Catalysts with unsymmetrical NHCs

A large number of second-generation Grubbs catalysts with unsymmetrically substituted NHC hgands have been synthesized [24]. However, the main challenge... 

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]. 

We believe that the principal advantage of (ADC)Pd-based catalysts as compared to Pd-NHCs, verified at the current stage of the studies, is the more modular character of the former species. Among routes leading to (ADC)Pd complexes, metal-mediated reactions of isocyanides with various nucleophiles permit a straightforward assembly of a wide range of well-defined metal carbene catalysts, including unsymmetrically substituted and chiral compounds. 

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). 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

Unsymmetrically substituted unsaturated NHC catalysts (silyl ether-mesityl 34 [86,87], perfluoroalkyl-mesityl 35 [86], ester-mesityl 36 [88]) can catalyze the RCM of dienes, however, with lower yields than those obtained with the parent catalyst 2 (Fig. 7). Subsequently, Blechert et al. reported the synthesis of alkyl-mesityl saturated NHC complexes 37 and 38. These catalysts show a generally lower reactivity than the corresponding symmetric catalysts 3 and 15 in RCM and CM [89]. 

---