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

Abstract

Synthesis of Pentafulvalene by Oxidative Coupling of Cyclopentadienide with Copper(I1) ChlorideStarting with a nearly quantitative coupling of cyclonondtetraenide 7 to 1, 1'-dihydrononafulvalene 8 by means of AgBF,, 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(1) or Cu(I1) salts, followed by deprotonation (-11) and oxidation (412); it has been realised in the case ofpentafulvalene (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 n-system being even more locdlised than that of simple pentafulvenes. In fact, 1 is extremely reactive in concentrated solutions above -50". Resides of polymerisations, Diets-Alder dimerisation 1 +19 followed by a rearrangement 19-20 takes place (Scheme 5 ) . 1.Einleitung. -Pentafulvalen (1) ist eine sowohl synthetisch wie theoretisch attraktive Verbindung, deren spektroskopische Daten wichtige Riickschliisse auf die n -Delokalisation und Ladungsdichteverteilung in gekreuzt konjugierten Systemen erlauben wiirden. Da 1 das n-System von Pentafulvenen aufweist, welches mit einer Butadien-Einheit iiberbriickt worden ist, konnte ein spektroskopischer Vergleich von 1 mit Pentafulvenen [ 1 J von Interesse sein.

The Fulvalenes

“…Their matrix sample was compared to one of authentic 5 supplied by Doering and showed absorption maxima near 295 and 310 nm in the matrix; more recent studies give the maxima at 287, 299, 313 and 416 nm. [58] Subsequently, Baird et al demonstrated that photolysis of diazocyclopentadiene proceeds via cyclopentadienylidene that can be diverted to ketene in the presence of CO (Scheme 4). [63,64] The loss of nitrogen is irreversible and the detailed study confirmed that 5 does not arise from carbene addition to unchanged diazo compound; a UV band was recorded at 296 nm.…”

“…[59] The reaction proves to be quite general and Cu II is often an alternative to Ag I . Thus the pathway of Scheme 1 can be markedly improved by using CuCl 2 in THF at -30°C for the cyclopentadienide coupling (dihydrofulvalene 14 is formed in 84 % yield) [58,71] and, after essentially quantitative deprotonation to 15 (BuLi), a second treatment with CuCl 2 and concentration provides a red solution (ca. 0.3 m) of spectroscopically pure 5.…”

“…[19] Coupling of the cyclopentadienyl anion gives dihydrofulvalene (14) that is double deprotonated (BuLi) to dianion 15, and subsequently oxidised (O 2 ). The principles embodied in this sequence are precisely those that gave the first pentafulvalenes [55][56][57] and, upon refinement, [58,59] have provided for the less common ring systems, 8 and 12 (see below). The early work, so nicely summarised by Bergmann, [5] is not repeated here save to note that deprotonation of cyclopentadiene by sodium ethoxide in the presence of a stabilised cyclopentadienone does not always work, [60] but that exposure to air of solutions of cyclopentadienyl anion show the characteristic UV absorptions of 5.…”

Ein Dominoprozess zu Dibenzopentafulvalenen mittels vierfacher Carbopalladierung

“…[2,3] Wurden sie ursprünglich als Kuriositäten mit ungewçhnlichen Bindungslängen und -winkeln wahrgenommen, [4] werden Pentafulvalene mittlerweile als Liganden [5] für Übergangs-metalle sowie als Zwei-Elektronen-Akzeptoren unter Entstehung von aromatischen Dianionen verwendet. [6] Während unsubstituiertes Pentafulvalen instabil bei Temperaturen über À78 8C ist, [7] zeigt das perchlorierte Derivat vollständig reversible Redoxeigenschaften.…”

“…[7,8] Weitere Synthesen erfolgten mithilfe von Grignard-Additionen an Ketone [9] sowie der Homokupplung halogenierter Spezies mittels Über-gangsmetallen. [10] Auch Diazoverbindungen, die leicht molekularen Stickstoff freisetzen, sind mçgliche Ausgangsverbindungen.…”