Synthese von Pentafulvalen durch oxidative Kupplung von Cyclopentadienid mittels Kupfer(II)‐chlorid

Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(II) Chloride Starting with a nearly quantitative coupling of cyclononatetraenide 7 to 1, 1′‐dihydrononafulvalene 8 by means of AgBF 4 , a simple general synthetic concept for fulvalenes is outlined ( Scheme 2 ), consisting in an oxidative coupling of ‘ Hückel anions’ like 2 and 7 to 1, 1′‐dihydrofulvalenes 10 with Ag(I) or Cu(II) salts, followed by deprotonation (→ 11 ) and oxidation (→ 12 ); it has been realised in the case of pentafulvalene ( 1 ; overall yield 61%; Scheme 3 ) and 1,2:5,6‐dibenzopentafulvalene ( 18 ; overall yield 66%, Scheme 4 ). NMR‐spectroscopic investigations show that 1 is a non‐aromatic compound with strongly alternating bond‐lengths, its π‐system being even more localised than that of simple pentafulvenes. In fact, 1 is extremely reactive in concentrated solutions above −50°. Besides of polymerisations, Diels ‐ Alder dimerisation 1 → 19 followed by a rearrangement 19 → 20 takes place ( Scheme 5 ).