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.