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, (3 E )‐diene tricarbonyl‐iron 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 (η 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 CC bonds together with a CO insertion and a formal carbene transfer step.