The Nature of N-Heterocyclic Carbenes

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]. 

The carbenes derived from six- [20-22] and seven-membered heterocycles [22-24, 70] of type 14 can be obtained both with an aliphatic or an aromatic backbone. The characteristic NMR resonances for the C2 carbon atoms in six-membered carbene heterocycles (8 236-248 ppm) [20, 21] have been recorded in the same range as the Ccarbene resonances for saturated imidazolidin-2-ylidenes of type 7. Dimerization to enetetramines occurs for some carbenes derived from a six-membered heterocycle [21] and this behavior is independent from the nature of the backbone (saturated or unsaturated), but depends strongly on the steric bulk of the N,N -substituents. 

N-heterocyclic carbenes have become a new tool in organometallic chemistry and the field of the application of NHC in catalysis has reached a certain level of maturity. Yet, it is astonishing that so many fundamental questions are still not completely solved. What exactly is the nature of the metal-carbene bond and (when) does 7r-backbonding play a significant role How can the shape of the NHC adequately be described and measured, so that ligands can systematically be compared with each other And (when) will we be able to predict a carbene s properties before we prepare it in the lab Research with NHC is still vibrant and it doesn t need an augur to predict that many exiting and unexpected results will be unveiled. 

An immobilized version of a Heck reaction catalyzed by Pd nanoparticles has been very recently described by Kaiimi and Enders [289]. The nanoparticles were obtained as a result of the covalent anchoring of a N-heterocyclic carbene palladium/ionic liquid matrix on a silica surfece and their nature was confirmed by TEM coupled with EDX analysis. The catalyst showed high thermal stability (up to 280 °C) and could be recycled four times for the reaction of bromobenzene with methylacrylate achieving a total turnover number of 36600. After carrying out a hot filtration process, the authors could not detect any Pd in the filtrate. The filtrate also showed no further reaction progress. Erom these findings the authors concluded that the reaction was, in their case, indeed catalyzed by the heterogeneous Pd particles and not from monomolecular Pd-complexes leached from the sur ce. 

On the nature of the active species in palladium catalyzed Mizoroki-Heck and Suzuki-Miyaura couplings (particularly, Pd complexes with imi-nopyridine and N-heterocyclic carbenes as ligands) 06ASC609. 

Figure 7.25 N-Heterocyclic carbenes catalyze reactions between ary aldehydes and enals. Depending on the nature of the catalysts, " "" 1,4-diones (Stetter products), y-lactones or even a 2+1 adduct (a hemiacetal) can be formed...

In the presence of an imidazolium salt and a base, oxidative cyclization of a Ni(0) species upon the diene and an aldehyde takes place first and forms an oxanickellacycle 25, which equilibrates with a seven-membered oxanickella-cycle 26, naturally possessing a cis double bond. a-Bond metathesis through 26 with hydrosilane affords (Z)-allylsilane (Z)-23. The role of NHC ligand (N-heterocyclic carbene, generated by H" " elimination from imidazolium C2H by a base) is not clear at present a Ni(0)-NHC complex is believed to effectively produce 26. 

To illustrate the substantial potential of this methodology, which should play an increasingly important role in the future, two very interesting examples for the application of achiral N-heterocyclic carbene (NHC)-catalyzed reactions on chiral substrates in natural product synthesis will be discussed. 

This chapter is devoted to the most significant developments of early-transition metal compounds ligated by N-heterocyclic carbene ligands (from group 3 to group 7 scandium to rhenium) [10]. When appropriate and available, the structural features of these metal complexes will be discussed. The synthetic pathways, bonding nature of the Ccarbene-M> reactivity, and applications of such derivatives (with a special emphasis on their use in catalysis) will be discussed [11]. 

The Lewis acidic nature of many group 13 compounds makes them ideal candidates for complexation with nucleophilic N-heterocyclic carbenes. Borane compounds in particular are useful for the stabilization of NHCs, protecting the lone... 

The nature of ligands, such as Boxs, phosphines, and N-heterocyclic carbenes (NHC), bound to a transition metal center usually afiects the selectivities and reaction rates in the organic reactions. Styring et al. described the preparation of a Pd (Il)-salen complex immobilized on Merrifield resin (42). The polymer catalyst facilitated the Suzuki-Miyaura cross-coupling reaction in a continuous-flow system (Scheme 7.32) [130]. The reactor continuously afforded biaryl adducts (43) from aryl... 

---