4,5‐Bis(dialkylamino)‐Substituted Imidazolium Systems: Facile Access to N‐Heterocyclic Carbenes with Self‐Umpolung Option

Abstract

The first synthesis of 4,5-bis-(dimethylamino)-substituted imidazolium compounds was developed, which is based on the reaction of a 1,2-diamino-1,2-bis(phosphonio)ethene with lithiated formamidines. This represents the first application of this class of ethene derivatives for the preparation of heterocycles. These N-heterocyclic carbene (NHC) precursors show a remarkably reduced basicity and nucleophilicity of their NMe2 groups, which is due to the strong anomeric interactions of the latter with the imidazolium core. According to DFT calculations, these NHCs are capable of self-umpolung if sufficiently strong acceptor substituents are introduced at the carbene center. To test the self-umpolung capabilities of the NHCs, various substituents were attached to the carbene center and the obtained compounds were characterized by single-crystal X-ray analysis as well as quantum chemical computations. Strong acceptor substituents are required to induce self-umpolung, such as in the phosphonio-substituted derivative, for which partial self-umpolung was found. The N,N′-bis(4-dimethylaminophenyl)-substituted imidazolium compound represents a special case, as it incorporates as much as three two-step redox systems within the NHC framework. This will probably result in a high electronic flexibility of the corresponding nucleophilic carbenes, especially when they serve as ligands in transition metal complexes.

Skeleton Decoration of NHCs by Amino Groups and its Sequential Booster Effect on the Palladium‐Catalyzed Buchwald–Hartwig Amination

“…Hence, they represent ideal standard archetypes from which an optimization of stereoelectronic properties can be envisioned. [9] In a logical continuation of our work on skeleton functionalization of NHCs, [10] we reasoned that the construction of NHCs 1 a,b and 2 a,b, formally derived from the IMes (a) and IPr (b) ligands by incorporation of one or two dimethylamino groups on the heterocyclic skeleton, [11] would constitute a rational strategy to increase the donor properties of the NHC and to unveil potentially better catalysts. [4] Moreover, modifications of the electronic donor properties by decoration of the imidazolyl skeleton are highly efficient, but often remain subject to synthetic limitations.…”

“…However, the synthesis of the 4,5-bis(dimethylamino)imidazolium salts [2 a,b·H](OTf) appeared less straightforward, since the procedure developed by Huber, Weiss et al [11] using a bis(phosphonio)diaminoethene reagent led only to either a very poor yield of [2 a·H](OTf) (less than Taking into account the steric bulk of the Mes and Dipp substituents in 7 a,b, we found that the in situ generated dichlorodiaminoethene 10 constitutes an ideal reagent for this synthesis. The 4-(dimethylamino)imidazolium triflates [1 a,b·H](OTf) were thus synthesized in a two-step sequence consisting of the alkylation of the formamidine 7 a,b by the a-chloroacetamide 8 and a selective triflic anhydride mediated intramolecular cyclization (Scheme 2).…”

Proton Transfer Reactions of Triazol-3-ylidenes: Kinetic Acidities and Carbon Acid pKa Values for Twenty Triazolium Salts in Aqueous Solution

“…20 Remarkably, despite the widespread use of azolium ion precursors to NHCs in organocatalysis and elsewhere, there have been relatively few literature reports of the solution kinetic or thermodynamic Brønsted acidities of these precatalysts, [21][22][23][24][25][26][27][28][29][30][31][32] although there have been studies of NHC nucleophilicities 33 and gas phase proton affinities. [34][35][36][37][38][39][40][41][42] Washabaugh and Jencks reported on the C2-H/D exchange and aqueous pK a values of a range of N-alkyl thiazolium ion analogues of thiamine, that are precursors to thiazol-2-ylidenes 1. [27][28][29] Amyes et al reported aqueous pK a values for the conjugate acids of N,N-dialkylimidazol-2-ylidenes 2 (R = R' = H or Me) and two benzoannelated variants.…”

The Highly Lewis Acidic Dicationic Phosphonium Salt: [(SIMes)PFPh2][B(C6F5)4]2

“…The fluoro-substituents occupy the axial positions with a F-P-F angle of 168.8(2)8 while the carbene and the phenyl-substituents occupy equatorial positions. [15] Compound 5 exhibits a dramatic 31 P downfield shift to 78.1 ppm with a P À F coupling of 1040 Hz. [13] One of the fluoro-substituents on the cation of 4 is removed upon treatment with Et 3 Si[B(C 6 F 5 ) 4 ]·2 (C 7 H 8 ) giving rise to the dication [(SIMes)PFPh 2 ][B(C 6 F 5 ) 4 ] 2 (5) and the expected by-product Et 3 SiF.…”