Five-membered heterocycle-derived carbenes

The 1,3-dipolar cycloadditions are a powerful kind of reaction for the preparation of functionalised five-membered heterocycles [42]. In the field of Fischer carbene complexes, the a,/ -unsaturated derivatives have been scarcely used in cyclo additions with 1,3-dipoles in contrast with other types of cyclo additions [43]. These complexes have low energy LUMOs, due to the electron-acceptor character of the pentacarbonyl metal fragment, and hence, they react with electron-rich dipoles with high energy HOMOs. 

Iwasawa et al. also developed a new reaction involving a three-component coupling process which affords five-membered heterocycles. This [2s+2sh-1c] cycloaddition reaction supposes the consecutive addition of an alkynyllithium derivative to a Fischer carbene complex followed by the addition of a third component which can be an aldehyde, an imine, an isocyanate, or C02 [119] (Scheme 74). 

In addition to the compounds listed above, a number of triazolium [36], thiazolium [37], and benzimidazolin-2-thione [32] derivatives as well as five-membered heterocycles with only one nitrogen atom [38, 39] or two phosphorus atoms [40] within the heterocycle, all of which are suitable for the generation of heterocyclic carbenes, have been described. 

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]. 

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

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

Carbenes Derived from Five-membered Heterocycles... 

Table 2.21. Preparation of five-membered, nitrogen-containing heterocycles by rearrangement of chromium-carbene-derived ammonium ylides.

The addition of other bifunctional nucleophiles also allowed the preparation, in moderate to high yields, of the five-, seven-, and eight-membered heterocyclic carbene species 55-58 (Fig. 9) which, in some cases, were formed along with minor amounts of other derivatives [63]. Related 1,2,3-diheterocyclizations involving (ethoxy)allenylidene complexes have been described, ethanol instead of HNMe2 being released in this case [62],... 

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. 

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. 

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