A laterally-fused N-heterocyclic carbene framework from polysubstituted aminoimidazo[5,1-b]oxazol-6-ium salts

A polysubstituted 3-aminoimidazo[5,1-b]oxazol-6-ium framework has been accessed from a new nitrenoid reagent by a two-step ynamide annulation and imidazolium ring-formation sequence. Metalation with Au(I), Cu(I) and Ir(I) at the C2 position provides an L-shaped NHC ligand scaffold that has been validated in gold-catalysed alkyne hydration and arylative cyclisation reactions.

The reaction mixture was stirred for 3 days at room temperature at which point the MeOH was removed under reduced pressure and the residue was suspended in 9:1 CH2Cl2/MeOH and filtered through a 5 cm pad of basic alumina.The filtrate was concentrated under reduced pressure and, where necessary, purified by flash column chromatography or recrystallisation.

General procedure 2 (GP2): gold-catalysed oxazole synthesis
Pyridinium ylide (1.2-1.5 equiv), dichloro(2-pyridinecarboxylato)gold (1-5 mol %), and ynamide (1.0 equiv) were added to a flask under an argon atmosphere.Toluene (0.1 M with respect to ynamide) was added (where ynamides were oils these were added as a solution in toluene) and the reaction mixture was heated at 90 °C for the indicated time.After cooling to room temperature, the reaction mixture was filtered through a 5 cm plug of silica gel (washing with CH2Cl2 and then EtOAc) and the filtrate was concentrated under reduced pressure and purified by flash column chromatography.

General procedure 3 (GP3): Synthesis of AImOx•HPF6 salts
Adapted from a literature procedure [10], under an argon atmosphere POCl3 (3.0 equiv) was added to a solution of oxazole (1.0 equiv) in m-xylene (0.1 M with respect to oxazole) and the reaction mixture was heated at 140 °C for 64 h.After cooling to room temperature, the impure AImOx•HCl salt was isolated by flash column chromatography (0→10% MeOH in CH2Cl2).
The resulting brown solid was taken up in refluxing H2O (≈ 10 mL) and the hot solution was filtered through glass wool, the flask was then washed out with additional boiling H2O (≈ 5 mL) which was also filtered through the glass wool.The aqueous filtrate was allowed to cool to room temperature and a saturated solution of KPF6(aq) (2.0 equiv) was added, leading to instant precipitation of the crude AImOx•HPF6 salt.The solid was collected by filtration, washed with ice-cold water and Et2O, and then recrystallised from acetone by addition of Et2O to give the AImOx•HPF6 salt.

General procedure 4 (GP4): Synthesis of (AImOx)MCl complexes
Adapted from a literature procedure [11], a vial was charged sequentially with AImOx•HPF6, the corresponding metal chloride, NEt3 (70 µL, 0.50 mmol) and acetone (2 mL) and heated at 60 °C for 18 h.The solvent was removed under reduced pressure and the crude mixture was taken up in CH2Cl2 (≈ 10 mL) and washed with H2O (2 × 10 mL).The organic phase was dried over Na2SO4, concentrated under reduced pressure and purified by flash column chromatography to give the (AImOx)MCl complex.
Product ratios were determined by 1 H-NMR spectroscopy with comparison to the literature spectra for 17 [17] and 18 [18].
H-NMR spectroscopy relative to an internal standard 1,2,4,5-tetramethylbenzene and with comparison to the literature spectra for .17; N, 6.15.Crystals suitable for single crystal X-ray diffraction were grown by slow evaporation of a solution of 13 in EtOAc.