NHCs ligand transfer reactions

The protocol of the allylic alkylation, which proceeds most likely via a c-allyl-Fe-intermediate, could be further improved by replacing the phosphine ligand with an M-heterocyclic carbene (NHC) (Scheme 21) [66]. The addition of a ferf-butyl-substituted NHC ligand 86 allowed for full conversion in the exact stoichiometric reaction between allyl carbonate and pronucleophile. Various C-nucleophiles were allylated in good to excellent regioselectivities conserving the 71 bond geometry of enantiomerically enriched ( )- and (Z)-carbonates 87. Even chirality and prochirality transfer was observed (Scheme 21) [67]. 

Due to the lability of the Ag-Ccm-bene bond, silver NHC complexes of type 25 are useful agents for the NHC transfer reactions to transition metals. The preparation of silver NHC complexes using the Ag20 method and the transfer of the NHC ligand to other metal centers like gold has become a standard procedure in NHC complex synthesis. The Ag20 method often gives access to NHC complexes where alternative syntheses are tedious or not successful. 

Metal-catalyzed oxidation of alcohols to aldehydes and ketones is a subject that has received significant recent attention [21,56,57]. One such method that utilizes NHC ligands is an Oppenauer-type oxidation with an Ir or Ru catalyst [58-62]. These alcohol oxidation reactions consist of an equilibrium process involving hydrogen transfer from the alcohol substrate to a ketone, such as acetone (Eq. 5), or an alkene. Because these reactions avoid the use of a strong oxidant, the potential oxidative instability of NHC ligands is less problematic. Consequently, these reactions represent an important target for future research into the utility of NHCs. 

Switching from palladium to rhodium, we encounter some very interesting chemistry. Zeng et al. [302] reacted the tiidentate PCP phosphino functionalised imidazolium salt with silver(I) oxide and subsequently transferred the carbene to rhodium(I) (see Figure 3.100). Careful selection of the rhodium precursor complex and reaction conditions enables tetrahedral, square bipyramidal and octahedral rhodium(I) and rhodium(III) complexes to be formed. As the authors explained, the activation of the C-Cl bond in methylene chloride in an oxidative addition reaction on rhodium(I) resulting in a rhodium(in) complex requires an electron rich rhodium(I) complex. The presence of a NHC ligand is advantageous in this respect. 

Reaction of this lO-S-3 [279] tetraazapentalene derivative with [Pd(PPhj) ] or [Rh(PPh3)3)Cl] results in the formal substitution of sulfur by the transition metal accompanied by a redox reaction (see Figure 4.93) [280], The endocyclic sulfur atom is transferred to a PPhj ligand (oxidation of phosphorus to PhjP=S). At the same time the transition metal is oxidis (palladium from 0 to +11 rhodium from +1 to +III), which leaves sulfur to be reduced by four electrons (it is -II in Ph I S and thus must have been +II in the tr-sulfurane starting material). It follows from this electron transfer analysis that the rt-sulfurane is indeed better desaibed as the sulfur complex of a doubly amide functionahsed NHC ligand. 

Another interesting approach to an NHC ligand with a chiral, bridging wingtip group was introduced by Perry et al. [45] and uses enantiomerically pure 1,2-diamino-cyclohexane as the scaffold. Reaction with chloroacetic acid chloride and subsequently with DIPP-imidazole yields the imidazolium salt that can be reacted with silver(I) oxide [46] to the respective silver(I) NHC complex. Subsequent carbene transfer to palladium(II) renders the chiral palladium(II) carbene transfer that can be used in catalysis (see Figure 5.9). 

NHCs with an alkoxy-anchor coordinated to alkali metals serve as excellent ligand transfer reagents. The reagent synthesized by Arnold et al. also showed an extraordinary stability and was successfully used in the preparation of [(NHC)Ti(Oi-Pr)3] 19 through a salt metathesis reaction (Equation (6.6)). This transfer reagent was also employed for synthesizing a related [(NHQaUI] complex. 

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