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 H 2 C = C = CHR 1 (R = H, C 6 H 5 ) 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 R 1 ‐ C = C ‐ R 2 4 (R 1 = C 6 H 5r , CH 2 OCH 3 ; R 2 = H r C 6 H 5 ) 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 ′.