Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(II) Chloride

Article abstract of DOI:10.1002/hlca.19860690719

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 'Hueckel 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 61percent; Scheme 3) and 1,2:5,6-dibenzopentafulvalene (18; overall yield 66percent; Scheme 4).NMR-spectroscopic investigations show that 1 is a non-aromatic compound with strongly alternating bond-lengths, its ?-system being even more localised than that of simple pentafulvenes.In fact, 1 is extremely reactive in concentrated solutions above -50 deg C.Besides of polymerisations, Diels-Alder dimerisation 1->19 followed by a rearrangement 19->20 takes place (Scheme 5).