Alternative NHC ligands

Alternatively, NHC ligands can also be transferred from triethylborane-carbene adducts [103] or complexes of the type [M(NHC)(CO)5] (M = Cr, Mo, W) [104], but these procedures are limited to special cases and are of less importance. The coordination chemistry of silver NHC complexes [105] and the advantages and applications of the Ag20 method [106] have recently been reviewed. 

Controlling the exact architectnre of polymers has always attracted attention in macromolecular chemistry. Snccessfnl synthesis of alternating copolymers nsing ring opening metathesis polymerisation is of great interest also from a mechanistic perspective. NHC ligands were fonnd to be ideal to tune the selectivity of the metathesis initiators. 

The cyclic voltammogram of complex 3 B"-Ni shows two quasi-reversible Ni(0)/(I) and Ni(I)/(II) redox waves at —2.5 V and —I.IV vs. Fc/Fc, respectively. Neither of the two oxidized complexes was isolable, most likely due to the masking of the low-valent nickel(O) center by the three ter -butyl substituents on the carbene ligand. The search for alternative routes to these complexes using less sterically bulky NHC ligands, such as the TIMEN isopropyl derivative, is under investigation. 

Metal complexes with M-heterocyclic carbene ligands were known long before the first stable NHCs were isolated. Wanzlick [5] and Ofele [6] demonstrated as early as 1968 that NHC complexes can be obtained by in situ deprotonation of azolium salts in the presence of a suitable metal complex without prior isolation of the free NHC ligand (Fig. 1). In these cases a ligand of the metal complex precursor (acetate or hydride) acted as a base for the deprotonation of the imidazolium cation. This method has been successfully transferred to other metal precursors containing basic ligands like [Pd(OAc)2] [97] and [(cod)lr(p-OR)2lr(cod)] [98, 99]. Alternatively, an external base such as NaOAc, KOf-Bu or MHMDS (M = Li, Na, K) can be added for the deprotonation of the azolium salt [100]. In general, the in situ deprotonation of azolium salts appears as the most attractive method for the preparation of NHC complexes as it does not require the isolation of the reactive free carbene or its enetetramine dimer. 

Due to the lability of the Ag-Ccm-bene bond, silver NHC complexes of type 25 are useful agents for the NHC transfer reactions to transition metals. The preparation of silver NHC complexes using the Ag20 method and the transfer of the NHC ligand to other metal centers like gold has become a standard procedure in NHC complex synthesis. The Ag20 method often gives access to NHC complexes where alternative syntheses are tedious or not successful. 

Alternatively, complexes 54 and 55 with abnormally bound NHC ligands have been obtained by oxidative addition of a C-H bond to Pd° and Pt° while blocking the C2 position of the azolium salt [155] or even without any blocking of the C2 position by oxidative addition of a C5-I bond of a donor-functionalized imidazolium cation (Fig. 19) [156]. 

NHCs have become popular ligands in coordination chemistry owing to the facile access to this type of ligands and to metal-NHC complexes. Most NHC ligands are prepared from azohum compounds such as imidazolium, triazolium, benzimidazo-lium, imidazolidinium, or thi azolium salts [1]. Alternatively, the reductive desulfurization of imidazolin-, benzimidazolin-, and imidazolidin-2-thiones to yield a variety of NHCs has been described. The preparation of suitable azolium salts and imidazolin-2-thiones is presented in Sect. 2.1. This is followed by the description of methods to liberate the free NHCs from these compounds. Today, stable singlet... 

In the quest for additional active catalysts for the Mizoroki-Heck reaction, the advent of N-heterocychc carbene (NHC) ligands was warmly welcomed [32], Transition metal-transition metal-phosphine complexes, and have therefore attracted considerable interest as competitive alternatives in Mizoroki-Heck chemistry, which requires high reaction temperatures. Since the seminal application of NHC ligands in Mizoroki-Heck arylations by Herrmann et al. [33], several research groups have introduced novel palladium catalyst-NHC ligand combinations. These were tested and assessed in standard couplings of simple iodo- or bro-moarenes 60 and activated acceptors such as acrylates 61 or styrene (63) [32], and a selection of impressive examples is summarized in Scheme 7.14. 

In 2002, Hoveyda and coworkers introduced an alternative concept to install chirality in ruthenium olefin metathesis complexes through a Ci-symmetric bidentate NHC ligand, bearing binaphtholate moieties (Figure 11.33). The NHCs in this type of complexes lacked backbone substitution and it was chelation that prevented free rotation of the ligand [118]. In this case, chiral information installed within the A-substituent was transferred directly to the ruthenium center. Unfortunately, these complexes were found to be less active because of the reduced Lewis acidity at the metal center, mainly due to the exchange of Cl... 

Otherwise not observed with alternate catalyst systems. A relatively small collection of the most popular NHC ligands have been utilized in the vast majority of developments in nickel chemistry, and broader examination of novel NHC motifs will likely lead to an increasing number of new reactions and selectivity features. Additionally, further improvements in the development of air-stable and easily accessible NHC-nickel precatalysts will further popularize the unique properties of these species and the reactions enabled by their use. 

Sonogashira coupling reactions of primary alkyl halides have been reported. " The key to this was the use of an NHC ligand with bulky alkyl groups on the two nitrogen atoms (Scheme 2.121) lAd 1.51 and ItBu 1.55. This reaction is an alternative to the classical alkylation of acetylides ions with alkyl halides, with the advantage that base-sensitive functionality is tolerated. 

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