CAACs

Analogs of NHCs such as cyclic diphosphinocarbenes [81] or alkyl-monoamino-carbenes (CAACs) [82, 83] were then designed. As compared to NHCs, their... 

Bertrand et al. demonstrated with the stable cyclic (aIkyl)(amino)carbenes (CAACs) 16 that only one nitrogen atom is sufficient for the stabilization of the carbene center. The resonance of the carbene carbon atom in carbenes of type 16 is... 

The cyclic version of alkylaminocarbenes, the cyclic alkyl aminocarbenes 10c (CAACs), have also been reported [36]. The rigid cyclic structure with very bulky substituents at the nitrogen atom increases the S/T energy gap (45.1 kcal/mol) [37]. Due to this electronic effect as well as to the more resistant substituent pattern, CAACs are much more resistant than the acyclic ones. 

Scheme 8 Metal complexes stabilized by CAAC ligands...

Very few catalytic reactions using complexes with stable non-NHC carbenes have been reported yet. Nevertheless, taking advantage of the specific properties of non-NHC carbenes, several catalytic reactions have been described, especially using CAACs. 

The push-spectator stabilization system enables one to employ various alkyl groups with different types of steric environment, which differentiate amino(alkyl) carbenes dramatically from the NHCs as ligands. Taking advantage of their steric and electronic properties, Bertrand et al. nicely demonstrated the utility of CAACs as ligands in the palladium catalyzed a-arylation of ketones. Depending on the nature of the aryl chloride used, dramatic differences were observed in the catalytic activity of Pd-complexes with CAACs featuring different types of steric environment [36]. 

As already mentioned, CAAC ligands can stabilize electron-deficient metal centers such as in cationic gold complexes. Complex 29 catalyzes a very unique reaction of enamines with acetylene, which produces a cumulene and an imine... 

Scheme 12 a-Arylation of ketones catalyzed by CAAC-Pd complexes... 

Scheme 13 Cumulene formation catalyzed by CAAC-Au complexes...

The cationic gold complex with CAAC ligand 29 can also catalyze the unprecedented hydroamination reaction of alkynes and allenes using ammonia [59, 60]. It was also demonstrated that it can catalyze the simple hydroamination reaction. It... 

Use the spectral decomposition to write A as CAC where A is the diagonal matrix of characteristic roots. Then, the Klh power of A is CAaC. Sufficiency is obvious. Also, since if some X is greater than one, Ak must explode, the condition is necessary as well. 

Stable cyclic (alkyl)(amino)carbenes (CAAC) have been developed by Bertrand et al. and can be readily prepared in a few steps starting from simple imines 16 (Fig. 4, Scheme 9) [64,65,82-84], A special feature of these 5-membered ring carbenes is their stabibzation by the help of a quarternary carbon next to the carbene. 

The cyclic variety of MACs are called cyclic alkyl amino carbenes (CAACs) and have an excellent steric element [198]. With a cyclohexyl wingtip group on the carbon end of the carbene - known as a wall of protection - they can sterically block one side of the transition metal centre in a transition metal carbene complex effectively introducing outstanding regioselectivity and stereoselectivity in catalytic processes [198] (see Figure 3.70). 

Figure 3.70 CAAC and NHC with cyclohexyl wingtip groups.

How might the foUowring compounds be prepared uaing Michael reaetnutii Sb w the nucleophilic donor and the elccimphilic aooeptor in each caac. 

From the decay scheme shown in Fig. 35, it is also clear that 1) every possible arrangement is favored as long as it forms in the cp direction, 2) insertion of Cu atoms in the surface is not favored, unless it is part of a Cu-Au trimer (bcb), 3) Cu atoms mostly remain in the overlayer anchoring cp chains (CAAC), 4) Au dimers tend to form in the surface plane, 5) no state where the simultaneous individual behavior of either Au or Cu atoms (bb-i-CC-i-AA) is favored by any chain, 6) individual Au atoms in surface sites are favored but probably due to the stabilizing effect of long, mixed chains in the overlayer and 7) the interplay between Cu and Au reflects in the fact that Cu atoms bracket Au atoms when forming chains in the overlayer, and Au atoms bracket Cu atoms in surface chains. 

CAACs) 16 that only one nitrogen atom is sufficient for the stabilization of the... 

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