Pyrrole durch metallinduzierte [3 + 2]‐Cycloaddition von 2‐Azaallenyl‐( = Iminocarben‐)Komplexen unter Übertragung einer CNC‐Einheit auf ein Alkin; ein Chromacyclopropen durch Cyclocarbonylierung eines 1‐Aminoalkins

Organic Syntheses via Transition Metal Complexes, 40. ‐ Pyrroles by Metal‐Induced [3 + 21 Cycloadditions of 2‐Aza‐allenyl (= Iminocarbene) Complexes with Transfer of a CNC Unit to Alkynes; a Chromacyclopropene Complex by Cyclocarbonylation of a 1‐Aminoalkyne The 2‐azaallenyl (= iminocarbene) complex L n M ⊖ ‐ CR = N ⊕ = CXR 1 1a [L n M = Cr(C0) 5 = OEt, R 1 = Ph] adds to the polarized alkyne Et 2 N ‐ C≡C‐CH 3 ( 3 ) to give a 2 H ‐pyrrole complex 4 in a metal‐induced [3+2] cycloaddition. On warming 4 yields the 2H‐Pyrrole 5 . Since the regiochemistry of the addition of the CNC unit is opposite to that of a nitile ylide, we consider 1 as an Umpolung equivalent (RC ⊕ = N ‐ C ⊖ XR 1 ) of a nitrile ylide (RC ⊕ = N ‐ C ⊖ XR 1 ). The 2H‐pyrrole 5 is kinetically unstable and isomerizes smoothly to the 1H‐pyrrole 7 under the influence of base by an intramolecular redox reaction. A novel (cyclopentadienyl)chromacyclopropene complex 8 resulting from a cyclocarbonylation of the iminoalkyne 3 is obtained as a minor side product. The structural data of 4 and 8 have been determined by X‐ray analyses.