Organische Synthesen mit Übergangsmetallkomplexen, 53 4‐Methylencyclopentene und 5‐Methylencyclohexenone aus „Fischer‐Carben”‐ Chromkomplexen, Allenen und Alkine

Organic Syntheses via Transition Metal Complexes, 53. — 4‐Methylenecyclopentenes and 5‐Methylenecyclohexenones from “Fischer Carbene” Chromium Complexes, Allenes, and Alkynes A method is described for the two‐step construction of 4‐methylenecyclopentenes 6 and 5‐methylenecyclohexenones 7 from Fischer carbene chromium complexes, allenes, and alkynes. In the first step a tri(methylene)methane complex 3 is generated by the addition of the carbene chromium complex (CO)5Cr = C(0Et)Ph (1) to the allene 2. The second step involves an addition of 3 to alkynes HC≡CR [5: R = Ph (a), SiMe3 (b)] at 75°C to give 6 and/or 7 in high chemical yields. The product ratios strongly depend on the substituents R.