Successive Protonation of an N‐Heterocyclic Imine Derived Carbonyl: Superelectrophilic Dication Versus Masked Acylium Ion

Abstract

Carbonyl cations are among the most commonly invoked reactive intermediates in organic synthesis. While Olah pioneered superacids to provide a “stable ion” environment for their study in situ, isolated examples are rare. Here, we disclose successive protonation of an N‐heterocyclic imine (NHI) derived carbonyl compound (IDippN)2CO, 2, to the monocation [(IDippN)(IDippNH) CO]+, [3]+, and the doubly protonated dication [(IDippNH)2CO]2+, [4]2+. [3]+ represents a rare example of an N‐protonated carbonyl cation and [4]2+ the first example of a superelectrophilic carbonyl dication. All three compounds have been characterized by X‐ray crystallography and IR spectroscopy, revealing stepwise strengthening of the C=O bond on protonation. The unique stability of these systems is attributed to the enhanced basicity and steric profile provided by the NHI substituents. In addition, we report the related singly NHI‐stabilized cation [IDippNCO]+, [5]+. Crystallographic and DFT analyses provide insight into the interaction between the carbonyl fragment and the NHI, which reveals that the [CNCO]+ unit (isoelectronic to CCCO) can be described as an acylium cation “masked” as a cumulene.

Redox Character and Small Molecule Reactivity of a Masked Titanium(II) Synthon

“…On the other hand, the imidazolin-2-iminato ligand displays a relatively long bond distance to the titanium center (Ti1–N1 = 1.900(2) Å) despite adopting a near linear coordination mode to the metal (Ti1–N1–C1 = 169.7(2)°) when compared to [(Et 2 N)­C­(NDipp) 2 ]­(Im′N)­TiCl 2 (Ti–N Im = 1.765(3) Å; Ti–N Im –C Im = 170.5(3)°). An obtuse Ti–N Im –C Im angle is typically suggestive of substantial metal–nitrogen π-overlap consistent with zwitterionic, imido-type TiNIm + bonding stabilized through imidazolium resonance contributions; , however, this bonding interaction is likely attenuated by the steric congestion of the titanium center in 4 ·THF. In line with this, the N1–C1 = 1.276(3) Å bond length of 4 ·THF is substantially shorter than the corresponding N Im –C Im = 1.326(5) Å bond in [(Et 2 N)­C­(NDipp) 2 ]­(Im′N)­TiCl 2 which does display TiNIm + bonding character.…”

“…The disparity in reduction character seemingly indicates the 3 N -ligand manifold of our [( Ket guan)­(Im Dipp N)­Ti] n + system has superior donor abilities over the dual amidoimines of [Me 2 NC­(N i Pr) 2 ] 2 TiCl 2 and [PhC­(NSiMe 3 ) 2 ] 2 TiCl 2 , which is likely due to the excellent σ- and π-donor abilities of the Im Dipp N – ligand. , To shed further light on this, the solution redox properties of 2 Cl were investigated through cyclic voltammetry using THF as solvent with [NBu 4 ]­[PF 6 ] as the supporting electrolyte. The cyclic voltammogram (CV) of 2 Cl (Figure S62) reveals an irreversible Ti­(IV)/(III) redox wave at −2.3 V (vs [Cp 2 Fe] 0/+ ), which is at the cusp of the Na/Hg amalgam redox potential (−2.38 V vs [Cp 2 Fe] 0/+ in nonaqueous solvent) .…”

Mesoionic N‐Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2

“…[6] Due to the considerably high ylidic character of the exocyclic imine moiety, NHIs are excellent donors and super nucleophiles. [7,8] For the last two decades, NHIs have drawn considerable attention. [9][10][11][12][13][14][15][16][17][18][19] NHIs (Figure 1b) have found diverse utilization in the design of excellent ligands for the isolation and stabilization of various actinide metal complexes in high oxidation states, [9][10][11] as well as numerous transition metal complexes.…”

“…in 1995 [6] . Due to the considerably high ylidic character of the exocyclic imine moiety, NHIs are excellent donors and super nucleophiles [7, 8] . For the last two decades, NHIs have drawn considerable attention [9–19] .…”

An Air-Stable “Masked” Bis(imino)carbene: A Carbon-Based Dual Ambiphile

“…N-Heterocyclic imines (NHIs) derived from conventional N-heterocyclic carbenes (NHCs) , are powerful 2σ, 4π electron donor ligands with a large steric profile. NHIs have recently received significant attention due to their ability to stabilize a wide range of highly electron-deficient main group systems. − However, the moderate electrophilic properties of these NHIs limit them to being purely Lewis basic ligands. On the other hand, CAACs have higher nucleophilicity than NHCs, but are immensely more electrophilic, − leading us to believe that an NHI based on a CAAC might be a suitable donor to tame an (imino)­carbene, but at the same time possess sufficient electrophilicity to act as a secondary Lewis acidic site .…”