The influence of electron delocalization upon the stability and structure of potential N-heterocyclic carbene precursors with 1,3-diaryl-imidazolidine-4,5-dione skeletons

Abstract

(2010). The influence of electron delocalization upon the stability and structure of potential N-heterocyclic carbene precursors with 1,3-diaryl-imidazolidine-4,5-dione skeletons. New Journal of Chemistry, 34 (7), 1295-1308. doi:10.1039 Aiming to develop synthetic protocols for the preparation of N-heterocyclic carbenes (NHCs) with increased π-acceptor character featuring N-pentafluorophenyl substituents, the molecular 2-chloro-1,3-bis(pentafluorophenyl)imidazolidine-4,5-dione (1a) was isolated from the condensation reaction of the corresponding formamidine with o xalyl chloride, instead of the expected, ionic 1,3-10 bis(pentafluorophenyl)-4,5-dioxo-4,5-dihydro-3H-imidazolium chloride. The nature of the extraannular N-substituents does not appear to exercise a significant influence on the structure of the products of this reaction, as evidenced by the similar structures observed with 2,6-dimethylphenyl (1b) and 2,6-diisopropylphenyl (1c) N-substituted analogues. These formal adducts of NHCs with hydrogen chloride demonstrate reactivity akin to that expected of alkyl halides. For instance, 1,3,1',3'-tetrakis(2,6-dimethylphenyl)-[2,2']diimidazolidinyl-4,5,4',5'-tetraone (2b) is formed via the reductive coupling of two units of 1b, while 1,3-15 bis(2,6-diisopropylphenyl)-4,5-dioxoimidazolidin-2-yl acetate (3c) is formed as the result of a metathesis reaction with mercury acetate. Chloride abstraction results in the formation of unstable imidazolium-4,5-dione salts (4a-c) that decompose rapidly, except in the case of the kinetically-stabilized 1,3-bis(2,6-diisopropylphenyl)imidazolium-4,5-dione hexafluorophosphate 4c, which could be observed spectroscopically. All imidazolium-4,5-dione hexafluorophosphate salts decompose to neutral 2-fluoro-1,3-bis(aryl)imidazolidine-4,5-diones (5a-c) via fluoride abstraction, demonstrating a clear preference for a four-coordinate 20 geometry about the diaminocarbon center in these compounds. 2-Methoxy-1,3-di(aryl)imidazolidine-4,5-diones (6a-c) were also prepared and they proved to be highly stable, failing to undergo thermolysis and yield the free NHCs. Computational analyses revealed that the instability of NHCs with an oxalamide skeleton, as well as that of their precursors, imidazolium-4,5-diones, results from a π-framework which extends over both carbonyl moieties and gives rise to a very low energy LUMO, rendering the compounds in question highly electrophilic.25

Anionic N‐Heterocyclic Carbenes with N,N′‐Bis(fluoroaryl) and N,N′‐Bis(perfluoroaryl) Substituents

“…1.40 in benzene), [25] in agreement with the extensive electron delocalization over the N À C À N and C À C À C fragments of the framework. The N(1) À C(4) and N(2) À C(2) bonds (1.476(3) and 1.487(3) , respectively) are very long for amide functionalities, and much longer than the corresponding distances observed in the molecular derivatives H (1.348(5)-1.381(2) ) [19] or in hexamethylmalonamide (1.34 ). [26] As observed in the precursor to K, [20] the competitive delocalization over the C À C À C backbone in 1 a-d reduces the multiple-bond character of the amidic C À N bond.…”

“…We have shown that the [3+2] ring-closing reactions of easily accessible nonfluorinated as well as poorly nucleophilic, fluorinated, and perfluorinated N,N-bisA C H T U N G T R E N N U N G (phenyl)formamidines [18] with highly electrophilic oxalyl chloride proceeds readily to give molecular products H instead of the expected ionic derivatives I. [19] These molecular products H displayed reactivity typical of alkyl halides, and in our hands failed to generate the targeted NHCs, J. Computational analyses showed that the oxalamide backbone significantly lowered the energy of the LUMO of the desired carbenes J, increasing their p-acceptor (electrophilic) character and decreasing their stability to the point where it rendered them too instable for isolation. Meanwhile, a series of closely related amido and diamidocarbenes have been isolated or incorporated in transition-metal complexes, including the anionic carbenes K, [20] the neutral derivatives L, [21] and the five-membered monoamidocarbenes M. [22] Although carbenes K-M are highly reactive compounds, it is clear that the destabilizing influence of the amido functionalities on the amidocarbene moiety in these compounds is inferior in magnitude to that observed in the oxalamide ligands J.…”

“…In an NMR tube, carbon monoxide was passed through a solution of [Rh- (15 mg, 23 mmol) in [D 6 ]DMSO (1 mL) for 2 h, after which time, the 1 H, 19 F, and 13 C NMR spectra were recorded and demonstrated the quantitative conversion of the starting material to the title product. The complex could be isolated as a solid by trapping the free carbene in situ as follows: KHMDS (7.0 mg, 0.035 mmol) was added to a THF solution (0.5 mL) of (NHC-F 6 )H (1 c) (15 mg, 0.035 mmol) and the mixture was agitated.…”

Ammonia Activation by a Nickel NCN‐Pincer Complex featuring a Non‐Innocent N‐Heterocyclic Carbene: Ammine and Amido Complexes in Equilibrium

“…Only few such examples have been structurally characterized to date, 17 and we propose that the competitive delocalization of the nitrogen lone pairs onto the exocyclic amidine moieties is responsible for a reduced stabilization of the carbene structure. 18 In support of this theory, the two C-N bonds of each pendant amidine group in 2 are almost equal in length. Treatment with potassium hexamethyldisilazide in toluene yielded nevertheless the NHC complex 3, with nickel retaining the chloride in its coordination sphere.…”

A theoretical study on the hydrogen adducts of diamidocarbenes and diaminocarbenes

“…Although they are typically known for their exceptional σ-donor abilities [5], π-interactions play a role in the bonding between NHCs and transition metals, as indicated by recent studies [6][7][8][9][10][11]. Hobbs and coworkers focused on preparing NHCs with reduced-energy lowest unoccupied molecular orbitals (LUMOs) to determine the importance of π-interactions between carbenes and transition metals [12]. By introducing an oxalamide into the heterocyclic backbone (e.g., 2 in Scheme 1), they discovered that the resulting carbene, referred to as N,N′-diamidocarbene (NAC), possessed a higher electrophilicity compared with typical NHCs.…”