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

Abstract

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) 2 Cl (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, 1 H NMR spectroscopic analysis indicates the formation of a novel bent allene or carbodicarbene (3b), which decomposes rapidly in solution at room temperature.

The Ambivalent Chemistry of a Free Anionic N‐Heterocyclic Carbene Decorated with a Malonate Backbone: The Plus of a Negative Charge

“…[22] Characteristically, such a value is approaching the chemical shift measured for the related O-methyl-substituted amino-acrylamidocarbene (d = 257.2 ppm) reported by Hudnall and coworkers. [29] Moreover, the splitting of the 1 H and 13 C NMR signals is in agreement with a loss of symmetry in the heterocyclic ring, revealing that the silyl group is not shuttled between the two oxygen atoms, at least on the NMR timescale.…”

“…These include the standard imidazol-2-ylidene IMes (2), its saturated imidazolin-2-ylidene SIMes (3), Bertrands "anti-Bredt" NHC 4, and the two more closely related neutral NHCs of [1]·Li, namely the diamidocarbene 5, and the amino-acrylamidocarbene 6. [29] The two orbitals playing a major role on the reactivity of a carbene are the in-plane, non-bonding occupied lone-pair of the carbon atom C1 (n s C1 ) and the vacant antibonding, ptype orbital centered on this carbon atom (2p p * C1 ). They generally constitute the HOMO and LUMO frontier orbitals of the free carbene.…”

“…The addition of phenylsilane to [1]·Li also occurred readily at room temperature to afford the "oxidative addition" product 26 in less than 1 hour (Scheme 8). The main spectroscopic characteristics of the insertion are the presence of a 29 Si NMR signal at d = À40.4 ppm, which lies close to the values reported for the NHC-PhSiH 3 activation products, [15,55] as well as a strong highfield shift of the carbenecarbon atom in the 13 C NMR spectrum to d = 64.5 ppm accompanied with the appearance of a broad singlet at d = 5.64 ppm assigned to the N 2 CHSi proton in the 1 H NMR spectrum, as already observed in compounds 22, 23, and 25. When 26 was submitted to an acidic aqueous work-up, a desilylation reaction with loss of the silyl moiety was observed, leading to compound 27, which is formally the hydrogenation product of the pyrimidinium betaine [1]·H.…”

Silver‐Catalyzed Amidiniumation of Alkynes: Isolation of a Silver Intermediate, Synthesis of Enamine Amido Carbene Precursors, and an Unprecedented Umpolung of Propiolamide

“…Bielawski and co‐workers reported that the six‐membered diamidocarbene (DAC) I was more electrophilic compared to typical NHCs (Figure 1), and favored NH and CH bond activation, reversibly fixing CO, coupling to isonitriles, and [2+1] cycloaddition of olefins, aldehydes, alkynes, and nitriles 4. The five‐memebered DAC II , reported by the groups of Bielawski4h and Ganter,5a–c and the six‐membered amino‐acrylamido carbene III , reported by Hudnall and co‐workers,6a also display similar electrophilic properties. We envision that decorating NHCs with a conjugated backbone containing an α,β‐unsaturated carbonyl moiety would provide the NHC with additional stability and exhibit unprecedented electronic properties, thereby showing distinct reactivity behavior.…”

First N‐Heterocyclic Carbenes Relying on the Triazolone Structural Motif: Syntheses, Modifications and Reactivity

“…The spectra indicatedt hat the newly formed species was the betaine 7 formed by formal loss of am ethyl cation from 16,w hich was subsequently transferred to the nucleophilic DMSO molecule resultingi nt he formation of the well-known cation Me(CD 3 ) 2 SO + .Ar elated slow demethylation was also observed by Hudnall and co-workers for their amino-acrylamido carbene precursor. [26] Thus, the triazolium salt 16 is obviously ag ood electrophilic methylating agent. To confirm this, 16 wast reated with as eries of N-nucleophiles such as triethylamine, methylimidazole, pyridinea nd DABCO, which were all cleanly converted to the corresponding N-methylated cationic derivatives( dicationic in the case of DABCO) while 16 was converted backt o7.H owever,w en ever observed am ethyl transfer from Ot ot he N-atom resulting in the formationo fc ation 20.F urthermore, it should be noted that this N-methyl derivative 20 did not show any alkylating reactivity at all.…”