Carbenes Derived from Five-Membered 6 Heterocycles

2 Carbenes Derived from Five-membered Heterocycles 

Most of the known N-heterocyclic carbenes are derived from five-membered heterocyeles with additional oxygen, sulfur or phosphorus heteroatoms. Stable NHCs with up to four heteroatoms and saturated or unsaturated five-membered heteroeyeles are discussed in the following Section. 

Imidazolin-2-ylidenes can be identified by the characteristic downfield resonance for the carbene carbon atom at 3 x 240 ppm in the NMR spectra 

Deprotonation of benzimidazolium salts or reduction of benzimidazol-2-thiones yielded benzimidazol-2-ylidenes or dibenzotetraazaful- 

To vary the electronic properties of the carbene carbon atom a number of carbo- and heterocycle-annulated NHCs were prepared and studied. Single pyrido-annulated NHCs (93-96) as well as the doubly pyrido-annulated carbene 97 were obtained by deprotonation of the corresponding azolium salts. NHCs such as the quinone-annulated carbene 98/ the naphtho-annu-lated carbene 99 and related polycyclic compounds have also been obtained from more extended ring systems. 

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In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Key to method (A) exchange reaction with tin heterocycle (B) hydride addition to diyne (C) oxidation of saturated ketone (D) bromination-dehydrobromination by pyrolysis (E) reaction of RLi or ArLi with exocyclic M-Cl of preformed diene (F) ring expansion reaction from cyclopentadiene derivative (G) LiAlHi reduction of exocyclic M-Cl (H) carbene insertion into five-membered cyclo-pentadiene derivative. Doering-Hoffman method (I) 1,6-cycloaddition of GeCU. 

The vast majority of A-heterocyclic carbenes contains the carbene in a five-membered ring with two adjacent nitrogen atoms. We consider platinum complexes containing these ligands first, and deal with the cases of those derived from a six-membered ring with one or two nitrogen atoms at the end of this section. 

Figure 1.2 Electronic configuration and resonance structures of carbenes derived from a five-membered heterocycle and containing an X2C carbene centre.

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