NHC-Ni complexes

Cyclopropyl ketones 32 and cyclopropyl imines 33 can also undergo [3+2] cycloaddition reactions with enones 34 in presence of NHC-Ni complexes to afford the corresponding cyclopentane compounds 35 (Scheme 5.9) [11]. The catalytic system is prepared in situ from the use of [Ni(COD),], SIPr HCl salt and KOBu, the reaction also required the use of Ti(O Bu) as an additive to improve yields and increase reactions rates. In most of the cases, th products 35 were obtained in good to excellent diastereoselectivities. 

The [2-I-2-I-2] cycloaddition reaction of diynes 40 and carbon dioxide 41 were successfully catalysed by a NHC-nickel (Scheme 5.12) [15]. The NHC-Ni complex was prepared in situ from [NiCCOD) ] and two equivalents of carbene. Pyrones 42 were obtained in excellent yields at atmospheric pressure of CO and mild reaction conditions. 

Scheme 5.12 [2-I-2-I-2] Cycloaddition reactions of diynes and CO catalysed by NHC-Ni complex...

Enynes 79 can also undergo cycloisomerisation reactions in presence of NHC/ transition metal complexes (Scheme 5.32). The cycloadduct 124 can be prepared either in presence of complex 110 [36] or in presence of a NHC-Ni complex (prepared in situ from a mixture of [ (COD) ] and IDTB 123 [37]). In the latter case, the active catalytic species is believed to be a Ni-H species that is generated via C-H activation of the carbene ligand. 

Figure 13.8 NHC-Ni complexes used for various Kumada couplings.

In 2006, Grubbs and co-workers described a high yielding decomposition product formed from the reaction of ligand 50 with potassium hexamethyldi-silazane (KHMDS) and then [(Ph3P)2Ni(Cl)Ph]. Instead of the expected NHC-Ni complex 51, species 52 was isolated, which resulted from the reaction of the Ni Ph moiety with the carbene carbon (Scheme 3.19). The corresponding... 

During their studies on the aerobic oxidation of Jt-allylchloro (NHC)nick-el(II) complexes, Sigman and co-workers demonstrated that conformational restriction around NHC metal bonds was relevant to catalysis and should be considered in terms of catalyst design and for rationalization of observed reactivity patterns. In particular, it appeared that a monodentate ligand with a lesser volume within the square plane was more likely to promote p-hydride elimination. The issue of conformational flexibility versus rigidity was then expected to directly impact the catalytic behaviour of NHC-Ni complexes. 

In this context, Radius studied the activation of organonitriles with NHC-Ni complex. They showed that [(IiPr)4Ni2(COD)] led to the efficient addition and C-CN cleavage of aromatic and aliphatic nitriles. This reaction irreversibly proceeded via p -coordination of organonitriles, followed by formation of Irons aryl cyanide complexes (Scheme 10.2). They applied this reaction to acetonitrile or trimethylsilylnitrile as well as adiponitrile, but with the latter, some by-reactions were observed. 

Aryl Amination, Aryl Thiolation and Hydrothiolation Mediated by NHC-Ni Complexes... 

The sulfur-hydrogen bond addition to alkynes represents a convenient access to vinyl sulfides. Nolan, Beletskaya and co-workers reported a new selective hydrothiolation process induced by homogeneous NHC-Ni complexes." They showed that [(IMes)Ni(Cp)Cl] was effective in catalyzing the reaction of terminal alkynes and various aryl thiols (Equation (10.10)). The process was found highly selective and the classical transfer of two thioaryl groups did not... 

The nickel-catalyzed reaction of a Grignard reagent with an aryl hahde, widely known as Corriu Kumada," constituted the first example of efficient crosscoupling reaction of aryl halides, long before the most popular Migjta Stille coupling. However, the classical protocol was not properly optimized and recent improvements were achieved using NHC Ni complexes. 

In 1999, Cavell and co-workers reported that the Suzuki coupling reaction could be catalyzed by NHC-Ni complexes, although not as efficiently as observed with Pd. The introduction of a bulky NHC on the catalyst precursor led to a significant increase in activity, although not as pronounced as in the Pd case. In fact, Blakey and MacMillan described the first efficient Suzuki-Miyaura cross-coupling reaction mediated by NHC-Ni in 2003. The original reaction used aryltrimethyl ammonium triflates as coupling partners and in the presence of CsF, the authors showed that IMes-Ni [1 1] led to the efficient preparation of various biphenyl derivatives (Equation (10.15)). 

Finally, the efficient Negishi coupling of unactivated aryl, heterocyclic, vinyl chlorides as well as aryl dichlorides catalyzed by binuclear and mononuclear NHC-Ni complexes was recently described by Chen and co-workers (Equation (10.18)). Elaborated heteroarene-functionalized NHC were found to be highly effective for the preparation biaryls and terphenyls in good to excellent yields under mild conditions. Notably, the binuclear nickel catalysts showed higher activities than mononuclear analogues, probably because of a bimetallic cooperative effect. 

Cycloadditions constitute a remarkable tool for the synthesis of 1,3-cyclohexadiene derivatives and related heterocyclic compounds. Surprisingly, no cyclotrimerization of alkynes (Reppe reaction) has been reported to date with NHC-Ni complexes. 

Zuo and Louie also found that NHC-Ni complexes catalyzed the rearrangement of cyclopropylen-yne derivatives. However, the selectivity of the reaction was strongly dependent on the NHC-Ni catalyst as well as on the substrate substitution. With a SIPr/Ni 1 1 system, a hindered vinylcyclopro-pylene-yne gave the isomerized seven-membered ring as the sole product while the corresponding methyl derivative afforded a disubstituted tetrahydrofuran derivative. The authors then described a successful and general access to the later by using the ItBu/Ni 1 1 system (Scheme 10.6). 

NHC-Ni complexes displayed moderate activities eompared to literature data in the polymerization of alkenes. In addition, they showed low selectivity... 

In the last decade, NHC-Ni complexes have become ubiquitous reagents in catalysis. As shown in this Chapter, sterically hindered NHC ligands have led to improved reactivity and selectivity compared to classical Ni-phosphine reagents. The activation of C-F bond is probably the best example of such improvements. Even if (S)IPr and (S)IMes appear to be the most commonly used NHCs with Ni, the design of new bi- or tridentate ligands as well as asymmetric units appears as a promising area of research. The future of NHC-Ni complexes will probably be sunny. 

An air-stable, well-defined (NHC)-Ni complex is another effective 3d-metal catalyst for the mild anaerobic catalytic oxidation of secondary alcohols with such functionahties as ether, tertiary amine, and alkene, using nonanhydrous, -degassed 2,4-dichlorotoluene as both oxidant and sol-... 

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