Bis-NHC-copper complexes

In parallel, Xia and coworkers have made an important contribution to the carbonylation area (Scheme 8.17) [46]. The double carbonylation of aryl iodides vyith amines highlighted [Cu(I)(IPr)j as the best choice for this reaction. Nevertheless, the combination of a NHC salt and [Cu(X)(NHC)j is required to reach high conversion. The influence of the halides clearly depicted a trend (I > Cl > Br). The reaction also occurred in the presence of [Cu(IPr)2] [BF4] alone [47]. When Nal is used as cocatalyst, the transformation is almost quantitative. The bis-NHC-copper complex is believed to be the active species. 

Af-Heterocydic carbene and phosphine systems were compared, and in some cases the bis-phosphine copper complex [Cu(NHPh)(dtbpe)] (dtbpe= l,2-bis(di-tert-butylphosphino)ethane) outperformed the NHC-based systems. Indeed, the transformation of aniline with acrylonitrile reached 95% conversion after 3h with dtbpe, whereas 12 h were required with [Cu(NHPh)(IPr)]. However, for the reaction of disubstituted cyclohexenone with aniline, [Cu(NHPh)(IPr)] outshone... 

Arnold and co-workers also reported the deprotonation of alkoxy imi-dazolium iodides with -butyl lithium to yield lithium alkoxide carbenes (Scheme 3).14 Single crystals of one of the complexes were grown from a diethyl ether solution, and revealed a dimer of LiL with lithium iodide incorporated to form a tetramer of lithium cations (7). The lithium-NHC bond distance of 2.131(6) A is similar to that of the lithium amide carbene 4. Also as in 4 there is distortion of the lithium-NCN bond which has an angle of 152.3°. The C2 carbon resonates at 200 ppm in the 13C NMR spectrum which is a relatively high-frequency, possibly as a result of the incorporated lithium iodide. The lithium salts were able to act as ligand transfer reagents and react with copper (II) chloride or triflate to afford mono- or bis-substituted copper(II) alkoxy carbene complexes. 

Mechanistic studies were undertaken to analyze the reactivity of bis-NHC systems, showing the formation of two intermediates during the first step a copper acetylide complex and an imidazoli(ni)um salt (Scheme 8.19). Interestingly, in... 

These three-component reactions performed in water are much scarcer. NHC copper(I) complexes [CuBr(SIMes)] (1 in Fig. 15.1) [13a] and [CuCl(SIPr)] (SIPr = Af,Af-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene, 7 in Scheme 15.9)... 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

The reactivity of dioxygen with nitrogen-coordinated copper(I) complexes has received extensive attention over the past two decades [53,54]. To date, analogous reactivity has not been realized for NHC-coordinated Cu(I). Ster-ically unhindered bis-carbene complexes of Cu(I) undergo rapid conversion to the corresponding ureas upon exposure to air in CH2CI2 solution (Eq. 4) [55]. This result suggests NHCs may not be universally applicable to metal-mediated oxidation chemistry. 

On the basis of the wide catalytic applications of NHC transition metal complexes [25], Nolan and coworkers have thoroughly studied the catalytic activity in CuAAC reactions of well-defined copper(I) complexes with general formula [CuX(NHC)]. Organic solvents, mixtures of EtOH/water, and pure water have been used as reaction media. In particular, it has been reported that complexes [CuBr(SIMes)] (1 in Fig. 15.1, SIMes=iVAf-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)) and [CuI(IAd)] (2 in Fig. 15.1, IAd=iVAf-adamantyl imidazol-2-ylidene) show a remarkable activity for the synthesis of a... 

Azide-tagged copper(I) complexes of NHCs analogous to the well-known bis [2,6-diisopropylphenyl]imidazol-2-ylidene (IPr) and AfdV -bis[2,6-diisopropylphenyl] imidazolin-2-ylidene (SlPr) in the reaction with propargyl alcohols, yield triazole functionalized copper(l)-NHC complexes with 75% and 69% yield. This strategy opens the possibility of diversifying the substitution patterns of these copper(l) complexes (Scheme 3.2) [32]. 

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