N-Heterocyclic Carbenes Derived from Six- or Seven-membered Heterocycles

4 N-Heterocyclic Carbenes Derived from Six- or Seven-membered Heterocycles 

Aside from the ubiquitous NHCs featuring a five-membered heterocycle, significant advances have been made regarding the preparation of NHCs derived from larger heterocycles. The first stable NHC featuring a six-membered aliphatic heterocycle 132a was prepared in 1999 followed by a number of differently A,A -substituted derivatives. ° Aromatic derivatives 133a are also 

NHCs featuring an aliphatic seven-membered ring scaffold of type 132b68c,69,i98 obtained by deprotonation of amidinium salts with 

134 and its zwitterionic complexes were also prepared. In their search for NHCs, which allow tuning of the electronic properties by a redox reaction, BielawskP and Siemeling prepared the diaminocarbene[3]ferrocenophanes 

The characteristic C NMR resonances for the carbene carbon atoms of the NHCs featuring six-membered heterocycles (132a, 133a) fall in the range 5 = 235-245 ppm, similarly to imidazoKn-2-ylidenes 86, whereas the carbene carbon resonances for 132b and 133b appeared more downfield ( = 258-260 ppm). = 

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