Organische Synthesen mit Übergangsmetallkomplexen, 42 Allene als Liganden in zweikernigen Eisencarbonyl-Komplexen; Carbonylierungen von Allenen sowie deren Kupplung mit Alkinen und Allenen; dynamische “Keten-Scheibenwischer”-Komplexe

Organic Syntheses via Transition Metal Complexes, 42. - Allenes in Dinuclear Iron Carbonyl Complexes; their Carbonylation and Coupling with Alkynes and Allenes; Fluxional “Ketene Windshield-Wiper” Complexes On photochemically induced metallation with pentacarbonyl iron allenes H2C = C = CHR 1 (R = H, C6H5) give dinuclear acyl iron complexes 2, in which the central carbon of the C = C = C unit is attached to a carbonyl group. 2 on warming yield π-ally1,σ complexes 3 by loss of carbon monoxide. The reaction 2 ⇌ 3 + CO is reversibel. 3 add alkynes R1 - C = C - R24 (R1 = C6H5r, CH2OCH3; R2 = Hr C6H5) with insertion of the C = C triple bond into the M - C σ-bond to give dinuclear ketene complexes 5. The latter are fluxional, as was shown by NMR measurements. The ketene unit is moving back and forth like a windshield wiper. Thermolysis of 5 under normal pressure affords ferracyclopentadiene iron complexes 6 (by 1,3-hydrogen shifts) and 4-methylene-2-cyclopenten-1-one 7 (by cyclocarbonylation). Thermolysis of 5 under CO pressure gives 7 as the sole products. Complexes 3 also add allenes 1 with insertion into the M–C σ-bond and formation of a C–C bond between the central carbon atoms of both allene units to give dinuclear bis(allyl) complexes 8 and 8′.