Organic chemistry of subvalent transition metal complexes: XI. Oxidative additions of nickel(0) complexes to carbon-carbon bonds in alkynes: Nickelirenes and nickeloles as catalytic carriers in the oligomerization of alkynes

Abstract

The formation of 2,3,4,5-tetraphenylnickelole-bis(triphenylphosphine) (IIIa) and 2,3,4,5-tetraphenylnickelole-bis(1,2-diphenylphosphino)ethane (IIIb), either from ( E, E)-1,2,3,4-tetraphenyl-1,3-butadien-1,4-ylidenedilithium (I) and the corresponding nickel(II) chloride-phosphine complexes (II) or from the reduction of η 4-tetraphenylcyclobutadienenickel(II) bromide dimer (XII) in the presence of phosphines, proceeds in good yields. Nickelole IIIa displays physical and chemical properties consistent with its structure and is a catalyst for the trimerization of diphenylacetylene. Nickelole IIIb is a highly associated structure but in its chemical response to alkynes, HOAc, O 2, Br 2, NaAlEt 2H 2 and heat displays the properties of a nickelole, rather than a cyclobutadienenickel(0) complex. Attempts to generate IIIb photochemically from η 4-1,5-cyclooctadiene(η 4-tetraphenylcyclopentadienone)nickel and diphos failed, but it was shown that structural types, such as η 4-tetraphenyl-cyclopentadienone(diphos)nickel (a model for the structure suggested by Hoberg and Richter for IIIb), are unstable. Oligomerizations of diphenylacetylene by bis(1,5-cyclooctadiene)nickel were retarded by conducting the reaction in THF or in the presence of diphos. This retardation permitted the interception of products ( cis-stilbene and ( E, E)-1,2,3,4-tetraphenyl-1,3-butadiene) diagnostic for the intermediacy of nickelirenes and nickeloles. Deuterium labeling verified the presence of carbon-nickel bonds. These trapping experiments, together with findings on the thermal behavior of nickeloles, are combined into a comprehensive view of the cyclotrimerization, cyclotetramerization and linear polymerization of alkynes by nickel(0).