content of 5 .…”

Switching of Reaction Pathway from C−C Rollover to C−N Ring‐Extension Annulation

“…It is important to mention that there was no indication of the presence of an N−H proton in the 1 H NMR spectrum, which eliminated the possibility of forming a protic NHC complex under these conditions. This was further confirmed by comparing the N‐C‐N bond angle value of 110.8(3)° in 5 with a known Cp*Rh‐complex (109.4(2)° for imidazolyl‐Rh versus 104.4(2)° for NHC‐Rh) . The intermediacy of 5 for the two different modes of reactivity was confirmed by performing two separate control experiments by treating 5 with alkyne 2 b in presence of Cu(OAc) 2 and Cu(BF 4 ) 2 which provided high yields of 3 b (91 %) and 4 b (85 %), respectively (Scheme b).…”

“…Eur.J.2017, 23,15529 -15533 www.chemeurj.org N-C-N bond angle value of 110.8(3)8 in 5 with ak nown Cp*Rhcomplex (109.4(2)8 for imidazolyl-Rh versus 104.4(2)8 for NHC-Rh). [12] The intermediacy of 5 for the two different modes of reactivity was confirmed by performing two separatec ontrol experiments by treating 5 with alkyne 2b in presence of Cu(OAc) 2 and Cu(BF 4 ) 2 which provided high yields of 3b (91 %) and 4b (85 %), respectively (Scheme 2b). Significantly,t he seven-membered double alkyne-inserted metallacyclic intermediate 6 for this new CÀNa nnulation reaction could also be isolated from areactionwith complex 5 and dimethylacetylene dicarboxylate (DMAD) as alkyne in the absence of Cu(BF 4 ) 2 (Scheme 2c).…”