Remote NHC

By convention, the carbenes displayed in Fig. 1.7 are normal NHCs as they are coordinated to the metal centre throngh the atom. By contrast, abnormal (also named non-classical or unnsnal) are those bound through the atom. Abnormal is also a term used for NHCs having a valence representation requiring additional charges. Remote is a term nsed to describe a carbene which does not have any heteroatom on the a-position to the carbenic carbon (Fig. 1.8) [57]. 

Recently a new type of metal bonded IV-heterocycUc carbenes named remote Af-heterocyclic carbenes (rNHCs) has been described [138]. In contrast to normal NHCs, rNHCs possess no heteroatom adjacent to the carbene carbon atom. The less efficient stabilization of the carbene carbon atom has so far prevented the isolation of a free rNHC which consequently have to be stabilized by coordination to a suitable metal center. 

Note Chirality in NHC ligands is normally more remote than that in phosphane ligands, relative to the metal centre of an appropriate metal complex. The reason is the inherently achiral nitrogen atom adjacent to the carbene carbon atom. 

The Chi group disclosed the first oxidative y-addition of enals to trifluo-romethyl ketones and enantioselective control via Lewis acid/NHC cooperative catalysis to give unsaturated 5-lactones. Enantioselective control involving the relatively remote enal y-carbon was achieved via Lewis acid and NHC cooperative catalysis (up to 81% yield and 94% ee). A reaction pathway... 

The Chi group recently achieved the first NHC-eatalyzed diastereo- and enantioselective [3 + 4] cycloaddition of azomethine imines and enals via 1,4-dipolarophile intermediates generated by oxidative catalytic remote y-car-bon aetivation of enals. Dinitrogen fused seven-membered heterocyclic products were produced with high enantiomerie purity (up to 81% yield and 99% ee). Racemic azomethine imines ean be used as 1,3-dipolar substrates to afford dinitrogen-fused seven-membered heterocyclic compounds with high enantiomeric purity (s-factor up to 339). The key vinyl enolate intermediate is generated by oxidative y-carbon activation of enal via NHC catalysis as the reactive 1,4-dipolarophile (Scheme 7.116). 

In 2012, Chi et al. disclosed an oxidative y-addition of enals to tri-fluoroacetophenone for the synthesis of unsaturated 5-lactones 114 under NHC catalysis. Scandium triflate/magnesium triflate as relatively strong Lewis acid cocatalysts were found to be effective for enantiocontrol involving the relatively remote enal y-carbon (Scheme 20.49). 

The a-C(sp )-H functionalization of carbonyl compounds is a well-known tool in organic synthesis, but functionalization of remote 3-C(sp )-H bond remains unusual (Scheme 2.40). For example, Pihko and co-workers developed Pd(II)-catalyzed oxidative 3-C(sp )-H arylation of P-keto esters with electron-rich arenes [233-236]. The Pd(II)-catalyzed dehydrogenative transformation of ketones to a,P-unsaturated ketones allows divergent P-C(sp )-H bond functionalization with a wide range of nucleophiles [237-239]. Two important independent reports on organocatalytic (aminocatalysis and NHC catalysis) P-C(sp )-H bond of aliphatic aldehydes were also achieved by Wang [240] and Chi [241]. 

Particularly interesting new work includes the development of remote and abnormal NHCs (see Chapter 5 for further details). Also, NHCs have not only been used as ligands in transition metal chemistry but also as nucleophilic organocatalysts. In an extension of this concept, Lavallo and Grubbs presented recently the first organometallic transformation catalyzed by NHCs. This illustrate nicely that almost 20 years after Arduengo s first report on an N-heterocyclic carbene, new NHCs as well as new applications for these interesting and versatile molecules are still and will be for some time the subject of intensive research. 

Poly-NHC Ligands Containing Abnormal, Remote, or Mesoionic Carbenes 216... 

Many more pincer-type hgands have been developed in the last years featuring abnormal, remote, or mesoionic NHCs, which will form the object of Section 2.4. 

The statements abnormal and mesoionic carbenes (aNHCs and MICs) are synonyms and are referred to free carbenes for which it is not possible to draw a structure resonance formula without formal charges (Fig. 6). Differ-endy, remote carbenes (rNHCs) are carbenes without any heteroatom in the a and positions of the carbene center. These three types of ligands are characterized by a different structure with respect to the one usuaUy owned by NHCs. In the case of normal carbenes (nNHCs), the carbene donor is stabilized by the presence in the a and a positions of two heteroatoms. To emphasize this difference, aNHCs, MICs, and rNHCs are also caUed... 

Rotation barriers around the NHC-Ni bond in [(NHC)Ni(Ti -allyl)(X)] species also reflected the bulk of the NHC, with increased barriers arising from a steric clash in the transition state where the NHC substituents must pass through the Ni coordination plane. Computed barriers were in good agreement with experimental trends and it was interesting to note the increased barrier for the " IMe system (AG = +17.8 kcal moC ) compared to the IMe analogue (AG = +15.2 kcal moC ) even though the extra steric bulk was imposed at the remote 4 and 5 positions. 

---