Carbenes, Fischer alkenylcarbene

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

Fischer alkenylcarbene complexes undergo cyclopentannulation to alkenyl AT,AT-dimethylhydrazones (1-amino-1-azadienes) to furnish [3C+2S] substituted cyclopentenes in a regio- and diastereoselective way along with minor amounts of [4S+1C] pyrrole derivatives. Enantiopure carbene complexes derived from (-)-8-(2-naphthyl)menthol afford mixtures of trans,trans-cycloipentenes and ds,ds-cyclopentenes with excellent face selectivity [75]. The mechanism proposed for the formation of these cyclopentene derivatives is outlined in Scheme 28. The process is initiated by nucleophilic 1,2-attack of the carbon... 

The potential of Fischer carbene complexes in the construction of complex structures from simple starting materials is nicely reflected in the next example. Thus, the reaction of alkenylcarbene complexes of chromium and tungsten with cyclopentanone and cyclohexanone enamines allows the di-astereo- and enantioselective synthesis of functionalised bicyclo[3.2.1]octane and bicyclo[3.3.1]nonane derivatives [12] (Scheme 44). The mechanism of this transformation is initiated by a 1,4-addition of the C -enamine to the alkenylcarbene complex. Further 1,2-addition of the of the newly formed enamine to the carbene carbon leads to a metalate intermediate which can... 

S+3C] Heterocyclisations have been successfully effected starting from 4-amino-l-azadiene derivatives. The cycloaddition of reactive 4-amino-1-aza-1,3-butadienes towards alkenylcarbene complexes goes to completion in THF at a temperature as low as -40 °C to produce substituted 4,5-dihydro-3H-azepines in 52-91% yield [115] (Scheme 66). Monitoring the reaction by NMR allowed various intermediates to be determined and the reaction course outlined in Scheme 66 to be established. This mechanism features the following points in the chemistry of Fischer carbene complexes (i) the reaction is initiated at -78 °C by nucleophilic 1,2-addition and (ii) the key step cyclisation is triggered by a [l,2]-W(CO)5 shift. 

Fischer carbene complexes 639 can be coupled with 3-alkynyl-2-heteroaromatic carbaldehydes to form initially the alkenylcarbene complexes 640, which undergo CO insertion to give the ketene intermediates 641, which upon ring closure affords heteroaryl-fused 277-pyran-2-ones (Scheme 147) <2001TL777, 2002JOC4177>. 

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