Imidazolin-2-ylidene, 1,3-disubstituted

Besides the addition of non-carbon nucleophiles to carbonyl and isonitrile complexes (Tables 2.2, 2.4), heteroatom-disubstituted carbene complexes can be prepared by direct addition of stable or latent carbenes to suitable complexes. The synthetic routes sketched in Figure 2.12 are those commonly used for preparing imidazoline-2-ylidene or imidazolidine-2-ylidene complexes. 

In many cases the synthesis of NHC complexes starts from iV,A/ -disubstituted azolium salts. Imidazolium salts as precursors for imidazolin-2-ylidenes are generally accessible by two ways complementing each other (i) nucleophilic substitution at the imidazole heterocycle or (ii) a multicomponent reaction building up the heterocycle with the appropriate substituents in a one-pot reaction. 

Similar trends were observed for RCM to access disubstituted olefins. However, this reaction was faster if catalyzed by complex 55d rather than by 55b. The overall activities were excellent and reached values close to those obtained with the standard Grubbs II catalyst precursors. Formation of trisubstituted olefins was slower and the triazolylidene-based complexes underperformed classical imidazolin-2-ylidene systems used in the original Grubbs II catalyst. 

Annoura, H. and Tatsuoka, T. (1995) Total synthesis of hymenialdisine and debromohymenialdisine stereospecific construction of the 2-amino-4-oxo-2-imidazolin-5(Z)-disubstituted ylidene ring system. Tetrahedron Lett., 36,413-416. 

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