N-heterocyclic carbenes stabilization

The basic nature of N-heterocyclic carbenes, stabilized by the presence of two adjacent N atoms, has been emphasized by several authors [38], Therefore, the yield and the selectivity of any organic synthesis (driven by the reactivity of a substrate vs. appropriate bases and carried out in imidazohum salts as solvents) can be affected by side reactions related to the acidity of C2-H unit in 1,3-dialkylimidazo-lium cation and to the basicity and nucleophilicity of the resulting N-heterocychc carbene. On this subject, noticeable work (related to the Baylis-Hillman, Knoevenagel and Claisen condensation, Homer reaction, etc.) carried out in RTlLs has been reported and discussed [37]. 

Ito, Kawakami and Sawamura recently described the borylation of al-lylic carbonates by B2pin2, catalyzed by bis(phosphine)copper(I) alkox-ides. It was proposed that bis(phosphine)copper(I) boryl species formed by alkoxide/boryl a-bond metathesis are key intermediates in the catalytic cycle [231]. Making use of related N-heterocyclic carbene stabilized precursors, Sadighi and co-workers have very recently isolated the thermally labile copper boryl complex (IPr)CuBpin (11.1) together with the products of oxygen atom, styrene and aldehyde insertion into the Cu-B bond (11.2-11.5 Scheme 24) [232,233,237]. The structure of 11.1 in the solid state reveals an approximately linear Cu(I) coordination geometry [ZB-Cu-C 168.1(2)°] and a Cu-B distance [2.002(3) A] which is somewhat shorter than the sum of the expected covalent radii [2.05 A] [106]. Yet further evidence for the... 

N-Heterocyclic Carbene-Stabilized Silylene Monohydride RSi ( NHC)H (R = rBuaSi, NHC = U,4, 5-tetramethylimidazol-2-ylidene)... 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

It has been speculated in the past that it might be possible to isolate the first Au(I) fluoride LAuF [182], if disproportionation into metallic gold and Au( 111) can be avoided by stabilizing ligands L, such as (PR3)3AuF [264]. This has just been achieved. Laitar et al. [185] were able to isolate a compound with an N-heterocyclic carbene ligand... 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

We reported the use of M-heterocyclic carbene complexes (NHC) for the catalytic activation of methane [55,56]. We found that solutions of N-heterocyclic carbene complexes of palladium(II) in carboxylic acids catalyze the conversion of methane to the corresponding methylesters. The high thermal stability of palladium(II) carbene complexes could be shown for complex 18 (Scheme 22), which we also structurally characterized [120]. An extraordinary feature is the unprecedented resistance of the palladium-NHC-complexes 18-22 under the acidic oxidizing conditions which are necessary for the CH-activation and functionalization. 

Amyes, T. L. et al.. Formation and stability of N-heterocyclic carbenes in water The carbon acid pK(a) of imidazolium cations in aqueous solution. J. Am. Chem. 

The vast majority of N-heterocyclic carbenes are based on 5-membered ring systems. It was found that sterically demanding substituents on the NHC are not only beneficial for the stability of the NHC, but also for its catalytic properties. Arguably, the most important and most often employed N-heterocyclic carbenes are imidazol-2-ylidenes IMes and IPr and the imidazolidin-2-ylidenes SIMes and SIPr (Fig. 3). The reactivity of the corresponding transition metal complexes is described in detail in the following sections. 

Other bidentate N-heterocyclic carbenes were used to form stable chelate complexes. A fine example is the use of palladium NHC complex 24 in the catalytic conversion of methane to methanol (Fig. 10) [111]. In this case the stability of the complexes is a requirement, since the reaction takes place in an acidic medium (trifluoroacetic acid) at elevated temperatures (80 °C) mediated by strong oxidizing agents (potassium peroxodisulfate). 

N-Heterocyclic carbenes (NHCs) - These are five-membered heterocyles in which a carbene function is stabilized by adjacent nitrogen (or sulfur) atoms on both sides of the carbene. They are often stable solids with sharp melting points and can be recrystallized from hydrocarbon solvents <2000CRV39>. For examples see Sections 24.3.1 and 244.2.5. 

The intention behind this research is the use of NHC stabilised palladacycles for use in homogenous catalysis [104], although the actual use in catalytic reactions was not reported yet. However, these compounds are designed to combine the advantageous stability of palladacycles with the high o-donor strength and steric demand of N-heterocyclic carbenes [93] [104]. 

The bond between carbon and rhodium is extremely stable, thus allowing mono- or biphasic hydroformylations without any excess of ligands. Due to this stability, for the first time ever an anchoring to a polymer support seems possible without leaching. These N-heterocyclic carbenes appear to be excellent ligands to stabilize catalytically active metals even under harsh temperature conditions, e. g. Heck C-C-coupling reactions at 130 °C [152 b]. 

Another indication for the Lewis acidic properties of NHSns is provided by the reaction of diben-zotetraazafulvalenes with stannylenes. Electron-rich enetetramines can be cleaved into N-heterocyclic carbenes. Pioneering work by Lappert has demonstrated that this cleavage, in the presence of an electrophilic metal center, leads to NHC complexes. Hahn and coworkers have used the electrophilic tin center in the NHSn 4 to cleave the dibenzotetraazafulvalene 5 with the formation of the zwitterionic stannylene-carbene adduct 6 (Scheme 2.8.5). Similar carbene adducts of di-aminogermylenes and diaminostannylenes have been described by Schumann et al. and Lappert et al. and even transient diorganogermylenes have been stabilized by formation of an adduct with an N-heterocyclic carbene. ... 

The majority of N-heterocyclic carbenes and their metal complexes are obtained from cyclic azolium derivatives (Fig. 8). Nevertheless, the first complex with a heteroatom stabilized carbene ligand, although it was not recognized as such, was prepared by Tschugajeff et al. as early as 1925 [157]. It was later identified as a heterocarbene complex [158]. Tschugajeff reacted the nucleophilic proton base... 

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