Organische Synthesen mit Übergangsmetall‐Komplexen, 56 Homopyrrole und Dihydropyridine aus N ‐Vinylaminocarben‐Eisenkomplexen und Alkinen durch Cyclisierung intermediärer 5‐Aza‐1‐metalla‐1,3,6‐triene

Organic Syntheses via Transition Metal Complexes, 56 1) . – Homopyrroles and Dihydropyridines from N ‐Vinylamino‐carbene Iron Complexes and Alkynes by Cyclisation of Intermediate 5‐Aza‐1‐metalla‐1,3,6‐trienes 2,3‐Homopyrroles 5 and dihydropyridines 7, 8 are obtained by the light‐induced addition of alkynes 4a – d (RC = CR 1 ; R, R 1 = H, CH 3 , C 6 H 5 , CO 2 CH 3 ) to the N ‐vinylaminocarbene iron complex (= 3‐aza‐1‐ferra‐1,4‐diene) 3 . The formation of the N‐heterocycles 5 and 7 as well as that of the pyrrolium carbonylferrates 6 can be explained by assuming the chelated 5‐aza‐1‐ferra‐1,3,6‐trienes A to be key intermediates in this reaction. Stereochemical considerations lead to the conclusion that the ring expansion of the 4 1/2‐membered ring of 3 to the 6 1/2‐membered ring of A occurs without prior ≡ decomplexation by the insertion of a C = C unit into the M = C bond. NMR measurements and MNDO calculations suggest that the bicyclic homopyrroles 5 are in equilibrium with the novel monocyclic homopyrrole species 9 . The structure of 6c has been established by X‐ray diffraction analysis.