Organische Synthesen mit Übergangsmetall‐Komplexen, 54 Cyclopentadiene aus 1‐Metalla‐1,3‐dienen und Alkinen durch Cyclisierung intermediärer 1‐Metalla‐1,3,5‐triene (Metall = Wolfram)

Organic Syntheses via Transition Metal Complexes, 54. — Cyclopentadienes from 1‐Metalla‐1,3‐dienes and Alkynes by Cyclization of Intermediate 1‐Metalla‐1,3,5‐trienes (Metal = Tungsten) We report on first examples of the formation of cyclopentadienes from a 1‐metalla‐1,3‐diene L n M=C‐C=C [L n M = W(CO) 5 ] and an alkyne in [3 + 2] cycloaddition multistep reactions. In a first step the alkyne Et 2 N‐C=C‐Me ( 2 ) adds to the 1‐metalla‐1,3‐diene (CO) 5 W = C(OEt)‐CH=CHPh ( 1b ) to give 1‐metalla‐1,3,5‐trienes (CO) 5 W=C(NEt 2 )‐CMe=C(OEt)‐CH=CHPh ( 3b ) and (CO) 5 W=C(OEt)‐CH=C(NEt 2 )‐CMe=CHPh ( 4b ). In a second step 3b cyclizes to thecyclopentadiene complex 5 , which has an η 1 ylide‐type structure as established by an X‐ray analysis. Hydrolysis of 5 leads to the formation of a 3‐aminocyclopentenone 7 . The 1‐metalla‐1,3,5‐triene 4b yields a cyclopentadiene complex 8 . In contrast to 5 the carbon skeleton of 8 has a connectivity different from that of the C 3 unit of the 1‐metalla‐1,3‐diene 1 .