Enynes reaction with Fischer carbene complexes

Scheme 4. Reaction mechanism of enyne with Fischer carbene complex...

The results of a series of reactions of Fischer carbene complexes with enynes are summarized in Tables 1 and 2. Cyclopropane synthesis is accomplished in the alkoxy series (Y = OMe) by the generation of a mixture of geometric isomers of enol ethers, whereas in the dialkyl-amino series, ketones are directly obtained after hydrolysis of the enamines. Higher yields have been obtained using the amino analog pentacarbonyl(l-pyrrolidinoethylidene)chromium [Y = N(CH2)J. - ... 

Table 1. Reaction of Fischer Carbene Complexes with Enynes...

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). 

The substituent effects on the alkene were investigated in the reaction of enyne 12 and chromium carbene complex 2c [8]. In the reaction of enyne -12a having a phenyl group on the alkene with Fischer chromium carbene complex 2c, metathesis product 13a was obtained as a main product along with cyclopropane 14 and cyclobutanone 15 (Eq.4). The reaction of Z-12a with 2c gave only... 

Alkenylpyrroles 23, which readily underwent hydrolysis to 24, have been prepared via reaction of enyne-imines 25 or the corresponding enyne-A.A -dimethylhydrazones with the Fischer carbene complex 26. The hydrazones were found to be the more useful synthons, generally giving higher yields <030L2043>. 

Coupling of a Fischer carbene complex with an alkene can generate a vinylcarbene intermediate 12 via an insertion-rearrangement reaction, which can then further react with a double bond. For intramolecular reactions of tethered enynes 10, the products formed are bicyclic cyclopropanes 14 intermolecular reactions lead to cycloalkenylcyclopropanes. 

Closely related to the ring-closing metathesis of enynes (Section 3.2.5.6), catalyzed by non-heteroatom-substituted carbene complexes, is the reaction of stoichiometric amounts of Fischer-type carbene complexes with enynes [266,308 -315] (for catalytic reactions, see [316]). In this reaction [2 + 2] cycloaddition of the carbene complex and the alkyne followed by [2 -t- 2] cycloreversion leads to the intermediate formation of a non-heteroatom-substituted, electrophilic carbene complex. This intermediate, unlike the corresponding nucleophilic carbene... 

The reaction of enynes with Fischer-type carbene complexes can also lead to the formation of cyclobutanones (Figure 2.23) [315]. The mechanism for this reaction is likely to be rearrangement of the intermediate, non-heteroatom-substituted vinylcarbene complex to a vinylketene, which undergoes intramolecular [2 -i- 2] cycloaddition to form the observed cyclobutanones. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

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