ORGANIC SYNTHESES VIA TRANSITION-METAL COMPLEXES .56. HOMOPYRROLES AND DIHYDROPYRIDINES FROM N-VINYLAMINOCARBENE IRON COMPLEXES AND ALKYNES BY CYCLIZATION 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 = CR1; R, R1 = H, CH3, C6H5, CO2CH3) 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 pi-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.