Unsymmetrical NHCs

Catalysts with an unsymmetrical NHC ligand featuring a vinylic side chain have the unique ability to metathesise their own ligand to form a metaUacycle as shown in Scheme 3.7 [119], Ring opening metathesis will then incorporate the monomers, e.g. cyclooctene, into that cycle until a cyclic polymer is cleaved by another intramolecular metathesis step. The catalyst is recovered and can restart this endless route to cyclic polymers [121]. 

The past decade has witnessed extensive modifications of Af-heterocyclic carbene ligands for ruthenium olefin metathesis catalysts. This includes symmetrical and unsymmetrical NHCs, 1,3- and 4,5-substitutions, introduction of heteroatoms into the backbone, NHC ring size variation, and introduction of chirality. Most of these changes were initially targeted to improve stability and activity of the catalyst, while recent approaches are mainly focused on affording well-defined stereoselectivity. However, the activity and stability of the ruthenium-based metathesis catalysts are not solely ruled by the type of neutral NHC ligand the anionic ligands, chelation mode, substrates used, and the reaction conditions naturally also influence catalytic properties. One of the main lessons learned from ruthenium olefin metathesis development is that there is no one catalyst fits all and every type of application must be studied in detail in order to discover the most efficient catalytic complex. 

An unusual unsymmetric NHC ligand was used to make homodinuclear ruthenium complexes 102 and 103 by Lemcoff and coworkers. These novel complexes showed an interesting propensity to promote dimer ring-closing metathesis (DRCM) (Figure 11.20) with the correct substrates [93]. 

Figure 11.31 Chiral Ru complexes 151 -154 bearing 4,5-cli-te/f-butyl-substituted unsymmetrical NHCs.

Complexes 23 and 24 bearing unsymmetrical NHC ligands have been prepared by replacing one of the mesityl substituent by a more electron-donating alkyl group [36] (Figure 4). The activity of these catalysts has been evaluated for the model RCM reaction of iV,A/-diallyl-/>-tosylamide and found to be similar to that of 3 and 6. Thus, the increased a-donation of the carbene did not result in maikedly enhanced catalytic activity. However, greater amounts of (Z) isomers were obtained witii 23a in comparison with 3 for a series of CM reactions. Improved diastereoselectivity was also obtained with these catalysts for a representative RCM reaction (Tables 2 and 3). 

SCHIFF BASE Ru-ALKYLIDENES -Ru-ALKYLIDENES BEARING UNSYMMETRICAL NHC LIGANDS... 

Scheme 16.7 Highly efficient alkyne hydration using unsymmetrical NHC ligands.

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