Amino-Acrylamido Carbenes: Modulating Carbene Reactivity via Decoration with an α,β-Unsaturated Carbonyl Moiety

Two 6-methoxy-4-oxo-1,3-diaryl-3,4-dihydropyrimidinium salts (2a and 2b) have been prepared as precursors to novel amino-acrylamido carbenes. Treatment of 2a (where the aryl groups are mesityl groups) with one equivalent of sodium hexamethyldisilazide in aromatic hydrocarbons affords the persistent amino-acrylamido carbene 3a, which has been characterized spectroscopically. This novel carbene has been trapped with Ir­(I) transition metal fragments as well as electrophilic carbon disulfide and nucleophilic isocyanides. Infrared spectroscopic studies carried out on 3a-Ir­(CO)2Cl (5a) indicated that this carbene exhibits a Tolman electronic parameter (TEP) of 2049 cm–1, a value that suggests that 3a is a stronger donor than both diamidocarbenes (DACs) and a recently reported amino-ureido carbene (DAC TEPs ≈ 2057 cm–1; amino-ureido TEP = 2058 cm–1), but similar σ-donating properties to a monoamido-amino carbene (TEP = 2050 cm–1). This result has been corroborated by DFT analyses carried out on all four species, which indicated that the HOMO and LUMO energies of 3a are comparable to the amino-ureido and monoamido-amino carbenes, whereas the DAC was shown to be more electrophilic with a much lower energy LUMO than the other three carbenes. Surprisingly, deprotonation of 2b (where the N-substituents are 2,6-diisopropylphenyl groups) does not afford the anticipated carbene. Indeed, 1H NMR spectroscopic analysis indicates the formation of a novel bent allene or carbodicarbene (3b), which decomposes rapidly in solution at room temperature.