NHC/phosphine system

Employing the same protocol as for the chelating NHC/phosphine system, nonchelating NHC ligands [60] gave high yields of trans coupling products for an array of aryl bromides (Scheme 15). 

Others have investigated the kinetics of amination reactions mediated by catalyst systems employing the new electron-rich monodentate ligands. In particular, Hartwig has shown that for catalysis by a 1 1 palladium to Xn tert-butyl)phosphine system, a mechanism in which oxidative addition of aryl chlorides follows coordination of base to the palladium competes with the standard nonanionic pathway. Finally, Caddick, Cloke, and coworkers have studied amination reactions of aryl chlorides performed by palladium complexes of N-heterocyclic carbene ligands. They found the rate to be limited by the oxidative addition step, which occurs first through the dissociation of an NHC ligand. 

Tsoureas et al. [61] employed a closely related system bearing an NHC/phosphine ligand in well-defined catalysts of LPdX2 formulation (X=Br,Me) or their corresponding cationic complexes. The results in the Heck coupling of aryl bromides and methyl acrylate were lower than the results using the in situ system. A maximum TON of 2,242 was achieved when the reaction was performed in NMP as solvent and NEt3 as base. 

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... 

Early complexes developed by Lee and coworkers for use in Suzuki couplings were developed bearing various functionalities. Polydentate ligands involving a tethered NHC/phosphine or NHC/pyridine system were isolated, and their... 

In 2001, Mori and coworkers reported a nickel-NHC-catalyzed system for the reductive coupling of aldehydes and 1,3-dienes that had distinct properties from a nickel-phosphine based system. Both systems generated the homoallylic silyl ether however, use of phosphines resulted in selectivity, while using an NHC reversed selectivity and resulted in the Z isomer (Figure 13.42) [94]. 

We have analyzed NCI in a series of trifluoromethyl halides, CF3X, where X = Cl, Br, I with dimethyl ether (DME), dimethyl sulfide (DMS), trimethyl phosphine (TMP) and imidazolin-2-ylidene (NHC). All systems were optimized using second-order Moller-Plesset perturbation theory with the aug-cc-pVDZ(-PP) basis set [81]. The wavefunctions were obtained at DFT level using the B3LYP functional and the 6-31++G basis set except for iodine, where the pseudopotential LANL2DZ was used. 

Herrmann and co-workers reported the activation of aryl C-F bonds by [(NHC)Ni] systems under Kumada-Corriu-type coupling conditions. Screening of catalytic conditions revealed that yields with [(IPr)2Ni] were poorer than when the catalyst was generated in situ from [Ni(acac)2]/IPr-HBF4. Interestingly, the electronic influence of the aryl fluorides on the reaction rate as demonstrated by Hammett ct -values indicated different pathways for the NHC-modifled catalytic system versus the analogous Ni-phosphine systems. 

It is worth noting that [2 + 2 + 2] cycloadditions of imines and alkynes have not been reported in the presence of NHC-Ni systems. These reactions seem to fail since Ogoshi et al. noted that the reaction of 2-butyne and A -benzene-sulfonylbenzaldimine did not proceed in the presence of a NHC-Ni catalyst (IMes or SIMes). This result contrasted with the results obtained by the same authors for the reaction catalyzed by Ni/phosphine systems. 

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