Cyclic carbenes CAAC

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... 

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). 

Catalysts bearing cyclic (alkyl)(amino)carbenes (CAACs) 30-32 (Figure 10) were shown to generate cross product 6 in the standard CM reaction with significantly lower E/Z selectivity than standard catalysts [30]. E/Z ratios of 1.5-2.5 were observed at conversions <60% and had minimally increased to 3 at 70% conversion. This contrasts with 3 and 4 where initial E/Z ratios of 3 increase to 6 by 70% conversion. While catalyst 32 reached 60% conversion in Ih at 25 °C, 30 and 31 took 32 and 45 h, respectively. This is indicative of the cis-trans selectivity being largely independent of catalyst activity and rather being influenced by metallacycle preference. 

Soleilhavoup, M., Bertrand, G. (2015). Cyclic (AIkyl)(Amino)Carbenes (CAACs) Stable Carbenes on the Rise. Accounts of Chemical Research. 48(2), 256-266. 

Cyclic(alkyl)(amino)carbene (CAAC) ligands are unique donor Hgands with an easily tailored steric environment and have yielded a variety of robust gold complexes with unusual catalytic activity with alkyne substrates [179-181]. Here a CAAC-gold complex (23) could mediate the hydroamination of 3-hexyne in a sealed tube at 160° C to give imine product as observed by H NMR spectroscopy (Scheme 15.23) [179]. 

Figure 4.2 When comparing cyclic alkyl amino carbenes (CAACs) to N-heterocyclic car-benes (NHCs), the LUMO is lower in energy and the HOMO is higher in energy.

Figure 15.1 Structures of common NHCs and cyclic alkyl amino carbenes (CAACs). Conventional numbering of the carbon atoms in the central ring is shown at the top left.

The isolation of the non-cyclic amino(aryl)carbenes and amino(alk-yl)carbenes demonstrated that singlet carbene centres can be sufficiently stabilized by only one a-nitrogen atom. In 2005, Bertrand and co-workers succeeded in preparing the first cyclic alkyl(amino)carbenes (CAACs Scheme 1.15). The precursor for CAAC 108 was obtained from an imine by deprotonation with LDA (LDA = lithium diisopropylamide) and subsequent reaction with 1,2-epoxy-2-methylpropane to give 106, which was converted into cyclic aldiminium salt 107 by reaction with trifluoromethanesulfonic acid... 

On the other hand, the insertion of two classes of stable singlet carbenes (i.e., NHCs and cyclic (alkyl)-(amino) carbenes CAACs) into Si-H bonds has been mechanistically investigated using DFT calculations. The results indicate that the corresponding Si-H insertion processes are kinetically favoured for both NHCs and CAACs and that, in both cases, the corresponding products are formed via a three-centre transition structure. 

Monoligating NHCs proved extremely successful in ketone arylation. The groups of Nolan and Bertrand reported catalysts 68 and 69-71, respectively (Figure 9.15). Whilst complex 68 was based on commercially available IPr and had a rather unremarkable structure, Bertrand s catalysts were the first reported complexes of so-called cyclic alkyl(amino)carbenes, or CAAC. These ligands display unique steric properties and they are also stronger a-donors... 

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