Die oxidative Addition von NickeI(0)-KompIexen an Kohlenstoff-Kohlenstoff-Bindungen im Cyclobutadien. Zur Frage der Überführbarkeit von Cyclobutadien-Nickel(O)-Komplexen in Nickelaringe / Oxidative Addition of Nickel(O) Complexes to Carbon-Carbon Bonds in Cyclobutadiene. The Question of the Interconvertibility of Cyclobutadiene-nickel(O) Complexes and the Nickelaring Systems

Abstract

Bis(triethylphosphine)(η⁴-tetraphenylcyclobutadiene)nickel (4) was synthesized by the reduction of (η⁴-tetraphenylcyclobutadiene)nickel(II)bromide (3) with t-butyllithium in the presence of Et₃P, and its structure was determined by X-ray crystallography. Furthermore, its reactivity towards CO, CH₃CO₂H, PhC≡CPh, LiAlH₄ and O₂ were investigated. 1,1-Bis(triethylphos-phine)-2,3,4,5-tetraphenylnickelole (14) was synthesized from (E,E)-1,4-dilithio-1,2,3,4-tetraphenyl-1,3-butadiene (15) and bis(triethylphosphine)nickel(II)bromide. Since the resulting crystals of the nickelole were not suitable for X-ray structure determination, the compound was characterized by elemental analyses, spectral data and carbonylation to yield tetraphenylcyclo-pentadienone (6).

Analogous reductions of (η⁴ -tetraphenylcyclobutadiene)nickel(II)bromide (3) in the presence of Ph₃P or Ph₂PCH₂CH₂PPh₂ , followed by carbonylation, led to 6 in 40% yield, demonstrating that about half of the cyclobutadiene rings in 3 undergo cleavage upon reduction to give the nickelole.

Reactions of the dilithium reagent 15 with NiBr₂ complexed with Me₂PCH₂CH₂PMe₂ ,Ph₃P or Et₂PCH₂CH₂PEt₂ , led to the formation of thermolabile nickeloles, as demonstrated by carbonylàtion which yielded 6. Warming of the nickeloles and subsequent treatment with CH₃CO₂H led to the formation of 1,2,3,4,5,6,7,8-octaphenyl-1,3,5,7-octatetraene (8) and, in one case, octaphenyl-cyclooctatetraene (5).

The relevance of these findings to the mechanism of the Reppe nickel-catalyzed oligomerization of alkynes is discussed.