Oxidative Coupling Of 2‐Alkyl‐6,6‐dimethylpentafulvenyl Anions

Abstract

~ ~_ _ _ _ _ _CuC1,-Induced oxidative coupling of 2-(rert-butyl)-6,6-dimethylpentafulvenyl anion 9 predominantly takes place at C(7) and C(5) to give [7-71 and (7-51 coupling products 15 and 16 in 35 and 47% yields, respectively (Scheme 3 ) whose structures are elucidated from ID-and 2D-NMR analysis. Compared with the product distribution observed for 6,6-dimethylpentafulvenyl anion 2 (Scheme I ) , no coupling at C(2)/C(3) of 9 is observed. This means that, besides electronic effects, steric effects are also important in oxidative couplings of fulvenyl anions. The same couplings occur in the case of 2,3-bis(6,6-dimethylfulven-2-yl)-2,3-dimethylbutane dianion 10 as well, but, due to electronic as well as conformational effects (Scheme 5), intermolecular coupling (to give polymers 17, Scheme 4 ) is strongly favored over intramolecular coupling. Mechanisms explaining base-catalyzed isomerization 15a + 15b e 1% (Scheme 6 ) as well as isomerization 16a e 16b (Scheme 7 ) are proposed.

Neue Wege zu Pentalen‐Vorstufen

“…Wir haben vor einigen Jahren gezeigt, dass sich Cyclopentadienide [20] und Isopropenyl-cyclopentadienide ( = 6,6-Dimethylpentafulvenyl-Anionen) [21] mittels CuC1, mit hohen Ausbeuten oxidativ kuppeln lassen. In analoger Weise lassen sich a,w-Di(cyc1o-pentadieny1)alkandiide umsetzen [22] [23], wobei die Ausbeute der Produkte eines intramolekularen Ringschlusses je nach Ringgrosse durch intermolekulare Kupplungen (zu Polymeren) herabgesetzt wird [23] .…”