Design and application of N,Nʹ-diamidocarbenes: expanding the constraints of stable carbene chemistry.
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Strategic incorporation of carbonyl groups into an N-heterocyclic carbene (NHC) scaffold via rapid and high-yielding methodologies culminating in classical deprotonation methods or the novel reduction of geminal dichlorides afforded stable and relatively electrophilic diamidocarbenes (DACs). Similar to transient, electrophilic carbenes and unlike NHCs, DACs underwent a variety of transformations with a broad range of small molecules including the unprecedented reversible coupling of carbon monoxide, metal-free transfer hydrogenations, and insertion into pnictogen-H bonds. Additionally, formal [2+1] cycloadditions were observed between DACs and a variety of alkenes, aldehydes, alkynes, and nitriles including the first examples between an isolable carbene and alkynes or electron-rich alkenes. DACs also effected the only uncatalyzed intermolecular C–H insertions of a stable carbene with non-acidic substrates to date. Beyond unprecedented reactivity scope, hydrolysis of the resultant diamidocyclopropenes or cyclopropanes afforded cyclopropenones or linear carboxylic acids. The latter constituted the formal, metal- and CO-free anti-Markovnikov hydrocarboxylation of an olefin and illustrated the ability of DACs to function as a reactive synthetic equivalent of carbon monoxide. Mechanistically, DACs were found to function primarily as nucleophiles in cycloadditions with alkynes, aldehydes, and various styrene derivatives and as an electrophile in the activation of primary amines. Thus, DACs may best be considered as ambiphiles which alters the paradigm of stable carbenes functioning exclusively as nucleophilic species. The reversibility of numerous small molecule-DAC interactions under mild conditions also suggested the potential of these systems in catalytic applications wherein dynamic association and release assume critical roles. Regardless, the reactivity associated with transient, electrophilic carbenes was largely achieved within a stable carbene scaffold and is envisioned to aid and influence the design and application of stable carbenes.