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 AgBF4, a simple general synthetic concept for fulvalenes is outlined (Scheme 2), consisting in an oxidative coupling of ‘Huckel 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 119 followed by a rearrangement 1920 takes place (Scheme 5).