The chemistry of metallacyclic alkenylcarbene complexes .3. C-C bond formation with carbon nucleophiles. Stereoselective dienylation of lithium enolates and cuprates and a novel cascade synthesis of highly substituted cyclopentenones from lithium acetylides

The cationic ferracyclic carbene complexes 1 react with carbon nucleophiles like lithium enolates and organocuprates to give the corresponding 4-substituted 1,(SE)-diene tricarbonyliron complexes 6 and 9. The reaction is thought to proceed by initial attack on the allyl terminus and subsequent ferra Claisen-Ireland rearrangement of the intermediate (eta(2)-alkene)carbene complex 4. This rationale is supported by the isolation and characterisation of analogous aminooxocarbene derivatives 8. With sterically demanding enolates products 6 are formed in diastereomeric ratios of up to 90:10, thus demonstrating the inductive power of the chiral metallacycle in 1. Lithium acetylides as carbon nucleophiles react with 1 according to a novel cascade to give regio- and stereospecifically 2,5,5-trisubstituted cyclopentenones 10 suggesting a concerted formation of four C-C bonds together with a CO insertion and a formal carbene transfer step.