1.3- Dienes reaction with iron carbene complexes

A similar type of reaction has been observed in the reactions of iron-carbene complexes with 1,3-dienes. In this case the direction of reductive elimination in the metal hydride intermediate corresponding to (193) is constrained to that which generates a conjugated 1,3-diene however, two isomeric products are also obtained from this reaction which are epimers about the face of the diene to which the iron tricarbonyl group is attached. Tliis reaction produces highly functionalized 1,3-dienyl complexes of iron in high yield under relatively mild conditions and will likely play a role in the development of the... 

Corey-Winter elimination dibenzobarralene (442). In a similar way, spirohex-4-ene was obtained in 80% yield from (443). Daub and his co-workers have synthesized several uncommon olefins, among these (442), from parent cyclic thionocarbonates, using iron pentacarbonyl as the efficient elimination promoter. Semmelhack and Stauffer have reported that bis-(cyclo-octa-l,5-diene)nickel (444) converts thionocarbonates of vic-diols into the corresponding olefins in excellent yields and with a high degree of stereospecificity. In connection with a discussion of the mechanistic pathways of this reaction, the authors presented evidence for the intermediacy of the carbene complex (445). 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Nucleophiles like lithium enolates and organocuprates can be added to the terminus of the allyl ligand of cationic cyclic allyl(carbene)iron complexes to give 4-substituted tricarbonyl[l,(3 )-diene]iron complexes. Subsequent to the nucleophilic addition, a ferra-Claisen rearrangement is supposed to be involved in the reaction mechanism. The free dienes can be released by treatment with ceric ammonium nitrate or alkaline hydrogen peroxide (Scheme 4-65). ... 

Scheme 4-65. Reaction of cyclic allyl(carbene)iron complexes with C-nucleophiles to afford substituted 1,3-dienes.

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